Waste stabilisation ponds have been a popular form of wastewater treatment in New Zealand both for large cities and small communities. Over 100 systems have been constructed ranging in size from over 500 ha to less than 0.1 ha. The largest pond system in New Zealand was at Manukau, Auckland and consisted of four ponds with a total of 530 ha. However, ongoing odour and midge releases and an algae parasite problem plus a requirement to reduce ammonia and total nitrogen in summer, led to the decision to decommission the ponds and allow the area to revert to tidal mudflats. The second largest pond system is at Christchurch and totals 226 ha. In contrast to Manukau, the Christchurch ponds have not caused significant odours and final effluent quality has been good. Christchurch has two parallel trains each with three ponds in series. It is proposed to retain and develop the ponds into a seven cells in series arrangement to reduce short-circuiting.
Odour emissions from the Manukau and Christchurch ponds have been measured by the use of olfactometry. Dispersion of odours has been modelled and the extent of “odour travel” determined on a statistical probability basis using actual meteorological data. It can be demonstrated that residential areas can co-exist with ponds, which are not overloaded, with separation distances of 200 metres.