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How Do The Different Types Of Septic Systems Work?

Conventional System - A pipe carries all wastewater from your home and it's sent to a storage tank where the solids and water are separated. The solids form sludge, while the water is sent to the drainage field where it is slowly released into the soil for filtration.

Gravity System - A conventional system which uses gravity to force the wastewater out into the drain field. These types of septic systems use gravel as a filter.

Pressure Distribution System - A conventional system which involves the use of a pump. The pump forces the wastewater out of the system to drain more effectively, so most new conventional systems use this method.

Aerobic Treatment Unit System - This type of alternative septic system uses oxygen which helps to break down the solids much faster, and results in cleaner water being discharged from the system. These systems do not experience septic system problems as often, but they must be closely supervised and maintained at all times.

Drip Irrigation System - This is one of the types of septic systems that are very flexible, and these systems can be placed in many different types of soil. When you have a drip irrigation system, the water is slowly distributed evenly through the drainage field. This makes sure the soil will not become saturated and the system is more effective.

Mound Septic System - This system uses elevated ground in some cases when there is not adequate soil for a conventional system. The drainage is directed to an area with a higher elevation, so that the water must filter through more soil or sand before reaching the groundwater.

Intermittent Sand Filter System - This system has extra filtration, because water is sent through an additional sand filter before it is released into the soil. These systems can be found in homes, commercial buildings, and even institutions.

Recirculating Sand Filter System - This system contains an additional sand filter as well, but it also involves another loop through the system so that the water discharged has been treated twice by the filters before release.

Membrane Bio Reactor - The newest form of system approved in Washington State. This system involves the filtration of effluent in the second compartment of the septic tank using a membrane bioreactor unit that forces waste water through microscopic pores on a membrane and leaves a highly treated effluent to be distributed to the drain field. This unit can treat up to level A without UV disinfection and is good for hard to treat lots with environmetally sensitive areas.

Ultraviolet Disinfection Systems - This is another of the disinfectant types of septic system and with this system ultraviolet radiation is used to treat and clean the water. When this radiation is absorbed by the microorganisms in the effluent and solids then the microorganisms cannot reproduce rapidly.

Glendon BioFilter System - This proprietary alternative septic system uses many layers of alternating sand and gravel placed in a container which is watertight and sealed. The wastewater is pumped into the box on the bottom, and the effluent must work through the various layers to the top of the box to reach the soil.

Chapter 24.05.040 WCC - SEPTIC DEFENITIONS

“Bed” means a soil dispersal component consisting of an excavation with a width greater than

three feet.

“Bedroom” means any enclosed room of 70 square feet or more that is not designated as a

kitchen, living/family room, utility room, office, bathroom, or dining room adjacent to the


“BOD” means biochemical oxygen demand, typically expressed in mg/L.

“Building sewer” means that part of the horizontal piping of a drainage system extending from the building drain, which collects sewage from all the drainage pipes inside a building, to an on-site sewage system. It begins two feet outside the building wall and conveys sewage from the building drain to the remaining portions of the on-site sewage system.

“CBOD5” means carbonaceous biochemical oxygen demand, typically expressed in mg/L.

“Cesspool” means a pit receiving untreated sewage and allowing the liquid to seep into the

surrounding soil or rock.

“Common point” as it refers to OSS means any interconnection of sewerage piping systems

whether inside or outside of a building or structure.

“Conforming system” means any on-site sewage system meeting any of the following criteria:

1. In full compliance with new construction requirements under Chapter 24.05; or

2. Approved, installed and operating in accordance with requirements of previous editions

of Chapter 24.05; or

3. Permitted through the waiver process which assures public health protection by higher

treatment performance or other mitigation methods.

“Covenant” means a recorded agreement outlining certain activities and/or practices that are

required or prohibited by a property owner.

“Conventional gravity system” means an on-site sewage system consisting of a septic tank and

a subsurface soil absorption system with gravity distribution of the effluent.

“Cover material” means soil placed over a soil dispersal component composed predominately of

mineral material with no greater than ten percent organic content. Cover material is

permeable soil that may contain an organic surface layer for establishing a vegetative

landscape to reduce soil erosion.

