Sewage Treatment
Sewage Treatment
Separation of liquid and biosolids
Sewerage and sewage must be defined at the outset because they are often used incorrectly. Sewerage is a system of pipes used to collect and carry sewage, which is the wastewater discharged from domestic premises. Sewage is wastewater discharged from a home, business, or industry. It is treated to remove or alter contaminants in order to minimize the impact of discharging wastewater into the environment. The operations and processes used in sewage treatment consist of physicochemical and biological systems.
Domestic sewage consists of human wastes, paper, and vegetable matter. This type of waste is organic because it consists of compounds containing carbon and can be broken down by microorganisms into simpler compounds, which are stable and not liable to cause a nuisance. Sewage can consist of 99.9% water and 0.1% solids.
Raw sewage is a health and environmental concern. It carries a host of bacteria and viruses, causing diseases such as typhoid, cholera, and dysentery. Decaying organic waste is broken down by microorganisms that require substantial amounts of oxygen. If raw sewage is released directly into rivers, lakes, and oceans, it will significantly, and often catastrophically, reduce the oxygen levels in the water, killing fish, native microorganisms, and plant life.
In natural sewage decay, organic waste is consumed by microorganisms such as bacteria and fungi. Initially, this decay is aerobic (requiring oxygen). If the quantity of material is too large, however, the oxygen is depleted and the decay mechanism becomes anaerobic (carried out in the absence of oxygen). Anaerobic decay is slower than aerobic decay, and produces toxic reduction compounds like methane and hydrogen sulfide. The natural process is acceptable for very limited amounts of sewage but impractical for the quantities produced by municipalities. As a result, bulk treatment methods have been developed.
In general, municipal sewage treatment is an iterative process. The process begins by screening out large solids, such as trash, with bars or large mesh screens. Next, grit is settled out in preliminary settling tanks. The sewage then proceeds to further separation by moving through a series of holding tanks where the heavy matter (sludge) falls to the bottom, where it is later removed, and the floating matter rises to the top where it can be skimmed off. Once filtered, sewage is sent to tanks where it is processed biologically, using aerobic organisms. In addition, sewage can be chemically treated to bring pH to an acceptable region, and to remove hazardous wastes.
These are methods of sewage treatment, but not all of them are employed at every sewage treatment plant. The specific methods of treatment is dependant upon both the location of final release and the nature of the sewage being treated.
Separation of liquid and biosolids
After trash and bulk contamination are removed from waste water by screening, the next step is the removal of suspended matter. This can be accomplished by several methods, the simplest of which is gravity sedimentation. Wastewater is held in a tank or vessel until heavier particles have sunk to the bottom and light materials have floated to the top. The top of the tank can be skimmed to remove the floating material and the clarified liquid can be drained off. In batch mode sedimentation, several tanks of sewage will go through the settling process before the accumulated sludge is removed from the bottom of the tank.
The settling process can be hastened by use of chemical precipitants such as aluminum sulfate. Gentle stirring with rods, another method, encourages the aggregation of a number of fine, suspended materials. As the clots of material grow larger and heavier, they sink. Suspended matter can be encouraged to float by exposing it to fine bubbles, a method known as dissolved-air floatation. The bubbles adhere to the matter and cause it to float to the surface, where it can be removed by skimming.
Another method of filtration, generally used after gravity sedimentation, is deep bed filtration. Partially processed liquid from the sedimentation tanks, called effluent, flows over a bed of graded sand and crushed coal. This material not only strains the larger particles from the effluent but also further clarifies it by removing fine particles via adhesion. The filtering material attracts these small particles of sewage by electrostatic charge, pulling them out of the main flow and resulting in significantly clearer liquid. Alternately, effluent can be filtered by a fine mesh screen or cloth, in a method known as surface filtration, or solid material can be pulled out by centrifuge.
At this point the original raw sewage has been essentially separated into two parts: sludge, or biosolids; and clarified effluent. Both parts still contain disease carrying, oxygen consuming pathogens, and need further processing. Earlier, the biological decay of raw sewage was discussed. Theoretically, both biosolids and effluent can be processed using biological treatment methods, but at this point cost considerations come into play. Biological treatment of dense sludge is time-consuming, requiring large tanks to allow complete processing, whereas that of effluent is fairly efficient. Thus, biosolids are generally processed by different methods than effluent.
