ON-SITE INDIVIDUAL SEWAGE TREATMENT (OSST)

On-Site Sewage Treatment

Individual sewage treatment systems are used in areas not served by a community wastewater treatment plant. In the United States, about one-fifth of the building stock relies on on-site individual sewage treatment systems to treat wastewater.

On-site sewage treatment (OSST) systems, traditionally called septic systems, usually consist of the building sewer, which leads from the building into a septic tank and then into a distribution box that feeds the fluid (effluent) into a drainage field or disposal field. Complex systems may contain a mound drain field system, an aerobic treatment unit, a gray water system tank, a grease interceptor, a dosing tank, and a solids or effluent pump. An OSST system can be installed beyond the building sewer on land of the owner or on another nearby site to which the owner has the legal right to install a system. OSST systems treat wastewater from rural and suburban homes, mo bile home developments, apartments, schools, retail facilities, and businesses that don’t have access to a community waste water treatment and disposal system. (See fgr.1.) An OSST system consists of a primary treatment component, such as a septic tank, and a disposal component, which is typically the drainage field. Household and human wastes flow in a pipe from the building's sanitary drainage system to a sep tic tank. Inside the septic tank, anaerobic and aerobic bacteria convert the wastes into minerals, gases, and a liquid waste called the effluent. Clarified effluent leaves the septic tank and flows in a pipe to a drainage field. If the drainage field piping is higher in elevation than the septic tank outlet, a pump tank and pump are required. The effluent is then distributed as evenly as possible throughout the drainage field where it percolates through the soil. Gravity is typically the driving force behind wastewater and effluent flow.

City and county health departments issue permits for the construction and alteration of OSST systems. These are based on an evaluation of the soil at the site. Consulting geological engineers are hired to conduct extensive soil evaluations for a particular site. State and federal soil scientists can provide assistance to health departments, agriculturalists, and others with local soil problems.

FGR.1 An on-site sewage treatment (OSST) system for a residence.

FGR.2 A rectangular septic tank is a watertight, covered container designed to settle out and hold solid wastes and partially treat wastewater with beneficial bacteria.

IMG 15.1 A precast concrete septic tank at the manufacturing plant.

( --) IMG 15.2 Lid of a two-compartment septic tank. Note the access openings and crane "picks" for lifting the lid.

( --) IMG 15.3 The interior of a three-compartment septic tank.

( --) IMG 15.4 Concrete baffles used to separate compartments in a septic tank.

( --) IMG 15.5 A 1250 gallon (4725 L) septic tank in hole before hole is backfilled. The three covers allow access to each compartment. Tank will be covered with soil.

( --) IMG 15.6 A polyethylene plastic septic tank.

Primary Treatment Equipment

Wastewater from a building is first treated in primary treatment equipment such as tanks or filters. In the primary treatment process, anaerobic digestion and settlement of solids in waste water takes place. Types of primary treatment equipment used in OSST systems include the following.

Septic Tank

The septic tank is a watertight, covered container designed to settle out and hold solid wastes and partially treat wastewater with beneficial bacteria. It represents a relatively low-cost, low maintenance method for primary treatment of wastewater. Sep tic tanks are constructed of concrete, metal, fiberglass, or plastic (fiberglass and polyethylene) and are commonly placed under ground with the top surface covered with grass. An access cover built into the top of the tank allows periodic inspection and removal of sludge and scum that collects in the tank.

A septic tank can be thought of as a separating device. It allows heavier solids to settle to the bottom of the tank and lighter particles such as grease and soap float to the top of the tank. The lighter particles form a layer known as the scum. Aerobic and anaerobic bacteria digest up to 50% of the wastewater solids that enter the septic tank. The remaining solids accumulate as sludge in the bottom of the tank. The bacteria require at least 24 hr for proper digestion of solids. The greater the surface area of the liquid in the tank, the more solids the bacteria can digest, thus improving the clarity (quality) of effluent discharged to the drainage field.

(See Fgrs 2 and 3 and Imgs 1 through 6.)

FGR.3 A steel septic tank.

