Common Wastewater Terms
The term activated sludge refers to the brownish flocculent culture of organisms developed in an aeration
tank under controlled conditions. Also, sludge floc produced in raw or settled wastewater by the growth of
zoological bacteria and other organisms in the presence of dissolved oxygen. A good quality of activated
sludge is shown by brown color, good settling characteristics, and DO present.
The capacity of water to neutralize acids, a property imparted by the water's content of carbonates,
bicarbonates, hydroxides, and occasionally borates, silicates, and phosphates.
A biological environment that is deficient in all forms of oxygen, especially molecular oxygen, nitrates and
nitrites. The decomposition by microorganisms of waste organic matter in wastewater in the absence of
dissolved oxygen is classed as anaerobic.
A biological environment that is deficient in molecular oxygen, but may contain chemically bound oxygen,
such as nitrates and nitrites.
Bacteria are living organisms (animals) that cannot be seen by the naked eye. They are a group of
universally distributed, rigid, essentially unicellular, microscopic organisms lacking chlorophyll. They are
characterized as spheroids, rod-like, or curved entities, but occasionally appearing as sheets, chains, or
Biochemical Oxygen Demand (BOD)
The BOD test is used to measure the strength of wastewater. The BOD of wastewater determines the
milligrams per liter of oxygen required during stabilization of decomposable organic matter by aerobic
bacteria action. Also, the total milligrams of oxygen required over a five-day test period to biologically
assimilate the organic contaminants in one liter of wastewater maintained at 20 degrees Centigrade.
A phenomenon that occurs in activated sludge plants whereby the sludge occupies excessive volumes and
will not concentrate readily. This condition refers to a decrease in the ability of the sludge to settle and
consequent loss over the settling tank weir. Bulking in activated sludge aeration tanks is caused mainly by
excess suspended solids (SS) content. Sludge bulking in the final settling tank of an activated sludge plant
may be caused by improper balance of the BOD load, SS concentration in the mixed liquor, or the amount
of air used in aeration.
Chemical Oxygen Demand (COD)
The milligrams of oxygen required to chemically oxidize the organic contaminants in one liter of
To have significant meaning, samples for laboratory tests on wastewater should be representative of the
wastewater. The best method of sampling is proportional composite sampling over several hours during the
day. Composite samples are collected because the flow and characteristics of the wastewater are
continually changing. A composite sample will give a representative analysis of the wastewater conditions.
A biological process by which nitrate is converted to nitrogen gas.
The biological decomposition of organic matter in sludge resulting in partial gasification, liquefaction, and
mineralization of putrescible and offensive solids.
The killing of pathogenic organisms is called disinfection.
Dissolved Oxygen (DO)
The oxygen dissolved in water, wastewater, or other liquid. DO is measured in milligrams per liter. If the
DO of a sample of water is 2 mg/L, it means that there are 2lbs of oxygen in 1 mil lb of water.
Solids that cannot be removed by filtering are dissolved solids.
A modification of the activated sludge process which provides for aerobic sludge digestion within the
Clumps of bacteria and particles that have come together to form clusters, or small gelatinous masses. The
floc mass in an activated sludge aeration tank generally consists of microorganisms.
In wastewater, a group of substances, including fats, waxes, free fatty acids, calcium and magnesium soaps,
mineral oils, and certain other non-fatty materials.
Milligrams per Liter (mg/L)
A unit of concentration of water or wastewater constituent. It is 0.001 g of the constituent in 1000 ml of
water. The unit parts per million is identical to milligrams per liter.
Mixed Liquor (ML)
The mixture of activated sludge, wastewater, and oxygen, wherein biological assimilation occurs.
Mixed Liquor Suspended Solids (MLSS)
The milligrams of suspended solids per liter of mixed liquor that are combustible at 550 degrees
Centigrade. An estimate of the quantity of MLSS to be wasted from the aeration tank of an extended
aeration plant may be determined by the rate of settling and centrifuge tests on the sludge solids.
The conversion of nitrogen matter into nitrates by bacteria. Nitrogen > Nitrogen is present in wastewater in
many forms: total Kjeldahl nitrogen, ammonia nitrogen, organic nitrogen.
The cycle of life, death, and decay involving organic nitrogenous matter is known as the nitrogen cycle. In
the nitrogen cycle ammonia is produced from proteins.
A simple compound of phosphorous and oxygen that is soluble in water.
A biological environment which contains molecular oxygen; aerobic.
