Metropolitan water utility Joburg Water has announced plans to expand the capacity of the Driefontein wastewater treatment plant, in Krugersdorp, from 55 Ml to 80 Ml, as well as to commission a biogas plant for power generation.

This will, however, be the maximum expansion potential for the plant as it is surrounded by residential areas, confirms Driefontein wastewater treatment works manager Jacqueline Ngwenya.  

She tells Engineering News that although the plant currently collects and treats 42 Ml of sewage a day, which is 75% of the nameplate capacity, the plant will soon need to accommodate the growing population in the areas it services, as well as increasing amounts of stormwater flowing into the plant as of late.

Commenting on the plans to commission a biogas plant, Ngwenya says it has already been built but is undergoing optimisation studies through the Council for Scientific and Industrial Research.

As soon as the studies are completed and show how the plant can function optimally, it will be commissioned and generate about 30% of the total plant’s power requirements.

Joburg Water in 2012 commissioned its first wastewater biogas-to-energy plant at the Northern Works facility, in Diepsloot, which was poised to save the city R100-million a year in electricity costs; however, the plant ran from November 2012 up to December 2019 at only 20% capacity, owing to insufficient gas having been supplied by Joburg Water.

The contractor for both biogas plants, WEC Projects, tells Engineering News that the Diepsloot biogas plant has not run since 2019 because Joburg Water never extended or re-awarded the operations contract. Similarly, the Driefontein biogas plant has not received sufficient gas from Joburg Water to run the plant.

"Neither of the current plants are running and they are basically neglected and not being cared for or maintained, despite South Africa being in the midst of an energy crisis. Joburg Water spent more than R70-million in capital expenditure for the two plants, which are not running and are effectively defunct. It will cost millions to have these plants repaired and restarted for continuous operation," WEC states.

The Driefontein plant is currently powered through the grid, with backup diesel generation capacity during loadshedding to power the most critical stages of the wastewater treatment process.

The Driefontein plant is one of six Joburg Water wastewater treatment plants and treats the sewage from the Roodepoort and Mogale City areas, which includes Muldersdrift, Magaliesburg, Kagiso and Krugersdorp.

It serves about 100 000 people.

The plant was commissioned in the 1970s starting with 15 Ml of capacity. It has consistently delivered a 98% effluent compliance rate against the Department of Water and Sanitation’s target of 90%.

On that note, Ngwenya says the effluent compliance rate typically drops to about 95% in times of loadshedding, since the plant has to release partially treated water in these instances.

Ngwenya assures the public that the resulting effluent is still clean, just not chemically treated.

With the amount of sewage that comes in continuously, it is impossible to “pause” the plant during loadshedding and, rather, Driefontein prioritises the critical treatment processes that need to keep running, and does not risk damage to the equipment owing to severe overflows.

The Green Drop status water treated at the fully automated Driefontein plant is discharged into the Crocodile river, while the remaining sludge from the treatment process is dried and composted for use in the agricultural sector.

The plant produces between 10 and 12 dry tonnes of compost a day.

The wastewater treatment process starts with preliminary treatment to sort out foreign items from the water, using a manual and a mechanical screen, respectively. The water then flows to primary sedimentation tanks, where it stays for about 15 days for sludge to ferment.

The clean overflow water flows into a balancing tank, which keeps the flow constant into the bioreactors. The two bioreactors on site, which respectively use a five-step and a four-step process to biologically digest microorganisms and treat the water to remove phosphates.

The water then progresses to secondary clarifiers and the disinfection stage, which involves treating the water with calcium hypochlorite in dosing tanks.

Ngwenya says flow is constantly measured throughout the plant as well as sampled in the early stages. An autosampler takes samples every 30 minutes to determine the strength of the sewage flowing in, which is important to detect illegal dumping of materials such as paint in the sewage network, as these materials can damage equipment at the wastewater treatment plant.

The plant also uses an online monitoring system to ensure proactive maintenance can be done and early fault detection can take place.

Ngwenya says the Driefontein Wastewater Treatment plant is arguably the best-performing wastewater treatment plant in Joburg Water’s portfolio, if not in the country.

She highlights that the plant has been fortunate in that it does not often struggle with challenges such as theft and vandalism, as well as extensive illegal dumping, as many other wastewater treatment plants do.

Ngwenya also lauds Joburg Water for its continued investment in infrastructure and technology, such as the Driefontein plant’s extension to 80 Ml of treatment capacity, particularly as environmental concerns are increasing globally and as the population grows.

She concludes that the extension project will take at least four years to complete.