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From poop to power

ASU professor discusses advances in recycling dirty water, and harvesting its content

Flowers in the toilet.
December 10, 2015

What happens after you flush the toilet is becoming a big deal.

In a just-published article in the science journal Nature, Arizona State University water treatment expert Bruce Rittmann and two colleagues propose a paradigm-shifting change in the treatment of wastewater, a shift they say could have a dramatic global impact. They outline ways to transition from conventional wastewater treatment, which removes contaminants and disposes of them, to advanced used-water resource-recovery methods that would be environmentally and economically advantageous.

In other words, your dirty water could be mined for useful and valuable resources — like nitrogen or phosphorous. 

The technologies for doing this are being explored today, but challenges remain before they can be used on a large scale and meaningful way. Rittmann, an engineering professor in ASU’s Ira A. Fulton Schools of Engineering and director of the Swette Center for Environmental Biotechnology in ASU’s Biodesign Institute talks about the new methods and what they can provide.

Question: These sound like very attractive and potentially useful technologies. Why aren’t they being implemented, or at least developed further, now?  

Answer: For decades, the conventional thinking was that anaerobic treatment processes are not efficient enough to treat domestic wastewater due to its low organic concentration and low temperature. Also, conventional aerobic treatment (e.g., activated sludge) has served us well as a means of “treatment only.” Only in recent years have we begun to question the assumption that the only goal is “treatment.” Since conventional processes did their assigned task well and energy costs were relatively low (most of the time), we didn’t have the impetus to do anything different.

In the past 10 years or so, a pull to reduce energy and to limit the greenhouse gas costs of treatment has changed our perspective. Combined with new materials (membranes and electrodes), we now have new tools to “push” development and to complement the “pull” of the desire to reduce energy and greenhouse gas impacts. The same reasoning exists for nutrient recovery — no “pull” until recently, and some new materials to give it a “push.”

Q: What are the environmental benefits of these technologies?  

A: By shifting from energy negative to energy positive, the anaerobic technologies seriously reduce the greenhouse gas emissions of treatment. Recovering nutrients prevents their discharge into surface waters and thus minimizes the acceleration of aging and dead zones in our lakes, reservoirs and oceans.

Q: What are the economic benefits of these technologies?  

A: The anaerobic processes can be used to generate energy not consume it. Electricity use is the largest non-personnel expense in treatment, and shifting it from a cost to a profit center has a huge economic benefit to a municipality. In addition, the anaerobic processes generate much less sludge that has to be treated and hauled off to the landfill. Currently, sludge treatment and disposal constitute the second largest operating expense. Recovering nitrogen and phosphorus also can provide an additional income stream if the quality of the products is good enough to sell. At a minimum, the sale of nitrogen and phosphorus products should offset the costs of removing them.

Q: What is the next step needed to convert wastewater treatment plants into resource generators?  

A: On the technology side, various technologies are at different stages. An anaerobic membrane bioreactor is pretty well advanced and in large-scale testing now. It should be ready to go full scale soon. The phosphorus- and nitrogen-recovery processes are commercially available for other applications, but need to be optimized and tested for nitrogen and phosphorus recovery from anaerobically treated effluent. The microbial electrochemical cells are at the pilot stage now and need significant development.

The most important steps are less technical and more economic and policy oriented. First, municipalities need to realize that they can dramatically reduce their costs of treatment and make their operations much more sustainable through these methods. They have to get out of the “business as usual” mindset. Second, society has to embrace using resources that are recovered from “used water.” They have to see that the economic and sustainability benefits are huge, and they have to break down regulatory and other barriers to using recovered materials. Third, we need markets for most of the outputs. While energy can be used internally to run the facility, the good outcome of being an energy exporter requires that the exports be valued in the market. Markets now are poorly developed or non-existent.

Q: Why is government involvement in this effort essential?  

A: We need the government to support research, development and large-scale testing. These are the essential risk-reduction steps to spur implementation of “disruptive technologies.” Also, government often will need to create policies that encourage recovery instead of suppressing it. These can include eliminating regulations that ban or disfavor recovered materials, as well as actively promoting early adoption, like with tax credits or even grants (or low-interest loans) to communities to install recovery technology.

Q: Even with solid R&D on these technologies, what about public acceptance? Is this a formidable barrier that needs to be overcome?  

A: The public has little idea what goes on with what they flush down the drain. They also do not know the costs of conventional treatment, economically or environmentally. I think that the public needs to learn that a sizable municipal cost can be eliminated by recovering these resources. They will be pleased to pay lower user fees because of it, and most will be very pleased to know that they are making their town more environmentally sustainable.

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