Water & Wastewater Treatment

18 | june 2014 | WWT | www.wwtonline.co.uk Project focus T he Viikinmäki wastewater treatment plant in Helsinki, Finland is extraordinary for many reasons. Firstly, like a number of plants in Nordic countries, this municipal facility is underground, offering significant advantages such as protection from extreme temperatures and the provision of stable conditions for process control and odour management. Secondly, and arguably more im- portantly, the plant's team is pushing the boundaries of modern thinking on the use of gas emissions data to improve wastewater process control. The de-nitrification of wastewater is common industry practice due to health and environmental issues. At Viikinmäki, the treatment process is facilitated by the activated sludge method, encompassing a trio of treat- ment stages: mechanical, biological and chemical. The plant also uses a biological filter deploying de-nitrifica- tion bacteria that enhances conven- tional nitrogen removal. However, few wastewater treatment plants will admit little more than a passing interest in what happens to gaseous emissions once they leave the liquid phase and escape into the atmosphere. A school of thought exists that suggests this is a widespread industry failing. Why so? Well, the wastewater industry goes to great lengths to remove NO3 nitrate, typically by ● new dawn for gas monitoring ● Finnish plant leads way ● extensive research in progress cycling an aeration system on and off. The cost of the electricity required to facilitate this process is in many instances thought to represent two- thirds of total energy consumption at a typical wastewater treatment plant (although the use of biogas from sludge digestion at Viikinmäki means the facility is 70% self-sufficient in terms of electricity). In an era dominated by process optimisation and the need STEED WEBZELL FReeLAnCe TeCHnOLOGY WRITeR Viikinmäki wastewater treatment plant is the biggest in Finland, processing circa 270,000 m³ of wastewater a day. It is proving a beacon of excellence in monitoring GHG emissions to help Helsinki tackle climate change. ● Monitoring dissolved nitrate before and a er treatment is an obvious check in wastewater process optimisation, but nitrogen leaving the liquid phase as N 2 O (and possibly NH 3 ) is o en ignored. The idea of monitoring N 2 O in the gas phase is to be able to account for all streams of nitrogen coming in and going out of the process, both from an environmental perspective and one of process control. • Drivers Nitrous emissions monitoring Analysis could boost process control 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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