SEPTIC INSTALLER WASHINGTON STATE
WOSSA FINAL EVALUATION 2026 TESTED
QUESTIONS GRADED A+
⩥ Acceptable Pipe Materials.
Answer: Vitrified clay, ductile iron, or plastic.
Anything after the tank or from distribution box has to be vitrified clay
or plastic.
⩥ Schedule 40 or Greater.
Answer: under roads, driveways, or parking areas
⩥ Gravity Slope.
Answer: Diameter: 4" pipe + min. slope of 12" per 100'
⩥ Building sewers in excess of 50' should have what?.
Answer: A C/O @ least every 50' from building to tank, distribution box,
or ATU
⩥ Creviced Limestone Formation.
Answer: SS system cannot be installed where bottom of system is less
than 4' from top of limestone
,If limestone is known to be present a soil test must be done @ least 4'
deeper than where the bottom of the trench will be
⩥ How far away can quick disconnect be?.
Answer: no more than 50' away from the device & w/in sight of
⩥ Experimental Use Permits.
Answer: min. of 10 experimental systems installed b4 unconditional
approval granted. Permit valid for 2 yrs & evaluated @ end of 2 yrs.
⩥ 2 options for garbage grinders.
Answer: 1. solids retention tank btwn WW source & septic tk. for
kitchen waste only. Needs to be 50% of the size of the designed septic
tank w/ a min. of 500 gals.
2. A septic tank receiving all flows sized as 50% of the septic tank size +
septic tank size. i.e., septic tk = 1500 gals. /2 = 750 + 1500 = 2250
(would need 2500 gal tk)
⩥ How long should a homeowner keep maint. records for their system?.
Answer: the life of the system
⩥ When should the 1st maintenance of a septic tank after installation be
completed for a residence?.
,Answer: Prior to or within 3 yrs. After initial inspection it can be done
once every 5 yrs. Can be done by everyone + homeowner
⩥ If sludge layer /suspended solids are greater than 33% . . ..
Answer: Must pump tank and perform maintenance
⩥ When should a septic system be evaluated initially after installation
for a non-residential bldg?.
Answer: prior to or w/in 3 yrs. After that it should be done @ least every
3 yrs.
Owner cannot do. has to be engineer, env. health practitioner, member of
dept, HD, or septic contractor
⩥ Aerobic Treatment Unit Evaluations.
Answer: 1 every 6 mos @ least (same ppl) + HOMEOWNER as long as
they follow manufacturer's requirements & proper maintenance
⩥ What is the maint. schedule for SFs & WSPs w/ surface discharges?.
Answer: @ least 1 per yr. + H.O. if rules r followed. Pumped when
solids & scum greater than 33%
⩥ Can a H.O. install & maintain a septic system on his/her property?.
Answer: Yes
, ⩥ Who can perform the excavation for a septic system?.
Answer: A licensed installer (employee while supervised) or the property
owner.
⩥ What is the min. size tank for flows UP TO 500 gpd?.
Answer: 750 gals.
⩥ If flows are greater than 500 gpd, how do you size the tank?.
Answer: Tank size should be = to @ least 1.5x the estimated sewage
flow
⩥ How do you size a tank(s) for flows greater than 1350 gpd?.
Answer: U must use multiple tks. in a series or multiple compartment
tks. where the 1st tank or the 1st comp. in a multicomp. tk. is 1/2-2/3 of
the total required capacity. of the tank(s)
⩥ How do you size the tank for a raised filter bed for a residence?.
Answer: It has to be an NSF 40 batch treatment system. The aeration
tank has to be 2x the daily flow
⩥ How do you size the tank for a RFB for a commercial bldg?.
Answer: The aeration tk. has to be 3x the daily flow.
WOSSA FINAL EVALUATION 2026 TESTED
QUESTIONS GRADED A+
⩥ Acceptable Pipe Materials.
Answer: Vitrified clay, ductile iron, or plastic.
Anything after the tank or from distribution box has to be vitrified clay
or plastic.
⩥ Schedule 40 or Greater.
Answer: under roads, driveways, or parking areas
⩥ Gravity Slope.
Answer: Diameter: 4" pipe + min. slope of 12" per 100'
⩥ Building sewers in excess of 50' should have what?.
Answer: A C/O @ least every 50' from building to tank, distribution box,
or ATU
⩥ Creviced Limestone Formation.
Answer: SS system cannot be installed where bottom of system is less
than 4' from top of limestone
,If limestone is known to be present a soil test must be done @ least 4'
deeper than where the bottom of the trench will be
⩥ How far away can quick disconnect be?.
Answer: no more than 50' away from the device & w/in sight of
⩥ Experimental Use Permits.
Answer: min. of 10 experimental systems installed b4 unconditional
approval granted. Permit valid for 2 yrs & evaluated @ end of 2 yrs.
⩥ 2 options for garbage grinders.
Answer: 1. solids retention tank btwn WW source & septic tk. for
kitchen waste only. Needs to be 50% of the size of the designed septic
tank w/ a min. of 500 gals.
2. A septic tank receiving all flows sized as 50% of the septic tank size +
septic tank size. i.e., septic tk = 1500 gals. /2 = 750 + 1500 = 2250
(would need 2500 gal tk)
⩥ How long should a homeowner keep maint. records for their system?.
Answer: the life of the system
⩥ When should the 1st maintenance of a septic tank after installation be
completed for a residence?.
,Answer: Prior to or within 3 yrs. After initial inspection it can be done
once every 5 yrs. Can be done by everyone + homeowner
⩥ If sludge layer /suspended solids are greater than 33% . . ..
Answer: Must pump tank and perform maintenance
⩥ When should a septic system be evaluated initially after installation
for a non-residential bldg?.
Answer: prior to or w/in 3 yrs. After that it should be done @ least every
3 yrs.
Owner cannot do. has to be engineer, env. health practitioner, member of
dept, HD, or septic contractor
⩥ Aerobic Treatment Unit Evaluations.
Answer: 1 every 6 mos @ least (same ppl) + HOMEOWNER as long as
they follow manufacturer's requirements & proper maintenance
⩥ What is the maint. schedule for SFs & WSPs w/ surface discharges?.
Answer: @ least 1 per yr. + H.O. if rules r followed. Pumped when
solids & scum greater than 33%
⩥ Can a H.O. install & maintain a septic system on his/her property?.
Answer: Yes
, ⩥ Who can perform the excavation for a septic system?.
Answer: A licensed installer (employee while supervised) or the property
owner.
⩥ What is the min. size tank for flows UP TO 500 gpd?.
Answer: 750 gals.
⩥ If flows are greater than 500 gpd, how do you size the tank?.
Answer: Tank size should be = to @ least 1.5x the estimated sewage
flow
⩥ How do you size a tank(s) for flows greater than 1350 gpd?.
Answer: U must use multiple tks. in a series or multiple compartment
tks. where the 1st tank or the 1st comp. in a multicomp. tk. is 1/2-2/3 of
the total required capacity. of the tank(s)
⩥ How do you size the tank for a raised filter bed for a residence?.
Answer: It has to be an NSF 40 batch treatment system. The aeration
tank has to be 2x the daily flow
⩥ How do you size the tank for a RFB for a commercial bldg?.
Answer: The aeration tk. has to be 3x the daily flow.
