BNR “Then” vs “Now” A Case Study - Kalispell Advanced Wastewater Treatment Plant

An advanced wastewater treatment plant was commissioned in 1992 for the City of Kalispell, which is located in northwest Montana and discharging to a sensitive Rocky Mountain watershed. The plant featured a biological nutrient removal (BNR) process and primary sludge fermentation to ensure complete biological phosphorus (Bio-P) removal. This plant was state-of-the-art BNR at the time and has performed exceptionally well since its commissioning having been awarded many federal, state, and local awards for its excellent effluent quality. Recent rapid growth within the community however has caused plant loading to near original design loading limits and consequently initiated a review of the facility. Stantec, the original designers of the BNR processes, were retained once again to review the process with two objectives in mind: 1) to determine the reserve capacity of the existing BNR process (i.e. bioreactors, secondary clarifiers, and primary sludge fermenter), and 2) provide a longer-term expansion/upgrade plan for the facility in order to provide treatment through to the year 2025.BioWin™ modelling was conducted on the existing Modified University Capetown (UCT) BNR process in order to determine any reserve capacity. Observations regarding favourable nitrifier growth rate, high bioreactor hydraulic residence time, and good sludge settleability and control, were considered in the analysis, and suggest the existing facility may treat an additional 35% BOD load in comparison to the original design loading. With just a fine-bubble retrofit, modelling suggests the existing process can extend treatment through to 2006 – 2007 before expansion of some sort would be required.Beyond 2007 plant expansion is required. Stantec is recommending certain BNR upgrades be implemented in order to simplify the BNR process and reflect advances in the state of BNR technology from “then” to “now”. The primary changes recommended include converting the bioreactor configuration from a Modified UCT process to a Modified Johannesburg process, and converting from two-stage primary sludge fermentation to single-stage primary sludge fermentation.