SR383 Performance of commercial rainwater and greywater systems (January 2020)

This study was undertaken to understand the actual performance and the practical and financial feasibility of commercial rainwater harvesting and greywater recycling systems in operation in New Zealand. Eight case study buildings were examined between 2014 and 2017. This included water audits, monitoring of water, rainwater and greywater quantity and quality and ongoing discussions on the management of the systems with building managers.

It was found that the buildings with the systems in operation were generally more water efficient to begin with, demonstrated by their water use intensity. Each building had its own set of design lessons, which provide a great opportunity for the design of any future buildings incorporating rainwater and/or greywater systems. Some of these included the way monitoring and mains switchover mechanisms are designed, energy efficient storage and distribution design and education and maintenance requirements for new building owners and managers.

In response to the findings in SR382, rainwater and greywater quality was tested over a 12-month period. The results of a health risk assessment concluded that there was a very low risk of infection from the flushing toilets and urinals. However, a representative level of risk that these systems pose cannot be established.

Overall, the utilisation of rainwater harvesting and/or greywater recycling systems in Auckland is typically feasible. Outside Auckland, fixed wastewater charges are hidden in council rates and as a result provide no incentive for water efficiency or conservation. On top of this lack of financial incentive, the lack of education, guidance and standards for the design and operation of the systems are creating institutional and educational barriers.