Recent experiments in our laboratory using both biofilm and suspended biomass reactors have demonstrated high rate nitrification at low pH with known autotrophic nitrifying bacteria originating from wastewater treatment plants refuting previous assumptions that nitrification is significantly inhibited at low pH. Since much of the earlier microbiological work regarding ammonia oxidising bacteria (AOB) physiology was carried out using Nitrosomonas europaea, this model bacterium's capability for high rate nitrification at low pH in a continuous biofilm reactor was tested. A biofilm reactor filled with sintered glass particles was inoculated with a pure culture of N. europaea. The reactor was first operated to high nitrification rates under conditions favourable to N. europaea (pH > 7; high ammonium concentrations). To eliminate inhibitory concentrations of nitrite at low pH, an enriched culture of Nitrospira (a nitrite oxidising bacterium) was then added. The transition from neutral to acidic conditions was attempted by sharply lowering the nitrification rate and by using a feeding solution containing insufficient buffer for complete nitrification. As opposed to other successful transitions, the pH in the N. europaea/Nitrospira reactor initially dropped only slightly and maintained pH > 6 for over two weeks. The reactor reached pH 4.5 only after four weeks. FISH results showed that while the percent of AOB and Nitrospira to eubacteria remained relatively constant at 51.1±8.2% and 40.8±6.4%, respectively, the AOB community changed completely in 60 days from 100% N. europaea to 100% Nitrosomonas oligotropha. Even though N. oligotropha was not intentionally introduced into the reactor, it is apparently much better adapted to conditions of low pH.

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