(20) Black, A. P.; Willems, D. G.: Electrophoretic Studies of Coagulation for Removal of Organic Color. Journal AWWA, May 1961, p. 589.

(21) Symposium-Water Quality Standards Discussion: Operator's Viewpoint, Baylis, J. R. Journal AWWA, September 1960, p. 1169.

(22) Baxter, S. S.: Recruiting and Keeping Competent Engineering and Management Personnel. Journal AWWA, October 1961, p. 1269.

(23) Luthin, J. C.: Consolidation of Water Systems in California. AWWA, November 1959, p. 1342.

Journal

(24) Schone, H. K.: Organization and Standards of the Oakland County Department of Public Works. Journal AWWA, April 1960, p. 426.

(25) Cohen, J. M.; Rourke, G. A.; Woodward, R. L.: Ferric Sulfate Coagulation in the Presence of Synthetic Detergents. Journal AWWA, October 1959, p. 1255.

(26) Aldrich, E. H.: Multiple-Treatment Units for Water Purification. Journal AWWA, December 1952, p. 1107.

(27) Timanus, C. S.; Spaulding, C. H.: The New Water Purification Plant for Springfield, Ill. Journal AWWA, 27:327, p. 1935. Also (28A), Spaulding, C. H. Conditioning of Water Softening Precipitates. Journal AWWA, November 1937, p. 1697.

(28) Lordley, H. E.: Use of Liquid Alum at Richmond, Va. September 1958, p. 1259.

Journal AWWA,

(29) Cohen, J. M.; Rourke, G. A.; Woodward, R. L.: Natural and Synthetic Polyelectrolytes as Coagulant Aids. Journal AWWA, April 1958, p. 463.

(30) Anonymous; Coagulant Aids for Potable Water Treatment. Journal AWWA, January 1962, p. 82.

(31) Riddick, T. M.: Discussion to Mechanisms of Coagulation, Black, A. P. Journal AWWA, April 1960, p. 502.

(32) Conley, W. R.; Pitman, R. W.: Test Program for Filter Evaluation at Hanford. Journal AWWA, February 1960, p. 205.

(33) Conley, W. R.; Pitman, R. W.: Innovations in Water Clarification. Journal AWWA, October 1960, p. 1319.

(34) Conley, W. R.: Experience with Anthracite-Sand Filters. Journal AWWA, December 1961, p. 1473.

(35) Aldrich, E. H.: Three Applications of Instruments and Automation. Journal AWWA, November 1961, p. 1371.

(36) Tuepker, J. L.; Hartung, H. O.: Effect of Accumulated Lime—Softening Slurry on Magnesium Reduction. Journal AWWA, January 1960, p. 106.

(37) Hartung, W. O.: Calcium Carbonate Stabilization of Lime-Softened Water. Journal AWWA, December 1956, p. 1446.

(38) Crow, W. B.; Wertz, C. F.: Techniques and Economics of Calcining Softening Sludges-Joint Discussion. Journal AWWA, May 1960, p. 322. (39) Funsk, A. W.: Ion Exchange Treatment at High Flow Rates. Journal AWWA, May 1959, p. 681.

(40) Adams, R. G.: Manganese Removal by Oxidation with Potassium Permanganate. Journal AWWA, February 1960, p. 219.

(41) Shea, H.: Iron and Manganese Removal at West View, Pa. Journal Pa. W. W. Oper. A., 1961, p. 23.

(42) Larson, T. E.: Loss in Pipeline Carrying Capacity Due to Corrosion and Tuberculation. Journal AWWA, October 1960, p. 1263, and other papers.

DESIGN AND CONSTRUCTION OF SEWAGE AND INDUSTRIAL

WASTE COLLECTION SYSTEMS

GENERAL

All communities produce waterborne wastes and are affected by the runoff of precipitation. Sewers perform the function of collecting waterborne wastes of domestic, commercial, and industrial origin, and storm waters, for conveyance to points of discharge or disposal, and are necessary to the health and welfare of the community.

