Can we recover valuable chemicals from sewage?
Will Tarpeh hopes to make the word “wastewater” obsolete.
An assistant professor of chemical engineering, with a courtesy appointment in civil and environmental engineering, Tarpeh is studying how to extract ammonia, sulfur and other chemicals from sewage. His immediate goal is to turn these waste chemicals into fertilizers, disinfectants and other products, while leaving the water safer and cleaner. We sat down with Tarpeh to ask how his current research could lead to industrial-scale processes that might one day even be used to perform chemical cleanups of polluted waterways.
What is your research vision?
We try to recover valuable chemical products from wastewater. We’re starting with raw sewage but we envision moving ahead to clean up polluted ponds and waterways. We are engineering new chemical processes to extract valuable chemicals like sulfur, nitrogen and phosphorous from water, and then use these to create valuable products like fertilizers, disinfectants and more. It’s recycling, but for water.
The ammonia in urine, for instance, can be used to make ammonium sulfate, a common fertilizer. We’re also looking into using ammonia extracted from wastewater to make disinfectants, such as for household cleaners. We’re also starting to look at being able to recover the sort of chemicals used in large-scale industrial processes, like hydrochloric acid, sodium hydroxide and sulfuric acid.
How did you get started on this path?
As an undergraduate at Stanford I majored in chemical engineering but I was also interested in African studies. I saw sanitation as a global challenge that we were nowhere close to solving. I started to think of how I, as an engineer, could contribute. So I looked for ways to re-imagine the toilet, not just as a structure for gathering waste that would have to be disposed of, but as a collection center for raw materials that we could reuse to produce valuable products.
How do you extract chemicals out of wastewater?
We do so in a couple ways. Let’s start with electrochemistry. We hook up batteries to electrodes and run electrical current through the wastewater to attract certain chemicals we want to retrieve. For instance, ammonia is positively charged. We can get ammonia to move through the water toward a negatively charged electrode. But if we just put the electrodes into the wastewater, the negative pole would attract all the positively charged chemicals — ammonia, sodium, potassium and others. So we protect the electrode with a series of membranes that, in this case, only allow ammonia to filter through. But the chemical engineering gets complicated. We have to use more than one membrane to extract ammonia, which has this unique property that allows it to become a gas. So, we first convert the ammonia to a gas and then filter this gas through a second membrane. This enables us to recover 94% of the ammonia and very little of anything else. We are now fine-tuning the process. It works, and that’s great, but we want to know why it works and how we can make it better.
Is the process profitable?
Not yet. Electrochemistry requires energy and, so far, the cost involved is greater than the value of the product we’ve recovered. We’re looking at ways to bring down the cost of the process by using solar energy as a power source. Solar would also make electrochemical recovery available in places that are off the grid. Making our process less energy intensive could be important on a global scale. We’re trying to estimate how many gigatons of energy might be saved or how much greenhouse gas can be eliminated if these technologies became widely adopted.
We’re also looking at another process called ion exchange, which is well known in chemical engineering. We’ve tweaked it specifically for urine treatment. Our ion exchanger is profitable. In Kenya, we are able to produce ion-exchanged fertilizer at a lower cost than industrial fertilizers sell for.
We’re moving forward with both approaches because they might make sense for different contexts. We think electrochemistry might be more scalable, but we have yet to show that with conclusive evidence. So far, we know that we can scale ion exchange, and that it’s easier to incorporate into existing infrastructure because it’s an established chemical engineering process.
Would each chemical you want to extract require its own process?
We hope to one day develop our electrochemical approach to extract different chemicals one by one in assembly-line fashion. We are already working on a system that recovers sulfur and then immediately afterward, nitrogen. Our big dream is to engineer several electrochemical extractions to occur one after another, in a low-energy, low-cost process that could occur in small-scale, decentralized settings. You wouldn’t have to have a million gallon per day wastewater treatment plant. You could do it at a neighborhood or even a building level.
By removing these chemicals, are you purifying the water in the process?
We are helping. But other steps are required to purify wastewater. What we help do is offset the cost of treatment. We think of wastewater as a series of raw materials including water. We can get out lots of valuable chemicals and then let other treatment processes finish turning the sewage into water that we could use to address water scarcity issues.