“Cuts and/or banks” means any naturally occurring or artificially formed slope greater than 100

percent (45 degrees) and extending vertically at least five feet from the toe of the slope to

the top of the slope as follows:

“Designer” means a person who matches site and soil characteristics with appropriate on-site

sewage technology licensed under Chapter 18.210 RCW and professional engineers

licensed under Chapter 18.43 RCW.


“Design flow” means the maximum volume of sewage a residence, structure, or other facility is

estimated to generate in a twenty-four-hour period. It incorporates both an operating

capacity and a surge capacity for the system during periodic heavy use events. The sizing

and design of the on-site sewage system components are based on the design flow.

“Development” means the creation of a residence, structure, facility, subdivision, planned unit

development, site, area, or any activity resulting in the production of sewage.

“Disinfection” means the process of destroying pathogenic microorganisms in sewage through

the application of ultraviolet light, chlorination, or ozonation.

“Distribution technology” means any arrangement of equipment and/or materials that distributes

sewage within an on-site sewage system.

“Drain field” see subsurface soil absorption system (SSAS) and soil dispersal component.

“Drainage ditch” means a linear excavation or depression constructed for the purpose of

conveying surface runoff or groundwater from one area to another.

“Drainrock” means clean washed gravel or crushed rock ranging in size from three-quarters inch

to two and one-half inches, and containing no more than two percent by weight passing a

US No. 8 sieve and no more than one percent by weight passing a US No. 200 sieve.

“Effluent” means liquid discharged from a septic tank or other on-site sewage system


“Expanding clay” means a clay soil with the mineralogy of clay particles, such as those found in

the Montmorillonite/Smectite Group, which causes the clay particles to expand when they

absorb water, closing the soil pores, and contract when they dry out.

“Expansion” means a change in a residence, facility, site, or use that:

1. Causes an on-site sewage system to exceed its existing treatment or disposal dispersal

capability, for example, when a residence is increased from two to three bedrooms or a

change in use from an office to a restaurant; or

2. Results in an increase of more than 50 percent of the existing floor space; or

3. Reduces the treatment or dispersal capability of the existing on-site sewage system or

the reserve area, for example, when a building is placed over a reserve area.

“Extremely gravelly” means soil with sixty percent or more, but less than ninety percent rock

fragments by volume.

“Failure” means a condition of an on-site sewage system that threatens the public health by

inadequately treating sewage or by creating a potential for direct or indirect contact between

sewage and the public. Examples of failure include:

1. Sewage on the surface of the ground;

2. Sewage backing up into a structure caused by slow soil absorption of septic tank


3. Sewage leaking from a septic tank, pump chamber, holding tank, or collection system;

4. Cesspools or seepage pits where evidence of ground water or surface water quality

degradation exists;

5. Inadequately treated effluent contaminating ground water or surface water.

6. Noncompliance with standards stipulated on the permit.

“Fecal coliform” means bacteria common to the digestive systems of warm-blooded animals that

are cultured in standard tests. Counts of these organisms are typically used to indicate

potential contamination from sewage or to describe a level of needed disinfection. Generally

expressed as colonies per 100 ml.


“Geotextile” means barrier material covering the gravel trench or bed. The geotextile shall be

nonwoven. The fabric shall be free of any chemical treatment or coating which reduces

permeability and shall be inert to chemicals commonly found in soil.

“Gravelly” means soils with fifteen percent or more, but less than thirty-five percent rock

fragments by volume.

“Gray water” means sewage from bathtubs, showers, bathroom sinks, washing machines,

dishwashers, and kitchen sinks. It includes sewage from any source in a residence or

structure that has not come into contact with toilet wastes.

“Ground water” means subsurface water occupying the zone of saturated soil, permanently,

seasonally, or as the result of the tides. Indications of ground water may include:

1. Water seeping into or standing in an open excavation from the soil surrounding the


2. Spots or blotches of different color or shades of color interspersed with a dominant color

in soil caused by reduction and oxidation of iron. These color patterns are redoximorphic

features, commonly referred to as mottling. Redoximorphic features often indicate the

intermittent presence of ground water and may indicate poor aeration and impeded

drainage. Also see “water table.

“Health officer” means the health officer of Whatcom County, or a representative authorized by

and under the direct supervision of the health officer as defined in RCW 70.05.

“Holding tank sewage system” means an on-site sewage system which incorporates a holding

tank, the services of a sewage pumper/hauler, and the off-site treatment and disposal for the

sewage generated.

