Challenge

Anticipating growth, the town of Kuna in an agricultural area of southwest Idaho decided to build a new wastewater treatment plant (WWTP) designed for enhanced biological nutrient removal. Discharges to the Lower Boise River system contain phosphorus, which ultimately impairs downstream water quality in the Snake River. Total phosphorus (TP) concentrations in Indian Creek, a Boise River tributary that included the WWTP's outfall, are more than 0.07 mg/L, so any technology Kuna installed would have to meet that limit. Other effluent requirements included a 5-day biochemical oxygen demand (BOD5), total suspended solids (TSS), and ammonia.

Solution

To meet the stringent effluent requirements, Kuna chose an MBR system from Siemens Water Technologies designed for a maximum monthly flow condition of 3.5 MGD but anticipated startup flows of only 0.3 MGD – more than a 10:1 turndown of the system.

The system consists of two biological treatment trains – each with an anaerobic reactor, anoxic reactor, two fine-bubble aerobic reactors, and a post-anoxic reactor – followed by four membrane tanks, each with a capacity to install 224 membrane modules.

Several turndown features were integrated into the MBR design, including multiple treatment trains, a dual point mixed liquor return system, temporary installation of mixing equipment, rotary lobe filtrate pumps, partial installation of membranes, multiple versus single pump configurations for turndown, and variable frequency drives on rotating equipment.

Results

Incorporating the above low flow design features into the Kuna MBR system resulted in excellent performance during startup. Influent flows are based on measured filtrate flow. For the first three months of operation, these flows averaged 0.31 MGD, 0.42 MGD and 0.38 MGD, which were very close to the predicted startup flow of 0.3 MGD.

The MBR system demonstrated very efficient BOD and TSS removal during low flow conditions. All measured values for effluent BOD and effluent TSS were reported as less than 3.0 mg/L during the first three months of operation. These are well below the average monthly limits of 30 mg/L.

The system also was able to maintain efficient nitrification during the low flow startup conditions, with effluent nitrates and nitrites averaging 8.32 mg/L and 5.97 mg/L in the first and second months, and ammonia averaging 0.04 mg/L in the second and third months. 

And the Kuna MBR system also achieved the required TP removal with phosphorus removal accomplished chemically using aluminum sulfate.