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Tackling wastewater emissions – More than just a smelly problem

Lesley Smith outlines Water New Zealand’s submission to the Productivity Commission on managing wastewater treatment plant emissions.

Lesley Smith

Wastewater treatment plant operators are used to thinking about the air emissions around wastewater treatment facilities. Odour from a wastewater treatment plant can be one of the surest bets for annoying your neighbours. However, a less commonly considered impact from the gases coming off our wastewater treatment plants is the one they have on climate change.

In greenhouse gas accounting parlance these are commonly referred to as “fugitive emissions”. Wastewater fugitive emissions are principally composed of methane and nitrous oxide, both potent greenhouse gases. Methane has approximately 25 times the greenhouse gas warming potential of carbon dioxide and around 298 times the potential of nitrous oxide.

The impact of our wastewater treatment plants became the subject of some frenzied research around the Water New Zealand office when the Productivity Commission, looking into transitioning New Zealand to a low emissions economy, recently posed the question; should wastewater treatment plants be included in the Emissions Trading Scheme?

The response from our members was mixed. Many pointed out that as the water sector sits on the front line of climate change impacts, all measures (including inclusion in the emissions trading scheme) should be on the table.

Others were more wary, asking; do we even know enough about our wastewater emissions for these to be included in the emissions trading scheme? What opportunities would we have to reduce emissions anyway?

It turns out the answers to these questions are; not really and heaps, respectively.

At an aggregate level, we have some informed guesses about roughly the size of our emissions. At an individual plant level, however, the picture becomes a little more woolly.

Take a quick browse through local authority greenhouse gas inventories and you might notice a stark absence of information on wastewater treatment plant emissions. To anybody who has compiled a greenhouse gas inventory this will probably come as no surprise.

Nowhere is New Zealand-specific guidance material on how to determine wastewater fugitive emissions published. For authorities using land treatment systems, a style rarely used outside of New Zealand, little exists in the international literature either.

However, not all authorities have let the absence of local guidance stop them from attempting to measure emissions. The Intergovernmental Panel on Climate Change document, Guidelines for National Greenhouse Gas Inventories 2006 contains methods for determining wastewater treatment plant emissions. With a few exceptions these mostly work in New Zealand as well.

Our country’s biggest wastewater treatment plant operator, Watercare, runs a highly robust emissions accounting framework. Emissions sources as diverse as embodied carbon in lime, biosolids, sludge dewatering and septic tanks are all accounted for on an annual basis, providing a potential example for other authorities who wish to get a better grip on their own wastewater emissions.

Water New Zealand has suggested that an important first step towards tackling wastewater treatment plant emissions is to get a better handle on our emissions and greenhouse gas methodologies for some of New Zealand’s particular treatment processes (especially land application).

For this to happen we can’t rely on local councils alone. Understanding carbon and nitrogen cycles is a difficult task, one where detailed expertise sits with central government agencies and academia. The mantra that you can’t manage what you can’t measure springs to mind.

If you have made it this far through the story – “you’re suggesting accounting?” – I hear you yawn. Accounting is merely the boring key to understanding and unlocking the many exciting emissions reduction opportunities that exist at wastewater treatment plants.

Our submission broadly suggests four categories of opportunities; energy recovery, energy efficiency, onsite effluent emissions labelling, and possibly nitrous oxide reduction through recovery systems. (See the breakout box for some of the more universally-applicable possibilities.)

Circling back to where we began, would including wastewater treatment plants in the emissions trading scheme bring about such changes? And at what cost?

Wastewater treatment plants have the important purpose of protecting public health and the environment, and accordingly treating effluent needs to remain the focus of their operation.

There are many trade-offs between energy, carbon and effluent quality. For example, energy UV disinfection systems, or aerated lagoons, improve effluent quality but come with a high energy penalty. Any moves that force trade-offs with public health in the name of emissions reductions should raise eyebrows.

Add to this, the existing pressures on already stretched local council resources and staff time and there is a question about whether a price signal alone is enough to create change in local authorities. The risk is that another line item is simply passed on through the rates bill to local communities.

• We’d love to hear from you if you have any further thoughts about what the sector should be doing to manage wastewater treatment plant emissions. Water New Zealand’s full submission to the Productivity Commission is available on both the Water New Zealand and the Productivity Commission’s websites.

• Lesley Smith is technical co-ordinator at Water New Zealand. lesley.smith@waternz.org.nz

This article was first published in the July 2018 issue of NZ Local Government Magazine.

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