High nitrification rate at low pH in a fluidized bed reactor with chalk as the biofilm carrier.

A typical steady state bulk pH of about 5 was established in a nitrifying fluidized bed with chalk as the only buffer agent. In spite of the low pH, high rate nitrification was observed with the nitrification kinetic parameters in the chalk reactor similar to those of biological reactors operating at pH>7. Various methods were used to determine the reasons for high rate nitrification at such low pH including (i) determination of bacterial species, (ii) microsensor measurements in the biofilm, and (iii) comparison of nitrification performance at low pH with a non-chalk fluidized bed reactor. Fluorescence in situ hybridization (FISH) using existing 16S rRNA-targeted oligonucleotide probes showed common nitrifying bacteria in the low pH chalk reactor. The prevalent nitrifying bacteria were identified in the Nitrosomonas oligotropha, Nitrosomonas europeae/eutropha, Nitrosospira and Nitrospira related groups, all well known nitrifiers. Microelectrode measurements showed that the pH in the biofilm was low and similar to that of the bulk pH. Finally, reactor performance using a non-chalk biofilm carrier (sintered glass) with the same bacterial inoculum also showed high rate nitrification below pH 5. The results suggest that inhibition of nitrification at low pH is highly overestimated.

Production of gaseous nitrogen compounds in a novel process for ammonium removal.

The production of gaseous nitrogen compounds, particularly the greenhouse gas nitrous oxide, was investigated in a novel process for ammonium removal from wastewater. The process is based on the adsorption of ammonium on zeolite followed by bioregeneration. The zeolite serves the dual purpose of an ion exchanger and a physical carrier for nitrifying bacteria which bio-regenerate the ammonium saturated mineral. An analysis of the nitrifying population composition in the reactor fed with simulated secondary effluent (NH4+ = 50 mg/l) revealed that about half of the bacteria in the biofilm were common ammonium oxidizers Nitrosococcus mobilis and Nitrosomonas, while the other half were nitrite oxidizers. The amount of nitrogen losses, under different conditions, and the identification of the emitted gases (N2 or N2O) were investigated in two sets of experiments: (I) batch experiments using biomass originating from the ion exchange reactor with and without the addition of nitrite, and (II) continuous experiments using the ion exchange reactor with zeolite as the biomass carrier. In the batch experiments, nitrite and oxygen concentrations were determined as the major parameters responsible for the formation of gaseous nitrogen gas during ammonia oxidation by autotrophic bacteria. Continuous experiments showed that the major parameter significantly affecting nitrogen losses was the amount of ammonium adsorbed by the zeolite during the ion exchange phase. The amount of ammonium adsorbed determines the ammonium concentration during the initial period of bioregeneration, which in turn directly influences oxygen demand and the resulting concentrations of oxygen and nitrite. It was concluded that the formation of nitrogen gas compounds in the ion exchange/bioregeneration process can be eliminated by adjusting the operational regime to have a shorter adsorption phase resulting in smaller amounts of ammonium adsorbed per cycle.