Wastewater Pretreatment Systems
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Karen Mancl, Professor Food, Agricultural & Biological Engineering
The Ohio State University
Sewage is the wastewater released by residences, businesses and industries in a community. It is 99.94 percent water, with only 0.06 percent of the wastewater dissolved and suspended solid material. The cloudiness of sewage is caused by suspended particles which in untreated sewage ranges for 100 to 350 mg/l. A measure of the strength of the wastewater is biochemical oxygen demand, or BOD5. the BOD 5 is a measure of the amount of oxygen microorganisms require in 5 days to break down sewage. Untreated sewage has a BOD 5 ranging from 100 mg/l to 300 mg/l. Pathogens or disease-causing organisms are present in sewage. Coliform bacteria are used as an indicator of disease-causing organisms. Sewage also contains nutrients (such as ammonia and phosphorus), minerals, and metals. Ammonia can range form 12 to 50 mg/l and phosphorus can range from 6 to 20 mg/l in untreated sewage.
Soil absorption systems do an excellent job of renovating wastewater to reduce BOD5, and filter out suspended solids and disease-causing organisms. Unfortunately, not every lot has suitable soil for a soil absorption system. In these cases, some other process must be used to renovate the wastewater before safe disposal.
Primary treatment separates suspended solids and greases from wastewater. Wastewater is held in a quiet tank for several hours allowing the particles to settle to the bottom and the greases to float to the top. Septic tanks are the most commonly used primary treatment unit for individual homes. The clarified wastewater flows on to the next stage of wastewater treatment.
Secondary treatment is a biological treatment process to remove dissolved organic matter form wastewater and reduce BOD5. Sewage microorganisms are cultivated and added to the wastewater. The microorganisms absorb organic matter form sewage as their food supply. Three approaches are used to accomplish secondary treatment: fixed film, suspended film and lagoon systems.
Fixed film systems grow microorganisms fixed to substrate. As organic matter and nutrients are absorbed from the wastewater, the film of microorganisms grows and thickens. Trickling filters, rotating biological contactors, and sand filters are examples of fixed film systems.
Fixed-film systems are easy to operate and produce clear effluent very low in BOD5 and ammonia. They tend to be medium sized and require some management of the wastewater distribution system.
suspended film systems stir and suspend microorganisms in wastewater. As the microorganisms absorb organic matter and nutrients from the wastewater they grown in size and number. After the microorganisms have been suspended in the wastewater for several hours, they are settled out as sludge. Some of the sludge is pumped back into the incoming wastewater to provide “seed” microorganisms. The remainder is wasted and sent on to a sludge treatment process. Activated sludge, extended aeration, oxidation ditch, and sequential batch reactor systems are all examples of suspended film systems.
A home aerobic unit is a suspended film system that serves one house. It usually consists of three chambers. The first acts as a septic tank to settle the solids. The second chamber contains an aerator or mixer to suspend and stir the microorganisms that renovate the wastewater. The final chamber settles out the microorganism and returns them to the second chamber.
Suspended-film system require continuous stirring and sludge management. They tend to be small in size.
Lagoon systems are shallow basins that hold the wastewater for three to six months to allow for the natural degradation of sewage. this system takes advantage of natural aeration and microorganisms in the wastewater to renovate sewage.
Lagoon systems tend to be large but require very little management. Lagoon system must be fenced to guard against drowning.
Final treatment focuses on removal of disease-causing organisms from wastewater. Treated wastewater can be disinfected by adding chlorine or with ultraviolet light.
Advanced treatment is necessary in some treatment systems to remove nutrients form wastewater. Chemicals are sometimes added during the treatment process to help settle out or strip out phosphorus or nitrogen. Some examples of nutrient removal systems include coagulant addition for phosphorus removal, air stripping for ammonia removal and denitrification for nitrate removal.
Sludge is generated through the sewage treatment process. Septage is the sludge that settles out during primary treatment in the septic tank, often have a strong odor and require treatment prior to disposal. Secondary sludges are the extra microorganisms from the biological treatment processes. The goals of sludge treatment are to stabilize the sludge and reduce odors, remove some of the water and reduce volume, decompose some of the organic matter and reduce volume, kill disease-causing organisms and disinfect the sludge.
Untreated sludges are about 97 percent water. Settling the sludge and decanting off the separated liquid removes some of the water and reduces the sludge volume. Settling can result in a sludge with about 96 to 92 percent water. More water can be removed from sludge by using sand drying beds, vacuum filters, filter presses, and centrifuges resulting in sludges with between 80 to 50 percent water. The dried sludge is called a sludge cake. Aerobic and anaerobic digestion are used to decompose organic matter to reduce volume. Digestion also stabilizes the sludge to reduce odors. Caustic chemicals can be added to septage and sludge to increase pH or it may be heat-treated to kill disease-causing organisms. Following treatment , liquid and cake sludges are usually spread on field, returning organic matter and nutrients to the soil.
Wastewater treatment processes require careful management to ensure the protection of the environment. Trained and certified treatment plant operators measure and monitor the incoming sewage, the treatment process and final effluent disposal.