FIGURE 6.-Smith & Loveless factory-built duplex pumping station, designed for capacities from 100 gallons per minute to 4,500 gallons per minute per pump with two or more pumps installed complete with all accessories. DESIGN AND CONSTRUCTION OF SEWAGE AND INDUSTRIAL WASTE TREATMENT WORKS GENERAL With the increasing growth and density of population and continued industrial expansion of our Nation, the need for adequate treatment and disposal of sewage and industrial wastes has grown apace. The increased water requirements that go along with this growth and expansion, and the resultant demand for cleaner streams, make waste. treatment imperative. Specifically, sewage and waste treatment are required to protect: Potable and industrial water supplies. Fish and aquatic life and wildlife. The esthetic values of the Nation's rivers and streams. Sewage treatment works must adequately remove or reduce the solids, bacteria, and chemical constituents of waste waters which interfere with legitimate uses of the receiving streams. The oxygendemanding constituents of industrial wastes, and those which cause objectionable tastes or odors in water supplies and taint the flesh of fish must be reduced to a satisfactory level. Oils and floating solids in industrial wastes must be reduced to the point where they will not create fire hazards, coat hulls of water craft, injure fish or wildlife or their habitat, or adversely affect public or private recreational development or other legitimate shoreline developments or uses. Also those materials which are toxic to human, fish, aquatic, or wildlife must be eliminated or reduced to known safe limits. THE PROBLEMS INVOLVED In order to achieve these objectives, adequate waste treatment plants must be provided to treat wastes before their discharge to streams. Municipal treatment works must be designed to treat the quantity and quality of wastes to be treated some 20 to 30 years in the future since they normally are built with debt financing. The establishment of this basis of design presents a problem which at best can only be solved by estimation. The design and construction of sewage and industrial waste treatment works poses further problems. The primary aim is to select a method of treatment to remove or adequately reduce the objectionable constituents of the waste to protect the uses for which the receiving stream is required, and do this as economically as possible. Selection of methods, equipment, and materials is limited by present knowledge and the number of choices. Current methods of sewage treatment are limited in their ability to remove or adequately reduce the objectionable constituent of the wastes to permit repeated reuse of downstream waters. The equipment and materials must withstand attack by complex wastes and corrosive atmospheres. At the same time, they must perform continuously and efficiently throughout the life of the facility, and provide economies in first cost and in operation and maintenance. Waste treatment practice. The treatment processes for the disposal of municipal or combinations of municipal and industrial wastes have become somewhat standardized in three phases: (1) Separation of liquids and solids, (2) treatment and disposal of solids, and (3) oxidation of the liquid. The first phase combined with the second is designated as primary treatment; the third phase as secondary treatment. After the combination of all three phases is known as secondary or complete treatment, the latter being an unfortunate misnomer since even the most efficient wastes processing methods presently known do not remove all oxidizable material, all objectionable dissolved solids or all suspended solids. The solids-liquid separation may be accomplished by a combination of screening, differential sedimentation, flocculation, and flotation. All of these processes are not necessarily incorporated into any one treatment works. Differential sedimentation may be effected in two stages. The first separates high specific gravity solids, known as grit. The second stage is designed to remove the lighter, more flocculent solids. Normally the grit, aside from being washed, is not subjected to further treatment. Solids removed in the primary sedimentation tanks may be treated in a number of ways prior to disposal. The liquid effluent from primary sedimentation basis is subjected to oxidation measures when further treatment is required before its disposal in the receiving stream. TRADITIONAL DESIGN AND CONSTRUCTION PRACTICES Primary treatment Screening. In the treatment of municipal wastes, the liquid-solids separation is normally accomplished by the use of screens and/or shredders, grit collectors, and settling tanks. The screens generally employed in sewage treatment works are racks of bars with clear openings of about 1 inch to 2 inches. These may be either manually or mechanically cleaned. The screenings may then be disposed of by burial or incineration, or they may be ground and returned to the sewage flow. Another means of handling these particles is by comminution; i.e., the reduction of the material to a smaller size by the use of a mechanical cutting or shredding device which functions in the flow. Grit removal.-Since the sand and cinders which are found in municipal sewage are heavier than the organic matter, they will settle faster. Removal may be accomplished by reducing the velocity of the sewage to 1 foot per second. At this velocity the heavier inorganic grit particles will settle, while the lighter organic material will not. The two most common methods of removing grit are to either pass the sewage through channels in which a velocity very close to 1 foot per second is maintained, or to pass it through basins in which the velocity is always 1 foot per second or less and then wash out from the grit such organic matter as may settle with it. Except in the smallest plants which may have manually cleaned basins of the first type, most grit chambers are mechanically cleaned. The mechanisms commonly in use include chain and bucket scrapers, plow-type scrapers, screw conveyors, airlift pumps, and bucket elevators. The washing devices which may be used sometimes employ air diffusion which would keep the organic matter in suspension, or classifying devices of the reciprocating rake, hydraulic cyclone, or screwconveyor type. Removal of suspended matter.