Waste Stabilisation Ponds (WSP’s) are still the most common form of wastewater treatment in New Zealand. However, the level of treatment acceptable a generation ago when many WSP’s were constructed is less accepted today. As resource consents become more stringent, Councils are faced with the choice of replacing existing WSP’s with more intensive treatment processes, or upgrading the WSP’s. In many cases, Councils do not have the capital and/or inclination to install more intensive treatment technologies. Instead, many Councils consider relatively low cost WSP upgrades.
This paper reviews the performance of New Zealand WSP’s which have been upgraded using a variety of technologies over the past decade or so. Many treatment technologies are reviewed, including AquaMats, floating wetlands, partitioned ponds, baffles, Actiflo, BioFiltro, wetlands, filtration, and ultra-violet disinfection. The key findings of this review are that upgrades of WSP’s which rely on natural treatment processes invariably retain one major disadvantage of WSP’s – inconsistent and unpredictable performance. In particular, if reliable year-round nitrification is required, upgrading WSP’s is considered to be a high risk option. Where WSP’s are upgraded using physical or chemical treatment processes, the level of treatment attainable is more predictable but still with limitations.