§ 18.21.190. Design standards for institutional sewerage systems.

A.

General Consideration of Use of Septic Tank Systems. Septic tanks may be used as a means of sewage treatment for nonresidential activities. However, experience indicates that the usefulness of the septic tank systems decreases as the size of the establishment served increases. A registered professional engineer shall be required to design all private sewage facilities serving institutions.

B.

Sewage Loading. The total quantity of sewage applied per day to the septic tank provides the basis for the determination of its size. Table 18.21.190 will be of assistance in estimating the daily sewage flow per capita for a variety of living and activity situations. Organic loading of sewage from restaurants, hospitals, nursing homes, and other commercial establishments will require an increase in tank size.

C.

Compartments to be Provided. Two compartments or two or more treatment units connected in series shall be utilized; shall be provided, the first created by a wall with a tee to permit liquid flow from the first compartment to the second one. The flow line of this intermediate fitting shall be at the elevation of the flow line of the outlet fitting of the second compartment. The fitting shall be three inches below the elevation of the flow line of the first compartment's inlet fitting. The capacity of the first compartment shall be one to two times the capacity of the second compartment. All tanks must be vented internally.

D.

Selection of Septic Tank Capacity. The net volume or effective capacity below the flow line of a septic tank for flows up to two hundred fifty gallons per day shall be at least seven hundred fifty gallons. For flows between two hundred fifty and five thousand gallons per day, the capacity of the tank shall be equal to at least three days sewage flow. For daily flows over five thousand gallons per day, the determination of the necessity for a waste discharge permit must be obtained from the commission.

E.

Pump Tank Usage. When treated sewage must be pumped to the disposal area, an effluent pump shall be placed in a watertight tank. A check valve shall be provided if the disposal area is upgrade and higher than the pump. The tank shall be provided with a highwater alarm having a power circuit separate from the pump. The electrical connections shall be hard-wired external to the pump chamber. Effluent pump tanks shall have a minimum capacity of one thousand gallons after the alarm is activated. For daily flows over five hundred gallons, a duplex pump configuration must be provided.

(Ord. 9448 (part), 1988)