Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | june 2014 | 19 for energy efficiency and carbon reduction, de-nitrification hurts. Why then, once all that effort and cost has been expended, are plants happy to simply let resulting gas emissions escape into the atmosphere? Aer all, according to a UN IPCC assessment report in 2007, a gas such as nitrous oxide (N2O) is a powerful greenhouse gas (GHG) which has been calculated to have 298 times the global warming potential of CO2 over a 100-year period. This somewhat offsets the good work of treatment plants worldwide to cut their carbon footprint and contrasts enormously with the regulatory monitoring of water quality emissions from such facilities. Mari Heinonen, the process manager at the Viikinmäki facility run by the Helsinki Region Environmental Services Authority (HSY), is a forward thinker on this subject, suggesting that it may soon be possible to use gas monitoring data, not only to see how much GHG is being produced, but with a view to improving process control. "Conventional monitoring and control systems centre on the accumulation of oxygen, nitrate and ammonia in the water," she says. "However, if high N2O gas levels are discovered for instance, this could be exploited to highlight a specific process issue." The monitoring and analysis of data for gaseous compounds such as N2O, NH3 and NOX should therefore be seen more widely as a complement to water analysis as it can clearly deliver a more holistic perspective of the entire cycle of nitrogen within wastewater treatment plants. "If the gaseous emissions are not very good it's self-evident that the process is likely not in balance," says Heinonen. "It's unclear yet what is going on or how we can help the process. There are several factors and stages happening at the same time, such as alkalinity and the amount of carbon present, and how these work together and interact." Heinonen and her team are actively engaged in this interesting area of research, and it forms part of the measurement data set currently under development at Viikinmäki. Aer all, being underground and utilising a single ventilation stack system presents a rather obvious advantage in terms of straightforward monitoring, although it could be argued that over-ground plants with covered basins could also implement similar research programmes. "Patterns of peaks and/or other Being underground and utilising a single ven- tilation stack system presents a rather obvi- ous advantage in terms of straightforward monitoring, although it could be argued that over-ground plants with covered basins could also implement similar research programmes. • Innovations ● The Gasmet CEMS uses an FTIR spectrometer to gather infrared spectra from waste gas streams firstly by obtaining an 'interferogram' of the sample signal via an interferometer. This evaluates all infrared frequencies at the same time to generate a spectrum that allows qualitative and quantitative data to be determined. The CEMS at Viikinmäki can display emissions data for CH 4 , N 2 O, CO 2 , NO, NO 2 , and NH 3 , for example. ● Gasmet's library of FTIR reference spectra extends to the simultaneous quantification of 50 gases or recognition of unknowns from more than 5000 gases. With this in mind, end users can reanalyse produced spectra via the instrument's Clacmet PC-based so ware to identify unknown gases – a significant benefit of FTIR ● The CEMS was supplied as a turnkey solution including an automatic sampling station, industrial computer, probes and gas analyser. Gasmet also delivered a Profibus- to-SCADA interface so that the Viikinmäki plant could take advantage of real time data. • Challenges ● Viikinmäki wanted to measure the impact of process control on the level of common GHG emissions such as carbon dioxide, methane and nitrous oxide. ● According to the plant at Viikinmäki, the gas emissions data from the initial portable FTIR analyser were very interesting but not representative of the annual emissions, and posed more questions than they answered.

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