Reuse of Water Pollutants

Extracting carbon, nitrogen and phosphorus from wastewater could generate resources and save energy, say Wen-Wei Li, Han-Qing Yu and Bruce E. Rittmann

Treating domestic and industrial wastewater so that it can be reused for drinking, irrigation and manufacturing is costly. The treatment of used household water from cooking, washing, cleaning and sanitation alone accounts for 3% of global electricity consumption and 5% of global non-carbon dioxide greenhouse-gas emissions (mainly methane). Industrial wastewater is more expensive to clean. Those proportions will rise in the next decade as the world's population grows and stricter water-quality standards are enforced by developing countries .

The costs could be more than recouped if valuable chemicals — including useful forms of carbon, nitrogen and phosphorus — were captured from wastewater. Water-treatment plants that harness methane could produce electricity rather than consume it, for instance. Scaled up, emerging technologies could efficiently and cheaply recover phosphate and ammonium for fertilizer.

What stands in the way of creating 'wastewater-resource factories'? Uncertainty — about which techniques are most useful and how to combine them. Here, we outline one possible strategy for domestic water (see 'Wastewater works'), illustrating how treatment plants that now cost millions of dollars a year to run could be retuned to generate more than US$1 million a year for communities. Similar schemes applied to more diverse industrial wastewater would deliver further benefits.

Down the drain

Domestic wastewater contains the detritus of our daily lives — faeces, fat, food scraps, detergents and pharmaceuticals. In chemical terms, 1 cubic metre of domestic wastewater contains 300–600 grams of carbon-rich organic matter (known as carbonaceous chemical oxygen demand, or COD), 40–60 grams of nitrogen (in the form of ammonium and organic compounds), 5–20 grams of phosphorus (in phosphates and organic compounds), 10–20 grams of sulfur (mainly as sulfate) and traces of heavy metal ions.

For the past century, the bulk of domestic wastewater has been treated using the aerobic 'activated-sludge process': it is whisked with air and bacteria to oxidize the pollutants. The process is simple and is effective at removing organic compounds, nitrogen and phosphorus. But it has a large energy and carbon footprint. A medium-sized plant (one that processes 100,000 cubic metres of water per day) consumes as much electricity as a Chinese town of 5,000 people (around 0.6 kilowatt-hours per cubic metre of wastewater) and emits as much CO2 as 6,000 cars per day.

Source: Nature

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Taxonomy

• Wastewater Collection
• Chemistry
• Energy Efficiency

1 Comment

1. This is how we will se polluants in the future... like resources, like they have to be. Congrats for the article!