The purpose of this research is to discuss water pollution from an engineering perspective.
The degree and type of treatment to be provided for controlling water pollution is dependent upon many factors, a major one being the water quality standards established for the receiving water. A classification system for freshwaters and saltwaters usually depends upon the ultimate government agency maintaining jurisdictional rights. Also important, however, is the future as well as the existing upstream and downstream water usage, the minimum flows, the types of sewers and wastewater characteristics, the assimilative capacity of the receiving waters, and the capability of the community to finance, operate, and maintain the facility as intended.
Federal and state regulatory agencies have recommended standards and guidelines for design of large sewage treatment plants and sewage systems. A preliminary basis of design, which has been for many years where drinking water supplies are not directly involved and other local factors are not adversely affected is the dilution principle, though by itself is no longer acceptable in the United States.
The dilution water available is in terms of ft3/sec/1000 equivalent population, with the water 100 percent saturated with oxygen. Under special conditions a lesser volume of dilution water may be sufficient to prevent the development of unsatisfactory conditions, such as when the stream has a turbulent flow or joins a larger watercourse after only a few hours’ flow.
On the other hand, five times the given dilution may be required if flow is through a densely populated area. In the final analysis the stream usage or classification will determine the degree of treatment required. Present opinion in the United States is that all sewage should receive a minimum of secondary treatment.
In Britain, the Ministry of Housing and Local Government reaffirmed in 1966 the Royal Commission’s “general standard” as a “norm” for sewage effluents: 5-day BOD 20 mg/1 and suspended solids 30 mg/1 with a dilution factor of 9 to 150 volumes in the receiving watercourse having not more than 4.0 mg/1 BOD. A higher effluent standard of 10 mg/1 BOD and suspended solids may be required if indicated. In any case, sewage effluents should not contain any matter likely to render the receiving stream poisonous or injurious to fish.
Plans of the area to be sewered should be complete and should include specifications and an engineering report giving the problem, objectives and design details. The plans must be prepared by a licensed professional engineer, drawn to a scale of 1 inch equal to not less than 100 feet or more than 300, with contours at 2-to-10-foot intervals. The discussion that immediately follows deals with sanitary sewers. Combined sewers should be redesigned as storm and sanitary sewers insofar as possible; extensions should be separate sewers.
The location of the sewers, with surface elevations at street intersections and changes in grade, are indicated on the general plan. The size of sewers, outfalls, slope, length between manholes, and invert elevations of sewers and manholes to the nearest O.01 feet are also shown on the general plans. For all sewer laterals 12 inches or larger and all main, intercepting, and outfall sewers, profiles including manholes and siphons, stream crossings, and outlets must be included. The horizontal scale must be at least equal to that on the general plan and the vertical scale not smaller than 1 inch equal to 10 feet. Detail plans to a suitable scale are required of all appurtenances, manholes, flushing manholes, inspection chambers, inverted siphons, regulators, pumping stations, and any other devices to permit thorough examination of the plans and their proper construction. The total drainage area and the area to be served by sewers should be shown on a topographic plan.
Sewers are usually designed for a future population 30 to 50 years hence, and for a per capita flow of not less than 400 gpd for submains and laterals and 250 gpd for main, trunk, and outfall sewers, plus allowance for industrial wastes. Intercepting sewers are designed for not less than 350 percent of the average dry weather flow.
Street sewers should not be less than 8 inch in diameter and at a depth sufficient to drain cellars, usually 6 to 8 feet. Vitrified clay or concrete sewers are deigned for a mean velocity of 2 fps when flowing full or half full, based on Kutter’s formula, with n – 0.013.
When cement asbestos pipe or cement – or enamel-lined cast-iron pipe is used, n – 0.011 or n – 0.013 may be used in design. In general, sewers should be laid on grades not less than 0.65 for 4 inch diameter and 0.053 for 24 inch diameter.
In some special situations there may be justification for the use of 6-inch sanitary sewers with full knowledge of its limitations. Compression-type joints for pipe 4 to 12 inch in diameter are highly recommended. Vitrified clay and cast-iron have a life of more than 50 years.
Manholes should be not more than 300 feet apart on 12-inch pipe or smaller and not more than 400 feet apart for 15-inch or larger pipe. Some standards permit a manhole spacing of 500 feet for sewers 18 to 30 inch, and 400-feet spacing for sewers 8 to 12 inch.
