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					AE-892 (Revised)

                                        Home Sewage
                                        Thomas F. Scherer
                                        Extension Agricultural Engineer

North Dakota State University, Fargo, ND 58105

    Introduction ...............................................3
    House Plumbing for Sewage ................4
    Septic Tanks ..............................................6
      Sizing .........................................................6
      Location and Installation .........................7
      Construction .............................................8
      Inlet and Outlet Baffles ............................9
      Access Holes in Septic Tanks .................9
      Holding Tanks .........................................10
    Pumping Stations ...................................11
      Sizing .......................................................11
      Sewage Pumps .......................................12
         Pump Controls .....................................13
    Septic Tank Outlet Sewer ...................13
    Soil Absorption Systems......................14
      Absorption Trenches ..............................14
         Trench Construction.............................15
            Rock-filled Trenches ........................15
            Gravelless Trenches ........................17
         Effluent Distribution .............................18
            Drop Boxes......................................18
            Distribution Boxes ...........................20              References
      Absorption Beds ....................................21              North Dakota State Plumbing Code,
                                                                            Chapter 62-03.1-03, 2001, North
      Sewage Mounds .....................................22                 Dakota State Plumbing Board,
         Sizing ...................................................25       Bismarck, North Dakota.
         Construction ........................................27          Design Manual: Onsite Wastewater
                                                                            Treatment and Disposal Systems,
    Lagoons .....................................................28         1980. Report number EPA-625/1-
                                                                            80-012, Environment Protection
    Alternate Septic Systems ...................29                          Agency (EPA), Office of Water Pro-
      Filtration Systems ..................................29               gram Operations, Washington DC.
      Alternative Drainfields ...........................29                Onsite Wastewater Treatment
                                                                            Systems Manual, 2002. Report
    Appendix: Percolation Test......................30                      number EPA/625/R-00/008,
                                                                            Environmental Protection Agency
                                                                            (EPA), National Risk Management
                                                                            Research Laboratory, Cincinnati,
                                                                          Residential Onsite Wastewater Treat-
                                                                            ment Systems: An Operation and
                                                                            Maintenance Service Provider
                                                                            Program, 2005. MWPS (Midwest
                                                                            Plan Service), 122 Davidson Hall,
                                                                            Iowa State University, Ames, IA.

T     his publication contains information on the design, installation and maintenance
      of individual home sewage treatment systems. It is meant to be a homeowner
reference document. An individual sewage system both treats and disposes of house-
hold wastewater. If a homeowner understands how the various components of a home
sewage system work, then a properly designed and installed system will function for
many years with a minimum of maintenance and upkeep.
     Home wastewater enters the septic tank, which separates solids from liquids.
Solids are held in the septic tank and liquids are conveyed to the final soil treatment
site. The septic tank is a “bioreactor” where microorganisms break down organic
matter in the wastewater to liquids, gases and solids. Gases are vented off through
the house vent stack. Solids are composed of both scum and sludge. Scum is lighter
than water and floats to the surface in the septic tank. The solid parts are heavier than
water and sink to the bottom of the tank. Bacteria feed on the wastes and the fraction
that can’t be decomposed is “sludge.” Sludge accumulates in the bottom of the septic
tank and must be removed periodically.
     The five parts of a sewage disposal system are: (1) the house plumbing, (2) the
sewer line from house to septic tank, (3) the septic tank, (4) the septic tank outlet
sewer pipe, and (5) the final soil treatment unit, which may be a soil absorption unit
or lagoon. All individual sewage treatment systems must comply with requirements
in the North Dakota State Plumbing Code. In some counties, a permit is required
before constructing a new home sewage treatment system or when repairing an
existing system. Before construction begins, check with your local zoning administra-
tor, sanitarian or health department.
     Whether you need a permit or not, concern for basic human health demands that
the location of individual home sewage system components meets certain require-
ments. For example, keep the septic tank and soil absorption unit at least 100 feet
away from any private well that is less than 100 feet deep, and at least 50 feet away
from wells more than 100 feet deep. Generally accepted safe distances are shown
in Table 1.
     When the sewage treatment system is installed, make a map of the installation.
Measure and record distances from the septic tank, septic tank cleanout and drainfield
to aboveground features such as buildings, fence corners or large trees. Then after the
area has grassed over, you still can find the component parts of the septic system.

  Table 1. Separation distance between the septic system components
  and water sources (all distances in feet).
                         Well less   Well more
                         than 100     than 100 Distribution Treatment   Property
                         feet deep   feet deep   device        area       lines    Buildings
  Building Sewer           100         50            -          -          -          -
  Septic Tank              100         50           5          10         10         100
  Distribution Device      100         50            -          -         10         20
  Treatment Area           100         50            5          -         10         10
  Well (shallow)            -           -          100        100         NA         NA
  Well (deep)               -           -           50         50         NA         NA
  Pressure Water Line       -           -           10         10         NA         NA
  Suction Water Line        -           -           50         50         NA         NA
  Surface Water Bodies     NA          NA          100        100         NA         NA
  NA – not applicable

House Plumbing for Sewage

T     he house plumbing system in-
      cludes waste pipes, vent pipes
and water traps (Figure 1). House
plumbing and home sewage treat-
ment systems must comply with
the North Dakota State Plumbing
Code. Following this code ensures
that a plumbing system will be safe
and operate properly.
     Waste and vent pipes usually
are the same pipe, with wastewa-
ter flowing downward and gases
rising in the pipe. A 3- or 4-inch
vertical pipe serves as the main
stack to carry wastes, water vapor
and gases from the house. The
main stack also acts as a vent for
gases that collect in the septic
tank. Gas from a septic tank has
a bad odor, may cause serious
illness, and in some situations,
can be explosive. In cold weather,
the gases exiting from the stack
contain water vapor that will form
a frost layer that can get thick
enough to close off the end of
the stack. Excessive snow on the
roof also can block the vent stack.
Closing off the vent stack will pre-
vent fixtures from draining properly.
The vent stack extending above
the roof should be insulated to
help prevent frost and snow from
closing it off.
     A water trap must be installed
in the drain line between each fix-
ture and the main stack. The trap
prevents sewer gases from coming
into the house through the fixtures.
Without a vent pipe, a full flow of
wastewater in the drain line could
siphon water out of the traps and
let sewer gases into the house.
Sometimes, during very windy

                                       Figure 1. House plumbing includes waste and vent pipes, as well as
                                       plumbing fixtures.
conditions, wind pressure on the           House sewer pipe should          insulation over the pipe during
vent stack can force sewer gas        have a slope between 1 percent        the installation process. The rigid
through the trap. Proper venting      and 2 percent. This is around a       foam insulation should extend
installed according to the plumb-     1- to 2-inch drop in 8 feet. On too   at least 1 foot on either side of
ing code will prevent this problem.   flat a grade, the liquid will slow     the pipe. Insulation generally is
     Adequate cleanouts are           down, allowing the solids to settle   not needed if the house sewer
necessary in the stack so the         out in the sewer pipe. On too         is more than 4 feet below the
plumbing and sewer line can           steep a grade, the liquids will       ground surface.
be serviced and cleaned. One          flow away from the solids.                 Don’t make sharp bends in
cleanout should be installed at the        The sewer line from the house    the house sewer system. When
base of the stack and a second        to the septic tank may be plas-       45- or 90-degree bends are
at the point where the sewer line     tic sewer pipe with glued joints      necessary, use long sweep (long
leaves the house. One cleanout        or cast iron with stainless steel     radius) elbows to allow a plumb-
may be enough if the stack is near    clamps or leaded joints. If using     er’s snake to get through the
the point where the house sewer       plastic pipe, the pipe must have a    sewer line. If long sweep elbows
leaves the building.                  pressure rating equal to Schedule     are not available, use several
     Cast iron or copper drain        40 or greater. The joints must be     22- and half-degree elbows.
systems may be found in older         glued so they are watertight and         Never, under any circum-
homes. Most new homes use             resist root penetration.              stances, allow basement
plastic sewer pipe listed in the           The house discharge sewer        footing drains to discharge
plumbing code. When working in        must be at least 4-inch diam-         into the house sewage sys-
older homes, avoid direct copper      eter pipe. This pipe must have a      tem. This water will overload the
to iron pipe connections since        uniform slope with no high or low     septic system. It can cause the
pinhole leaks may develop in the      spots. A common place for frost       water and sewage to back up into
iron due to galvanic action. Use      to accumulate in the house dis-       the house. Run basement footing
insulated connectors between          charge sewer pipe is just after the   drain water to a sump. Install a
copper and iron pipe to reduce        pipe passes through or under the      sump pump and pump the water
this problem.                         basement wall. Where the house        out away from the drainfield.
                                      sewer pipe is above the frost line,
                                      this problem can be corrected
                                      by putting 2-inch-thick rigid foam

