Document Sample
EXERCISE _____ Powered By Docstoc
					                                                                                 SEWAGE TREATMENT
                                                                                           Page 12-1

Adapted from:
   Belcik, F.P., “Tour of Water and Sewage Treatment Plants”, Laboratory in Environmental Biology, p. 97-108.
   Clean Water for Today: What is Wastewater Treatment?, brochure by Water Environment Federation

This lab will enable the student to understand:
         the importance of water quality to environmental health
         the chemical, physical and biological processes involved in water treatment
         the importance of water as a resource and part of ecosystems
         the problems associated with various water contaminants

Water is essential to the life processes of all organisms. Even though it is the most abundant
substance on the surface of the earth, only 0.03% is readily available freshwater that can be used
by humans. The small percentage that is available may contain parasites, toxic chemicals or
other harmful particulate matter. It is the role of water treatment facilities to remove these
contaminants. The water availability and treatment capacity of an area is a very important factor
in economic development. In many areas of eastern North Carolina and other parts of the world,
water is the limiting factor to urbanization. Our society rarely attributes the appropriate
importance to quality water.

In nature, bacteria and other small organisms like algae and fungi are attracted to any wastewater
as a food source. By consuming this food, new organisms are reproduced and other small
molecules, like carbon dioxide, ammonia, nitrite and nitrate, are released which serve as a food
source for higher organisms. Oxygen is required by the bacteria to break down this material, and
a body of water provides this oxygen by absorbing it from the atmosphere. Faster moving
streams and other turbulent water acquire more oxygen than slow moving streams and flat water.
Another source of oxygen within the water is the release of oxygen by aquatic plants during their
photosynthetic processes. Only a finite amount of oxygen is capable of entering the system.
Once oxygen has entered the water column, it is called dissolved oxygen.

In a natural system, small amounts of wastewater are consumed by bacteria which are capable of
restoring water quality, removing small amounts of oxygen that do not endanger fish and other
organisms. The levels of dissolved oxygen are quickly replenished by natural sources. When the
volume of wastewater discharged into streams increases, more bacteria are needed to consume it;
more oxygen is removed from the system; and natural means are unable to regenerate the
dissolved oxygen in the water needed to support life. Urbanization in many areas increases the
volume of wastewater discharged to a level that exceeds the capabilities of a natural system,

Wastewater treatment facilities are based on natural systems and designed to perform two basic
functions: speed up the natural purification processes that occur in our streams, rivers and lakes
and reduce toxic contaminants that might otherwise interfere with the natural processes. The
goal is to produce a stream of treated water that is safe for return to the environment.
                                                                             SEWAGE TREATMENT
                                                                                       Page 12-2

Wastewater is defined as the flow of water from a community. It is 99.94% water and only
0.06% is dissolved or suspended solids. Wastewater contains disease-causing bacteria, viruses
and other microorganisms which are removed through treatment. Knowing the importance of
dissolved oxygen to aquatic life, an important measurement is the amount of oxygen required by
the bacteria to effectively consume solid waste. This measurement is called biological oxygen
demand (BOD). The higher the BOD, the more oxygen is demanded by the bacteria to consume
the waste. In keeping with the goal of wastewater treatment plants, by reducing BOD, the
number of bacteria are reduced because there are fewer solids in the water to breakdown.
Substances that are not consumed by microorganisms are not included in BOD; they include
pesticides, heavy metals and inorganic molecules that serve as nutrients for aquatic plants.

Wastewater treatment plants have a number of common features which are presented in a block
flow diagram in Figure 12-1.

         Raw Wastewater

                                     Primary Settling         sludge       Sludge Thickener
                                          Tank                                  Tank

                                     Secondary Trmt
                                   Biological Filtration

                                                                       methane     .
                          sludge      Final Settling
                                                                           Land Application
                                                                             of Biosolids
                                                clarified water

                                    Final Treatment

                                  Figure 12-1

The New Bern city sewage treatment plant utilizes many of the same processes. Their operation
is presented in Figure 12-2 and a description of each step is also provided.
                                                                        SEWAGE TREATMENT
                                                                                  Page 12-3

                                 Figure 12-2

Preliminary Treatment Raw wastewater is pumped through intake screens before the raw water
pumping station transfers it to the settling basin. The intake screens remove leaves, sticks, rags
and other large, floating debris to prevent damage to the pumps. The settling basin removes
heavier particulate matter, such as rocks, grit and stones which would create unnecessary wear on
the equipment. A chemical called sodium aluminate is added to precipitate (solidify) any

Primary Treatment - Primary Settling Tank

The primary settling tank holds a large volume of water, slowing the flow to allow suspended
solids an opportunity to settle on the bottom of the tank. This includes the precipitated
phosphorous. A scraper on the bottom of the tank removes sludge (solids) and sends it to a
sludge thickener. Approximately 60% of the sludge is removed from wastewater at this point. A
skimmer removes grease from the surface of the tank which is also sent to the sludge thickener.
To this point, physical processes have been used to remove solids from the waste stream,
screening, skimming and settling. The next processes employ biological systems. Water is
oxygenated as it flows over the side of the basin, supplying oxygen to the bacteria that break
down the remaining waste particles.

