ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - MUNICIPAL WASTEWATER potx

727 MUNICIPAL WASTEWATER Sewage is the spent water supply of a community. Because of infiltration of groundwater into loose sewer pipe joints, the quantity of groundwater is frequently greater than the quan- tity of water initially consumed. Sewage is about 99.95% water and 0.05% waste material. A weaker (more dilute) sewage results from greater per capital water consumption. Industrial wastes will contribute to sewage strength. Sewage flow varies with time of day and will be higher during daylight hours. Larger cities will exhibit less variation with time than smaller cities. Many small communities will have a flow in the late night hours that is due almost entirely to infiltration. Per capita production will vary from less than 100 gal- lons per day for a strictly residential community to about 300 gallons per day for a highly industrialized area. The concept of “population equivalent” is frequently applied in evaluating industrial waste contributions to sewage flow and planning for hydraulic, solids, and biochemical oxygen demand loadings. Wastewater treatment facilities have high initial capital costs due to the need for large tanks, equipment and land area. The designed life of a treatment plant is normally equal to the life of the bonded indebtedness of the project. It is expected that capacity will be just reached at the end of this period. In a community with combined sanitary and storm sewers it is often found necessary to bypass waste- water flows during periods of heavy rain or, in low lying areas, during high tides. Excess water may be held in deten- tion basins until normal flow resumes and then treated in the plant. Bypassed flow can be screened and chlorinated before discharge. This subject is receiving increasingly close attention. Strength of sewage is usually expressed in terms of the following parameters: total solids (sometimes called volatile suspended solids, VSS), filterable solids, and biochemical oxygen demand (BOD). Many methods are available for determination of the foregoing. However, in order that oper- ating and research data reported by workers in widely sepa- rated areas be comparable, it is necessary to have analytical methods which are simple, give consistent results and are widely accepted. This need is filled by Standard Methods for the Examination of Water and Wastewater. Contained in each edition are methods for analysis of water and waste- water which have been accepted by committees of experts in various fields. Standard Methods is revised about every five years. Analysis of a typical American sewage is given below: Total solids 600 mg/l a) suspended solids 200 mg/l 1) settleable solids 120 mg/l 2) colloidal solids 80 mg/l organic 60 mg/l mineral 20 mg/l b) Filterable solids 400 mg/l 1) colloidal solids 40 mg/l organic 30 mg/l mineral 10 mg/l 2) dissolved solids 360 mg/l organic 100 mg/l mineral 260 mg/l c) BOD (5 days 20°C) 1) suspended 42 gms/cap. × day settleable 19 gms/cap. × day non-settleable 23 gms/cap. × day 2) dissolved 12 gms/cap. × day Total 54 gms/cap. × day Assuming an average daily flow of 100 gallons per capita, 54 gms/cap. × day = 0.12# BOD/cap. × day. This is a strictly domestic sewage. Per capita BOD values can vary from about 0.10#/day to above 0.25#/day. A commonly accepted value used for estimation is 0.17#/cap. × day. In the above analysis, the determination of solids, min- eral and organic, gives an indication of the loadings to be placed on the plant. Volatile solids give the organic loading and mineral solids are those which must be accommodated by sedimentation equipment. BOD is a measure of the loadings placed on the oxygen resources of the receiving waters. Several methods have been used over the years for determination of the amount of oxygen necessary for stabilization of the waste. Permanganate and other oxidizing agents were formerly used. One method still finding application, but only as a general indication, is the putrescibility, or methylene blue, test. Methylene blue, a dye, decolorizes in the absence of dissolved oxygen. This test is only quasiquantitative, but finds use in day to day operation control of wastewater treatment plants. The method used today is an incubation and dilution method. Dissolved oxygen is determined at the beginning and end of a 5 day period by the Alsteberg azide modification of the Winkler method. C013_007_r03.indd 727C013_007_r03.indd 727 11/18/2005 10:43:39 AM11/18/2005 10:43:39 AM © 2006 by Taylor & Francis Group, LLC 728 MUNICIPAL WASTEWATER Water can hold about 10 mg/l dissolved oxygen. Since BOD values can vary from less than 1 mg/l for a clean stream to many thousands of milligrams per liter for some industrial wastes, it is necessary to dilute stronger wastes. The dissolved oxygen concentration of a seeded nutrient dilution water is determined. A measured quantity of waste is then added to a known volume of dilution water. Total sample volume is usually about three hundred milliliters. The mixture is then incubated at 20°C for five days. At the end of this period the dissolved oxygen concentration of the mixture is again measured. BOD is then equal to the reduc- tion in dissolved oxygen divided by the dilution factor. Figure 1 shows the exertion of BOD over an extended period (more than 50 days). The trajectory displayed is one which might be followed when waste is added to a rela- tively clean stream and the watercourse acts as an incuba- tion bottle. Such a complex reaction is of little utility for day to day control. In order to make the concept of BOD more useful, a simplified model is developed. The path followed in the idealized model is displayed in Figure 2. The lag phase is eliminated by use of seeded dilution water. In this phase, microorganisms which utilize the waste material for food are becoming acclimatized and assuming dominance in the system. In the carbonaceous phase the dominant organ- isms are feeding upon the more easily degraded compounds. The nitrogenous phase, in which the character of the food and the microbial population change, does not usually begin for at least ten days. This portion is of interest in research on kinet- ics but can be ignored in the following. It is hypothesized that a simple monomolecular rate constant can describe the car- bonaceous phase and the reaction is asymptotic to a limiting value L at infinite time. L is known as the ultimate BOD and is a mathematical artifact. The rate of reaction is assumed pro- portional to the BOD still remaining, d d 1 y k t Lyϭ Ј Ϫ(). Integration and taking of proper limits gives yL e L kt kt ϭϪ ϭϪ ϪϪ ЈЈ ()( ).1110 11 Phelps’ relation for decolorization of methylene blue with time y L t ϭϪ1 0.794 is equivalent to the model equation when k 1 equals 0.10/day. In practice, the 5 day 20°C BOD is used to describe the strength of a waste. Simple enumeration of the BOD value tells nothing about the path by which the number was reached since L and k 1 are not specified. Sewers and treat- ment plants are not isothermal entities but it is necessary that a common basis be established. One of the reasons advanced for the choice of the five day period is the fact that almost all rivers in England are within