We all know the high environmental load of traditional swimming pools; the energy required to heat them; the chlorine required to keep them clean; the large amount of water that is required to fill them in the first place and refill them after they have been drained down for the winter. It’s also common knowledge that a designer can ensure a pool is more sustainable by specifying a pool cover, installing solar thermal panels to heat the pool and considering ozone instead of chlorine as the disinfection method. But all of that does not need to concern us in the environmental building sector because if a client wants a pool, there is a perfect solution available on the market and that is a natural swimming pool. Right? Well wrong actually!
I was recently asked to provide a critical analysis of the water strategy for a large multi-million pound dwelling that, as with many such sustainable properties, had not addressed water beyond rainwater harvesting and dual flush loos. On first reading the one thing we didn’t have to worry about was the swimming pool as it was going to be a natural pool. But when we began to analyse the pool specification in detail some worrying facts emerged.
Natural swimming pools
A natural pool consists of a swimming area and a planted area, (the filtration zone)1. The plants extract any nutrients that run into the water, thus restricting the growth of algae. A pump is used to circulate the water through a filter to aid cleaning. Natural swimming pools have several environmental advantages; no chemicals are required to clean the pool, thus preventing the formation of hazardous chemicals;2 they do not require draining down over the winter and refilling in the summer, thus saving water; they use less energy as they are not heated; they provide a habitat for insects and amphibians in the shallow planted zone; and birds use the pool for drinking and bathing.
Natural pools are often designed to be larger than a traditional pool; in this case a surface area of 124m2 (compared to an average traditional pool size of 45m2). You would assume that a natural swimming pool would be quite happy (indeed prefer) to be filled with alternative sources of water to mains water. However, when we asked about filling the pool from the commercial fish ponds on the site we were told the nutrient levels in the water would be too high. So this pool would require 153m3 of mains water to fill – the equivalent of a daily use of 420 litres of water for a whole year! We asked about topping up with rainwater from the green roof and were again told that nutrient levels would be too high even though no fertiliser would be used on the roof.3
Evaporation, even from an unheated pool, is significant. Calculating evaporation rates is complicated, with many variables. The main factors that affect evaporation rates from domestic outdoor pools are pool surface area, temperature difference between the water and air, humidity levels and wind. Research in Australia has shown that the average daily evaporation rates from an unheated pool are 6.4 mm/day (6.4 litres/m2/day) for the six hottest months in Melbourne. The same research shows that a pool in Perth, in the coldest months, loses 3,000 litres a month over a surface area of 42m2.4 In the absence of any comparable data for the UK I calculated evaporation rates from the pond using both sets of data (with changes to better reflect UK conditions) and came up with some depressing figures.5 With a pool surface area of 124m2, and a rate of 2.1mm evaporation a day, 260 litres a day is lost to evaporation (against the average water use in the UK of 150 litres/person/day. Over four months this is 31,200 litres (31m3). Adding evaporation across the remaining eight months of the year (albeit at a lesser rate) total yearly losses are over a third of the pool volume. So far, so bad, and it gets worse… To keep the pool clear the water is continuously pumped through a filter. A pool this size will require a pump drawing 400 Watts. That is 3,504Wh of electricity a year, the average household’s total electricity use!
So am I saying no ponds at all? No, of course not. That would make for very poor garden design. But these highly engineered natural pools are not the solution. On this site, the response by the architects to our findings was swift and decisive. They reduced the size of the pool to 45m2 for the swimming area with a planting zone of 12m2, cutting evaporation rates, original fill water and electrical load by over 50%. And although the natural pool is currently still in the specification, the architects agreed to our suggestion to provide a natural pond as well, filled and topped up with rainwater from the green roof. The level of the pond will rise and fall with the seasons and the available rainfall. Any overflow from the pond will drain naturally into the ground via swales and rain gardens. A natural pond increases the biodiversity on this site as well as providing another focal point, and uses the runoff from the roof in the best possible way.
- The size of the planted area depends on the pool type but most have a planting zone of between 15-25% of the surface area of the pool.
- Chlorine in pool water reacts with organic compounds in the water such as sweat and urine to produce a host of hazardous chemical compounds that include nitrogen trichloride, aldehydes
- Both of the main suppliers of natural pools in the UK stated this.
- As Melbourne in the summer is hotter than the UK, I used a figure of one third the evaporation rates (i.e. 2.1mm/day) and over four months rather than six months. Using the Perth data as it stands but for all twelve months, yearly evaporation losses would be 36m3.