In risky water?
A Quantitative Risk Assessment (QRA) can calculate the human health risks of wastewater – and produce surprising results.
QRA highlights bird problem
Wastewater from the Beachlands/Maraetai sewage treatment plant, east of Auckland, gets into the Te Puru stream and estuary through an adjacent wetland and farm pond.
Earth Consult Ltd and Manukau Water are working on a proposed upgrade of the plant, and asked us to assess the health risks for people swimming in the estuary or taking shellfish from around there, and on livestock drinking water taken from the stream.
We evaluated the risks associated with a range of micro-organisms found in wastewater which often cause gastro-intestinal or respiratory infections in humans, and we used faecal coliforms as a bacterial indicator for the risk to livestock.
The QRA showed that point-source discharge from the treatment plant posed a negligible risk to people swimming or eating shellfish, or to livestock. The main risk was from birds fouling the water in the farm pond.
The people factor
Increasingly, NIWA is combining QRA with ‘comparative risk assessment’. In this approach we:
- compile a list of perceived hazards from wide community consultation
- convene an informed group to assess and compare the risks posed by those hazards
- communicate this back to the community.
NIWA scientist Graham McBride led such a process to assess the human health risks in Lake Taupo and its surrounding area for the 2020 Taupo-nui-a-Tia project. An expert group identified 39 risks, mostly from a list of over 100 potential threats (hazards) drawn up by the community. Most of these were found to pose only a low or medium risk. The highest relative rank went to faecal micro-organisms in roof tank water from bird droppings.
‘It seems that people felt genuinely involved, especially when identifying threats, and this enhanced community acceptance of the results. Effective communication and willingness to consider different views are key factors,’ says Mr McBride.
Assessing risks for Brisbane Water
Brisbane Water manages the wastewater treatment plants in Queensland’s capital city. NIWA scientists, along with ESR, looked at the information already available to Brisbane Water for assessing the public health risk posed by effluent from the Luggage Point and Gibson Island plants. As a result, we recommended further risk assessment procedures. We also reviewed other large coastal wastewater treatment plants to give Brisbane Water some benchmarks for rating their monitoring and management systems.
QRAs can:
- assess whether water poses any significant human health risk even when it does not exceed guidelines for bacterial indicators
- identify which pathogens and which aspects of effluent discharge pose the highest human health risks
- evaluate design options for wastewater treatment and outfall length.