people answer is every 20 minutes. Well this is correct for E. coli under proper lab conditions. Each organisms has a potential maximum growth rate (Mmax). Of course this maximum growth rate is on a specific media, temperature, pH, oxygen concentration, and abundance of nutrients. In wastewater, we are dealing with a much more wonderful, wild environment with much more variation in environments and nutrient sources. K-rate strategists organisms - those that use fast growth on commonly used substrates - tend to double every 30 minutes to 2 hours. The more niche organisms such as Ammonia Oxidizing Bacteria (AOB), also called r-rate strategists, can take 6 hours to days to divide. This is why a system has to operate at a longer MCRT to maintain ammonia removal when compared to a non-nitrifying system.
In addition to growth rates, cell yield must be considered when thinking about wastewater treatment. A commonly cited yield is 0.5 g biomass per 1 gram influent soluble BOD5. While the 0.5 g/g BOD5 may be more uniform across systems than individual microbial growth rates, you do see variation in cell yields. Systems with lower F/M will have lower cell yields from endogenous respiration - where microbial growth slows, cells lyse, and cell maintenance requires a larger portion of available energy (food). Another important fact is the makeup of influent BOD5, COD, or TOC (whichever is used). Remember that these are a proxy measurement standing in for multiple compounds in any given influent. Some compounds require more energy to degrade or make biologically available. In this case, cell yield and growth rates are lower.
So what does this mean for operating a wastewater treatment system? To determine true growth and yield rates, you need to carefully measure influent, effluent, and perform a mass balance. Don't just run at one point in time. With good monitoring and use of spreadsheets, we can look at time series trends and moving averages that gives better indications for growth and yield.
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
Click to set custom HTML