A typical septic tank is divided into two compartments to enhance the treatment process. The first compartment is usually larger than the second. For larger flows, two or more non-compartmented tanks can be arranged in series. A septic tank is equipped with baffles to prevent the scum and sludge from leaving the tank. The baffles allow only the liquid effluent between the scum and sludge to leave the tank and enter the drainage field.

The size of the septic tank is based on the amount of wastewater flowing into it from the building or group of buildings it serves. A septic tank must permit detention of incoming sewage for a minimum of 2 days based on maximum daily flow. For a home, this flow is based on the number of bedrooms, the square footage of the home, and the use of water-saving fixtures. For commercial installations, the flow is usually based on the number of persons visiting or working in the facility and the processes used.

A septic tank should be inspected annually by removing the cover. Sludge and scum that collects in the septic tank must be removed (pumped out) regularly. In a residence with a properly sized septic tank, this must be done about every 3 to 5 years. Kitchen garbage disposals and similar devices are strongly discouraged in a conventional septic system because they introduce an extremely high amount of solids to a system.

Additionally, chemical household cleaners and too-frequent pumping of the septic tank destroy the beneficial bacteria and the ability of the septic tank to properly treat wastewater.

Aerobic Tank

Aerobic tanks are a substitute for a septic tank. They consist of a trash tank, an aeration chamber, and a settling chamber. Some systems require a trash tank to be installed external to and in front of the unit. Premanufactured aerobic tanks use wastewater treatment processes similar to municipal wastewater treatment processes. The clarified effluent is then usually discharged into a drainage field.

Aerobic tanks work more effectively than septic tanks and thus can be smaller in size. To remain effective, aerobic treatment components require regular maintenance and continuous monitoring. Additionally, an aerobic tank must be used on a regular basis to maintain the aerobic unit's microbe digestion process. Treatment bacteria survive on a constant flow so aerobic systems should not be used for weekend vacation homes and similar non-regular uses.

Pump Tank

A pump tank is a watertight container used to temporarily store clarified effluent before it flows into a drainage field. Wastewater is first treated in an aerobic or septic tank. The effluent then flows by gravity into the pump tank. When the level of stored effluent reaches a preset elevation, a float switch turns on the pump. The pump discharges the effluent to the drainage field several times a day. Excess effluent produced during periods of high generation of wastewater can be stored in the pump tank temporarily and then discharged in the drainage field during low generation periods. A remote alarm sounds if the effluent level in the tank becomes too high. Pump tank materials are typically concrete; plastic (fiberglass and polyethylene) tanks are also used.

Sand Filters

A sand filter is a lined, impermeable container containing a bed of granular material that provides additional treatment of effluent as it flows from the primary treatment tank to the drainage field. They are usually placed underground with the top surface covered with grass. At sites that have near-surface bedrock or a high water table, sand filters are usually constructed with above ground concrete walls.

There are two types of sand filters: the intermittent sand filter and the recirculating intermittent sand filter. In an intermittent sand filter, the effluent is dispersed throughout the upper portion of the granular bed through perforated pipes. The effluent then flows by gravity through the granular material until it reaches another series of perforated pipes, where it flows to a pump tank. The effluent is then pumped from the pump tank to the drainage field. A recirculating intermittent sand filter requires a recirculating pump tank. The pump is used to mix filtrate with incoming septic tank effluent. The effluent is circulated several times through the sand filter media before it’s pumped to the drainage field.

Trash/Grease Tank

A trash tank is occasionally used in conjunction with an aerobic tank. The trash tank removes materials that treatment microorganisms are unable to degrade. Several types of aerobic tanks used for small wastewater flows have the trash tank en closed inside the unit. Grease tanks are used with septic and aerobic tanks, usually in commercial applications.

Cesspool

A cesspool is a covered underground container that receives untreated sewage directly from a building and discharges it into soil. Openings in the cesspool walls allow untreated sewage to pass through and seep into the surrounding soil. Because of health concerns tied to the discharge of raw sewage, use of a cesspool is considered unacceptable today in most applications in developed countries.

