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0.24 to over 2.5% as many plants are already energy optimised and produce biogas, and the latest national emission factor for Denmark is 0.84% N2


O-N/TNinlet with a spread between 0.24 –


1.24%” Dr Holmen Andersen expects that many UK WWTP will be similar.


“Using sensors to monitor nitrous oxide is key to understanding and controlling process emissions,” agrees Nick Blamire Brown, Technology Consultant at Isle Utilities. “Increasing emissions monitoring and modelling of any changes to processes allows operators to make informed adjustments to improve operational efficiencies”.


Figure 2: Water industry emissions from different activities (MtCO2 equivalent) (from the 2018-19 UK Carbon Accounting Workbook).


In terms of introducing mitigation meirisasures, it’s often easier to reduce emissions by improving existing technologies and systems e.g. efficient pump scheduling, than to change the whole process or technology used i.e. adding a new process to the treatment train says Mr Blamire Brown. But sensors play an important role in planning and evaluating the effectiveness of any interventions. For example, in Denmark, energy neutral WWTP (producing more energy than they consume) have typically saved 30% of energy through energy saving measures, and 70% of energy through real-time sensors, real-time controllability and advanced process controls. Isle Utilities has undertaken an evaluation of 183 of the available technologies to reduce carbon emissions in the water industry, focusing on process and fugitive emissions, a few of the shortlisted technologies are outlined below.


A game changing sensor developed in Denmark patented in 2012 is still the only nitrous oxide sensor in the world. Nitrous oxide makes up 8% of global GHG emissions and encompasses 26% of the emissions from the water sector and up to 90% of a single WWTP’s GHG emissions can be from wastewater treatment. Worryingly the N2


to increase due to increased consumption of farmed food. The deammonification process in particular produces nitrous oxide (Figure 3). Even a single N2


O emissions for the whole process, with calibration twice a month. “It is very important to measure N2 Figure 3 The production of nitrous oxide during wastewater treatment (© Unisense, 2021).


O content in wastewater is projected O sensor in one activated sludge tank


is enough for some online, continuous, real time measurement and control of N2


O


online and over a long time, because there will be fluctuations during each day, over seasons and even years, associated with temperature changes and process types,” reasons Dr Holmen Andersen, CTO at Unisense, “in my experience as much as 90% of N2


O will be produced over the spring and summer months, when there is a higher turnover of ammonia.”


One way to reduce nitrous oxide production is by diluting the ammonia present by allowing higher levels of activated sludge in the mixed liquor than standard practice. At Biofos, the largest utility in Denmark, this reduced emissions of nitrous oxide by


80% which equates to the mitigation of 16,200 tonnes of CO2 equivalent per year (presented at IWA2021 conference, VARGA project, https://projekt-varga.dk/en/front/). Another mitigation method which has been piloted is to replace alternating nitrification and denitrification cycles with simultaneous cycles (Figure 4). After allowing the plant to adapt to the new regime, a 26% reduction of nitrous oxide emissions was achieved. In addition, air flow was reduced by more than 35%, leading to cost savings. The dissolved oxygen (DO) setpoint was reduced from 1.5 to 0.3 mg/l, whilst still meeting effluent quality standards. To achieve this level of advanced process control you need high frequency N2


O monitoring data on a minute-by-minute basis. A modelling programme can be used to model the risk of N2


Figure 4: A pilot project tracking nitrous oxide emissions and dissolved oxygen showing a change from alternating nitrification/denitrification cycles to simultaneous nitrification/denitrification at a WWTP (© Unisense, 2021).


monitoring and controlling their emissions.


Many countries have already started decarbonising their electricity grid or have targets to do so. The proportion of power from renewable sources is increasing, so that over time the upstream, indirect CO2


emissions from power will decrease.


However, companies can be more proactive than just waiting for this to happen by generating their own renewable power for their operations on site e.g. by installing wind or solar power. Carbon dioxide emissions from grid electricity can be tackled by purchasing 100% renewable energy from a reputable, certified renewable energy provider which can provide Renewable Energy Certificates (RECs) for each MWh. It is generally agreed that Scope 1 process emissions are now the main emission challenge to water industry net zero targets.


So if process or direct emissions (Scope 1) remain the largest remaining challenge to water companies, which factors need to be considered? Wastewater treatment processes produce large amounts of methane (with a Global Warming Potential (GWP) ~25 times greater than CO2 of ~265 times that of CO2


1


and release of these gases is obviously a priority. Biomethane (‘biogas’) can be collected and sold for injection into the natural gas grid. But although biogas production might appear to be a sustainable option, it actually can lead to increased production of nitrous oxide (a more potent GHG than methane) due to the low carbon and high ammonium levels present in the wastewater from the biogas production, as pointed out in the 2019 IPCC GHG update. So you would need to offset the harvesting of biogas in order to become carbon neutral.


“Process emissions are not yet well understood - the general view is that the UK water industry Carbon Accounting Workbook is underestimating actual process emissions of nitrous oxide and methane by a factor of 10,” highlights Dr Steve Palmer, Stantec “so the proportion of Scope 1 emissions is likely to shift from the current estimated baseline.”


), but also nitrous oxide (with a GWP ) (Figure 2). Minimising the production


International emission factors for estimating nitrous oxide emissions per total nitrogen load entering a WWTP also need more research, according to Dr Holmen Andersen, “The updated IPCC emission factor is 1.6% N2


O-N/TNinlet but our monitoring at WWTP in Denmark has found emission factors varying from


However ‘energy neutrality’ does not take into account the carbon footprint, whereas a ‘carbon neutral’ plant will be carbon neutral for both energy and process emissions


O


emissions using historic SCADA data. Algorithms such as fuzzy logic can predict when emissions will occur in biological processes and which mitigation measures are needed to control them. Reductions of 40% and 70% in N2


O have been achieved in 2


WWTP respectively, by implementing simple adjustments to DO set point values and basic operating rules. In Land van Cuijk, Netherlands, a reduction of 90% in N2


O has been achieved using


this risk model to: improve nitrification (reduce ammonia peaks); improve denitrification (reduce nitrate); and improve biological phosphorus removal (from better DO control) (Figure 5). This also resulted in a net reduction in consumption of grid electricity.


Rather than first trying to get perfect monitoring in place, Matt Gordon Engineering Manager (Digital) at Suez says in his experience it is best to use existing monitoring and start modelling and making iterative improvements to performance e.g. to optimise pump operation. “Rather than measuring twice and cutting once as is traditional, my motto is that you can measure once and cut [waste] twice,” says Mr Gordon.


“It has helped us enormously to use software which pulls all assets and sensing data into dashboards, for live predictive demand analysis and early warning of a weather event on its way or keeping track of assets which have been taken out of commission.” Says Chris Ames, Network Analyst and Hydraulic Modelling team lead at Welsh Water. Welsh Water has adopted


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