Public health Policy
For a high density commercial building (such as
a compact, high rise, city centre development), the rainwater harvesting system would achieve only 10% to 15% of the overall WC fl ushing demand of the building dependent upon the storage provision. In comparison, for a low density commercial building of the same population (but low rise, with a larger roof area), the rainwater harvesting system could achieve between 40% and 50%. Given such percentages, any rainwater harvesting
system would actually need to be supplemented with fresh water, and pipework systems will, in part, need to be duplicated. A further irony is that, due to the increasing trend for green roofs, there is also less run-off water available for rainwater harvesting. Studies have also shown that the run-off from green roofs can be too high in biological residues to be suitable for water fl ushing. Grey water systems are technically more complex
than rain water systems utilising either mechanical filtration or settling tanks and UV treatment. These require maintenance and supervision to ensure that the water is of suitable quality for distribution around the building. These systems also consume energy, which can be similar to the energy consumed per cubic metre by a large scale water treatment plant. It could well be that the energy required to clean it is greater than the amount of water it provides, so it might be a better investment in time and resources to reduce leaks in the mains water distribution network instead. All of this needs to be considered very closely by
engineers, before assuming that rainwater or grey water harvesting automatically offers energy saving advantages. In domestic situations, rainwater harvesting is much
simpler. A water butt provides a rough and ready solution for watering the garden. However, CSH applies equally to an individual house as it does to a block of fl ats. This can create anomalies in the credits assigned for particular practices. For example, in terms of rainwater recycling, it is obviously sensible to only recycle rain water at the lower levels within multiple dwelling developments to achieve a balance between the volume collected and distribution effi ciency. In this situation, to obtain the required CSH credit for
a dwelling not served by the rainwater harvesting system, it will be necessary to install low-fl ow water appliances, as the benefi t of the rainwater harvesting system cannot be averaged across the block. Other anomalies can be found within both the CSH and BREEAM Commercial. For example, Part L-compliant software, such as TAS, estimates hot water consumption based on metre square of fl oor area of a particular room type. Therefore a hotel room hot water consumption varies depending on the room size and not the number of occupants or the type of fi ttings. Solar thermal and CHP systems must be confi gured based on the predicted water consumption requirements and therefore the Part L results are not particularly suitable for sizing these systems. However, with energy performance certifi cates and energy strategies being based on Part L calculations,
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ap90x133_cibse_supp_0910.indd 1 September 2010 CIBSE Journal 10/08/2010 10:17 55
there is temptation to use this approach at the conceptual stage of the project. All of these can deliver very good theoretical results according to the rating system applied, but very poor results in real-life performance. In situations like this, solutions may become driven by their credit-worthiness rather than best engineering or construction practice, or user needs – and sticking to the letter of the CSH, or of BREEAM could instead lead to duplication of systems and increased capital costs. Of course, the latest changes to Part G of the Building Regulations will introduce water efficiency on a mandatory basis. With the exception of publicly funded properties and the requirements of some planning authorities, until now the need to comply with CSH has been voluntary. But very soon, compulsory water effi ciency measures could have a massive impact on the design and construction of new homes. It remains to be seen whether this impact will create
a long-term shift. After all, once you have completed construction and achieved Part G, there is no way of enforcing continued compliance. ●
Contributing authors, all from Hilson Moran:
Chris Springett, senior associate public health design engineer; Andrew Russell, associate public health design engineer; Steve Johnson, associate sustainability consultant; and Emma Flower, sustainability consultant.
Get it wrong
in either environment, and the occupants will soon be changing the fi ttings, post occupancy
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