Septic Tanks

S    eptic tanks have been used
     for on-site wastewater treat-
ment for more than 120 years.
A septic tank can have single or
multiple compartments. Single-
and two-compartment septic tanks
generally are used with individual
home sewage treatment systems.
Household wastewater enters
the septic tank through the house
discharge sewer pipe (Figure 2).
After passing through the inlet baf-
fle, the solids separate from the
liquid as the sewage flows slowly
through the septic tank. Some sol-
ids settle to the bottom of the tank
and others float in the scum layer
at the top. Bacterial action partially   Figure 2. Typical septic tank. Sludge builds up in the bottom and scum
decomposes the solids.                   floats on the liquid surface.
    The material in the septic
tank separates into three distinct
    1. A top layer of floating scum
    2. A middle liquid zone              tarians call effluent on the ground      Sizing
    3. A bottom layer of sludge          surface “day lighting.” Human               Septic tanks are rated accord-
     The scum layer consists             effluent on the ground surface           ing to liquid-holding capacity, not
primarily of cooking fats and            can be a source of dangerous            total capacity. The liquid capacity
oils, soap scum and products of          water-borne diseases and pro-           of a septic tank is the volume of
decomposition that are lighter           duce offensive odors. The effluent       effluent it holds below the tank
than water. The greatest amount          must be delivered to a properly         outlet. Liquid capacity often is
of bacterial action occurs in the        designed and constructed drain-         called the “working capacity” of a
sludge layer, which consists of          field or lagoon for treatment.           septic tank. For a house, the re-
solids heavier than water.                   All household wastewater            quired working capacity is based
     The liquid discharged from a        must go to the septic tank. House-      on the number of bedrooms, not
septic tank is called effluent. Ef-       hold wastewater sometimes is            the number of people in the house
fluent from a properly maintained         referred to as “black water” or         at the time of construction. Each
septic tank is slightly cloudy and       “gray water,” depending on which        bedroom can hold two people,
contains fine suspended solids,           appliance or fixture it came from.       so the standard three-bedroom
bacteria and nutrients. Sep-             No matter the source, do not let        house could have six people gen-
tic tank effluent must not be             water and other similar wastes          erating household wastewater.
discharged directly to the               bypass the septic tank. Grey water
ground surface or into sur-              containing soap or grease sent
face waters. Professional sani-          directly to a drainfield will plug the
                                         soil pores quickly.

                                            Location and                           ground surface. A shallow sep-
                                            Installation                           tic tank and drainfield provides
                                                 The septic tank should be         easy access to the component
                                            at least 10 feet away from the         parts, plus the drainfield is more
                                            house. The tank should be aligned      efficient at treating effluent. For
                                            straight out from the point where      new houses, the North Dakota
                                            the discharge sewer line leaves        Plumbing Code recommends us-
                                            the house. Installing the tank so      ing a basement sump (Figure 3)
 Table 2. Recommended                       it is level, with no slope in any      designed to handle wastewater if
 septic tank liquid capacities.             direction, is important. For pump-     the septic tank and drainfield can
                        Minimum             ing and cleaning, the septic tank      work by gravity and the home has
                        Working             should be situated near a drive-       basement toilets and fixtures. For
                        Capacity            way or other access road. Most         these types of systems, the sewer
              Minimum for Homes
              Working with Garbage          septic pump trucks carry between       pipe leaves the house through
 Number of    Capacity* Grinders#           50 and 100 feet of hose, so the        the basement wall. The sump lifts
 Bedrooms     (gallons) (gallons)
                                            tank should be accessible from         the water to the sewer pipe. If the
 3 or less      1,000         1,500         this distance. Select a location       house sewer pipe is less than
 4              1,250         1,750         away from high vehicle traffic          4 feet below ground surface, it
 5              1,500         2,000
                                            areas. Never locate septic tanks       needs to be insulated in the trench
 *Working capacity is the tank volume
 below the outlet. An additional internal
                                            under sidewalks or patios where        for frost protection. Ready-to-in-
 volume equal to 20 percent of the          the tank is inaccessible for pump-     stall basement wastewater sump
 working capacity is needed for floating     ing.                                   stations are available in most
 scum storage.
 #Garbage grinders require septic                A home sewage treatment           home supply or hardware stores.
 tanks with about 50 percent greater        system works best and is more               Having the lift pump in the
 volume and often a second compart-
 ment is required.                          easily maintained if both the septic   basement has some major ad-
                                            tank and drainfield are near the        vantages. First, that allows the

Figure 3. Shallow septic tank installation.

wastewater from the upper part          at a shallow depth. In areas where       Construction
of the house to flow by gravity to       the water table can rise above the
                                                                                      Septic tanks are built using
the septic system. This has proven      top of the septic tank, homeown-
                                                                                 corrosion- and decay-resistant
to be valuable during a flood or         ers must seal around the inlet and
                                                                                 materials. If installed properly,
power outage. Second, the hom-          outlet pipe holes, any joints, the
                                                                                 they will be watertight for a long
eowner doesn’t need a lift station      septic tank cover and manhole ac-
                                                                                 time (30 years or more). Precast
that pumps all the effluent from a       cess so they are watertight.
                                                                                 reinforced concrete tanks are the
house. Third, a basement sump               Wastewater from the house
                                                                                 most common. However, tanks
only turns on when basement             and bacterial action usually
                                                                                 also may be constructed using
fixtures are used. Last, it’s acces-     provide enough heat to keep the
                                                                                 poured-in-place concrete or built
sible for maintenance and repairs.      septic tank from freezing, even
                                                                                 from concrete blocks with the
     Many older houses have a           when located above the frost
                                                                                 cores filled with concrete rein-
septic tank installed lower than        depth. When the top of a septic
                                                                                 forced with rebar. Concrete block
the basement floor level. New            tank is installed within 2 feet of the
                                                                                 tanks must be sealed with at least
houses can have a deep septic           ground surface, covering the top
                                                                                 two coats of concrete plaster.
tank, providing a high water            of the tank with 2 inches of rigid
                                                                                 Fiberglass and durable plastic
table is not present (Figure 4).        foam insulation board will help re-
                                                                                 septic tanks also are available.
If suitable slope is not present, a     tain heat in the septic tank. When-
                                                                                 They must be installed by care-
lift-pump station must be installed     ever the system is built above frost
                                                                                 fully following the manufacturer’s
at the outlet side of the septic tank   depth, laying all sewer pipes at a
                                                                                 instructions so they will withstand
so the drainfield can be installed       uniform grade with no high or low
                                                                                 soil and water pressure.
                                        spots is very important.

