As the name would suggest, above ground septic tanks differ from traditional septic tanks as they are installed and used above the ground’s surface. They are not built for or designed for an effluent leech field and drainage system like underground septic tanks. Rather, aboveground septic tanks are wastewater holding tanks only. They are primarily used to collect and hold sewage derived from human waste, known as blackwater, from toilets, and greywater from sinks and showers until it is time for them to be pumped and serviced.
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Above ground septic tanks are small-to-mid-range capacity waste holding tanks designed to hookup to a plumbing system for receiving wastewater in temporary setups or locations unsuitable for underground septic tanks. Some models come prepared for direct hookup with piping, couplers and clamps. Other types require the buyer to prepare and install tank fittings. Most aboveground septic tanks come with at least 1 threaded lid, (i.e., manway) and 2 ports for plumbing hookups. Some models can have up to 4 ports. More ports allow more inlet and outlet connections to be made and are ideal for multi-unit applications where multiple tanks are plumbed together in succession to increase capacity and extend service frequency.
Above ground septic tanks are often used in areas without municipal sewer lines. Examples include construction sites, temporary job sites, job shacks, agricultural settings, remote homes, trailers, cottages, cabins, hunting posts, campgrounds and RV locations without other hookups or facilities. Aboveground septic tanks have several names that the containers are known by. Their different names also serve as an indication of common application uses. Above ground septic tanks are known as waste holding tanks, job trailer and job shack waste tanks, cottage, trailer, camper and motorhome septic tanks.
Aboveground septic tanks are made and available in holding volume capacity that ranges from 250 gallons up to gallons. The limited capacity range keeps the tanks smaller in size, making them easier to handle, transport and install. Aboveground septic tanks are used until the tank reaches capacity and then must be emptied. They are often emptied when no more than 2/3 full as it is not recommended to allow an aboveground septic tank to become completely full.
When at max capacity, the tank must be serviced. A septic tank pumping service is usually called to have the holding tank pumped just like an underground septic tank. This is due to how heavy the tank will be at full capacity. In example, a 2/3 full 250 gallon holding tank will weigh well over 1,000 pounds. If maintenance is performed early, a used holding tank may be able to be hoisted or lifted and placed in a truck bed or trailer for transport to a dumping site.
Above ground septic tanks are opaque, often black, tan or yellow in color. They do not feature exterior gallon indicators. To determine if a holding tank is full and it is time to empty, above ground septic tanks must be visually inspected. Tanks come fitted with manways for this to be done.
Above ground septic tanks are manufactured with a low profile design so they can be placed beneath structures such as job trailers, decks and general crawlspaces. Their low profile design also improves access for installing hookups and performing maintenance. Septic handling tanks are molded into a rectangular, wide-area shape that helps distribute the tank’s weight, making it easier to maneuver and allows most models to be moved with just a few people.
Available above ground waste holding tanks are manufactured from quality high-density polyethylene (HDPE) resin. Although made from premium polyethylene plastic, they are not US FDA certified for use with potable (drinking) water and should not be used for drinking water applications. The HDPE construction of above ground septic tanks produces a heavy duty container that does not rust, resists weathering, resists corrosion from sewage gasses and chemicals, and is durable against punctures, drops and impacts. The tanks are engineered for above ground installation and use and should not be buried.
HDPE holding tanks are also fabricated with ultraviolet UV inhibitor compounds that stabilize the plastic against damage and deterioration that exposure to sunlight would otherwise occur. UV blocking compounds approve aboveground holding tanks for outdoor use. UV blockers increase outside service life, allowing the waste tanks to be installed in locations that are not completely away from sun exposure. However, due to the sewage waste these holding tanks are used for, isolating them from sun exposure and heating is often preferred.
The design and size of a septic system can vary widely, from within your neighborhood to across the country, due to a combination of factors. These factors include household size, soil type, site slope, lot size, proximity to sensitive water bodies, weather conditions, or even local regulations. Below are ten of the most common types of septic systems used, followed by illustrations and descriptions of each system. The list is not all-inclusive; there are many other types of septic systems.
See below for illustrations and descriptions of various types of conventional and alternative septic systems. More detailed information on specific technologies can be found in Fact Sheets.
Conventional Systems:
Alternative Systems:
A septic tank is a buried, watertight tank designated and constructed to receive and partially treat raw domestic sanitary wastewater. Heavy solids settle to the bottom of the tank while greases and lighter solids float to the top. The solids stay in the tank while the wastewater is discharged to the drainfield for further treatment and dispersal.
A conventional decentralized wastewater treatment system consists of a septic tank and a trench or bed subsurface wastewater infiltration system, known as a drainfield. A conventional septic system is typically installed at a single-family home or small business.
The gravel/stone drainfield is a design that has existed for decades. The effluent is piped from the septic tank to a shallow underground trench of stone or gravel. A geofabric or similar material is then placed on top of the trench so sand, dirt, and other contaminants do not enter the clean stone.
