Types of Wastewater Systems
Anaerobic filters are used as part of a treatment train designed to minimize nitrate concentration in areas where discharge of nitrates to surface water or ground water is a concern. Anaerobic filters convert nitrate (NO3) to gaseous forms of nitrogen (N2, N2O, NO). The key design consideration for anaerobic filters is to ensure that the carbon-to-nitrogen ratio is sufficient for denitrification. Good performance can be obtained by treating septic tank effluent with a nitrifying (usually sand) filter before the anaerobic filter.
At-Grade Soil Absorption Systems
At-grade soil absorption systems are similar to the subsurface soil absorption systems, but bedding material (usually gravel) is placed at the ground surface rather than below ground and is covered with soil fill material. At-grade systems are used in areas with relatively high ground-water tables or shallow bedrock.
Decentralized wastewater collection and treatment systems serving two or more dwellings, but less than an entire community. Sometimes, the wastewater from several homes are pretreated on-site by individual septic tanks before being transported through alternative sewers to an off-site, nearby treatment unit that is relatively small compared to centralized systems.
Constructed wetlands are engineered systems designed to optimize the physical, chemical, and biological processes of natural wetlands for reducing BOD and TSS concentrations in wastewater. Wastewater from a septic tank flows through a pipe into the wetland, where the wastewater is evenly distributed across the wetland inlet. Sedimentation of solids with the media substrate occurs. Constructed wetlands are reliable for BOD and TSS removal, and may contribute to nutrient removal when used after a nitrifying unit process.
Disinfection refers to the destruction of disease-causing organisms called pathogens (e.g., bacteria, viruses) by the application of chemical or physical agents. Disinfection may be necessary where other types of treatment are inadequate to reduce pathogen levels to the required regulatory standards for surface discharge. The most common types of disinfection for decentralized systems are:
- Chlorination Systems
Chlorination occurs by mixing/diffusing liquid or solid chlorine forms with wastewater. Chlorination is considered to be the most practical disinfection method for on-site wastewater treatment because it is reliable, inexpensive, and easy to use; however, dechlorination may be needed to prevent the dispersal of residuals that may be harmful to aquatic life.
- Ultraviolet Disinfection
In an ultraviolet treatment system, high intensity lamps are submerged in wastewater or the lamps surround tubes that carry wastewater. Disinfection occurs when the ultraviolet light damages the genetic material of the bacterial or viral cell walls so that replication can no longer occur. Care must be taken to keep the surface of the lamps clean because surface deposits can shield the bacteria from the radiation, thus reducing the performance of the system. Ultraviolet radiation is a highly effective technique especially attractive in cluster systems where the effluent cannot include any residuals or where there are overriding concerns with safety.
Effluent Distribution Systems
Effluent distribution systems are essential components of subsurface wastewater treatment systems. These systems deliver wastewater to soil infiltrative surfaces either by gravity or by pressure distribution. Pressure distribution: Pressure dosing systems distribute water over more infiltrative surface and provide a resting period between doses that increases the life and performance of the leach field. Dosing pumps provide the pressure; the latter requires additional maintenance demands.
Fixed Growth Systems
In fixed growth systems, aerobic microorganisms attach and grow on an inert media. Wastewater flows across a slime layer created by the attached microorganisms, which extract soluble organic matter from the wastewater as a source of carbon and energy.
Intermittent Sand Filters (ISF)
Intermittent sand filter consists of sand media with a relatively uniform particle-size distribution above a gravel layer. An ISF reduces BOD and TSS concentrations to 10 mg/L or less. Wastewater passes through the filter and drains from the gravel to the collector. Uniform distribution of influent is very important to filter performance. Influent is dosed to the surface 4 to 24 times per day, with best performance from higher numbers of smaller doses. The sand filter material may be left exposed or covered with removable covers. A septic tank (or other pretreatment system) is required to remove settled solids and grease, which can clog the sand. Covers are used in cold climates. If sand filter material is left exposed, it must be checked regularly for litter, vegetation growing on the surface. It may require raking periodically. An uncovered system is susceptible to potential odor problems. Less frequently, the sand may require removal/replacement of the top layer.
