Current through Register Vol. 24-18, September 15, 2024
(1)
Sewage
tank loads. Sewage tanks must be designed and constructed to withstand
all structural, hydraulic, hydrostatic, earth loads, and any anticipated
traffic loads. They must be designed and constructed so they:
(a) Do not collapse, deform, or crack when
subjected to the anticipated loads when the tanks are either full or
empty;
(b) Support a dead load
equivalent to at least three feet of earth cover with a unit density of at
least 110 lb/ft3 and a 2,500
lbf/wheel load concentration over the critical elements
of the tank. Tanks installed with more than three feet of earth cover must be
reinforced to support the additional load;
(c) Account for minimum hydrostatic load of
62.4 lbs/ft3 and support earth backfill and
hydrostatic pressures. Minimum lateral load calculations must include pressures
due to effective weight of adjacent earth backfill and hydrostatic loads
assuming the water table is at ground level;
(d) Allow for septage pumping during high
groundwater conditions. Internal hydrostatic pressures must be included in the
calculations to allow for septage pumping during high groundwater conditions
assuming a water table is at ground level;
(e) Counteract buoyancy effects, assuring an
adequate flotation safety factor in high groundwater areas. The design engineer
shall submit to the department calculations to demonstrate the tank's ability
to counteract buoyancy effects and include this information as part of the
sewage tank installation instructions; and
(f) Withstand a wheel load of 16,000
lbf/wheel with fourteen feet axle spacing consistent
with a HS20-44 loading as designated by AASHTO, if designed as a "traffic
bearing tank."
(2)
Construction materials. Sewage tanks must be designed and
constructed of solid, durable and watertight materials that do not corrode or
decay. Steel sewage tanks are prohibited. Acceptable materials include:
(a) Concrete for cast-in-place tanks;
and
(b) Concrete, fiberglass,
polyethylene or other solid, durable, watertight material that does not corrode
or decay for prefabricated tanks.
(3)
Connections and components.
Sewage tanks must be designed and constructed using structurally sound and
watertight access connections or components, either into the tank or through
the tank's walls. Sewage tank connections and related components include:
(a) Inlet and outlet fixtures;
(b) Electrical conduits; and
(c) Access ports, inspection ports, and
risers.
(4)
Inlets, outlets, and intercompartmental fittings or baffles.
(a) Sewage tank inlets, outlets, and
intercompartmental fittings must:
(i) Provide
effective scum storage and sludge retention; and
(ii) Be constructed of a durable material and
attached to the walls of the tank in a secure and corrosion resistant
fashion.
(b) All inlet
and outlet devices must have sanitary tees constructed of:
(i) PVC conforming to or exceeding the
requirements of ASTM D 3034; or
(ii) ABS conforming to or exceeding the
requirements of ASTM D 2680.
(c) All sanitary tees must have a minimum of
four inches inside diameter. For a larger capacity tank, the diameter must be
greater to accommodate the design flow.
(d) Concrete baffles are allowed if cast with
the tank pour. Concrete baffles installed after the tank has been poured are
not allowed.
(5)
Seals and gaskets. Seals and gaskets for inlet, outlet, and
intercompartmental fittings must be resilient, watertight, corrosion-resistant,
and flexible. Seals meeting ASTM C-1644, or equivalent must be used to join the
tank wall and the PVC piping to prevent leakage at the wall
connection.
(6)
Water-tightness. Sewage tanks must be watertight and prevent
surface drainage and groundwater from entering into the tank or connected
chambers. The department and local health officers are encouraged to require
testing sewage tanks in the field at installation.
(7)
Air space and venting.
Sewage tanks must provide air space to allow gases to vent through the main
building sewer vent or other plumbing vent stacks to the atmosphere.
(a) Air space must be above the liquid
surface in the tank back and through the tank's inlet.
(b) Sewage tanks must maintain at least a
one-inch air space between the underside of the top of the tank and the top of
any of the inlet, outlet, or intercompartmental fitting to vent
gases.
(c) Sewage tanks that do not
adequately vent through the building plumbing vent stacks must:
(i) Use a carbon-filtered vent above the
ground surface; or
(ii) Bury the
end of the vent in a gravel trench in a manner adequate to prevent infiltration
from groundwater or surface water.
(d) Use another sewage tank venting method
approved by the department according to the requirements under WAC
246-272C-0500.
(8)
Confined space.
Designs must take into account whether the space is a confined space. Confined
spaces must comply with the department of labor and industries' requirements in
chapter 296-809 WAC, Confined spaces.
(9)
Forms or processes.
Manufacturers of prefabricated tanks may use any form or process to construct
the tank, provided the tank meets or exceeds the standards and requirements in
this section through WAC
246-272C-0250.
(10)
Coatings. Coatings,
sealants or liners may be added to the inside or outside of the sewage tanks to
enhance corrosion protection and water-tightness of the tanks. All coatings,
sealants, or liners must be rated and warranted by the manufacturer for use
with sewage or sewage effluent.
(11)
Access openings and risers.
Access openings must be large enough for a person with equipment to easily
clean, maintain, remove, and replace sewage tank components.
(a) The minimum diameter of the sewage tank
opening must be:
(i) Eighteen inches for tanks
with a liquid volume of less than or equal to two thousand gallons;
and
(ii) Twenty inches for tanks
with a liquid volume greater than two thousand gallons.
(b) Maximum distance between access points on
a tank must be ten feet center-to-center.
(c) Access openings must be located above the
inlet and the outlet.
(d) Access
openings must be located directly above any pumping or dosing equipment, or
effluent screen or filter.
(e)
Risers must be a minimum of twenty-three inches in diameter.
(f) Connection of the riser to the tank and
the connection of additional riser sections must incorporate joint grooves or
adapters to prevent lateral movement and to remain watertight.
(g) Access and riser openings must be covered
with a lockable lid or other type of secured lid to prevent unauthorized
entry.
(h) Access risers and lids
must be structurally sound to withstand the anticipated site-specific load
conditions of the riser.
Statutory Authority: RCW 43.20.050(2) and (3). 09-23-119,
§ 246-272C-0210, filed 11/18/09, effective
12/19/09.
