Water staff reports mixed wastewater plant capacity results; growth impacts uncertain

City water/wastewater staff told council a recent town hall focused on wastewater loading and that compiled data show week-to-week variation that makes a single yes/no answer about future capacity difficult.

Staff presented composite weekly influent sampling and explained the plant’s design criteria (expressed as pounds per day of biochemical oxygen demand and hydraulic capacity). The presenter said the plant has performed “abnormally well” relative to design in some scenarios but that spikes can push the plant near design limits. A model projection shown to council suggested 200–300 additional homes could put the plant at or above design criteria in some cases but not in others, depending on current inflow and loading patterns.

Staff said the plant’s hydraulic loading is relatively low now (the plant rated at 1,500,000 gallons per day while current flows were about 0.5 million gallons per day), which provides extra treatment time; however, inflow/infiltration and other local issues can increase influent strength. Staff recommended reducing existing inflow and addressing users who may be in violation of ordinance requirements to free capacity before approving additional subdivisions.

Council heard that the town hall had about 16 participants and that staff intend to hold similar events annually. No formal land-use approval or development permits were taken during the discussion; staff said development capacity depends on mitigation of existing loading problems and that further data review will inform future decisions.