LIQUID-SOLID SEPARATION

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1 Fact Sheet T1 Performance & Cost for Decentralized Unit Processes TREATMENT SERIES DECENTRALIZED WASTEWATER SYSTEMS What is Liquid-Solid Separation? Liquid-solid separation is typically the first unit process used in a wastewater system. As the name suggests, the primary purpose is to separate liquid wastewater from the non-liquid waste constituents. In individual onsite systems, liquid-solid separation is provided by a septic tank. Because of greater flows and the multitude of inappropriate materials that get flushed down the drain, municipal systems have not just one but a series of processes that separate liquid wastewater from non-liquid waste products. Many wastewater professionals refer to liquid-solid separation as primary treatment because it is the first process in converting wastewater back into water. This factsheet will focus on liquid-solid separation technologies that are appropriate for residential and small community wastewater management systems. For all intents and purposes, liquid-solid separation occurs in a tank that is configured and sized to accept the wastewater flow and retain it for a sufficient amount of time for the process to occur. For this factsheet, tanks used for liquid-solid separation will be termed primary tanks when serving a community and septic tanks when they serve an individual residence or other building. A special tank known as an Imhoff tank may also be used. Although it is designed differently from the septic tanks and primary tanks discussed here, its function is the same. Liquid-solid separation is an essential treatment component whether the wastewater management system is serving one home or a whole city. In rural housing, satisfactory liquid-solid separation occurs in a septic tank. Illustration of similarities between one of the first septic tanks (Mouras 1883 There are several options for clustered housing developments version, Top) and a modern version and small communities. One option may be to include a septic (Bottom). The significance of liquid-solid separation has been recognized for tank at each house and transfer only effluent to a common centuries.

2 2 treatment location. This arrangement is known as an effluent sewer system. Alternately, the raw For more information, see: wastewater (solids included) may be collected and Fact Sheet C1: Gravity Sewer Systems conveyed to a common primary tank for liquid-solid Fact Sheet C2: Pressure Sewer Systems separation via traditional gravity sewers, low Fact Sheet C3: Effluent Sewer Systems pressure sewers, or vacuum sewers. These four Fact Sheet C4: Vacuum Sewer Systems Collection system options are described in other Fact Sheets included in this series. Sedimentation and flotation are the primary processes that occur during liquid-solid separation. When raw wastewater enters a primary or septic tank, non-liquid waste constituents will settle or rise depending on their density. The floating layer is called the scum layer and the settled solids form the sludge layer. A clarified effluent zone develops between the two layers of solids. The tank outlet is designed to draw effluent from the clarified zone. This separation technology can reduce the solids content by 60 to 80%. Because much of the material captured in the tank is organic, approximately 50% of the organic load is removed by during liquid-solid separation. Effluent from primary tanks and septic tanks typically contains 140 to 220 mg/l BOD 5, 45 to 70 mg/l TSS, and mg/l FOG. The performance of primary treatment components determines the nature (and performance) of subsequent components used in a treatment system. Septic tanks used today typically include an effluent screen installed in the outlet end of the tank. The screen is designed to capture solids that may still be suspended in the effluent as it exits the tank. There are many different proprietary screens available in the market today and most are designed to capture solids in the range of 1/32 to 1/16 inch in diameter. Tanks fitted with effluent screens must have an access at or near the finished grade to allow a service provider to remove and clean the screen on a regular basis. Effluent screens must be accessible for maintenance. The tank must be large enough to retain the wastewater in a relatively quiet state for two or more days to allow settling and flotation to occur. This concept is known as detention time and is an important design consideration. Excessive flow creates turbulence that can disrupt the settling process. Thus, tank volume, size, shape, and inlet baffle configuration are each designed to minimize turbulence and prevent the migration of solids. Accumulated solids are stored in the tank until they are periodically removed. Septic tanks and primary tanks are pumped when solids occupy approximately 40% of the tank s volume. The removed

