An equipment failure at the Moa Point wastewater treatment plant last Wednesday sent raw sewage into the sea directly off Wellington’s south coast.
Since Thursday night, the untreated wastewater has been flowing into Cook Strait a kilometre from the coast.
Wellington Water is advising that people stay away from local beaches and not collect kai moana. A rāhui is in place over the affected area.
It could be months before the plant is fully repaired, and before people can go swimming at south coast beaches.
The SMC asked experts to comment.
Associate Professor Ricardo Bello Mendoza, Department of Civil and Environmental Engineering, University of Canterbury, comments:
“The Moa Point wastewater treatment plant’s floods last week serve as a reminder of how critical wastewater infrastructure is for the functioning of our cities. Events like these, resulting in raw sewage being dumped into the coast, threaten public health and damage the natural environment.
“It is also a reminder that we need to invest in our wastewater infrastructure to provide the quality service that New Zealanders deserve. Infrastructure New Zealand has estimated that $120 to $185 billion in investment is required in our water infrastructure over the next 30 years. One of the drivers for investment is climate resilience. Heavy rain in the last few weeks has put pressure on wastewater treatment plants around the country. In Christchurch, personnel at the wastewater treatment plant in Bromley have been fighting odour problems caused by recent heavy downpours. Fortunately, the situation seems to be improving and is under control. Heavy rain events are likely to occur more frequently in the coming years, and we need to be prepared to mitigate their effects. We also need to learn from this unfortunate lesson in wastewater engineering. A comprehensive investigation into the causes of the Moa Point problem is required to prevent similar events in the future.
“Unfortunately, once a wastewater treatment facility is damaged to the extent of Moa Point’s, there is little that can be done in terms of alternative treatment. Sadly, raw wastewater is likely to continue to flow into the Cook Strait via the long outfall pipe for several weeks.”
Conflict of interest statement: “The author declares no conflict of interest.”
Associate Professor Hamish Mackey, Department of Civil and Environmental Engineering, University of Canterbury, comments:
“There is little I could add to what [Wellington Water chief executive] Pat Dougherty has already mentioned. The Wellington Water plan and timeframes are reasonable. We are talking significant and complex infrastructure, and hazardous and challenging working conditions to get the plant online again. While treatment plants keep key replacement items on site, there is a limit to what is practical and financially viable, meaning certain specialised mechanical and electrical equipment that has been damaged will likely need to be shipped from overseas.
“There is little in the way of alternative options to treat/manage the magnitude of sewage arriving at the plant other than direct outfall discharge under a catastrophic plant failure. The Moa Point wastewater plant has been receiving average flows of 65,000 m3 per day. That’s equivalent to 26 Olympic swimming pools, or in the order of 4000+ sewage tankers. In wet conditions that could increase by 5-10 times. For very small plants, temporary treatment systems providing basic settling of solid material or aeration could possibly be used to reduce organic loads, but for flows of Moa Point’s magnitude, this is not practical. The space required for such systems, the construction times and the cost would all be prohibitive.
“The best approach will be to get various elements of the plant functioning again as quickly as possible to provide partial treatment as each treatment unit is brought online. This will be dependent on the exact design of the plant and the options to bypass various treatment stages.”
Conflict of interest statement: No declaration received.