Land treatment of sewage effluent for Rotorua : an evaluation of spray irrigation of effluent into Whakarewarewa State Forest Park

Abstract

Selection of a sewage treatment and disposal method is a problem currently being faced by Rotorua City. The city is built on the shores of Lake Rotorua, a lake highly valued for tourism and recreation, but also the recipient of the city's sewage effluent. The lake's eutrophication and accompanying declining water quality have highlighted the conflict in its use.

A large proportion of nutrient input to the lake comes from sewage effluent. Consequently, the water right requires the removal of a substantial proportion of the effluent's phosphorus load. To achieve the necessary phosphorus removal, an alternative treatment method is required to the one presently used in Rotorua.

Because land treatment of effluent is not currently being considered by Council, this report conducts preliminary evaluation of a land treatment option for Rotorua.

The land treatment option considered in this report is spray irrigation of effluent into Whakarewarewa State Forest Park. Preliminary investigation is conducted of existing sites and the likely implications of a land treatment system. Preliminary estimates are presented of design requirements and costs.

The proposed land treatment alternative is compared with the alternatives currently being considered by Council. The comparison uses the framework of cost-benefit analysis. Because only financial costs of the options are measured, the analysis alone does not identify the best alternative.

This study suggests land treatment may be a cost competitive alternative capable of achieving higher phosphorus removals than those required by the water right. However, a concern with the proposed land treatment site is its location within a State Forest Park. Because of its location, its suitability for effluent application is questioned.