five days flow of the sea. Waste once discharged to the infinite ocean is no longer of interest. Times have changed and there is great concerns for effects of waste on the “infinite ocean.” In the literature reference to BOD means 5 day 20°C BOD (BOD 5 ) unless specifically stated otherwise. Recently, some popular writers have erroneously used the term “ biological oxygen demand.” There is no such parameter. The reaction rate constant k 1 actually describes a series of complex microbiological reactions. In the initial stages of biological stabilization of sewage, carbonaceous material is oxidized to carbon dioxide and nitrogenous material is oxi- dized first to nitrite and then nitrate. One of the measures of degree of stabilization of organic matter in an effluent is the nitrate concentration. However, recent theories concerning eutrophication have raised serious questions concerning the desirability of high effluent nitrate concentrations. time lag phase carbonaceous phase B.O.D mg/l nitrogenous phase FIGURE 1 B.O.D. mg/l L y L – y y time FIGURE 2 C013_007_r03.indd 728C013_007_r03.indd 728 11/18/2005 10:43:39 AM11/18/2005 10:43:39 AM © 2006 by Taylor & Francis Group, LLC MUNICIPAL WASTEWATER 729 Sewage contains the waste of a city and its inhabitants. It is possible to find almost any microorganism in sewage. In fact, in the sewer system can be found quite unexpected creatures. About 1960, sewer workers in New York City found a number of live and exceedingly unfriendly alligators in that city’s sewers. These animals, gifts to city residents, had been disposed of through household toilets. The warm, wet environment of the sewers, rich in food, was excellent for rapid growth. Traditionally, public health practice applied to water san- itation has placed major emphasis on prevention of typhoid fever, the causative organism of which is S. typhosa. This disease is spread by the pathway of anus to mouth. S. typhosa is a fairly delicate organism and is usually not found in high concentrations. It is felt that if pollution arising from human intestinal discharges is removed to a high degree, then S. typhosa will also be removed. As BOD is used as an indi- cator of pollutional loading on oxygen resources, so is the indicator organism E. coli used as an indication of pollu- tional loading due to human intestinal waste. E. coli is a normal inhabitant of the human intestinal tract. It is excreted in huge numbers and the presence of this organism in receiv- ing waters may be evaluated quantitatively. The lactose broth-brilliant green bile test is commonly used to determine the presence of E. coli. A common soil organism, A. aero- genes, gives a false positive test and further confirmatory tests are necessary. Serial dilutions of the water being tested are prepared and, on the assumption that one organism is responsible for a positive test, the Most Probable Number (MPN) of organisms is determined. The MPN is based on statistical reasoning. Work by Kupchick, using the enzyme urease to determine the presence of uric acid, has shown a high degree of correlation between the concentration of this acid in wastewater and the MPN. Most pathogenic organisms are not hardy and do not com- pete well for food. The use of E. coli as an indicator organism is in the way of a margin of safety. This is consistent with Phelps’ concept of multiple barriers. Microorganisms form the basis of secondary, or biologi- cal, wastewater treatment. Stated briefly, microorganisms establish themselves on trickling filter slime or in activated sludge liquor and feed on waste material in the sewage. Large particles are removed in the primary settling por- tion of treatment. The larger particles, grease, etc. are in some ways not as objectionable as the colloidal or truly dissolved materials. The larger particles are, of course, objectionable from an aesthetic point of view, but the smaller particles place more of an immediate load upon the receiving waters. In almost all areas of environmental engineering surface area is one of the controlling parameters. Microorganisms, in carrying out their vital processes, utilize the waste as food and the smaller particles offer greater surface area per unit volume. Microbial activity is correspondingly greater and the oxygen required is also greater. Microorganisms require a readily available source of oxygen. Aerobic conditions are said to exist when the oxygen is in the form of dissolved molecular oxygen or nitrates. At the point of exhaustion of nitrates and nitrites the system is in the anaerobic state and the oxygen sources are then sulfates, phosphates, borates, etc. Reduction of sulfates to give mercaptans (HS − ) carriers with it charac- teristic vile odors. Secondary treatment is an essentially aerobic process while conventional sludge digestion is an anaerobic process. In the trickling filter an activated sludge processes microorganisms extract their food from the flowing waste- water. By means of extracellular enzymes large molecules are broken down so that they may pass through the microbial cell wall by diffusion. The food is further broken down for cell synthesis and energy by means of intracellular enzymes. End products are largely carbon dioxide and water. The waste material, now part of the cell mass, is removed in the final stage of treatment. The primary reason for treating any waste is the need for avoiding nuisance and dangerous conditions in the environ- ment. It is necessary to remove some of the waste so that the remaining can be discharged to the surroundings. This is, in effect, disposal by dilution. Discharge standards are usually based on concentration and total quantity. Sewage purification works were formerly constructed for reasons based primarily on public health. More modern thinking has expanded the original rationale to include pro- tection of oxygen resources of the receiving waters. If the second criterion is satisfied, the first will almost always be also satisfied. Demand for clean waters is increasing even more rapidly than population. Water will be used many times in its passage to the sea. Lakes are essentially a closed system. Leisure time is increasing and the question of water pollution has reached the point where aesthetics is now a significant parameter in planning. It is no longer possible to treat a watercourse as a separate entity. The approach now finding wider and wider application is that of basin manage- ment. This is the systems approach. An excellent example of this is the Ohio River Sanitation Commission. Here it has been possible to rise above local jealousies and self inter- est. The results are most gratifying and should serve as an example to other areas. It is unfortunate, but true, that one heavy pollution source can undo the efforts of many groups with social responsibility and foresight. Because objectives in waste management can change drastically at political boundaries, it has occasionally been necessary for intervention by representatives of larger politi- cal entities when pollution problems effect, for example, several states. Federal agencies in the Unites States and other countries have come to play an increasingly larger role in waste management, particularly when problems do not respect political