Sewage is the wastewater released by residences, institutions and businesses in a community. It is 99.94 percent water, with only about 0.06 percent of the wastewater dissolved and suspended pollutants. The cloudiness of sewage is caused by suspended particles, which in untreated wastewater, is from 100 to 350 mg/l. A measure of the strength of wastewater is the biochemical oxygen demand, or BOD5. The BOD5 measures the amount of oxygen microorganisms require over five days to break down the sewage. Untreated sewage has a BOD5 ranging from 100 to 300 mg/l. Pathogens or disease-causing organisms are present in sewage. E. coli are used as an indicator of disease-causing organisms. Sewage also contains ammonia and phosphorus. Ammonia levels can range from 15 to 50 mg/l, and phosphorus levels can range from 6 to 20 mg/l in untreated sewage.
Sewage treatment is the multistage process that renovates wastewater (Figure 1) before the wastewater enters a body of water or is reused on the land through irrigation. The goal is to reduce or remove the pollutants and kill disease-causing organisms. Each receiving body of water has limits on the amount of pollutants it can safely accept without degradation. Therefore, each sewage treatment plant must hold a permit listing the allowable levels of BOD5, suspended solids, E. coli and other pollutants it is allowed to discharge. The discharge permits are called NPDES (National Pollutant Discharge Elimination System) permits.
|Figure 1. The wastewater treatment process.|
Screening, grinding and separating large solids and debris from wastewater is the first stage in wastewater treatment. Sticks, rags, large food particles, sand, gravel and even toys that are flushed down the drain must be removed from wastewater to protect pumps and other wastewater treatment equipment. Preliminary treatment equipment includes bar screens, comminutors (larger versions of garbage disposals) and grit chambers. The collected debris is usually disposed of in a landfill.
Quiet tanks are used to settle out solids and separate grease from wastewater in the second stage of treatment. Wastewater is held in large tanks for several hours, allowing heavy particles to sink to the bottom and grease to float to the top. Solids are drawn off the bottom, and grease is skimmed off the top and sent on to be treated as sludge. The clarified wastewater flows on to the next stage of wastewater treatment. Clarifiers and septic tanks are usually used to provide primary treatment.
Microorganisms are mixed with wastewater to provide the biological treatment of wastewater. Naturally occurring microorganisms are cultivated and feed on the pollutants in the wastewater. Organic matter is consumed and ammonia is transformed to nitrate during secondary treatment. Three approaches are used to accomplish secondary treatment: suspended film, fixed film and lagoon systems.
Suspended Film Systems
Wastewater is stirred and aerated with microorganisms in suspended film treatment. The clarified wastewater becomes cloudy as microorganisms are poured into the sewage. For several hours, mixers and aerators stir the wastewater/microbe mixture as the organic matter and ammonia is consumed. Next, the microorganisms are removed from the wastewater by letting the mixture flow to a settling tank. The microbes sink to the tank bottom, are collected and are returned to the mixing tank. The clear, treated wastewater flows out of the tank to the next stage of treatment. Suspended film systems are called activated sludge, extended aeration, oxidation ditches or sequential batch reactors.
Fixed Film Systems
Wastewater is sprayed over rocks, sand or even pieces of plastic that are covered with a thin film of microorganisms (Figure 2). As the wastewater flows over the microbial film, organic matter and ammonia are consumed. Fixed film systems are called trickling filters, sand bioreactors or rotating biological contactors.
|Figure 2. Microbes covering media to form a biological film for wastewater treatment in a fixed film system.|
Large, shallow ponds collect and hold wastewater for months as naturally occurring microorganisms consume the organic matter and stabilize the wastewater. Lagoon systems may be naturally aerated or be equipped with a mechanical aerator to gently stir the wastewater to ensure good contact between the microorganisms and the pollutants.
Chemicals added to the wastewater or extra treatment processes are sometimes added to settle out or strip out phosphorus or nitrogen from wastewater. Chemicals called coagulants can help bind up and settle out phosphorus. Air stripping, pH adjustment or aerobic/anaerobic treatment processes are sometimes used to remove nitrogen from wastewater.
