Fundamentals of Design – Water Management


Water is vital to life, yet it is often taken for granted. Many city dwellers don’t know where their water comes from or where it is disposed of – until something goes wrong. With threats of drought, water restrictions and “watershedding”, it’s worth reflecting on best practice for water management in building design.

Far more potable (drinking quality) water is used in buildings than is needed, highlighting that there is room for water use within buildings to be better managed without compromising creature comforts.

Demand management – the low hanging fruit

Efficiency is the first principle of all sustainable building interventions and is often the simplest to implement. Water is no exception and the cheapest water savings are achieved by specifying efficient fittings. Low-flow taps, dual flush systems, even waterless urinals feature in most green certified buildings. These interventions alone, depending on the overall profile of the building, can translate into water savings of up to 20%.

But whether or not efficient fittings can be specified throughout depends on the building purpose, user norms, and requirements and perceptions of the technology. Jaco Kemp, sustainable buildings specialist at Arup, says it’s a challenge to deliver efficient sanware in the hospitality industry, for example, because a focus on luxury may preclude bathroom water-efficiency.

Second on the list of basic interventions is installing meters with a monitoring and leak detection function. In a study published in 2012 by Lugoma, van Zyl and Ilemobade, which quantified on-site losses, they note: “Municipalities are not responsible for private systems on which on-site losses occur and may even benefit from the losses in these systems through increased water sales.” The findings of the study, conducted in Johannesburg suburbs, reported 64% of the 182 sites evaluated had measurable on-site leakages.

Monitoring and leak detection, installed at a relatively small cost, can have a significant impact on reducing water consumption and saving money that would otherwise be ‘washed away’.  Terramanzi owner Fabio Venturi says, “A Building Management System (BMS) with leak detection is essential for providing quick and accurate feedback, which the project team can respond to and mitigate potential flooding damage and water losses”.

Supply management – alternative water sources

There are three potential alternatives to municipal supply: harvested rainwater, grey water and blackwater.

Rainwater is relatively clean, requiring only simple filtration before it can be used for non-potable applications. Many systems typically include a first flush diverter that uses the first rain in the storm event to clean larger debris off the roof. However, Steve Parker of Pure Rain Technologies points out “the water diverted in the ‘flushing’ design is an incredible waste of the resource. On a continent that produces lower than average rainfall, the thought that we may waste up to 50% of our collection cleaning our roof seems crazy”.  He believes the designers of the first flush filters overlooked the fact that “about 97% of all rainfall events worldwide are classed as ‘light events’“. He adds: “It is critical to maximise your light event capture to harvest usable quantities of the resource.”

Sotiralis Consulting suggests a pluvial system for harvesting off roofs. It allows debris to settle out and then removes water from the surface to storage. Sotiralis’ Andries Denner says: “The client can benefit if the roof can be utilised as an attenuation pond that captures rainwater and reduces the flow rate of storm water. The downstream storm water system then only has to be designed for the reduced flow rate, which is a cost benefit to the client due to reduced pipe sizes. The storm water can then be collected in a rainwater harvesting tank for later use.”

Deciding how to use rainwater depends on seasonal rainfall patterns. Essential questions to answer include: when does the rain fall; and how much of it lands on hard surfaces from which it can easily be collected? This will vary according to site location.

The amount of water required by each water usage and the application, which can maximise the consumption of the alternative source with the optimum amount of infrastructure, must also be considered. This will vary according to occupancy type and usage profiles. For example, a middle class residential property with no pool and relatively small landscaping area may use as much as 60% of their water for bathing. In commercial buildings, HVAC (if water-based heat rejection is used) or irrigation (depending on the extent and type of the landscaped area) may use the lion’s share, and in hotels and hospitals, where linen is typically washed after single usage, laundry can have the heaviest impact.

