Wastewater Collection Systems
The first sewers for the collection of wastewater were storm sewers that were installed to collect rainwater in the commercial centers of large cities. In modern times London was the first major city to use sewers. London's example was then followed by Paris, New York, and Boston. Household wastes were collected and hauled in wagons for disposal outside the city.
As indoor water supplies became common, the storm sewers were used to carry off domestic wastes as well as storm water. These combined sewer systems succeeded in removing wastes from the immediate vicinity of the households, but by discharging them into the nearest watercourse, created serious problems of water pollution, particularly during dry periods when the water level in the streams was low.
When wastewater treatment was introduced in the middle of the 19th century, separate sanitary sewer systems for domestic wastewaters were installed. A sanitary system collects the wastewaters from household, commercial, industrial, and institutional establishments and conducts them through a sanitary sewerage, or wastewater, collection system to a treab11ent site.
The storm sewers required for handling surface runoff empty into the nearest watercourse. Collection systems for domestic and industrial wastewaters involve part of all of the following elements: plumbing systems, connections from plumbing systems into the collection system, sewers, manholes, and pumping stations. Because the materials in domestic and industrial wastewaters may be corrosive, the sewers are usually made out of vitrified clay tile, cement-asbestos mixtures, centrifugally cast concrete, or rigid plastic materials. Sanitary sewers usually range in size from a minimum of S inches (200 mm) to 6 to 8 feet (1.5 - 2.4 meters) in diameter.
Storm water collection systems collect surface runoff through inlets in gutters in city streets, through inlets in large paved areas, or through collectors from building roofs. Storm sewers range in size from 15 inches (350 mm) in diameter to conduits 10 feet (3 meters) or more across. Small storm sewers may be made of precast concrete or corrugated steel, and large sewers are often of concrete constructed in place.
When a city, like New Orleans is below the level of the waters surrounding it, all the rain that falls on the city is collected in storm sewers and is pumped out of the city.
Usually water is moved through sewers by gravity, and the sewers must have enough slope to prevent the sedimentation of wastewater solids.
Slopes may vary from 6 inches to 15 feet (150 mm - 4.5 meters) per mile (1.6 km). To avoid the excessive excavation that would result from maintaining adequate sewer grades in cities built on flat terrain, pumping stations may be installed at points throughout the system. Where combined sanitary and storm sewers are used, it is not feasible to conduct the large volumes of mixed wastewaters to a treatment plant so that special storm overflows leading directly into the receiving water are necessary.
These overflows discharge substantial amounts of raw wastewater to the receiving waters during rainstorms, causing considerable pollution of the water. For this reason, combined storm and sanitary systems are no longer being built and attempts are being made to separate combined systems where they do exist.
Development of Wastewater Treatment
Human body wastes are naturally decomposed and stabilized by microorganisms, either in the ground or in the topsoil or receiving waters. In the middle of the 19th century, when rivers flowing past the major urban industrial centers in Europe and the United States became heavily polluted, the first wastewater treatment on a community-wide scale was initiated. This treatment, which utilized natural decomposition, consisted of using the wastewater on agricultural lands, and it is still practiced in some parts of the world. The acreage necessary for such disposal is great, usually from 20 to 200 acres of land per thousand population.
With the growth of densely populated urban areas, however, the space required for natural decomposition became scarce and special processes for treatment were developed.
Modern wastewater treatment is generally divided into three phases: primary, secondary, and tertiary. Each of these steps produces sludge, which can be disposed of or used for various purposes.
Primary treatment, or plain sedimentation, developed in the early 1900's, removes only the settleable solids from wastewaters.
This process is considered to be the absolute minimum of treatment that every community must afford. By 1970, about 9% of the sewered population in the United States was still without any treatment at all while about 33% of the sewered population was provided with only primary treatment.
