Absorption Area: that area determined by multiplying the length and width of the bottom area of the disposal trench.
Absorption System: a secondary treatment system including conventional drainfields, alternative systems, and experimental systems used for subsurface disposal of pre-treated waste effluent.
Absorption Trench: excavations less than or equal to 3 feet in width where the distribution system is laid for the purpose of distributing pretreated waste effluent into the ground.
Alternative Systems: wastewater treatment and disposal systems approved by the Department to be used in lieu of conventional systems.
Bedrock: Material that cannot be readily excavated by hand tools, or material that does not allow water to pass through or that has insufficient quantities of fines to provide for the adequate treatment and disposal of wastewater.
Bedroom: any room that is or may be used for sleeping. An unfinished basement is considered as an additional bedroom.
Certified Installers: installers of wastewater treatment and disposal systems who have passed an annual examination to ensure sufficient knowledge of the sewer regulation.
Cesspool: a seepage pit without a septic tank to pretreat the wastewater.
Class I Exam: the exam that an installer must pass to become certified to install conventional systems and alternative systems.
Class II Exam: the exam that an installer must pass to become certified to install conventional systems and replacement systems.
Community Wastewater Treatment and Disposal System: a public wastewater treatment and disposal system which serves a non-transient population characterized by residential development.
Construction Season: March 1st through November 30th.
Contaminate: an increase in the concentration of chemicals, viruses, or bacteria in water to a degree which is likely to affect present or future beneficial uses of the water or which violates any applicable ground water or surface water standard.
Conventional System: a subsurface wastewater treatment and disposal system which consists of a septic tank and a drainfield.
Disposal Trench: an excavation in which the piping for an absorption system is laid for the purpose of distributing pre-treated waste effluent into the ground.
Distribution Box: a receptacle that receives septic tank effluent and distributes it equally into two or more header pipes leading to the absorption area.
Dosing Tank: a water‑tight receptacle placed after the septic tank or other treatment device approved by the Department, equipped with an automatic siphon or pump designed to discharge effluent.
Drainageway: a course or channel along which stormwater moves in draining an area.
Drainfield-equivalent secondary treatment: secondary treatment that is similar to or better than the treatment provided by a drainfield meeting all requirements of Section V(B) and (C). Absorption beds qualify only when minimum separations are met and there is not enough room for a drainfield. Seepage pits qualify only when minimum separations are met and there is not enough room for a drainfield or absorption bed. If a replacement system will not meet minimum separations, the Department can require elevated systems or alternative treatment systems that improve the quality of wastewater before discharge.
Dwelling or residence: any structure, building, or portion thereof, which is intended or designed for human occupancy and supplied with water by a piped water system.
Escarpment: any slope greater than 50% which extends vertically six (6) feet or more as measured from toe to top.
Experimental Systems: a wastewater treatment system, which is neither conventional nor alternative, which needs to be evaluated and approved by the Department using rigorous scientific methods.
Failed Wastewater Treatment and Disposal System: A system shall be deemed to be a failed system whenever the absorption system fails to accept waste at the rate of application, no longer provides the treatment and/or disposal for which it was intended, when a septic tank suffers structural failure, or whenever a system violates section I (A) of this regulation.
Floodplain: that portion of land adjacent to a water-way which is inundated when the water-way overflows on an average frequency of once per one hundred years, including all land area designated by the Federal Emergency Management Agency as being in the 100 year floodplain on the National Flood Insurance Rate map.
Flood‑prone Areas: areas where information indicates that the land is subject to flooding in a 100-year flood event but not included on Flood Insurance Rate Maps.
Groundwater Table: the upper surface of the zone of water saturation of a geologic formation. The upper surface of a perched water table is included in this definition.
High Seasonal Groundwater: The depth from the natural ground surface to the upper surface of the zone of saturation, as measured in an unlined hole or perforated monitoring well during the time of year when the water table is the highest. The term includes the upper surface of a perched water table.
Holding tank: a watertight receptacle that receives wastewater for retention and does not, as part of its normal operation, dispose or treat the wastewater.
Impervious layer: any layer of material in the soil profile that has a percolation rate slower than 120 minutes per inch.
