Investigations were undertaken at the Western Treatment Plant (WTP), near Melbourne, Australia, to find ways of increasing overall ammonia and nitrogen removal rates in the WTP lagoon systems. Immobilisation of nitrifying bacteria in biofilms was one approach explored. Preliminary tests showed that algal/bacterial biofilms capable of achieving ammonia removal rates of 3 to 4 μg N/cm2·h would form on support surfaces immersed in the WTP lagoons. A laboratory-scale investigation was carried out to characterise the influence of parameters such as pH, temperature, COD level, dissolved oxygen concentration and incubation depth on biofilm performance. This study was followed by a pilot-scale investigation in a series of experimental ponds at the WTP. This compared the performance of three ponds, two containing 9360 m2 and 18240 m2 respectively of a geotextile biofilm support material and one containing no biofilm support material (the control pond). Ammonia removal rates comparable to those obtained in the preliminary tests were obtained when the biofilm support material was within the top 500 mm of the lagoon, i.e. in the photic zone. COD and suspended solids levels in the effluents from the biofilm containing ponds were substantially lower than those in the control pond effluent.

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