Wye connections should be installed for each existing and future service connection as the sewer is being laid. A tight connection must be assured.
Where required, inverted siphons should be not less than 6 inches in diameter nor less than 2 in number. Sufficient head should be available to provide velocities of not less than 3 fps in sanitary sewer or 5 fps in combined sewers. Accessibility to each siphon and diversion of flow to either one would permit inspection and cleaning.
Pumping stations should contain at least three pumping units of such capacity to handle the maximum sewage flow with the largest unit out of service. The pumps should be selected so as to provide as uniform a flow as possible to the treatment plant. Two sources of motive power should be available. Small lift stations should have duplicate pumping equipment or pneumatic ejectors with auxiliary power.
In all cases, raw sewage pumps should be protected by screens or racks unless special devices are approved. Proper housing for electric motors above ground and in dry wells should provide good ventilation, preferably forced air, and accessibility for repairs and replacements. Wet wells or sumps should have sloping bottoms and provide for convenient cleaning. Select water-level pumps controls with care because they are the most frequent cause of pump failure.
Sewage treatment works should be designed for a population at least 10 years in the future, although 15 to 25 years is preferred, and a per capita flow of not less than 100 gpd plus acceptable industrial wastes. Actual flow studies should govern. Plants should be accessible from highways but as far as practical from habitation and wells or sources of water supply. The required degree of treatment should be based on the water quality standards and objectives established for the receiving waters and other factors, as pointed out earlier.
The two major sewage treatment design parameters are the 5-day biochemical oxygen demand and suspended solids of the wastewater to be treated and the removal expected of the treatment process. The 5-day BOD is usually assumed to be 0.17 lb/capita/ day and the suspended solids 0.20 lb with the daily flow 100 gpd/capita. Studies at 78 cities suggest 0.20 lb BOD, 0.23 lb suspended solids, and 135 gpd/capita as being more representative. Design should be based on actual wastewater strength and flow.
Scale drawings of the units comprising the sewage treatment works, together with such other details as may be required to permit review of the design, examination of the plans, and construction of the system in accordance with the design, must be prepared and submitted to the regulatory agency having jurisdiction. Provision for measuring the flow and sampling the sewage and an equipped laboratory for examinations to control operation should be included in the original plans. The design must include disinfection of the plant effluent if the receiving stream or body of water in the vicinity of the outfall is used for water supply, shellfish propagation, recreation, or other purposes that may be detrimentally affected by the sewage disposal.
The average dry weather flow of sanitary sewers is approximately equal to the discharge rate of runoff from a rainfall having an intensity of 0.01 inch/hour. But 0.02 to 0.03 inch of rainfall is needed to wet the ground before there is a runoff. The rate of flow in a combined sewer is approximately equal to 100 times the rainfall intensity in in./hr. times the dry weather flow, up the capacity of the sewer. Therefore, if the average dry weather flow is 1 mgd and the rainfall intensity is 2 in./hr. for a sufficient time to cause runoff from the area under consideration, the rate of flow would be 200 mgd.
The economic futility of trying to design a treatment plant to treat combined sewer flows is obvious. On the other hand, combined sewers contribute significant amounts of sediment, oil, salts, and organic matter. Discharge, or overflow from combined sewers, immediately after a heavy rain may have a BOD of several
hundred milligrams per liter and MPN of hundreds of thousands. Even after the initial flushing the pollution discharged is still substantial. Hence complete stream pollution control cannot be realized unless combined sewer overflows are retained and/or adequately treated before discharge.
The importance of sewer inspection before backfilling has not been given sufficient emphasis. Too many properly designed plants have created problems almost as serious as those they were intended to correct. Some plant flows equal or exceed the design flow before the system is completed because of improper sewer and house connection construction; roof, footing, and area drainage connection; street drainage connection; and similar practices. Inspection during construction should assure tight service connections and sewer construction by requiring full compliance with pipe, joint, bed, and backfill specifications.
