The Alpine wastewater treatment plant is a membrane bioreactor (MBR) type of activated sludge facility designed to treat a maximum day flow of 0.4 MGD and be expanded to 0.8 MGD.
The primary treatment facilities consist of two parallel 2 mm rotating screens with integral washer compactor and screenings bagging system, influent flow measurement, and a raw wastewater lift station.
The secondary/tertiary treatment process has two flow trains each consisting of a flow equalization/anoxic/pre-aeration basin and a membrane biological reactor. The raw sewage is comingled in the anoxic basin with the mixed liquor returned by gravity from the MBR. Denitrification, removal of nitrate from the process stream, occurs when the heterotrophic bacteria convert nitrates (NO3) to nitrite and nitrogen gas as part of cellular respiration. The flow then enters a pre-aeration basin where the dissolved oxygen is increased to 2.0 mg/l with fine bubble aeration and variable speed blowers utilizing a DO probe to control the process. The wastewater is then lifted into the membrane bioreactor at a constant flow which is adjusted once per day based on influent flow.
Each membrane bioreactor (MBR) contains 1600 submerged flat plate membranes that filter the mixed liquor, retaining the solids in the bioreactor and allowing the clear effluent to pass through. Each membrane unit in the MBR has integral air diffusers to furnish air for process oxygen requirements, membrane cleaning, and mixing requirements. The bioreactor can be operated at a mixed liquor suspended solids (MLSS) concentration of 8000 to 12000 mg/l.
The clear effluent flows by gravity through an ultra-violet (UV) treatment process for final disinfection and is then discharged to the Snake River at Palisades Reservoir. The flat plate membranes provide greater than 6-log removal of bacteria and 4-log removal of viruses, so disinfection requirements are very low.
Typical effluent has BOD5 and TSS concentration below 5 mg/l, ammonia less than 1 mg/l, nitrate less than 10 mg/l, turbidity less than 0.1 NTU, and fecal coliform non-detected.
Biosolids are wasted to an aerobic digester where they are treated to meet Class B standards. The solids concentration in the digester is thickened to 3% via another submerged membrane to increase the storage capacity of the digesters. The digested biosolids are placed into a trailer mounted gravity filter that retains the solids and allows the water to pass through the filters and back to the treatment plant headworks. The biosolids at a concentration of 15% are then used at the plant site as a soil amendment or hauled to the landfill.