Rotting algae produces sewage plant stench
Rotting algae was responsible for a stench described as an “odour event”, a council committee has heard.
At the Nelson Regional Sewerage Business Unit (NRSBU), operations manager Brad Nixon explained the “odour event” on February 17 at Bell Island took place after the plant had been experiencing a high load for the week.
“Unfortunately, this coincided with programmed maintenance on the primary clarifier, which meant they were unable to reduce the load to the ponds by diverting some of it through to the secondary treatment system on the island,” Nixon said.
The plant then lost one of the large algal species that was predominant in the pond, that caused the odour.
The deterioration in pond health was “pretty short and sharp and crept up on us pretty quickly,” Nixon told councillors.
The algae that had been affected “sort of rose to the surface and effectively started rotting on the surface”, producing odour.
Wastewater load, Clarke explained in response to questions from Stuff, is a combination of “flow and concentration”. Flow is the rate that wastewater is discharged, and concentration is the level of contaminants in the water that have to be removed when the water is treated.
In the days prior to the odour event, the load at the plant was higher than normal because of the concentration of the wastewater received from contributors, Clarke said.
The NRSBU has four large contributors - Alliance Group meat processors, the two councils, and Nelson Pine Industries Ltd.
Nixon said the issue with the algae was a biological process, which they had “very few levers” to control.
On Wednesday and Thursday, there was no indication to expect a significant change in the pond condition, he said.
“We knew that it was slightly deteriorated, but it still seemed to be working fairly well.”
However, some of the key information they had was already a few days old at the time.
Nixon explained that algal counts were sent to the North Island, and could take two or three days for the samples to arrive and the data to return.
Once it became apparent that the pond condition was deteriorating rapidly, the operators dosed the pond with sodium nitrate that reversed the odour in a short time, he said.
Bad smells from the Bell Island sewage treatment plant, or from biosolids, have seen some residents complain and one labelling an affected stretch “poo corner”.
Treated “biosolids” from the Bell Island plant are reused as fertiliser and applied to council plantation forestry land on nearby Moturoa/Rabbit Island.
A report presented to the unit noted that there were three odour notifications for the period to end of January.
In each case, the source was considered to potentially be biosolids storage or application, the report said, but it was not possible to confirm the odour due to the time between the notification being made and NRSBU staff being informed.
On January 12 and 13, multiple Richmond residents took to social media to complain about a wafting stench, with one labelling the intersection of Nayland Rd and and Saxton as “poo corner”.