What are the primary factors impacting biological treatment efficiency?

Biological waste treatment units are designed to provide an environment that promotes rapid microbial growth and metabolism. Our job in running the system is to ensure we provide the best possible growth environment, but what constitutes the ideal growth environment? I have a standard list that includes the following:

Aerobic Systems

• Dissolved Oxygen (DO)
  
• Temperature
  
• pH
  
• Alkalinity
  
• Ammonia
  
• Total Nitrogen (TKN)
  
• Phosphorus (ortho & total)
  
• COD/BOD/TOC (measure of organic loading concentration)
  
• Influent makeup (look at ease of biodegradation or potential toxicity)
  
• Metals (especially heavy metals)
  
• Chlorides or salinity
  
• Residence time in biological unit
  
• Biological solids in the unit (MLSS/MLVSS)
  
• Sludge age (days)
  
• Position on growth curve (determined by microscopic exam and oxygen uptake rates)
  

Anaerobic/Anoxic Systems

• Redox potential (mV)
  
• pH
  
• Tempreature
  
• Alkalinity
  
• Methane in off gas - if available
  
• Carbon dioxide in off gas- if available
  
• COD/BOD/TOC (loading)
  
• Residence time
  
• Metals
  
• Influent makeup (look for difficult or toxic compounds)
  
• Biological solids
  
• Ammonia & nitrogen in/out - (ammonium, nitrite, nitrate, TKN)
  
• Phosphorus in/out
  
• Trace micronutrients (if methane production is unexpectedly low)