By Dr. Melita Jazbec, Research Principal, Institute for Sustainable Futures, UTS
The energy, water and waste sectors are all striving to achieve net zero emissions by 2050. A recent study from researchers at the University of Technology Sydney (UTS) found that urban organic waste has a significant role to play in meeting this goal.
Recent Intergovernmental Panel on Climate Change (IPPC) assessment reports2 have stated that the transition to renewable energy from fossil fuels would only address 54 per cent of greenhouse gas (GHG) global emissions. The remaining 46 per cent of the anthropogenic GHG are the result of the production of goods and the management of land – this includes the food system and organic waste generated along its value chain.
Around three-quarters of GHG emissions for the waste sector are produced by organic waste disposed of in landfills, where it decomposes to generate methane. However, only a small amount (eight per cent) of this landfill gas is captured to generate energy 3. To tackle organic waste in landfills, the waste sector has set targets to firstly, half the generation of food waste, and secondly, divert the organic wastes going to landfill via separate organic waste collection and processing.
At the same time, the water sector is aiming to become energy self-sufficient. This could be achieved by utilising anaerobic digestion to process sludge at wastewater treatment plants (WWTP). Anaerobic digestion is a biological process that generates biogas (a mix of methane and carbon dioxide) and digestate. This biogas can then be utilised for energy generation or as a replacement for natural gas. Digestate, which contains nutrients (nitrogen, phosphorous and potassium), is normally used as soil conditioner.
Achieving energy self-sufficiency
Energy self-sufficiency has been successfully achieved at WWTPs internationally. For example, Ejby Mølle WWTP in Denmark4 has optimised operating conditions to enhance nutrient release and subsequently increase biogas and energy generation. Other WWTPs have effectively increased generation of biogas through co-digestion of wastewater sludge with other external organic waste feedstocks. Combining wastewater sludge with organic and green household wastes; organic waste from hospitality (e.g. restaurants and catering); from food manufacturing (e.g. dairies, slaughterhouses); and manure from farms, transforms WWTPs into circular economy hubs or biorefineries. A great example is Billund BioRefinery in Denmark5, which processes food waste collected from households and farms, and in return provides energy to households and fertiliser to farms. Co-digestion of multiple feedstocks often results in generation of excess energy (more than the WWTP needs) enabling WWTPs to become energy self-sufficient and export any excess energy.
Billund BioRefinery also explored other circular economy solutions that add value to the business. These include capturing carbon dioxide through algae production and subsequently harvesting and utilising the algae. The company also explored production of bioplastics and proteins from biogas, and use digestate as a soil conditioner, which replaces the need for synthetic and non-renewable fertilisers. Furthermore, Billund is generating additional income by sharing this knowledge and assisting other businesses to implement similar technologies.
Unlocking the potential of urban waste
In our recent study1, we identified the energy potential of the urban organic wastes that could be co-digested at three selected Sydney WWTP’s.
We developed a methodology to quantify urban organic wastes by sector type, including commercial and industrial waste, for which there is no publicly available data. We also identified energy potential hot spots in the areas surrounding WWTPs, which could assist planners in the development of organic waste collection and processing infrastructure.
In this research, we estimated that urban organic waste from the study area could generate 37.8 billion litres of biogas per year, equivalent to the natural gas used by 30,000 households in a year1.
The generated bioenergy would be 4.5 times higher than the energy currently generated by solar panels in this area. Almost half of all food waste from the study area could be processed at the three Sydney WWTPs.
Utilising the existing anaerobic digestors at the WWTPs would mitigate the need to build new infrastructure to process food waste and divert it from landfills. These three WWTPs alone could fill as much as 20 per cent of the planned anaerobic digestion infrastructure that needs to be built by 2030 to process Sydney’s projected food waste.
Australia generates 7.7Mt of food waste along the value chain, of which only 40kt are processed through anaerobic digestion and just 736kt of food waste (mainly industrial and commercial) are processed at the wastewater treatment plants. While the main method of processing food waste is composting (2.2Mt including home composting), almost half of food waste still ends in landfills (3.3Mt)6 . Most of the food waste in landfills are from households and hospitality.