Increased Use: the enlargement or change in use of a structure served by a Wastewater Treatment and Disposal System where the enlargement or change in use would potentially increase the effluent flow from the structure. Increased use includes but is not limited to the enlargement of a residence by adding one or more spaces which can be used as bedrooms. It also includes increasing a room or building's total square footage in way that could lead to increased use in the future. The Department has the sole discretion to determine if an enlargement or change in use is an increased use.
Secondary Treatment: a biological wastewater treatment process occurring after solid/liquid separation in a septic tank or equivalent.
Significant Improvement: when a structure has suffered 50% or greater destruction and is being replaced. The destruction can be intentional or unintentional, resulting from things like fire, flood or remodeling.
Individual Wastewater Treatment and Disposal System: any wastewater treatment and disposal system that serves one or two single‑family dwelling units.
Limiting layer: bedrock, an impervious layer, or seasonally high groundwater.
Living unit: the area under one roof occupied by a household. For example, a duplex is considered two living units.
Lot: synonymous with “tract” or “parcel.”
Mobile Home: A transportable structure constructed without a permanent foundation.
Multi-User Wastewater Treatment and Disposal Systems: those wastewater treatment and disposal systems which serve or are intended to serve three (3) to nine 14 connections where the total number of people served does not exceed twenty-four (24). In estimating the population served, the Department shall multiply the number of living units times the county or census tract average or persons per living unit based on the most recent census data.
Natural Soil: soil that has developed through natural processes, and where no fill material has been added.
Non-Community Public Wastewater Treatment and Disposal Systems: public wastewater treatment and disposal systems which serve a transient population such as a restaurant or bar.
Parcel: a part of land which is created by a division of land or a space in an area used for recreational camping vehicles or mobile homes.
Percolation Test: a standardized test used to determine soil permeability. This test is described in Appendix B.
Permit: a written authorization issued by the Department, permitting the construction, alteration, extension, or operation of a wastewater treatment and disposal system under this regulation.
Public Wastewater Treatment and Disposal Systems: those wastewater treatment and disposal systems which serve fifteen (15) or more connections or twenty‑five (25) or more people for a period of at least 60 days out of the calendar year. In estimating the population served, the Department shall multiply the number of living units times the county or census tract average or persons per living unit based on the most recent census data.
Public Nuisance: any condition which affects an indefinite number of persons, or all the residents of a particular locality, or all persons coming within the extent of its range or operation by being injurious to health, annoying, or indecent or offensive to the senses, although the extent of the effect on individuals may vary.
Replacement System: a wastewater treatment and disposal system that is installed to replace an existing system.
Scarify: to break up and loosen the surface of the soil.
Sealed pit privy: an enclosed receptacle designed to receive non-water carried toilet wastes into a lined vault.
Seepage Pits: deep excavations used for the subsurface disposal of pre-treated effluent. Covered porous walled chambers are placed in the excavation and surrounded by rock.
Septic tank: a storage settling tank in which settled sludge is in immediate contact with the wastewater flowing through the tank while the organic solids are decomposed by anaerobic action.
Site Evaluation: an evaluation to determine if a site is suitable for the installation of a subsurface wastewater treatment and disposal system.
Site Evaluator: any individual who has a knowledge of soils and how they relate to the design and function of subsurface wastewater treatment and disposal systems and who is approved by the Department to conduct site evaluations.
Slope: the rate that a ground surface declines in feet per one hundred (100) feet. It is expressed as percent of grade.
Soil Profile: a description of the soil strata to a depth of ten (10) feet using the USDA soil classification system.
Soil Texture: the amount of sand, silt or clay, measured separately in a soil mixture. (For individual definitions, see Appendix A)
STEP: Septic Tank Effluent Pump.
STEP Septic Tank: a septic tank designed to accept pumping equipment to pump effluent into a municipal sewer system which meets the design criteria established by the City of Missoula Public Works division.
Structure: that which is built or constructed, an edifice or building of any kind, or any piece of work artificially built up or composed or parts joined together in some definite manner, including but not limited to dwelling units, mobile homes, sleeping quarters, business establishments, grandstands, amphitheaters, and warehouses.
