Mechanical equipment failures have been blamed for the discharge of partially-treated waste into the Umbilo River.
Ethekwini municipality said in a statement that the Umbilo wastewater treatment works, which discharged into the river, had experienced the breakdown of two processing units.
“These were mechanical equipment failures (syphon failures on the clarifier and centre bearing failure on primary settling tank) that have led to the malfunction of these two units."
The units had to be put offline so repairs could be made, which resulted in the plant only functioning at 50% and overloading of the remaining primary settling tank and clarifier.
This, said the city, led to the process being compromised, with the remaining units unable to handle the load and partially treated effluent being discharged to the river.
The city said repairs had started and the units were now back online.
“It should take a further two weeks to recover the process to full compliance standards."
The effluent is currently being disinfected to reduce public health impacts.
"The river will be aerated to increase oxygen levels and assist aquatic life. It will also be monitored and sampled daily with samples being sent to the laboratory for testing,” the city said.
It warned against using water from the river.
Chairman of the Dusi-uMngeni Conservation Trust Dave Still said the spill would inject large amounts of nitrogen and phosphorus into the water, which could help feed the growth of invasive plants like water hyacinth, which impacts negatively upon native ecosystems.
“The water in the river is already highly polluted. I think it would be safe to perhaps water your garden with it, but not food crops.”
He said that it was important to maintain infrastructure, and have an adequate number of inspectors on the ground monitoring the processing works.
“At the same time, people abuse the system by dumping items in their toilets and in manholes that are not meant to be disposed of that way. This taxes an already strained system.”
He said aerating the water would help.
“These kinds of leaks flatten the oxygen content of the river, which can be harmful for the organisms living there.”