High-Level Alarms

A high-level alarm is used to alert the homeowner or building operator if liquid inside a tank reaches a level that is higher than it would be if the pump were operating normally or if the liquid inflow is greater than the maximum pumping capacity of the pump. High-level alarms are used in sump, sewage, or effluent installations. These alarms can either be purchased separately or as part of a preassembled package.

Secondary Treatment and Disposal Equipment

A drainage field provides secondary treatment and is the final disposal location for clarified effluent from wastewater. A piping network carries effluent from a septic or pump tank to the drainage field for further treatment within the soil or disbursement into the air. Types of drainage fields are described below.

Absorption Drainage Field

An absorption drainage field consists of rows (called lines) of underground pipes through which the clarified effluent passes.

Perforated pipes are laid in shallow underground beds or narrowly cut trenches filled with gravel, then are typically covered with grass. Effluent from a septic or pump tank is discharged into the drainage field, where it’s distributed to the soil bed or trench and percolates through the soil. Absorption drainage field soils must have good absorption and filtration qualities.

Dense clay soils and gravel soils don’t absorb or filter the effluent properly and therefore are considered unacceptable soils.

(See fgr.4 and IMG7.)

A distribution box receives the effluent from the septic tank and distributes it equally to each individual line of the drainage field. The box is connected to the septic tank with a watertight sewer line. Perforated pipe (pipe with holes), frequently called drain tiles because they used to be made of clay tile pipe, disperse the clarified effluent throughout the drainage field. The effluent then gravitates into gravel or other media covered with a geo-textile fabric and loamy soil. Vegetation covering the soil absorption system uses the water and nutrients to grow. The quantity of effluent flowing to the drainage field and the percolation rate determines the drainage field size.

An absorption drainage trench is an absorption field that consists of one or more individual trenches containing the drainage pipes. Trenches typically have a maximum width of 3 ft with a minimum 6 ft between trenches. An absorption drainage bed is an open drainage field without trenches containing the drainage pipes. A drainage field may take any of a number of shapes, depending on the contours (slope) of the ground, the size of the lot and the location of any well or stream on the property.

FGR.4 An absorption drainage field consists of rows (called lines) of underground pipes through which the clarified effluent is distributed. Effluent seeps into and percolates through the soil.

IMG 7 An absorption drainage trench. Trench will be covered with soil. ( --)

IMG 8 Gravel-less drain pipe during construction. Pipe will be covered with soil. ( --)

Gravel-Less Drain Fields

A gravel-less drain field distributes effluent into the soils through gravel-less drain pipe instead of gravel. As a result, these systems are also called no gravel or no rock drain field systems. Like a conventional OSST system, gravel-less pipe requires only a septic tank to pre-treat the wastewater. When installed, the pipe is not surrounded by gravel or rock. These systems can be installed with small equipment and in hand-dug trenches in areas with steep slopes where conventional gravel systems would not be possible. This pipe takes many forms, including geo-textile fabric-wrapped pipe and open-bottomed chambers. (see IMG8).

Fabric-wrapped drain field systems use proprietary, large diameter, corrugated, and perforated polyethylene tubing covered with permeable nylon geo-textile filter-like fabric to distribute water around the pipe. The pipe is available in 8 and 10 in (200 and 250 mm) diameters and 10 and 20 ft (3 and 6 m) lengths. Effluent can circulate behind the filter fabric, within the pipe corrugations, allowing it to discharge into the soil from all sides of the pipe. A variant of the fabric-wrapped pipe is to produce it from permeable synthetic materials (i.e., expanded polystyrene foam chips) that strain the effluent.

The pipe is laid in a trench that is backfilled with native soil. It requires a well-aerated soil; it cannot be installed in clayey soils. The area of fabric in contact with the soil provides the surface for the septic tank effluent to infiltrate the soil. The pipe is placed in a 12 to 24 in (300 to 600 mm) wide trench.

Multiple trenches are connected with a solid 4 in diameter pipe.

The pipe is flexible, which enables it to be placed in curved trenches excavated to a specified elevation on a sloping site.

Each trench must also have a cleanout/inspection port to allow sludge to be pumped out and air to enter the pipe.

