Petrochemicals giant Sasol’s industrial sludge bioremediation project aims to move into full commercial scale over the next two years to treat 200 000 t/y of industrial sludge.

The project uses aerobic composting to convert the sludge from its Bioworks water treatment plants, in Secunda, into fertiliser, which it will use to grow bulking material.

The company uses the bulking material as a food source for composite bacterial populations, which also convert the sludge into inert fertilisers.

Sasol has tested five different waste sludge streams to date and each one requires particular bacterial populations to be converted into fertilisers.

The sludge contains some oily compounds and some residual heavy metals. The sludge is currently either incinerated or sent to a landfill.

Sasol Technology environment technology manager Dr Sarushen Pillay explains that the Secunda plant, as a zero-effluent plant, developed the bioremediation process as a way to enable Sasol to sustainably reduce its environmental footprint in alignment with the objectives of the Waste Act and the Air Quality Act.

Wastewater treatment sludges can be composted, but Sasol was not sure that its industrial wastewater sludges could be composted in a similar manner. It started with a small, 30 t test of the concept in 2012 and proved that, while complex, industrial sludges can be composted.

“This process will use 200 000 t/y of sugar graze to treat 200 000 t/y of sludge into 80 000 t/y of fertiliser. Most of the mass lost is through evaporation of water.”

Bioremediation business Sola Fidei Manufacturing, developed through Sasol’s supplier development initiative, has developed a method to activate specialised microbial populations, specifically heavy-metal-composting bacteria, to target, assimilate and biochemically transform the potentially harmful trace elements in industrial waste sludge into immobilised and environment-friendly forms.

Sasol had to apply for exemption from the new standards of the Air Quality Act, but would be able to meet the more stringent emission standards of the Act once its bioremediation project was at full capacity in 2016.

“The bulking material is harvested and shredded and laid out in composting windrows. The various sludges are then mixed with specific bacteria and poured onto the sugar graze beds, with each windrow used to compost a specific sludge stream. “The windrows are turned every few days to aerate the process.”

At various stages, the temperature of the windrows is allowed to rise, killing the bacteria. A windrow-turning machine, pulled by a tractor, turns and doses the windrow with new bacteria populations suited to that stage of the composting process.

“We have designed and tested this process over two years. “Our bulking, material-growing lands were monitored prior to and after being dosed with the fertilisers, and our results show an improvement in soil conditions. “Sasol will use 2 000 ha of land, dosed with this bioremediated fertilisers, around its Secunda plant to grow the sugar graze. The land is not currently under cultivation and is owned by Sasol,” says Pillay.

The process was designed to be labour intensive and the full-scale project will provide employment for 150 people in the community, as part of Sasol’s employment creation initiative. The training they receive provides them with composting, planting, farming and tractor-driving skills, as well as a safe working experience, says project manager Wynand Kruger.