After settling out, biosolids can be removed from the bottom of the sedimentation tank. These tanks may have a conical shape to allow the sludge to be removed through a valve at the tip, or they may be flat bottomed. The sludge can be dried and incinerated at temperatures between 1,500 and 3,000°F (816 and 1,649°C), and the resulting ashes, if non-toxic, can be buried in a landfill. Composting is another method of sludge disposal. The biosolids can be mixed with wood chips to provide roughage and aeration during the decay process. The resulting material can be used as fertilizer in agriculture. Properly diluted, sludge can also be disposed of through land application. Purely municipal sludge, without chemicals or heavy metals, makes a great spray-on fertilizer for non-food plants. It is used in forestry, and on such commercial crops as cotton. It must be monitored carefully, though, so that it does not contaminate ground water.
Biomanagement of effluent
Though the biological treatment methods described here can be applied to any raw sewage, for the reasons described earlier, they are generally only used to process effluent that has already had the bulk of the solid material removed. As such, it is mostly water, though still containing unacceptable levels of pathogens and oxygen-consuming organisms.
An early method of biological treatment used natural soil. Sewage was allowed to percolate down through the soil where it was processed by aerobic organisms. Such treatment methods were not practical for the large volumes of sewage produced by towns of any appreciable size. If a significant amount of solids accumulated, the reaction would become anaerobic, with the attendant disadvantages of odor and slow decay.
Contact gravel beds are an improved form of the natural soil method. This type of processing is usually performed in batch mode. Gravel beds several feet deep enclosed in tanks are charged with effluent. Voids in the gravel guarantee aeration, and the aerobic decay process proceeds more rapidly than the natural soil method. After a batch is processed, the beds can be left empty so that the gravel can re-aerate.
A more efficient version of contact gravel beds are percolating or trickling filters, still in common use. Effluent is trickled over gravel beds continuously, and the voids between the gravel provide aeration. The beds rapidly become charged with a slime layer containing complex ecosystem made up of bacteria, viruses, protozoa, fungi, algae, nematodes, and insects. The various life forms in this biological mat maintain a balance, some feeding on the effluent, some feeding on one another, keeping the filter from becoming clogged. The new grown solid material can be flushed out with the purified water, then removed in settling tanks called humus tanks.
Scientists studying biological treatment methods at the beginning of the twentieth century discovered that if sewage is left in a tank and aerated, with the liquid periodically removed and replaced with fresh sewage, the sludge that settles in the tank will develop into a potent microorganism stew. This material, known as activated sludge, can oxidize organic sewage far more rapidly than the organisms in trickling filters or contact gravel beds.
In activated sludge processing systems, the effluent is introduced at one end of a large tank containing activated sludge and is processed as it travels down to the outflow pipe at the far end. The mixture is agitated to keep the sludge in suspension and ensure adequate aeration. Air can be bubbled through the tank to introduce additional oxygen if necessary. After outflow, the processed liquid is held in sedimentation tanks until the sludge settles out. The now purified water is then released to a river or other body of water and the settled sludge is removed and returned to the main processing tank. Over time, the activated sludge accumulates, and must be treated in the same way as the biosolids discussed earlier.
Algal ponds are a variation on the activated sludge method. Algae on the surface of a pond of effluent aerate the liquid by photosynthesis. The bulk of the processing is still performed by bacteria.
Some wastes contain too high a level of toxic materials to be processed using biological methods. Even small amounts of toxic chemicals can kill off activated sludge or other biological systems, causing the municipality to restart the culture while the sewage waits to be processed. If wastes are too wet to incinerate, wet air oxidation can be used in which oxygen and hot effluent are mixed in a reactor. Another process for dealing with toxic waste is vitrification, in which the material is essentially melted into glass by a pair of electrodes. The material is inert and immobilized, and can be buried with a higher degree of safety than in its previous state.