A chamber-type drain field system consists of several arch-shaped, open-bottomed plastic chamber segments connected to form the underground drain field network. Chamber sections are placed in an open trench and backfilled with native soil. Distribution pipes hung from the inner chamber walls distribute and disperse the effluent to the open-bottomed trenches, where it infiltrates the soil. These systems are also called leaching chambers, galleys, and flow diffusers.

A typical leaching chamber consists of several high density polyethylene injection-molded arch-shaped chamber segments. Chamber segments are available in a variety of shapes and sizes. A typical chamber has an average inside width of 16 to 40 in (0.4 to 1.0 m) and an overall length of 6 to 8 ft (1.8 to 2.4 m). The chamber segments are usually about 1 ft (300 mm) high, with wide slotted sidewalls. Because leaching chamber systems can be installed without heavy equipment, they are easy to install and repair.

Chamber-type drain field systems can provide more efficient storage than conventional gravel systems because the empty chamber provides additional storage volume to handle peak effluent loads. The additional storage volume may reduce over all drain field costs and the number of trees that must be removed from the drain field area. Maintenance requirements are comparable to those of aggregate drain field systems.

Evapotranspiration Bed or Trench Drainage Field

An evapotranspiration (ET) bed or evapotranspiration trench drainage field treats wastewater by evaporating the water from the soil and by transpiring the water into the air through plants and grasses. Vegetation covering the soil prospers on nutrients introduced by the effluent. An ET drainage field typically consists of two sets of perforated drainage pipes and gravel, soil lining (if needed), and topsoil backfill for vegetation growth.

The surface area of the ET drainage field is divided into two bed sections. By periodically alternating flow to a section of bed, the other section of bed is periodically rested.

ET drainage fields are used at sites with impermeable soils such as dense clays or with very permeable soils such as sandy gravel or fractured limestone. Heavy elastomeric (rubbery plastic) membrane sheeting is commonly used for liners in a lined ET system. Impermeable dense clay soils don’t need to be lined and are less expensive than lined ET drainage fields.

ET drainage fields don’t perform well in wet regions where rainfall rates exceed evaporation rates.

Low-Pressure Dosing Drainage Field

Low-pressure dosing (LPD) drainage fields typically consist of narrowly cut 6 to 12 in wide trenches, containing small-diameter PVC dispersion pipes. LPD systems also include a pump tank and an electrical control system. As effluent flows from the septic tank into the pump tank, the water level rises inside the tank.

When a fixed water level is reached in the pump tank, the pump introduces water into the drainage field. In the drainage field, the effluent is introduced into the drainage field through small holes in the drainage pipe. The soil provides additional wastewater treatment. Soil particles filter the effluent, and microbes in the soil kill the bacteria and pathogens.

Pumped effluent low-pressure dosing drainage fields are a basic LPD trench system intended for single-family homes and cannot be used for commercial or institutional facilities.

They consist of a septic tank, pump tank, and drainage field.

The pump periodically doses a measured quantity of effluent evenly within the drainage field. Use of a pump allows the drainage field to be located upslope from the septic tank. A 2% maximum slope (2 ft vertical rise in 100 ft vertical run) of natural grade is allowed with this type of system.

Absorption Mounds

Absorption mounds consist of septic tank(s), a pump tank, effluent pump and controls, and an above-grade drainage system. The drainage system consists of perforated drainage pipes laid in a deep layer of sand to sandy loam soil that is placed above the natural soils. The mound created by this deep layer soil is a topsoil cap and grass cover. Effluent from the pump tank flows through holes in the perforated drainage pipes and into the mound. The effluent moves downward through the mound by gravity flow and is partially treated by the time it reaches the natural soil.

Mounds are used at sites that have shallow soils. They typically require a minimum depth of 12 in of natural soil to any limiting layer such as bedrock or high water table. Natural surface slopes greater than 10% (10 ft vertical rise in 100 ft vertical run) are not suitable for absorption mounds. The main sizing standard for mound construction is the contact area of the fill sand with the existing soil. For slowly permeable clay and clay loam soils, a loading rate of from 0.20 to 0.25 gallons per day per square foot is usually satisfactory. This typically means that a large drainage area is required for absorption mounds, which can substantially increase land area requirements for this type of drainage field system.

