Homework Solution for Module 15 Waste Water Treatment Plants 1. Why do we have to treat wastewater? Because the dilution of a river is usually not sufficient to purify sewage and industrial pollution well enough for downstream users to use it. 2. What is the difference between municipal and industrial wastewater? Industrial wastewater comes from a specific industry, or factory. It may have specific chemical streams in the wastewater. Municipal wastewater comes from homes and businesses via sewers. Municipal wastewater may include industrial wastewater if the industry is allowed to discharge to the city sewers. 3. Define effluent Effluent is liquid that flows FROM something. In the laboratory, the term is used to describe the liquid coming out of a flow experiment. In the context of wastewater treatment, effluent is defined as the flow coming out of the plant and discharging into the river. Other examples include treated water being discharged from a factory, OR the flow coming from a lakeside cabin into a lake. Effluent can be treated or untreated. 4. Define receiving water Receiving water is a stream, river, lake, ocean, or groundwater, into which treated or untreated wastewater is discharged. 5. What might downstream users be using the river water for? Downstream uses might be drinking water, irrigation, fish hatchery, shellfish farming, stock and wildlife watering, swimming, navigation, hydroelectric power generation, etc. 6. Describe the importance of the operator s role with respect to downstream water users. In your answer, describe why downstream water users may consider the water treatment plant (or the operator) an adversary. The operator and his/her supervisors is the only people that stand between the downstream users and possible disaster. The operator must be aware of the water quality needs of the downstream users. He should remain in contact with them so they believe at a personal level that he has their interests in mind. He may have to contact them and warn them if (heaven forbid) he has had a system failure and he is discharging above his NPDES limits; such a warning will be received much more favorably if he is in regular contact with them. Even though the operator is protecting downstream users from the total impact of the community the operator serves, the operator is effectively a polluter from the downstream perspective. The water treatment plant discharges pollution, even though it is much less than if the plant weren t there.
7. Define organic waste. Organic waste is from a plant or animal source. It is waste that contains carbon. Organic wastes generally can be consumed by bacteria. 8. Define inorganic waste. Inorganic waste is from a mineral source. It does not contain carbon. Examples of inorganic wastes are silt, clay, and chemicals and metals from industrial operations. Bacteria may be able to change the chemical form of these wastes, but it usually takes more time than the residence time of a water treatment plant. 9. In some communities, the municipal plant may receive water from an industrial operation, and treat this water along with the domestic wastes. What industries produce organic wastes? Industries that produce organic wastes include vegetable and fruit processing plants, dairy processing plants, pulp and paper plants, meat processing plants, etc. (When you drive up the Lewiston Grade to Moscow, you may see big splotches green grass that are obviously irrigated land on the north side of the river. This is the result of land application of pea and lentil processing waters. By employing land application, the producers are putting the nutrients into the irrigated plants, instead of into the river.) 10. What industries produce inorganic wastes? Sand and gravel operations, mining operations, and industries that discharge inorganic chemicals. 11. What is the difference between primary and secondary treatment? Primary treatment removes the sinkers and the floaters the two visible wastes that can be removed physically. Secondary treatment includes methods to convert dissolved and suspended wastes into a form that is easily separated from the water, such as a settleable solid. Settleable solids can be removed by sedimentation and filtration. 12. Define aerobic bacteria Aerobic bacteria are those that will survive only in an environment with available oxygen (that is, water in air or in dissolved air). Aerobic bacteria cannot use oxygen that is bound in a chemical compound. 13. Define anaerobic bacteria
Anaerobic bacteria can thrive in an environment with no available oxygen. These bacteria derive their oxygen by breaking down other compounds that contain oxygen, such as sulfate, nitrate, or phosphate. 14. What is an Imhoff Cone? An Imhoff Cone is a clear, cone-shaped container with graduated lines on it. It is used to measure the amount of settleable solids in a given volume of water. 15. Define biochemical oxygen demand (BOD) BOD is the amount of oxygen used by organisms in a water body (or a water sample) as the organisms break down decomposable organic matter under aerobic conditions. BOD has units of mg/l, representing the mg of oxygen used per L of water. (Careful: your book says it is a rate but it isn t. The units aren t per time.) 16. What causes oxygen depletion when organic wastes are discharged to water? In your answer, list the bad outcomes if oxygen depletion occurs. When oxidizable wastes are discharged to a stream, bacteria decompose these wastes. Because the environment is aerobic, the bacteria are aerobic, and use oxygen for their respiration as they bread down the organics. If the food supply is ample, the bacteria will reproduce until they exhaust the oxygen supply. When this occurs, fish and other living things in the stream die, and anaerobic bacteria begin to multiply. The anaerobes release gases such as methane, hydrogen sulfide, which stink. The decomposing fish and other creatures killed by the depletion also stink. This condition is called septic. 17. Define septic Septic is the condition produced by anaerobic bacteria. If severe, the wastewater produces hydrogen sulfide, turns black, gives off foul odors, and contains little or no dissolved oxygen. 18. Define nutrient cycle The transformation of a nutrient such as nitrogen or carbon from one form to another by biological and biochemical means until the nutrient is returned to its original form. 19. What nutrients are usually of concern in receiving waters? Nitrate, sulfate, phosphate, and carbon. 20. In the nitrate cycle, which step produces ammonium? When fish die and decompose, ammonium is created.
