Wastewater treatment chemistry helps manage greenhouse emissions
Wastewater treatment plants produce greenhouse gases. Such plants leak methane, which itself is a major greenhouse gas, plus nitrous oxide (N2O). Although the amounts of N2O are small, equivalent volumes have nearly 300 times the potential greenhouse effect of carbon dioxide. Under the Paris Agreement, such plants must reduce their emissions and become climate-neutral. This will involve cleaning up the N2O. The problem is that N2O production pathways are complex, and difficult to determine where sewage treatment processes are concerned. The underlying biology is not especially difficult given pure bacterial cultures in controlled laboratory settings. However, sewage treatment plants consist of a complex and changeable mix of many bacterial species having different metabolisms. Changing influent composition and the physical aeration of wastewater plants complicates matters further. Therefore, an understanding of N2O problem in laboratory conditions does not necessarily translate to the operation of a real plant.
N2O problem under real conditions
The EU-funded AMACONOE project examined the N2O problem of wastewater treatment plants, to help control emissions. The team compiled the world’s most comprehensive database of operational parameters from full-scale plants, and conducted advanced statistical and process-modelling analyses of the data. The unprecedented size and complexity of the database allowed analyses that had never been possible before. Evaluation of the model suggested design and operational improvements for wastewater plants. Researchers found that autotrophic ammonia-oxidising bacteria produce N2O during a plant’s aerobic (oxygenated) treatment phases, and that nitrate (NO3) is produced in the same phases due to nitrogen-oxidising bacteria as expected. More importantly, the team found that elevated nitrate concentrations at the start of the anoxic (low-oxygen) treatment phase were crucial to the elimination of N2O build-up in the liquid. “Here we hypothesised that heterotrophic organisms will prefer to use nitrate first,” explains Professor Gürkan Sin, project coordinator, “and only after that is finished will they use and eliminate liquid N2O. Hence, it is important to manage both aerobic and anoxic activities in the plant to have a balanced operation.”
Practical control measures
This means that wastewater plants must carefully control aeration regimes both with respect to duration and intensity. The project developed technologies to make this possible. The first was a method of managing aeration during the aerobic phase, to minimise the rate of N2O production, using just the right amount of aeration. Additionally, the team developed a way of controlling elimination of N2O during the anoxic phase, by controlling the external carbon source. In practice, this means intermittent addition of influent wastewater during anoxic phases. “Many plants already have a system known as supervisory control and data acquisition SCADA,” adds Sin. “On top of that, some plants may have a sophisticated control algorithm or technology. So our findings can be implemented, either from scratch or by revising existing control and automation procedures in the plants.” Thus, operators will be able to analyse their plants and choose the right strategy to limit N2O emissions. The outcome has been specific techniques for limiting the production of N2O during wastewater plant operation. Being able to control this will help mitigate climate change. This research was undertaken with the support of the Marie Skłodowska-Curie programme.
Keywords
AMACONOE, plant, N2O, wastewater, biology, sewage, greenhouse gases, nitrous oxide
