Nitrifying biofilms: structure, co-operation and performance

Dempsey, Michael J. (2000) Nitrifying biofilms: structure, co-operation and performance. In: 147th Ordinary Meeting of the Society for General Microbiology, 12-15 September 2000, Exeter, UK.

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Abstract

An expanded bed, nitrifying bioreactor designed to remove ammonia from effluents has been in operation for several years. It was inoculated with a mixed microbial population from a nitrifying trickling filter, and it took several weeks for visible biofilms to form on the support material, coke. This biomass support material is non-toxic, hard-wearing, cheap and porous. Its porosity makes it ideal for the attachment of adhesive microorganisms, which first colonise the pores and then overgrow the particles, to form biofilms approximately 0·5 mm thick. This results in a biomass concentration of up to 40 g dm-3 (dry weight estimated from volume). Transmission electron microscopy of detached biofilms revealed a mixed population, mainly of bacteria, occurring primarily as colonies of similar cells. The use of Response Surface Methodology to simultaneously optimise the temperature and pH of the nitrification process revealed these optima to lie in the range 14-16°C and pH 7·8-8·0. When operated at these optima, the biofilms were capable of oxidising a 140 mg dm-3 ammonia feed at a maximum rate of 120 mg dm-3 h-1; which equated to achieving complete nitrification at a dilution rate of 0·85 h-1. This was equivalent to a residence time of less than 1·2 h, a recirculation ratio of 70 and the removal of 2.0 mg dm-3 per pass. With a 280 mg dm-3 ammonia feed, the maximum rate of 21 nitrification was 145 mg dm-3 h-1; which equated to achieving complete nitrification at a dilution rate of 0·5 h-1. This was equivalent to a residence time of about 2 h, a recirculation ratio of 120 and the removal of 2.4 mg dm-3 per pass. The amount of ammonia oxidised per pass was limited mainly by ammonia-limitation and the insolubility of oxygen in water, as the bioreactor relied on aeration during recycle of the effluent. The nitrification capacity of the biofilms was found to be equivalent to an ammonia removal rate of up to 5·5 kg NH3- N m-3 d-1.