Improving the effluent discharge from oxidation ponds in New Zealand with membrane polishing

Many of New Zealand’s Waste Water facilities are faced with the prospect of having to upgrade their existing treatment plants to meet higher influent flows and stricter effluent quality guidelines to obtain resource consent for their effluent discharge. Their challenge is to find innovative solutions that work with their existing plant equipment. Many facilities already use oxidation ponds to reduce the biological loads and suspended solids levels in the effluent, but seasonal variations in flow can have a large impact on the effectiveness of these ponds. Additional filtration is required to improve the solids removal. Traditional water filtration methods, like sand filtration, are not sufficient to meet the stricter guidelines in variable conditions and often require large amounts of space that are not available at existing treatment facilities. Two New Zealand plants, at Hikurangi WWTP and Dunedin Airport, have met this challenge by complementing their oxidation ponds with low-pressure membranes. This paper explores the innovative engineering designs employed to upgrade their existing equipment, and the motivations and results for choosing a low-pressure membrane solution to reduce the impact of the effluent discharge on New Zealand’s natural water resources.