boundaries. The discussion presented here involves primarily technology of wastewater treatment and the underlying philosophy will not be extensively covered. It has been mentioned that larger particles in wastewater can be removed by physical means. Removal of colloidal and dissolved components requires other methods and this must be accomplished economically. Treatment is classified as primary if it is without biological basis. Secondary treat- ment is generally accepted as biological treatment. A few regulatory authorities have endeavored to classify treatment C013_007_r03.indd 729C013_007_r03.indd 729 11/18/2005 10:43:40 AM11/18/2005 10:43:40 AM © 2006 by Taylor & Francis Group, LLC 730 MUNICIPAL WASTEWATER degree on the basis of degree of removal. This relegates some biological treatment to primary status. Many experts do not agree with this change in definition. A widely accepted defi- nition of tertiary treatment is the use of any process, in addi- tion to conventional secondary treatment, for the purpose of further removals. From the foregoing it should be noted that chemical treatment, popular in the past, is a primary treat- ment process. Water carriage of waste was practised in the Minoan civilization of Crete. It may be said that sanitation practices reflect the level of a civilization. Certainly, sanitary practices of the Middle Ages were of a rather low order. Methodical control of waterborne diseases was not attained until the 19th century. It is of historical interest to note that knowl- edge of the use of creosote for odor control at the Carlisle, England sewage purification works gave inspiration to Joseph Lister for the birth of asepsis in surgery. The process by which wastewater is purified can best be understood by following the waste from its source through the collection system and treatment plant. Organic material discharged to a watercourse will eventually undergo stabi- lization. This is accomplished by natural processes (unit operations) and a wastewater treatment plant basically sets up, under controlled conditions, the processes which act in the river. Indeed, it has often been said that a sewage treat- ment plant is a river in miniature. In the river heavy particles settle out and lighter particles float to the surface. Biological decomposition takes place. Oxygen present is used by organ- isms that accomplish decomposition. Some of the settled material will be resuspended, increasing the organic load- ing. While oxygen is being withdrawn by BOD this resource is being replenished at a rate proportional to the deficit. The oxygen deficit is the difference between the amount that can be held at saturation (about 10 mg/l) and the amount actually present. Stated mathematically, the oxygen concentration in a river as a function of time is d d D t ky kDϭ Ј Ϫ Ј 12 . This expression considers only the effects of BOD and atmo- spheric reaeration. A more complete equation can be writ- ten but effects of oxygen production by algae and oxygen reduction by benthal (bottom) deposits are not of major significance here. Solving the above gives D kL kk D t kt kt kt ϭϪϩϫ ϪϪ Ϫ Ј 1 1 0 10 11 2 − {} 10 10 . This expression is commonly known as the oxygen sag equation and is displayed in Figure 3. It is of interest here because it illustrates the rationale underlying waste treat- ment requirements. Waste is treated so that undesirable conditions do not develop in the receiving waters. In effect, a limit has been placed on the allowable oxygen deficit. Regulatory authorities usually require that a minimum dis- solved oxygen level be maintained. Normally, this will be stated as a percentage of dissolved oxygen saturation. This is the largest permissible critical deficit. Game fish may require a minimum of 5 mg/l D.O. while scavengers can survive in a much lower quality water. The critical deficit is given by D kL k c kt c ϭ ϫ Ϫ 1 2 1 10 . Once the maximum deficit is specified, the BOD loading on the stream can be immediately estimated. The allowable BOD loading will be that impressed on the watercourse by the wastewater treatment plant effluent. It is of interest to observe the effect of the condition of the river at the point of discharge. A river in poor condition will have a large ini- tial deficit, D 0 . This can raise the treatment requirements. It can be seen that it is necessary to integrate the efforts on a basin wide basis. Parameters other than just dissolved oxygen must be controlled by the treatment processes. This is accom- plished in some, or all, of the following steps. Decomposition of the waste begins in the collection system. Ease, or difficulty, of treatment depends to a large degree on the condition of the sewage when it reaches the plant. Some substances are not permitted in the sewage system. Gasoline and other flammable substances, oil, hexavalent chromium are examples of prohibited substances. These can damage either the collection system or the treatment plant and processes. The legal vehicle by which such materials are excluded is known as a sewer ordinance. It is most economical to collect sewage by gravity flow. If topography does not permit, pumping will be necessary in order to cross high points and to avoid excessively low flow velocities and deposition of waste material in the pipes. (1) (2) D c reoxygenation D.O D critical deficit deoxygenation time (1)+(2) D.O. s at.’ D O D=O FIGURE 3 C013_007_r03.indd 730C013_007_r03.indd 730 11/18/2005 10:43:40 AM11/18/2005 10:43:40 AM © 2006 by Taylor & Francis Group, LLC MUNICIPAL WASTEWATER 731 A combined system will, at times, exceed the hydraulic capacity of the treatment plant and flow must be either bypasses or held in detention tanks until heavy flow has subsided. Multiple treatment units are provided and treatment is not interrupted during periods of maintenance or repairs. Protection must be provided for pumps against large objects, such as floating pieces of wood. Coarse racks, with clear openings of more than 2 inches, may be placed at the entrance to the plant. Racks to be placed in advance of grit chambers and settling tanks will have clear openings of 1 to 2 inches. In smaller plants racks are cleaned by hand while larger plants have mechanically cleaned racks. Disposal is by burial, incineration or digestion with sewage sludge. Mechanically cleaned racks have smaller clear openings because head losses are lower with continuous cleaning. Racks with clear openings of 1 to 2 inches can be expected to give from 20 to 100 ft 3 of solids per 1000 people annually. Comminutors macerate floating material into sizes sufficiently small to be easily handled by centrifugal pumps. Racks and screens with very small openings have been almost completely replaced by comminutors. Rate of flow into the plant will vary over a wide range during any 24 hour period with smaller plants exhibiting greater variation. Flow is measured by Parshall flume, Venturi meter or Sutro (keyhole) weir. The Parshall flume, sometimes called the open channel venturi, is most commonly used. The device operates on the principle of critical flow and mea- surement of water depth upstream of the flume throat. The governing equation is of the form Q = cWH 3/2 , where Q is the discharge per unit time, W is the throat