Nitrogen Removal Systems
Several types of treatment processes are capable of removing nitrogen in wastewater. Nitrogen removal systems are used in on-site treatment trains or ensure protection of ground water as well as coastal waters recharged by ground water. Biological nitrogen removal requires aerobic conditions to first nitrify the wastewater, then anaerobic conditions to denitrify nitrate-nitrogen to nitrogen gas. The successful removal of nitrogen from wastewater requires that environments conducive to nitrification and de-nitrification be induced and positioned properly. Three types of nitrogen removal systems are described below:
- Separation of Black Water and Gray Water. Black water (toilet water) can be segregated from other sources of household wastewater (gray water) for separate treatment and disposal. A separate plumbing system within a house is required. Black water, which contains 80% or more of the nitrogen in household wastewater, can be discharged directly to a holding tank, the remaining gray water is discharged to a septic tank/soil absorption system.
- Nitrification/Denitrification Trickling Filter Plant. Septic tank effluent is recycled by a pump to a low-loaded, plastic-media trickling filter for aerobic treatment; and nitrification can occur. Filtrate from the trickling filter returns to the lower anaerobic septic tank effluent, providing an environment conducive to biological denitrification.
- Recirculating Sand Filters. Recirculating sand filters also can provide consistent nitrogen removal (See “Recirculating Sand Filter’’ below).
Non-sand filters function similarly to sand filters but use materials other than sand as the filter medium, including natural media such as peat and bottom ash, and synthetic media such as expanded polyurethane foam and honeycombed plastic to reduce levels of TSS, BOD, and fecal coliforms. Most non-sand filter media are packaged in units or placed in enclosures and use pressure dosing to distribute the effluent in the filter.
Recirculating Sand Filter (RSF)
A recirculating sand filter uses relatively coarse sand or gravel media for filtration of wastewater. The wastewater is dosed from a recirculating tank, which receives septic tank effluent and returned filtrate. A portion of the filtrate is diverted for disposal during each dose. RSFs are suitable in areas too small for conventional soil absorption systems or with shallow depths to groundwater or bedrock. RSFs can be used for reducing TSS, BOD, fecal coliform, and nitrogen. RSFs are reliable, requiring little maintenance in comparison to activated sludge systems.
Sand mounds are used when soil depth is too shallow for a conventional septic tank and leach field system. The sand mound filters septic tank effluent before it reaches the natural soil. Sand fill is placed above the ground surface, and a pipe distribution system and pressure dosing is used to distribute the effluent. A septic tank or other pretreatment is required to remove settled solids and grease.
A buried tank designed and constructed to receive and pretreat wastewater from individual homes by separating settleable and floatable solids from the wastewater. Grease and other light materials, collectively called scum, float to the top. Gases are normally vented through the building’s sewer pipe. An outlet blocked off from the scum layer feeds effluent to a subsurface soil absorption area or an intermediate treatment unit.
Subsurface Soil Absorption Systems
A typical soil absorption system consists of perforated piping and gravel in a field or trench, although gravel-less systems can also be used. Soil absorption systems are normally placed at relatively shallow depths (e.g., < 2 ft). Excellent TSS, BOD, phosphorus, and pathogen removal is provided in the unsaturated soil which surrounds the infiltrative surfaces. If properly sited, designed, constructed, and maintained, subsurface soil absorption systems are very reliable and can be expected to function for many years.
Suspended Growth Systems
Suspended growth treatment systems are variations of the activated sludge process in which microorganisms are suspended in an aerated reactor by mixing. Oxygen is supplied to oxidize organic carbon and, possibly, nitrogen compounds. Effluent is discharged either to surface water or subsurface systems. Suspended growth systems can be engineered as package plants to serve clustered residential housing, commercial establishments, or small communities with relatively small flows.
Used to reduce BOD, pathogens, and nitrogen levels, trickling filters are composed of a bed of porous material (rocks, slag, plastic media, or any other medium with a high surface area and high on permeability). Wastewater is first distributed over the surface of the media where it flows downward as a thin film over the media surface for aerobic treatment and is then collected at the bottom through an underdrain system. The effluent is then settled by gravity to remove biological solids prior to being discharged.