Waste not, want not

tauringhuang.

Aug 18th 2008
From Economist.com

Wastewater irrigation is better than you think


NORMALLY, news about the environment can be pigeonholed into one of two categories: a big one, labelled “Bad” and a smaller one, with a heading along the lines of “Encouraging” or at least “Not Quite as Bad as You Thought”. But your correspondent has no idea where to file a report on the use of wastewater in agriculture released this week by the International Water Management Institute (IWMI), a research centre.

The report looks at 53 cities in the developing world, to see to what extent local farmers water their crops with untreated sewage or industrial effluent, either directly or through contaminated local water sources. The results are striking. In more than two-thirds of the cities, over half the agricultural land is irrigated with wastewater—400,000 hectares (988,000 acres), all told. The study estimates that some 20m hectares around the world are watered in this way.

AP

Waste not, want notThat certainly sounds like a strong contender for the bad-news slot. After all, it is a reminder that many cities treat none, or just a tiny proportion, of their sewage. And growing crops in sewage is a good way to transmit diseases to farmers and their customers—which is all the more worrying as even simple ailments like diarrhoea are often killers in poor countries. Then there are all the nasty chemicals present in industrial effluent, including poisonous metals such as cadmium and mercury, that can find their way into the food chain if applied to crops.

But Colin Chartres, the head of IWMI, is quick to point out that the report’s findings are not uniformly depressing. For one thing, in the cities surveyed, over 1m farmers make a living by growing crops in this way. Their earnings help support around 4.5m dependants. Worldwide, perhaps as many as 200m farmers rely on wastewater for at least some of their water.

This form of agriculture, it turns out, is especially beneficial to women. In some cities, women comprise more than 70% of the total number of farmers. Many also trade vegetables, and sell them wholesale. For those who worry about the status of women in the developing world, farming with wastewater looks like a good thing.

Many urban farmers, it is true, simply tap local rivers for their water. In such cases, it is hard to argue that the rivers’ contamination with wastewater does them any good. But others use pure wastewater for irrigation, suggesting that they have no alternative. They are benefiting not only from free water, but also from free fertiliser—albeit in the form of human excrement.

And then there are the customers. In Accra, Ghana, for example, 200,000 people—a tenth of the population—buy vegetables grown with wastewater every day. In other words, wastewater agriculture provides an important source of nutrition for a good chunk of the population. Without such supplies, vegetables would definitely be more expensive. In some countries, there might be little alternative supply at any price, due to poor transport infrastructure or a lack of suitable agricultural land.

The environment benefits too. Spreading wastewater over fields, and allowing it to leach back through the soil into local waterways, turns out to be a reasonable way to purify it. The process filters out all the organic contaminants, and much of the nitrogen and phosphates that would otherwise contribute to algal blooms and dead zones further downstream. It is certainly preferable to dumping wastewater straight into the nearest big river or lake.

IWMI has not done a thorough cost-benefit analysis, so it is hard to say with certainty that the extra costs of agriculture based on wastewater, in the form of time and lives lost to disease, outweigh the benefits, in terms of livelihoods, nutrition, and environmental protection. But Mr Chartres is clear that banning the use of wastewater in agriculture would be a mistake.

Instead, he suggests that cities where the practice is widespread make an effort to minimise the risks of wastewater use. They could run information campaigns, to ensure that customers know to wash their purchases thoroughly. They could train farmers to allow the sediment to settle out of the wastewater before using it—thereby avoiding the worst of the faecal matter.

And local authorities could also try to tackle industrial pollution at source—an easier and cheaper process than building an entire water treatment infrastructure from scratch. If a few of the cities in question could manage any of that, the report would slot neatly into the “Encouraging” hole.