On monitoring lattergas

The 2019 Refinement to the 2006 Guidelines for National GHG Inventories makes the importance of Tier 3 facility level monitoring clear as best practice for countries with good data and advanced methodologies. We may debate whether our countries have either good data or advanced methodologies in wastewater treatment but if not us, in privileged high income countries then who... surely, we need the ambition! Besides, without facility level monitoring we cannot understand or mitigate emissions.

However there is no clear guidance for what to do next if you have the means to undertake Tier 3 facility level monitoring (which, to reiterate, we must find the means to do if we're serious about a safe future for humanity).

We just had a great IWA webinar on N2O and Danish experience. Check back in for the recording soon. This picked up on how to monitor N2O, drawing on decades of Danish sector experience, highlighting the benefits of regulator support and funding, consistency and common understanding around monitoring approaches and emissions reporting. Within the IWA Climate Smart Utilities GHG Monitoring subgroup we're writing a practical guide for N2O monitoring for utilities just now. This will draw on the IWA Quantification and Modelling of Fugitive Greenhouse Gas Emissions from Urban Water Systems publication last year (see Masterclass webinars on this too - search 'Masterclass') but will be more of a practical guide for utilities - for those just starting and those who have some years under their belt - with real experts in the field contributing to this. We're aiming to publish this by the end of the year... all help appreciated if you want to join the working group!

The topic of monitoring N2O is really gaining attention just now (sadly, not enough in the realms of the UWWTD recasting). Lately, I've been asked a lot about monitoring for N2O and specifically about hoods and off-gas monitoring systems. This is, I think, because there's plenty on liquid phase monitoring and there's not much out there on hood based off-gas monitoring systems (in particular their operation and practicalities) if you're a utility or practitioner. It's a bit of a vacuum!

So here, I'm sharing I've found in my recent literature and geographical travels on the topic. I'm not an expert - but some info I found useful and reflections.

In my mind, there are no prizes for mitigating process emissions first - the only way to win is to do it at pace together! The more we share the better -please add your own thoughts and links to info you've found useful or correct me where I'm wrong; we are all trying to be experts.

Starting simply - liquid phase N2O monitoring

The recent webinar provided a great summary of the wealth of Danish sector work around liquid phase N2O monitoring. And indeed, almost all of the nitrous oxide monitoring at practical utility level and at sector scale to date has been with liquid phase monitoring. This is for good reason - it is a cost effective, relatively simple solution which can be deployed fairly quickly and easily.

Using the liquid N2O concentration, temperature and gas flow along with reactor system geometry, this allows us to calculate an emission derived based on an empirical model from work by Foley et al (2010) at the University of Queensland and application of Henry's law. (William Henry went to Edinburgh University as it turns out and, would you know, did his dissertation on uric acid - which may be elevated in our bodies when we consume excess protein; the N in protein being the bit that produces N2O. We all eat too much protein, by the way, which could reduce N2O emissions ... another controversial, health, agriculture and environmental water quality-related topic for another day).

So - we can estimate emissions from aerated and non-aerated zones. For sites with surface or coarse bubble aeration this estimation is more challenging but possible.

The folks at Unisense Environment who invented the monitoring technology have a great repository of information online here including invaluable English language technotes around sensor placement (here and here), monitoring, the Danish national monitoring programme and, importantly, successful mitigation led by Utilities. This truly allows info to be shared with the world because most of the global progress in monitoring and mitigation at utility level is in Denmark and most of the information remains in Danish!

In addition to the practical info out there, having first hand experience with the widely used liquid phase N2O sensors has been invaluable personally - there's plenty to learn and a huge thanks to Mikkel, Bastian, Pernille and the team at Unisense who are always there to support. Also kudos to Alec Kimble and the Bi-Zen team who do great flow and loads work and optimisation studies and who have been willing to embark on liquid phase N2O monitoring and N2O InSites, learning together - more on this another time!

