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Understanding Dredging

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$227million upgrade of Potsdam wastewater treatment works begins

2023-08-14 Paul Levey

140823 // patsdam_wwtw.jpg (79 K)

Photo Credit: City of Cape Town

Delayed for more than 10 years by a succession of bidder appeals, construction workers finally cut ground on 2 August 2023 marking the start of the long-anticipated upgrade of the Potsdam wastewater treatment works (WWTW), Cape Town, South Africa.

Commencing around two months later than planned, the Western Cape’s second largest infrastructure project is due for completion in 2027, with operational trials expected during 2026. Set to increase the plant’s daily capacity to treat wastewater from 47 to 100 million litres, the upgrade has long been sought to cope with Cape Town’s burgeoning population.

There are also plans to dredge the Milnerton Lagoon to remove pollution build-up in sediment that has accreted over decades due to poorly treated effluent from the plant, sewer spills from local pump stations, run-off from stormwater drains, and upstream settlements. The dredging, along with the infrastructure upgrades, are key components of a multi-faceted programme to restore the environmental health of the waterbody. 

Work to re-engineer reed beds adjacent to Potsdam WWTW is already nearing completion. The natural filters, in tandem with man made walls and valves, will be better able to prevent polluted stormwater from reaching the Diep River, as the retained water will be pumped back to the plant for treatment.

Dredging is already being undertaken to improve the performance of Potsdam’s maturation ponds. Designed to retain effluent before its release, the ponds have largely been by-passed due to the build-up of polluted sediment. Once cleaned, the ponds will retain effluent that will be released into the Diep River after being subjected to a final UV treatment. This layered approach is expected to  result in a marked improvement on the quality of effluent being released into the lagoon.

Incrementally closing off pollution sources to the lagoon will be accompanied by the installation of membrane technology to ensure high treatment standards. The ultimate objective is to create the necessary conditions to allow the lagoon to be dredged to remove polluted sediment. The dredging is expected to improve the lagoon’s water quality, which has long posed health risks to local residents, and its ecology, which has seen multiple fish die-offs caused, in part, by algae blooms which deplete oxygen in the water.

City officials will work alongside experts in estuarine science and remediation, amongst others, to restore the lagoon and protect it from future pollution.

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