3 3 materials are residuals known as septage. See the Fact Sheet on Residuals Management for further information on the management of this material. For more information, see: Fact Sheet D7: Residuals Management Compatibility with the Community Vision Having liquid-solid separation as a treatment component is not an option; however, the location where liquidsolid separation is accomplished can vary. Use of septic tanks dispersal option must be Selection of any wastewater in conjunction with individual, cluster development or considered within the context community systems is a viable option and septic tanks can be of a community s broad, longrange plans for land use. effectively used with all types of collection systems. Since they Changes in development can be installed to accommodate single or multiple patterns, population density, connections, their use does not inhibit increasing wastewater livability, and delivery of volume. services will occur as a result If an existing community is faced with a significant of the choices made and these number of soil-based dispersal system failures, a potential must all be taken into account. solution is to collect the wastewater and combine the individual dispersal components into a community system. Wastewater that has already undergone liquid-solid separation is easier and less costly to convey. If liquid-solid separation takes place at the individual sites, final treatment and dispersal can occur at a central location. The degree of treatment needed prior to dispersal depends on the limitations of the dispersal site. Infrastructure and access for maintenance and management of residuals (solids retained in the septic tanks) must always be part of the consideration. Land Area Requirements Tanks are typically sized to provide accommodate twice the expected daily volume of wastewater or two days of detention time. A onethousand gallon tank (suitable for daily wastewater volumes up to up to 500 gallons per day) typically measures about 4 feet wide by 8 feet long. Larger volumes obviously require larger tanks and occupy more space.

4 4 Construction and Installation Primary tanks and septic tanks are installed below ground. Prefabricated tanks are available and may be constructed of concrete, fiberglass or plastic. Larger tanks may be built in place using concrete. Independent of the material of construction, tanks must meet appropriate strength requirements to withstand the exterior soil pressures and interior liquid pressures. It is important that they are constructed of high quality materials so that they remain structurally sound and watertight. Excavations for modular tanks must be performed in accordance with applicable safety regulations. Workers must not enter excavations that may be subject to cave-in unless appropriate stabilization measures are taken. Proper bedding and backfilling procedures must be used to ensure a level and stable installation. In areas where shallow groundwater is present, tanks must be installed to prevent flotation. All tanks must have flexible, watertight seals at all locations where pipes enter and exit and a cast-in-place or mechanically-attached access riser to grade with tight fitting lid. Operation and Maintenance Stored solids (septage or residuals) must be removed on a regular basis. The removal (pumping) frequency is determined by the level of use by the source. Service providers must have knowledge and skills needed to measure depth of sludge and scum to determine when tanks need pumping. A properly operated and installed primary tank or septic tank should have no chemical requirements (i.e., additives).

5 5 The size and depth of the tank are a significant safety concern for the service provider. Gases, such as hydrogen sulfide, methane, and carbon Regular service is important for all systems to ensure best long term performance to protect public dioxide, result from anaerobic (without oxygen) digestion that occurs in the tank. These gases health and the environment. This also protects the investment. Frequency of operation and maintenance is dependent upon wastewater volume, rela- create a hazardous and corrosive environment. tive risk to public health and the environment as well Tanks are considered confined space and must as the complexity of any pretreatment components never be entered without the proper training and used prior to dispersal. equipment. Costs for Liquid-solid Separation Primary tanks and septic tanks do not require power gravity is the primary source of energy. The exceptions are when a tank level alarm is included and/or when a pump is installed to convey the effluent to the next component. Costs for septic tanks depend upon a variety of factors including subsurface site conditions, location of and access to the site, and the type of tank. Deeper installations require stronger construction and will be more expensive, as will tanks installed where vehicular traffic is expected. Table 1 is a cost estimation for the materials, installation, and maintenance of a residential septic tank. These costs assume that the topography is relatively flat, the contractor would charge 20% for overhead and profit, and there are no sales taxes on materials. Engineering fees and other professional services are not included in the costs. The size of the tank is based on two days of detention storage. Maintenance costs were based on a part time service provider and the cost of septage removal. Septage removal was estimated at $360 per 1,000 gallons. Table 1. Estimated cost to install and maintain a septic tank at a single-family residence. Materials and installation 1,000 gallon tank, delivery, and connections $2,800 $4,200 Annual electricity ($0.15 per kw-hr) Assumes no pump -0- Annual O&M 60-yr life cycle cost present value (2009 dollars) Septage removal every 7 yrs and service provider cost Based on maintenance assumes tank will last 60 years $70 - $110 $5,400 - $8,000