21. If no dissolved oxygen is present in a receiving water, what is the endpoint (a) of the carbon cycle? (b) Of the sulfur compounds? (a) methane, a flammable gas (b) hydrogen sulfide gas, which smells like rotten eggs 22. What does NPDES stand for? National Pollution Discharge Elimination System a system that regulates discharge into navigable waters from all point sources of pollution. All point sources must have an NPDES permit. (A point source is distinguished from a non-point source by the extent of the discharge. A point source comes out of a pipe or ditch; a non-point source discharges all along the river bank.) 23. What types of things might have discharge limits on a NPDES permit that is granted for a municipal wastewater treatment plant? Suspended solids, BOD, number of coliform bacteria, and temperature. NOTE: Temperature is an important factor for the Moscow, ID permit, because the plant is just upstream of the Washington State Line. The state of Washington wants Paradise Creek (the waterway in question) to support fish, and temperature adversely affects fish. It s too expensive to refrigerate the effluent, so the city of Moscow and the UI use as much of the WTP effluent as possible to irrigate the lower-elevation playing fields. In using land application, Moscow is employing the same solution as the pea processors on the Snake River, except that the problem parameter is temperature, not nutrients. Signs are posted so thirsty athletes don t drink from the sprinklers effluent from the Moscow WTP does not meet drinking water standards. 24. Name 3 waterborne diseases that are caused by pathogenic organisms. You should name 3 from the following list: Typhoid, cholera, dysentery, polio, giardia and hepatitis. 25. When water is flowing through the city sewers to the sewage treatment plant, what problem can occur if it takes a long time? If the flow time is long and the weather is hot, all the oxygen will be depleted, and the water will go septic. Hydrogen sulfide can cause an odor problem all the way en route, can damage concrete in the plant, and makes the wastes more difficult to treat. 26.Why can combined sewers be a problem? Combined sewers carry storm water and sewage in the same system. They can be a problem during heavy rains the treatment plant may be overloaded and unable to treat all the water, and then have to release some untreated water into the receiving stream.
27. Why is the screening step included in waste water treatment? The screening step is employed to keep wood, rags, roots, and sand from damaging the pumps and other equipment. 28. What does detention time in a clarifier accomplish? Detention time in a clarifier, or sedimentation tank, provides sufficient time for the sinkers to get to the bottom, and the floaters to get to the top. 29. How are scum and sludge removed from a clarifier? Scum is removed with a scum skimmer, and sludge is removed with a scraper blade, or plow, and pushed into a sump at the bottom of the clarifier. 30. What occurs during secondary treatment? In secondary treatment, the wastewater is treated by microbes. Secondary treatment processes include trickling filters, rotating biological contactors, activated sludge, and secondary clarifiers. 31. How are the large populations of microorganisms in the effluents from the trickling filters, rotating biological contactors, and activated sludge tanks removed from the flow? By settling in a secondary clarifier 32. What two steps are performed on sewage sludge before it is disposed? Digestion and dewatering. 33. When are waste treatment ponds used instead of sedimentation tanks, trickling filters, etc.? When you have a lot of land in a sparsely populated area, which allows you to perform anaerobic treatment without upsetting residents with the odor. 34. How long a detention time is required within waste treatment ponds in order to remove solids and harmful bacteria? Usually a month or more is required within waste treatment ponds in order to remove solids and harmful bacteria