width, H is the water depth, and c is a constant. While c changes with throat width, it is closely constant for a constant throat width. Flume liners of reinforced fiberglass are all but replacing steel and concrete liners. Ease of fabrication, close tolerance, and corrosion resistance are advantages cited. The true Venturi operates on closed pipe (pressure) flow and is usually found in larger plants. The Sutro, or keyhole, weir is shaped as its name suggests. Its principal advantage is maintenance of a constant upstream velocity over a wide flow range, but it does have a high energy loss and metering is lost when the opening is submerged. It is necessary to remove grit in order to protect pumps against excessive wear and to maintain capacity of sludge digesters. It has been found that digesters in plants serving low lying sandy areas can, if grit removal is not efficient, lose up to a third of capacity in but a few years. Grit cham- bers must operate in a fairly narrow velocity range of from 0.75 to 1.25 ft/sec. Above this range deposited material is scoured back into suspension and below the lower value organic material settles out. The resulting material, called detritus, is unsuitable for landfill uses due to its highly putrescible nature. Grit chambers are usually designed to remove particles with a specific gravity of 2.65 and a mean diameter of 0.02 cm. Because flow variation with depth fol- lows a parabolic function, Q = cWH · H 1/2 , the grit chamber is often given an approximately parabolic shape and better velocity control is attained. The amount of grit collected per million gallons flow is found to vary from 1 to 12 cubic feet. Grit is removed manually in small plants and continuously by mechanical means in larger plants. Settling tanks are provided for removal of larger, heavier organic particles, oil, and grease. Oil, grease, and other materials lighter than water are skimmed continuously from the surface and led to digestion. Both circular and rectan- gular surface configurations are used. Rectangular tanks of the flow through variety have length to width ratios of 4/1 to 6/1. Circular tank size is usually limited by structural requirements of trusses carrying skimming devices. Tank depths vary from 7 to 15 feet. Bottoms are sloped about 1% in rectangular tanks and about 8% in circular tanks to facili- tate sludge removal. Design is on the basis of hydraulic loading. A com- monly used figure is 1000 gal/day × ft 2 surface area. It can be expected that a BOD removal of 30% will be achieved in a well operated primary sedimentation unit. If treatment includes only screening, sedimentation, and chlorination of effluent, the treatment is classed as primary. Primary treatment, while inadequate for most areas, is better than no treatment. The adequacy of secondary treat- ment is now being seriously questioned. Nonetheless, it fits the economics of the situation. Only in the 1960’s did wastewater treatment become of interest to any but a small number of people. Sanitary engineers were wont to say “It may be sewage to you but it is bread and butter to me.” Theirs was not a profession to which much glamour was attached. Financing bodies were reluctant to invest adequate sums in waste treatment facilities. Hopefully, this has now changed. There are two main processes utilized for biological (secondary) treatment. These are (1) the trickling filter and (2) activated sludge. The trickling filter is not a true filter. It can best be described as a pile of stones, or other coarse material, over which sewage flows. This is the most widely used biological treatment process. Present day biological treatment technol- ogy is a logical development from sewage farms (irrigation areas) to intermittent sand filters to contact (fill and draw) beds to trickling filters and activated sludge units. Numerous modifications of the basic processes have evolved but the underlying principles remain unchanged. In biological treatment a suitable environment is provided so that micro- organisms may thrive under controlled conditions. The suit- able environment is one rich in food and maintained in the aerobic state. The zoogleal mass remains fixed on the filter media in the trickling filter while the sewage flows past. In the activated sludge process the sewage and organisms flow together. In both cases the microorganisms come from the sewage itself. Traditional secondary treatment plants operate in the declining growth phase. Irrigation by sewage provides water return and some waste stabilization but this means of sewage disposal is in conflict with sound public health practice and ought not be used where there is a possibility that sewage can pass with little change into the groundwater table. Irrigation is best applied in arid regions. When it is utilized for food growing areas, care must be taken so that edible plants and fruit are C013_007_r03.indd 731C013_007_r03.indd 731 11/18/2005 10:43:41 AM11/18/2005 10:43:41 AM © 2006 by Taylor & Francis Group, LLC 732 MUNICIPAL WASTEWATER not contaminated. Odors are a problem and removals decline markedly in cold weather. Intermittent sand filters are much like the slow sand fil- ters used for potable water production. The sewage is applied to a sandy area and allowed to percolate downward. Raw sewage may be applied at rates as high as 80,000 gal/acre × day and secondary effluents at rates as high as 800,000 gal/ acre × day. Application of the secondary effluent would be considered tertiary treatment. Biological films that form on the sand grains undergo continuous stabilization. It is neces- sary to rest the bed between dosings so that objectional con- ditions do not develop. Surface accumulations of solids must be periodically removed. This method is not recommended for areas underlain by fissured limestone. Fill and draw beds operate as the name indicates. A tank, packed with coarse material, is filled with sewage and allowed to stand full. It is then drained and allowed to rest. Air is drawn into the bed during filling and emptying. Loadings are about 200,000 gal/acre × ft × day. There is little application of this method today in treatment of municipal wastewater but it does find use in industrial waste treatment. Fill and draw beds are a batch operation and the trickling filter is a continuous operation. Sewage is distributed over a trickling filter by slowly revolving arms equipped with nozzles and deflectors. Some earlier plants had fixed nozzles but this is no longer done. Revolving arms are driven by hydraulic head. Sewage dis- charged is allowed to flow slowly downward through the bed. Air is down into the bed by temperature differential, thus maintaining a supply of oxygen for the process. Filter media is usually stone. Sizes are in the range of 1 to 4 inches. Packing of this size permits air to be drawn into the bed and the bed is not clogged by biological slime. There appears to be a trend toward more use of plastic filter media. Filter depths range from 3 to 14 feet. A common depth is 6 feet. After passage through the filter the sewage is collected in tile underdrains. These underdrains serve two purposes: (1) collection of filter effluent and (2) circulation of air into the filter. The underdrains discharge to a main collec- tion