Spray Distribution

Spray distribution systems spray the disinfected effluent onto the ground surface in a manner similar to a lawn irrigation system. They require an advanced treatment process that purifies the water, a pathogen-removing disinfecting system (chlorinating is the most common method), a pump tank, distribution pipe, and a disposal area containing the spray heads. The wastewater must be treated to secondary-quality effluent level, which usually requires an aerobic tank for primary treatment or an intermittent sand filter. Spray systems can be used in almost all soil conditions (subject to regulatory approval), but typically have higher cost and maintenance requirements compared with most other system types.

Leaching Chamber Drainage Field

Leaching chambers are proprietary, commercially produced plastic chambers pre-molded into a dome shape. They can be easily assembled and placed in trenches. Depending on site conditions, leaching chamber drainage fields can require less land area than other conventional absorption drainage fields.

Leaching chambers distribute effluent to the soil in a manner similar to conventional gravel-filled trench systems.

Each chamber dome supports the soil above it while maintaining sufficient volume inside for effluent storage. The sides are louvered and the bottom is open to allow for passage of effluent into surrounding soils. Leaching chamber domes usually vary in width from 15 to 36 in.

FGR.5 A septic tank to seepage pit system. The seepage pit has openings in its walls that allow effluent to pass through and seep into the surrounding soil.

IMG 9 A concrete seepage pit.

Subsurface Drip Drainage Field

Subsurface drip systems consist of a septic tank, a pump tank, a filtering device, and a drip distribution system. An aerobic tank is typically used for the primary treatment process. When a sep tic tank is used, additional treatment in a sand filter is required.

Treated wastewater is pumped through a filtering device that removes larger particles to reduce clogging of the drip emitters.

The filtered effluent is distributed to tubing laid just below the ground surface. Effluent is introduced to the soil through emitters spaced along and inserted into the tubing wall, where bacteria and pathogens are removed. Emitters can become clogged and an ongoing maintenance contract is required. Subsurface drip systems can be installed on steep slopes and at sites containing dense clay soils and soils overlying shallow bedrock.

Seepage Pit

A seepage pit is a deep underground container that receives clarified effluent from a septic tank. It has openings in its walls that allow effluent to pass through and seep into the surrounding soil. (See fgr.5.) A seepage pit functions like an absorption drainage field except that effluent seeps through openings in the sidewalls of the pit rather than through the bottom of a trench or bed. A seepage pit tends to be deeper than an absorption bed or trench. It’s designed on the basis of the sidewall area. It can serve as an alternative to a drainage field, but only when surrounding soil has a good percolation rate (usually below 30 min/in) and the water table is very deep.

A seepage pit can be a precast concrete or molded plastic unit. It can also be constructed of individual special concrete masonry units (CMU) laid with the cells (holes) placed horizontally to allow the effluent to seep into the ground. See IMG9. The tapered cells of the block are set with the widest area to the outside to reduce the amount of loose material behind the lining that might fall into the pit. The bottom of the pit is lined with coarse gravel a minimum of 1 ft (0.3 m) deep before the block or concrete is placed. Between the block or concrete and the soil is a minimum of 6 in (0.15 m) of clean crushed stone or gravel. The top of the pit should have an opening with a water tight cover to provide access to the pit if necessary. The construction of the pit above the inlet pipe should be watertight.

General Regulations

Typically, an OSST system should not be installed, repaired, or rehabilitated where a community sanitary sewer system is available or where a local ordinance requires connection to a community system. An OSST system is generally not permitted when a building site is located within 200 ft from any community sewer. When a community sanitary sewer becomes available within 200 ft, any building then served by an OSST system should connect to the community sanitary sewer within a time frame or under conditions set by the governmental authority.

Generally, it’s illegal to discharge any wastewater from OSST systems (except under a governmental permit) to any ditch, stream, pond, lake, waterway, or drain tile, or to the surface of the ground. In older buildings, existing systems having any of these prohibited discharges are typically required to eliminate these discharges by constructing a system in compliance with current requirements.

Prev: COMMUNITY WASTEWATER TREATMENT AND DISPOSAL

Home   Similar articles    top of page