-Most of the flocculent suspended solids remaining after screening and grit removal are comparatively easily separated from sewage by plain sedimentation. Settling is a desirable step before discharging the wastes to the secondary treatment or oxidation units, or to a stream which can receive sewage subjected to only primary treatment. Settling tanks are designed as horizontal or as vertical (up-flow) flow tanks, either rectangular or circular in plan. Normally mechanical equipment of the chain and scraper or plow type is placed in the basins for continuous or intermittent sludge removal so that the deposited solids do not have the opportunity to undergo putrefaction in the basins. Two-story tanks (Imhoff tanks) may be used, generally in smaller plants, in which settling takes part in the upper chamber, and the lower story is used for sludge digestion. Secondary treatment Secondary treatment of municpal wastes invariably involves oxidation, affected by aerobic micro-organisms. The necessary organisms are normally present in sewage and it is merely necessary to create an optimum environment to encourage their domination and multiplication among the microbial life. Carbohydrates, fats, and proteins present in sewage are food sources for organisms of all types. By oxidizing this organic matter to carbon dioxide, water, nitrates, and ash under controlled conditions, the treated sewage may be discharged to many streams without detrimental effects. Complete oxidation is seldom possible; however, results may be obtained which will allow the stream to purify itself in time. By controlling the environment for the micro-organisms in the treatment plant, types that are selective in their metabolism can be developed to produce optimum results. Trickling filter. In trickling filters, a rock, slag, or other fixed porous medium is provided as a base for organisms to live in a gelatinous matrix which forms on the media. The sewage normally is applied as a spray by rotating distributors. The filter media is generally 3 to 10 feet deep and is placed on a floor provided with a drainage system. Filters usually are designed and constructed circular in plan with vertical walls. Walls have been built of concrete, stone, brick, clay tile, precast concrete segments, and steel plate. The walls are constructed so that air will be admitted through the drainage system and up through the filter media. Sometimes in colder climates these filters have also been covered to prevent freezing. Activated sludge process.-The activated sludge process, in contrast to trickling filters, involves biological synthesis of the solids normally suspended in the sewage. In the activated sludge process, it is necessary to keep the biological medium (sludge) in contact with the waste by circulating it and to provide an excess of free air (oxygen) to maintain aerobic conditions. In some systems circulation and aeration are effected simultaneously by introducing compressed air into the basins containing the waste and the sludge. In others both aeration and circulation are accomplished by mechanically stirring the basin contents. With both trickling filters and the activated sludge process, final settling tanks are necessary to separate the sludge solids and microorganisms from the oxidized sewage. These settled solids are then either recirculated to the process or returned to the primary settling basins for disposal with the primary sludge. The solids and oxidizable materials remaining in the treatment plant effluent, by either biological process, must be assimilated by the stream to which the discharge is made. Secondary effluent does not always satisfy the environmental conditions in the stream. Nitrate nitrogen, phosphates, and sulfur compounds enrich the nutrient level in the stream and often results in excessive algae and other plant growth which then becomes a problem to be dealt with. Sludge digestion Disposal of the solids from the primary and secondary sedimentation tanks is one of the most troublesome features of waste treatment works. In the digestion process, the sludge (93 to 97 percent water) usually is digested for a period of 30 to 50 days in a special tank under anaerobic conditions. At the end of that time a degree of organic stability has been attained by the microbiological activity in the tank, and the sludge may be disposed of with little or no odor or nuisance. Temperature control, about 95° F., is necessary for efficient digestion. Heating facilities have in the past been in the form of heating coils inside the tank; however, present practice is to use external heat exchangers. As fresh solids are pumped to the digesters, the supernatant liquor (water separated from the solids) overflows and is diverted back to the plant to be treated with the raw sewage. Gas from the process, mostly methane, carbon dioxide, and small quantities of hydrogen sulfide, is collected and used for heating the digesters. The excess gas may be wasted by burning or in larger plants it is often used to drive gas engines and electric generators. Throughout the process, the presence of explosive mixtures of gas and air is a potential hazard, and the utmost precautions need be taken. Digestion tanks are normally constructed of concrete, either poured in place or prestressed, and are generally circular in plan, but have been square and hexagonal. They may be built with fixed-in-place steel or concrete covers, or with covers which float on the liquid. These covers are usually steel which support either a wood and composition roof, or a roof of insulated metal. Sludge disposal.-Digested sludge, although stabilized with respect to the organic matter, still contains 85 to 95 percent moisture. The high water content offers obstacles to disposal methods because onsite methods usually involve total destruction by incineration and offsite methods require transportation. Here again, a liquid-solids separation problem occurs. Where land area is not at a premium and the climate is favorable, spreading on well-drained sand drying beds for drying is commonly used. If the climate factor is unfavorable, it can be offset by enclosing the sand beds in greenhouses. Another method of disposing of digested sludge when ample suitable land is available is by lagooning. Lagoons can provide permanent storage, or they may be periodically cleaned. Where economics or the nuisance |