Effluent filters through the stone and is then further treated by microbes once it reaches the soil below the gravel/stone trench.
Gravel/stone systems are relatively large in overall footprint and may not be suitable for all residential sites or conditions.
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Gravelless drainfields have been widely used for over 30 years in many states and have become a conventional technology replacing gravel systems. They take many forms, including open-bottom chambers, fabric-wrapped pipe, and synthetic materials such as expanded polystyrene media. The gravelless systems can be manufactured with recycled materials and offer a significant savings in carbon footprint.
An example of a gravelless system is the chamber system. The chamber system serves as an alternative design to the gravel/stone system. The primary advantage of the chamber system is increased ease of delivery and construction. They are also well suited to areas with high groundwater tables, where the volume of influent to the septic system is variable (e.g., at a vacation home or seasonal inn), in an area where gravel is scarce, or in areas where other technologies such as plastic chambers are readily available.
This type of system consists of a series of connected chambers. The area around and above the chambers is filled with soil. Pipes carry wastewater from the septic tank to the chambers. Inside the chambers, the wastewater comes into contact with the soil. Microbes on or near the soil treat the effluent.
The drip distribution system is a type of effluent dispersal that can be used in many types of drainfields. The main advantage of the drip distribution system is that no large mound of soil is needed as the drip laterals are inserted into the top 6 to 12 inches of soil. The disadvantage of the drip distribution system is that it requires a large dose tank after the septic tank to accommodate the timed dose delivery of wastewater to the drip absorption area. Additional components, such as electrical power, are necessary for this system, requiring an added expense and increased maintenance.
Aerobic Treatment Units (ATUs) use many of the same processes as a municipal sewage plant, but on a smaller scale. An aerobic system injects oxygen into the treatment tank. The additional oxygen increases natural bacterial activity within the system that then provides additional treatment for nutrients in the effluent. Some aerobic systems may also have a pretreatment tank and a final treatment tank including disinfection to further reduce pathogen levels.
The benefits of this system are that it can be used in homes with smaller lots, inadequate soil conditions, in areas where the water table is too high, or for homes close to a surface water body sensitive to contamination by nutrients contained in wastewater effluent. Regular life-time maintenance should be expected for ATUs.
Mound systems are an option in areas of shallow soil depth, high groundwater, or shallow bedrock. The constructed sand mound contains a drainfield trench. Effluent from the septic tank flows to a pump chamber where it is pumped to the mound in prescribed doses. Treatment of the effluent occurs as it discharges to the trench and filters through the sand, and then disperses into the native soil.
While mound systems can be a good solution for certain soil conditions, they require a substantial amount of space and periodic maintenance.
Sand filter systems can be constructed above or below ground. Effluent flows from the septic tank to a pump chamber. It is then pumped to the sand filter. The sand filter is often PVC-lined or a concrete box filled with a sand material. Effluent is pumped under low pressure through the pipes at the top of the filter. The effluent leaves the pipes and is treated as it filters through the sand. The treated wastewater is then discharged to the drainfield.
Sand filters provide a high level of treatment for nutrients and are good for sites with high water tables or that are close to water bodies, but they are more expensive than a conventional septic system.
Evapotranspiration systems have unique drainfields. The base of the evapotranspiration system drainfield is lined with a watertight material. After the effluent enters the drainfield, it evaporates into the air. Unlike other septic system designs, the effluent never filters to the soil and never reaches groundwater.
Evapotranspiration systems are only useful in specific environmental conditions. The climate must be arid and have adequate heat and sunlight. These systems work well in shallow soil; however, they are at risk of failure if it rains or snows too much.
A constructed wetland mimics the treatment processes that occur in natural wetlands. Wastewater flows from the septic tank and enters the wetland cell. The wastewater then passes through the media and is treated by microbes, plants, and other media that remove pathogens and nutrients. The wetland cell typically consists of an impermeable liner, and gravel and sand fill, along with the appropriate wetland plants, which must be able to survive in a perpetually saturated environment.
A wetland system can work via either gravity flow or pressure distribution. As wastewater flows through the wetland, it may exit the wetland and flow into a drainfield for further wastewater treatment into the soil.
A cluster (or community) decentralized wastewater treatment system is under some form of common ownership and collects wastewater from two or more dwellings or buildings. It conveys the wastewater to a treatment and dispersal system located on a suitable site near the dwellings or buildings. It is common to find cluster systems in places like rural subdivisions.
Within a region, county, city, town, or neighborhood, the systems used to treat wastewater can vary significantly and may include both centralized and decentralized treatment options. The most appropriate system is selected based on factors such as cost, land availability, population density, and environmental conditions. In this illustrated example of a town, several types of decentralized systems are shown: a conventional septic system with a drainfield serving a single home, an onsite cluster system serving a commercial shopping center, and an offsite cluster system serving multiple houses and an apartment building.
The table below the graphic describes each of the four examples of decentralized and centralized wastewater treatment.
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