So - for liquid phase monitoring there is good practical info available if you're a utility or in industry. These systems typically provide you with two sensor bodies and heads per controller and should be mounted, like any other sensor, for ease of access and maintenance. Great manuals and video guides here, though still lots of learning to share.

What's under the gas hood?

I think the questions on off-gas hood based systems come - and also the evidence we see from the trials and tribulations of utilities trialling hood based systems - because there are very few suppliers of integrated hood and monitoring systems designed for wastewater WRRF N2O monitoring. And, by and large, these have remained academically supported and geographically focused within the reach of those undertaking the research - likely given the effort and cost required to design, manufacture, install, improve and maintain the systems.

Whilst suffering a lack of utility application, it is very true that process unit level off gas monitoring (hood systems plus on or off line analysis) has been the foundation of the majority of academic studies to date on N2O. Key work here from circa 2009 includes that from Kartik Chandran whose US EPA Reviewed and endorsed protocol for an N2O measuring system was used in early US work. It will be great to hear from Kartik in our upcoming WEFTEC N2O workshop - and I hope to see the US making headwinds in this air space (!) again! The protocol (which includes both hood and also liquid phase measurement) is often referred to in off-gas monitoring work for N2O and it has been further adapted - again largely in academic studies, globally.

It's worth noting that off gas monitoring of N2O from enclosed plant ventilation systems has been undertaken by a number of utilities for many years - in particular by Sweden and Finland. There are numerous academic and research institute studies, and plenty of great collaboration with industry and good practical examples - including comparison with liquid phase measurement - here from Baresel et al. (2016). You can also see Christian present on 10 years of quantification in this Unisense Environment webinar which also covers some great monitoring and mitigation work from the Nordics.

Back to the process unit off gas monitoring - e.g. hoods not ventilation shafts. There is much to read on the 2009 US EPA protocol and all the published academic variations on this since but what I've been interested in is the practical application of hood systems such that they can support N2O monitoring and mitigation progress now at typical utilities with the typical challenges at WRRFs and typical supply chain resources/constraints.

In theory, as the folks at Unisense Environment I think will agree, accurate off gas monitoring is a gold standard when it comes to N2O. It directly measures what you're trying to measure (N2O emitted) and has co-benefits like insights into system oxygen mass transfer. However in practice, off gas systems are not widely used beyond academia and I'd heard about their challenges - costs, challenges of keeping the system free of water vapour, different hood designs, estimating air flows, which analysers, downtime, cost and resource requirements.

To learn a bit more first-hand, and understand why to date practical application for permanent or semi-permanent use by water utilities was so limited, I went to 2 full scale installations with hood based monitoring.

In August 2022 and thanks to our ongoing collaboration with the University of Queensland, my excellent Jacobs Melbourne colleague Aprilia who designed the plant, and Melbourne Water, I visited the great 160S plant in Victoria, Australia. Then in May 2023, after reaching out to the Eawag team having read their great work developing new national EFs for Switzerland, thanks to a link through the N2O buffs at Brunel, I had the pleasure of a visit to the Eawag urine diverting toilets and one of their utility hood-based off-gas monitoring installations, ARA Altst?tten.

Both the Aussie and Swiss hood based off-gas N2O monitoring systems were developed through multiple academic studies and PhDs. And versions of the system designs are available open source. This is positive! The UQ design implemented is based on the system described in this published case study (one of the first full scale mitigation case studies published, from the great team including Haoran Duan and Liu Ye and others at UQ as well as South Australian Utility SA Water). This hood based off-gas monitoring system has been rolled out across multiple Australian utilities with ongoing support from UQ and local supply chain manufacture and maintenance.

The Eawag system I visited at Altst?tten is similar to that described here https://opendata.eawag.ch/dataset/off-gas-monitorng-system-for-wwtp and is being used by Eawag and also the great spun out team at Upwater to monitor and mitigate at scale in Switzerland.