channel which, in turn, discharges to the final settling (humus) tank. The importance of the function of the final settling tank can be seen by an examination of what occurs in the filter itself. A new filter is “broken in” by applying sewage as in normal operation. After a time the microbial (zoogleal) mass establishes itself on the filter media and carries on the work of waste stabilization. Waste material in the flowing sewage (food) is first absorbed into the zoogleal mass and then assimilated by the microorganisms. Much of the organic waste material has, at this point, been utilized for cell syn- thesis and energy. There must be continuous removal of filter slime or the process becomes sluggish due to a lower feeding rate of old organisms. Since waste material is now a part of the filter slime there must be a means provided for removal of sloughed off organisms or the waste material, now in different form, would still appear in the plant efflu- ent and little constructive would have been accomplished. The required removal is carried out in the secondary settling tank. A rate of application lower than that of the primary tank is necessary here because of the different character of the material to be removed. Rates in this portion of the system are in the range of 600 gal/ft 2 × day. A portion of the effluent is recirculated, as shown in Figure 4. This is done in order to (1) smooth out flow, (2) keep the food concentration more constant, (3) lower the film thickness and, thus, control the psychoda fly, and (4) reseed the applied sewage with acclimatized organisms. The psychoda, or filter, fly is a very small insect which breeds in trickling filter slime. It does not bite but can be extremely bothersome because it does get into the nose and mouth. The range of flight is short but the creature can be carried great distances by the wind. Control of the fly in its developmental phase can be achieved by flooding the filter periodically or through chlorination of influent. Trickling filters can be classified on the basis of (1) hydrau- lic loading per unit area and (2) applied pounds of BOD per 1000 ft 3 of filter volume. The low rate trickling filter, with hydraulic loadings of 2 to 4 million gallons per acre per day (mgad) and 10 to 20 pounds BOD per 1000 ft 3 , is usually found in use in smaller plants. With proper operation, BOD removals of 80 to 85% can be routinely expected. Raising the applied sewage to 10 mgad produced greater BOD removals per unit filter volume but the effluent organic concentration was found to be high. Influent organic concentration was reduced by greater effluent recirculation and lower effluent organic concentration was realized. Units that operate in the 10 to 40 mgad range are called high rate trickling filters. BOD load- ings are up to 90 pounds per 1000 ft 3 , but removals to be expected are in the range of 65 to 75%. In the 4 to 10 mgad range operational difficulties were frequently encountered and this range was avoided for many years. It appears that, in this range, the hydraulic application was inadequate to keep the filter slime from attaining exces- sive thickness. Many plants had operated well in this range, but other plants had many problems. The solution seems to have been reached with use of relatively large, 2 to 4 inches, filter stones. Experimental plants using plastics media have recently achieved very high removal efficiencies (97%) at hydraulic loading rates of 100 mgad. Much of the microbial mass is in the recirculated effluent and these plants are, in effect, modifications of the activated sludge process. Organic influent Headworks Primary Sed. Biological Treatment Secondary Sed. out recirculation excess to digester to digester Primary Treatment CI 2 FIGURE 4 C013_007_r03.indd 732C013_007_r03.indd 732 11/18/2005 10:43:41 AM11/18/2005 10:43:41 AM © 2006 by Taylor & Francis Group, LLC MUNICIPAL WASTEWATER 733 loadings are in the region of 100 pounds BOD per 1000 ft 3 filter volume. Filter packing by plastic media is finding wider appli- cation. Design criteria, however, call for quite deep filters and this appears to be uneconomical in terms of power requirements. Modifications of the trickling filter process over the years have dealt with improvements in media, air circula- tion, and loadings. One of great interest is that proposed by Ingram. Sewage is introduced at various levels in a very deep filter in an attempt to distribute the load more uniformly over the whole filter depth. Hydraulic loadings up to 500 mgad have been successfully achieved. A new development in trickling filter technology is the Rotating Biological Contractor (RBC). A rotating drum is partially immersed in wastewater. A zoogleal mass devel- ops on the drum surface, functioning in the same manner as trickling filter slime. Such installations may be completely enclosed in plant buildings, thus avoiding any effects of extremes in outside temperature. An excellent example of the application of this process for upgrading the municipal treatment works at North Bergen, New Jersey. Activated sludge serves the same function as trickling filter slime. The major difference lies in the filter slime being fixed to the filter media while the activated sludge is carried along with the flowing wastewater. Development of the acti- vated sludge process began with attempts to purify sewage by blowing air into it. It was observed that after prolonged aera- tion, flocs composed of voraciously feeding organisms devel- oped. This floc settled after aeration was stopped. Addition of fresh sewage to tanks containing the settled sludge produced high purification in a practical time. The name activated sludge was assigned this means of waste treatment. At first, this was operated as a fill and draw system. Research showed that continuous operation could be practiced and this is the present means or operation. The process involves: 1) return of activated sludge to the aeration tank influ- ent and discharge of excess sludge to digestion 2) aeration of the sludge-sewage mixture to maintain purification and 3) settling of the aeration tank effluent to remove floc before final discharge. Step (3) is necessary for the same reason as the comparable portion of the trickling filter process-removal of waste mate- rial transferred to the microbial cell mass. Floc is formed in the tank through aerobic growth of unicellular and fila- mentous bacteria. Protozoa and other organisms will also be found in the floc. This is a strictly aerobic process and air requirements are high. Two aeration systems are used, (1) diffused air units and (2) mechanical aeration. Air dif- fusers are more commonly used in North America but mechanical aeration systems may be found in plants of less than 1 million gallons per day (mgd) capacity. Both methods of aeration perform three functions, (1) transfer of oxygen to the mixture and maintenance of aerobic conditions, (2) intimate mixing of floc and sewage, and (3) keeping the floc in suspension. Aeration tanks are normally rectangular in cross section, 10 to 15 feet deep and 30 feet wide. Length to width should be greater than 5 to 1 in order to avoid short circuiting. Detention periods are from 4 to 6 hours. Air is introduced from diffusers in