“Hydraulic loading rate” means the amount of effluent applied to a given treatment step, in this

regulation expressed as gallons per square foot per day (gal/sq.ft./day).

“Industrial wastewater” means the water or liquid carried waste from an industrial process.

These wastes may result from any process or activity of industry, manufacture, trade or

business, from the development of any natural resource, or from animal operations such as

feedlots, poultry houses, or dairies. The term includes contaminated storm water and

leachate from solid waste facilities.

“Infiltrative surface” means the surface within a treatment component or soil dispersal

component to which effluent is applied and through which effluent moves into original,

undisturbed soil or other porous treatment media.

“Installer” means a qualified person approved by a health officer to install or repair on-site

sewage systems or components.

“Intermediate septage holding tank” means a septage holding tank used by a licensed pumper

intended for intermediate storage of septage prior to final disposal at a permitted facility.

“Large on-site sewage system (LOSS)” means any on-site sewage system with design flows, at

any common point, greater than 3,500 gallons per day.

“Lot” means the entire parcel of land with fixed boundaries in single or joint ownership, which

area is for the use of the occupants of the building to be served by the proposed sewage

disposal system. Easements may be included in determining the boundaries of the lot.

“Maintenance” means the actions necessary to keep the on-site sewage system components

functioning as designed.

“Massive structure” means the condition of a soil layer in which the layer appears as a coherent

or solid mass not separated into peds of any kind.

“May” means discretionary, permissive, or allowed.


Mobile home park” means a plot of ground in which three or more sites are intended for

permanent occupancy by mobile homes.

“Moderate structure” means well-formed distinct peds evident in undisturbed soil. When

disturbed, soil material parts into a mixture of whole peds, broken peds, and material that is

not in peds.

“Monitoring” means periodic or continuous checking of an on-site sewage system, which is

performed by observations and measurements, to determine if the system is functioning as

intended and if system maintenance is needed. Monitoring also includes maintaining

accurate records that document monitoring activities.

“O&G” means oil and grease, a component of sewage typically originating from food stuffs

(animal fats or vegetable oils) or consisting of compounds of alcohol or glycerol with fatty

acids (soaps and lotions). Typically expressed in mg/L.

“On-site sewage system (OSS)” means an integrated system of components located on or

nearby the property it serves, that conveys, stores, treats, and/or provides subsurface soil

treatment and dispersal of sewage. It consists of a collection system, a treatment

component or treatment sequence, and a soil dispersal component. An on-site sewage

system also refers to a holding tank sewage system or other system that does not have a

soil dispersal component.

“Operating capacity” means the average daily volume of sewage an OSS can treat and disperse

on a sustained basis. The operating capacity, which is lower than the design flow, is an

integral part of the design and is used as an index in OSS monitoring.

“Operation and maintenance specialist” means a qualified person approved by the health officer

to perform operation and maintenance inspections on on-site sewage systems or


“Ordinary high-water mark” means the mark on lakes, streams, and tidal waters, found by

examining the beds and banks and ascertaining where the presence and action of waters

are so common and usual, and so long continued in all ordinary years, as to mark upon the

soil a character distinct from that of the abutting upland with respect to vegetation, as that

condition exists on the effective date of the regulation codified in Chapter 24.05, or as it may

naturally change thereafter. The following definitions apply where the ordinary high-water

mark cannot be found:

1. The ordinary high-water mark adjoining marine water is the elevation at mean higher

high tide; and

2. The ordinary high-water mark adjoining freshwater is the line of mean high water.

“Ped” means a unit of soil structure such as blocks, column, granule, plate or prism formed by

natural processes.

“Permit” means a written document issued by the health officer authorizing the construction,

installation, or alteration of a sewage disposal system.

“Person” means any individual, corporation, company, association, society, firm, partnership,

joint stock company, or any governmental agency, or the authorized agents of any such

entities. Employees of persons holding a valid license under this Chapter are included in

and covered by the license and a company may designate an employee as a qualified

professional representing the company.

“Planned unit development” means a development characterized by a unified site design,

clustered residential units and/or commercial units, and areas of common open space.

“Platy structure” means soil that contains flat peds that lie horizontally and often overlap. This

type of structure will impede the vertical movement of water.


