You might want your bath to be full of bubbles – but excessive foam at Murchison’s wastewater treatment plant could impact the rural Tasman town’s sewerage processing.
By Max Frethey of Local Democracy Reporting
Wastewater operators have observed 14 instances of excessive foaming over the last 18 months, but more may have occurred when no one was present.
The foaming was caused by disruptions to the treatment system’s microbial balance, likely caused by unusual discharges into the sewerage network.
Recent investigations by Tasman District Council identified wastewater with a "very high" pH of 10+ entering the treatment plant, which was believed to be the source of the problem.
The problem was not new to Murchison, with the foaming having previously occurred.
Council waters and waste manager Mike Schruer said the origin of the past foaming issue was suspected to be a meth lab, but that was never confirmed.
"From time to time we work with the police and do sampling and testing of our waste stream, but [it’s] really hard to track down."
The source of the high pH discharges has been traced to a group of about six homes, but access issues on private land and intermittent foaming and discharges complicate further tracking efforts.
PH is a scale that specifies how acidic or alkaline a solution was, with low pH indicating acidity and a high pH indicating alkalinity.
New monitoring equipment is being installed to build a clearer picture of the discharges.
Wastewater engineers at the council said the effect of the foaming thus far has been "relatively minimal", largely resulting in additional cleaning and monitoring.
That involved sending a boat and an extra wastewater operator to Murchison to assist, taking them away from their normal tasks and creating additional travel costs for the council.
However, the engineers warned that the issue could further destabilise the aeration pond – in the worst case, causing its ecology to "crash".
A crash had not yet happened, but one would reduce treatment capacity, resulting in poorer-quality treated wastewater and potential odour issues, and would take a long time to fix.
"Think Christchurch, just on a much smaller scale."
If foaming persisted for an extended period, it could also flow from the aeration pond to the oxidation pond and upset the balance of algae and bacteria in the pond.
That had happened once in the last 20 months.
Similarly to the aeration pond, if the oxidation pond was impacted, wastewater could be treated to a lower quality before it was discharged from the treatment plant.
While these challenges were believed to be caused by a high pH environment, wastewater with a low pH could also bring risks.
Low pH discharges could release significant amounts of hydrogen sulphide gas into the headspace of wastewater pipes.
The gas was toxic and could cause death at relatively low concentrations, in addition to causing reactions that would eventually corrode wastewater infrastructure.
– Local Democracy Reporting is local body journalism co-funded by RNZ and NZ On Air
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