Uses suitable for non-potable water include HVAC make-up water, flushing of toilets, irrigation and laundry. While the water requirements of some of these applications, such as irrigation and HVAC, may fluctuate daily or seasonally, other applications, such as flushing and laundry, will remain constant. Prioritising the constant water requirements would mean the water storage capacity could be reduced to what is needed for a few days. “The tank should be sized to store two to three days of the water demand. Smaller developments (requiring less than 10m3 storage capacity) may be able to utilise Jo-Jo tanks but bigger developments may require larger tanks, which would make concrete tanks more feasible,” says Denner. “Another critical point is to ensure the tank is above natural ground level to ensure water can overflow directly out of the building in case of an emergency.”

At a level up on the cost and maintenance scale, capturing and filtering grey water may render better results in low-rainfall areas. As a source of water it is also more reliable than rainfall, especially for building typologies where regular bathing is part of the usage profile. But viability may hinge on how accurately the quantity of water generated from grey water consumption can be estimated. Reflecting on his experiences at No. 1 Silo at the V&A Waterfront in Cape Town, Kemp says it was predicted much more grey water would be generated from the showers than was the case in reality. The resulting system was oversized and they are looking to take on the grey water from neighbouring buildings to fully utilise the system’s processing capacity.

On-site blackwater treatment plants with water recapturing for non-potable usage is the Rolls Royce of water treatment systems and can be the last cog that neutralises a building’s water-associated environmental impact. Andrew McDonald, senior engineer at Arup, says up to 99% of water can be recovered when a blackwater treatment plant is installed.

Until recently, there has been widespread resistance from the industry to installing blackwater treatment because of the associated capital cost, complexity and on-going maintenance implications.  While any recycled water can be polished to potable water, the results on a domestic scale treatment plant are generally not guaranteed and due to pathogen presence, are considered high-risk in blackwater systems. Yet, innovative projects are starting to explore options with some notable results. “Blackwater treatment is the way of the future,” says McDonald.

Pushing Innovation

An excellent example where water-independence has been achieved is Estuaries Plaza in Century City, Cape Town. The project targeted Green Star SA certification relatively late in the build programme and after an extensive screening process, opted to implement a fully independent water system to maximise the points available for implementable credits. The system collects all wastewater, which is treated via a three-stage process: a bio digester, an HWT organic filtration system and then a sophisticated reverse osmosis (RO) machine. “Initially only the biodigester and RO system was planned, but through commissioning it quickly became evident that additional intermediate treatment was required to ensure efficiency and predictability. The team then installed an HWT organic filtration system, which includes composting worms. The final output is now purer than bottled water,” explains Venturi.

He sums up the take-home experience at Estuaries Plaza as “a big investment for the client, but taking into consideration the reality of the current water crisis, it was the right choice, and our client’s property investment is truly future-proofed”.

There is no one size fits all and most projects will assemble components in response to the characteristics of the building’s need and context to produce an integrated system.

The Silo District has taken advantage of a contextual resource by using water from the cold Atlantic ocean to pre-cool the water used in the HVAC system. “Due to the salt content, the water is corrosive, so we had to separate it from the mechanical system with a heat exchanger,” Kemp says. Water in the building HVAC system is on a closed circulating loop, while water is taken from the sea in an open loop to pre-cool the water in the air-cooling system.

Speaking on efficiency, Kemp adds: “If you specify an air-cooled chiller, then the HVAC won’t use any water, but a water-based system is more energy efficient, so there is a trade-off. If you can use non-potable sources in a water-based system, then you limit both the energy and the water consumption.” The system implemented at No. 1 Silo has been so successful it is being rolled out to all the Silo buildings.

Whether a project is a small budget-conscious enterprise or a leading example of innovation will dictate what savings can be achieved. Simple, cost effective solutions exist that will reduce the water-impact and can be implemented on all projects regardless of scale or green mandate. Given the urgency of South Africa’s pending crisis, these savings have significant benefits. By further matching optimal usage to alternative supplies, significant reductions can be achieved with only a small additional green budget, but true water independence on a building scale is a noteworthy area for development and innovation.

By Peta Brom

For the full article, see earthworks magazine Issue 30, February-March 2016.