such a way as to set up a spiral flow pat- tern, thus aiding in mixing of floc and sewage and helping to prevent dead spaces in the tank. It was found that oxygen requirements decreased as the waste proceeded through the tank. The number of diffusers was, therefore, increased at the beginning of the unit and decreased at the effluent end. This is now the common practice and is known as tapered aeration. Mechanical aeration has the same function as air diffusers but is accomplished by rotating paddles or brushes. Peripheral velocity is about 2 ft/sec. Floc returned to the aeration tank has the purpose as trickling filter slime but floc concentration can be varied as operation needs dictate. Returned sludge varies from about 10 to 30%. Mixed liquor suspended solids (MLSS) will vary from 600 mg/l to 4000 mg/l, on a dry weight basis. An important parameter in routine process control is the ratio of the volume of MLSS to the dry weight of MLSS. This is known as the sludge volume index (SVI) and is in the range of 50 to 100 in well operating plants. When the value approaches 200 operating difficulties can be expected. Factors which promote or inhibit microbiological growth are important and these include pH, temperature, and oxidation- reduction potential (ORP). Hydrogen ion potential, pH, will have a great effect on the dominant species of organisms. Bacteria predominate above pH of 6.5 while fungi assume greater importance below this value. There must be adequate buffering capacity if metabolic products are acidic. Modifications of the basic activated sludge process have come about for solution to specific operating problems. The municipal treatment plant at Peoria, Illinois received a waste high in carbohydrates. The resulting nitrogen defi- ciency caused a light and poorly settling activated sludge floc with attendant poor waste stabilization. Kraus, for whom the modification is named, aerated digester supernatant. This, added to the influent, gave a nitrifying activated sludge. The result was a readily settleable sludge with improved organic removals. New York City has plants scattered throughout the five boroughs, treating more than 1 billion gallons of sewage per day. A major modification resulting from experimentation with plant operation has come from this city. In conventional plants sewage was added at one end of the aeration tank and allowed to flow through. This gave a high initial microbial food supply and correspondingly high oxygen requirement. The New York City modification involves introduction of sewage at intervals along the tank. This smooths out the food supply and lowers the oxygen requirements. The sewage is added at discrete steps along the unit and the name applied is step aeration. A low mixed liquor suspended solids concentration of 200 to 500 mg/l is maintained in the high rate process. This gives a high food to microbial mass ratio. This keeps the C013_007_r03.indd 733C013_007_r03.indd 733 11/18/2005 10:43:42 AM11/18/2005 10:43:42 AM © 2006 by Taylor & Francis Group, LLC 734 MUNICIPAL WASTEWATER floc in the active growth phase, but excess food will be dis- charged in the effluent. BOD removals are only 50 to 60%, but in some areas this is acceptable. New York City applied this method successfully because of a weak sewage and low temperatures. Philadelphia and Los Angeles met with indifferent success because of stronger sewage or higher temperatures. A process originated simultaneously and independently by Smith and Eckenfelder made use of a phenomenon observed by many researchers but dismissed as experimen- tal error. When activated sludge and raw sewage are mixed together in an aeration vessel there is a noted reduction in BOD, followed by a rise and then another reduction. The first decrease had been ignored by most research workers. Smith and Eckenfelder found that this was due to adsorp- tion of waste material onto the activated sludge floc. This came from material desorbed from colloidal particles. Plants at Austin, Texas, and Bergen County, New Jersey were con- verted from overloaded to underloaded by changing to the biosorption process. A recent activated sludge process modification is the Deep Shaft Process. As its name suggests, two concen- tric deep shafts (120–150 m) are sunk into the ground. Wastewater is injected into one of the concentric shafts and the effluent is withdrawn from the other. A constant ambient temperature is maintained due to the surrounding geological formations. Compressed air is injected at the bottom, giving high dissolved oxygen concentrations and provided intimate mixing. Waste sludge is removed in clarifiers as in conven- tional activated sludge plants. Putrescible material collected from the primary settling tanks and excess sludge from humus tanks must be disposed of cheaply and efficiently. This material is highly unstable and a potential nuisance source. Because it is putrescible it can be stabilized by biological means, serving as food and energy sources for microorganisms naturally found in the sludge. Raw sludge is about 95% water, but the water is not easily removed. As the sludge is broken down the water content is lessened, and the volume is markedly reduced. A rough rule is that sludge volume is reduced by half when water is low- ered from 95 to 90%, and by two thirds when reduced from 95 to 85%. Fresh sludge has a gray color and can be easily pumped. Its odor is most disagreeable, being due principally to mercaptans. Digested sludge is black in color, granular and has a slight tarry odor. Sludge digestion is carried out in order to reduce the volume of sludge to be handled, and reduce the number of pathogens. Sludge is usually withdrawn at regular intervals from primary and secondary tanks and led by gravity to a sludge well. It is then pumped to the digester. Mixing is very important for efficient sludge digestion. Temperature is equally important. Since destruction of sludge is carried on by microor- ganisms, kinetics of their life processes will be temperature dependent. It has been found that sludge temperature of about 95°F will give acceptably short detention times. Even shorter detention times for the same quality of digested sludge can be achieved with temperatures of about 125–130°F, but this temperature range is not widely used for reasons of econom- ics. Above 95°F an increase in detention time is noted, up to 110°F, and then again a decrease. The reason for this is the changing character of the predominant organisms. Heating of sludge for efficient digestion is carried out in one of two ways. The older installations have hot water coils in the periphery of the tank, and heat is transmitted to the digesting sludge. Mixing was felt to be adequately effected by turbulence due to gas generation. Mechanical mixers have been used. It was found, however, that mixing was not sufficient. In addition, heating of entire tank contents was not achieved due to “baking” of sludge in the vicinity of the heating coils. A second method of sludge heating and mixing was developed, involving the use of external heat exchang- ers. Sludge is pumped from the digestion tank through a heat exchanger and returned to the tank. Two objectives are accomplished (1) efficient mixing of sludge, thereby reduc- ing