Like the 2009 US EPA protocol, both of these off gas measurement systems have evolved from earlier versions, with continual refinement, and are used in conjunction with analysis and data insights from subject matter experts. Whilst a single hood and analyser set up might be simple; multiple hoods and an integrated monitoring system for multiple gases designed for permanent install is not - there's a reason these have evolved through many PhDs!

The other hood based system for which there are some good available design details is that which colleagues at RHDHV have been using and which I haven't yet visited was designed by the folks at TU Delft - you can read about it in this great publication from Edward van Dijk. This system is applied as a single hood, multi-gas, system for (granular) SBR systems - compared with the cycling multi-hood based measurement systems I visited for continuous plug-flow reactors. Whilst SBRs have more homogeneity than plug flow, it is recognised that spatial variation in SBRs is relevant too (Duan et al., (2020) and UQ and SA Water team point out here - noting variation is likely due to substrate gradients from feeding and non-ideal mixing).

So having shared the above open source info around hood systems that I'm aware of (please add yours!), some of my takeaways from actually seeing the two systems in action are below:

Hood mobility and operability

The need for hoods to float (!) and be moved/tethered is key - how can they be installed, lifted, secured and relocated? Can it be done by hand or does it require mechanical plant - e.g. scissor lifts - this will also depend on reactor construction, design and elevation.

The layout of reactors at the Swiss site and hood design meant they could be lifted by hand (see below); in Oz the system required plant to be brought to site to move the hood(s) between zones but they could be moved spatially within an existing zone to some degree.

How many hoods and where

How many hoods and where they go is an obvious question (for any type of monitoring). The Swiss system can go up to 14 in a single analyser setup - or anything below this; the site we went to had one tethered per zone in a 3 zone lane. The Oz set up also had multiple fixed tethered hoods and one which could be moved spatially within a zone. Both installs were for long term (12months +) monitoring - they want to capture first hand seasonal variation and also trial mitigation at these sites.

Of course, as some have trialled, you could just take a single hood and use it for oxygen and/or N2O characterisation in different locations or as a cross check of your liquid phase monitoring, noting challenges around getting an accurate measure of air flow discussed subsequently (particularly if you're moving the hood around I'd say but I know there's more to learn here).

The installs I visited were for long term monitoring. You could consider more temporary hood (or temporary liquid phase N2O monitoring) installs too. This is counter to the science base that confirms the importance of long term monitoring but there could be good reasons for this. There's unanswered questions - can we afford monitoring absolutely everywhere, how do we prioritise, how can we mitigate in some demonstrable way at pace...? Do we want to monitor to estimate accurately the facility EF and/or to work out how to mitigate and implement (and sustain) the right actions for mitigation? The why is important.

For liquid phase monitoring, measuring in anoxic zones is not a problem and assumptions about mass transfer from liquid surface from dissolved N2O can be calculated.

For hood systems this is trickier and the folks at Eawag used a little fish tank aerator below their anoxic zone hood to get some air flow (image of this almost above). Alternatives for anoxic zone monitoring as described in the US EPA protocol include hood designs with provision for sweep and tracer gas - more complexity, not being used by utilities in practice.

Whilst we know N2O emissions will happen from aerobic zone stripping, anoxic zones are interesting as these can both produce and reduce N2O and whilst Danish work has shown low concentrations relative to aerobic zones, this may not always be the case (particularly if they swing!). This isn't to say we should definitely be measuring N2O in anoxic zones - there will always be constraints and our job is to help inform decision making within these using best science!

In terms of location of hoods, there's good work here by work from Wenzel Gruber and the Eawag and ETH Zürich teams here. This also highlights the importance of long term monitoring and within site variability - as we know - and which is also very well described in recent liquid phase monitoring publications from the Danish and Finnish sector work (the linked topic of reject water an important one for another post!). This point may be another reason to make some N2O systems very or somewhat mobile and moveable for large sites with many lanes. To explore variation down the lane (particularly in our UK highly tapered plug flow systems) ... maybe they will even fly!