“Premises” means any building or structure and the property on which it is located and

surrounding area utilized by persons as a residence, a place of business or place of

sponsored public assembly and includes established picnic or camp grounds.

“Pressure distribution” means a system of small diameter pipes equally distributing effluent

throughout a SSAS, as described in the WDOH ”RS&G for Pressure Distribution Systems,”

latest version. A subsurface drip system may be used wherever the regulation requires

pressure distribution.

“Professional engineer” means a person who is currently licensed as an engineer under the

provisions of Chapter 18.43 RCW.

“Proprietary product” means a sewage treatment and distribution technology, method or

material subject to a patent or trademark.

“Public domain technology” means a sewage treatment and distribution technology, method, or

material not subject to a patent or trademark.

“Public sewer system” means a sewerage system:

1. Owned or operated by a city, town, municipal corporation, county, or other approved

ownership consisting of a collection system and necessary trunks, pumping facilities and

a means of final treatment and disposal; and

2. Approved by or under permit from WDOE, WDOH and/or the health officer.

“Pumper” means a person approved by the health officer to remove and transport wastewater or

septage from on-site sewage systems.

“Record drawing” means an accurate graphic and written record of the location and features of

the OSS that are needed to properly monitor, operate, and maintain that system.

“Recreational vehicle” means a vehicular-type unit as defined by the Department of Labor and

Industries, designed for temporary living quarters for recreational, camping or travel use,

which either has its own motor power or is mounted on or drawn by another vehicle.

“Recreational vehicle park” means a plot of land in which three or more sites are occupied or

intended for occupancy by recreational vehicles for travel, recreational or vacation uses.

“Repair” means restoration, by reconstruction or relocation, or replacement of a failed on-site

sewage system.

“Report of System Status” means a WCHD operations and maintenance report filed by a WCHD

licensed O&M specialist completed at intervals outlined in 24.05.160(A)(4).

“Reserve area” means an area of land approved for the installation of a conforming system and

dedicated for replacement of the OSS upon its failure.

“Residential sewage” means sewage having the constituency and strength typical of wastewater

from domestic households.

“Restrictive layer” means a stratum impeding the vertical movement of water, air, and growth of

plant roots, such as hardpan, claypan, fragipan, caliche, some compacted soils, bedrock

and unstructured clay soils.

“Rock fragment” means rock or mineral fragments having a diameter of two millimeters or more;

for example, gravel, cobbles, stones, and boulders.

“RS&G” means Recommended Standards and Guidance documents published and updated by


“Seepage pit” means an excavation more than three feet deep where the sidewall of the

excavation is designed to dispose of septic tank effluent. Seepage pits may also be called

“dry wells”.


“Septage” means the mixture of solid wastes, scum, sludge, and liquids pumped from within

septic tanks, pump chambers, holding tanks, and other OSS components.

“Septic system” see On-site sewage system (OSS).

“Septic tank” means a watertight pretreatment receptacle receiving the discharge of sewage

from a building sewer or sewers, designed and constructed to permit separation of

settleable and floating solids from the liquid, detention and anaerobic digestion of the

organic matter, prior to discharge of the liquid.

“Sewage” means any urine, feces, and the water carrying human wastes, including kitchen,

bath, and laundry wastes from residences, buildings, industrial establishments or other

places. Also see “residential sewage.”

“Sewage quality” means contents in sewage that include:

1. CBOD5, TSS, and O&G;

2. Other parameters that can adversely affect treatment. Examples include pH,

temperature, and dissolved oxygen;

3. Other constituents that create concerns due to specific site sensitivity. Examples include

fecal coliform and nitrogen.

“Sewage tank” means a prefabricated or cast-in-place septic tank, pump tank/dosing chamber,

holding tank, grease interceptor, recirculating filter tank or any other tanks as they relate to

on-site sewage systems including tanks for use with proprietary products.

“Shall” means mandatory.

“Shoreline Management Program Area” means upland areas within 200 feet of the Ordinary

High Water Mark (OHWM) of the shoreline of all streams with a mean average flow of 20

cubic feet per second, all lakes over 20 acres in size, all marine shorelines, and all

associated wetlands and floodways in accordance with the Whatcom County Critical Area


“Significant periods of the year” means six months or longer.

“Soil dispersal component” means a technology that releases effluent from a treatment

component into the soil for dispersal, final treatment and recycling.

