About solids yield - what determines kg of biosolids per kg of loading

Cell yield is the amount of biological solids per unit of loading. Loading can be described as specific chemicals (ex. glucose or ammonia) but often we use BOD5 or COD as a proxy given the mixed nature of wastewater influent.  Cell yield is higher when the energy source (food) provides more energy for cell division. 

For domestic wastewater BOD5, we usually use 0.5 - 0.6 grams biomass per gram BOD5 in the influent as the yield (Y). In this case biomass includes living cells, extra cellular polymers (EPS), and all substances in the floc (MLSS). As handling and disposing of biological solids is expensive, often we would like to lower yield or amount of wasted biosolids.

Potential ways to decrease solids yield:

• Increase sludge age (Lower F/M) - by increasing biological solids in the system, you are moving along the growth curve to endogenous respiration or decline phase growth. With very low F/M, cells consume adsorbed organics and extracellular materials. This is exactly what happens in an aerobic digester. Costs for running in extended aeration mode include more energy requirements for maintaining D.O. and clarifier solids capacity to handle the increased solids flux (more solids to clarifier).
• Fixed film or MBBR addition - The addition of biofilm technology to an activated sludge system works by increasing biological solids. The film part keeps the biological solids out of the secondary clarifier. You still need to budget for more oxygen requirements from added biomass in the aeration basin.
• Bioaugmentation - You often read about adding enzymes, micronutrients, or 'bugs' to a system and seeing reduced cell yield. This works in higher F/M conditions where adding microbes helps move along the growth curve. It also helps when influent components, such as FOG, create conditions where water and insoluble compounds build up in the MLSS resulting in larger volumes of solids (bacteria yield is actually the same but you have less water and EPS in the wasted solids). 
  

Biological solids production is a given in a wastewater system. You can reduce solids by operating at decline/endogenous phase growth, but this requires energy for D.O. , maintaining solids in suspension (or providing a MBBR/fixed film support), and having secondary clarifiers capable of handling the biological solids  without carryover. No matter what strategy you use to minimize biological solids yield, you must eventually waste solids. What we call old sludge contains more particulates, dead cells, and inorganics than target MLSS. When you start consuming EPS that binds foc, the fine particles we call pin floc start to increase effluent turbidity and TSS.