Figure 4. Deep septic tank installation. Suitable for use where a high water table is not present. Always
check with the tank manufacturer to make sure the tank has the load- bearing capacity to handle the soil

    Septic tanks should be de-                Baffles must be durable and        ground level, extensions must be
signed for a liquid depth of not         corrosion-proof. Durable concrete,     attached to the top of the tank to
less than 3 but not more than 6          fiberglass or plastic are excellent     bring the cover within 12 inches of
1/2 feet. The horizontal distance        materials. Never use steel baffles,     the ground surface. In high water
between the inlet and outlet of          as they will corrode quickly. If       table areas, the extensions and
rectangular tanks should be about        baffles are bolted in place, use        cover must be watertight. If the
three times the width of the tank.       only stainless steel bolts.            manhole access is at ground level,
Incoming solids will settle out               The bottom of the inlet baffle     it should have a locking bracket
in this distance and not flow out         should be at least 6 inches below      or be chained shut with a padlock
to the drainfield. Circular tanks         the liquid surface when the tank is    (Figure 5).
should have at least a 5-foot            full. The bottom of the outlet baffle        To check the septic tank’s
inside diameter.                         should be at least 18 inches below     operation, the tank should have
    The liquid depth in a septic         the liquid surface. The top of the     two inspection ports, each 4 to 6
tank is the distance from the outlet     baffles must be open and extend         inches in diameter. One should
pipe to the bottom of the tank. The      no closer than 1 inch to the tank      be over the inlet baffle and one
septic tank inlet should be at least     cover. This is necessary for proper    over the outlet baffle (Figure 2).
3 inches above the outlet. The           venting of gases from the tank.        This allows for easy inspection
tank must have room above the                                                   to determine if sewage is flowing
liquid level for scum accumulation.                                             into or out of the tank properly
A minimum of 20 percent of the
                                         Access Holes                           or if plugging has occurred. The
tank’s liquid depth should be left       in Septic Tanks                        inspection ports must be capped
as freeboard between the outlet              Access to septic tanks is          at or just above ground level. The
level and the cover of the septic        required for inspection and pe-        caps prevent gas from escap-
tank. For most manufactured              riodic cleaning. One manhole at        ing and children from dropping
tanks, this will be about 12 inches.     least 15 inches in diameter must       objects into the tank.
                                         be on the top of the tank. The              Septic tanks must be cleaned
                                         manhole should have a concrete         regularly to remove the sludge
Inlet and                                cover with an earth covering of        that accumulates in the bottom of
Outlet Baffles                            at least 6 inches, but not more        the tank. Note that the manhole,
    Inlet and outlet baffles are criti-   than 12 inches. If the top of the      not the inspection ports, must be
cal parts of a correctly installed       tank is more than 12 inches below      used for cleaning the tank. For
septic tank (Figure 2). The inlet
baffle directs incoming sewage
downward into the liquid zone of
the septic tank. The outlet baffle
allows sewage effluent to flow out
of the liquid zone, retaining the
scum in the tank. Many new septic
tanks have a removable filter
incorporated into the outlet baffle.
The filter prevents nondecompos-
able materials (condoms, sanitary
napkins and other items) from
reaching the drainfield.
    Baffles may be constructed
as an integral part of the septic
tank or fastened to the tank after
construction. Four-inch diameter
plastic tees often are used as
baffles (Figures 2, 3 and 4).
                                         Figure 5. Methods of securing a manhole cover to prevent
                                         unauthorized entry.

most houses, cleaning every three     be pumped on a regular basis,         The cover must have a lock or be
years is satisfactory. However, a     and reducing daily water use in       chained shut to prevent children
house with a garbage grinder that     the house will contribute to lower    or other unauthorized individuals
the occupants use may require a       annual pumping costs.                 from entering it (Figure 5).
one- to two-year cleaning interval.       The holding tank must be              Since the tank is watertight, it
Garbage grinders increase the         watertight and hold at least 400      could try to float out of the ground
organic matter flowing into the        gallons for each bedroom in the       right after being pumped if the
septic tank.                          house. However, for any house,        local water table is within 3 feet of
                                      the minimum tank size should          the ground surface. In locations
                                      hold at least 1,000 gallons. For a    with high water table conditions,
Holding Tanks                         common three-bedroom house            earth anchors may be necessary.
     Some houses are in areas         occupied by four people, a 1,200-     Keeping the tank above the water
where a septic tank and drainfield     gallon tank will provide about        table and pumping sewage from
will not work. Locations with a       six to 10 days worth of storage.      the house if necessary usually are
high water table, bedrock close       The holding tank must be situ-        best.
to the surface and very small lots    ated so it is accessible to a pump        Installing a meter to measure
with insufficient drainfield area are   truck under all weather condi-        water flow into the house, and
the most common reasons. Often        tions and where accidental spills     thus the amount of water flowing
the only option is to use a holding   during pumping will not create a      into the holding tank, may be nec-
tank for household wastewater         nuisance. The tank must be level      essary. By periodically checking
(Figure 6). People with holding       and placed on firm, settled soil       the water meter, you will be able
tanks must practice good water        capable of bearing the weight of      to determine when the tank is full.
conservation so the tank doesn’t      a full tank. The riser of a holding   A better method would be to use
fill quickly. Holding tanks have to    tank must come to the surface.        a high-water alarm. These can
                                                                            be either electronic with a remote
                                                                            reader in the house or a floating
                                                                            indicator that protrudes above
                                                                            ground surface.

Figure 6. Holding tank for household effluent. Must be pumped on a
periodic basis (usually every two weeks to a month).

Pumping Stations

P    umping stations are required
     in situations where the ef-
fluent cannot flow by gravity and
must be lifted to a destination.
Many new houses are using lift
stations in the basement, and
outside the house the wastewa-
ter flows by gravity through the
septic tank to the drainfield. Many
manufacturers make complete
ready-to-install pump stations for
basements. Many older on-site
systems have pumping stations
that pump all the water coming
from a septic tank to the drainfield
(Figure 7). Pumping is required
with a deep septic tank installation
and shallow soil absorption unit or
when the soil absorption unit is at
a higher elevation than the septic
tank. A mound soil absorption sys-
tem will require a pumping station.
A pumping station consists of two
main parts — the watertight tank
and the pumping system that in-        Figure 7. Septic tank effluent pumping chamber. Working capacity
cludes the pump, on/off switches,      should equal about one-fourth the daily sewage volume. Reserve plus
alarm system and wiring.               working capacity should equal about one day’s sewage volume. This
                                       allows time to correct any pumping problems.

     The tank containing the pump      A reasonable reserve capacity is     off setting. Ranges of 12 inches
must be watertight. If not, ground     three-quarters of the daily esti-    to 30 inches are common. The
water may seep into the tank           mated wastewater volume.             following table gives some capaci-
and the excess water easily will           For example, a three-bedroom     ties of circular tanks in gallons per
overload the drainfield. Whether        house with a design volume of        foot of depth.
the pumping station is in the base-    450 gallons per day (gpd) requires
ment or outside, the tank should       a tank with a working volume          Inside Diameter   Gallons per foot
have enough volume below the           of about 110 gallons (volume              (inches)          of depth
inlet pipe to hold about a day’s       between pump-on and pump-off)               18                13
worth of wastewater. The tank          and a reserve capacity of about             24                23
should hold about one-fourth the       330 gallons. The working volume             30                36
                                                                                   36                53
daily wastewater volume between        plus reserve capacity equals                42                72
the on and off levels of the pump      about 450 gallons, or one day’s             48                94
controls. Some reserve capac-          amount of wastewater storage.               60               147
ity in the pumping tank must be            Pump controls often have a              72               212
available in case of pump failure.     limited range between the on and

