Cell Yield (Y) in wastewater systems

Cell yield (Y) is the amount of new cell mass created per unit of substrate removed. In wastewater, we usually define substrate as BOD5, COD, or TOC. In most cases a true defined substrate is unknown becuase the influent is a mix from multiple streams. So over the years, data has been collected from multiple systems with differing influent makeup, temperatures, F/M ratios, pH, and any number of other environmental factors. I'll distill some of the information below:

Aerobic Yield

• With no information on F/M ratio or influent makeup, I use 0.5 g cells/g BOD5 in the influent.
• With starch and other readily degradable influents, you can increase this to 0.6 g cells/g BOD5.
• High F/M rations (less biomass, more substrate) - you have a higher cell yield which decreases gradually as the F/M lowers - N.B. it is not a linear relationship between F/M and Y. The lower cell yield is actually do to endogenous respiration (cell death) that occurs in the confines of the treatment system so it does have some advantages & disadvantages (I'll post more on this later).
• In practice you see cell yields anywhere from 0.4 - 0.6 in most cases.
  
  

Anaerobic Yield

• From an energy generation standpoint, anaerobic respiration results in less "energy" for cell growth than aerobic systems. The result is less cell yield per unit of substrate.
• Normal cell yield (Y) is 0.06 g cells/g substrate - which is approximately 10x less than yields fro aerobic systems. This is why high strength wastewaters are often treated with anaerobic digestion.

Now for a general table of Cell Yield