Biofilm development for a nitrification process

Lannigan, Kevin C. and Dempsey, Michael J. (2000) Biofilm development for a nitrification process. In: 147th Ordinary Meeting of the Society for General Microbiology, 12-15 September 2000, Exeter, UK.

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Abstract

Ammonia released into the aquatic environment causes a series of problems, including toxicity to fish and oxygen depletion through nitrification, with an increased risk of eutrophication through consequent nitrate production. Sources of ammonia include industrial, agricultural and sewage treatment effluents; and its removal requires an industrial nitrification process. A key feature of nitrifying bacteria is their slow growth rate (Nitrosomonas td ˜ 7 h; Nitrobacter td ˜ 13 h), which means that to retain them in conventional suspension culture systems, the residence time must exceed their doubling time. This would result in processes of excessive volume, unless the cells are retained in some way. Cell retention by natural immobilization has advantages over other methods, in terms of cost and long-term stability. Immobilization onto small support particles provides a large surface area for cell attachment (3,620 m2 m-3 for 1 mm spheres) and these cells can multiply to form thick biofilms. It also allows for expansion of the bed of particles, so that each one is suspended and surrounded by the upflowing aqueous medium. This reduces any substrate or product transport delays to diffusional limitation within the biofilm, by minimising the boundary layer. Immobilization onto a particulate support material and operation as an expanded bed allows the dilution rate to exceed the maximum growth rate of the cells without them being washed out. Previous work has shown that coke is a suitable support material for microbial colonisation, although the best type for immobilization of nitrifying bacteria is not known. In this study, five types of coke were evaluated by feeding them from a common reservoir of artificial effluent, which was recirculated through the system. This “effluent” was fed separately to five columns (2·5 x 50 cm), each containing 50 cm3 of coke, so that 50% bed expansion was achieved. The system was inoculated with a mixed culture of nitrifying bacteria, obtained from an expanded bed bioreactor, which had been running for six years. The system was fed at a low rate, with a growth medium containing ammonium sulphate (420 mg dm-3 [NH3-N]) and trace elements in a phosphate buffer. One type of coke was colonised at a much higher rate than the others, with a maximum increase of over 1mm per day in the static bed height, which equated to 1% of the original bed height per day.