Nitrous Oxide Monitoring in a Typical Nitrifying Activated Sludge Process
Spernal Sewage Treatment Plant
N2O (nitrous oxide) emitted from wastewater treatment facilities is considered a problem due to its environmental impact. N2O is a potent greenhouse gas (GHG), with a global warming potential that is roughly 300 times higher than that of carbon dioxide. Wastewater treatment facilities are a significant source of N2O emissions, as the biological wastewater treatment processes create conditions that are conducive to N2O production. Worldwide studies have shown that it can make up 50-90% of a single treatment plants’ GHG emissions. For this reason, reducing the N2O emissions at wastewater treatment facilities is a sustainability target of highest importance.
Severn Trent is the UK’s second biggest water company, serving 4.4 million homes and business customers in England and Wales. One of their plants is Spernal STP, a medium sized sewage treatment plant in Warwickshire. With a discharge consent on ammonia, and resulting focus on nitrification in nitrogen treatment, it is a typical site for the UK and many other parts of the world.
The treatment plant serves as Severn Trent’s “Resource Recovery and Innovation Centre”, where the company evaluates emerging technologies that are compatible with a low energy, circular economy approach. As such, it was Severn Trent’s first site to be equipped with dissolved nitrous oxide sensors. The goal is to gain a better understanding of the site’s scope 1 climate impact and the potential for reducing emission by means of process optimizations.
Spernal STP has a dry weather flow capacity of 27,760 m3/day and encompasses several parallel treatment trains with primary settlement, activated sludge tanks, followed by several filtration and settling steps. The mixed liquor flows to one of several activated sludge tanks. The tanks are divided into an anoxic zone followed by 3-pass aerated zones (Fig. 1), where most of the nitrification takes place.
Severn Trent implemented N2O monitoring in the aerated part of the sludge tanks. The sensors are maintained as part of the site’s regular measuring routine. The two N2O sensors are approx. 25% and 50% into the aerated zone (Fig. 1). This location is known to give a very representative value for the emissions of a whole aeration zone.
The data discussed in this paper is based on one month’s data collection and is shown in Figure 2. It can be concluded that N2O formation is mainly dependent on NH4 loading. During the first 2½ weeks, N2O concentrations were high in both passes, with the highest values in pass 2. This is likely due to an accumulation of nitrites, which coupled with local zones of low oxygen can trigger nitrifier denitrification to produce N2O.
After this period there is a significant drop in the mean ammonium inlet concentration. Subsequently, N2O concentrations in pass 1 are almost negligible, with lower N2O concentrations in pass 2 as well. With a lower load, the ammonium-based aeration controller safely moves the ammonium load toward the second pass. This results in a slower ammonium turnover and reduced nitrite accumulation. This combination leads to a lower N2O production. The load change is also reflected in the effluent ammonium concentrations (Fig. 2).
After analyzing the data, we identified two potential methods for reducing N2O emissions. The first is to improve load balancing where possible, while the second is to distribute NH4 turnover throughout the lane by using aeration cascading that relies on N2O sensor input.
Severn Trent have ambitious plans to be at the forefront of the UK Water Sector’s Net Zero initiative and the circular economy. This includes investments in energy savings and production as well as advanced nutrient recovery – all while reducing the carbon footprint of their operations. With the implementation of real-time N2O monitoring, Severn Trent is now gathering in-depth knowledge and understanding of their sites’ performance and can realize opportunities to reduce greenhouse gases while optimizing the use of energy and resources to the benefit of their customers and the environment.