Discharged wastewater tests published
Queenstown Lakes District Council has published the first test results of treated wastewater being discharged into the Shotover River.
Direct discharge from its Shotover Wastewater Treatment Plant began on Monday.
The results, which can be found on the QLDC website, are well within consented limits set by regulator Otago Regional Council (ORC), council bosses say.
E.coli levels were below 10cfu (colony-forming units) per 100ml. QLDC’s consent limit requires 90% of results* to be less than 260cfu/100ml, which is also the nationally accepted safe levels for contact recreation.
Total suspended solids were 14mg/l, compared to QLDC’s consent which requires 95% of results* to be less than 50mg/l .
QLDC Infrastructure Operations Manager Simon Mason said the results reflected a well-operating treatment plant.
“For compliance purposes we test samples taken immediately downstream of the last treatment process within the plant, in this case the UV disinfection step," Mason says.
"Any sampling completed further along the water’s journey to the river are not reliable indicators of the actual treatment performance given they can be affected by external factors unrelated to the functioning of the plant.
"For example, animal faeces such as from birds and dogs are known to cause spikes in E.coli readings."
That appears to reference the results of samples collected by local news website Crux, in the middle of the new discharge channel around 100 metres from the plant. Lab results show 570 cfu’s per 100 ml of e coli in the Crux samples.
However, Mason says the council will now be undertaking monitoring upstream and downstream of the discharge point to ensure it has a full understanding of any effects of the discharge.
"These results will be shared with ORC as regulator and made publicly available on our website. Typically, these tests are processed in around two weeks by the external lab.”
Mason confirmed that onsite testing of the treated wastewater is historically conducted daily, and that this would continue under the new discharge regime.
“These onsite tests are indicative of operational performance in that they would quickly show if there were any issues with the quality of wastewater at the end of the treatment process. However, we also send samples to an external laboratory for analysis on a weekly basis.
"Compliance results are shared with ORC (as regulator) on a monthly basis in accordance with the conditions of our existing resource consent."
E.coli levels were below 10cfu (colony-forming units) per 100ml (10cfu being the lowest concentration that can be reliably measured). This compares to QLDC’s consent limit which requires a geomean and a 90th percentile* of less than 260cfu/100ml, and the nationally accepted safe levels for contact recreation (also at 260cfu/100ml).
Total suspended solids were 14mg/l compared to QLDC’s consent which requires an annual mean of less than 30mg/l and a 95th percentile of less than 50mg/l respectively.
* meaning the percentage of results matching a specific ‘percentile’ that need to be below the upper test limit (i.e. 90th percentile mean 90% of results need to be below the limit, etc).