A large compound formed of several orthophosphate molecules connected by phosphate-storing
Wastewater before it receives any treatment. Reactor A tank where a wastewater stream is mixed with
bacterial sludge and biochemical reactions occur.
Settled activated sludge returned to mix with incoming raw or primary settled wastewater. When the return
sludge rate in the activated sludge process is too low, there will be insufficient organisms to meet the waste
load entering the aerator.
Return Activated Sludge
Activated return sludge is normally returned continuously to the aeration tank. Recycling of activated
sludge back to the aeration tank provides bacteria for incoming wastewater. Its should be brown in color
with no obnoxious odor and is often also returned in small portions to the primary settling tanks to aid
sedimentation. Settled activated sludge is generally thinner than raw sludge. Some activated sludge will be
wasted to prevent excessive solids build up.
In the activated sludge process, a measure of the length of time a particle of suspended solids has been
undergoing aeration, expressed in day. It is usually computed by dividing the weight of the suspended
solids in the aeration tank by the weight of excess activated sludge discharged from the system per day.
The purpose of sludge digestion is to separate the liquid from the solids to facilitate drying. The proper pH
range for digested sludge is 6.8 - 7.2. Sludge Index Properly called sludge volume index (SVI). It is the
volume in millimeters occupied by 1 g of activated sludge after settling of the aerated liquid for 30 minutes.
The continuous aeration of sludge after initial aeration for the purpose of improving or maintaining its
A division box that splits the incoming flow into two or more streams. A device for splitting and directing
discharge from the head box to two separate points of application.
Domestic wastewater is 99.9% water and 0.1% solids. Fresh wastewater is usually slightly alkaline. If the
pH of the raw wastewater is 8.0, it indicates that the sample is alkaline. If wastewater has a pH value of 6.5,
it means that it is acid. Wastewater is said to be septic when it is undergoing decomposition.
What is wastewater, and why treat it?
We consider wastewater treatment as a water use because it is so interconnected with the other
uses of water. Much of the water used by homes, industries, and businesses must be treated before it is
released back to the environment.
If the term "wastewater treatment" is confusing to you, you might think of it as "sewage
treatment." Nature has an amazing ability to cope with small amounts of water wastes and pollution, but it
would be overwhelmed if we didn't treat the billions of gallons of wastewater and sewage produced every
day before releasing it back to the environment. Treatment plants reduce pollutants in wastewater to a level
nature can handle.
Wastewater is used water. It includes substances such as human waste, food scraps, oils, soaps and
chemicals. In homes, this includes water from sinks, showers, bathtubs, toilets, washing machines and
dishwashers. Businesses and industries also contribute their share of used water that must be cleaned.
Wastewater also includes storm runoff. Although some people assume that the rain that runs down
the street during a storm is fairly clean, it isn't. Harmful substances that wash off roads, parking lots, and
rooftops can harm our rivers and lakes.
Why Treat Wastewater?
It's a matter of caring for our environment and for our own health. There are a lot of good reasons
why keeping our water clean is an important priority:
Clean water is critical to plants and animals that live in water. This is important to the fishing
industry, sport fishing enthusiasts, and future generations.
Our rivers and ocean waters teem with life that depends on shoreline, beaches and marshes. They
are critical habitats for hundreds of species of fish and other aquatic life. Migratory water birds use the
areas for resting and feeding.
Wildlife Recreation and Quality of Life
Water is a great playground for us all. The scenic and recreational values of our waters are reasons
many people choose to live where they do. Visitors are drawn to water activities such as swimming,
fishing, boating and picnicking.
If it is not properly cleaned, water can carry disease. Since we live, work and play so close to
water, harmful bacteria have to be removed to make water safe.
The major aim of wastewater treatment is to remove as much of the suspended solids as possible
before the remaining water, called effluent, is discharged back to the environment. As solid material
decays, it uses up oxygen, which is needed by the plants and animals living in the water. "Primary
treatment" removes about 60 percent of suspended solids from wastewater. This treatment also involves
aerating (stirring up) the wastewater, to put oxygen back in. Secondary treatment removes more than 90
percent of suspended solids.
Wastewater Treatment Plant: Primary Treatment of Wastewater
The Primary Treatment Process
1. Screening: Wastewater entering the treatment plant includes items like wood, rocks, and even
dead animals. Unless they are removed, they could cause problems later in the treatment process.
Most of these materials are sent to a landfill.
