Comment

Basic and crucial treatment process data, such as specific growth rate for nitrifying organisms (µn), and the dependent solids retention time (SRT), are omitted in the MBR analysis.

Response

The power of a modern, state-of-the-art process model is that although nitrifier growth rate is much more complex than the textbook calculations imply, this complexity can be simulated with a computer. For reference, Biowin’s default maximum growth rate for Ammonia Oxidizing Bacteria (AOB, nitrifiers) is 0.9/day. The base rate expression for nitrifier growth process is the product of the maximum specific growth rate, the ammonia oxidizing biomass concentration and a Monod expression for ammonia. This base rate is modified to account for environmental conditions (e.g., zero growth at low dissolved oxygen and inhibited by nitrous acid), nutrient limitations (phosphate, inorganic carbon, other cations and anions) and pH inhibition. The MBR models were run at several different SRTs (12 to 30 days, to capture the range of temperature and loading conditions experienced in Big Sky). One of the primary advantages of MBR technology is that it enables longer-than-usual SRTs to ensure nitrification is complete. This information will be included in the Final Draft.