The bacteria responsible for most ammonia and nitrite removal in wastewater treatment systems - new data

In most of my training and consulting materials, I have spoken of nitrification as a combination of two distinct microbial genera - Ammonia oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB) - which were normally listed as Nitrosomonas sp. (AOB) and Nitrobacter sp. (NOB). Other genera were mentioned but considered secondary (if ever mentioned). The organisms and reactions are given below: 

What we have been teaching is not exactly correct in most wastewater systems! With more advanced genetic testing, we have found that most chemoautotrophic nitrification is being done by Nitrospira sp. which can do both steps. Therefore, Nitrospira can be thought of as AOB & NOB cultures. Nitrosomonas are much less common (lower percentage of total reads in testing). 

No matter which organisms are removing ammonia and nitrite, the following checklist comes from working wastewater treatment plants that achieve ammonia and nitrite oxidation.

• Typically in the field 70-80% organic reduction should have occurred prior to trying nitrification.  Usually COD levels at effluent should be 100-150 mg/L while BOD < 40 mg/L.
• The majority of organics must be degraded since the biomass contains 93-97% heterotrophs and 3-7% chemoautotrophs.  Thus, if high carbon content is present, the heterotrophic organisms will out-compete the slower growing autotrophic nitrifying bacteria for essential nutrients.
• Rule of Thumb:  As the BOD:TKN ratio decreases, nitrification kinetics increase.
• Dissolved Oxygen (DO) is vitally important.  Although critical DO (in vitro) is 0.2 mg/L, field DO should never drop below 0.5 mg/L.  Optimal range is >2.0 mg/L for no inhibition whatsoever. 
• The oxygen required to oxidize 1 gram of NH3-N to NO2-N is approximately 3.5 grams O2.  From NO2-N to NO3-N, it is 1 gram of O2 per gram of NO2-N.
  
  

Activated Sludge
pH                                                      6.5-8.5, 7.2 - 7.8 is optimum for MLSS
Temperature                                        10 - 38 Deg C, 30 Deg C optimum
Effluent BOD5                                    < 30 mg/L
Effluent COD                                     <100-150 mg/L
Effluent TOC                                      <45 mg/L
MLSS                                                 2,500 mg/L
Sludge Age (MCRT)                             5 - 15 days
DO                                                    >2.0 mg/L, 2.0 - 4.0 in systems subject to shock