2. Pumping: The wastewater system relies on the force of gravity to move sewage from your home
to the treatment plant. So wastewater-treatment plants are located on low ground, often near a
river into which treated water can be released. If the plant is built above the ground level, the
wastewater has to be pumped up to the aeration tanks (item 3). From here on, gravity takes over to
move the wastewater through the treatment process.
3. Aerating: One of the first steps that a water treatment facility can do is to just shake up the
sewage and expose it to air. This causes some of the dissolved gases (such as hydrogen sulfide,
which smells like rotten eggs) that taste and smell bad to be released from the water. Wastewater
enters a series of long, parallel concrete tanks. Each tank is divided into two sections. In the first
section, air is pumped through the water.
As organic matter decays, it uses up oxygen. Aeration replenishes the oxygen. Bubbling oxygen
through the water also keeps the organic material suspended while it forces 'grit' (coffeegrounds,
sand and other small, dense particles) to settle out. Grit is pumped out of the tanks and taken to
4. Removing sludge Wastewater then enters the second section or sedimentation tanks. Here, the
sludge (the organic portion of the sewage) settles out of the wastewater and is pumped out of the
tanks. Some of the water is removed in a step called thickening and then the sludge is processed in
large tanks called digesters.
5. Removing scum: As sludge is settling to the bottom of the sedimentation tanks, lighter materials
are floating to the surface. This 'scum' includes grease, oils, plastics, and soap. Slow-moving rakes
skim the scum off the surface of the wastewater. Scum is thickened and pumped to the digesters
along with the sludge.
Many cities also use filtration in sewage treatment. After the solids are removed, the liquid sewage
is filtered through a substance, usually sand, by the action of gravity. This method gets rid of
almost all bacteria, reduces turbidity and color, removes odors, reduces the amount of iron, and
removes most other solid particles that remained in the water. Water is sometimes filtered through
carbon particles, which removes organic particles. This method is used in some homes, too.
6. Killing bacteria: Finally, the wastewater flows into a 'chlorine contact' tank, where the chemical
chlorine is added to kill bacteria, which could pose a health risk, just as is done in swimming
pools. The chlorine is mostly eliminated as the bacteria are destroyed, but sometimes it must be
neutralized by adding other chemicals. This protects fish and other marine organisms, which can
be harmed by the smallest amounts of chlorine.
The treated water (called effluent) is then discharged to a local river or the ocean.
Another part of treating wastewater is dealing with the solid-waste material. These solids are kept
for 20 to 30 days in large, heated and enclosed tanks called 'digesters.' Here, bacteria break down (digest)
the material, reducing its volume, odors, and getting rid of organisms that can cause disease. The finished
product is mainly sent to landfills, but sometimes can be used as fertilizer.
Reclaimed Wastewater: Using Treated Wastewater for Other Purposes
The use of reclaimed wastewater helps us in two ways:
1. Reclaimed water can supply needed water for some purposes.
2. Reclaimed wastewater frees up fresh water that can be used somewhere else, such as for drinking
California is a good place to go to see how reclaimed wastewater is being used. The East Bay
Municipal Utility District has a working water reclamation project that benefits the community in these
Conserves drinking water
Reduces pollution into San Francisco Bay
Provides water for irrigation and industrial purposes
Their project results in a savings of about 5.5 billion gallons per year. Eventually the project will
save enough water to provide drinking water to 83,000 households.
So, what exactly is reclaimed wastewater used for? A lot of it goes toward watering golf courses
and landscaping alongside public roads, etc. Some industries, such as power-generation plants can use
reclaimed wastewater. A lot of water is needed to cool power-generation equipment, and using wastewater
for this purposes means that the facility won't have to use higher-quality water that is best used somewhere
Wastewater Treatment Outline
Municipal sewage is the liquid waste from homes, businesses, and industries.
It contains water, organic matter, mineral nutrients, microorganisms, and pathogens.
It is usually treated and released into a river, lake, or ocean (receiving water).
B. REASONS FOR TREATMENT
For ecological and public health reasons, the impurities must be removed prior to release.
Pathogens must be removed for public health.
Organic matter and nutrients must be removed to prevent degradation of the receiving water.
C. RAW SEWAGE
Municipal sewage comes primarily from toilets, bath, laundry, dishwashers, garbage disposals, car
About 150-200 gal/day/person, mostly water.
Household wastewater is delivered by the sanitary sewer system to the wastewater treatment
Gravity driven with occasional lift stations.
E. STORM DRAINS
In modern systems storm water runoff and domestic sewage are kept separate.