the amount of inadequately digested sludge, (2) more uniform temperature throughout the tank, thus reducing digestion time. The use of external heat exchangers has almost completely supplanted heating coils and internal mixers in new plant design. Sludge gas generated during digestion is approximately 72% methane and 28% carbon dioxide. Hydrogen and H 2 S are present in trace amounts. Gas thus generated has a calo- rific content of about 600 BTU/ft 3 . About 10 ft 3 of gas are produced per cubic foot of raw sludge digested. Generally, the amount of sludge gas produced is sufficient to provide heat used in maintaining digesting sludge at the required temperature, heating plant buildings, provide hot water and incineration of digested sludge, when practiced, and fuel and generators. Volatile acids, reported as acetic acid, are perhaps the most important parameters in control of sludge digestion. Volatile acids below 1000 mg/l occur in a healthy digestion process. Volatile acids of 6000 mg/l indicate a malfunction- ing process. pH values of 6.8 to 7.2 are optimum. Values less than 6.8 usually are due to excessive volatile acid produc- tion. In the past liming of malfunctioning tank contents was practiced in an effort to adjust pH to about 7.0. However, the change in volatile acids production was due to changing dominant process microorganisms. The lowered pH and high volatile acids concentrations were a sign of a sick process, rather than the cause. Digested sludge is reasonably inert but it must be fur- ther dewatered and the question of final disposal of raw and digested sludge is one of the most pressing with which envi- ronmental engineers must deal today. Sludge can be dewatered on open or covered drying beds. Open beds are exposed to the air and drying is accomplished by drainage and evaporation. Covered beds resemble a greenhouse. Temperatures are rather high and this aids evap- oration. In both cases sludge is allowed to flow over sand beds and let stand for a suitable period. The dried sludge is then scraped from the beds. Sludge can be dewatered by vacuum filtration. Filter drums rotate slowly, picking up wet sludge at the bottom. A slight vacuum is applied and the water drawn off is returned to the C013_007_r03.indd 734C013_007_r03.indd 734 11/18/2005 10:43:42 AM11/18/2005 10:43:42 AM © 2006 by Taylor & Francis Group, LLC MUNICIPAL WASTEWATER 735 plant stream. At the end of the cycle the dewatered sludge is removed by a scraper. Dried sludge can be incinerated, taken to a landfill dis- posal site, composed or subjected to superoxidation. Some coastal cities barged sludge to sea. This was to have ceased in 1981 but was permitted to continue for ten years. It is not allowed after 1991. Incineration of sludge has the potential for air pollution problems and often there is local opposition to installation of an incinerator. Incinerators are expensive to build and operate. Land disposal is expensive and disposal sites frequently are considerable distances from the gener- ating wastewater treatment plants. Groundwater pollution can occur. Landfills which can accept digested sludge are in short supply. Transportation costs are a quite significant part of total disposal costs. Composting has been suggested as a possible ultimate solution. The requirement of relatively large land areas and odor production are problems. A promising approach involves superoxidation. Here the sludge is treated with a strong oxidizing agent. Volume is reduced greatly and the end product is stable and inoffen- sive. Transportation costs are thus reduced. It is common practice to chlorinate effluent for bacte- rial control. Regulations vary from state to state, but most regulations require chlorination to a specified residual. Requirements usually vary from season to season, the most stringent rules governing the swimming season. A phenomena not yet fully understood is that of after- growth, wherein bacterial count is fairly low immediately after effluent discharge but then suddenly rises to a high figure. In some plants chlorination of the influent is practiced for the purpose of odor control. Chlorination of storm water overflow is commonly prac- ticed. In some cases storm water overflow is subjected to simple sedimentation and/or screening, storage and chlori- nation, then discharged after cessation of the storm. One commonly used definition of tertiary treatment is any treatment in addition to secondary (biological) treatment. Tertiary treatment is practiced when an effluent of much higher quality is required than is attainable with conventional biological treatment. They type process used will depend on the final effluent quality necessary and the economics of the total process. Commonly used tertiary treatment processes are listed below: 1) Sand filtration 2) Microstrainers 3) Oxidation ponds 4) Foam separation 5) Activated carbon adsorption 6) Chemical clarification and precipitation 7) Ion exchange Disposal of human and kitchen wastes in areas not served by sewers and wastewater treatment plants presents unique problems. Disposal must be in the immediate vicinity of the source of the wastes. In adequate controls are not exercised a closed system may results. An example is Suffolk County (Long Island), New York. Septic tanks are widely used, and there is strong local opposition to the considerable expense of installing sewers. Effluent from the septic tanks found its way into the ground water which is the supply for much of the county. Eventually, the problem was graphically pointed up by the appearance of foaming detergents in water issuing from the tap. In more primitive societies waste disposal is a matter of convenience. A “cat hole” or communal straddle trench is utilized and covered when capacity is reached. This is the same as the practice with privies and cesspools. Privies, as the name implies, are simply open pits with a structure to provide privacy. Human excreta is deposited into the pit and is slowly stabilized. Stabilization is slow, due principally to the presence of urine. Pits may be open earth or concrete vault. Drawbacks for both types are odors and fly problems. For the unlined pit there is the additional problem of ground water pollution. Older privy construction allowed access to flies around the edge of the pit. For proper protection against flies there must be a tight seal around the edge of the pit and adequate screening of openings in the privy structure itself. In the 1930s a large number of the older privies were replaced by concrete vault types. Today, such methods of waste disposal are found only in the smaller rural communi- ties where there is no municipal collection system. Cesspools are simply pits into which waste is allowed to flow. The term leaching pit is sometimes used. Water seeps into the ground, leaving solid matter in the pit. Construction is of two types. A pit may be unlined, or it may be lined with sewer pipe laid on end. Almost nowhere in the United States are cesspools permitted by health authorities. Septic tanks are widely used in smaller towns and out- lying suburbs of larger cities. They are a combination sedi- mentation tank and anaerobic digester. Sanitary and kitchen wastes flow into the tank and grease and light material rise