Modern practice is to design and construct two separate sewer systems sanitary and storm water. This report is primarily concerned with sanitary sewer systems since it is the wastes in these sewers which can adversely affect public health and which contribute most to water pollution. In the design of sanitary sewers, the engineer must develop estimates of the quantity of flow to be conveyed at some time in the future, since these facilities are not easily enlarged.

THE PROBLEMS INVOLVED

Part of the problem in estimating flow is to make allowance for (1) ground water infiltration, since sewer joints are never made perfectly watertight; (2) roof and foundation drainage, because there are always connections of this sort to sanitary sewer systems even though such connections may be illegal; and (3) peak flows, since water usage and consequently sewage flow is not constant throughout the day, week, or even year. Exclusion of ground water, roof, and foundation drainage from sanitary sewer systems is most important for economic and functional reasons. Excessive flows require increased investment in both sewers and sewage treatment facilities.

In planning the system, the engineer must strive to obtain as economical a plan as he can. Gravity conduits are utilized wherever the topography permits. However, where gravity flow cannot be achieved, pumping equipment is necessary in order to deliver the wastes to the point of treatment and disposal. Furthermore, the engineer must design a sewer system in such a manner that the velocities within the conduits will be sufficient to keep the sewage solids in suspension. The layout of the system requires a careful study of the tributary area in order that all properties are served in the most effective and economical manner.

The preparation of construction drawings and specifications and the construction of the facilities pose further problems. Among the most important of these is the selection of materials to be used. The following factors must be considered:

(1) Flow characteristics, friction coefficient.

(2) Life expectancy and use experience.

(3) Resistance to scour.

(4) Resistance to corrosion and to attack from acids, alkalines, gases, solvents, etc.

(5) Ease of handling and installation.

(6) Strength to carry structural loads.

(7) Type of joint (watertightness and ease of assembly).

(8) Availability and ease of installation of fittings and connec

tions.

(9) Availability in sizes required.

(10) Cost of materials, handling, and installation.

The methods of construction available to the contractor as well as the conditions which may be encountered must also be considered.

TRADITIONAL DESIGN AND CONSTRUCTION PRACTICE

The design of a sanitary sewer system must take into account the problems enumerated above and insofar as possible, resolve them. The size, slope, and depth of the sewer and the materials of its construction must be determined. Attention must also be given to obtaining proper design of appurtenances such as manholes, junction chambers, and other structures to minimize turbulence and head loss, and to prevent deposition of sewage solids.

Pipes and conduits. It is obvious that care should be exercised in providing proper pipe and conduit sizes, in the selection of materials, and in the details necessary to achieve soundness of construction. Vitrified clay, concrete, asbestos-cement, cast iron, steel, and brick masonry are the materials that normally have been used for sewer construction. Not all of these materials can be used under all conditions.

Joints must be designed and constructed to be watertight, resistant to root penetration, resistant to corrosion, have a reasonable degree of flexibility, and be durable.

Pumping stations. The number and location of pumping stations required must be selected with care. This decision may quite possibly be based upon the maximum desirable depth of sewerlines below the ground surface. This limitation may be determined by soil conditions, rock, or ground water levels. In any case, the construction, operation, and maintenance costs of a pumping station must be weighed against the costs of construction and maintenance of sewerlines.

Sewage pumping stations serving more than 50 homes are normally of the wet well-dry well type of construction. The sewage enters the wet well, sometimes through a bar screen, where 5 to 10 minutes storage capacity is provided. The pumps, generally of the nonclog centrifugal type, are installed in the dry well with all the necessary piping, valves, and controls. The motors may be close coupled to the pumps or raised out of the way of any possible flooding by the use of extended shafts. Normal practice is to install more than one pumping unit with the capacity of each such that, if any one pump is out of operation, capacity remains to handle peak flows.

USE OF NEW MATERIALS, EQUIPMENT, AND DESIGNS AND NEW APPLICATION OF OLDER MATERIALS, EQUIPMENT AND DESIGNS

In recent years there have been several new developments with respect to pipe sewers. Most of these have been concerned with the improvement of pipe joints to prevent ground water infiltration. Some of the materials for this purpose have been in common use for other purposes for many years. In addition there have been several new materials introduced in recent years for use in sewer pipe, but these have been primarily for smaller lines and house connections, or for use under abnormal conditions of one sort or another.

