PDFWAC 246-272B-06150

Design flows.

(1) The design engineer shall use the following minimum design flows, which include the appropriate peaking factors, for developments or portions of developments serving residential facilities such as single-family residences, multi-family dwellings, and mobile home parks with total LOSS design flows of 14,500 gpd or less:
(a) Modifications of existing developments:
(i) 120 gpd per bedroom; or
(ii) 240 gpd per existing dwelling unit, residence or mobile home lot or space, whichever is greater.
(b) New residential developments and new mobile home parks:
(i) 120 gpd per bedroom; or
(ii) 360 gpd per lot or space, whichever is greater.
(2) The design engineer shall use 270 gpd minimum design flow per dwelling unit, residence or mobile home lot or space for residential developments or portions of developments serving residential facilities, with total LOSS design flow greater than 14,500 gpd.
(3) For commercial and nonresidential developments or portions of developments that serve commercial and nonresidential facilities, the department may accept the following:
(a) Design flows listed in Table 4 in this section; or
(b) Design flows from other generally recognized sources, on a case-by-case basis.
Table 4: Typical Design Flows for Nonresidential Facilities
Type of Facility
Design Unit1
Design Flow (gpd)
Schools with showers and cafeteria2
per student
16
 
Schools without showers and with cafeteria2
per person
12.6
 
Schools without showers and without cafeteria
per person
10
 
Boarding schools2
per person
75
 
Temporary farm worker housing2
per person
50
 
Hotels, motels, and B&Bs
per room
130
 
Restaurants2
per seat
50
 
Truck stops and Interstate restaurants2
per seat
180
 
Offices, with showers
per person, per 8-hr shift
25
 
Offices, without showers
per person, per 8-hr shift
15
 
Nursing homes2
per bed
200
 
Elder care facilities, retirement homes2
per bed
100
 
Laundromats2
per machine
500
 
Churches, without kitchen
per seat
3
 
Churches, with kitchen2
per seat
5
 
Day care centers
per person
20
 
Picnic areas
per person
5
 
Campgrounds, RV parks with flush toilets only, no laundry or wet sewer hookup
per camp site
50
 
Campgrounds, RV parks with flush toilets, showers, laundry, no wet sewer hookup
per camp site
75
 
Campgrounds, RV parks with flush toilets, showers, and wet sewer hookup, with or without laundry2
per RV space
120
 
Trailer dump stations2
per dump
40
 
Resort cabin
per person
40
 
Bar, cocktail lounge2
per seat
20
 
1.
Does not include employees and staff, unless indicated.
2.
Indicates potential for waste strength to exceed that of residential strength sewage, requiring Treatment Level E or higher.
(4) For the department to evaluate alternate design flows, a written request and the following information must be submitted:
(a) The preceding year's actual metered flow data read at intervals acceptable to the department to capture seasonal and peak usage; or
(b) Comparable flow data from similar existing facilities, including:
(i) From a minimum of three similar developments; and
(ii) Using a peaking factor if average flows are recorded.
[Statutory Authority: RCW 70.118B.020. WSR 11-12-035, § 246-272B-06150, filed 5/25/11, effective 7/1/11.]