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Tertiary treatment takes secondary-treated effluent and "polishes" it through fine sand and filters. The process produces a product that has 95 percent of the suspended matter removed.

Sand filtering, UV disinfection and the removal of nutrients (nitrogen and phosphorus) are all part of the tertiary process. Tertiary treatment is required of plants that discharge into freshwater receiving waters.

The Wahiawa WWTP, which discharges to the Wahiawa Reservoir, is a biological, tertiary-level facility, one of the highest levels of wastewater treatment. Laie WWTP, recently acquired by the City, is another facility that utilizes tertiary treatment.