Cell yield (or biological solids production) from biological waste treatment

Treatment of wastewater results in the removal of organic components measured as BOD/COD/TOC and the final conversion of organics into carbon dioxide, water, and new microbial cells. The new cells form the mixed liquor suspended solids in activated sludge or the biofilm in fixed film systems. In any case, normal operation includes keeping a fixed amount of microbial biomass to treat influent loading. So some of the cells must be wasted (keep a stable food/microorganism (F/M) or sludge age (MCRT).

How must wasting needs to be done is calculated based on cell yield per unit of organic treated. Using BOD5 or soluble BOD as the influent loading, we often plug in 0.5 g of new cells per gram of BOD5. However, this is not a constant or fixed number and can easily vary from 0.3 - 0.7.  What causes variation in cell yield? I will list a few:

• Composition of the influent - more complex/difficult organics result in less cell yield
• F/M ratio - lower F/M ratios result in less growth
• MCRT - works the same way as F/M
• Dissolved Oxygen - aerobic has much higher cell yield than facultative or anaerobic metabolism
• Environmental factors - temperature, pH, metals, etc in the water

The greatest impact on cell yield in aerobic systems comes from manipulating the F/M or MCRT to an "older sludge". However too old a sludge can start to cause problems with water clarification (biosolids start to become pin floc) and there is also a utility costs to maintaining oxygen with excess solids (much harder to quantify) - so I usually leave the proper MCRT question at keeping strong floc or biofilm in the system.