Urban storm water runoff
Storm water is another issue in sewage treatment. During rainstorms, the water washing down the buildings, streets, and sidewalks is collected into the sewers. The sewage treatment plant can process a portion of the storm water, but once the plant reaches overflow, the water is often released directly into the environment. Most systems are not designed to process more than a small percentage of the overflow from major storms.
Stormwater overflow is a major source of pollution for urban rivers and streams. It has a high percentage of heavy metals (cadmium, lead, nickel, zinc) and toxic organic pollutants, all of which constitute a health and environmental hazard. It can also contain grease, oil, and other automotive product pollution from street runoff, as well as trash, salt, sand, and dirt. Large amounts of runoff can flush so-called dry weather deposition from sewer systems, causing overflow to contain the same types of pathogens as raw sewage. The runoff is oxygen-demanding, meaning that if routed directly into rivers, streams, and oceans, it will rob the water of the oxygen needed to support life.
Historically, storm water runoff has not been considered part of the sewage treatment plan. Most municipal sewage treatment facilities have only minimal space for storing runoff, after which it is routed directly into receiving waterways. Government and engineers are studying various ways of lessening the problem, including construction of catch basins to hold runoff, flushing sewers regularly to reduce dry-weather deposition of sewage, implementing sewer flow control systems, and a number of strategies to reduce deposition of litter and chemicals on city streets. Economical methods of creating storage tanks and performing preliminary and secondary treatment of the runoff water are being developed. According to some estimates, it could cost the United States as much as $300 billion for combined sewer overflow and urban storm water runoff control. It is left to be seen how much more the environmental effects of uncontrolled runoff will cost.
Septic tanks
Not all homes and businesses are connected to municipal sewage systems. Some are too remote, or in towns too small for sewage systems and treatment plants. In such cases, septic systems must be used.
A septic system consists of a septic tank, a drain field or leach field, and associated piping. Gray water from washing and black water from household toilets runs through watertight sewage pipe to the septic tank. Anaerobic decay takes place in the septic tank, primarily in a layer of floating scum on top of the sewage. An outlet pipe leads to the drain field. The sewage undergoes final processing in the drain field, including filtration and aerobic decay.
When sewage reaches the septic tank, solids settle out of it. Anaerobic bacteria, yeast, fungi, and actino-mycetes break down the biosolids, producing methane and hydrogen sulfide. Fine solids, grease and oils form a layer of scum on the surface of the liquid, insulating the anaerobic community from any air in the tank.
There are numerous septic tank designs. The primary requirements are that the tanks be watertight; and that they have inspection/cleaning ports and are large enough to contain three to five days worth of sewage from the household. This ensures that the anaerobic creatures are able to process the sewage prior to its release to the drain field, and that the tank does not fill up and/or overflow—a rather revolting prospect. This outflow pipe is normally at a lower level than the inflow pipe and at the far end of the tank from the inflow pipe, to ensure that only processed sewage is released. Many septic tank designs include baffles or multiple chambers to force the black water through maximum processing prior to release to drain field.
Aerobic decay of the sewage takes place in the drain field. The outflow from the septic tank, called effluent, still contains pathogens. Effluent travels through a network of pipes set in gravel several feet below ground. The sections of pipe are slightly separated at the joints, allowing the liquid to seep out. The soil and gravel of the drain field filter the effluent and expose it aerobic bacteria, fungi, and protozoa that feed on the organic material, converting it to soluble nutrients. The liquid eventually either percolates down to the water table or returns to the surface via evaporation or transpiration by plants.
Roughly 4 ft (1.2 m) of soil are needed to process effluent, although authorities differ on the exact number, which varies with the makeup of the drain field soil. In other words, effluent passed through a couple yards of soil is pure enough to drink. To ensure a significant margin of safety, a drain field must be from 50 to 400 ft (15.2 to 121.9 m) from the nearest water supply, depending on the soil and the number of people served by the aquifer.
