For a city to be healthy, it needs a functional sanitation system with a network of underground bulk sewers that connect households to wastewater treatment works (WWTWs). The Zandvliet WWTW form a significant part of the City of Cape Town’s sanitation infrastructure. In 2013, the works was in dire need of an upgrade to treat the ever-increasing inflow from its catchment and to produce effluent to an acceptable standard.
The Black-Mac sewer is a 14.4 km-long bulk sewer constructed in 1983, consisting of epoxy tar coated asbestos cement pipes, ranging in diameter from 400 mm to 1,000 mm. The sewer was originally designed to convey sewage to the Macassar WWTW, but a significant portion of the flow was diverted to the Zandvliet WWTW in the 2000s. The pressure on the Zandvliet works could be reduced by diverting flow back to the Macassar works, which could potentially be achieved with minor modifications.
Zutari was appointed to carry out a detailed condition assessment of the Black-Mac outfall sewer to determine the viability of this option. This was critical to understand the scope and cost-effectiveness of this solution, especially as almost 8 km of the sewer had not been used for more than a decade. State-of-the-art multi-sensor inspection technology was used to obtain a detailed view of the sewer’s condition along its entire route.
The condition assessment found that the level of corrosion – and therefore the extent of structural deterioration – increased along the length of the sewer. This information was used to evaluate the structural integrity and remaining useful life of the sewer, as well as its ability to convey future flows. It was found that the upper and middle reaches of the sewer had adequate capacity, but the lower reaches of the sewer did not.
The design team’s work enabled the client to make an informed decision on the optimum solution, which was a combination of the rehabilitation of the existing sewer where possible, based on its remaining useful life and available hydraulic capacity, and the replacement of the remaining section with a new outfall sewer.
“Applying an innovative approach, combined with the latest technologies, is a cost-effective way to breathe new life into aging infrastructure,”
The project involved rehabilitating 3.7 km of 800 mm diameter pipe of the Black-Mac sewer using thermally cured-in-place-pipe (CIPP) liner. This trenchless technology increases the remaining life of the sewer and improves its hydraulic performance. A non-circular pipe section under the National Route 2 (N2) highway was rehabilitated using high-performance ultraviolet (UV) reinforced CIPP liner, a first in South Africa. The lower 3.7 km of the Black-Mac sewer was replaced with a new outfall sewer comprising high-density polyethylene (HDPE) lined concrete pipes, ranging in diameter from 1,050 to 1,200 mm, with depths of up to 6 m.
The project was successfully commissioned in 2018, diverting flows of up to 30 Ml/d away from the Zandvliet WWTW to the Macassar WWTW.
“Applying an innovative approach, combined with the latest technologies, is a cost-effective way to breathe new life into aging infrastructure,” says Zutari Professional Civil Engineer, Jacobus Kriegler.
*The Aurecon Africa business has been officially renamed Zutari as at 21 July 2020. Zutari acquired Aurecon Middle East on 20 November 2020.