This paper reviews the performance of two waste stablisation ponds (WSP) systems in the South Island of New Zealand that have been upgraded to multiple ponds-in-series to improve effluent quality. Results of monitoring are provided which show that it is possible to achieve relatively low ammonia (approximately 1 g/m3) and total nitrogen (approximately 10 g/m3) effluent concentrations through the use of nitrification filter beds (rock trickling filters) and sand filters. Evidence suggests that the nitrification and denitrification processes in the extra biofilm surface area provided by the rock filters or rock bank protection is primarily responsible for the improved effluent quality. The paper also compares the WSP results with effluent quality predicted by published formulae. It is concluded that these formulae do not reliably predict the performance of WSP systems and the development of universally applicable design guidelines would be useful.

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