Reducing greenhouse gas emissions to improve the long-term sustainability of wastewater treatment plants is of vital interest to operators of these facilities worldwide.

Large wastewater treatment plants (WWTPs) emit a significant quantity of greenhouse gases (GHG) and the global impact of climate change is forcing design decisions to be increasingly driven by considerations of these emissions and long-term sustainability.

Leading engineering, sciences and project delivery firm, Sinclair Knight Merz  has collaborated on a study  of the GHG footprints of three large WWTPs, as part of the W2W Alliance for the Water Corporation (Western Australia) in the WA capital, Perth highlights the need to look at the “bigger picture” to achieve truly sustainable design options in the long term. The W2W Alliance combines the international and local engineering design and construction expertise of Black & Veatch, Thiess and SKM with the Water Corporation in tackling on one of Western Australia’s most substantial wastewater programs, to upgrade treatment plants at Beenyup, Subiaco and Woodman Point.

The analysis showed that the greenhouse gas footprints of the Woodman Point, Beenyup and Subiaco WWTPs returned widely varying results according to the accounting model used.

The three plants were each assessed using the Water Services Association of Australia (WSAA) standard model, the Australian Greenhouse Office (AGO) model and a comprehensive model that includes a more thorough list of the inputs and outputs of wastewater treatment facilities.

Traditionally, the AGO model has only taken the consumption of power into consideration to calculate a plant’s GHG footprint, whilst the WSAA model also accounts for methane leaks and nitrous oxide (N₂O). (This latter gas is important because in terms of its greenhouse footprint, it is 320 times more potent than CO₂.)

The comprehensive model includes the WSAA data plus all further potential sources of GHG emissions, such as the CO₂ produced through activated sludge treatment, the CO₂ associated with off-site mineralization of biosolids (when applied to land as a soil enhancer) and the embedded CO₂ in chemicals used on-site.

The full carbon footprint shows that while power is a measure of the energy used to operate a WWTP, a range of other items need to be taken into consideration including the chemicals that are consumed. While the plants themselves do not produce the chemicals or the energy used to manufacture them, the demand for chemicals supports an industry that consumes a lot of energy and therefore is part of the broader impact of WWTP operations.

At the Woodman Point WWTP, it was proposed to dose ferric chloride into the primary sedimentation tanks to reduce the aeration power requirements in the aeration tanks. According to the AGO and WSAA models, GHG emissions would be reduced by 90% and 32% respectively, indicating a highly sustainable design option. However the comprehensive model predicted only a 10% reduction in GHG production, due to the increased amount of chemicals required and the extra sludge produced, suggesting mitigation efforts could perhaps be better focused elsewhere.

Comparisons of the GHG footprints of the three plants according to the comprehensive model showed that the Woodman Point plant had a high emissions value – more than twice as large as the Beenyup WWTP. This is despite the fact that both plants have similar loading rates, receiving around 122ML/d at similar wastewater composition. Additionally, the Woodman Point plant generates power on site using digester gas.

The large difference is due to the significant quantities of N₂O emitted from the aeration basins in the Woodman Point sequencing batch reactor (SBR). This was constructed in 2001 using ‘state of the art’ SBR technology. The technology was considered to be more efficient at the time due to the fact that both nitrification and denitrification could be performed in a single, low dissolved oxygen aeration phase (referred to as SND or simultaneous nitrification and denitrification), rather than separating the two biological phases, as is the case at both Beenyup and Subiaco WWTPs.

Research into N₂O emissions by Murdoch University in 2005 revealed that Woodman Point emits around 15 times more N₂O from its aeration basins than both Beenyup and Subiaco WWTPs. So while SBR technology has a number of advantages, including lower capital cost, a major disadvantage is that N₂O appears to be emitted in significant quantities.

The study also determined that while the Subiaco WWTP is half the size of the other plants, its carbon footprint is not proportionately smaller. This indicates that the plant is quite inefficient, which is in contrast to widely-held perceptions of its performance.

The key to determining truly sustainable design options in the longer term is therefore about understanding the bigger picture and focusing on the items of greatest impact. It is not sufficient to focus on power consumption alone. In addition to this, the question in any study of GHG footprint is always where to draw the boundaries. In this study, SKM decided to extend the boundaries beyond the fence line in recognition of the broader impacts that WWTPs have on the community. The broader that the boundary is drawn, the more truly sustainable decisions will be.

This represents a great revelation for those attempting to gain an unequivocal picture of where a plant’s emissions originate and their relative sizes. For SKM, this provided an enhanced understanding of what to target and led to a deeper study into N₂O emissions in the bio-treatment section of Woodman Point. Our aim was therefore to focus on decreasing those emissions, something that is relatively easy to achieve through operational changes.

When constructing a GHG reduction strategy it is therefore crucial to appreciate this more holistic view. Without the benefit of this wider analysis, it is easy to waste resources on targeting smaller technical questions such as pump efficiency, for example. Yet improved pumps would win only a one or two percent improvement in overall GHG footprint. By targeting the bio-treatment for Woodland Point, however, the plant’s emissions will be cut by half.

The results of the study will also be invaluable when the proposed carbon trading scheme comes into effect. While the details are yet to be released, companies that have documented the steps they have taken to proactively reduce their carbon footprint will earn credits for their efforts that could be in the order of millions of dollars per year.

This carbon credit aspect is important in light of the SKM study since it means that developing a comprehensive GHG reduction strategy across multiple WWTPs can lead to even larger returns. Focusing on N₂O reduction was an obvious starting point for the Perth plants, but SKM is currently working on an even more comprehensive strategy that will tackle operations across all three facilities.

The baseline that the study provided has enabled us to understand what we need to focus our efforts upon and how can we assist WWTP operators going forward.

© Sinclair Knight Merz
Requests to re-publish achieve articles should be made via information@globalskm.com
For copyright and disclaimer notices, see Terms of Use.