Storm water is collected by storm drains.
Mixing the two results in overloaded STP during storms and release of raw sewage.
F. INDUSTRIAL WASTES
Industries may also contribute to municipal sewage after on-site pretreatment.
Or they can release directly to receiving water after complete treatment
Clean Water Act prohibits negative impact of industrial waste on either the STP or the receiving
Regulatory agencies monitor compliance
G. WASTEWATER TREATMENT PLANT
Domestic sewage delivered to plant by sanitary sewer system.
Purpose is to reclaim water by removing impurities
Sewage treatment plant = wastewater treatment plant = water pollution control facility
Engineered to operate efficiently, effectively, and inexpensively
Takes advantage of simple inexpensive physical (Stokes Law and gravity) and biological (aerobic
respiration) processes to accomplish its goal
H. WATER PROTECTION LEGISLATION
Pollution control laws
Clean Water Act of 2004
II. COMPOSITION OF DOMESTIC SEWAGE
raw sewage is 99.9% water
this water must be restored to near its original condition
recovery of water is the purpose of the wastewater treatment process
B. GRIT AND DEBRIS
sand and gravel
inadvertently introduced to the sewer system through cracks and leaks
prior to separation of storm drains and sanitary sewers it was abundant
plastic bags, rags, some paper (not toilet paper), sanitary napkins, condoms
this is stuff you should not flush down the toilet
(toilet paper disintegrates into its component fibers)
C. PARTICULATE ORGANIC MATTER
particles of various sizes
feces, garbage from disposal, disintegrated toilet paper
contributes to BOD
contributes to turbidity
particulate matter will settle in still water
2. Biochemical Oxygen Demand
the amount of oxygen required to oxidize all organic matter to CO 2 and H2O
C6H12O6 + O2 -->H20 + CO2
it is a measure of the amount of organic matter present
a major STP function is to oxidize the organic matter and remove the BOD
BOD should be oxidized in the STP rather than in the receiving water
B. BOD and Receiving Water
bacteria may exhaust DO by respiring BOD
results in anoxia, fish kills
organic matter may accumulate on bottom
C. Nutrients and the Receiving Water
nutrients stimulate growth of algae
algae produce more organic matter and increase BOD
D. COLLOIDAL AND DISSOLVED ORGANIC MATTER
much of the organic matter is colloidal or dissolved
does not settle
also contributes to BOD and to turbidity
E. DISSOLVED INORGANIC MATERIAL
primarily mineral nutrients
ammonia, nitrate, and phosphate
from feces, urine, kitchen wastes, laundry (phosphates), car washes
water-borne pathogens are present in domestic sewage
typhoid, cholera, salmonellosis, diarrhea, infectious hepatitis A, polio, dysentery,
gairdiasis, and many parasitic worms.
removal of pathogens prior to release is a major function of the STP
*a safe drinking water supply is the major reason for the good health enjoyed by
heavy metals, pesticides, exotic toxins
illegal materials from homes, businesses, hospitals, schools, laboratories, industries
objective is removal of impurities from wastewater to reclaim clean water
process relies heavily on the tendency for particles to settle
whenever possible soluble materials are converted to particulates and settled
liquid and solids are separated by settling and treated separately
objective is accomplished primarily by inexpensive physical and biological processes
gravity is used to transport waste whenever possible
settling (gravity again) is used to remove particulate organic material (POM) from water
bacterial respiration is used to remove dissolved organic material (DOM) from the water
in general, particulates are easy to remove, solutes are difficult
2. Treatment Plant
a. Input (Raw Wastewater)
*turbid, malodorous, unsanitary
*suspension of debris, organisms, pathogens, toxins, minerals, organic material
b. Output (Finished Wastewater)
clean, transparent, odorless, colorless, sanitary, safe water with low BOD, and pathogen-
ideally with low nutrient levels
3. Level of Treatment
there are several levels of treatment
o Primary treatment
o Secondary treatment
o Tertiary treatment
there are several levels of treatment
complete treatment, with all stages, removes all four major components (DIM, DOM,
POM, and pathogens)
most STPs in developed nations remove all but DIM
more and more treatment plants also remove DIM
much of the third world does not even have collection systems
raw wastewater pumped to top of system and flows downhill through the plant
removes grit and debris
to prevent clogging or damage to valves, pumps, and pipes
a. Bar Screen
parallel steel bars or grid through which water flows
debris trapped on screen is removed and sent to landfill or incinerated
