Wash. Rev. Code § 90.48.162
Any county or any municipal or public corporation operating or proposing to operate a sewerage system, including any system which collects only domestic sewerage, which results in the disposal of waste material into the waters of the state shall procure a permit from the department of ecology before so disposing of such materials. This section is intended to extend the permit system of RCW 90.48.160 to counties and municipal or public corporations and the provisions of RCW 90.48.170 through 90.48.200 and 90.52.040 shall be applicable to the permit requirement imposed under this section. A permit under this chapter is not required for large on-site sewage systems permitted by the department of health under chapter 70A.115 RCW or for on-site sewage systems permitted by local health jurisdictions under rules of the state board of health.
RCW 90.48.162