The Merebank plant is unique in that it can handle both domestic and industrial wastewater
The high percentage of industrial waste coming in meant that eThekwini required the pilot plant to have two waste trains, from two industrial sewers and one domestic one, explains Wade Swannell, Chemical Engineer – Water, Hatch – the company responsible for the process design of the anaerobic digestion pilot plant. “The reasoning behind the pilot plant was to test the ideal operating conditions for the digestion of the particular sludge that needs to be processed here, as Merebank differs from traditional domestic wastewater treatment works,” Swannell explains. In terms of the general process flow, there is a continuous feed into a flow tank. This is pumped proportionally to primary settling tanks so as to simulate the diurnal flow of the sewer. Sludge thickeners process the sludge further, following which it is pumped to digesters. “All the while there is a constant overflow going back to the sewer,” Swannell elaborates.
The main challenge associated with the project has been its small size, with a nominal capacity of 14 m3/hour. “It is difficult to size equipment that small, which is why we opted for a semi-batch process design. Despite being small, we still put in the same design effort as with a larger project, especially as it has quite a complex control system,” Swannell points out.
The pilot plant has already been commissioned, and is currently in the one-year contractor’s defects liability period. “While our technical role is now at completion, we are looking to stay involved from a research-and-development perspective, as we are quite interested in the results that will come out of the pilot plant.” Swannell highlights that the pilot plant is expected to provide valuable data on the dual treatment of domestic and industrial wastewater, which poses a particular challenge in water-stressed South Africa.
