Operators often call the biological treatment units the "bug farm", where they monitor and care for a huge population of living microbes. The organisms growing in the system are present based on:

• Influent composition & concentration
• Operating environment - pH, Temperature, D.O., MCRT, F/M, system design

While there is a great degree of variation in the species of microbes, we can classify many organisms based on their ecological niche in the biological treatment unit. Here are the most common functional groups.

• Heterotrophic Floc Formers – want microbes that uptake soluble organics. Best represented by the sponge analogy. Rapid uptake of organics with actual metabolism as the organisms pass through the system. Ideal organisms form biofilm/floc. This is composed of polysaccharides, proteins, DNA, adsorbed particulate organics, and other biological polymers. Want the “perfect” slime – that gives stable floc without holding too much water (viscous bulking). Manage populations with F/M or MCRT
  • Keeping design F/M favors bacteria with rapid uptake of soluble BOD (organics)
  • Stored organics in EPS used as “food” outside the abundant zone
  • Too little EPS = pin floc or old sludge, too much EPS = non-filamentous or zoogleal bulking
• Niche Bacteria that require operator attention
  
  • AOB/NOB – managed by adjusting MCRT & F/M – Alkalinity, D.O. Key concept is that both AOB & NOB are slow growing cultures so need lower F/M and longer MCRT.
  • PAO – function in similar niche to Thauera/Zooglea. Rapid uptake of soluble organic acids for use as “food” later. To favor PAO over Thauera, have true anaerobic zone (no nitrate) with high organic acids.
  • Denitrifying – very common pathway – nitrate/nitrite removal optimized by low D.O. (anoxic conditions) with soluble organics. Relatively high energy yield so these are common and can even thrive inside “aerobic” reactors.
  • Sulfur Oxidizers/Sulfur Reducers - sulfur oxidizers have convert sulfides into sulfur and eventually sulfate. If you have sulfides in the influent, the SOX are important for AOB/NOB growth. The Sulfur Reducers (SRB) are found in anaerobic zones and generate sulfides - often found in collection systems & anaerobic digesters
    
  • Filaments - Microbes that can grow into high surface area filamentous morphology. Filamentous bacteria in small quantities help floc formation by acting as a biological rebar to strengthen the floc. Once the filaments are bridging floc or extending free into the solution, you start to see filamentous bulking that can be a problem in secondary clarification. 
    
  • Foaming bacteria - this includes Nocardia and M. parvicella. Problem foaming is caused when influent contains insoluble organics (fatty acids or grease). These organisms use hydrophobic EPS to accumulate the fatty acids to support their growth.  These are slower growing organisms than most heterotrophic wastewater bacteria, so they compensate for slow growth by exploiting their ability to grow on insoluble organics.

Biological phosphorus removal requires an operating environment that promotes organisms capable of storing soluble phosphorus inside their cells. Once in the cells, the phosphorus is removed by wasting the biological solids.
 
PAO – organisms with the ability to “store” polyphosphates as energy reserve inside the cell. When faced with abundant organics, they use bound energy (phosphate) to adsorb/absorb organics for later use.  Other organisms can store soluble organics, so the key to phosphorus removal is having the correct mix of organic acids and an anerobic/nitrate free zone before moving to an aerobic zone.  Once the environmental conditions are correct in A2O and other biological phosphorus removal systems PAO specific organisms become favored – most common in WWTP is Candidatus accumulibacter.
 
C. accumulibacter releases phosphorus to fuel the uptake of short chain VFA which are produced by anaerobic metabolism of soluble BOD by bacteria in either the collection system to a lesser extent in the anaerobic digester (methane production reduces VFA concentrations). Long residence time collection systems will yield needed VFA. However, odor control programs using nitrate can prevent VFA as well as H2S formation. Also, if nitrate makes it to the anaerobic zone, PAO organisms will have competition from NRB. 

PAO organism populations can be monitored using molecular tools like other biological nutrient removal cultures. We have noticed that PAO populations drift with changes in residence times, VFA concentrations, and nitrate/nitrite levels. Using a Microbial Community Analysis test for periodic monitoring helps operations improve system control.
 
Interesting note – aerobic/NRB bacteria in aerobic WW systems also use a similar organics uptake/storge mechanism. In conventional activated sludge systems, we see Thauera & Zooglea genera which uptake soluble organics near the influent, store the organics in EPS, and use the EPS to fuel growth in lower F/M conditions that exist outside the influent zone. It is when you don’t have enough time with low F/M that the EPS continues to build creating non-filamentous or zoogleal bulking.
 
Key Points to Promote PAO Growth

• PAO are favored when you combine VFA & Anaerobic (nitrate free) conditions before an aerobic zone.
• Phosphorus is released by the cells in the anaerobic zone and uptake soluble phosphorus in the aerobic zone.
• Need to ensure sufficiently low nitrate/nitrite in the return MLSS to the anaerobic zone.
• Check influent VFA concentrations – you must have enough to promote the unique PAO metabolism.