A shed at Stratford's wastewater treatment plant in Victoria Road houses the pump that collects the samples. Photo / Ilona Hanne
Explainer: Stratford's wastewater tested positive for Covid-19 last week. What does that mean and how does that testing happen?
What is meant by the term wastewater, and why is it being tested?
Wastewater is the water from toilets, showers, baths, basins, sinks and laundries that goes from homes and businesses to
the wastewater treatment plant. Testing helps health authorities know if there might be Covid-19 in the community, even before someone tests positive for it.
Who does the testing?
The Institute of Environmental Science and Research (ESR) is in charge of testing the country's wastewater, looking for the presence of viral fragments of Sars-CoV-2 (the virus that causes Covid-19) after samples are sent to its laboratory in Wellington from sites throughout Aotearoa New Zealand.
How are the samples collected?
In Stratford, it is collected from the town's wastewater treatment plant in Victoria Rd. This is done by automatic composite sampling, which is ESR's preferred method. For this, a pump at the treatment plant automatically collects a small volume of wastewater every 10 minutes over 24 hours, that combined liquid is then sent to Wellington for testing.
In plants that don't have the facility for automatic composite sampling, the sample is instead collected by what is called grab sampling. In grab sampling, the sample is taken at a point in time, rather than spread over a longer period.
What happens next?
When the ESR scientists receive the samples, they get to work. Because viruses, including Covid-19, can be in both the water and the solids (poos) they use methods that can recover viruses from both. It is then concentrated down to about half a teaspoon of solid matter to be tested. The process then is the same as it is for testing a swab taken from a person. Called a PCR (polymerase chain reaction) test, it is a laboratory method to make large numbers of copies from a very small sample of genetic material. So the test can find really small amounts of virus genes in a sample.
How often is Stratford's wastewater tested?
Recent testing has been weekly, however, this has moved to daily since last Friday's positive result was identified, says Stratford District Council chief executive Sven Hanne.
Does last week's positive result mean someone in Stratford has Covid-19?
No. Taranaki District Health Board medical officer of health Dr Jonathan Jarman says the positive result could indicate one of several things. It could be due to recently recovered cases returning to the region from MIQ who are non-contagious but still shedding the virus, from visitors to the town, or it could signal undetected cases in the community.
Did this test cover septic tanks too?
No. Not everyone in the district is connected to the town's sewerage system, some are on septic tanks. Currently the system services over 2200 properties, or 97 per cent of the Stratford urban area. Waste from septic tanks is disposed of into the same treatment plant, via the septic tank dump site in Esk Rd; however, the dump station was not used in the timeframe leading to the positive result.
What about motorhomes visiting the area - could they have used the dumping station?
Yes, and monitoring at the motorhome dump site shows a total of eight motorhomes did use it in the timeframe under question.
Can people catch Covid-19 from wastewater?
No.There's no evidence the virus causing Covid-19 can be transmitted through wastewater, either before or after treatment. The viral fragments themselves are not infectious.
How can people keep themselves safe from Covid-19?
Dr Jarman says testing and vaccination are our best protection.
"Testing will be key to detecting community transmission and if we get onto it quickly, we might be able to cope better if Covid-19 arrives here. But of course, vaccination is our best protection. The higher our vaccination rates are in Taranaki, the less impact it will have on our healthcare services, our businesses, and our everyday living.
"Please remember to be extra vigilant wearing face coverings, scanning in everywhere you go, washing hands or sanitising, and social distancing as much as possible."