b. Grit Settling Chamber
flow velocity is reduced slightly so that sand and gravel settle
the first of several steps that use gravity to separate particles from liquid
POM remains suspended
settled solids taken to landfill
D. PRIMARY TREATMENT
*removal of particulate organic material (POM)
*this reduces BOD (and nutrients)
*POM is allowed to settle in quite water
primary clarifiers have low flow (2 m/hour)
particles settle to bottom (30-50% of POM)
clarified water flows over the sides
solids (=raw sludge) are sent to sludge facility (more later)
clarified water sent to secondary treatment
*grease and fat skimmed off the top
waste has been separated into sludge and water
they will be treated separately
E. SECONDARY TREATMENT
removal of dissolved organic material
this further reduces the BOD (and nutrients)
2° treatment is a biological process
aerobic bacteria respire DOM and convert it to CO 2 and H2O
C6H12O6 + O2 --> CO2 + H2O
reduces the BOD in the STP instead of the receiving water
several methodologies are available
all provide microorganisms with abundant oxygen for respiration of DOM
a. Trickling Filters
the oldest of the methodologies
large tanks filled with a substrate (medium) that provides abundant surface area
exposed to air (oxygen)
medium may be rocks or synthetic
supernatant from primary clarifiers is applied to the media and allowed to flow over
bacteria growing on the surfaces respire the DOM
biological community with several trophic levels
bacteria are the base of the food web
no producers; the energy supply is allochthonous (it is DOM from your home)
bacteria, protozoa, rotifers, insect larvae
insect larvae include rat-tail maggots, filter flies, horseflies
these do not contribute importantly to respiration but can be a nuisance
filters are periodically flooded to kill insect larvae
trickling filter ecosystem lacks producers
relies on allochthonous production (BOD)
there is a little photosynthesis on the surface layer of rocks (negligible)
trickling filters can achieve 85-90% BOD removal
b. Activated Sludge
the most widely used system
more complex, more expensive
requires less space
a little more effective
employs a large tank through which clarified sewage flows
clarified wastewater enters one end
inoculated with aerobic bacteria (from sludge)
bacteria assimilate DOM and reproduce rapidly
DOM thereby is converted to POM
some is respired to CO2 and H2O
Pg (DOM) = Pn (POM) + R (CO2 + H2O)
effluent is sent to secondary clarifier to remove sludge (POM)
the sludge is "activated" sludge
some of it is used to inoculate the incoming supernatant
2° supernatant goes to tertiary treatment or post-treatment
excess sludge is wasted back to the sludge treatment facility (more later)
activated sludge can remove 90-95% of DOM
c. Rotating Biological Contactors
a new secondary treatment method
is really specialized type of trickling filter
1° supernatant flows over large rotating disks
the disks increase surface area
bacteria grow in thick layers on the disks
F. TERTIARY TREATMENT
removal of inorganic nutrients
especially phosphorus, sometimes nitrogen
chemical, physical, or biological
no single, simple, inexpensive process
usually expensive, unlike 1° and 2° treatment
most STPs do not have 3° treatment
becoming more and more common as required by EPA and states
necessary to prevent eutrophication of receiving water
sometimes only the limiting nutrient need be removed
most, but not all, methods convert DIM to particulates, then settle
e. Precipitation of Phosphorus
2° supernatant treated to precipitate or coagulate P
aluminum sulfate, aluminum chloride, ferric chloride, or an organic polymer
precipitate or coagulant is settled, separated, sent to landfill
f. Biological Nitrogen Removal
relies on several species of nitrogen-metabolizing bacteria
nitrogen, in various oxidation states, figures in the energy metabolism of many bacteria
incorporated into special activated sludge tanks
tank must be partitioned to maintain different oxygen regimes
ammonifying bacteria mineralize organic nitrogen to ammonia under anoxic conditions
C-NH2 è NH4
chemoautotrophic nitrifying bacteria convert NH4 to NO3 under aerobic conditions
NH4 è NO3
nitrifying bacteria use NH4 as an energy source
heterotrophic denitrifying bacteria use NO3 as an oxidizing agent to oxidize reduced
carbon compounds for energy
this reduces NO3 to N2
NO3 è N2
N2 released into atmosphere
g. Biological Removal of Phosphorus
phosphorus has no gaseous phase
growing bacteria in activated sludge assimilate and store PO 4
this converts soluble P to particulate P
particulate P is removed by settling (2° clarifiers)
plants can be irrigated with treated wastewater
St. Petersburg, FL irrigates 4000 acres of lawns, parks, golf courses with 2° effluent
