Evaluating waste streams for nitrifier inhibition

Chemostat used as SBR reactor for bench testing of influents

Keeping stable Ammonia Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB) populations in wastewater can be a challenge. Some factors that can result in population instablity include:

• Low or high temperatures
• High organic loading - 80% of BOD5 typically needs removal before nitrification starts
• Low D.O.
• Sludge age is less than needed to support populations of slower growing AOB/NOB
• Inhibitory compounds (many things can slow or totally inhibit AOB/NOB growth)

Over the past several weeks, I have been watching Aster Bio's lab run a very intesting test for examining waste streams for chronic toxicity or inhibition. Each reactor was run as a sequencing batch reactor (SBR) and we monitored outlet ammonia, nitrite, and nitrate. We also diretly measured the AOB/NOB populations in the MLSS using qPCR testing. The goal was to determine the impact on AOB/NOB populations of both the individual waste stream with varying dilutions.

What I have learned or seen proof of working during the testing

• AOB cultures are more sensitive to changes in waste makeup & inhibition than NOB
• Changes in influent makeup often cause a drop in AOB/NOB numbers. But without significant inhibition, the AOB/NOB populations will naturally increase - this time lag can be reduced by adding AOB/NOB culture slurry.
• Inhibition is rarely just one single chemical in wastewater. Several inhibitory compounds can combine to inhibit growth.
• Various pre-treatment and in-biological unit options exist to reduce the toxicity and promote AOB/NOB growth. Optimizing the growth environment for nitrifiers is important including removing BOD5, degrading inhibitory compounds (phenol, biocides, surfactants), and keeping environmental factors in the optimal range.