Subdivision: means a division of land or land so divided that creates one or more parcels containing less than 20 acres, exclusive of public roadways, in order that the title to or possession of the parcels may be sold, rented, leased, or otherwise conveyed and includes any re-subdivision and any condominium or area, regardless of size, that provides permanent multiple space for recreational camping vehicles or mobile homes.
Useable acreage: The total area of a lot minus flood plain, flood prone area and area where slope exceeds 25%.
Wastewater: liquid waste which may include chemicals, household, commercial or industrial wastes, human excreta, animal and vegetable matter in suspension or solution, discharged from a dwelling, building, or other establishment.
Wastewater Treatment and Disposal System: any wastewater system including individual, multi-user, and public systems which receives human excreta, liquid waste, or both; treats the effluent; and disposes of the effluent through application into or onto the soil, or into any device, sealed vault, or holding tank. Included within the scope of this definition are pit privies, subsurface drainage systems, and alternative and experimental systems that are addressed by this regulation.
Zone of Saturation: that area beneath the ground in which all open spaces are filled with groundwater.
A site evaluation shall be conducted in the location of each proposed system. The following factors must be evaluated: size and shape of the lot, soil conditions, slope of natural and finished grade, depth to groundwater, proximity to existing and future water supplies, proximity to existing systems, proximity to state water, floodplain and flood prone areas, escarpments, area available for expansion or replacement of the system. Special guidelines govern the following criteria:
|1. Public or multi-user wells/springs||100||100||100|
|2. Other wells||50||50||50|
|3. Property Boundaries||10||10||10|
|4. Foundation Walls||10||10||10|
|5. Suction lines||50||50||100|
|7. Stream, Lake , or Irrigation Ditch, springs||50||50||100|
|8. Roadcuts, escarpments||10 (d)||10||25 (f)|
|9. Floodplain or flood prone area||10||100||100|
|10. Slopes > 25% (e)||10 (d)||10 (d)||25|
|11. Subsurface drains||10||10||10|
|12. Water Lines||10||10||10|
|13. Drainfields/sand mounds (c)||10||10||-|
|14. Surface water, springs||50||50||100|
|1 bedroom||150 gpd|
|2 bedrooms||225 gpd|
|3 bedrooms||300 gpd|
|4 bedrooms||350 gpd|
|5 bedrooms||400 gpd|
|Each additional bedroom||add 50 gpd|
Flow rates for Industrial and non-residential establishments shall be as determined by design in accordance with DEQ 4, Chapter 5.
NOTE: Any space or room such as a den, study, storage area, or any area that can easily be converted to a bedroom shall be considered an additional bedroom.
|3 bedrooms||300 gpd|
|4 bedrooms||350 gpd|
|5 bedrooms||400 gpd|
|Each additional bedroom||add 50 gpd|
|Texture||Est. Perc Rate
|Gravelly sand or very coarse sands||< 3 (a)||0.8(a)|
|Loamy sand, coarse sand||3 - < 6||0.8|
|Medium sand, sandy loam||6 - < 10||0.6|
|Fine sandy loam, loam, silt loam||10 - < 16||0.5|
|Very fine sand, sandy clay loam||16 - < 31||0.4|
|Clay loam, silty clay loam||31 - < 51||0.3|
|Sandy clay, clay, or silty clay||51 - < 121 (b)(c)||0.2|
|Clays, silts, silty clays (soil is reported throughout the soil profile) (USE EVTA BED)||≥ 121 (d)||0.15|
|Clays or silts, pan evaporation rates do not
allow for EVTA use
Lineal feet = gallons effluent per day application rate X width of trench in feet
|Units||Gravity Maximum||Gravity Minimum|
|Slope of drainfield lines||percent||0||0|
|Slope of trench bottom||percent||0||0|
|Slope of header pipe||percent||0||0|
|Depth of coarse material
|Size of coarse material||inches||2.5||.75|
|Space between centers||feet||--||7|
|Number of laterals||--||2|
|PIPE DIAMETER||SCH 40||CLASS 200||CLASS160|
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