The link between stormwater and sanitation

In South Africa, wastewater and stormwater are conveyed in separate pipelines. And while it can be an environmental disaster and health hazard for stormwater and sewage to meet, it is advisable to manage both as part of an integrated urban water system. By

Kirsten Kelly

In government, multiple departments are involved in water supply, wastewater and stormwater management processes, and too often they are siloed. For instance, there are cases where stormwater is not even within a water department but falls under the roads department,” says Dr Kirsty Carden, associate professor in the Department of Civil Engineering and interim director of the Future Water Research Institute at the University of Cape Town.

“A roads department will generally view stormwater as a liability that must be directed off the road surfaces as quickly as possible because it can be hazardous. For water and wastewater

Dr Kirsty Carden, associate professor: Department of Civil Engineering and interim director: Future Water Research Institute, University of Cape Town departments, stormwater can also be a liability, as it is a major source of contamination to drinking water sources; however, that same water falling on the roads, if properly managed, can be a resource that recharges groundwater sources. No water should ever be considered a waste,” she continues.

A safe and secure water supply, along with drainage and sanitation for public hygiene and flood protection have always ranked highly among the primary needs of urban dwellers.

However, for most cities, stormwater management is the ‘poor cousin’ of the municipal water services and is chronically underfinanced. Water supply services are prioritised, but less emphasis is placed on drainage or sanitation and wastewater treatment services. This results in vast quantities of partially treated or untreated, polluted water entering the environment, causing water security issues (in terms of both supply and quality) going forward.

Separation of stormand wastewater

The perfect separation of stormwater and wastewater is impossible to achieve. “This is partly because pipes are never perfect and there will always be some form of leakage, particularly with sewer pipe systems. The potential for cross contamination between the systems will always exist,” states Carden.

“Sewer overflows are extremely common in South Africa and are worsened by load-shedding when pump stations do not work, leading to sewage being discharged to drainage systems. A lot of the drainage infrastructure in this country comprises open channels and canals that connect to rivers, to waterways and to the sea, which also receive stormwater discharges from the traditionally designed, hydraulically efficient piped systems. With sewer overflows and other forms of wastewater, these stormwater systems become transporters of solid waste and contaminated water. And therefore, the quality of stormwater is directly linked to the quality of our sanitation systems,” she adds.

Stormwater can also add complications to wastewater systems. South Africa has separate stormwater and sewer systems. Some individuals or companies illegally connect their stormwater to the sewer system, or stormwater pipes may leak. This can potentially overload the wastewater treatment works and dilute incoming sewage, which paradoxically makes its treatment difficult owing to lower concentrations of organic matter in the water.

Stormwater harvesting

If managed properly and treated, stormwater would not be polluted and could be a resource. Carden recommends cities adopt stormwater harvesting – the collection and storage of rainfall run-off in open ponds or aquifers. “It can be an alternative water resource that supplements traditional urban water supplies. Stormwater harvesting can improve water security and increase resilience to climate change in urban areas. It can also prevent frequent flooding and provide additional benefits to society – such as creating amenities and preserving biodiversity.”

Stormwater harvesting schemes all make use of some form of storage system. Some make use of retention ponds with permanent water storage. Others make use of detention ponds; these are normally dry except following large storm events when they temporarily store stormwater to reduce downstream flooding.

Detention or retention ponds are used to store run-off volumes. This results in the reduction of downstream flows and decreased flooding. Stormwater can infiltrate into the ground from these ponds, or it can be injected into boreholes so that it can be captured and stored in aquifers for later abstraction – a process known as managed aquifer recharge.

There are further opportunities for stormwater managers to actively manage systems using real-time control. This can be done in a way that, prior to a predicted storm event, the storage is partially emptied, resulting in an increase in the flow rates in the river ahead of the storm, but a decrease in the peak flows during the storm, which could prevent flooding. In this way, additional storage capacity is created for stormwater harvesting purposes.

A study of the Liesbeek River Catchment in Cape Town found that stormwater harvesting had the potential to reduce the total current residential potable water demand of the catchment by more than 20% if the stored stormwater were used for purposes like irrigation and toilet flushing. For such a reduction to take place, the vast majority of residents would be required to make use of harvested stormwater. This would likely necessitate changes to the regulations related to the supply of water in the city.

There has only been one large-scale example of successful, long-term stormwater harvesting in South Africa. This is in the town of Atlantis on the country’s west coast. This low rate of adoption of stormwater harvesting is likely due to a range of socio-institutional challenges. These include resistance to innovative approaches, fragmented and underfunded water management institutions, a lack of political will, and a shortage of capacity required to operate and maintain the harvesting process. There are, however, signs of increasing interest of utilising stormwater as a resource.

Improved stormwater management practices

Carden adds that there are new approaches to stormwater management and the management of water as a whole in cities. “The concept of ‘sponge cities’ allows for water to infiltrate into the ground through networks of blue and green infrastructure that provide more resilience to droughts and floods. Hybrid systems integrate green infrastructure and conventional grey infrastructure, with nature-based solutions like sustainable drainage systems (SuDS) allowing for a further level of treatment for water that does not make its way to a treatment plant. It is time to admit that by relying solely on conventional engineered approaches, we are failing to achieve SDG 6.”

The sponge city absorbs rainwater, which is then naturally filtered by the soil and allowed to reach urban aquifers; allowing for extraction of water from the ground through wells that can be treated and used for city water supply.

Carden believes that there would be a better uptake of stormwater management practices if all citizens had basic levels of water literacy, where there is an understanding of where cities obtain their water from and the role of urban stormwater management. There is a growing recognition that attenuation on-site is the solution to stormwater quantity and quality management in the built urban environment, but optimal attenuation approaches need to be identified for South Africa. Alternative reuse options used internationally may provide sustainable solutions to stormwater management in South Africa – where the focus on reuse is currently on groundwater recharge, industry and mining.

“In the City of Cape Town, stormwater management has been brought within the Water and Sanitation Department. There is an intentional move from the City to coordinate different water streams and manage them together. This is a great start. There needs to be more institutional arrangements that allow for the better control of stormwater and include it as a water resource,” states Carden.

She believes that there also needs to be a mindset shift regarding sanitation. “We think of sanitation as toilets, but it is much more. It encompasses all of the ways in which we deal with water and waste in the environment. In Brazil, sanitation includes the supply of drinking water, sanitary sewage, as well as the proper handling and disposal of solid waste and the management of stormwater – even control of disease vectors like rats and mosquitoes.”

Water supply, sanitation and stormwater are one interconnected system. Each part cannot be viewed alone and needs to be managed as part of the system. These parts can interact with each other negatively, for example, through the contamination of drinking water by poor sanitation, as well as positively, with wastewater and stormwater potentially providing a viable water supply. Understanding and managing stormwater and sanitation as an integrated system will lead to the prioritisation of public health, environmental improvement and quality of life outcomes.