A modern system for primary treatment entails collecting the wastewaters, conveying them to a central point for treatment, using screens to remove large objects and grit chambers to remove grit, and using primary sedimentation tanks to remove the suspended settleable solids. This type of system produces about one third of a gallon (1.3 liters) of wet sludge per person per day, and facilities for handling and disposing the sludge are also needed. Primary treatment reduces the concentration of suspended solids by about 60% and reduces the BOD by about 35%, but removes very little of the other constituents of the wastewater.
Secondary treatment involves the addition of a biological treatment phase following plain sedimentation. In the United States, about 58% of the sewered population is served by some form of secondary wastewater treatment. At best, this treatment removes about 85% to 95% of the organic matter in wastewater.
It has little effect on dissolved materials or on the nutrients that stimulate the growth of algae in the receiving waters. Thus, efficient secondary treatment for a community of one million persons still permits the discharge of untreated wastewater equivalent to that of a community of 100,000 persons. It also discharges all the community's nutrients and dissolved solids, as well as any contaminants that may be added to the water by industrial plants in the community.
There are two basic methods used in modern secondary treatment: the trickling filter and the activated-sludge process. In small communities, secondary treatment is usually accomplished by the trickling-filter method, which evolved from two earlier methods: the sand filter and the contact bed. In larger communities it is generally accomplished by the activated-sludge process.
Early sand filters were beds of fine sand, usually 3 feet (1 meter) deep, through which the wastewater slowly seeped. As it seeped through the sand, the organic matter was decomposed and stabilized by the microorganisms in the wastewater. Sand filters required about 4 acres (1.6 hectares) of sand beds for each thousand people. Because of this large space requirements, sand beds are seldom used.
The contact bed, consisting of many layers of stone, slate, or other inert material, provided a relatively large surface area for the growth of microorganisms. It operated on a fill-and-draw basis, and the organic matter delivered during the fill period was decomposed by the microorganisms on the bed. The oxygen required by the microorganisms was provided during the resting period, when the bed was exposed to the air.
The trickling filter came into use in the early 1900's. In the modern version of this system, the wastewater is applied to the filter through rotary distributors and it is allowed to trickle down over large stone or plastic beds that are covered with microorganisms. The beds are not submerged , thus air can reach the organisms at all times. The area requirements for trickling filters are about 5 to 50 acres (2-20 hectares) per million people.
In the second decade of the 20th century a more efficient method of biological treatment was developed, the activated sludge process. In this process, heavy concentrations of aerobic microorganisms, called biological floc or activated sludge, are suspended in the liquid by agitation that is provided by air bubbled into the tank or by mechanical aerators. Final sedimentation tanks are needed to separate the floc material from the flowing liquid. Most of the biologically active sludge is then returned to the aeration tank to treat the incoming water. The high concentration of active microorganisms that can be maintained in the aeration tank permits the size of the treatment plant to be relatively small, about 1 to 5 acres (0.4-2 hectare ) per million population.
Tertiary treatment is designed for use in areas where the degree of treatment must be more than 85% to 95% or where the wastewater, after treatment, is reused. It is primarily intended to further clean, or polish, secondary treatment plant effluents by removing additional suspended material and lowering the BOD, generally by filtration. This polishing, however, has little impact on the dissolved solids, including the nutrients, synthetic organic chemicals, and heavy metals. To eliminate these constituents of wastewater, other methods of treatment have been devised. These processes include coagulation and sedimentation, precipitation, adsorption on activated carbon or other adsorbents, foam separation, electrodialysis, reverse osmosis, ion exchange, and distillation.
The accumulated solid materials, or sludge, from wastewater treatment processes amount to 50 to 70 pounds (22-31 kg) per person per year in the dry state or about one ton (0.9 metric ton) per year in the wet state. A city of one million persons produces about 35,000 tons (31,500 metric tons) of dry sludge per year.
Sludge is highly capable of becoming putrid and can itself be a major pollutant if it is not biologically stabilized and disposed of in a suitable manner. Biological stabilization may be accomplished by aerobic or anaerobic digestion.
