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.]