Some areas use incineration for the disposal of sludge. Earlier incinerators proved expensive to operate and for this reason many of the plants were abandoned. Many grass-roots organizations also disapprove of incinerators because of health reasons. Incinerators release carcinogenic (cancer-causing) and toxic chemicals from their smoke stacks, including heavy metals (such as arsenic, lead, cadmium, mercury, chromium and beryllium); acid gases, including hydrogen fluoride; partially-burned organic material such as polyvinyl chloride (PVC), herbicide residues and wood preservatives; other organic chemicals, including polycyclic aromatic hydrocarbons (PAHs); and dioxins and furans. One analysis identified 192 volatile organic compounds being emitted by a solid waste incinerator. A new form of incinerator has been developed since that study, which is based on the use of a fluidized bed that is proving more successful.
Under the Clean Water Act (CWA), sewage treatment plants and factories must obtain pollution permits, or legally binding agreements, which limit the volumes and types of pollution discharged into U.S. lakes and rivers. These permits form the basis of virtually all water-pollution tracking and reduction, as well as enforcement of water pollution laws. They must be renewed at least every five years and, with each new permit, the amount of polluted discharge allowed is to be lowered toward the eventual goal of
KEY TERMS
Active sludge —Sewage that has been aerated in a tank and has developed powerful organic oxidation capabilities.
Aerobic —Requiring or in the presence of oxygen.
Algal ponds —A variation on the active sludge method in which aeration is performed by algae photosynthesis.
Anaerobic —Describes biological processes that take place in the absence of oxygen.
Biosolids —Feces.
Black water —Sewage that contains biosolids, e.g. water from the toilet.
Drain field —Underground layer of soil and gravel where aerobic decay of septic tank effluent takes place.
Effluent —Liquid that flows from a septic tank or sedimentation tank; pathogens—bacteria and viruses capable of causing disease.
Grey water —Sewage that does not contain biosolids, e.g. water from the kitchen sink or the shower.
Leach field —Drain field.
Percolating filters —A sewage treatment system in which effluent is trickled over gravel beds and efficiently purified by bacteria.
Septic tank —Tank in which anaerobic decay of sewage takes place.
Sewerage —Piping and collection system for sewage.
zero pollution. At the beginning of 2000, The Friends of the Earth (FOE) and Environmental Working Group conducted a review of the publicly available water-pollution records from the 50 states and the District of Columbia. The FOE rated states on a pass-fail basis. The grade assigned to each state was based on the percentage of expired permits as of the start of this year. States with more than 10% of their permits expired were failed based on a 10% maximum permit backlog set by the United States Environmental Protection Agency (EPA). They found that 44 states and the District of Columbia failed their criterion. Since that time, other reviews have found similar results. So, by the mid 2000s, the majority of states still fail with their water-pollution permits.
The average person produces roughly 60 gal (227 l) of sewage daily, including both black and grey water. Municipal treatment plants and septic systems use mechanical and biological treatment methods to process out most of the pathogens and oxygen-consuming organisms. Toxic wastes are more difficult to remove, and are present in significant volumes in largely untreated storm water runoff. In particular, industrial effluent presents environmental and health risks. It falls to individual citizens to be responsible in useage and disposal of these substances, which eventually find their way back into the environment.
There are a number of public and private sector sewage treatment facilities in the United States. According to 2004 data from the U.S. Census Bureau, the United States has 5,934 sewage treatment facilities in the country. The industry consists of businesses primarily engaged in operating sewage systems or sewage treatment facilities, which collect, treat, and dispose of wastes (sewage).
See also Poisons and toxins; Waste management.
Resources
BOOKS
Bitton, Gabriel. Wastewater Microbiology. Hoboken, NJ: Wiley-Liss and John Wiley & Sons, 2005.
Duncan, Mara, and Nigel J. Horan, eds. Handbook of Water and Wastewater. San Diego, CA, and London, UK: Academic, 2003.
Franson, Mary Ann H. ed. Standard Methods for the Examination of Water and Wastewater. Washington, DC: American Public Health Association, 2005.
Hvitved-Jacobsen, Thorkild. Sewer Processes: Microbial and Chemical Process Engineering of Sewer Networks. Boca Raton, FL: CRC Press, 2002.
Report to the Chairman, Committee on Environment and Public Works, U.S. Senate. Securing Wastewater Facilities: Utilities have Made Important Upgrades but Further Improvements to Key System Components May be Limited by Costs and Other Constraints. U.S. Government Accountability Office, 2006.
Kristin Lewotsky