Wednesday, January 25, 2012

Reuse of Municipal Waste Water Has Increased Substantially Over The Past 30 years

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.

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