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Septic Installer Test for Washington State (WOSSA) | Questions & Verified Answers | Comprehensive WOSSA Septic Installer Exam Review Study Guide PDF | 2026

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Prepare for the Washington State WOSSA Septic Installer Exam with this comprehensive study guide featuring exam questions and verified answers designed to strengthen your understanding of onsite sewage systems and Washington State installation requirements. This detailed review covers high-yield topics including septic tank requirements, installation practices, setbacks, site preparation, soil and site evaluation, topography, system components, pressure distribution, mound systems, sand filter systems, holding tanks, construction details, wastewater treatment principles, permitting, inspections, maintenance, troubleshooting, safety, regulatory requirements, and Washington State code provisions relevant to septic installers. Ideal for WOSSA certification candidates, septic installers, wastewater professionals, environmental health specialists, contractors, inspectors, and onsite sewage system professionals, this resource is perfect for preparing for certification exams, practice tests, quizzes, and comprehensive WOSSA reviews while improving knowledge retention, strengthening technical skills, and maximizing exam readiness.

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State (WOSSA) EXAM Questions and Answers (Verified Answers) Study Guide (latest version
ANSI American National Standards Institute




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




CBOD Carbonaceous Biochemical Oxygen Demand, expressed in mg/L.




FC Fecal Coliform, expressed in number of colonies 100/ml.




LOSS large on-site sewage system




NSF Nation Sanitation Foundation International




O&G Oils and Grease, component of sewage typically originating from food stuffs (animal
fats of vegetable oils) expressed in mg/L.



RS&G Recommended Standards and Guidance




SSAS a subsurface soil absorption system




TAC Technical Advisory Committee established in WAC 247-272A-0400




TN Total Nitrogen, expressed in mg/L.




TSS Total Suspended Solids. A measure of the total suspended solids in a given
solution, expressed in mg/L.



USEPA United States Environmental Protection Agency




Additive a commercial product added to an on-site sewage system intended to affect the
performance or aesthetics of an on-site sewage system



Approved Acceptable to the authority having jurisdiction. Local health officer or the department

,State (WOSSA) EXAM Questions and Answers (Verified Answers) Study Guide (latest version
BED a soil dispersal component consisting of an excavation with a width greater than 3
feet



Building Sewer that part of the horizontal piping of a drainage system which starts from the end of
the building drain and which receives the discharge of the building drain and
conveys it to a public sewer, private sewer, individual sewage disposal system or
other point of disposal. 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.


Cesspool a covered hole or pit for receiving drainage or sewage, as from a house allowing
liquid to seep into the surronding rock or soil



Conforming System an on-site sewage system of component meeting rules of: New construction
requirements
Approved and installed within previous requirements
Permitted by WAC-246-272A-0420


Cover Material Soil placed over dispersal component, made up of mineral material with no greater
than 10% organic content. Cover may contain organic surface material, an organic
landscaper to help reduce and contain soil erosion.


Cuts and Banks a naturally occurring or artificially formed slope greater than one hundred percent
(forty-five degrees) and extending vertically at least five from the toe of the slope to
top of the slope.


Department Washington State Department of Health




Designer Person who matches site and soil characteristics with approved components




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 a on-site sewage system component is based on the design
flow.


Development The creation of a residence, structure, facility, subdivision, area, or similar activity
resulting in the production of sewage.



Disinfection The process of destroying pathogens in sewage through the application of
chlorination, ultra-violent light, or ozonation.



Distribution Technology any arrangement of equipment and/or material that distributes sewage within and
on-site sewage system.



Drainfield subsurface soil absorption system, and soil dispersal component




Drainrock cleaned washed gravel or crushed rock ranging in size from 3/4" to 2.5", and
containing no more than two percent by weight passing a US No.8 sieve and no
more than one percent by weight passing through a US No. 200 Sieve.

,State (WOSSA) EXAM Questions and Answers (Verified Answers) Study Guide (latest version
Effluent liquid waste discharged to a septic tank or other on-site sewage system




Expanding Clay means a clay soil with the mineralogy of clay particles. Clay particles expand when
absorbing water



expansion a change in residence, facility or site that results in the quantity of sewage to
exceed the original design



Extremely gravely soils with 60% or more, but not less than 90% rock fragments by volume.




Failure a condition of an on-site sewage component that threatens the public health by
inadequately treating the sewage, or creating a potential direct or indirect contact
between sewage and the public.


Fecal coliform bacteria bacteria found in the intestinal tracts of warm-blooded animals that are cultured in
standard tests. Generally expressed as colonies per 100 ml.



Gravely Soils with 15% or more, but less than 35% rock fragments by volume.




Gray Water Sewage from bathtubs, showers, bathroom sinks, kitchen sinks, washing machines,
and dishwashers.



Ground water Subsurface water in the zone of saturated soils, seasonally, permanently, or as the
result of the tides.



Holding tank sewage system an on-site sewage system that a holding tank with no discharge outlet




hydraulic loading rate the amount of effluent applied to a given treatment step, expressed as gallons per
square foot per day, (gal./sq.ft./day)



industrial waste Waste from production of consumer goods, mining, agriculture and petroleum
extraction and refining



Infiltrative surface 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 a person approved by the local health office to install on-site sewage systems or
components



Local Health Officer The provincial or municipal health officer.

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