    The capacity of rectangular          tanks. Sump pumps sold in home                When choosing a pump for a
tanks in gallons per foot of depth       supply and hardware stores for            mound soil absorption system,
can be calculated by multiplying         basement drain water are not              size the pump for a delivery of
length in feet by width in feet by       recommended for use in sewage             about 7.5 gpm per 100 square
7.5. For example, a rectangular          lift stations. Pedestal sump pumps        feet of rock bed area. For exam-
tank with an inside width of 4 feet      with an open motor should not be          ple, a mound for a three-bedroom
and inside length of 5 feet has          used except in an emergency.              house has about 300 square feet
a capacity of 4 x 5 x 7.5, or 150              All lift pumps are designed to      of rock bed area, so the pump
gallons per foot of depth. With          be submersible. Pump bodies are           should have a flow rate around
a 12-inch on-off setting for the         commonly made from cast bronze,           27 gpm. The pump must have this
pump, this tank would handle a           cast iron and plastic. All bolts, nuts    capacity at the required head. The
three-bedroom house easily. A            and screws are stainless steel.           required head will be the eleva-
circular tank would need a diam-         The pump must be set on a con-            tion difference in feet between the
eter of at least 48 inches, but if the   crete block or pedestal in the tank       pump discharge and the mound,
on-off setting were 30 inches, a         bottom so grit and other solids are       plus friction loss in the pipe, plus
30- to 36-inch diameter tank could       not drawn into the pump and sent          5 feet. For mound systems, use
be used.                                 to the drainfield.                         a minimum of 1½-inch diameter
    Pumping station tank materi-               Pump capacity is rated by how       plastic pipe. At a pumping rate
als include concrete (similar to         much flow rate can be produced             of 27 gpm, the friction loss in the
holding tanks), concrete culvert         versus the amount of head (verti-         pipe will be about 5 feet of head
sections and complete, ready-to-         cal lift plus friction losses) it is      loss per 100 lineal feet of pipe.
install plastic units. Metal tanks do    lifting. For example, a pump with
not last very long because effluent       a 3/4 horsepower motor might
is very corrosive. Installations with    pump 40 gallons per minute (gpm)               Example: A pump must
an open bottom, such as concrete         against a lift of 15 feet. With a lift         be selected to lift 27 gpm
culvert sections, must have a            of 25 feet, the same pump will                 of effluent from a pump
watertight, cast-in-place concrete       have a flow rate of only 15 gpm.                chamber to a mound for
floor. All joints between culvert               Flow rate usually is not a limit-        a three-bedroom house.
sections must be sealed so they          ing factor in pump selection when              The mound is 200 feet
are watertight. Flotation may be a       pumping to trenches or an absorp-              from the pump tank and
problem with precast tanks under         tion bed. However, the maximum                 10 vertical feet above the
high water table conditions. Under       lift capability of the pump may be             elevation of the pump
this condition, soil anchors may         a limiting factor. Always determine            discharge. The pump
be required to prevent upward            the vertical lift by measuring from            must overcome a head
movement.                                the pump discharge to the pipe                 of 10 feet (for elevation
    A secure manhole cover               outlet in the drainfield. Select a              difference) + 10 feet (for
must be located on the top of the        pump with a maximum lift capa-                 friction loss) + 5 feet =
pumping tank. The cover must be          bility at least 5 feet higher than             25 feet. The chosen pump
lockable to prevent children from        this elevation difference. Use a               must deliver about 27
removing the cover.                      1¼-inch or larger diameter flex-                gpm at about 25 feet of
                                         ible plastic pipe from the pumping             head.
                                         station to the drainfield. The plastic
Sewage Pumps                             pipe must be buried below frost
Many manufacturers make lift             depth with a uniform grade back               Install the pump with a union
pumps specifically for sewage             to the pumping chamber. In the            or quick disconnect coupling near
effluent. Lift pumps must be              winter, the water in the line should      the top of the pump tank. This
durable and corrosion-resistant          drain back to the pump station            makes installation and removal of
with sealed motors and electrical        to prevent freezing. Low spots in         the pump easier. Do not install a
connections. They must be able to        shallow buried pipe will freeze.          check valve in the outlet pipe from
withstand the acidic and corrosive                                                 the pumping station. If the pump
environment present in sewage                                                      has a built-in check valve, remove

it. Loop the outlet pipe with a          piggyback into the control cord
¼-inch weep hole drilled at the          plug. A weatherproof box outside
                                                                                 Septic Tank
low point of the loop. The weep
hole allows the water in the pipe
                                         the pumping tank must be used
                                         for an electrical outlet to serve the   Outlet
to drain back to the pump tank
after the pump shuts off.
                                              Pumps must have a failure          Sewer
                                         alarm system to warn the home-
■ Pump Controls
     All wastewater pumps need
to be turned on and off based on
                                         owner if the pump stops pumping.
                                         The alarm system senses pump            T    he outlet sewer pipe carries
                                                                                      sewage effluent from the
                                                                                 septic tank to a pump chamber
                                         failure when water rises above
the liquid level in the tank. The                                                or the drainfield. The outlet sewer
                                         the pump-on control. The sensor
most common pump on/off control                                                  pipe must be watertight where it
                                         is another mercury level control
is a mercury level control switch                                                leaves the septic tank and at least
                                         switch in a watertight bulb. It usu-
sealed in an effluent-resistant                                                   4 inches in diameter. Plastic pipe
                                         ally is set 3 to 6 inches higher
plastic or rubber bulb (Figure 8).                                               (either PVC or ABS) should be
                                         than the pump-on water level sen-
The length of the cord between                                                   schedule 40. Plastic pipe with thin-
                                         sor. The pump failure alarm circuit
the attachment point and the mer-                                                ner wall thickness often will slump
                                         should be installed on an electri-
cury bulb determines the water                                                   when the soil settles. Lay the pipe
                                         cal circuit separate from the pump.
level where the pump turns on                                                    at a minimum grade of 1/8 inch
                                              A homeowner can select from
and off.                                                                         per foot. No maximum grade is
                                         among several alarm methods
     A licensed electrician must in-                                             required for the outlet sewer pipe
                                         to warn of pump failure. Remote
stall all electrical wiring for a pump                                           since it carries only liquid. The
                                         alarms, located in a convenient
station. Electrical outlets must not                                             outlet sewer pipe must be laid
                                         place in the house or garage,
be installed inside a pump tank.                                                 to grade with no low spots where
                                         are common. They can be pro-
State plumbing code requires                                                     effluent can collect and freeze.
                                         grammed to make a sound similar
all electrical connections to be
                                         to a smoke detector, flash a light
outside the pump tank. On/off
                                         or call a telephone number. Many
control switches commonly use
                                         alarms are at the pump tank on
piggyback plugs where the control
                                         a pole and use either a light or
cord plugs into an electrical outlet
                                         buzzer to warn of pump failure.
and the pump electrical cord plugs
                                         Some use a light aimed at a win-
                                         dow in the house.

Figure 8. Liquid level pump controls. All electrical connections must
be outside the pumping chamber and at ground surface.

Soil Absorption Systems

T     he soil absorption system has
      to work all year. That means
it must infiltrate effluent during
                                        system. The infiltration rate of the
                                        soil usually was given in “minutes
                                        per inch” or mpi. The lower the
                                                                                least 24 inches (2 feet) below the
                                                                                trench bottom. However, 36 inches
                                                                                or more of separation between
wet springs and cold winters.           mpi, the higher the infiltration rate.   the high water table and trench
Effluent from the septic tank is         However, percolation tests have         bottom is preferred. Trenches are
about 99 percent water but it also      proven to be unreliable in many         most suitable for soil textures with
contains biological material (small     situations. Currently, many local       percolation rates of 60 minutes
particles). Additional treatment of     health districts require a regis-       per inch (mpi) or less (Table 3).
the biological material occurs in       tered soil classifier to determine       Trenches may be used on 61 to 90
the soil absorption system (Figure      the texture of the soil and local       mpi soils, providing a high water
9). The area of soil absorption         groundwater hydrology for de-           table does not exist and adequate
must be sized so it can infiltrate       sign purposes. Percolation tests        trench length is used. For soils
the daily wastewater flow from           still are accepted in some health       with percolation rates of 61 to 90
the house, as well as effectively       districts or if a house lot has been    mpi, increase the trench area by
decompose the biological materi-        created with fill soil. The proce-       25 percent over the area required
als in the effluent.                     dure for doing a percolation test is    for a soil with a 60 mpi percolation
     The ability of the soil to treat   in Appendix A.                          rate. For soils with percola-
and infiltrate the effluent is based                                              tion rates greater than 90
on the texture and local hydrology                                              mpi, trenches should not be
at the depth where the effluent
                                        Absorption Trenches                     used as a soil absorption
will be introduced to the soil. In          Trenches are the most com-          system without consulting
the past, a “percolation” test was      mon and effective drainfield. They       the local health sanitarian.
performed and the results were          can be used in areas where the             The square footage (length
used to size the soil absorption        historic high water table is at         and width) of required absorp-

Figure 9. Location of the biological treatment area under a drainfield.