Infiltration flow tests in wet ground of each line before acceptance, or exfiltration flow tests in dry ground, will determine compliance with specifications. The exfiltration is a more severe test; large volumes of water are needed and must be disposed of. Under a head of 2 feet of water the exfiltration was found to be 4.8 percent greater than shown by the infiltration test; under a 4 foot head, 19.8 percent greater; and 27.3 percent greater under a 6 foot head. Low pressure air test at 3.0 psi with air loss not greater than 0.0030 cfm/ft2 of internal pipe surface has certain advantages and can also be used.
The total leakage should not exceed 150 gpd/mi/in. of internal diameter of pipe over a 24-hour test period, in a well-laid line. Recommended Standards for Sewage Works states that leakage shall not exceed 500 gpd/mi/in. of pipe diameter. A figure of 200 to 400 gal/acre of sewered area/day is also used. Rubber ring joints or the equivalent on vitrified clay tile, concrete, and asbestos cement pipe, and hot-poured joints in dry trenches can give good results.
A community camp, institution, factory, school, or other establishment that constructs sanitary sewers and treatment plants should immediately set up rules and regulations prohibiting sewer connections by other than responsible individuals. This must be supplemented by effective enforcement to guarantee exclusion of groundwater, surface water, and rainwater from the sanitary sewer system in order to protect the investment made and accomplishment the objective of the treatment plant. Sanitary sewers that, in effect, become combined sewers almost always cause backing up in basements after storms and sometimes overflow through manholes onto the street and into stores, basements, and son on, with resultant damage to oil burners, electric motors, and personal property. Cellars remain damp and become contaminated with sewage. Treatment plants become overloaded, requiring the bypassing of diluted sewage to the receiving stream or body of water, resulting in danger to water supplies, recreational area, fish life and property values. This, of course, mollifies in part the purpose of sewage treatment. In recent years, greater attention has been given to the proper construction of sanitary sewers and to the elimination of roof leader connections to the sanitary sewer, a major cause of hydraulic overloading.
The effect of roof leader and submerged manhole flows is more fully appreciated by some comparisons. A 4-in. rainfall in 24 hr. can be expected in the United States roughly south of the Ohio River, once in 5 yr. If 180 dwellings (1000 ft2 roof area) connected their roof leaders to the sanitary sewer, the resultant flow would equal the capacity of an 8-inch line; that is, 460,000 gpd with a slope of 0.4 ft/100 ft. Five manhole covers with six vent and pick holes, under 6 in. of water, will also admit enough water to fill an 8-in. sewer.
The design of new industrial plants should incorporate separate systems for sewage, cooling, water, waste, and storm- or surface-water drainage. In some cases combination of sewage and liquid waste is possible, but this is dependent on the type and volume of waste. At an existing plant and at a new plant, a flow diagram should be made showing every step in a process, every step in a process, every drain, sewer, and waste line. Dye or temperature tests will help confirm the location of flows. Radioactive isotope tracers might also be used under controlled conditions. Where pipelines are exposed, they can be painted definite colors to avoid confusion. In general, clean water from coolers, roof leaders, footing and area drains, and ice machines can be disposed of without treatment. Water conservation such as use of cooling towers and recirculation of the water, use of air-cooled exchangers, and wastewater pretreatment and reuse will reduce the wastewater problem.
A knowledge of the industrial process is fundamental in the study of a waste problem. The volume of flows from each step in a process, the strength, chemical characteristics, temperature, source, and variations in flow are some of the details to be obtained. The existing or proposed methods of disposal, opportunities for wastage, drippage, and spillage should be included. Sometimes revision of a chemical process can eliminate or reduce a waste problem. Possible waste-control measures are salvage, in-plant waste reduction, reclamation, concentration of wastes, flow equalization and new methods.
A method of simplifying a waste problem is to spread its disposal over 24 hr. rather than over a 4- or 6-hr period. This is accomplished by the use of a holding tank to equalize flows and strength of waste, accompanied by a constant uniform discharge over 24 hr. Where needed, aeration will prevent septic conditions and odors. If necessary, chemical mixing can be incorporated, followed by settling and uniform draw-off of supernatant by means of a flexible or swing joint pipe. The pipe may be lowered uniformly by mechanical means such as a clock mechanism or motor, or a float with a submerged orifice, to give the desired rate of discharge. The mass diagram approach may be used to determine the required storage to give a constant flow over a known length of time. Sludge is drawn from the hopper bottom of the settling tank to a drying bed or to a treatment device before disposal.