Waste not, want not
Managing water demand in buildings requires the same kind of thinking as epitomises good practice in energy management, say Ant Wilson and Charlotte Parkes.
The United Nations predicts that half of the world’s population will face ‘water stress’ by 2030. Even some areas of the UK are technically ‘water stressed’ and likely to get worse. Consequently, water conservation measures are increasing in importance.
However, while building accreditation schemes such as BREEAM and the Code for Sustainable Homes imply that rainwater harvesting and re-use of grey water are highly desirable, they should not be the first option. In fact, we would encourage an approach that parallels measures to minimise energy consumption, based on the ‘reduce, re-use, recycle’ model — albeit with the recycling element falling to the water companies in most cases.
Clearly, the obvious starting point is to minimise water consumption, which has additional benefits of reducing pumping energy, as well as the environmental impact of treating water at the water works. Reducing hot water use also has the added benefit of reducing the energy requirements for heating the water.
There are growing drivers for water management in buildings, for example the Planning Policy Statement on Climate Change and DEFRA’s Future Water call out to local and regional planning bodies to consider the availability of water resources and set relevant local standards for water efficiency. Earlier this year the proposed 2009 Approved Document for the Building Regulations Part G was released, introducing water efficiency requirements for new homes. This, in line with the Code for Sustainable Homes, proposed the efficiency requirement to be calculated in accordance to a new Government ‘national calculation methodology’, the Water Efficiency Calculator for New Dwellings.
There is significant variation in how water is used in different types of buildings.
In typical offices, most of the water consumed goes down WCs and the urinals, so these are important areas to address.
In residential schemes, hotels and shared accommodation the importance of hot water increases as a result of higher bath and shower use.
There are existing, albeit limited, CIRIA (Construction Industry Research & Information Association) water targets for commercial buildings, and the BREEAM assessment schemes apply different calculator- or standards- based approaches for different types of buildings.
As with other services, the first line of defence is good design. Appropriate measures include minimising draw-off lengths, selecting appropriate systems and giving particular emphasis to hot-water systems because of the energy embedded in hot water. It also makes sense to use the sanitaryware options available, such as aerating or spray taps and aerating or low-water flow showers. The latter is very important with so many more people cycling to work and taking a shower on arrival.
Other options include percussion taps or infra-red sensor taps with a sensible time delay, waterless urinals, low-flush and vacuum WCs. In typical offices, most of the water consumed goes down WCs and urinals so these are important areas to address. Flushing urinals can be combined with presence detectors to ensure that flushing only occurs when the urinal has actually been used. Detectors or timers can also be used to shut off the supply to toilets completely out of hours, so that any water loss through undetected leaks is minimised.
Careful design and specification is essential to retain the usability of the systems. Design parameters such as system pressures need to be considered in the design approach.
When all water-saving measures have been considered, it is time to address the opportunities for re-use, such as grey water and rainwater harvesting. Once again, it is worth noting that re-used water displaces treated water and reduces the environmental impact at the water-treatment works.
Traditionally, harvested rainwater has been used for irrigation, but there is increasing use of this resource for flushing as well. It is vital to get the storage volumes right to account for the demands and, into the future, changing rainfall patterns.
The British Standard for rainwater harvesting (BS 8515:2009) intermediate approach (there is a simplified approach for simple domestic applications) is based on 5% of annual rainwater yield. This is the equivalent of 18 days’ storage and calculated as the average daily rainfall on the collecting area multiplied by the run-off coefficient (dependent on the collector/roof pitch and surface) multiplied by efficiency of the hydraulic filter multiplied by 18 days.
Where the calculated average daily rainwater yield matches the demand, 18 days’ storage should ensure that most of the demand is supplied by the collected rainwater. However, models simulating the performance for rainwater harvesting systems show that the benefit of large storage sizes reduces as the tank sizes increase, with diminishing returns. Therefore, tank size can be reduced, resulting in cost and material savings, with only a small decrease in system effectiveness. Conversely if the tanks are undersized, a small decrease in the storage volume results in a large decrease in water-saving.
Looking to the future, climate-change projections suggest that not only is average rainfall expected to fall by 10% in the south east of England, it is also likely to be more variable. Consequently, we can expect to experience more extreme rain-free periods and intense rainfall, so there is an argument for more detailed design practices (such as computational models and analysis) considering the annual rainfall patterns. Detailed design approaches may also be beneficial in situations where the demands are irregular or seasonal — such as tourist venues, some non-domestic premises and schools.
It is also important to bear in mind that rainwater can be contaminated as it runs off surfaces such as roofs, bird faeces being a common culprit, so ultra-violet treatment may be required.
In the re-use of grey water, and water from showers, baths and basins there are a number of considerations.
The first consideration is that the grey water cannot be stored for more than three days without treatment; such treatment can be expensive and can carry its own environmental impact.
One way to reduce the costs and impacts of treatment is to use a short-retention storage system to collect grey water from washbasins on one floor, apply very basic treatment, and store it in tanks to flush toilets and WCs on the floor below.
These types of basic treatment systems introduce other issues such as the acceptability of grey water re-use and levels of treatment.
Grey water from wash basins has the potential to be contaminated by whatever people have on their hands, but if used for flushing should not be a problem. However, some people are aware that there can be a ‘spray’ from a flushing WC, and this may need to be taken into account.
The complexities of using grey water and rainwater harvesting, therefore, serve to strengthen the argument for conserving as much water as possible through reduction measures. For example, waterless urinals and low-flush WCs reduce the water needed for flushing, so there would be little point in investing in water re-use.
In fact, a recent study by the Environment Agency showed that in dwellings using rain or grey water re-use increases CO2 emissions for water supply, use and disposal.
Questions are also emerging in the industry about the true environmental benefits of re-using water — though more research is needed. So while re-using water remains an area of contention it makes sense to focus on the design practices that we know will deliver beneficial effects.
Ant Wilson and Charlotte Parkes are with AECOM.
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