Equipment:	Centra-flo™ Gravity Sand Filter with Blue PRO® Four (4) Model CF-64
System Size:	0.35 MGD average flow
Deliverable:	Less than 0.05 mg/L total phosphorus
Installation Date:	Start-up in September 2010
Location:	Plummer WWTP, Idaho

The new Plummer WWTP replaced an old lagoon plant for a small town. Upon startup, the Blue PRO^® system achieved <0.050 mg/L total phosphorus with a chemical dose of only 10 mg/L iron for the entire plant.

Plummer, Idaho arrived at a total phosphorus discharge limit of <0.050 mg/L to protect water quality in the river that flows through town, and makes its way into Lake Coeur d’Alene. This low phosphorus discharge concentration was agreed upon through cooperation with the United States Environmental Protection Agency (EPA), the Idaho Department of Environmental Quality, and the local Coeur d’Alene Tribe. The entire Lake Coeur d’Alene, Idaho and Spokane River, Washington watershed area is a region heavily impacted by nutrient loading, particularly phosphorus from both point and non-point sources. The same impact can be seen in the Grand River and Lake Simcoe Watersheds.

The new WWTP is a packaged biological nutrient removal (BNR) plant, with sequencing aeration and continuous clarification, including an anaerobic selector. The secondary system is intended to lower phosphorus to <1 mg/L biologically. This is followed by Blue PRO^® tertiary reactive filtration for phosphorus removal and UV disinfection. Final future design flow for the plant is 0.315 MGD.

The Blue PRO^® tertiary reactive filtration system consists of two continuous backwash, upflow filters with hydrous ferric oxide (HFO) coated sand for adsorption of phosphorus arranged in series. The reject streams from both filters are recycled to the secondary system that allows for the uptake of phosphorus by the excess adsorptive capacity leftover in the HFO waste particulates. The final fate of the HFO particulates is in the waste solids.

Performance testing at the plant was complete in October 2010, with results at 0.020-0.024 mg/L total phosphorus. Additionally, 24-hour composite testing for 7 consecutive days produced an average result of 0.036 mg/L total phosphorus. As plant operations stabilize the effluent concentration is expected to drop further.

In the reactive filtration system, ferric sulfate or ferric chloride is dosed in front of the filters to allow the continuous regeneration of the HFO coated sand. At Plummer, this iron dose is 6 mg/L as Fe in the first filter, and 4 mg/L as Fe in the second filter. Besides this 10 mg/L Fe, there is no other phosphorus removal chemical dosed in the system, and no pH adjustment is necessary.