DECADES ago, your correspondent visited one of the larger sewage works in the Thames Valley to learn how the new biodegradable detergents, with their long hydrocarbon chains, were affecting the plant's filtration processes. The plant was coping just fine, he was informed. And the output was so good, it was piped straight back to local reservoirs for redistribution.

Each drop of water used by Londoners subsequently passed through the plant for reprocessing at least six times before eventually escaping to the sea. The engineer in charge was convinced that, with further refinement, the sewage works would be capable of recycling the same water indefinitely—with the quality improving with each treatment cycle. Offered a glass of the finished product, your correspondent thought it tasted a good deal better than the chalky liquid that spluttered from London taps (see “From toilet to tap”, September 26th 2008).



In America, the assumption is that, if recycled at all, reprocessed effluent is used strictly for irrigating golf courses, parks and highway embankments, or for providing feedwater for industrial boilers and cooling at power stations. The one thing water authorities are loathe to discuss is how much treated sewage (politely known as “reclaimed water”) is actually incorporated in the drinking supply.



The very idea of consuming reprocessed human, animal and industrial waste can turn people's stomachs. But it happens more than most realise. Even municipalities that do not pump waste-water back into aquifers or reservoirs, often draw their drinking supply from rivers that contain the treated effluent from communities upstream.



A survey done in 1980 for the Environment Protection Agency (EPA), which looked at two dozen water authorities that took their drinking water from big rivers, found this unplanned use of waste-water (known as “de facto reuse”) accounted for 10% or more of the flow when the rivers were low. Given the increase in population, de facto reuse has increased substantially over the past 30 years, says a recent report on the reuse of municipal waste-water by the National Research Council (NRC) in Washington, DC.



Along the Trinity River in Texas, for instance, water now being drawn off by places downstream of Dallas and Fort Worth consists of roughly 50% effluent. In summer months, when the natural flow of the river dwindles to a trickle, drinking water piped to Houston consists almost entirely of processed effluent.



The main problem is not changes in the weather (though global warming hardly helps), but population growth. The American population has doubled, to over 300m, since the middle of last century—and is expected to increase by a further 50%, to 450m, over the next half century. Meanwhile, households as a whole have been consuming water at an even faster rate, thanks to the housing boom and the widespread use of flushed toilets, dish washers, washing machines, swimming pools and garden sprinklers.



Then there is the ongoing migration within America from the cooler climes of the north-east and mid-west to the sunbelt of the south. Since 1970, Arizona, California, Florida, Nevada and Texas have seen their populations surge by 85% to 400%. This exodus to warmer, dryer parts of the country has coincided with a decline in the construction of hydrological infrastructure—dams, aquaducts, tunnels, pipelines and reservoirs—for collecting, storing and transporting water to precisely those parched places.



The fact is, there are simply no more ambitious water projects remaining to be tackled like those of the early 20th century, which pumped water from the Colorado River and the snow-capped Sierra Mountains across hundreds of miles of desert to the thirsty cities of the American south-west (see “Water, water everywhere”, June 25th 2010). Today, few lakes and rivers within pumping distance of the country's conurbations remain untapped. Meanwhile, dams that help purify effluent in rivers—by holding back water for months on end so that microbial and photochemical processes can do their job—are being dismantled to restore natural habitats and protect threatened species.

Over the past quarter of a century, the amount of water used in the United States has remained stable at around 210 billion gallons (795m cubic metres) a day. While consumption by households has tripled since the 1950s, the amount of water used to irrigate agricultural land and feed industry has declined. Farmers have embraced more efficient sprinkler systems, put more crops under glass, planted more drought-resistant varieties, and profited from selling their surplus water to nearby towns. On the industrial side, the use of thermo-electric power—with its need for cooling water—peaked in 1980 and is now below its 1970 level. Meanwhile, many old water-using industries have upgraded from steam to electric power or moved offshore.



Conservation has also helped ease the demand for fresh water, though it comes nowhere near offsetting the thirst of the sunbelt's surging population. The only conclusion is that, like it or not, people will have to get used to drinking their own effluent.



In doing so, the least of the troubles water districts face are technological. The know-how for filtering and purifying waste-water is as advanced in America as anywhere, though installation still lags. In Britain and much of the rest of Europe, water authorities insist not only on primary and secondary treatment of raw sewage (to remove suspended solids and organic matter, and add disinfectants), but also require tertiary processing (to remove nutrients, biodegradable products and even traces of pharmaceuticals and other organic compounds).



Thanks to the Clean Water Act of 1972, when Congress voted over $24 billion ($130 billion in today's money) for the construction of modern sewage plants across the country, America has begun to catch up. Today, over a third of the population drinks water that has been at least tertiary-treated, though the rest have to put up with water processed to only secondary level or less.



This is not necessarily cause for concern. Many rural communities do not need tertiary sewage treatment. Regions with heavy rainfall may not either. The Safe Drinking Water Act of 1974 sets the same standard for potable water throughout America, irrespective of whether it is derived from pristine mountain streams, recycled effluent from sewage works, or de facto reuse from upstream communities.



The recent NRC report makes it clear that any possible health hazards—caused by exposure to chemical contaminants or disease-causing microbes in reprocessed waste-water—do not exceed (and, in some cases, may be significantly lower than) the risks inherent in existing drinking water.



Finding the money to keep clean water flowing, though, is quite another matter. A decade ago, the EPA reckoned that, with many of the country's drinking-water and sewage works coming to the end of their working lives, up to $450 billion would need to be invested in new plant between 2002 and 2020. If there is to be greater reliance on recycled waste-water, that may not be nearly enough.



Getting the great American public to accept having waste-water in its drinking supply is a bit of a problem, too. As the NRC report notes, people have been trained for generations to think of their water supply and their waste disposal as two quite separate, and unrelated, undertakings. No-one is sure how they will come to terms, if ever, with the notion that the two are part and parcel of the same thing. But if only people could be persuaded to take the taste test, your correspondent is certain they, too, would find that recycled waste-water can be every bit as sparkling and tasty as the freshest mountain dew.