A proposed additional 1000 mm diameter pipe in the Markman main sewer system, designed by waste water infrastructure experts AfriCoast Consulting Engineers, is set to dramatically improve the system and avert future potentially disastrous pipe bursts.
The existing sewage system between Markman and Fish Water Flats is considerably over capacitated and recent leaks in the sewage infrastructure from other pipes crossing the same river have meant ecological and health hazards for the Swartkops River estuary and surrounding communities.
AfriCoast project manager Gerrie van de Merwe said the R30-million augmentation will dramatically improve the flow and ensure longevity and sustainability of the system for many decades to come.
In January this year, after critical breaks on other sewers allowed a free flow of raw sewage and industrial waste into the river, the Swartkops River Trust issued a public plea for the redesign and replacement of the sewage infrastructure in Nelson Mandela Bay. Van de Merwe said plans for the augmentation had in fact been set in motion in 2008 but stalled due to funding prioritisation.
“The hydraulic capacity of the existing pipe is no longer sufficient to support demand on the line, which has increased dramatically over the past 25 years, as populations in the area have grown. The current length of 1 510 m of pipe carries all the flow. The existing capacity is 680 litres per second and in peak wet weather the flow increases to more than 1 100 litres per second.
“Additionally, the older concrete or asbestos cement pipes have been corroded by the sewage over time, which makes them vulnerable to breaks. Alternative pipe materials are under consideration and the final design will curtail the risk of pipe failure. We decided to retain what we already have and add to the existing system to increase capacity, while at the same time creating a back-up, as well as a way in which to perform ongoing maintenance and repairs,” he said.
In their pre-design study AfriCoast engineers also determined that the existing grit chamber in Bluewater Bay was slowing the velocity of the flow in the Markman Main Sewer, thereby reducing the capacity of the pipeline.
“The existing grit chamber is smelly and the build-up of gases is potentially hazardous. Our recommendation is to remove the grit chamber – which will also benefit Bluewater Bay residents – and instead have designed a dual pipe system. We will be installing a second 1000 mm diameter line and will retain the old one in order to give dual system capability. This will vastly improve maintenance capabilities as well as improve flow and reduce the risk of pipe failure.”
The dual system will run from upstream of the Swartkops River bridge, crossing down to the Fishwater Flats Waste Water Treatment Works. It will provide the flexibility to switch between the new and old lines, a capability which the municipality has not had available before.
One of the problems in adequately maintaining the system has been that the sewer cannot be closed like a tap. Now, with the additional line, the flow can be diverted and engineers will be able to repair promptly where necessary. They will also be able to inspect the condition of old sewer pipes with CCTV cameras and effect repairs where necessary.
The new line will run alongside the N2 national road, rather than through the vulnerable wetlands estuary as the existing lines do. AfriCoast took extensive risk and environmental assessment studies into account in engineering the new sewage system, to avoid further disturbance to potentially important fossil heritage and the environmentally sensitive saltwater marsh.
Modernised operations at the Fishwater Flats Wastewater Treatment Works, which are currently being enlarged, will also go a long way to improving the municipal sewage operation. The existing Markman line ends on an inverted syphon system, which Van de Merwe explained is not ideal and will be altered to allow free discharge to the new inlet works to improve the flow.
“The new inlet works is being built at a deeper level, so that there won’t be a syphon system but instead a free discharge to allow gravity flow conditions along the length of the pipeline.”
The project is estimated to create at least 25 jobs during construction and will also provide opportunities for local SMMEs. Construction is planned for completion in 2017.
Alan Straton
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