In aerobic digestion, the solids are decomposed over long periods of time in the presence of aerobic microorganisms. In anaerobic digestion, which is much more common, the solids are placed in an airless tank containing anaerobic organisms. Methane, carbon dioxide, and water are produced as a result of anaerobic digestion.
The methane is often recovered for fuel to heat the tank (and thus increase the rate of anaerobic digestion) or to produce power. Where the gas is used to drive engines for power, the cooling water of the engine is used for heating the tank, increasing the efficiency of the process. Digestion of sludge, whether anaerobic or aerobic, reduces the volume of sludge and renders it nonputrescible.
Digested sludge, whether in its wet form or after being dried on open beds, is a useful soil builder but has little value as a fertilizer. Wet digested sludge is often used for the reclamation of arid and barren soils in parks, along highways, and similar public places. However, caution must be exercised when these sludges are used for edible crops because disease-causing organisms may survive the processing. Only heat drying at high temperatures assures that the sludges are free of these organisms.
In modern wastewater treatment plants, mechanical dewatering of sludge by vacuum filters, centrifuges, or other devices is becoming widespread.
The dewatered sludge may then be heat dried if it is to be reclaimed or it may be incinerated. In large communities where large amounts of sludge are produced, mechanical dewatering and incineration are commonly practiced.
The reclamation of sludges for their organic value was actively pursued at one time. A treatment plant built in Milwaukee, in the 1920's markets heat-dried sludge, sold under the name Milorganite, as fertilizer. With the development of inexpensive, easily applied chemical fertilizers, the market for sludge dwindled, and no new treatment plant can expect to sell all of its sludge.
The unintentional reuse of wastewaters occurs often because wastewaters are generally discharged into streams and lakes that are used as sources of water supply. In the United Statcs, about 40% of the population is served by waters containing municipal or industrial discharges.
Sometimes wastewaters are intentionally used for replenishing groundwaters, for industrial processes or cooling, for the creation of recreational lakes and other facilities, or for irrigation. Direct reuse of wastewaters for drinking purposes is not practiced. In areas where the water supply for drinking purposes is limited, wastewaters may be reclaimed for secondary uses, such as lawn nitrogen removal
To the and park irrigation, industrial processes, toilet flushing, clothes washing, and fire fighting.
By far the greatest potential for wastewater reclamation is for irrigation, particularly in arid regions. For irrigation, quality requirements are minimal, and wastewaters may be used after primary or secondary treatment, though not to irrigate crops that are to be eaten raw. Irrigation with wastewaters is thus appropriate for such crops as cereals, hay, trees, and cotton. Well treated wastewater effluents from a community of about 1,000 persons can irrigate 10 to 25 acres (4-10 hectares) of land.
Wastewater treatment plant effluents are extremely useful for industry, particularly as cooling water. The required quality of the wastewater depends on the industry and the particular use. Industrial plants often install their own treatment facilities to improve and regulate the quality and quantity of the wastewater they use.
Recreational usage of wastewater effluents is widely practiced, especially in arid regions. Wastewater effluents have been used for many man-made lakes, including those in Golden Gate Park in San Francisco, California, and the lakes for swimming, boating, and fishing in Santee, California there the lakes and ponds are to be used for bathing or water sports, the wastewater has to be disinfected.
The use of wastewater treatment plant effluents for groundwater recharge is increasingly popular as an ultimate method of disposal and for the conservation of water resources. However, the quality requirements for recharge are exceedingly high, and some form of tertiary treatment may be required. The recharge of groundwaters may be accomplished by spreading the wastewaters in large basins where they may percolate into the ground. In areas where the aquifer (the water-bearing bed) is well below the surface of the ground, the wastewaters may be injected into deep wells. In places where recharge is important in protecting or developing groundwater resources, the capture of storm water for recharge is becoming increasingly popular.