Like children given broccoli or candy at Halloween, bacteria prefer to grow on the "tasty" compounds first. In this case, heterotrophic organisms grow fastest on soluble compounds with glucose often being the "ideal" energy source. The key concept here is - net energy yield! Cells have to produce enzymes and undertake other biochemical processes all of which take energy when growing. These enzymes and biochemicals transform organics into simple compounds that fuel cellular machinery and yield energy.
Here are a few general rules of bacteria calculus:
- Simple soluble organic compounds are degraded first. Tells why insoluble FOG tends to accumulate.
- Aerobic processes yield more cellular energy than anaerobic.
- While complex organics such as lignin or Polynuclear Aromatics Hydrocarbons (PAH) are biodegradable, the bacteria using these compounds much be either slow growing niche organisms or have exhausted the "easy" energy sources
- AOB/NOB (nitrifiers) are among the chemotrophic organisms that occupy a niche - they may have some other pathways, but most of their energy comes from oxidation of ammonia and nitrite. This has lower energy yield than most organics, so AOB & NOB have lower growth rates.
So how do you use bacterial calculus to predict behavior in wastewater?
- High energy reactions will happen first. As energy sources are exhausted, the population will shift to other energy sources.
- Low F/M - means that you are forcing the bacteria to work on the lower energy reactions sooner than would be in a high F/M system.
- Low F/M - means that niche organisms have more of a chance to acquire macronutrients and other growth factors as competition with fast growing organisms is reduced.