Like many New Zealand organisations, councils and water utilities are at the start of their own journey to plan and budget for reducing their carbon footprint to meet the Zero Carbon Act target of net-zero greenhouse gas emissions by 2050. A carbon footprint is the greenhouse gas (GHG) emissions footprint expressed as carbon dioxide equivalent (tCO2-e).

In order to begin this journey, it is important to confidently understand the extent of these emissions by developing a carbon baseline that is used as the foundation for a carbon roadmap which can show what pathways are available to practically meet the target.  

Various international studies show direct (Scope 1) emissions – those emitted from owned or controlled assets – are released from various points of a wastewater treatment plant (WWTP). The Intergovernmental Panel on Climate Change (IPCC) has synthesised these studies, to develop a guideline for quantifying emissions from different types of WWTPs[1].

These direct emissions include two potent greenhouse gases – methane (CH4) and nitrous oxide (N2O) which are often released at very low concentrations. Over 100 years their respective global warming potential (the amount of warming per unit of emission) compared to carbon dioxide for CH4 and N2O is 25 times and 298 times respectively. As a result, even if small amounts of these gases are emitted from a WWTP, they can contribute significantly to a plant’s operational carbon footprint. To date, these direct CH4 and N2O emissions have been overlooked in carbon footprint assessments – it is time to draw back the curtains.

We choose to adopt the IPCC guideline to initially estimate direct emissions released from WWTPs. The guideline is an internationally verified methodology containing default emissions factors based on international studies where onsite testing of emissions factors is carried out.

As onsite testing is yet to be conducted at any NZ WWTP - we are forging new ground when it comes to applying the IPCC guidelines to the NZ context.

What's contributing to the carbon footprint of your WWTP?

Beca has been working with WWTP owners around the country to baseline two emissions types:

  • Operational carbon - emissions associated with the operations categorised into Scope 1 (direct CH4 and N2O, on-site energy use), Scope 2 (electricity) and Scope 3 emissions (e.g. biosolids management, chemical consumption)
  • Capital carbon - emissions associated with the creation of assets projects, those embodied in materials and used in construction processes (tCO2-e).

These baselines can be utilised as the foundation for a carbon roadmap, where the carbon and cost impact of the different options can be realised to help prioritise projects, programmes and make informed decisions, in pursuit of net-zero carbon by 2050.

Scope 1 Emissions

Operational carbon baseline assessments with Hamilton City Council (Pukete WWTP) and Queenstown Lakes District Council (Project Shotover WWTP) showed that the direct, CH4 and N2O emissions make up over 80% of each plant’s Scope 1 operational emissions. The remainder of Scope 1 emissions are typically associated with onsite energy use.

For both Pukete and Project Shotover, dissolved CH4 is likely generated in the sewer network and released at turbulent points in the plant and the nitrogen removing reactors are a source of N2O emissions. The residual carbon and nitrogen in the effluent are likely to generate CH4 and N2O emissions respectively, in their discharge environments[1].

Pukete’s solids stream process units, the largest contributor to direct emissions, are also the greatest opportunity for reduction. The council plans to improve the anaerobic digestion and biogas system to reduce their operational carbon footprint. The baselining process emphasised that this investment will be impactful for their journey to net zero carbon.

Project Shotover’s greatest opportunity for emission abatement sits with the facultative pond system, a key source of CH4 emissions. It is soon to be decommissioned as part of the Project Shotover WWTP Upgrade Stage 3, which would enable the WWTP to meet the 2030 Zero Carbon Bill target of 10%[2] reduction in biogenic CH4 emissions.

Figure 1 visually shows the probably sources of direct emissions across a typical wastewater treatment plant, summarising those described from the case studies above.  

Figure 1. Direct Sources (Scope 1) of Methane and Nitrous Oxide Emissions from a Wastewater Treatment Plant

These are just two of the WWTPs we have been working with from around New Zealand. The results so far show us that, even when accounting for uncertainty, direct CH4 and N2O are significant to a plant’s carbon footprint and cannot be overlooked.

Scope 3 Emissions

Carbon baseline assessments typically included Scope 3 emissions to help WWTP owners understand the upstream and downstream emissions associated with their plants.

Biosolids management emissions are a carbon hotspot if the biosolids of the WWTP are trucked and disposed of in landfill. Around 90% of the biosolids in NZ are sent to landfill – this carries a significant carbon impact for the industry but is likely to be the greatest reduction opportunity. Biosolids management strategies, such as beneficial re-use for landscaping, could be an opportunity to offset a WWTP’s carbon emissions.

Where does the Water Industry go next?

Case studies around the world present reduction opportunities, but without an accurate carbon baseline it is difficult for a WWTP to understand where the hotspots are and what capital investment to prioritise.

There is uncertainty with using default activity data and emissions factors in the IPCC guidelines. We employ the following options to decrease this uncertainty, and improve the accuracy of the carbon baseline assessments to support more robust decision making:

  1. if available, use WWTP specific activity data (measured flows and loads) in conjunction with default emissions factors
  2. use of emissions factor from a literature study of a similar plant where onsite testing has been conducted
  3. another avenue for sourcing emissions factors is from the latest wastewater modelling software versions
  4. on-site testing at every site in a range of operating conditions to develop unique emissions factors for each plant

Although all options can establish a strong starting point for emissions estimation, the last option will give a WWTP owner the most confidence in their baseline emissions.

Due to the low concentration levels of emissions, particularly for N2O, appropriate onsite testing equipment and associated logistics are difficult and expensive in New Zealand. Investment in the equipment at an industry level, could alleviate the cost constraints that each WWTP owner would otherwise incur.

If such investment was made, New Zealand specific benchmarks could be developed for WWTPs to measure their progress against.  

How to find out more information?

At the New Zealand Water Conference coming up in November 2020, Caroline Hope from Beca, Emily Sopers and Evan Vaughters and David Hight from Hamilton City Council presented on the options for estimating emissions and on-site emissions testing in New Zealand. With a bit of luck, in this post-COVID New Zealand, they will implement the testing before the conference and present results.

The Water New Zealand Climate Change Special Interest Group will be working hard in the short term to support WWTP owners with information and resources to better understand their emissions.

[1] IPCC. (2019). Wastewater Treatment and Discharge, Chapter 6, Volume 5, 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. USA: Intergovernmental Panel on Climate Change.

[2] NZ Government. (2019, November 13). Climate Change Response (Zero Carbon) Amendment Act 2019,. Retrieved from New Zealand Legislation:

[1] IPCC. (2019). Wastewater Treatment and Discharge, Chapter 6, Volume 5, 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. USA: Intergovernmental Panel on Climate Change.