Conventional and Alternative on-site sewage systems

From Wastewater Management Program Office of Environmental Health and Safety

Washington State http://www.doh.wa.gov/ehp/ts/Approved_Systems_List_Nov-2000.PDF

The links below are not an endorsement of the products but are presented only for information on the type of systems available.

A conventional on-site sewage system consists of a septic tank and gravity flow or pressure distribution to a gravel-filled drainfield

Alternative Sewage System Descriptions are:

Aerobic Treatment Units

Aerobic treatment units provide aerobic biodegradation or decomposition of wastewater by bringing the wastewater in contact with air. These units come in different configurations and sizes, and incorporate a variety of mechanical (and non-mechanical) methods to enhance aerobic biodegradation of wastewater. Included are air pumps, air injectors, and biological-contact surfaces (such as pipes, fabric, grids, and rotating disks).

Typical Applications: Site soil that is poor for sewage treatment. Aerobic treatment units are less reliant upon existing original soil for treatment, but still dependent on the soil for disposal of the treated wastewater. High quality pre-treatment performance may allow reduced installed drainfield size to reduce the size and cost of initial installation.

Alternating and Dosing Systems

Dosing System: A system that employs a dose-rest cycle within a conventional gravity system by means of a dosing tank and a dosing device, such as a pump or siphon. The arrangement allows the dosing tank to fill to a predetermined level at which point the dosing device periodically discharges the volume contents to a drainfield or other approved disposal component.

Alternating Drainfields: Similar to dosing systems in that dose-rest cycles are provided. However, the rest cycle is long enough for complete drying and oxidation of the clogging layer. The flow from the pretreatment device is intermittently directed into two or more separate drainfields.

Typical Applications: Where continuous gravity flow is not feasible or desirable or where pressure distribution design is not used. Can be applied anywhere conventional drainfield design could be used.

Composting Toilets

Composting toilets are designed to store and compost, by aerobic bacterial digestion, human urine and feces, which are non-water-carried. Toilets may include necessary venting, piping, electrical, and/or mechanical components.

Typical Application: Where development area is limited. Separating, treating and disposing of grey-water and blackwater separately can have advantages: composting toilets can reduce total wastewater volume by about 50%, and greywater may be treated and disposed of through conventional or alternative means, depending upon site conditions, soil conditions, and scope of development.

Gravelless Drainfield Systems

A drainfield system using preformed structures or gravel-substitute to provide void space for passage and storage of effluent, and to provide an interface with the exposed infiltrative surface. These are functions performed by gravel in the conventional drainfield. Three types of systems are approved: gravelless pipe systems, gravelless chamber systems, and gravel-substitute systems. Site, soil, application, design and installation requirements differ for the three system types.

Typical Applications: Where cost or availability of gravel is a factor. Gravel is heavy and difficult to move by hand, but in some settings, use of large, heavy equipment is destructive to landscape, plantings, etc. Some materials / systems lend themselves well to root-level irrigation of shrubs, flowers, and trees. Other applications would be where there is concern about fine materials entrained with gravel, and where there is a desire to access the infiltrative surface for monitoring and maintenance.

Holding Tank Sewage Systems

A water tight tank designed to hold the entire daily operational waste flow (plus reserve capacity) from an institutional or small commercial facility, together with controls, alarms and pump-out features to facilitate easy and reliable pumping of the sewage from the tank. These tanks are usually constructed of pre-cast concrete but may be fiberglass or polyethylene or poured-in-place concrete.

Typical Applications: Generally these options have limited application: parks, and recreational facilities, temporary or seasonal facility operation, etc., but may be useful in other settings depending on need, site limitations, and desired service intervals.

Incineration Toilets

Self-contained devices which reduce non-water-carried human urine and feces to ash and vapor, including the necessary venting, piping, electrical and/or mechanical components. The process is fueled by gas, fuel oil, or electricity.

Typical Applications: Where development area is limited. Separating, treating and disposing of grey-water and blackwater separately can have advantages: incineration toilets can reduce total wastewater volume by about 50%, and greywater may be treated and disposed of through conventional or alternative means, depending upon site conditions, soil conditions, and scope of development.

Mound Systems

These wastewater treatment systems are characterized by sand media placed upon the ground surface, with effluent being treated before discharge from the sand media into the underlying soil. They share the principal attributes of intermittent sand filters except that the media is not contained within a structure. This technology is generally used at sites with shallow soil conditions over a restrictive layer or elevated groundwater table. Proper operation requires influent to be distributed over the media in controlled, discrete doses. In order to achieve accurate dosing, these systems require either a pump or siphon system with associated pump chambers, electrical components and distribution pipe-work. Current Recommended Standards and Guidance require the use of timed dosing of the effluent and timed resting periods.

Typical Applications: Site soil that is poor for sewage treatment. Mound systems are less reliant upon existing original soil for treatment, but still dependent on the
soil for disposal of the treated wastewater.

Sand Filters

Wastewater treatment systems characterized by a relatively large container and means for distributing septic tank effluent atop a layer, or layers, of graded sand (or gravel) where, as the wastewater moves downward, it undergoes biochemical degradation. There are many different designs of sand filters, but they can generally be divided into two types: single-pass filters, and multiple-pass filter. The RS&G’s for the sand filter technologies address three single-pass sand filters (intermittent, sand-lined drainfield trench, and stratified) and one multiple-pass filter (recirculating gravel filter system).

Typical Applications: Site soil poor for sewage treatment (systems are less reliant upon existing original soil for treatment, more for disposal of the treated wastewater). High quality pre-treatment performance may allow reduced installed drainfield size, meeting limited area constraints for some sites.

Vault and Pit Privies

A non-portable toilet enclosed in a vented outdoor structure. Vault privies have a waste storage chamber, which is watertight or installed in impermeable material. Pit privies have a subsurface waste storage chamber that is not watertight.

Drip systems

A drainfield system using drip tubing can be used in place of the traditional drain line. Many manufactures produce drip tubing for wastewater distribution systems

Biofilters

An aerobic biofilter is designed for the biological treatment of wastewater. The waster water is sprayed over an absorbent synthetic filter medium designed to optimize the biological degradation of wastewater.