5 | T1 Liquid-Solid Separation v2.3
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. There are several options for clustered housing developments and small communities. One option may be to include a septic tank at each house and transfer only effluent to a common treatment location. This arrangement is known as an effluent sewer system. Alternately, the raw wastewater (solids included) may be collected and conveyed to a common primary tank for liquid-solid separation via traditional gravity sewers, low pressure sewers, or vacuum sewers. These four 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 BOD5, 45 to 70 mg/L TSS, and 10-30 mg/L FOG. The performance of primary treatment components determines the nature (and performance) of subsequent components used in a treatment system.
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 materials are residuals known as septage. See the Fact Sheet on Residuals Management for further information on the management of this material.
Compatibility with the Community Vision
Having liquid-solid separation as a treatment component is not an option; however, the location where liquid-solid separation is accomplished can vary. Use of septic tanks in conjunction with individual, cluster development or community systems is a viable option and septic tanks can be effectively used with all types of collection systems. Since they can be installed to accommodate single or multiple connections, their use does not inhibit increasing wastewater volume.
If an existing community is faced with a significant number of soil-based dispersal system failures, a potential 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 one-thousand 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.
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).
The size and depth of the tank are a significant safety concern for the service provider. Gases, such as hydrogen sulfide, methane, and carbon dioxide, result from anaerobic (without oxygen) digestion that occurs in the tank. These gases create a hazardous and corrosive environment. Tanks are considered confined space and must never be entered without the proper training and 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.
The prices give costs included n in this document are for comparison purposes only. The actual cost for Liquid-solid Separation will vary significantly depending on site conditions and local economics. For detailed cost investigations, consult the Cost Estimation Tool associated with these materials.
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
Electricity ($0.15 per kW-hr) / Assumes no pump / -0-
O&M / Septage removal every 7 yrs and service provider cost / $70 - $110
60-yr life cycle cost present value
(2009 dollars) / Based on maintenance – assumes tank will last 60 years / $5,400 - $8,000
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 / $31,200 - $47,000 / $62,000 - $94,000 / $312,000 - $468,000
Electricity ($0.15 per kW-hr) / -0- / -0- / -0-
O&M / $1,000 - $1,500 / $2,000 - $3,000 / $10,000 - $15,000
60 year life cycle cost present value
(2009 dollars) / $66,000 - $98,000 / $313,000 - $197,000 / $656,000 - $984,000
References
1. Onsite Sewage Treatment Program, University of Minnesota. 2009. Manual for Septic System Professionals in Minnesota. St. Paul, MN.
2. Crites, R., and G. Tchobanoglous. 1998. Small and Decentralized Wastewater Management Systems. McGraw-Hill, Boston, USA.
3. U.S. EPA. Onsite Wastewater Treatment Systems Manual. EPA 625-R-00-008. Office of Water, Washington, D.C.
4. RSMeans. 2008. Building Construction Cost Data, 67th Annual Edition. Construction Publishers and Consultants, Kingston, MA, USA.