Quantitative microbial Risk Assessment - Recent Advances in New Zealand and Their Application to Moa Point WWTP Bypass Discharges

Quantitative Microbial Risk Analysis is increasingly being used for the assessment of Public health risk in the New Zealand water sector. It may now be considered a ‘Best Practice’ when assessing public health risk associated with wastewater discharges. Traditionally, the health risks associated with such discharges are assessed against faecal indicator bacteria concentrations (FIBs). These have been established from epidemiological studies that may not adequately mimic conditions, particularly near outfalls. QMRA, together with advancing enumerative laboratory techniques has allowed the outfall-associated public health risk assessment to focus directly on pathogens. Another significant benefit of QMRA methodology is that it allows the particular effect of an activity to be isolated from the gross effect of allsimilar activities in the same geographical space (e.g., stormwater discharges).

This paper, based around a recent QMRA of bypass flows from the Moa Point (Wellington City) Wastewater Treatment Plant (WWTP), discusses recent advances in the use of QMRA in New Zealand. In particular, a) the adoption of human Norovirus as a key risk organism b) derivation of a non-point source model for the risk of respiratory illness and c) the move from ‘Risk of Infection to ‘Risk of Illness’ assessments.

We also discuss issues that have arisen that will require further research as this powerful risk assessment methodology gains further acceptance and, as is already the case in some situations, ‘required’ status.