Secondary Treatment

High Rate Trickling Filter. This is a biological oxidizing bed which is used to convert non-
settleable colloidal [insoluble] particles and dissolved solids to settleable solids. Wastewater is
sprayed over rocks sized between 2-4.5 inches stacked 8-10 feet deep. Microorganisms grow on
their surface and they are responsible for breaking down organic material. The spaces between
the rocks provides plenty of air space in the bed. The water flow through recirculation pumps
where part of the flow is diverted back to the filters to help regulate the flow rate. Excess flow
moves onto the next stage in the process.

Final Settling Tank. Bacteria continues to collect on suspended particles causing them to settle
to the bottom of this tank. The settled material (sludge) is sent back to the primary treatment
tank, along with the bacteria. The addition of these organisms to the incoming raw sewage
speeds up the decomposition process. These sludge particles attract the new sewage, causing floc
(settleable material) to form. This process is called flocculation. Clarified water overflows from
this tank and is sent to final treatment.

Final Treatment

In the chlorine contact tank, chlorine gas is added by a flash mixer into wastewater flowing from
the final settling tank. The chlorine contact tank is a 30-minute holding tank which ensures the
final destruction of bacteria. Chlorine is a destructive compound, in order to neutralize it,
sodium bisulfite [NaHSO3] is added. However in the presence of small nitrogen-containing
                                                                         SEWAGE TREATMENT
                                                                                   Page 12-4

molecules, it combines to form a carcinogenic compound called chloramines. Therefore, the
removal of chlorine is extremely important in the treatment of wastewater.

On a daily basis, a number of water quality tests are performed on water that is being returned to
the river. In addition to chlorine, the water is tested for ammonia, phosphate, and coliform
bacteria. The presence of this bacteria would indicate a failure in the disinfection process. BOD
and total suspended solids (TSS) are also monitored in water being discharged into the river,
other indicators of the performance of the system.

The sensitivity of the Neuse River Basin requires that advanced waste treatment operations be
performed to remove more of the nutrients that stimulate the plant and algae growth that plague
our region. Typically, conventional primary and secondary treatment methods remove less than
half of the ammonia produced in the process. The New Bern plant currently discharges 9 ppm
into the Neuse River. In the year 2000, a new limit is being established at 3 ppm. This limit is
not possible under the present operation; therefore, changes in the secondary treatment system
are proposed that will break down the ammonia further to nitrate a nutrient that is less harmful to
the receiving river.

Sludge Thickener and Digesters

The sludge thickener tank is a holding area that gives sludge more time to settle out and thicken
as excess water is removed. Excess water is routed to Preliminary Treatment and mixed with the
untreated water coming into the plant. Thickened sludge is directed to digesters 1 and 2 where it
is heated to 95 oC. Anaerobic bacteria attack raw sludge breaking down organic material into
inorganic nutrients. The third stage is completed in digester III where another type of bacteria
consumes more resistant nitrogenous materials. This process releases large volumes of methane
gas part of which is recycled and part of which is used as fuel for boilers to maintain the
temperature in digesters between 95-105F. When digestion is complete, sludge is pumped to the
sludge truck fill station and is carried away from the plant and used as a nutrient supplement on
local agricultural fields. If necessary, digested sludge is pumped to the sludge drying beds where
it can be dried and then hauled away.

Concentrated solids (sludge) can be placed in landfills, incinerated, applied to the land or
composted for use as a soil conditioner. Every other month, 0.5 million gallons of biosolids are
produced by the New Bern wastewater treatment plant. They have a land application program in
place that is strictly regulated. The land must not produce food for human consumption, only
certain types of soil qualify for land application, and the soil must be tested on a regular basis to
ensure it does not exceed a level of nitrogen that it can safely process.
                                                                        SEWAGE TREATMENT
                                                                                  Page 12-5

QUESTIONS - answer in complete sentences

1. How does a wastewater treatment plant resemble nature?

2. What are the two purposes of a wastewater treatment plant? What is its primary goal?

3. On average, ____________________ gal untreated sewage reaches the plant on a daily basis;
   approximately   ____________              gal are removed from the river to process raw
   wastewater. Information from trip.

4. What does the screening process accomplish? Why is this done?

5. What is sludge? Once the process is complete, what is it used for?
                                                                       SEWAGE TREATMENT
                                                                                 Page 12-6

6. What function does bacteria serve in the treatment of raw sewage?

7. Why is oxygen an important factor in the sewage treatment process?

8. What role does chlorine play? When is it added?

9. What problems might arise if chlorinated wastewater was discharged into local rivers?
                                                                      SEWAGE TREATMENT
                                                                                Page 12-7

10. For what is methane, a byproduct of the process, used?

11. What water quality tests are performed at the wastewater treatment plant before the water is
    released back into the environment?

12. Which is better, a high BOD or low BOD being released into the water? Why?