New pipe and joint materials. Factory-made joints using resilient materials for vitrified clay pipe can be grouped into three basic material combinations: (1) Those joints in which the same resilient material is used on both bell-and-spigot ends with one varying in hardness from the other (see fig. 5), (2) those joints using different types of resilient materials in the bell-and-spigot ends, and (3) those joints which rely upon gasket or compression ring. The last type of joint is also used with concrete pipe. The resilient materials used include plasticized resins of polyvinyl chloride, plastisol, a polyester plastic, and a phenolic plastic. Gaskets or compression rings are normally made of rubber or neoprene.

FIGURE 5.-Polyvinyl chloride joint for vitrified clay pipe as manufactured by American Vitrified Products Co.

The new pipe materials which have been developed and are available in sizes generally less than 8 to 12 inches in diameter include plastic, polyvinyl chloride, Fiberglas reinforced epoxy resin, polyethylene and styrene. Bituminized-fiber pipe has been used for house connections and a limited number of other applications. Many of these new materials were developed to offset such problems as corrosion, watertightness, and resistance to chemical attack.

Improved construction methods.-It has often been common practice to build concrete sewers of the larger diameter in place. These sewers have on occasion been lined with vitrified clay, bituminous, or plastic liner plates for corrosion resistance. In recent years a number of new machines have been developed for casting concrete sewers in place by the slip-form method. One such machine operates in an excavated trench and consists primarily of a traveling form with facilities for distributing and vibrating the concrete as it is poured. Concrete is

supplied to the machine hopper by transit mixers. Equipment is available to construct pipe 24 to 60 inches in diameter at a maximum depth of 16 feet.

Another machine, operating in a trench excavated to the outside diameter of the pipe to be laid, is propelled at the desired speed, laying the pipe without joints by using aluminum forms supported on the inside with full circle struts. One company provides a machine for building concrete pipe in place, using an inflatable inside form. Pipe with diameters of 12 to 30 inches may be constructed with this machine. Pumping equipment and stations.-A number of advances have been made in recent years to improve the design of pumping equipment and stations, and to reduce their initial capital cost as well as operating and maintenance costs. Significant reductions in capital costs have been achieved by the use of prefabricated pumping stations. Since in many cases these stations are less costly than on-the-site construction and equipment installation, it has become more economical to use more lift stations with shallower sewer trench cuts, rather than fewer lift stations with deeper cuts.

A number of manufacturers supply these stations which basically consist of a standard pump chamber with an entrance tube. (See fig. 6.) These stations are built of epoxy resin coated steel and are factory assembled to include pumping equipment, an electrical panel, starting equipment and control switches, lights, a ladder, a sump pump, a blower and dehumidifier, cathodic protection, and the necessary pipe and fittings to the outside of the chamber. Optional equipment includes elevators for access to the station. These stations normally are equipped with two pumps, however, it is possible to install three or four. These pumps may be either close-coupled nonclog sewage and trash pumps, or pneumatic ejectors. The unit is delivered to the job site and placed on a prepared concrete slab. After piping and wiring connections are made and earth is backfilled around the unit, it is ready to operate.

Among the newer developments in pumping equipment for sewage and industrial wastes are a torque-flow pump which has a recessed impeller mounted out of the path of flow to enable passage of solids the size of the suction pipe, a two-vane open impeller pump which is self-priming to 15-foot lifts, and a number of completely submersible pumps so designed that the entire pump and motor may be submersed in the sewage.

Among the advantages of variable speed pumping are (1) it normally allows a lesser number of pumps to be installed, and (2) the size and depth of the wet well required can be reduced. A variable speed pump control, which is essentially a water rheostat, has been developed. The drive motor (wound rotor type) speed is controlled by the use of a series of stainless steel plate resistors in a two-compartment container. These resistors are wired into a circuit with the wound rotor motor. The submergence of the plates is determined by the volume of sewage to be pumped, and the design of the shape of these resistor plates is such that the pump motor operates at a speed which will cause the pump to discharge at a rate matching the flow.

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