United States Environmental Protection Agency Water Engineering Research Laboratory Cincinnati OH 45268 Research and Development EPA/600/S2-85/133 May 1986 &EPA Project Summary Alternative Sewer Studies This report provides new design and operational information on two of the most effective and widely applied alter- native sewer systems—small-diameter gravity and pressure sewers. The infor- mation provided here will help system designers and operators avoid or rectify problems resulting from sulfides and downhill hydraulics that could otherwise represent major impairments to the successful application of these technologies. This Project Summary was developed by EPA's Water Engineering Research Laboratory, Cincinnati. OH. to an- nounce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction Small communities in need of new or expanded sanitary sewers are faced with a severe financial burden. Low popula- tion densities and unfavorable geological situations increase per capita costs of conventional sewers, which often ac- count for up to 80% of the total capital costs of a new wastewater management system. Conventional sewers are expensive. To insure that raw sewage flows freely, conventional sewer systems use large- diameter pipes set in the ground at minimum slopes. Pumping stations are often required as well. Extensive exca- vation is usually necessary to achievethe desired slopes. Flat terrain, high ground- water, and waterfront areas all add to construction costs and difficulties. Finally, infiltration and inflow (l&l) of extraneous water cannot be eliminated entirely in large pipes. The added wastewater vol- ume and solids mean that the treatment plant must have a greater capacity than would be required to treat only the dry weather flow. Alternative approaches to sewering that address some of the problems en- countered with conventional systems can reduce collection and treatment costs. Three types of alternative sewers are discussed below: • pressure sewers • vacuum sewers • small-diameter gravity (SDG) sewers Discussion Alternative sewers offer the dual ad- vantages over conventional sewers of small-diameter pipes and a greater ability to follow the natural topography without risk of clogging, which reduces excavation and construction costs. Furthermore, all three of these sewers provide reduced I/I. The two major types of pressure sewer systems are grinder pump(GP) and septic tank effluent pump (STEP). These two systems differ in the onsite equipment, layout, and quality of the wastewater conveyed to the pressure sewer. In GPs, solids are ground to a slurry and dis- chargedthrough pressure lines. In STEPs, wastewater from a home first flows into a septic tank from which treated effluent is pumped to pressurized lines. Vacuum sewers use a central vacuum source to constantly maintain a vacuum on small-diameter collection mains. Per- iodically, the pressure differential created by the vacuum source draws a slug of sewage from a holding tank at each home into the line. When sufficient volume of sewage is collected at a central vacuum station, it is pumped to the treatment plant or main interceptor. Like the STEP system, a small-diameter gravity (SDG) sewer is used with indi- vidual septic tanks. Because solids are removed by the septic tank, pipes of 4 in. in diameter can be used at very shallow slopes without risk of clogging. The effluent requires little or no pumping, generally flowing by gravity to the treat- ment facility. Despitetheir many advantages, several concerns have been raised about alter- native sewers. The most important of these potential problems are: ------- • Excess sulfide generation, and • Two-phase flow in pressure and SDG sewers Sulfide generation affects all types of sewers. The problem manifests itself in unpleasant odors and corrosion produced by hydrogen sulfide. Years of experience have gone in to the design of conventional sewer systems to minimize sulfide gen- eration. Experience with sulfides in con- ventional sewer systems has raised two main concerns a bout alternative systems: (1) That the septic effluent in SDG and STEP systems may be more prone to sulfide generation, and (2) that the an- aerobic nature of the pressure and the SDG sewers may contribute to sulfide generation. Two-phase flow refers to a hydraulic problem of particular concern in pressure systems. In downhill sloping sections of pressure sewers, gas bubbles present in the pipeline can adversely affect flow. The typical solution is to install air-release valves at summits within the pipeline. However, in many cases this technique does not work efficiently, and additional steps must be taken to sol vet he problem. Conclusions Although a significant number of pres- sure and SDG sewers have been de- signed and constructed, there remain some significant gaps in understanding the technology. These studies provide some insights into two of the major technology gaps. The major conclusions are as follows: 1. GP systems can produce sulfides at a rate of three to four times that of STEP systems and about twice that of conventional sewer force mains because of the high organic strength of the wastewater. 2. STEP systems show unexplained losses of sulfide and gains in dis- solved oxygen based on analyses performed in this study and prev- ious data for septic tank effluents. 3. Both pressure systems (i.e., GP and STEP) can be expected to have some sulfide con cent ration in their waste- waters, with values varying from 1 to 1 4 mg/L based on this study. 4. GP sulfide concent rat ions will gen- erally increase in the direction of mainline flow, but random locations of service lines and branches may mask this trend 2 5. Concentrations of sulfides in pres- sure sewers cannot yet be quanti- fiably predicted because of the empirical nature of the available equations and their derivation from weaker conventional wastewaters. 6. SDG sewers should not be designed to minimize pipe sizes, to flow full for substantial periods, or to pro- liferate substantial inundated sec- tions of mainline if sulfide mini- mization is desired. 7. For conventional gravity sewers, equilibrium sulfide concentrations result from long pipe segments of relatively uniform conditions. Ap- plying the equilibrium equation for conventional gravity sewers to SDG sewers results in concentrations comparable with those observed in SDG sewers. These concentrations are much lower than the higher levels reported to occur in septic tanks. 8. Because of the phenomenon of conclusion No. 7, SDG sewers appear to be capable of producing terminal wastewater sulfide con- centrations lower than those of pressure sewers. 9. Conventional placement of air-re- lease valves at high points of a pressure sewer system does not preclude the entrainment of air, which results in headlosses greatly exceeding design calculations. 10. In downhill runs where the pres- sure main intersects the dynamic hydraulic grade line (HGL), a hy- draulic jump is formed that gen- erates gas bubbles that pass on to downstream segments of the main. 11. Placement of sewage-type auto- matic air-re I ease valves at points at least 14 pipe diameters below hy- draulic jump locations was effective in removing entrapped air and reducing headlosses to near theo- retical levels. 12. Backpressure sustaining valves were found to be inadequate for control of downhill hydraulics in the pressure sewer because of high capital cost, intensive maintenance requirements, and unreliable opera- tion. 13. On downhill runs with irregular terrain that provide numerous op- portunities for the formation of smaller hydraulic jumps, standpipes were shown to be inexpensive and reliable. The standpipes used large- diameter downlegs to prevent the escape and conveyance of air bub- bles into the downstream segment of the mains and automatic air- release valves at their summits to expel the trapped gases. 1 4. Soil absorption beds were success- fully used for the vented gases from the air-release valves to prevent hydrogen sulfide odors. Recommendations The designer needs improved capability to predict sulfide concentrations in pres- sure and SDG sewers. Toward this end, comprehensive studies of sulfides should be made from the septic tank to the terminus of a number of these systemsto identify the gains and losses of sulfide concentration, to quantify the mechan- isms responsible, and to develop predic- tive equations. Once this goal is accom- plished, a study should be undertaken to develop a corrosion-based methodology for evaluating the alternatives of design- ing a transition station or modifying a receiving conventional sewer. Such a methodology would provide a quantitative solution to one of the major design obstacles in the design of pressure and SDG sewers that terminate at larger conventional sewers. A need also exists for more quantitative assessment of the requirements for, location of, and design and operating and maintenance requirements for air-re- lease valves in pressure and SDG sewer systems. Further assessment is also needed for soil absorption and other low- maintenance odor control methods ap- propriate for these alternative sewers. The full report was submitted in partial fulfillment of Contract No. 68-03-3057 by Urban Systems Research and Engineer- ing, Inc., under the sponsorship of the U.S. Environmental Protection Agency. ------- |