The power of poop: revolutionizing wastewater treatment

All along we've been wasting all our waste! That's the message from a recent commentary¹ in Nature that describes how we can produce energy from all the undesirables we flush down our sinks and toilets. According to Chinese and US researchers, investing federal research money to scale up already-existing technologies could transform waste-water treatment from an energy-consuming necessity to an energy-producing luxury.

The cost of reuse

Feces, food scraps, fat, detergents...the list of items we throw down the drain is endless. But these organic materials and chemicals don't just disappear; water washes them through our pipes to the closest treatment facility to recycle as much wastewater as possible.

The present sanitation processes used to reuse water for drinking, irrigation, or manufacturing have revolutionized the cleanliness of human civilization and allowed for relatively clean, dense urban environments. To achieve such a feat, treatment plants typically toss wastewater with a mix of air and bacteria, an aerobic process (oxygen is present in the air) that oxidizes so-called pollutants to remove nitrogen, phosphorous and organic compounds from the water. This process leaves behind a thick sludge as residue but provides cleaner water for domestic and industrial use.

These advances come with a cost. Sanitizing wastewater for household use alone - cooking rice, washing dishes, flushing, etc. - accounts for 3% of global electricity production and 5% of all greenhouse gas emissions. That's equal to the daily emissions from 6000 cars per treatment plant, and these percentages will only increase as developing countries attain higher water quality standards. The nasty sludge produced from the aerobic process must also be incinerated or stuffed into landfills, which accounts for almost half of all costs.

Reclaiming our valuables

Transforming this industry requires a perspective shift. Instead of viewing all the flushed egg shells, vegetable skins, and detergents as useless waste, what if we valued the potential energy stored in the chemical bonds of organic materials and collected the nitrogen and phosphorous for fertilizer production? On average, one cubic meter of wastewater holds a half-pound of carbon-rich organics and tens of grams of nitrogen, phosphorous, and sulfur. Reclaiming these energy and minerals could morph wastewater treatment from an expensive but necessary recycling process into an electricity- and resource-producing industry.

Even better, most of the necessary technology for this transition is already available! The key change requires switching the traditional, aerobic process to an anaerobic process in which microbes metabolize organic materials in an oxygen-poor environment. This process reduces complex, organic compounds into simpler ones that are used to create methane, which can then be burned to produce electricity.

So why haven't we already been using anaerobic processes to produce electricity? The authors of the commentary claim that the primary obstacle is a lack of knowledge and financial resources to figure out how to combine already-existing technologies that currently operate on a small scale. Wastewater is a particularly challenging case for anaerobic processes to work efficiently because organics appear in relatively low concentrations in the water.

Technologies are already in development to address this difficulty, such as a porous membrane used to trap organic matter and microbes in a small space together while letting water and other particulates pass through. This increases the rate of methane production because the microbes can more easily find and break down the carbon-based compounds. Also, some microbes transfer electrons from their inner cells to outer layers during organic metabolism. These electrons can be captured by electrochemical cells to directly produce electricity.

Nitrogen and phosphorous recovery methods are also improving. Phosphate ions can be captured in porous materials like zeolite, and electric fields can be applied in treatment facilities to separate phosphate and nitrogen ions from the rest of the water to collect them for fertilizer. Nitrogen would be especially important globally because nitrogen-based fertilizers are produced using the extremely energy-intensive, Haber-Bosch process.

A call for support

According to the authors, we are on the cusp of a revolution in how we deal with wastewater. Research has creative new ideas to improve anaerobic processes, and now we only need to find ways scale these technologies up to plant-level production. The trapping membranes are passable but tend to clog as input levels increase. Electron capture from metabolizing microbes can be a very slow process. But these are solvable problems if federal research grants focus on improving this technological aspect of public health.

The economic benefits could be sizable as well. In one example given by the authors, incorporating this new technology in an average treatment facility in China would transform the plant from a 50,000 kWh consumer per day to a 17,000 kWh producer. In addition, the plant could recover tonnes of phosphorous and nitrogen each day to assist agricultural industries.

This is a perfect example for initial federal investment to provide long-term economic dividends. Greenhouse gas emissions are not priced correctly to illuminate the true cost of current wastewater facilities. By introducing regulation including the costs of emissions for waste disposal, countries could incentivize scaling anaerobic technology and expedite a transition to energy-producing plants. Our waste has never looked so beautiful.

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