The use of algal-bacterial biofilms to enhance nitrification rates in lagoons: experience under laboratory and pilot-scale conditions

Investigations were undertaken at the Western Treatment Plant (WTP), near M elbourne, Australia, to find ways of increasing overall ammonia and nitroge n removal rates in the WTP lagoon systems. Immobilisation of nitrifying bac teria in biofilms was one approach explored. Preliminary tests showed that algal/bacterial biofilms capable of achieving ammonia removal rates of 3 to 4 mug N/cm(2) h would form on support surfaces immersed in the WTP lagoons . A laboratory-scale investigation was carried out to characterise the infl uence of parameters such as pH, temperature, COD level, dissolved oxygen co ncentration and incubation depth on biofilm performance. This study was fol lowed by a pilot-scale investigation in a series of experimental ponds at t he WTP. This compared the performance of three ponds, two containing 9360 m (2) and 18240 m(2) respectively of a geotextile biofilm support material an d one containing no biofilm support material (the control pond). Ammonia re moval rates comparable to those obtained in the preliminary tests were obta ined when the biofilm support material was within the top 500 mm of the lag oon, i.e. in the photic zone. COD and suspended solids levels in the efflue nts from the biofilm containing ponds were substantially lower than those i n the control pond effluent.