“Soil log” means a detailed description of soil characteristics providing information on the soil’s

capacity to act as an acceptable treatment and dispersal medium for sewage.

“Soil scientist” means a person certified by the American Society of Agronomy as a Certified

Professional Soil Scientist.

“Soil type” means a numerical classification of fine earth particles and coarse fragments as

described in WCC 24.05.110(B) (5), Table II.

“SSAS” Subsurface soil absorption system means a soil dispersal component of trenches or

beds containing either a distribution pipe within a layer of drainrock covered with a

geotextile, or an approved gravelless distribution technology, designed and installed in

original, undisturbed, unsaturated soil providing at least minimal vertical separation as

established in this regulation, with either gravity or pressure distribution of the treatment

component effluent.

“Standard methods” means the 20th Edition of Standard Methods for the Examination of Water

and Wastewater, prepared and published jointly by the American Public Health Association,

the American Water Works Association and the Water Environment Federation.

“Strong structure” means peds are distinct in undisturbed soil. They separate cleanly when soil

is disturbed, and the soil material separates mainly into whole peds when removed.


“Subdivision” means a division of land or creation of lots or parcels, described under Chapter

58.17 RCW, now or as hereafter amended, including both long and short subdivisions,

planned unit developments, and mobile home parks.

“Subsurface drip system” means an efficient pressurized wastewater distribution system that

can deliver small, precise doses of effluent to soil surrounding the drip distribution piping

(called dripline) as described in the WDOH “RS&G for Subsurface Drip Systems.”

“Subsurface soil absorption system” (SSAS) means a soil dispersal component of trenches or

beds containing either a distribution pipe within a layer of drainrock covered with a

geotextile, or an approved gravelless distribution technology, designed and installed in

original, undisturbed, unsaturated soil providing at least minimal vertical separation as

established in this regulation, with either gravity or pressure distribution of the treatment

component effluent.

“Surface water” means any body of water, whether fresh or marine, flowing or contained in

natural or artificial unlined depressions for significant periods of the year, including natural

and artificial lakes, ponds, springs, rivers, streams, swamps, marshes, and tidal waters.

“Timed dosing” means delivery of discrete volumes of sewage at prescribed time intervals.

“Treatment component” means a technology that treats sewage in preparation for further

treatment and/or dispersal into the soil environment. Some treatment components, such as

mound systems, incorporate a soil dispersal component in lieu of separate treatment and

soil dispersal components.

“Treatment level” means one of six levels (A, B, C, D, E, & N) used in these rules to:

1. Identify treatment component performance demonstrated through requirements specified

in WAC 246-272A-0110; and

2. Match site conditions of vertical separation and soil type with treatment components.

Treatment levels used in these rules are not intended to be applied as field compliance

standards. Their intended use is for establishing treatment product performance in a

product testing setting under established protocols by qualified testing entities.

“Treatment sequence” means any series of treatment components that discharges treated

sewage to the soil dispersal component.

“Trench” means a soil dispersal component consisting of an excavation with a width of three

feet or less.

“TSS” means total suspended solids, a measure of all suspended solids in a liquid, typically

expressed in mg/L.


“Unit volume of sewage” means:

1. Flow from a single-family residence;

2. Flow from a mobile home site in a mobile home park; or

3. Four hundred fifty gallons of sewage per day where the proposed development is not

single-family residences or a mobile home park.

“USEPA” means United States Environmental Protection Agency.

“Vertical separation” means the depth of unsaturated, original, undisturbed soil of soil types 1-6

between the bottom infiltrative surface of a soil dispersal component and the highest

seasonal water table, a restrictive layer, or soil type 7 as illustrated below by the profile

drawing of subsurface soil absorption systems:

“Very gravelly” means soil containing thirty-five percent or more, but less than sixty percent rock

fragments by volume.

“Water table” means the upper surface of the ground water, whether permanent or seasonal.

Also see “ground water.”

“WCHD” means the Whatcom County Health Department.

“WDOE” means the Washington State Department of Ecology.

“WDOH” means the Washington State Department of Health.

“Well” means any excavation that is constructed when the intended use of the well is for the

location, diversion, artificial recharge, observation, monitoring, dewatering or withdrawal of

ground water for agricultural, municipal, industrial, domestic, or commercial use.

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