As used in this chapter:

(1) "At-site treatment" means treatment and discharge of combined sewage at the CSO site.

(2) "Baseline annual CSO volume and frequency" means the annual CSO volume and frequency that is estimated to occur based upon the existing sewer system and the historical rainfall record.

(3) "Best management practices" means use of those practices which will best reduce the amount of pollution caused by nonpoint sources so that pollutant loadings in combined and storm sewer flows during rainfall events are minimized.

(4) "Combined sewage" means the mixture of sanitary sewage, infiltration, and inflow.

(5) "Combined sewer" means a sewer that has been designed to serve as a sanitary sewer and a storm sewer, and into which inflow is allowed by local ordinance.

(6) "Combined sewer overflow (CSO)" means (a) the event during which excess combined sewage flow caused by inflow is discharged from a combined sewer, rather than conveyed to the sewage treatment plant because the capacity of either the treatment plant or the combined sewer is exceeded.

(7) "CSO reduction plan" means a comprehensive plan for attaining the greatest reasonable reduction of CSOs at the earliest possible date. The requirements for a CSO reduction plan are as further described in this chapter.

(8) "Department" means the department of ecology.

(9) "Disinfection" means the selective destruction of disease-causing and bacterial indicator group organisms.

(10) "Domestic wastewater facilities" means any CSO treatment or control facility included under the definition of domestic wastewater facilities as defined in chapter 173-240 WAC.

(11) "In-line storage" means storage of sewage within the sewer pipes through the use of regulators and gates.

(12) "Infiltration" means the addition of groundwater into a sewer through joints, the sewer material, cracks, and other defects.

(13) "Inflow" means the addition of rainfall-caused surface water drainage from roof drains, yard drains, basement drains, street catch basins, etc., into a sewer.

(14) "NPDES" means the National Pollutant Discharge Elimination System.

(15) "Off-line storage" means storage of sewage adjacent to the sewer pipe in a tank or other storage device.

(16) "Primary treatment" means any process that removes at least fifty percent of the total suspended solids from the waste stream, and discharges less than 0.3 ml/l/hr. of settleable solids.

(17) "Sanitary sewer" means a sewer that is designed to convey sanitary sewage and infiltration.

(18) "Sanitary sewage" means the mixture of domestic, commercial, and industrial wastewaters.

(19) "Secondary treatment" means any process that achieves the requirements of 40 C.F.R. Part 133 as supplemented by state rule and guidance.

(20) "Storm sewer" means a sewer that is designed to convey surface water drainage caused by rainfall.

(21) "Storm sewer/sanitary sewer separation" means construction of new storm sewers or new sanitary sewers so that sanitary sewage and surface drainage are conveyed in different sewers.

(22) "The greatest reasonable reduction" means control of each CSO in such a way that an average of one untreated discharge may occur per year.