There’s rather more to designing a sustainable building than specifying low-embodied energy materials and making sure it will require little energy and maintenance in use. Low water demand and the ability to get the nutrients discharged in the sewage back to the land are important too.
The two main nutrients we need to be concerned about are phosphorus and nitrogen. There’s rather too much of both around at present for the good of the environment, particularly when they get into rivers and lakes. Apparently, thanks to vehicle emissions, more nitrogen compounds now come out of the air as acid rain and particles than farmers put on their fields in the 1950s. Indeed, a recent article in New Scientist quoted scientists as saying the build-up of man-made nitrogen compounds in water and in the air was a more serious threat than that posed by climate change. One of those interviewed called for a Kyoto-type agreement to limit the production of nitrogenous fertilisers.
Phosphate pollution presents a more localised problem than nitrogen but the effluent from sewage treatment plants and the run-off from farms caused serious algal blooms on the great West of Ireland lakes a few years ago. The situation has recently improved a little because many sewage works in the area have been equipped to remove phosphates. But the main source, the run-off from the land continues unabated and it is not clear what effect the single farm payment system will have. The optimists think that less fertiliser will be applied now that farmers get paid regardless of output, while the pessimists fear that as small farms are bought up and amalgamated, their new, younger, ‘more progressive’ owners will apply more fertiliser than the previous older ones did. Since a kilo of phosphate in water can produce half a tonne of vegetable matter, the knowledge that Irish farmers spread 96,000 tonnes of the stuff annually is rather alarming.
If space is really limited, you can opt for a "living wall" in which the plants grow in pockets, one above the other.
It’s therefore important to keep nitrogen and phosphorus out of water flows. However, in view of the excessive quantities of both that are about, it might seem odd to insist that all new buildings should be equipped to conserve and recycle both nutrients – unless, that is, the excess can be expected to disappear during the buildings’ lifespans. And that, of course, will almost certainly be the case. The output of nitrogenous fertilisers will be sharply reduced as natural gas supplies become scarce. Also, as oil output declines, smaller quantities of nitrogen oxides will come from vehicle exhausts. This will ease environmental pressures and farmers will revert to relying on legumes and animal wastes such as urine to put nitrogen into their soil.
With phosphorus, the problem is much more serious as there is no natural phosphorus cycle to return the nutrient to a depleted soil. Human intervention is necessary but, at present rates of extraction, the world’s reserves of phosphate ore will be exhausted in 130 years. That may seem a long time, but, as with all such ores, the richest sources are mined first and, as the years go by, it will take more and more energy to replace the 4-20 kg of phosphorus which every hectare of farmland loses annually depending on the crops or animals produced on it and sold away. Folke Gunther, a Swedish expert in this area, reckons that phosphate fertiliser prices could well increase 100-fold. “It’s no exaggeration to state that phosphorus is the most important nutrient” he says. “The world will never run out of it but if this generation disperses it, future generations may not have the energy to recover as much as they need.”
Continue to Page 2