Air flow is still critical for estimating off gas emissions

Like liquid phase N2O monitoring, where we're relying on system air flow data (at best per zone which assumes uniform per diffuser air flows within that zone), hood systems also require this info. Both hood systems I saw had hood based air flow measurement - though these come with challenges and it seems that whilst hood air flow can be successfully monitored directly on the hood itself, there remain challenges in installing reliable instruments, having certainty of their accuracy and in keeping them functional in a fairly adverse working environment, at reasonable cost.

Not something we discussed at the two sites but from other studies and speaking with utilities trying to compare liquid and gas phase methods, hoods can move around, making hood air flows hard to calibrate and intended comparisons between gas and liquid phase methods challenging. Morten from Aarhus Vand alluded to this in the recent webinar - they got there in the end but accurate estimation of the air flow measurement on the off-gas hood and aligning this with the aeration system measured air flows (being used for liquid phase calculation) took a long time. The end result was 10-15% difference in gas hood and liquid phase (+ emission calculation) which is positive. What is sufficient and how much checking is necessary? Certainly air flow has a massive impact on N2O mass emissions and the more tapered your system the trickier. And often we may not have individual zone air flows. So perhaps hoping for well installed and aiming for calibrated thermal mass flow meters is useful here... checking you have this can be a pain but is good practice for any kind of N2O monitoring as they can be inaccurate and this can mean significantly under estimated mass emissions potentially. The knowledgeable UU Senior OT Engineer Danny says of thermal mass meters - It's very easy with this equipment to produce >20% errors with poor installation. An unhelpful side effect :). This is still going to be better than the inaccuracy around using standardised (not actual) N2O EFs to estimate emissions... but being aware so we don't get carried away with our significant figures seems prudent.

To reiterate - the accuracy of air flow assumptions to be combined with gas phase concentrations for hood based off-gas analysis is as important as it is for liquid phase measurement - so if you end up estimating it from process air manifold or zone flow measurement, perhaps also using control valve opening data and characteristics, then take care to understand the potentially issues around accuracy in your calculations. But if we know direction of travel to mitigate emissions then surely we shouldn't get too caught up here; particularly set in context of the dire statistics around the impacts of climate crisis. Our kids are surely going to wish we just reduced N2O emissions and addressed the systemic associated issues (N cycle) - not that we spent ages monitoring without mitigating because we didn't quite know enough yet or couldn't quite get the accuracy sufficient for updating our benchmarking carbon accounting workbooks.

How much hood?

Hoods come in different shapes and sizes. The floating type are best, as large as practicable given the reality that FBDA systems are anything but uniform (of course spatial irregularities impact on any kind of N2O measurement so liquid phase also). Hood size is important - though trade-off in design; low profile systems with minimal headspace, mitigating against issues from foam and in stability seem to be key considerations. Given how dynamic the system we are considering is, the time delays between sample capture and sample analysis and any delays with other data (e.g. air flow) we're trying to align with is important to understand. The time to cycle between analysis gases and hood locations for the single gas analyser in hood and off-gas systems is going to be greater than the continual liquid phase measurement from liquid phase systems and in both cases, you may be aggregating with a measured gas flow to get a mass emission - but all of this needs to be considered in context of what is observed in dynamic changes in N2O. We're not after perfection, just climate action!

Gas sample lines are small and can freeze, also need to avoid blockages and moisture

It is important that gas analysis takes place without moisture interference in the small diameter tubing. This can be through heating, cooling, moisture traps, maintaining continuous flows. Whilst small diameter tubing can be run long distances, this may increase risks of freezing which need to be mitigated.

Gas analysers and calibration

There are many gas analysers - these range in price, availability and calibration requirements. Analysers require calibration with multiple gases but this can be programmed to be done remotely or with regular site visit (not so regular as required 2 monthly liquid phase instrument calibration though).