requires an expensive independent distribution system
f. Wetland Ecosystems
effluent can be routed through a constructed wetland for natural nutrient removal
Orlando, FL uses a 1200 acre constructed wetland
>1,000,000 marsh plants planted in a maze of shallow ponds and marshes
creates an attractive wildlife habitat
Phinzy Swamp Nature Park in Augusta is a constructed wetland
sometimes increase DO
sometimes improve appearance
few pathogens survive 1° and 2° treatment
disinfection kills the few, if any, that remain
required for public health reasons
several methodologies available
the most commonly used methodology
chlorine gas usually used
toxic and dangerous
sodium hypochlorite (bleach) can be used and is safer
residual chlorine kills organisms in receiving water
effluent must be dechlorinated
SO2 usually used for dechlorination
*but unstable and explosive
*must be generated on-site
*requires construction of an expensive ozone generating plant
iii. Ultraviolet Irradiation
UV irradiation disinfects
has no effect on the receiving water
a final filtration step sometimes added
removes fine particulates and reduces BOD
there may be a final aeration step
reduces BOD and elevates DO
clear, colorless, clean effluent with low BOD
quality is often higher than that of receiving water
if not for public resistance, this water could be potable
it would be much easier and cheaper to convert this effluent to drinking water than to
start with raw water from a lake or river
H. SLUDGE TREATMENT
recovered water goes into a receiving water
but what can we do with the solids?
particulates have been removed at several steps in the process
particulates collectively known as raw sludge
it must be treated
disinfected, BOD reduced, odor reduced, dried, disposed of
black, reduced, foul-smelling, thick, potentially with pathogens
raw sludge comes from 1° and 2° clarifiers
it is dewatered in a sludge thickener (=sludge clarifier)
particulates are allowed to settle
water is decanted to influent (sent back to beginning)
solids treated by various technologies
3. Sludge Treatment Technologies
a. Anaerobic Digestion
an old, still used method
dewatered sludge sent to large concrete anaerobic digestors
anaerobic bacteria reduce the BOD
convert organic solids to gasses (CH4, CO2, H2, and several other stinky gasses)
much of the BOD is in these gasses
this gas mixture is biogas and it is similar to natural gas (CH 4)
biogas used to warm the digestors or burnt in the atmosphere (CO 2 and global warming)
after 4-6 weeks treated sludge is stable, humus-like, and nutrient-rich, pathogens
it still has a high water content
poured onto sand drying beds
dried sludge can be used as soil supplement
can be pressed into sludge cake for storage, transport, application using manure spreaders
b. Land Application of Liquid Sludge
dewatered, disinfected sludge can be applied directly to some crops or forests
thickened sludge is disinfected using lime to raise pH to 12 for 2 hours
Greenwood maintains a 350-acre hay farm for treated sludge disposal
the hay is sold to local farmers
life of the farm is limited by build-up of heavy metals in the soil
dewatered sludge is mixed with wood chips (timber industry waste)
wood chips increase surface area exposed to oxygen
aerobic bacteria break down the organic material
sludge is converted to humus-like material and pathogens eliminated
requires about 6-8 weeks
wood chips are reused
a similar process uses waste paper instead of wood chips
the paper becomes part of the product
no need to separate substrate from humus
e. Pasteurization and Drying
filter-pressed to dewater
resulting sludgecake is dried in ovens to pasteurize it
used for soil conditioner by farmers or homeowners
may be formed into pellets which may be sold as soil conditioner
*Note: municipalities never make a profit selling wastes
*the principle motivation is to get rid of the product and, if possible, recoup some
*the motivation is usually to save space in the landfill
IV. SEPTIC TANKS
many homes are not connected to a sewage treatment facility
they use individual septic tanks and drain fields
resemble a combined clarifier and anaerobic digester
object is to separate solids from liquid
waste flows into underground tank
solids settle to the bottom and are partly digested anaerobically
liquid flows off the top into an underground drainfield
liquid flows into the soil
soil organisms reduce BOD
overlying vegetation thrives
solids must be pumped out periodically
1. Make a comment on the wastewater controversy regarding Tijuana River in Mexico.
2. Make a report on using reclaimed wastewater for other purposes. Is there any hazard or danger for
3. Make comment on “Reusing Treated Wastewater in Domestic Housing: the Toronto Healthy