The GBCSA defines a net-zero/net-positive water building as one “designed, constructed and operated to greatly reduce total water consumption and then use harvested, recycled and reused water such that the amount of water consumed is the same as the amount of water produced (net zero). Or if the water recycled or produced is greater than the water consumed (net positive).”
Georgina Smit, head of technical at GBCSA, points out that building design options should not be limited. “Net-zero water can be achieved via several technical solutions, but key ingredients include an integrated design approach and focusing on optimising efficiency first. Then secondly, considering alternative supply solutions. Site conditions and design response for each building should be unique and considered in terms of optimising the building’s efficiency,” she says.
• Reducing demand by employing innovative and efficient technologies that consume less water.
• Generating alternative water supply sources to offset purchased potable water.
• Treating wastewater on-site and reusing it appropriate to the use.
• Implementing green infrastructure that allows stormwater to infiltrate to the original water supply.
“Net-zero carbon, which deals with operational energy, is easier to justify than net-zero water,” explains Marc Sherratt of Marc Sherratt Sustainability Architects in Johannesburg. Local water is good quality and relatively cheap, compared to soaring electricity costs, so the financial incentive just isn’t there. “Net-zero carbon has a payback period, in some cases only three to five years. That’s why we’re seeing the net-zero carbon rating being the most popular at present.”
Sherratt is the founder and sustainability architect on Vleihuis Residential Development in Linden, Johannesburg. It was GBCSA-rated Net-Zero Carbon – (pilot) Level 2, Net-Zero Water – (pilot) Level 2 and Net Positive Ecology – (pilot) Level 1 ratings in September 2018, for the design. “We took a development approach by buying the land and rezoning and developing the design ourselves,” he explains. “Covid significantly delayed our progress. However, we’re hoping to break ground in six months with our show unit.”
Five units in the development have a 1 350m2 floor area, set in a landscape traditionally dominated by grassland, wetland and koppies. Restoring the indigenous wetland ecology of the site was the starting point for “reversing local extinction through sustainable architecture. The new residential units are designed to sit sensitively in the wetland landscape like a bird’s nest”.
It’s the water aspects that are relevant. “The wetlands provide evaporative cooling of around 2°C to surrounding air temperature – the change projected due to climate change – so this created landscape will provide resilience of the design for the future. The wetland won’t just be for aesthetic pleasure, but to store and filter drinking water for residents, to provide aquaculture services and a wildlife sanctuary for indigenous wetland species. The architecture sits raised above the wetland with its water flowing underneath each unit.”
Sufficient storage is a challenge with net-zero water, according to Sherratt. “With Vleihuis, instead of unsightly tanks, we used a created wetland. How you deal with stormwater is then crucial, as generally, this is your cleanest renewable water source.”
“Sustainable Urban Drainage (SUD) systems are an alternative way of dealing with stormwater to the conventional approach: putting it in a pipe and getting it out as fast as possible. SUDs try and slow water, keeping it near the surface using natural means and absorbing water back into soils and plants onsite,” he explains. “Water should be seen as a resource, not a waste product.”
“Of course, residential is the hard one to crack,” he adds. “To get people to spend on sustainable technology that has a long-term horizon, which also usually involves a change in consumption behaviour.”
“The goal of the net-zero tool is reduced environmental impact. With net-zero water certification specifically, the focus is to reduce water consumption to self-sustaining or even regenerative levels,” says André Harms, founder of Ecolution Consulting. He was the sustainability engineer on The District, a commercial office building in Woodstock, Cape Town, that was GBCSA-certified in 2018 (until 2021) and achieved its Net-Zero Water (pilot) Level 2 Occupant Consumption (modelled) rating.
As an existing, operational building when the alternative water solutions were implemented and Net-Zero Water certification was pursued, it presented some design challenges. “These include first ensuring the building’s infrastructure (flush and flow fittings) are as efficient as possible, before quantifying the water consumption and suitably sizing the water treatment plant,” Harms clarifies.
“Identifying and allocating space to plant, equipment and storage tanks was another challenge, as space in commercial buildings is typically constrained, so conflicting demands are often placed on open areas.”