Experience in operation and maintenance

Having local support for operation and maintenance of off-gas systems seemed important for both installations - much can be done remotely in terms of monitoring after install but local hands-on experience is critical when issues arise. Both systems were maintained with support from the parties who built the units. Remote monitoring of systems to pick up on issues seems important for any system (gas or liquid phase) though the same could perhaps be said for any of our online instrumentation - our industry will need a step change to pay better attention to process health in order for us to do our best on N2O emissions I think.

Remaining uncertainties

Accuracy of the system (including all components) is important for hood-based off gas systems - not just analyser accuracy. If you can be happy with your gas flow, you will be getting a direct measurement from a hood based system. The evidence suggests that we can also achieve accuracy without the complexity by using liquid phase monitoring (and same reliance on gas flow). If you want to understand production mechanisms in your anoxic zones, this is easier with liquid phase, harder with hoods.

In addition to the air flow assumptions discussed above, for any kind of aerobic phase N2O monitoring, the assumptions about how many bubbles from how much diffuser surface area are arriving under your hood (or to your zone for your liquid phase monitoring) remains another challenging question.

I think what is key here is to know the limitations of our assumptions and do what we can to check the accuracy and quality of data we are relying upon. There will be plenty more academic research to come but based on those who have been at it for many years I think it's fair to say we know enough for practical monitoring and mitigation now. There will continue to be assumptions around air flows as mass transfer, around similarities within and between lanes, even for permanent monitoring. Perhaps a combination of permanent/semi-permanent and temporary liquid phase and hood-based off-gas monitoring could be used intelligently to support the data we need for improved reporting and mitigation and to also challenge some of our basic asset specifications and design and refurb standards certainly here in the UK - including potentially many aspects of civils, process and MEICA design.

In conclusion

This summary isn't about whether hood systems or liquid phase are better - they likely both have their place depending upon utility constraints - cost, expertise, local support, project and programme ambition.

To date liquid phase systems have certainly been more obtainable, affordable and practical and are what are being used in the most widespread monitoring and mitigation globally. As noted to start, there's much practical info on liquid phase systems in the useful literature from Unisense Environment, and some also from the industry national monitoring programme within reports published by the Danish EPA (MUPD website - search lattergas and be prepared to machine translate!).

Seeing first-hand two hood based off-gas monitoring systems for continuous multi-point monitoring by utilities really helped put these systems in context - and highlighted challenges and opportunities.

There remains lots to learn for us all but we know enough to take action now. Building on decades of research, we now see monitoring of N2O mainstreaming at least for progressive utilities and cities (who have the means to do this) which is great. It's nice to have so much info available - some bits harder to find than others. Sadly, the regulation on these emissions is missing in action, despite that no other sector will decarbonise for us. But I reckon our job is to collaborate for change - showing what is achievable if we have support and incentives, carrots and sticks to monitor and mitigate.

Get involved!

As an aside, for those US and Canada based, there are now liquid phase systems going in which is exciting (in no small part in Canada due to my own super Jacobs colleagues) and if you're at WEFTEC you can meet with the Unisense Environment team - it's a bit bittersweet that the topic is gaining enough traction across the pond that we're expending precious carbon miles to attend from Europe and further afield...

There is also the 2nd ever WEFTEC pure N2O technical session Tuesday 3:30-5pm - hosted by Julian Sandino and Jose Porro and with some great presentations. If you're caught up in the wave of carbon diverting nutrient removal in the US, please come and reflect on the very significant N2O impacts of this - our shortcuts need to be ones that actually safeguard a future.

Sharing learnings, literature and importantly practical experience from measurement and mitigation is so important. It is for this reason that Ellen from RHDHV and I come together to help co-founded the UK and Ireland Community of Practice for Process Emissions (open to UK and Irish utilities) - based on Ellen's great experience with the Dutch CoP for N2O. This idea of shared learning is also why I am involved in the IWA GHG monitoring sub-group within the Climate Smart Utilities initiative (open to all).

If you have something to share on any aspect of N2O, or someone shares something useful with you - please consider joining a group or finding a forum to share it more widely!

Together we can become the experts needed and our children will surely thank us for it as they become experts at dinosaurs and so much more.