Page 27 - Landscape129

IRRIGATION FEATURE

Can stormwater harvesting provide a viable source of water for municipal supply? A

project funded by the Water Research Commission (WRC) used an urban catchment
in Cape Town to explore the possibilities. This article by Sue Matthews has been
reproduced here with permission from The Water Wheel, a publication of the Water

Research Commission. It has been edited due to its length.

Groundwater flow and abstraction from wastewater treatment works, leaking But I would argue that with wastewater
were also modelled, and water quality sewers, informal settlements, solid waste we don’t know what people are doing in
considerations assessed. The results sites, cemeteries, as well as industrial and their homes, like taking drugs or discarding
indicated that the proposed MAR would agricultural areas. substances down drains, and those things
allow the Zeekoe catchment to support 140 are not tested for in the existing regime.
boreholes, each pumped at 3.5–8.1 litres Incentive from Day Zero Wastewater is not just sewage and grey
per second to ensure that the flow fields The ‘day zero’ drought provided the water, but what you don’t test you don’t
were drawn largely from areas around the incentive that was needed to refocus on know, and what you don’t know won’t scare
stormwater ponds. This would provide a the Cape Flats aquifer, which now features you!” he quips.
combined mean annual groundwater yield strongly in the City of Cape Town’s new “The other issue is reliability, which I do
of 29–33 Mm3, of which approximately water programme; this was designed to agree with, because stormwater is only
30% would be attributable to stormwater fulfil the commitment made in the 2019 really available in the winter rainy season
infiltration. Water quality would improve water strategy to develop 300 megalitres in Cape Town, while wastewater flow is
somewhat during the time in storage, with per day of additional water supply capacity quite consistent year-round.” What’s more,
natural die-off of pathogens, although by 2030. The first wellfield, Strandfontein the variability in stormwater flows is likely
disinfection treatment would still be West, is almost complete and will start to increase in future, as climate change is
needed for potable uses. contributing to the municipal supply by predicted to cause more extreme events,
Okedi is excited to see whether this mid-year. The abstracted groundwater will such as droughts and floods.
research will show that the concept is be treated on site in a package plant with Okedi assessed the impact of climate
practically possible. His own economic a capacity of five megalitres per day, before change on demand and stormwater yield in
analysis on the viability of stormwater being introduced directly to the potable the Zeekoe catchment using modelled data
harvesting indicated that the most cost- distribution network. The MAR component provided by UCT’s Climate Systems Analysis
effective option would be MAR for potable of the Cape Flats Aquifer Project is only Group. The data suggested an annual mean
water supply using the existing water planned to be operational by 2026/7, reduction in rainfall of 40–200 mm by the
reticulation system. although construction of the pre-treatment year 2100, along with a 3–5°C rise in mean
Athough the City of Cape Town plant for treating wastewater effluent to a daily temperature, which will increase
committed to make “optimal use of high standard before it is injected into the evapo-transpiration. This will likely result
stormwater and urban waterways for aquifer, is expected to be completed in in a 15–50% decrease in stormwater yield
the purposes of flood control, aquifer 2024. in the catchment, but it will be more than
recharge, water re-use and recreation”, it Other projects within the new water offset by the increase in hard, impervious
has since revealed the intention to use programme include the Table Mountain surfaces that can be expected with land use
treated wastewater effluent, rather than Group Aquifer Project, the refurbishment change. Should the mean imperviousness
stormwater, for MAR of the Cape Flats and expansion of the Atlantis Aquifer of the entire study area increase from
aquifer. For more than 40 years, it has used Scheme, the Berg-Voelvlei River the current estimate of 45% to 75% as
both stormwater and treated wastewater Augmentation Scheme, the Faure New developments replace agricultural land
to recharge a similar sandy aquifer on the Water Scheme (a water re-use scheme and other open spaces, a 91% increase in
west coast through a series of infiltration that will treat effluent from the upgraded stormwater yield is predicted.
ponds and recharge basins that form part of Zandvliet wastewater treatment works for “If we are able to capture that water, it
the Atlantis Water Resource Management potable supply) and desalination. would make a significant contribution to
Scheme. Okedi says that City of Cape Town water future water security and be very useful
In the case of the Cape Flats aquifer, staff had explained that the reason for for sustainability,” says Okedi. “We could
the potential of using treated wastewater choosing treated wastewater effluent augment the city to become a giant
effluent for MAR was thoroughly over stormwater for the Cape Flats aquifer catchment so that any rain that falls here is
investigated in a WRC-funded project MAR was due to variability in both quality infiltrated into the ground and stored there
during the 1970s – ‘The Reclamation, and quantity. “They would rather work for later use. So, it’s vital that we protect
Storage and Abstraction of Purified Sewage with wastewater because they are sure of our greenbelts and ponds, as these will be
Effluents in the Cape Peninsula’, led by the the water quality – it’s not very variable our access to the unconfined aquifer to
CSIR. But the aquifer was not developed as compared to stormwater, and with their preserve water for the future.” n
a municipal water resource, and over the testing regime they don’t have a good
years additional research indicated that it handle on what’s in stormwater, in terms of Photo by John Wilson
was under increasing threat of pollution heavy metals and emerging contaminants.

Landscape SA • Issue 129 2023 25