tion trench for a given house and            ■ Trench Construction                      where effluent enters the trench. A
lot is based on the daily average                 Trenches are constructed with         trench can be 200 feet long if the
wastewater flow and soil texture              a backhoe. Commonly, a 24- to              effluent is delivered to the center.
at the depth of the trench bottom.           36-inch-wide bucket is used.               On sloping ground, trenches must
If the soil texture is a mixture, for        A wider bucket makes a wider               follow the contour of the slope so
instance a fine sand with some                trench that increases the trench           the bottom of the trench is level
silt mixed in, design for the finer           bottom area and reduces the                along its full length (Figure 10).
soil. Be sure to construct at least          length of trench. Do not allow any              Never construct trenches in
this amount of absorption trench.            wheel tracks in the trench since           loam or clay loam soils under wet
Many homeowners and installers               the compaction will seal the sur-          conditions. At the depth where the
feel this is more trench than they           face, greatly reducing the effec-          trench bottom will be, obtain a soil
require and install less trench to           tiveness of the absorption trench.         sample and the soil wetness. If the
save money. This usually ends up                  The bottom of each trench             soil can be rolled into a thread 1/8
being false economy. The system              must be level throughout its full          inch in diameter without breaking,
fails within a number of years and           length. A level trench bottom al-          it is too wet to dig trenches. Wet
then additional work must be done            lows effluent to infiltrate uniformly        soil will compact and smear, seal-
to upgrade the system. Recom-                for its entire length. If the trench       ing the trench and greatly increas-
mendations for trench bottom area            bottom has any slope, all the ef-          ing the chance of failure. If the soil
are based on a long-life treatment           fluent will collect at the low spots.       is dry enough for construction, it
system, not a temporary solution.            This can lead to premature failure         will crumble when you try to roll it
Built into the trench bottom area            or day lighting of effluent. In most        into a thread.
requirements is reduced infiltration          situations, the maximum length             Rock-filled Trenches
capacity through time due to the             of any one trench should not be                Trenches using crushed rock
accumulation of biological mate-             more than 100 feet from the point          for overburden support may be
rial in the trench bottom.
                                                                                        constructed 18 to 36 inches wide

    Table 3. Effluent loading rate of an absorption trench based on soil texture.
    A registered soil classifier should determine the soil texture at the depth where the
    bottom of the trench will be located.
                                                 Percolation      Depth of rock below the distribution pipe
    Soil Texture                                 (minutes/inch)    6”           12”          18”         24”
                                                                  Square feet of trench bottom per bedroom
    Sand and loamy sand                          1 to 5           125          100           85          70
    Sandy loam                                   6 to 15          190          150          125         110
    Fine sand, very fine sand, loam               16 to 30         250          200          165         145
    Silt and silt loam                           30 to 45         300          240          200         170
    Clay loam, sandy clay, silty clay loam       45 to 60         330          265          220         190
    Clay                                         60 to 120        650          515          440         375
                                                                   Trench bottom area loading rate, gal/ft2/day
    Sand and loamy sand                          1 to 5           1.2          1.5          1.8           2.1
    Sandy loam                                   6 to 15          0.8          1.0          1.2           1.4
    Fine sand, very fine sand, loam               16 to 30         0.6          0.75         0.90          1.05
    Silt and silt loam                           30 to 45         0.5          0.63         0.75          0.90
    Clay loam, sandy clay, silty clay loam       45 to 60         0.45         0.57         0.68          0.80
    Clay                                         60 to 120        0.23         0.29         0.34          0.4

and 6 to 48 inches deep. The         it drainfield rock). Washed rock is    rate and lead to premature failure.
depth of crushed rock is depen-      important because most rock has       The depth of soil cover over the
dent on the depth of the trench      fine clay attached. When un-           rock will depend on the depth of
(Figure 11) and distribution pipe.   washed rock is used, the clay will    the distribution pipe.
In the trench, use ¾- to 2½-inch     be washed off by the effluent and          Four-inch diameter plastic
diameter crushed rock that has       end up in the bottom of the trench.   sewer pipe with ½-inch diameter
been washed (some suppliers call     This can reduce the infiltration       or larger holes spaced 12 inches
                                                                           apart or closer is used for distribu-
                                                                           tion pipe (Figure 12). It is available
                                                                           in home supply and hardware
                                                                           stores. The distribution pipe can
                                                                           have a slight slope (1- to 2-inch
                                                                           drop in 100 feet) to help distribute
                                                                           the effluent over the full length of
                                                                           the trench. The pipe should have
                                                                           at least 2 inches of crushed rock
                                                                           over it. The rock under the pipe
                                                                           distributes effluent over the trench
                                                                           bottom and sidewalls, allowing the
                                                                           liquid to infiltrate into the soil. The
                                                                           pipe is oriented in the trench with
                                                                           the holes downward. When using
                                                                           pipe with a double row of holes,
                                                                           place the pipe with the holes
                                                                           downward in a 5 o’clock and 7
                                                                           o’clock position.
                                                                               The rock must be covered to
                                                                           prevent soil from seeping in and
                                                                           plugging the spaces between the
                                                                           rocks (Figure 13). Several prod-
                                                                           ucts can be used. The most com-
                                                                           mon is red rosin paper or a geo-
Figure10. Trenches constructed on the contour of a slope. The bottom       textile fabric. A 4- to 6-inch layer
of the trenches must be level.                                             of hay or straw also can be used.
                                                                           Don’t use plastic (black or clear)
                                                                           to cover the rocks in the trench.
                                                                           After covering the rock, backfill
                                                                           the trenches with earth. Overfill
                                                                           the trenches with 4 to 6 inches of
                                                                           backfill to allow for settling.

Figure11. Cross section of a rock-filled absorption trench.

     Figure12. Gravel-filled trench with plastic drop box in the
     foreground.                                                           Figure13. Gravel-filled trench
                                                                           covered with geotextile fabric.

Gravelless Trenches
    Gravelless trench systems use
plastic pipe or chambers (Figures
14 to 17) instead of crushed rock
for overburden support. No rock
is required in the trenches with
these products. Gravelless sys-
tems are becoming more common
because they can be installed with
a smaller crew, require less heavy
equipment and provide greater
effluent storage volume in the
trench. Like any trench system,       Figure 14. Cross section of a gravelless trench design using plastic
the trench bottom must be level       pipe (10- or 12-inch outside diameter) encased in a geotextile fabric
for good distribution of effluent.     sock.
    Trench bottom area is deter-
mined from Table 3 using the 6
inches of gravel column. Both the
pipe and chamber system designs       could put in three trenches each     of just more than 3 feet. This is
use an equivalent of a 3-foot-        100 feet long or four trenches       equivalent to a 3-foot-wide trench
wide trench. For example, say we      each 75 feet long.                   because the biomat treatment
want to install a gravelless trench       The gravelless pipe system       area forms on the sock on the
system for a three-bedroom house      uses plastic corrugated pipe         outside of the pipe and increases
with a silt loam soil at the bottom   covered with a nondegradable         the infiltration area. The chamber
of the trench. From Table 3, the      geotextile fabric sock. It can be    system also has a maximum buri-
necessary bottom area would be        installed in trenches a minimum      al depth of 4 feet. The chamber
300 square feet per bedroom for a     of 18 inches wide to a maximum       design has various permutations
total of 900 square feet. Using an    depth of 4 feet. The outside         that range in width from 18 inches
equivalent trench width of 3 feet     diameter of the pipe is 12 inches,   to 3 feet. Selection is based on
requires 300 feet of trench. We       which provides a circumference       site requirements and soil texture.

                                                                      ■ Effluent Distribution
                                                                           Most trench drainfields have
                                                                      two to four separate trenches re-
                                                                      quiring some method of equitably
                                                                      distributing the effluent between
                                                                      the trenches. Two methods com-
                                                                      monly used are drop boxes and
                                                                      distribution boxes. Drop boxes
                                                                      are used on sloping land where
                                                                      the trenches are aligned with the
                                                                      contour of the slope (Figure 9).
                                                                      Distribution boxes are used on
                                                                      level land where the elevations of
                                                                      all the trench bottoms are about
     Figure 15. Cylindrical gravelless pipe in shallow trenches.
                                                                      the same.
     Note the geotextile cover.                                       Drop Boxes
                                                                          Drop boxes are the preferred
                                                                      method of effluent distribution
                                                                      and may be used on near-level
                                                                      or sloping terrain (Figures 18, 19
                                                                      and 20). Drop boxes commonly
                                                                      are made from concrete or plastic.
                                                                      They have an inlet, two outlets
                                                                      that distribute the effluent to the
                                                                      trench and an outlet that takes the
                                                                      overflow to the next drop box. The
                                                                      inlet and outlets commonly are
                                                                      4 inches in diameter to accom-
                                                                      modate plastic pipe. Drop boxes
     Figure 16. Cross section of a gravelless trench design using a   allow a trench to be utilized fully
     plastic chamber (sometimes called a chamber unit).               before any effluent goes to the
                                                                      next trench. Drop boxes are re-
                                                                      quired for trench systems installed
                                                                      on hillsides. Even distribution of
                                                                      effluent over the entire drainfield
                                                                      is very important. Otherwise, all
                                                                      the effluent would collect in the
                                                                      low spots and overload the soil. If
                                                                      a trench becomes overloaded and
                                                                      effluent is coming to the surface,
                                                                      the outlets from the drop box can
                                                                      be blocked to allow the trench
                                                                      time to recover its ability to infil-
                                                                      trate effluent.

