Wetlands could remove viruses from wastewater
A wetlands complex could be the simplest, most natural way to remove human viruses and bacteria from Gisborne’s treated wastewater.
Medical Officer of Health Dr Bruce Duncan, who chairs the Wastewater Technical Advisory Group (WTAG), says using plants and sunlight in the design of a wetland makes sense.
He and fellow WTAG member molecular biologist John Mackay are looking at how simple things like plants, sunlight, bark chips and some metals could help naturally destroy viruses and bacteria in the environment.
“We know sunlight, our natural UV, kills viruses. Rather than blasting treated wastewater with an electrically powered UV disinfection system, we could use sunlight as it flows within wetlands,” Dr Duncan says.
Their work is part of Gisborne District Council’s joint-funded trials to see if a wetlands complex is a realistic alternative to ultraviolet disinfection. This was to be added to the city’s wastewater treatment by the end of 2014 but WTAG recommended a wetland as the best alternative. By 2020, WTAG and Gisborne District Council hope to see the city’s treated wastewater – also called effluent – flow to land instead of sea. Wastewater treatment is being upgraded to ensure cleaner water in the bay.
Dr Duncan says domestic sewage contains a variety of bugs, including bacteria and viruses. These can be divided into the normal, beneficial organisms in everyone’s gut, and those causing disease.
“We want to protect human health and wellbeing by removing these bacteria and viruses.”
Current wastewater treatment has some impact on viruses but some go through the biological trickling filter (BTF) unchanged and will survive in the environment.
“A lot of viruses, including norovirus, which causes diarrhoea and vomiting, will die if water remains in a wetland for three days. But other viruses like adenovirus – a cause of colds, rashes and diarrhoea – can survive for up to 60 days. Adenovirus won’t be wiped out by the BTF. When we remove adenovirus from domestic wastewater, we can be confident we have got rid of many other viruses.”
Mr Mackay’s tests show the BTF is taking out 90 percent of bacteria and viruses.
“But the remaining 10 percent is still a lot, hence the need for a final clean-up step through wetlands. Being smaller and more robust, viruses are typically not removed by the BTF as efficiently as bacteria. We want to know how well the wetlands trial plant deals with viruses."
John Mackay (left) and Dr Bruce Duncan check the growth of plants growing in barrels as part of a trial at the Gisborne Wastewater Treatment Plant site.