Cape Town’s ‘Drought of the Century’ forever changed the City’s urban water management landscape. The Western Cape was declared a disaster area in May 2017 and the City of Cape Town (CoCT) faced severe level 6b water restrictions – up to 50 litres per person per day, and residents and businesses alike experienced considerable water tariff increases. The possibility of ‘Day Zero’ was a major threat to business continuity and precipitated a necessary discussion on water security and the value of water as resource. Business-as-usual in water management was no longer an option.
Cape Town’s water comes almost entirely from surface water resources (i.e. rainfall run-off into dams), which is captured and stored in six major reservoirs around the city. Supply dependence on surface water resources can reduce supply resilience to climatic shocks, such as drought.
In light of tariff increases and supply stresses, reducing domestic and commercial water demand, as well as associated water costs, has become important for industries, homeowners and businesses. Consumption in toilets, taps, showers and irrigation typically comprise 60-80% of potable use in domestic and commercial areas and targeting these water uses became the focus for demand reduction strategies.
Interestingly, business continuity, rather than savings on utility bills, became a primary motivation for de-centralised alternative supply.
When approaching water management challenges, aligning with best practice (locally and internationally) is important. A number of guiding principles are incorporated in all JG Afrika projects when considering alternative supply and on-site treatment.
Water Sensitive Design (WSD)
WSD is a widely accepted concept internationally that addresses limitations of conventional urban water management. It integrates all aspects of the water cycle with urban design to provide economic, environmental and social (sustainability) benefits. These principles form a framework through which sustainable water management can be achieved.
‘Fit-for-purpose’ use is important when selecting a suitable alternative supply for a local site. Not all water supply needs to be a potable (drinking) standard. The application, available quantities and associated risk should determine the level of treatment incorporated. Non-potable use within buildings often necessitates altering plumbing networks – a process that is the easiest to incorporate during the design stage.
Source diversification provides resilience against climatic shocks, such as drought. This requires identifying and matching suitable alternative sources with appropriate application(s).
The Water Management Hierarchy
WSD principles can be implemented through a JG Afrika strategy termed ‘The Water Management Hierarchy’ comprising three stages.
i. After a mandatory baseline assessment is undertaken to develop a site water balance that provides an understanding of water use on site, JG Afrika first focuses on reducing demand. This can be done by, inter alia, installing efficient fittings; addressing leaks; educating staff/users and encouraging behavioural change; as well as managing system pressures. Importantly, reducing demand is emphasised before implementing alternative supply solutions. This step is critical in decreasing quantities of alternative supply required and, in so doing, reducing installation, operation and maintenance costs, as well as utility bills, while also facilitating good stewarding of precious water resources. Many JG Afrika clients have saved over 50% in water use after implementing these measures.
ii. The second stage entails reusing greywater and rainwater in “fit-for-purpose” applications, such as toilet flushing and irrigation
iii. Alternative supply from more conventional sources, such as borehole abstraction in conjunction with sustainable drainage systems managed aquafer recharge, river abstraction and treated wastewater reuse, are assessed in the final stage as a last resort.
Alternative supply: SUN greywater system
These principles were applied in the design and operation of a greywater system at Stellenbosch University (SUN). One of the largest of its kind in Africa and a Water Category winner at the 2019 South African Institution of Civil Engineering (SAICE) Western Cape Regional Awards, the system was designed to provide fit-for-purpose water for SUN.
Once both installation phases have been completed, the network will flush over 1 300 toilets used by about 25 000 university students to meet a significant portion of campus water supply and supplement campus irrigation. During term time, up to 75 m3/day of greywater can be treated and reused (Phase 1). This capacity could be increased to between 150 and 200 m3/day after Phase 2.
Eight representative buildings on campus were assessed and modelled. Water characteristics from each type were then extrapolated across campus to other similar buildings and calibrated against utility data to develop a comprehensive campus water balance. Interventions focused on the top 40 users, comprising 80% of total water demand and the WSD principles were then applied according to the Water-Management Hierarchy. Notably, campus interventions introduced as part of the first “reduce” stage of the Water-Management Hierarchy decreased potable water use during the drought by more than 50%.
Alternative water supplies were then investigated. The Water Masterplan identified treated greywater reuse on campus as a viable alternative supply. JG Afrika was appointed to implement a campus-wide greywater reuse system for toilet flushing and irrigation.
In this system, shower greywater from selected residences is isolated from blackwater and redirected into the collection system via sumps, manholes and grit traps and distributed to a treatment plant.
The treatment plant on site is able to treat, store and distribute up to 100 m³ of greywater a day at a peak supply of 6 l/s. Treatment steps include primary sedimentation, aeration, solids removal/physical filtration and disinfection/sterilisation by means of hydrogen peroxide dosing. The treated water is stored in tanks at the treatment plant for daily use and in future (Phase 2), excess greywater will be boosted into the existing irrigation network.
Treated greywater is pumped to a header tank with a booster system situated on the roof of a residence to pressurise the non-potable network, which includes a municipal potable supply backup. Plumbed directly into the toilets, this network plans to be expanded to supply and collect from additional campus buildings during the second phase of the project.
Alternative supply: JG Afrika’s rainwater system
During drought conditions in the Western Cape, JG Afrika’s Cape Town office decided to install a rainwater-harvesting system to provide an alternative source of water should municipal supply cease to be readily available.
However, implemented as early as 2011, JG Afrika’s own demand-side management at its office had already recorded a 73% saving in water use. Retrofitted old water fixtures with water-saving items began in 2013 through a series of water saving interventions, including reduced irrigation time and waterless urinals. Further measures, such as hold-flush toilets, low-flow taps and showers, were undertaken in 2016 and 2017. Educational information on effects of the drought and responsibilities of the consumer was distributed to staff and engagement on the suitability of installed fixtures was facilitated regularly with employees. Water efficient retrofits kept office water use below level 6b water restriction targets and reduced utility bills considerably.
Once demand had been reduced, a rainwater harvesting system – comprising 30 kl of storage, activated carbon filtration and UV-sterilisation and a booster system, was installed for flushing toilets and irrigation. This system enabled “off grid” use for between six and eight months of the year and increased municipal saving to 83% from the baseline year. The combined savings realised by the rainwater harvesting system and efficient fixtures under drought tariffs enable a payback of 3 – 4 years for all water optimisation measures. With an alternative supply available, the risk of closing the office should “Day Zero” arrive was also eradicated and business continuity guaranteed.
WSUD principles, applied to the Stellenbosch University Campus using ‘The Water Management Hierarchy,’ improved the campus water sustainability. The SUN greywater was designed to improve campus supply resilience and provide ‘fit for purpose’ water.
JG Afrika demonstrated that demand reduction measures – regardless of implementation scale – can be simple, cost-effective and result in better than expected savings. Installation of efficient fixtures typically do not require behaviour change and only minor maintenance. These measures can be implemented by a local plumbing team and do not usually depend upon additional technical assistance. Furthermore, rainwater harvesting when used alongside efficient fittings can be highly effective in maintaining business continuity during a drought and reducing utility bills.
Implementing similar measures to these two case studies on a larger scale could considerably alleviate pressures on regional and national water supply and enable water savings in other offices, homes and campuses. JG Afrika will continue to focus on improving resource efficiencies and pushing the limits in water management.
Biggs is a civil engineer in JG Afrika’s Municipal Infrastructure and Sustainability divisions