     Figure 17. Chamber-type gravelless system being installed on a
     very small house lot. Normally the chambers would be used in

Figure 18. Drop box showing inlet and outlet pipe elevations.

Figure 19. Location of drop boxes used for effluent distribution to an absorption trench system.

                                                                          Distribution Boxes
                                                                              Distribution boxes are used
                                                                          only on level terrain (Figure 21).
                                                                          Distribution boxes normally are
                                                                          made from concrete or plastic.
                                                                          They have an inlet and usu-
                                                                          ally three outlets for each of the
                                                                          absorption trenches. The inlet and
                                                                          outlets commonly are 4 inches in
                                                                          diameter to accommodate plastic
                                                                          pipe. All of the outlets from the
                                                                          distribution box are set at the
                                                                          same elevation, thus having a
                                                                          base under the box of crushed
                                                                          rock or gravel to keep it level is
                                                                          very important. However, in prac-
                                                                          tice, due to frost action or flooding
              Figure 20. Concrete drop boxes used to distribute           conditions, keeping all outlets at
              effluent to absorption trenches. Note that the
                                                                          the same elevation throughout
              distance between the trenches is 6 feet, the minimum
              separation distance. The trench on the right has been       the life of the system is virtually
              backfilled and the trench on the left is ready to be         impossible. If the box gets tilted,
              covered with hay, straw, untreated building paper or        the trench served by the low-
              a geotextile fabric.

Figure 21. Distribution box used for effluent distribution to an absorption trench system.

est outlet will receive the great-       Absorption Beds                           The absorption bed should be
est amount of effluent. For this                                                dug with a backhoe. No wheeled
                                              Absorption beds basically are
reason, distribution boxes may be                                              or tracked equipment should be
                                         rectangular pits dug into the soil
used only where the elevation of                                               driven on the bottom of the bed.
                                         (Figure 22). Depending on the
the lowest trench is high enough                                               The bottom of the absorption bed
                                         house lot, depths can range from
to back effluent up to the distribu-                                            must be level in all directions.
                                         1 to 4 feet. They cannot be used
tion box without surface seepage                                               A minimum of 6 inches of rock
                                         in locations having slopes greater
occurring.                                                                     should be placed in the bottom of
                                         than 6 percent. Absorption beds
     A common sign of problems                                                 the bed. Use washed rock that is
                                         are not as effective at treating
with a distribution box is if the soil                                         ¾ to 2½ inches in diameter. The
                                         effluent as trenches because the
gets soggy over one trench but                                                 rock must be washed to remove
                                         absorption bed has less sidewall
stays dry over the others. To check                                            clay particle. When unwashed
                                         area for infiltration. Therefore,
for this problem, open the top of                                              rock is used, the clay will be
                                         about 25 percent to 50 percent
the distribution box and check the                                             washed off by the effluent and end
                                         more bottom area is required than
water flow to the outlets. You may                                              up in the bottom of the bed. This
                                         for trenches. Even with the larger
have to dig around the box and                                                 can reduce the infiltration rate and
                                         bottom area requirement, absorp-
level it again. After leveling the                                             lead to premature failure.
                                         tion beds require less total yard
box, the outlet going to the trench                                                The pipes within absorption
                                         area than trenches and can be
that was soggy can be plugged                                                  beds are normally 4-inch diameter
                                         used on small house lots. Soil tex-
temporarily (two to four weeks) so                                             perforated plastic. The pipes must
                                         ture and the number of bedrooms
the trench can be rested.                                                      be level, 4 to 6 feet apart and 1½
                                         in the house determine the neces-
                                                                               to 3 feet from the edge of the bed.
                                         sary bottom area of an absorption
                                                                               The pipes normally are joined
                                         bed (Table 4).

Figure 22. Absorption bed construction and layout.

at the ends to form a continu-          sell and Richard Witz designed          On low infiltration rate soils, the
ous loop, although they can be          in 1947 at North Dakota State           basal area must be large enough
terminated as shown in Figure           University. Through the years, the      to allow percolation of the daily ef-
22. A four-way tee will distribute      design parameters and shapes of         fluent volume into the soil surface.
the effluent but a distribution box      mounds continually have changed         In high water table conditions,
also can be used. All joints must       as more research information            the elevated bed allows biologi-
be glued. Washed rock then is           became available. Present param-        cal treatment to take place before
placed over the distribution pipe       eters for mound design allow for        the effluent infiltrates to the water
to a depth of 2 to 4 inches. After      higher water usage and differ-          table.
the distribution pipe is covered        ent construction techniques than             Effluent is pumped to a mound
with rock, a soil separator must be     many earlier designs.                   system and distributed across
placed above the rock. Any of the           A sewage mound is a special         the rapid infiltration bed under
following materials are acceptable:     drainfield design used in locations      pressure (Figure 25). The effluent
a 4- to 6-inch layer of hay or straw,   with high water tables (within 2        is pumped to the mound through
untreated building paper (called        to 3 feet of the surface) and slow      1½-inch diameter or larger plastic
red rosin paper) or a geotextile        infiltration rate soils. A sewage        pipe. The pump tank should be
specifically designed for drain          mound takes advantage of the            large enough and the pump con-
fields. Cover the bed with 6 to 18       higher infiltration rate of surface      trols set so a dose equal to about
inches of topsoil and form a crown      soils, compared with subsurface         one-fourth of the daily wastewa-
to account for any settling and         soils. A sewage mound is con-           ter volume is discharged to the
also allow the bed to shed water.       structed above the existing ground      mound when the pump starts. A
Shallow absorption beds often will      surface. A mound is basically a         three-bedroom house has a de-
have a permanent mound. Don’t           high infiltration rate bed set on        sign load of 450 gpd, so the pump
plant trees or shrubs on top of the     top of a sand fill that has been         should discharge about 110 gal-
absorption bed.                         spread over the existing ground         lons per dose. This provides a rest
                                        (Figure 23). The high infiltration       period between doses and allows
                                        rate bed can be constructed using       the effluent to infiltrate before the
Sewage Mounds                           clean rock or gravelless systems        next dose. In addition, it increases
    Sewage mounds can trace             (Figure 24). The area where sand        pump life. Frequent starting and
their origins to the first “NODAK”       makes contact with the existing         stopping of a pump will reduce the
mounds that J. Clayton Rus-             ground is called the basal area.        life of the motor.

             Table 4. Effluent loading rates for absorption beds based on soil texture.
             A registered soil classifier should determine the soil texture at the depth where
             the bottom of the bed will be located.
                                                          Percolation                    Bottom Area
                                                             Rate         Loading Rate   Per Bedroom*
             Soil Texture                                (minutes/inch)    gal/ft2/day    square feet

             Sand and loamy sand                            1 to 5           1.0             150
             Sandy loam                                    6 to 15           0.65            230
             Fine sand, very fine sand, loam               16 to 30           0.5             300
             Silt and silt loam                           30 to 45           0.40            375
             Clay loam, sandy clay, silty clay loam        45 to 60          0.35            430
             Clay                                         60 to 120          0.18            830
             *Based on 150 gallons per day per bedroom

Figure 23. Construction features of a sewage mound.

Figure 24. Cross section of a sewage mound showing two types of gravelless leaching systems.

Figure 25. Septic tank, pumping chamber and sewage mound.

