When troubleshooting nitrification, it is essential to first ensure your basic general parameters are applicable. The pH should generally be kept greater than 6.8 and less than 8.3. Alkalinity, which acts as the “food” for nitrifying bacteria, requires a residual of greater than 75 mg/L in the filtered mixed liquor at all times. It is important to test alkalinity residuals in the filtered mixed liquor rather than the final effluent, as alkalinity may be recovered if denitrification occurs in the clarifiers.
Sludge age is highly temperature-dependent. In colder weather, it should generally be greater than six to eight days, while in warmer weather, greater than three to four days is typical. Rapid changes in temperature interfere with the growth of nitrifying bacteria, and nitrification rates are greatly reduced as temperatures decrease. Regarding hydraulic retention time (HRT), bench-scale testing shows it is common for nitrification to occur within five to six hours.
Finally, dissolved oxygen is recommended to be 2 mg/L or greater in most systems. However, this is site-specific, and some systems may require less or more. Pay close attention to septic or anaerobic conditions, as ammonia has the potential to be re-released. If there is low ammonia in the mixed liquor and an increase after the clarifier, the sludge blanket in the clarifier may be septic.
Common troubleshooting steps and microbial health
In the event of a loss of nitrification, it is common to start with a slight decrease in wasting, which increases the sludge age. Next, you should assess microbial health through a microscopic evaluation. Place extra emphasis on floc structure, higher life form organisms, the health of filamentous bacteria, and estimated overall bacterial viability.
There are also optional tests available to evaluate microbial health. Some plants utilize ATP testing; however, this is often case-specific logistically. DNA testing can determine if nitrifying bacteria are present within the biomass. In most municipal treatment systems, it is common to have a DNA read of greater than 1% nitrifying bacteria in treatment plants with successful nitrification. Certain markers — such as excessive sulfur-reducing bacteria or an excessive percentage of bacteria capable of fermentation to propionic acid — are sometimes helpful for greater insight.
Inhibition and bench scale testing
If you suspect toxicity, inhibition testing can provide excellent insight. If a mixed liquor sample is collected and aerated overnight, supplying oxygen via an aeration stone and retesting ammonia may help provide insight related to the “time and numbers” aspect of nitrification. Basic testing may also be done by various bioaugmentation companies in which high amounts of nitrifying bacteria are added to the mixed liquor. If ammonia concentrations deplete during this test, this may indicate there are simply not enough nitrifying bacteria present within the system.
Other bench-scale testing may be done by feeding a known healthy influent to the mixed liquor of the plant, or by feeding the influent of the plant with difficulty nitrifying to a known healthy biomass with nitrifying bacteria at the bench scale. In these instances, simulating treatment plant conditions as best as possible is desired. It is important to consider that inhibition is often intermittent and difficult to catch, and a successful nitrification test does not rule out previous inhibition or nitrification toxicity.
Addressing confirmed inhibition or toxicity
If inhibition or nitrification toxicity is successfully confirmed, there are a few specific actions to take:
- Test for quats: Testing for quaternary ammonia compounds within the biomass and free liquid is often implemented. If quats are present, elimination of the source and/or the addition of quat “blocking” agents are common practice.
- Consider aggressive wasting: If the biomass has high quat accumulation, there are instances in which aggressive wasting of sludge and reseeding are needed to prevent legacy toxicity.
- Neutralize PAA: Ensure any peracetic acid is neutralized prior to biological treatment.
Pursue additional testing: More extensive testing from treatability labs or broad-scale testing for priority pollutants is recommended at this point.
About the author: Ryan Hennessy is the principal scientist at Ryan Hennessy Wastewater Microbiology. He was trained and mentored by Dr. Michael Richard for over 10 years in wastewater microbiology, and serves as a microbiology services consultant. Hennessy is a licensed wastewater treatment and municipal waterworks operator in the state of Wisconsin and fills in as needed for operations at several facilities. He can be reached at ryan@rhwastewatermicrobiology.com. Hennessy's new book Wastewater Microbiology: Filamentous Bacteria Morphotype Identification Techniques, and Process Control Troubleshooting Strategies is now available on Amazon.
