Energy from Wastewater

Producing energy from waste is hardly a new idea, as illustrated by efforts such as the landfill methane-to-LNG project I mentioned in November and by numerous demonstration projects to turn various forms of solid waste into liquid fuels. However, I don't normally think of wastewater as having any noteworthy energy potential to extract. An emailed press release from the American Chemical Society announcing the publication of a paper on this subject in Environmental Science & Technology opened my eyes. Based on tests of wastewater streams in Britain, it might be possible to produce at least as much energy from wastewater as is normally used in the process of treating it. That's an intriguing prospect, although we should put it in perspective.

The press release and the paper included a number of statistics, some more accessible than others. One of the two wastewater streams tested in the UK had an energy content of 7.6 kilojoules per liter, which as the ACS noted is equivalent to each gallon being able to light a 100 watt bulb for 5 minutes. (The other sample contained twice that much energy.) That still doesn't sound very impressive until you factor in some other statistics in the report, such as the 12.5 trillion gallons per year of wastewater processed annually in US sewage treatment plants, at an energy cost of around 1.3% of US electricity consumption. That suggests that in theory US treated wastewater might generate as much power, net of processing requirements, as we currently get from biomass power plants fired on wood, which supplied just under 1% of our electricity last year.

Actually harnessing that potential wouldn't be simple. As you might imagine, and as the paper confirms, wastewater is hardly uniform. It contains a wide array of compounds that would require different techniques to produce energy in useful forms such as hydrogen, methane, or direct electricity. The more techniques required at one facility, the bigger the investment involved. Another complication relates to the low concentrations in which these energy precursors are found in wastewater. The team at Newcastle University measured them at levels in the hundreds of parts per million--well below 1%. That implies the need for either separation before processing or handling very large volumes. Either approach adds to cost or complexity. Still, the idea of producing energy from this source at the sewage treatment plant goes one step beyond the usual logic of waste-to-energy projects: Not only is the cost of the feedstock low or effectively negative, but it requires no additional transportation; it's already flowing through the plant.

Extracting energy from wastewater won't solve all of our energy or environmental problems, and it might prove to be more costly than it's worth. However, the paper's conclusion, that this merits a closer examination in a wider variety of settings, seems entirely justified. It makes even more sense in the context of our growing need to make better use of the water portion of this waste.