    Sewage mounds always                   The distribution pipes must      where the effluent enters the
should be designed with a pres-        have a ¼-inch diameter hole          pressurized distribution system
surized effluent distribution           every 40 inches in the bottom of     from the pump delivery pipe. To
system (Figure 26). A pressurized      the pipes. The pipe joints and end   protect from freezing conditions,
distribution system evenly distrib-    caps must be glued. Otherwise,       the distribution pipe and delivery
utes the effluent over the entire       they will pull apart during con-     pipe must be designed to drain
basal area and helps prevent           struction, under pressure or later   when the pump is off. This can be
overloading in any one spot under      from settling and frost heave. The   accomplished by using a ¼-inch
the mound. The plastic pipes           pipes are connected at the center    diameter weep hole in the delivery
can be set in either a rock bed or                                          pipe in the pump tank.
inside gravelless pipe. Gravelless
pipe manufacturers make prod-
ucts specifically for use in sewage
    The pressure distribution
system is constructed from either
1¼- or 1½-inch PVC pipe. Pipe
diameter is important because
too large a diameter (such as 2
inches) could result in uneven
distribution of the effluent. For a
sewage mound requiring a 6-foot
or narrower rapid infiltration area,
two parallel pipes are used. For a
sewage mound requiring a 7- to
10-foot-wide rapid infiltration area,
three parallel pipes are used.
When using plastic gravelless sys-
tems instead of rock, the pressure
distribution pipe is attached inside
the chamber or pipe.
                                       Figure 26. Pressure distribution system using two parallel pipes with
                                       a center feed. The pump in the pump chamber supplies effluent under

■ Sizing
    The basal area of the mound
                                           Example: for a three-bedroom house producing
(contact area of the fill sand with
                                           450 gallons per day of effluent, the contact area
the existing soil) determines all
                                           between the fill sand and the existing ground should
the other mound design parame-
                                           be between 1,800 feet2 (450 gpd ÷ 0.25 gpd/feet2)
ters. The basal area is determined
                                           and 2,250 feet2 (450 gpd ÷ 0.20 gpd/feet2). Figure
by the texture of the existing soil.
                                           27 shows a mound for a three-bedroom home. The
For slowly permeable clay and
                                           basal contact area for this mound on level ground is
clay loam soils, use a design ef-
                                           2,150 square feet. The amount of clean, washed sand
fluent loading rate of from 0.20
                                           needed is about 72 cubic yards.
to 0.25 gallon per day per square

Figure 27. Sewage mound on flat terrain. The dimensions were calculated based on the effluent from a
three-bedroom house with the mound built above a clay-loam to clay soil.

    The rapid infiltration area is      the effluent distribution pipe width        Mounds should be located
determined using the intake rate       at 4 to 6 feet. For our three-bed-    on flat areas or the crests of hills;
of medium sand. Medium sand            room home example, the rock           however, they can be built on slop-
has a loading rate of 1.2 gallons      bed or gravelless systems will be     ing terrain (Figure 28). However,
per day per square foot. Using the     about 375 feet2 ÷ 6 = 63 feet long.   soil texture and percolation rate
three-bedroom home example,            A 6-foot-wide rock bed 12 inches      will determine the maximum slope.
the required rock bed area is 375      thick will require about 14 cubic     For example, if the soil texture of
feet2 (450 gallons per day ÷ 1.2       yards of washed rock. For gravel-     the topsoil is clay loam with a per-
gallons/day/feet2). If a gravelless    less systems, two rows of plastic     colation rate of 120 mpi, then the
system is used instead of rock, it     chamber or two rows of gravel-        ground slope should not exceed
needs to be sized to provide 375       less pipe, each 63 feet long, are     3 percent (3-foot vertical drop in
feet2 of rapid infiltration area. For   required.                             100 feet). If the percolation rate is
soils with low permeability, keep                                            between 60 and 120 mpi, then the

Figure 28. Sewage mound on sloping terrain. For most soils, the maximum slope should be no greater than
6 percent (6-foot vertical drop in 100 feet). The dimensions were calculated for the amount of effluent from
a three-bedroom house with the mound built above a clay-loam to clay soil.

ground slope should not exceed 6       Once the surface is prepared, no      or straw, untreated building paper
percent, and if the percolation rate   wheel traffic can be allowed in the    (red rosin paper) or a geotextile
is 30 mpi or less, then mounds         basal area. Wheel traffic will seal    material designed for septic sys-
can be constructed on slopes up        the soil. Do not work in wet soil     tem drainfields.
to 12 percent.                         conditions because wet soil will           For gravelless distribution,
    When a mound is constructed        compact, smear and seal the soil.     place the gravelless pipe or cham-
on a slope, only the sand-soil              Next comes the fill sand. The     ber on top of the level sand. Place
contact area under the distribu-       fill sand should be washed and         the distribution pipe in the gravel-
tion area (rock bed or gravelless)     checked to be sure it contains        less carriers and connect to the
and down-slope from there can          no more than 10 percent               delivery pipe from the pump tank.
be considered basal area. Under        fines. To test, put 2½ inches of       Double-check all connections. Test
sloping soil conditions, the ends      sand in a quart jar and add water     the pump and distribution system
and up-slope portion of the mound      until about three-quarters full.      before covering. Cover the grav-
receive very little effluent and        Cover and shake to mix the sand       elless system with a geotextile
do not contribute to the area of       and water. Let the mixture stand      fabric and put a cap of heavy soil
infiltration.                           for an hour and measure the silt      on top (Figure 24).
                                       and clay accumulation on top of            Cap the mound with a loam
■ Construction
                                       the sand. If the depth is ¼ inch      or loamy sand soil (Figures 23
     Mounds require very diligent
                                       or less, the sand is clean enough     and 24). Make the cap 12 inches
and careful construction practices.
                                       for use in the mound. This test       high at the center of the bed and 6
Mounds have been known to fail
                                       should be done before the sand is     inches high at the end of the bed.
due to two main causes — too
                                       delivered to the site. Pit run sand   Taper the cap down the sides of
small of a basal area for the efflu-
                                       varies widely, even from the same     the mound.
ent flow from the house, and poor
                                       area of a pit, and should not be           Last, cover the mound with 6
construction practices. To ensure
                                       used.                                 inches of good top soil and seed
a mound works as planned,
                                           Place the clean sand start-       to grass. Don’t plant trees or
construction practices must be
followed very closely. Plus, the ho-   ing at one end and work toward        shrubs on top of the mound. Wa-
meowner must use water wisely.         the other end. Drive on top of the    ter-tolerant shrubs may be planted
     The first step in mound con-       sand as you progress. Shape the       around the base. Permanent lawn
struction is to mow the grass or       sand with a backhoe with tracks       sprinkler systems should not
vegetative cover over the basal        or a crawler tractor but make sure    be installed near enough to the
area to a maximum 2-inch height        you have at least 6 inches of sand    mound to throw water on it.
and remove all the cuttings. Then      under the tracks. Wheeled ve-              Construct mounds to follow
dig in the effluent line from the       hicles should not be used for this    the contour of the existing ground.
pumping station. The line must         operation. Do not allow the tracks    Never place a mound in a low
                                       to run directly on the earth. After   area where water will accumulate.
be installed below frost level
                                       forming with the tractor blade,       If the mound is on sloping ground,
or sloped uniformly back to the
                                       level and do the final shaping by      use a berm on the uphill side to
pumping chamber so it drains
                                       hand. The top of the sand bed         divert runoff water around the
after the pump shuts off. The ex-
                                       should be level the full length of    mound. Mounds can be construct-
cavated trench must be backfilled
                                       the mound.                            ed to complement your landscap-
and the soil firmly compacted to
                                           For rock beds, a trench about     ing design. Shrubs at the base of
prevent effluent from flowing along
                                       12 inches deep and 6 feet wide        the mound will use water and help
the pipe.
                                       should be formed the required         trap snow. If the mound is built in
     Ground preparation comes
                                       length on top of the sand. Place      the fall, it should be covered with
next. The ground must be ripped
                                       6 inches of ¾- to 3-inch diameter     straw or hay the first winter to
or scarified with a chisel plow or
                                       rock in the trench. Place the pres-   prevent freezing.
the teeth from the backhoe bucket.
                                       sure distribution pipe on the rock
                                       and cover with 2 inches of rock.
                                       Cover the rock with one of the fol-
                                       lowing: a 4- to 6-inch layer of hay


S     mall lagoons (Figure 29) can
      be used to contain septic
system effluent in some locations.
                                      freeboard of 2 feet. The sides
                                      of the lagoon should have a 3-1
                                      slope or more. A circular 3,000-
Lagoons are a viable option when      square-foot lagoon with 3 feet
the nearest neighbor is at least a    working depth and 3-1 side slopes
quarter of a mile away. Lagoons       would have a 62-foot diameter at
should be constructed only in         its working depth and a 74-foot di-
high clay content soils. A lagoon     ameter at the top of the dike. The
should act as a container and not     lagoon also may be square
as an infiltration galley. The water   or rectangular.
in a lagoon should be removed by          A lagoon must be fenced to
evaporation.                          exclude children and animals. An-
    The lagoon surface area           nual maintenance requires check-
should be sized at about 1,000        ing for damage and to make sure
square feet per bedroom. A            animals are not burrowing into the
lagoon serving a three-bedroom        sides of the lagoon. Cattails and
house needs about 3,000 square        other plants in the lagoon help
feet of water surface area. The la-   use the nutrients in the water, but
goon should have a working depth      their roots can create channels for
of about 3 feet and a minimum         water to seep away.

