The 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 importantly, 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 treatment stages: mechanical, biological and chemical. The plant also uses a biological filter deploying de-nitrification bacteria that enhances conventional 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 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 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? After 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.
After 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 issues may indicate concerns that could help bring about better process control, but more scientific research is required that deliberates the destiny of nitrogenous compounds beyond simply those contained in the wastewater,” states Heinonen. “Yes, the extraction of nitrogen from wastewater is an important goal, but if the outcome is high N2O emissions, the process may require managing in a different way.”
For now, the Viikinmäki wastewater treatment plant, which is the biggest in Finland processing circa 270,000 m³ of wastewater a day, is proving a beacon of excellence in monitoring GHG emissions to help Helsinki tackle climate change.
Monitoring has been in place at the facility since 2007 following the implementation of the European Regulation on Pollutant Release and Transfer Register (E-PRTR). Initially, Viikinmäki modelled its yearly gas emissions by analysing grab samples.
However, the facility took a step forward by hiring a portable multi-gas FTIR (Fourier Transform InfraRed) from Gasmet to assess emissions in more detail for research purposes.
The results proved interesting enough for the plant to purchase a continuous emissions monitoring system (CEMS) from Gasmet in late 2012; technology that was selected due to its potential to monitor multiple gases simultaneously.
“The system has performed admirably with the need for hardly any maintenance,” says Heinonen. “Only a few minutes each day is required for zero point calibration with nitrogen. Furthermore, the process is fully automated. Water vapour calibration is performed once every year at the bare minimum, but in typical circumstances no other calibration is required.”
Leveraging the data collected since the CEMS was installed at the end of 2012, Heinonen has determined the yearly emissions for methane to be approximately 350 tonnes, and about 134 tonnes for nitrous oxide.
Therefore, the emissions per cubic metre of wastewater equate to 3.5 g and 1.34 g of methane and nitrous oxide respectively.
Precious little data has ever been published on the GHG emissions resulting from wastewater treatment. According to the plant at Viikinmäki, it is the only facility of its type in the world undertaking monitoring of this ilk. As a consequence, any results published are certain to be of huge industry interest. However, just imagine if this process could be put into reverse, using gaseous emissions data to control and refine the treatment process – the benefits to the wastewater sector would be little short of enormous.