(1)
OSS must only be designed by a
professional engineer, licensed under chapter
18.43 RCW, or an
OSS designer, licensed under chapter 18.210 RCW, except:
(a) If at the discretion of the local health
officer, a resident owner of a single-family residence not within 200 feet of a
marine shoreline is allowed to design an OSS for that residence; or
(b) If the local health officer performs the
soil and site evaluation, the health officer may design the OSS.
(2) The
designer shall use the
following criteria when developing a design for an
OSS:
(a) All sewage from the building served is
directed to the OSS;
(b) Sewage
tanks are in compliance with chapter
246-272C WAC;
(c) Drainage from the surface, footing
drains, roof drains, subsurface stormwater infiltration systems, and other
nonsewage drains is prevented from entering the OSS, the area where the OSS is
located, and the reserve area;
(d)
The
OSS is designed to treat and disperse the
sewage volume as follows:
(i) For single-family residences:
(A) The operating capacity is based on 45 gpd
per capita with two people per bedroom;
(B) The minimum design flow per bedroom per
day is the operating capacity of 90 gallons multiplied by 1.33 to account for a
33 percent surge capacity. This results in a minimum design flow of 120 gallons
per bedroom per day;
(C) The local
health officer may require a factor greater than 33 percent to account for
surge capacity;
(D) The minimum
design flow of the OSS is 240 gpd; and
(E) The local health officer may require an
increase of the design flow for dwellings with anticipated greater flows, such
as larger dwellings; or
(ii) For single-family residences with one
additional dwelling served by the same
OSS:
(A) All requirements in (d)(i) of this
subsection apply;
(B) The minimum
design flow for one additional dwelling is 120 gallons per bedroom;
and
(C) The local health officer
may require an increase of the design flow for dwellings with anticipated
greater flows; or
(iii)
For three or more dwellings served by the same
OSS:
(A) All requirements in (d)(i) of this
subsection apply;
(B) The minimum
design flow for the first dwelling is 240 gallons per day;
(C) The minimum design flow for each
additional dwelling is 120 gallons per bedroom;
(D) The local health officer may require an
increase of the design flow for dwellings with anticipated greater flows;
and
(E) The local health officer
shall require documentation including, but not limited to, an easement,
covenant, contract, or other legal document authorizing access for
construction, operation, maintenance, and repair; or
(iv) For other facilities, the design flows
noted in "On-site Wastewater Treatment Systems Manual," USEPA,
EPA-625/R-00/008, February 2002 must be used. Sewage flows from other sources
of information may be used in determining system design flows if they
incorporate both an operating capacity and a surge capacity;
(e) The
OSS is designed to address
sewage quality as follows:
(i) For all
systems, the
designer shall consider:
(A)
CBOD5, TSS, and O & G;
(B) Other parameters that can adversely
affect treatment anywhere along the treatment component sequence. Examples
include pH, temperature, and dissolved oxygen;
(C) The sensitivity of the site where the
OSS
will be installed. Examples include areas where
fecal coliform constituents can
result in public health concerns, such as shellfish growing areas, designated
swimming areas, and other areas identified by the local management plan
required in WAC
246-272A-0015; and
(D) Nitrogen contributions. Where nitrogen
has been identified as a contaminant of concern by the local management plan
required in WAC
246-272A-0015, it must be
addressed through lot size, treatment, or both.
(ii) For
OSS treating
sewage from a
nonresidential source, the
designer shall provide the following information
showing:
(A) The sewage is not industrial
wastewater;
(B) The sewage effluent
quality and identifying chemicals found in the sewage effluent are not found in
sewage effluent from a residential source; and
(C) A site-specific design providing the
necessary treatment equaling required treatment of sewage effluent quality from
a residential source;
(f) The vertical separation used to establish
the treatment levels and application rates. The selected vertical separation
must be used consistently throughout the design process; and
(g) Treatment levels:
(i) Requirements for matching
treatment
component and method of distribution with soil conditions of the
soil dispersal
component are listed in Table VI of this section. The treatment levels
correspond with those established for treatment components under the product
performance testing requirements in Table III of WAC
246-272A-0110. The method of
distribution applies to the
soil dispersal component.
(ii) Disinfection may not be used:
(A) To achieve BL1 or BL2 in type 1 soils;
or
(B) BL3.
Table VI
Treatment Component Performance Levels
and Method of Distribution1
|
Vertical Separation in
inches
|
Soil Type
|
|
1
|
2
|
3-6
|
|
12 [LESS THAN] 18
|
A & BL1 - pressure with timed
dosing
|
B & BL2 - pressure with timed
dosing
|
B & BL2 -pressure with timed
dosing
|
|
[GREATER THAN EQUAL TO] 18 [LESS THAN]
24
|
B & BL2 - pressure with timed
dosing
|
C & BL3 - pressure with timed
dosing
|
C & BL3 - pressure with timed
dosing
|
|
[GREATER THAN EQUAL TO] 24 [LESS THAN]
36
|
B & BL2 - pressure with timed
dosing
|
C & BL3 -pressure with timed
dosing
|
E - pressure with timed dosing
|
|
[GREATER THAN EQUAL TO] 36 [LESS THAN]
60
|
B & BL2 - pressure with timed
dosing
|
E - pressure
|
E - gravity
|
|
[GREATER THAN EQUAL TO] 60
|
C & BL2 - pressure
|
E - gravity
|
E - gravity
|
1 The treatment component
performance levels correspond with those established for treatment components
under the product testing requirements in WAC
246-272A-0110.
(3) The
coarsest textured soil within the vertical separation selected by the designer
determines the minimum treatment level and method of distribution.
(4) The
local health officer shall not
approve designs for:
(a) Cesspools;
or
(b) Seepage pits.
(5) The local health officer may
approve a design for the reserve area different from the design approved for
the initial OSS, if both designs meet the requirements of this chapter for new
construction.
Notes
Wash. Admin.
Code §
246-272A-0230
Amended by
WSR
24-06-046, Filed 3/1/2024, effective
4/1/2024
Statutory Authority:
RCW
43.20.050. 05-15-119, § 246-272A-0230, filed
7/18/05, effective 7/1/07.