Figure 29. Cross section of a small farm lagoon sized to handle the sewage effluent from a typical
three-bedroom home.

Alternative Septic Systems

H     omeowners have many alter-
      native methods for treatment
and disposal of wastewater from a
                                        Filtration Systems
                                             Filtration systems physi-
                                        cally trap suspended solids in the          Most alternative drainfields are
house. Some alternative systems
                                        wastewater and provide a me-            variations of the trench, absorp-
replace the septic tank or modify
                                        dium to speed up the process of         tion bed and mound systems out-
the operation of the septic tank
                                        decomposition. They are like mini       lined in this publication. At-grade
to speed up or improve the initial
                                        versions of a typical municipal         systems are used in high water
treatment of the household waste-
                                        waste treatment system. Filtration      table areas, areas with shallow
water. Some alternative methods
                                        systems can be part of the septic       bedrock or other infiltration prob-
to the traditional drainfield improve
                                        tank or just downstream from            lems. They are a combination of
the infiltration of the treated water.
                                        the septic tank. Sand filters have       trench and mound technology. The
Many of these alternative systems
                                        been used for many years and            trench bottom is ground surface.
are designed as complete pack-
                                        they can be configured as single-        The vegetation is removed and
                                        pass (waste flows through only           the ground is roughed up or scari-
     Generally, alternative systems
                                        once) or multiple-pass systems.         fied where the trench bottom will
are more expensive than tradition-
                                        Filtration systems that use peat        be. Then gravel is added and the
al systems and practically all of
                                        have come on the market in the          distribution pipes are laid in place
them require electrical power on
                                        last 10 years and are used as a         and covered with gravel and then
a continuous basis. However, in
                                        single-pass system. Other filtration     a layer of topsoil.
some locations where a traditional
                                        systems use artificial or synthetic          Alternating drainfields is
septic system cannot be installed,
                                        materials to filter the effluent and      another method of distributing
they provide a viable alternative.
                                        treat the biological solids. These      wastewater. Two drainfields are
If you are in a health district with
                                        mostly are multiple-pass systems.       constructed. Each has about 50
on-site septic system rules, check
                                        In some parts of the U.S., artificial    percent to 100 percent of the area
with your local health sanitarian
                                        wetlands have been used to treat        needed for the house. While one
before installing an alternative
                                        household wastewater. However,          drainfield is in use, the other is
system. Some may not be ap-
                                        research on artificial wetlands in       resting. Typically, one will be used
proved. In areas with no sanitar-
                                        the northern-tier states has not        for about two years, then the hom-
ian, check with the North Dakota
                                        been successful.                        eowner will switch to the other. A
Health Department’s Environmen-
                                             Except for some single-pass        special box with a two-way valve
tal Health Section. Some of the
                                        filtration, all the others have to use   or sliding gate controls the flow
more common alternative systems
                                        a pump to circulate the effluent for     from the septic tank.
are listed here.
                                        multiple passes. Use of a pump on           Drip irrigation is being used
                                        a continuous basis increases en-        to distribute treated wastewater
                                        ergy costs and requires an alarm        in some parts of the U.S. The drip
                                        system. Often the systems cannot        lines usually are buried about 1
                                        be operated unless the pump is          foot below the soil surface. Drip
                                        working.                                lines can plug very easily, so
                                                                                the wastewater must be treated,
                                                                                filtered and chlorinated before it
                                                                                is pumped to the drip lines. These
                                                                                systems can be very expensive
                                                                                both to install and maintain.

Appendix: Percolation Test
    Percolation tests are made at locations where          and bottom of the holes with a knife or nails driven
the water table is at least 2 feet below the surface       into a board to counteract the sealing action of the
or where the drainfield will be constructed in “fill’        spade or auger. Clean out the loose dirt in the bot-
soil. The percolation test helps determine the ability     tom of the hole. Place 2 inches of coarse gravel in
of a soil to absorb effluent. The rate of water drop in     the hole.
a test hole determines the percolation rate (Figure      2. Carefully pour at least 12 inches of water into the
30). A fast drop indicates a high percolation rate          hole. Add water through a hose connected to a fun-
while a slow drop indicates slow percolation. Soils         nel. The bottom of the hose should rest on the top
that have slow percolation rates need large soil ab-        of the rock to prevent washing of the soil in the test
sorption areas to treat a given quantity of effluent.        hole. Washing will loosen fine materials and seal
    Prior to running a percolation test, make soil          the hole. Keep water in the hole at least four hours
borings to a depth of at least 5 feet to determine          (overnight would be better). Refill if necessary to
the soil texture (amount of sand, silt and clay) at         keep the water at the 12-inch level. In clay soils,
potential locations for the drainfield. Then choose          water must be kept in the hole at least 12 hours
the location with the best soil characteristics.            for soil swelling to take place before measuring the
Generally, soils with higher sand content are better        percolation rate.
choices for the drainfield area. Once a location is
                                                         3. Do the percolation test. Adjust the water level so
selected, dig at least three holes at opposite ends
                                                            it is about 6 to 8 inches above the gravel layer.
of the site and do a percolation test in each hole.
                                                            Measure the water level from a fixed point every
    The following steps outline the procedure used
                                                            30 minutes or as often as required and calculate
in running a percolation test:
                                                            percolation rate in minutes per inch. The percola-
1. Dig a 6- to 8-inch diameter hole to the depth            tion rate is calculated by dividing the time interval in
   of the proposed drainfield. Roughen the sides             minutes by the water level drop in inches.

                                                           Example: If the water level takes 34 minutes
                                                           to drop 1c inches, the percolation rate is:
                                                                 34 minutes
                                                                            = 30.2 mpi
                                                                 1c inches

                                                             Refill the hole as required. Calculate the percola-
                                                         tion rate for each reading. Continue taking readings
                                                         until three percolation rates vary by no more than 10
                                                         percent. For example, if you had three readings of
                                                         28.4, 27.4 and 29.5 mpi, 10 percent would be about
                                                         2.8 mpi, which means they are within 10 percent of
                                                         each other. Use the slowest value for each test hole to
                                                         get the average percolation rate for designing the soil
                                                         absorption field.

                                                                        Test 1    Test 2     Test 3     Use
                                                           Hole 1        28.4      27.4       29.5      29.5
                                                           Hole 2        31.1      32.8       33.5      33.5
                                                           Hole 3        33.4      34.1       32.9      34.1
Figure 30. Soil percolation test hole and water                                             Average     32.4
level measuring apparatus.


      Use the space above to make a map of your house, septic tank
            and soil absorption system components and well.
                  Include distance to septic tank and soil absorption components from two fixed points,
                                             such as the corners of the house.

Installing Contractor _____________________________________________________________
                        Address             ____________________________________________________________
                        Telephone           __________________________________

System Description
            Gravity ____________                        Pressurized ___________
            Septic Tank ______________ gallons                                  Manhole ________                  Inspection Ports _________
            Pumping Chamber _________ gallons

Soil Absorption System (Number of Feet, or Area Covered)
            Trenches __________                        Mound ____________
            Bed ______________                        Other __________________                             Depth ________ ft

            Distribution Box __________                      Drop Boxes __________
            Pump _______________ GPM at _____________ feet of head

For more information on this and other topics, see:
County commissions, North Dakota State University and U.S. Department of Agriculture cooperating. Duane Hauck, director, Fargo, N.D. Distributed in
furtherance of the acts of Congress of May 8 and June 30, 1914. We offer our programs and facilities to all people regardless of race, color, national origin,
religion, gender, disability, age, veteran’s status or sexual orientation; and are an equal opportunity institution. This publication will be made available in
alternative formats for people with disabilities upon request, (701) 231-7881.                                                                        2.5M-8-06

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