United States Environmental Protection Agency Research and Development Water Engineering Research Laboratory Cincinnati OH 45268 EPA/600/S2-86/022 May 1986 c/EPA Project Summary Small Diameter Gravity Sewers: An Alternative for Unsewered Communities Richard J. Otis A 3-year laboratory and field study was undertaken to evaluate small di- ameter gravity sewers as a wastewater collection alternative to conventional sewers. The objective of the study was to develop design criteria that would minimize costs and construction and yet maintain reliable, trouble-free oper- ation. The study included a review of the design, construction, operation, and costs of operating systems, de- tailed characterization of the small di- ameter gravity system in Westboro, Wisconsin, and a laboratory investiga- tion to develop hydraulic design crite- ria. Study results indicate that small di- ameter gravity sewers or effluent drains are a viable and less costly alter- native to conventional gravity sewers in unsewered areas. This Project Summary was devel- oped by EPA's Water Engineering Re- search Laboratory, Cincinnati, OH, to announce 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 Many unsewered communities are facing severe financial hardships in providing proper wastewater facilities for their citizens because of the high costs of conventional sewerage. The most costly component is the installa- tion of the wastewater collection sys- tem, which can represent as much as 90 percent of the total capital costs of the facilities. To reduce these costs, communities have pursued develop- ment of less costly collection alterna- tives such as pressure and vacuum sew- ers. The most recently introduced collec- tion alternative is the small diameter gravity (SDG) sewer. Unlike conven- tional sewers, SDG sewers only collect the liquid portion of the wastewater. Grit, grease, and other troublesome solids that might obstruct the sewer are separated from the flow in interceptor tanks upstream of each connection. Thus, the hydraulic design need not be based on water carriage of solids. This feature could permit significant cost savings in construction. A field and laboratory study was un- dertaken to assess the potential of efflu- ent drains as a less costly alternative to conventional sewers in small communi- ties and urban fringe developments. The objectives of this study were to: • Review current effluent drain de- sign practices, • Evaluate unit costs of construction for effluent drain components, • Monitor and characterize an operat- ing system, • Evaluate the effects of drain diame- ter, flow velocity, and depressed drain sections on the operating characteristics, and • Develop design guidelines and maintenance procedures. Results A review of 10 operating systems in the United States revealed that the ex- perience with SDG sewers has been very favorable. Construction cost sav- ings are estimated to range from 0 to 50 percent over conventional gravity ------- sewers. The use of fewer and smaller appurtenances were possible to facili- tate maintenance by less skilled person- nel and to reduce replacement costs. At the treatment plants, headworks and primary sedimentation were eliminated or reduced in size. Reported problems with the operation of SDG sewers were few and easily corrected. The most fre- quently reported disadvantages was the need to enter private property to install and maintain the interceptor tanks. No significant inherent problems have appeared with SDG sewers. Ob- structions have not occurred in any of the systems, despite the lack of routine cleaning. Slime growths do adhere to the pipe, but they slough off continu- ously and are easily carried by the flow. Odors can be a problem where turbu- lent flow occurs, such as in lift stations or in lines on steep gradients; but odor control measures have been effective. Building sewers that are in poor condi- tion and cracked interceptor tanks with poorly fitting covers are a major poten- tial source of infiltration and grit. The use of existing tanks should be carefully considered, and new tanks should be thoroughly inspected. Routine maintenance needs of SDG sewers appear to be fewer than those for conventional sewers. Because SDG sewers do not carry larger solids and other debris, lift stations and conduits do not need regular cleaning. Only the interceptor tanks require regular clean- ing. If cleaning of the SDG sewer be- comes necessary, mechanical equip- ment or hydraulic jetting equipment is appropriate. Simple flushing with clear water is very effective. Flush volumes of 4 to 5 pipe volumes at a sufficient head to create a 1.5- to 2-ft/sec flow velocity is sufficient. Such volume can be supplied by a fire tank truck. The costs of SDG sewer construction were significantly less than those of conventional gravity sewer construc- tion. Bid costs from the projects re- viewed averaged $39.92/ft of drain in- stalled and $4,335 per connection (May 1982 dollars). These costs include ser- vice connections but exclude all non-construction costs. The national av- erage for conventional gravity sewer construction (adjusted to May 1982) is $46.68/ft, which excludes service connections but includes all non- construction costs. Installed pipe and interceptor tank installation accounted for more than 50 percent of the effluent drain construction costs. Recommended Design Guidelines Based on this work, design criteria were recommended for SDG sewers and appurtenances (Tables 1 and 2). The full report was submitted in par tial fulfillment of Contract No. 68-03 3057 by Rural Systems Engineering Inc., under the sponsorship of the U.S Environmental Protection Agency. Table 1. Recommended Design Guidelines for SDG Sewers Parameter Recommendation Comments Design flow Pipe diameter Open channel flow Pressure conduit flow Slope Velocity Open channel flow Pressure conduit flow Pipe roughness coefficient Manning's m Hazen-Williams Cn Alignment 1.0 gpm/connection 2 in. minimum 4 in. minimum No minimum No minimum 0.5 fps minimum 0.015 100 Curvillinear May be reduced if "forced storage" is pro- vided. No connection may be larger in diameter than the collector pipe. Inflection gradients are permitted, but overall gradient must be suffi- cient to carry design flow. Horizontal and vertical planes Table 2. Recommended Appurtenances for SDG Sewers Appurtenance Recommendation Comments Interceptor tank size Inlet/outlet Material Access Cleanouts Manholes Lift stations 1,000 gal minimum Baffled with 0.25 ft minimum drop across tank Concrete Manhole over inlet with watertight cover to grade At junctions, termini, high points, major changes in direction, and regular intervals Major junctions and inspection points Drop inlets and air vent to buried gravel bed for odor control Liquid surface area should be 40 ft2 minimum. Outlet is larger than pipe diameter. Must be watertight and structurally sound to with- stand anticipated loads. May be buried if necessary to prevent tampering. Because of their low cost, liberal use is suggested. Aids in locating pipe. Use should be limited and covers should be water- tight. Large solids-handling sewage pumps are not re- quired. ------- Richard J. Otis is with RSE Group. Madison, Wl 53706. James F. Kreissl is the EPA Project Officer (see below). The complete report, entitled "Small Diameter Gravity Sewers: An Alternative for Unsewered Communities." (Order No. PB 86-167 335/AS; Cost: $16.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Water Engineering Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 BULK RATE POSTAGE & FEES EPA PERMIT No G-: Official Business Penalty for Private Use $300 EPA/600/S2-86/022 0169064 Mr"6"0" • «0«0* ------- |