Most textbook descriptions of the nitrification process in wastewater treatment refer to Nitrosomonas species as the AOB and Nitrobacter species as the NOB and this simplification can serve the process operator and troubleshooter well as the two groups have well characterized growth conditions.
Nitrifying bacteria are autotrophs, they use inorganic sources of carbon (such as carbon dioxide and carbonate ion) to produce biomass in contrast to the great majority of the other microbes in the treatment system (heterotrophs) which typically use a variety of organic substances both as an energy and carbon source.
Enforcing industrial pretreatment standards for domestic sewer discharges of substances known to negatively affect the slower growing nitrifiers.
The autotrophs grow and reproduce much more slowly than the heterotrophs, e.g. Nitrosomonas may reproduce (divide) once in eight hours compared to a fast-growing heterotroph that may divide every 20 minutes. In addition, the autotrophs are more sensitive to the growth conditions such as pH, temperature and the presence of toxic compounds.
Maintaining optimal conditions in a wastewater treatment plant is not always practical since it can be cost prohibitive. Instead, several measures can be used to help maintain the nitrifying populations in the face of variable conditions.
These measures include:
- Maintaining a higher than normally desired biomass concentration in the biotreater aeration zone, and then building up an even higher sludge concentration in the biotreater to help hold the slow growing autotrophs in the system under anticipated adverse conditions ( like colder weather).
- Using bioaugmentation with separately grown, concentrated microbial inocula to augment the natural seeding and growth of the autotrophs in the system.
- Keeping tight control over the pH of the system. Lower pH’s (acidic conditions) are particularly adverse.
- Maintaining excess dissolved oxygen in the aeration zone at all times. The autotrophs compete with heterotrophs for dissolved oxygen and the heterotrophs are more efficient at scavenging oxygen at low concentration. A dissolved oxygen concentration of 2 mg/L should be maintained.
- Keeping high concentrations of substances known to be toxic to the autotrophs such as excessively high ammonia concentrations or toxic heavy metal ions such as copper and chromium out of the wastewater entering the system. This can be helpful in industrial settings such as petroleum refineries where relatively high concentrations of ammonia are present in the untreated wastewater and where separation of side streams is more feasible.
- Enforcing industrial pretreatment standards for domestic sewer discharges of substances known to negatively affect the slower growing nitrifiers.