As anerobic digestion processing of urban organic waste not only generates energy but also captures valuable nutrients, why are we not utilising this opportunity more?
Overcoming barriers
In our study1, consultation with stakeholders identified several barriers. These include operational challenges, such as dealing with inconsistency of feedstock, feedstock contamination (e.g. PFAS, microplastics), and necessary additional pre-processing.
Respondents also described regulatory hurdles (especially in relation to application of digestate to land), issues with zoning, and complex and lengthy contractual agreements in the waste sector.
Many stakeholders were also concerned about the lack of incentives to make this business case viable. There are no subsidies in place for generation of biomethane, despite it being a renewable resource. To be financially viable, there is a need for scaling up, and establishing the new logistics presents additional costs.
Stakeholders also reported that there is already a competition for the organic wastes and there is a challenge in establishing a sustainable gate fee that can compete with the current relatively low landfill disposal fee.
However, stakeholders also saw a lot of opportunities in processing urban organic waste at WWTPs. A main benefit is that WWTPs are located close to sources of the organic waste, which omits the need for it be transported to composting facilities that are generally located on the city’s outskirts.
With the emerging technologies promising a better separation of organic wastes and decontamination of the digestate, anaerobic digestion would provide similar benefits as composting (producing soil nutrients and fertilisers)and more, as it also generates biogas, an important component in the renewable energy mix.
Stakeholders stated that there is need to conduct further feasibility studies to understand costs, gate fees, the economy of scale, and the economics of capturing carbon. They also expressed the urgency to gather more up-to-date and high-quality data on the available feedstocks that could be used for generation of biogas.
In addition, solving barriers to utilise digestate as a fertiliser would bring additional revenue for the WWTPs. Currently, WWTPs are disposing of biosolids at cost.
An additional challenge has been presented due to the current strategic direction, and requirement, for the collection of food waste with garden organics (FOGO) from the households.
As FOGO cannot be fed directly into anaerobic digestor, it requires costly additional pre-processing. This makes anerobic digestion more expensive, due to capital and operational costs.
Finally, there is also a need to better understand international benchmarks of best practice, how Australia compares with these, and what are the pathways for Australia to reach the world scale standards. These could include favourable feed-in tariffs, capital grants, guaranteed access to the gas pipeline network at a reasonable fixed cost. Setting renewable gas targets with support will create demand.
Urban organics present an attractive opportunity as a source of biogas, especially if processed in the existing infrastructure at the WWTPs. This pathway will not only solve problems that waste sector faces, but will also assist the water sector in becoming energy self-sufficient and contribute to the transition to renewable energy, with the possibility to generate energy behind the meter or feed into the existing gas networks.
Featured image: Natsicha Wetchasart/shutterstock.com
Acknowledgement: Co-authors of the study Dr Andrea Turner and Dr Ben Madden, Funding: RACE for 2030 CRC and it’s partners: Sydney Water, NSW EPA, NSW DPIE
1 Jazbec, M., Turner, A., Madden, B., and Nghiem, D.L. (2023). Mapping Organic Waste in Sydney: Advancing Anerobic Co-Digestion for Energy Generation and GHG Reduction. Fast track Project for Research Theme B5 Anaerobic digestion for electricity, transport and gas. RACE for 2030 CRC
2 Pathak, M., et al., Technical Summary, in Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. 2022, Cambridge University Press.
3 Pickin, J., Wardle, C., O’Farrell, K., Nyunt, P., and Donovan, S. (2020) National Waste Report 2020. Department of Agriculture, Water and the Environment.
4 Jazbec, M., and Turner, A., 2020 Wastewater gas recovery opportunities in a circulareconomy, report prepared by the Institute for Sustainable Futures, University of Technology Sydney, for Sydney Water
5 Jazbec, M., and Turner, A., 2018, Creating a circular economy precinct, report prepared by the Institute for Sustainable Futures, University of Technology Sydney, for Sydney Water.
6 https://www.fial.com.au/sharing-knowledge/food-waste
