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.

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​Prior to using metagenomics testing in wastewater, most engineers assumed the MLSS microbial had a constant microbial composition. MLSS would contain the fastest growing, best bacteria for a given influent. Occasional environmental or influent shift would alter the biomass, but it would always return to single ideal microbial mix.

With the use of metagenomics testing, Aster Bio has found that systems have a changing microbial makeup depending upon their position on the growth curve. 

Looking at the difference in K-strategist and r-strategist microorganisms allows use to understand the changes in MLSS composition. r-strategists have the ability to grow rapidly and thrive when “food” is plentiful. Biomass development from lag through log phase growth centers around the population explosion of r-strategists. In wastewater terms, a r-strategist has a fast doubling rate, tolerance to more toxic compounds, and thrives on a range of organics. Once the system reaches steady-state or decline phase growth the microbial population pressures start to favor k-rate strategists. Unlike r-strategists, the k-strategists have lower growth rates and ability to exploit ecological niches. 

Operating an activated sludge system requires wasting to maintain a healthy biomass (MLSS). While conventional activated sludge and contact stabilization units have small aeration basins with high cell yield per unit of water treated, they normally have sizable aerobic digesters to continue organic decomposition and take the biomass further down the growth curve where cells begin to lyse (decline or endogenous phase growth). Many industrial wastewater systems and smaller municipal systems now use extended aeration where part or nearly all of the aerobic digester function is moved into a larger main aeration basin. This means lower F/M or loading rates per unit of MLSS. 

With extended aeration, cell yield is lower and cells actually lyse and enter endogenous respiration in the aeration basin. Often in an attempt to lower solids disposal costs, operators reduce wasting below design rates. The idea is the bacteria will continue to thrive by lysing more cells, leaving a higher non-volatile percentage in the biomass. While this can save money in the very short run, here is why not wasting ends up costing money:

• Extreme low F/M results in weak floc and turbidity. Often we see polymer feed on the secondary clarifiers to remove the "fines".
• Without EPS (biological polymers that create floc) at normal levels, the biomass loses some of the normal adsorption capability - ability to act like a sponge for uptake of insoluble organics and also becomes more susceptible to toxic shocks without the protective EPS.
• Oxygen required to keep excess solids in the system. Energy required for aeration is often one of the biggest variable costs in running an activated sludge systems. While respiration rates decline in extreme low F/M operations, oxygen is still required to keep the biomass in the aerobic zone. A healthy biomass balances respiration rates and total solids to maximize your utility costs.

Most activated sludge treatment units still rely on secondary clarifiers to settle biological solids for recycle and wasting. A slow settling rate caused by bulking results in loss of biomass over the clarifier weir - causing effluent TSS compliance issues and a loss of MLSS concentration in the aeration basin. The following is a short list of the most common settling problems with both cause & effect (From Jenkins et al. (1984).