As used in this chapter, unless the context indicates otherwise:

(1) "Seven-day average" means the arithmetic mean of pollutant parameter values for samples collected in a period of seven consecutive days. The department may use pollutant parameter values for samples collected in a calendar week for determining compliance with permit conditions.

(2) "Thirty-day average" means the arithmetic mean of pollutant parameter values for samples collected in a period of thirty consecutive days. The department may use pollutant parameter values for samples collected in a calendar month for determining compliance with permit conditions.

(3) "BOD" means five-day biochemical oxygen demand.

(4) "CBOD" means five-day carbonaceous biochemical oxygen demand.

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

(6) "Department" means the Washington department of ecology.

(7) "Director" means the director of the Washington department of ecology.

(8) "Discharge standard" means a minimum performance requirement established in regulation by the department. Effluent limitations for a pollutant parameter shall not be less stringent than the applicable discharge standard.

(9) "Domestic wastewater" means water carrying human wastes, including kitchen, bath, and laundry wastes from residences, buildings, industrial establishments, or other places, together with such groundwater infiltration or surface waters as may be present.

(10) "Domestic wastewater facility" means all structures, equipment, or processes required to collect, carry away, treat, reclaim, or dispose of domestic wastewater together with such industrial waste as may be present. In the case of subsurface sewage treatment and disposal, the term is restricted to mean those facilities treating and disposing of domestic wastewater only from:

(a) A septic tank system with subsurface sewage treatment and disposal and an ultimate design capacity exceeding fourteen thousand five hundred gallons per day at any common point; or

(b) A mechanical treatment system or lagoon followed by subsurface disposal with an ultimate design capacity exceeding three thousand five hundred gallons per day at any common point.

Where the proposed system utilizing subsurface disposal has received a state construction grant or a federal construction grant under the Federal Water Pollution Control Act as amended, such system is a "domestic wastewater facility" regardless of size.

(11) "Effluent concentrations consistently achievable through proper operation and maintenance" means:

(a) For a given pollutant parameter, the 95th percentile value for the thirty-day average effluent quality achieved by a wastewater facility in a period of at least twenty-four consecutive months, excluding values attributable to equipment failures, operational errors, overloading, and other unusual conditions; and

(b) A seven-day average value equal to 1.5 times the value derived under (a) of this subsection.

(12) "Effluent limitation" means any restriction, prohibition, or specification established by the department in a permit or administrative order on:

(a) Quantities, rates, percent removals, and/or concentrations of physical, chemical, or biological characteristics of wastes which are discharged into waters of the state; and

(b) Management practices relevant to the prevention or control of such waste discharges.

Effluent limitations shall be derived from discharge standards and other relevant factors identified in chapter 173-220 WAC.

(13) "Expansion" means the construction of additional treatment units to accommodate hydraulic flow and/or pollutant load for the purpose of increasing the existing design capacity of the wastewater facility.

(14) "Fecal coliform" means the group of coliform bacteria which originate in the intestinal tract of warm-blooded animals.

(15) "Industrial wastewater" means the water or liquid carried wastes from industrial or commercial processes as distinct from domestic wastewater. These wastes may result from any process or activity of industry, manufacture, trade, or business, from the development of any natural resource, or from animal operations such as feedlots, poultry houses, or dairies. The term includes contaminated stormwater and also leachate from solid waste facilities.

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

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

(18) "Interfere with" means a discharge by an industrial user which, alone or in conjunction with discharges by other sources, inhibits or disrupts the domestic wastewater facility, its treatment processes or operations, or its sludge processes, use or disposal and which is a cause of a violation of any requirement of the domestic wastewater facility's permit (including an increase in the magnitude or duration of a violation) or of the prevention of sewage sludge use or disposal by the domestic wastewater facility in accordance with the following statutory provisions and regulations or permits issued thereunder (or more stringent state or local regulations): Section 405 of the Federal Water Pollution Control Act, the Solid Waste Disposal Act (SWDA) (including Title II, more commonly referred to as the Resource Conservation and Recovery Act (RCRA)), and including state regulations contained in any state sludge management plan prepared pursuant to Subtitle D or the SWDA, the Clean Air Act, the Toxic Substances Control Act, and the Marine Protection Research and Sanctuaries Act.

(19) "Permittee" means the entity to which the department issues a permit.

(20) "pH" means the negative logarithm of the hydrogen ion concentration.

(21) "Sanitary sewer" means a sewer which is designed to convey domestic wastewater and infiltration.

(22) "State" means the state of Washington.

(23) "Trickling filter" means a fixed growth biological treatment system in which wastewater is sprayed over the top surface of a column of rock or synthetic media. This definition does not include fixed growth biological systems which have a supplemental biological treatment system, other than a waste stabilization pond(s), for the principal wastewater stream.

(24) "TSS" means total suspended solids.

(25) "TSS concentrations achievable with waste stabilization ponds" means a TSS value, determined by the department, which is equal to the effluent concentrations achieved ninety percent of the time within the state or appropriate contiguous geographical area by waste stabilization ponds that are achieving the levels of effluent quality for BOD specified in WAC 173-221-050 (2)(a).

(26) "Waste stabilization pond" means basins built by excavating the ground and by diking for the purpose of treating wastewater under conditions that favor natural biological treatment and accompanying bacterial reduction. This includes domestic wastewater facilities which are classified as stabilization ponds, or aerated lagoons per the department's Criteria for Sewage Works Design.

(27) "Wastewater facility" means all structures and equipment required to collect, transport, treat, reclaim, or dispose of domestic, industrial, or combined domestic/industrial wastewaters.

(28) "Waters of the state" means all lakes, rivers, ponds, streams, inland waters, groundwaters, salt waters, and all other waters and watercourses within the jurisdiction of the state of Washington.

(29) "Water quality standards" means the standards set forth in chapter 173-201 WAC.

(30) "Wet weather" means the time during and immediately following rainfall events which cause large quantities of inflow.