6 6 Table 2 estimates the cost of a primary treatment system for three sizes of communities 5,000, 10,000 and 50,000 gpd. For this example, it was assumed that tankage is being used to provide liquid/ solid separation. The tank volume is based on two times the daily flow, and the installation contractor would charge 20% for overhead and profit. Engineering and other fees are not included in the costs. The maintenance cost is based on a part-time service provider, a 5-year septage removal cycle, and that the tank will last for 60 years. Table 2. Estimated cost for a community-scale tank for liquid/solid separation. Daily Wastewater Volume (gpd) 5,000 gpd or 20 homes 10,000 gpd or 40 homes 50,000 gpd or 200 homes Materials and Installation Annual Electricity ($0.15 per kw-hr) $31,200 - $47,000 $62,000 - $94,000 $312,000 - $468, Annual O&M $1,000 - $1,500 $2,000 - $3,000 $10,000 - $15, year life cycle cost present value (2009 dollars) $66,000 - $98,000 $313,000 - $197,000 $656,000 - $984,000 The costs provided in this document are for comparison purposes only. The actual cost for a collection system will vary significantly depending on site conditions and local economics. For localized cost investigations, consult the Cost Estimation Tool associated with these materials. References 1. CIDWT Installation of Wastewater Treatment Systems. Consortium of Institutes for Decentralized Wastewater Treatment. Iowa State University, Midwest Plan Service. Ames, IA. 2. Crites, R., and G. Tchobanoglous Small and Decentralized Wastewater Management Systems. McGraw -Hill, Boston, USA.

7 7 3. Loudon, T.L., T.R. Bounds, J.C. Converse, T. Konsler and C. Rock Septic Tanks Text in (D.L. Lindbo and N.E. Deal eds.) Model Decentralized Wastewater Practitioner Curriculum. National Decentralized Water Resources Capacity Development Project. North Carolina State University, Raleigh, NC. 4. Onsite Sewage Treatment Program, University of Minnesota Manual for Septic System Professionals in Minnesota. St. Paul, MN. 4. RSMeans Building Construction Cost Data, 67 th Annual Edition. Construction Publishers and Consultants, Kingston, MA, USA. 5. Seabloom, R.W., T.R. Bounds, and T.L. Loudon Septic Tanks Text. in (M.A. Gross and N.E. Deal, eds.) University Curriculum Development for Decentralized Wastewater Management. National Decentralized Water Resources Capacity Development Project. University of Arkansas, Fayetteville, AR. 6. U.S. EPA. Onsite Wastewater Treatment Systems Manual. EPA 625-R Office of Water, Washington, D.C. This Fact Sheet was prepared by members of the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT), including: John R. Buchanan, PhD, PE University of Tennessee Nancy E. Deal, MS, REHS NC State University David L. Lindbo, PhD, CPSS NC State University Adrian T. Hanson, PhD, PE New Mexico State University David Gustafson, PE University of Minnesota Randall J. Miles, PhD University of Missouri These materials were reviewed by the WERF Project Subcommittee including: Tom Groves NE Interstate Water Pollution Control Commission Mike Hines Southeast Environmental Engineering Jim Kreissl Environmental Consultant Jack Miniclier Charles City County Barbara Rich Consultant Eberhard Roeder Florida Department of Health Larry Stephens Stephens Consulting Services Jeff Moeller WERF Senior Program Director 4/10 Water Environment Research Foundation 635 Slaters Lane, Suite G-110 Alexandria, VA