to the top. Heavier particles settle to the bottom where anaerobic stabilization occurs. Deflector plates are provided at inlet and outlet in order to minimize short circuiting. Effluent flows to a tile field where disposal is into the earth. The tile field is composed of perforated field tile fed by a manifold. The tile is underlain with granular material, usually gravel. Care must be taken that the earth does not become clogged by material car- ried over from the septic tank. Septic tanks are being replaced as more and more areas are served by municipal systems. Health authorities do not look with favor on septic tanks. Capacity will be a function basically of the number of per- sons or units served. Some experts feel that, in no case, should capacity be less than 1500 gallons. Lesser volumes are permit- ted in many codes and the thought that 1500 gallons ought to be the minimum permitted arouses home builders and land developers. Periodically it is necessary to employ a scavenger service for emptying the tank of accumulated solids. Solids thus collected may be discharged to a convenient treatment system or directly to a wastewater treatment plant. C013_007_r03.indd 735C013_007_r03.indd 735 11/18/2005 10:43:42 AM11/18/2005 10:43:42 AM © 2006 by Taylor & Francis Group, LLC 736 MUNICIPAL WASTEWATER The Imhoff Tank has not been discussed earlier because it is similar in many respects to the septic tank. Use is gener- ally confined to small communities and isolated installations. Operation is a combination of sedimentation and anaerobic digestion. This tank was invented by Karl Imhoff, who first used them in the Essen District in Germany in 1907. The tank is composed of two chambers, one above the other. Surface configuration may be circular, square or rectangular. Depth is 25′ to 35′. Sewage flows through the upper chamber, at a low veloc- ity (about 1 fps). Solids settle out and slide through a slot into the bottom chamber. Detention period is about 2 hours. Solids accumulating in the bottom, or digestion, chamber have an initial water content of 85 to 95%. After proper diges- tion of about 60 days the water content is reduced to about 50% and the volume is greatly reduced. Gases produced during digestion are vented to the atmosphere by gas vents located at the tank sides. Solids buoyed up by gas are pre- vented from escaping to the upper tank by deflector plates. Attempts were made to hasten digestion by heating the lower compartment but were of limited success due to over- turning of the tank contents. Some rectangular tanks are arranged so that the direction of flow can be reversed, with outlets becoming inlets and vice versa. In recent years manufacturers of waste treatment equip- ment have endeavoured to supply complete treatment plants for small communities or developments and isolated instal- lations. Basically, these plants, called package plants, supply primary treatment and sometimes some biological treatment on a small scale without requiring extensive operating super- vision. It is felt that such treatment is to be preferred to septic tanks or only primary treatment (Imhoff Tanks, for example), but such installations are not the ultimate solution. REFERENCES 1. Anderson, E. and W.T. Lockett, J. Soc. Chem. Ind. London , 33, 523, 1914. 2. Bewtra, J.K., Biological Treatment of Wastewater, Encyclopedia of Envi- ronmental Science and Engineering , 4 th Ed., Vol. 1, Gordon and Breach, Inc., New York. 1998. 3. Bewtra, J.K. and H.I. Ali, Physical and Chemical Treatment of Wastewa- ters, Encyclopedia of Environmental Science and Engineering , 4 th Ed., Vol. 2, Gordon and Breach, Inc., New York. 1998. 4. Bewtra, J.K., Recent Advances in Water-pollution-control Technology. Advances in Environmental Science and Engineering , 1, Gordon and Breach, New York, 1979. 5. Camp, T.R., Trans. ASCE , 3, p. 895, 1942. 6. Cecil, L.K., Water reuse, Encyclopedia of Environmental Science and Engineering , 2, Gordon and Breach, Inc., New York, 1983. 7. Cecil, L.K., Water reuse and disposal, Chemical Engineering , May 5, 1969. 8. Disposal of Municipal Sewage. House Report No. 20 12th Report by Committee on Government Operation, House of Representatives, March 24, 1965. 9. Dobbins, W.E., J. of the Sanitary Engineering Division, ASCE , 90, No. SA 3, Proc. Paper 3949, June 1964. 10. Eckenfelder, W.W. and D.J. O’Connor, Biological Waste Treatment , Pergamon Press, Ltd., London, 1961. 11. Fair, G.M., J.C. Geyer, and D.A. Okun, Water and Waste Water Engi- neering , 1 and 2, John Wiley and Sons Inc., New York, N. Y., 1968. 12. Guttman, H.N., Microbiology, Encyclopedia of Environmental Science and Engineering , 4 th . Ed., Vol., 2, Gordon and Breach, Inc., New York. 1998. 13. Imhoff, K., Taschenbuch der Stadtentwässerung , R. Oldenbourg Verlag, Munich, 1960. 14. Imholff, K. and G.M. Fair, Sewage Treatment 2nd Ed., John Wiley and Sons, Inc., New York, N.Y., 1956. 15. McKinney, R.E., Microbiology for Sanitary Engineers , McGraw-Hill Book Co., New York, N. Y., 1962. 16. Nolte, W.F., Effects of elevated pressure on secondary treatment of wastewater, Advances in Environmental Science and Engineering , 4, Gordon and Breach, New York, 1981. 17. Pfafflin, J.R., Water: Sewage, The Encyclopedia of Chemical Technol- ogy , Interscience Publishers, Inc., New York, N.Y., 1981. 18. Pfafflin, J.R., Water: Reuse, The Encyclopedia of Chemical Technol- ogy , Interscience Publishers, Inc., New York, N.Y., 1981. 19. Phelps, E.B., Stream Sanitation , John Wiley and Sons, Inc., New York, N.Y., 1944. 20. Pollock, D.C. and M.A. Wilson, Development of Deep Shaft Effluent Treatment Process in North America , Second World Congress of Chem- ical Engineering, Montreal, 1981. 21. Qasim, S.R., Wastewater Treatment Plants—Planning, Design and Operation, Holt, Rinehart and Winston, New York, 1985. 22. Salvato, J., Environmental Sanitation , 2nd Ed., John Wiley and Sons, Inc., New York, N.Y., 1972. 23. Sawyer, C.N. and P.L. McCarty, Chemistry for Sanitary Engineers , 2nd Ed., McGraw-Hill Book Co., New York, N.Y., 1967. 24. Standard Methods for the Examination of Water and Waste water , 17th Ed., APHA, 1990. 25. Streeter, H.W. and E.B. Phelps. A study of the pollution and natural purification of the Ohio river. Public Health Bulletin 146, US Public Health Service, Washington, D.C., 1925. 26. UK Patent No. 2,128,980. 27. US Patent No. 4,464,257. 28. US Patent No. 4,500,428. JAMES R. PFAFFLIN Gillette, N.J. CAMERON MACINNIS University of Windsor. C013_007_r03.indd 736C013_007_r03.indd 736 11/18/2005 10:43:42 AM11/18/2005 10:43:42 AM © 2006 by Taylor & Francis Group, LLC MUNICIPAL WATER REUSE: see WATER REUSE MUNICIPAL WATER SUPPLY: see WATER TREATMENT . Ed., Vol. 1, Gordon and Breach, Inc., New York. 1998. 3. Bewtra, J.K. and H.I. Ali, Physical and Chemical Treatment of Wastewa- ters, Encyclopedia of Environmental Science and Engineering , 4 . REFERENCES 1. Anderson, E. and W.T. Lockett, J. Soc. Chem. Ind. London , 33, 523, 1914. 2. Bewtra, J.K., Biological Treatment of Wastewater, Encyclopedia of Envi- ronmental Science and Engineering. L.K., Water reuse, Encyclopedia of Environmental Science and Engineering , 2, Gordon and Breach, Inc., New York, 1983. 7. Cecil, L.K., Water reuse and disposal, Chemical Engineering , May 5,