INSIDE SA’S LARGEST MBR PLANT

Operating over capacity, the Stellenbosch Wastewater Treatment Works (WWTW) was putting the surrounding environment and community at risk. Stellenbosch Municipality undertook to upgrade the plant, and the result was the creation of what is currently the largest membrane biological reactor (MBR) WWTW in South Africa.

Effluent from the Stellenbosch WWTW flows into the Eerste River, which is essential for the Cape Winelands’ agricultural and agritourism communities. In 2011, the WWTW was operating over capacity, dilapidated, struggling to meet effluent compliance, a hazard to the environment and a nuisance to the surrounding community, ultimately placing the livelihood of the communities surrounding the Eerste River at risk.

There was a critical need for the WWTW to be upgraded and its capacity extended to cater for flows from the town up until 2035. Stellenbosch Municipality therefore made the strategic and critical decision to upgrade the WWTW with a limited budget at hand. The rapid urban expansion of the town of Stellenbosch also placed additional pressure on the WWTW.

The main objectives were to increase capacity, improve the effluent quality and reduce the foul odour. Zutari, as consulting engineer, undertook the planning and design of the treatment works in 2014, as well as site supervision when construction began in 2015.

Designing a world-class plant

Zutari investigated various options to increase the capacity and upgrade the processes to ensure a high-quality effluent and mitigate odours, while ensuring the upgrade remained within budget.

Following various planning and design workshops, a project concept was co-created by Zutari and the Stellenbosch Municipality, comprising a fully automated plant embodying modern and reputable processing technologies that would ensure a highquality effluent, robustness, as well as operational and maintenance efficiency.

The design process was structured to enable Stellenbosch Municipality to be an integral part of the blueprint during the planning and design stages. This enabled the municipality to be involved in technology selection, design, operation and maintenance considerations, as well as the aesthetic layout and appearance of the facility.

Membrane technology was selected in order to guarantee a high-quality effluent. The MBR process is an advanced wastewater treatment process that uses ultrafiltration membranes for liquid-solid separation, instead of conventional clarifiers. The design was also developed to maximise the use of existing infrastructure, and to seamlessly integrate the refurbished infrastructure into the new plant, thereby reducing the overall capital cost of the project.

The use of MBR technology was beneficial in terms of the small footprint of the bioreactor, particularly considering the spatial constraints of the existing site, and the production of high-quality effluent, which surpasses the standards prescribed in the wateruse licence issued by the Department of Water and Sanitation.

The WWTW was designed to cater for a combination of domestic and industrial wastewater, as well as handle seasonal changes in the influent loading due to the agricultural harvesting season, when organic loads increase by 25%. Varying key process parameters such as sludge age and concentration during the harvesting season ensures high-quality, compliant effluent.

A superior effluent quality also presents immediate opportunities for the reuse of treated effluent, which is in line with the municipality’s water conservation and demand management strategy.

World-class technology

The project comprised an upgrade of the plant to a full biological nutrient removal process that is capable of handling up to 35 Mℓ/day average dry weather flow, as well as ensuring compliance with the National Water Act (No. 36 of 1998). This upgrade comprised a new inlet works, a new 27 Mℓ/day MBR lane that can handle a peak flow of 67.5 Mℓ/day, the rejuvenation of the existing plant