Suitably sizing a system to cater for an unknown future can be tricky. Both in terms of the alternative water supply (in this case, basement dewatering/drainage water, which can fluctuate with changing rainfall and groundwater) and demand within the building.
The District’s net-zero water design innovation was that its water filtration plant has the capacity to generate 140kL of potable water daily from four sumps in the basements. This water is “cleaned to WHO standards and injected into the current reticulation”, rendering the building independent of municipal water. Harms says The District’s plant was intentionally oversized compared to the building’s demand, to be able to provide treated water for other buildings or needs.
Owner Growthpoint Properties was approached for insights as to why the 2018 design certification (until 2021) for The District has not been renewed. “The Net-Zero Water certification for The District was primarily motivated by two factors, the first being the record-breaking drought facing the City of Cape Town, due to three years of inadequate rainfall. As a result, dam levels were at 25% capacity by January 2018,” says Grahame Cruickshanks, Growthpoint Properties’ head of sustainability and utilities.
“The second, equally significant factor, was that The District seven-story building is built on top of a naturally occurring spring that flows into the basement. These conditions were unique to both that time and location and may not be repeated at other properties.”
Cruickshanks says the primary benefit for The District 2018 tenants was a reliable water supply, to ensure they could maintain business continuity during Cape Town’s water restrictions. “During the drought, relaxations to regulatory compliance cleared the path for the water project, based on technical and financial feasibility studies at that time. Subsequently, the City reinstated these regulations, making the project significantly less viable.”
“The cost of meeting regulatory requirements, primarily due to the time it takes to obtain licenses and the associated consultant fees, has made the water processing plant financially unfeasible,” he concludes. “In addition, excess water from the on-site water installation is currently discharged as wastewater and not used in the building. We have, as a result, placed the re-certification of The District as Net-Zero Water on hold until we can come up with a viable solution.”
Smit acknowledged that there are still technical challenges to easily achieving Net-Zero/Net Positive Water buildings. “Part of the challenge relates to the building location and local weather patterns, and how that supports water harvesting,” she says. “But furthermore, and in some cases, the lack of ideal payback periods and incentives also play a role.”
“A mind shift is required by seeing buildings not exclusively as dependent on external services and resources, but as an opportunity to identify avenues where the building and its systems can become independent from the grid and add to resilient cities’ provision of services and utilities,” suggests Harms.
Annelidé Sherratt (Marc’s wife) is head of department of green building certifications at Solid Green Design Consulting. The company is involved with a project that pushes sustainable boundaries. Legaro Medical Centre is at 76 Corlett Drive, Melrose North. The building is targeting the first international Living Building Challenge (LBC) rating system water accreditation for a medical facility, the first LBC Petal Certification for a local commercial office and a GBCSA 6-Star Green Star Medical suites v1.1 Design certification. Certifications were originally scheduled for completion in 2020.
Legaro Medical Centre is aiming to be the “most sustainable building in Africa”. Mrs Sherratt says at between 90-100 points, they are aiming for the highest 6-Star Green Star rating possible. The rating will include Net-Zero Carbon – Level 2 (Base Building and Occupancy) and Net-Zero Water – Level 2 (Base Building and Occupancy).
From a water perspective, the Net-Zero Water – Level 2 (Base Building and Occupancy) means the building is designed so “all water to be used on site will come from a non-municipal supply – rainwater and grey water”. It will be done via rainwater harvesting from a roof area of 790m2, a reclaimed water system, stormwater management system and wastewater treatment.
“We are targeting Net-Zero Water. This means 100% of the project’s water needs must be supplied by captured precipitation or other natural closed loop water systems, and/or by recycling used project water. And be purified as needed, without the use of chemicals,” she adds.
Stringent requirements also mean “105% of the project’s energy needs must be supplied by on-site renewables on a net annual basis, without the use of on-site combustion”. In addition, they have to sub-meter major energy end users (heating, cooling, lighting, fans/pumps, plug loads, vertical transportation and domestic hot water).
A tall order indeed but encouraging for net-zero goals and water specifically.