Killing disease-causing organisms is the last stage of wastewater treatment. Chlorine is an effective disinfectant and will kill disease-causing organisms with enough time. Treatment plants will have baffled tanks to provide the contact time needed for the chlorine to complete the disinfection process. Because chlorine is toxic to aquatic life, the chlorine must then be neutralized before being discharged into a body of water. Ultraviolet light is also used in treatment plants for disinfection.
Sludge Treatment (Also Called Biosolids)
Sludge contains the pollutants removed in the wastewater treatment process. Primary sludge is the solids and grease collected in primary treatment. The extra microorganisms grown during secondary treatment are wasted and are considered secondary sludge. Precipitates of phosphorus and other pollutants generated during advanced treatment are also sludge.
The goal of sludge treatment is to stabilize it to reduce odor; remove water to reduce volume; and disinfect it to kill disease-causing organisms. Once treated, sludge is recycled through land application to reuse the organic matter and nutrients captured during wastewater treatment to amend soil.
Since untreated sludge is about 97 percent water, much of sludge treatment involves separating the water from the solids. Settlers, filters, dryers, presses and centrifuges are all used to reduce the water content of sludge. To stabilize to reduce odors, sludge is digested either by aerating it for many days or by digesting it for a few months in anaerobic digesters that produce methane gas. To disinfect sludge to kill disease-causing organisms, caustic chemicals are added (like lime), or the sludge is heated.
Wastewater Treatment Plant Operators
The wastewater treatment process requires careful management to protect the environment from pollution. Trained and certified operators are required to monitor the incoming sewage, the treatment process and the final effluent discharged. Wastewater treatment plant operators must be certified by the Ohio Environmental Protection Agency (EPA); more than 12,000 people hold a certification. Certified operators must complete training, be tested and complete continuing education every year to maintain their certification.
Wastewater Treatment Regulations
Clean water has been a concern nationwide since the early 1970s. In 1972, U.S. Congress adopted the Federal Water Pollution Control Act (also called the Clean Water Act) to protect the waters of the United States. Through this law, the U.S. EPA and corresponding state agencies were given the responsibility to regulate activities that threaten the nation’s water resources.
In the Federal Water Pollution Control Act, U.S. Congress adopted comprehensive water policy for the nation and set a national goal to eliminate the discharge of pollution to navigable waters. To reach the goal, U.S. Congress established the regulatory framework:
- No one has the right to pollute the navigable waters of the United States. Dischargers must have a permit to pollute.
- Permits set pollutant limits, and a violation carries a penalty of fines or imprisonment.
- The permits expire and must be renewed every 5 years.
In Ohio, the Ohio EPA issues permits and enforces the federal law. Two permits are required for most systems. The first is a Permit to Install (PTI), which needs to be obtained before the planned system can be constructed. The second is a discharge permit (NPDES permit) that lists all of the pollutant discharge limits. NPDES permits must be renewed and include more restrictions every 5 years as the nation continues to move to meet the goal of eliminating the discharge of pollutants to navigable waters.
However, small systems that treat wastewater onsite and do not have a discharge are permitted by local health departments under regulations adopted by the Ohio Department of Health. Local health departments issue permits for onsite systems serving one-, two- or three-family dwellings. Ohio EPA and local health departments work to regulate small discharging systems. They also work together to regulate nonresidential systems that treat up to 1,000 gallons of wastewater per day.
Penalties for violating water pollution laws range from a public nuisance to an illegal discharge. A public nuisance is created by the discharge of raw sewage from a building. It is a third-degree misdemeanor punishable by up to 60 days in jail or a $500 fine, plus the requirement to remove the sewage and abate the nuisance (Ohio Revised Code 3767.13). An illegal discharge is a discharge to a waterway without a permit or a discharge to a waterway that exceeds the limits of the permit. An illegal discharge carries a fine of up to $10,000 with each day being a separate violation for civil penalties. If an illegal discharge is found to be a criminal violation, the penalty is up to $25,000, plus one year imprisonment (Ohio Revised Code 6111.07).
Ohio EPA can also issue a “connection ban” that prohibits the construction of any further homes or businesses if the pollution is being discharged to the waters of the state (Ohio Administrative Code 3745-11).
By regulating the discharge of pollutants to Ohio’s waterways, streams and lakes are cleaner; water supplies and recreational waters are protected; and fish and other wildlife can flourish.
|Figure 3. Permits and regulatory agencies responsible for sewage systems in Ohio.|