Bacteria Electrify Sewage, No Methane Needed

by Emil Morhardt

Bacteria are good at getting energy out of sewage; that’s what wastewater treatment plants are mostly about…converting the organic carbon that we didn’t extract from the food during it’s passage through our guts into something that won’t pollute the water bodies we dump the treated wastewater into. In the closed anaerobic digester tanks you can see at any wastewater treatment plant the microorganisms are busy converting it into methane. Sometimes this methane gets used onsite to generate power, or is further processed and piped off as “biogas” for some other use, maybe even to power city buses. More often than not it is just released into the atmosphere where, although it can no longer pollute any water, it is a powerful greenhouse gas. What if we could skip the methane production step and just generate electricity directly from the sewage by sticking electrodes in it? Sounds impossible, but there is new science that is making it happen, at least at laboratory scale. Xie et al. at Stanford University have constructed what they call a microbial battery that makes just as much electricity out of a given amount of wastewater as you can get from first using the microorganisms to produce methane, then burning it…without the intervening gas handling and power plant, not to mention the likely leaks of methane to the atmosphere in the process. The secret is a solid-state cathode which makes the system act like a rechargeable battery, with exoelectrogens—microorganisms that oxidize the electron-donating chemicals in the sewage and transfer the electrons to the anode. The electrons then pass through an external circuit as an electrical current, on their way to the cathode. Voila! Electricity that can be used for anything you like.

Xie, X., Ye, M., Hsu, P.-C., Liu, N., Criddle, C.S., Cui, Y., 2013. Microbial battery for efficient energy recovery. Proceedings of the National Academy of Sciences 110, 15925-15930.

The Three Best Practices to Decrease N Losses in EU-27 Cause High Costs to Farmers, But Are Successful

Oenema et al. (2009) performed a modeling-based experiment to determine the environmental and economic costs of implementing the three most promising measures to abate N losses in agricultural production within the 27 Member Nations of the European Union (EU-27). The three general categories of measures include balanced fertilization (BF), low-protein animal feeding (LNF), and ammonia emissions abatement (AEA), with preferred order of implementation BF, LNF, and AEA. Both practices of BF and LNF were found to decrease N inputs as they simultaneously increased N output in useful products. AEA caused a decrease in ammonia emissions, but this was coupled with an increase in other types of N emissions, therefore not seen to be effective when used alone without other measures. All three measures were not without additional costs to the farmer in time and lost income, as well as to society in the form of higher priced goods.—Maria Harwood
Oenema, O., Witzke, H.P., Klimont, Z., Lesschen, J.P., Velthof, G.L., 2009. Integrated assessment of promising measures to decrease nitrogen losses from agriculture in EU-27. Agriculture, Ecosystems and Environment 133, 280–288.

To determine the environmental effects of the different measures, the MITERRA-EUROPE model was used, along with the CAPRI model to determine the economic costs (Oenema et al. 2009). These three measures were selected based upon their evaluated effectiveness to increase N use efficiency and decrease N losses through ammonia and nitrous oxide emissions to the atmosphere and N leaching into groundwater and surface waters. Nitrogen use efficiency (NUE) is defined as the N output in useful products (harvested crops, milk, meat) as a percentage of the total N inputs (N fertilizer). The three measures were evaluated against the N losses and NUE increases from “business as usual” in 2000 and 2020.
AEA was only considered in conjunction with the other measures because a decrease in ammonia emissions results in “pollution swapping” where N increases in other areas. All three measures were found to have varying degrees of effectiveness across EU-27 given the differences in the type of crop or animal production and soil types, although implementation always resulted in reduction of N losses, but not without costs to farmers and consumers in the form of lost profits and welfare. BF was determined to be the best measure to implement because of its low relative cost and large benefits in the form of decreased N losses, mostly in areas determined as nitrate vulnerable zones (NVZ). LNF had high costs associated with it because in the baseline measures there already was a relatively low amount of N in the animals excretion, therefore lower protein content feed is not a viable solution because the feed already contains close to the optimum amount. AEA also had high costs associated with implementation, but due to the phenomenon of pollution swapping, this measure could only be implemented when coupled with the other measures, further increasing the overall costs. These measures all have been proven to have demonstrable effects on lowering N losses to the environment, but not without decreases to farmers’ income as high as 25% of the total EU spending on agriculture.