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Water Pollution Control -
A Field Trip Christine
L. Case
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| Objectives |
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Background See the figure |
Sewage is the used water supplies of homes including garbage disposal and washing machine waters. The amount of sewage produced by each individual varies greatly, however, an average of 400 liters (0.4 m3) per person per day is considered normal for most United States cities. Sewage consists of 99.8% water, the remaining 0.2% consists of 20% inorganic matter (mostly road debris), and 80% organic material. Removal of the organic material is of major concern in the sewage treatment operation. Sewage treatment can be divided into three categories: primary, secondary, and tertiary. Primary treatment consists of removal of solid material from sewage, the solids may be taken to a dump, the remaining water is chlorinated and discharged into a receiving body of water. Primary treatment cannot be the sole type of sewage treatment (Clean Water Act, 1972), all water must have secondary treatment or better. Secondary sewage treatment involves removal of solid matter, aerobic bacterial digestion of organic material, chlorination of the water, and discharge. The purpose of secondary treatment is to remove organic matter that could be used by bacteria in the receiving body of water. This organic matter is called biochemical oxygen demand or BOD because bacteria can deplete the oxygen in water when they use the organic matter for food. The most efficient means of digestion of the organic material is the activated sludge process. Activated sludge systems remove 75 to 95 percent of the BOD from sewage. Trickling filters, another means of digestion, remove 80 to 85 percent of the BOD. The effluent from secondary treatment contains some residual BOD, about 50 percent of the original nitrogen, and 70 percent of the original phosphorous. These nutrients can pollute water. Tertiary treatment is designed to remove essentially all of the BOD, nitrogen, and phosphorous. Following secondary treatment, nitrogen is removed by bacterial conversion to ammonia or nitrogen gas. Phosphorous is precipitated out with ferric chloride. Sand filters and activated charcoal remove small particulate matter and dissolved chemicals. Finally, chlorine is added and the water can be used for drinking. It is necessary to emphasize the importance of recycling the natural resources of the Earth in order to meet the increasing demands for these resources. Water from Sierran watersheds should be reused before being sent to the ocean. Following secondary treatment, the water could be used for irrigation and outdoor washing. Nutrients in this reclaimed water would be recycled to land to fertilize the soil, replacing nutrients depleted by agriculture. In water, these nutrients stimulate algal and bacterial growth leading to eutrophication; returned to land, they can save energy and further soil minerals. |
| Location | Visit your local sewage treatment plant. |
Procedure
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The ____________________ Water Pollution Control Facility is a secondary plant. The influent is pumped from ____________________ pumping stations. At the pumping stations, nonbiodegradable materials are removed by gravity and floating matter is skimmed off the surface. Larger matter and paper are ground up. The solids are taken to a solid waste dump. At the STP, the sewage flows through a Grit Chamber where tar and gravels are removed; these large inorganic materials would cause excessive strain on later operations. Grit is then removed. One-hundred fifty to 200 gallons of grit per day are taken to a solid waste dump. Liquid goes to the Aeration Tanks where bacteria degrade organic matter. The tanks are aerated to provide oxygen for the bacteria. Aerobic decomposition is more efficient than anaerobic (without oxygen). This plant uses an activated sludge process wherein the water from the aeration tanks is mixed with sludge from the clarifier and aerated, the aerobic conditions increase the rate and efficiency of microbial decomposition. Using oxygen, bacteria can completely degrade organic material to inorganics whereas, anaerobically, many organic molecules remain undegraded. Solids from the Vacuators are mixed with a flocculant such as alum, ferric chloride (FeCl2) or Hercofloc® to cause suspended or colloidal solids to coalesce and settle out. The settled material and scum are combined as sludge and go to the Digesters. In the Digesters, anaerobic bacteria degrade organic material. The most abundant bacteria are coliforms. (This term includes members of the genera Citrobacter, Escherichia (coli), and Enterobacter.) Coliforms are present in the large intestines of mammals and the excess bacteria are excreted in fecal matter. These bacteria metabolize carbohydrates to yield organic acids, carbon dioxide (CO2), and methane (CH4) under anaerobic conditions. Of the gases produced in a Digester, 70% is methane. When burned, methane produces about 66% of the heating efficiency of natural gas. 16,000 ft.3 of methane are produced each day. This methane is used to heat a boiler which in turn heats the digesters to 35° to 37°C and to run engines. The Digesters are heated to the optimum growth temperature for coliforms - human body temperature. The Digesters are maintained at pH 7.0. Digestion takes place for approximately 20 days. Sludge is periodically removed from the Digesters by centrifugation, dried, and mixed with rice hulls and coffee grounds to be used as a soil amendment. These are nonbiodegradable or are difficult to degrade requiring longer time or different bacterial species and are therefore not recycled by sewage treatment. Sludge contains few viable bacteria because of its low water content. It is relatively low in nitrogen and alone, makes a poor fertilizer. Water (supernatant) from the Digesters is returned to the headworks. Water from the Aeration Tanks goes to the Clarifier where it settles for about 2 hours. Sludge settles to the bottom and is removed by weirs. The sludge is mixed with floc and used in the Aeration Tanks. The sludge provides the necessary bacteria for degradation in the Aeration Tanks. Effluent water from the Clarifiers is mixed with gaseous chlorine (Cl2) to disinfect it. Disinfection removes most bacteria but is relatively ineffective against viruses, protozoans, and helminthic parasites. Chlorine is added to approximately 1mg/mL residual concentration; the residual is removed during dechlorination. Chlorine is detrimental to many marine organisms and must be removed before final discharge. The effluent is pumped out the outfall pipe which is located ______________. The last 500 feet of the pipe is a diffuser to disperse the effluent over a greater volume of water in order to dilute the wastewater. |
| Observations & Data Record | Summer | Winter | |
| What is the capacity of this plant? | |||
| How much water is processed during an average day? | |||
| How much water is processed during a rain storm? | |||
| Difference |
| Aerobic | Anaerobic | O2 doesn't matter | ||
| Biological degradation | ||||
| Chlorination | ||||
| Clarifier | ||||
| Dechlorination | ||||
| Sedimentation | ||||
| Sludge |
| Before treatment | After treatment | % efficiency | ||
| Settleable solids | ||||
| BOD | ||||
| Coliforms |
| Where do each of the following go after treatment? | ||
| Settled solids | ||
| Sludge | ||
| Water |
| Questions |
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