Full scale co-digestion of wastewater sludge and food waste: Bottlenecks and possibilities
Introduction
Energy security, resource depletion, and pollution prevention are three of the most vexing challenges of our time. They often manifest themselves in the form of climate change, economic crisis, and geopolitical instability [1], [2]. These challenges call for a fundamental paradigm shift in resource and environmental management, which has resulted in the emergence of the circular economy concept. In a circular economy, products at the end of their service life or waste materials are turned into resources for another purpose, thus closing loops in industrial ecosystems and minimizing waste [3]. It is estimated that by shifting to a circular economy, several European countries can potentially reduce their national greenhouse-gas emissions by 70%, grow their workforce by 4%, and decrease their dependency from resource and energy import [4].
The transformation toward a circular economy can be already seen in the wastewater servicing sector. Traditionally, wastewater treatment plants (WWTPs) have been designed with the end-of-pipe treatment philosophy of meeting discharge standards to receiving water bodies while waste produced during treatment is destined for landfill or incineration. Conventional WWTPs typically include activated sludge treatment of wastewater and anaerobic digestion of the produced sludge. Most WWTPs adopting anaerobic digestion for sludge treatment face similar problems namely low organic loading and biogas (methane) yields due to the low biodegradability of wastewater sludge. In addition, WWTPs are often designed with spare capacity to cater for variation in the wastewater flow and future population growth [5]. Recent progress in water conservation and slow or even declining population growth in many developed countries have left many wastewater treatment facilities with spare digester capacity that is not and will not be utilised in the future.
Wastewater treatment is vital for environmental protection, but it is also an energy intensive activity. The treatment of municipal wastewater accounts for about 3% of global electricity consumption and 5% of global greenhouse gas emission [2], [6]. Thus, it is not surprising that the wastewater industry has actively explored options to move toward energy-neutral operation.
A typical energy demand for biological wastewater treatment is in the range of 20–30 kWh per person equivalent per year [7]. On the other hand, given the current engineering limitations, energy recovery via anaerobic digestion of wastewater sludge is only about 15–18 kWh per person equivalent per year. In other words, under an optimal condition, WWTPs can potentially achieve up to about 65% energy self-sufficiency by fully utilising their sludge for energy recovery [7], [8]. In practice, a typical WWTP can currently offset 20–30% of the energy consumption and greenhouse gas emission [7], [9]. Thus, a viable and pragmatic approach is to co-digest municipal organic wastes in combination with municipal wastewater sludge using the spare digestion capacity at WWTPs to increase biogas production [10], [11], [12]. This approach does not only allow WWTPs to be energy-neutral but also reduce the cost of municipal organic waste management [13] while facilitating nutrient recycling [14], [15].
Waste disposal options by landfilling or incineration are expensive and not compatible with the concept of a circular economy. Ongoing leachate management and extensive monitoring for potential groundwater and air pollution are required during active landfill operation and even up to 50 years post-closure [16]. On the other hand, extensive air pollution control is required for the waste incineration. Incineration of waste materials also results in significant greenhouse gas emission. In addition, both landfilling and incineration options offer very limited possibilities for resource recovery. Contrary to the traditional end-of-pipe treatment philosophy, in a circular economy, waste materials in general and the organic wastes in particular are a rich vein of resources in terms of energy and nutrient waiting to be tapped.
The opportunity to utilise spare capacity at WWTPs for municipal organic waste co-digestion is immense. Nevertheless, co-digestion at WWTPs also faces many significant hurdles that must be resolved through the dissemination of full-scale operation experience in both waste separation and biogas technology. Indeed, despite the rapid increase in the number of co-digestion studies at laboratory scale in the literature over the last couple of years [17], the number of pilot- and full-scale studies (particularly those dealing with operational issues) is still very limited. Of a particular note, waste materials can be highly variable in composition and data from laboratory-scale experiments using a small sample are not readily transferable to full-scale operation.
It is noteworthy that the bulk of organic waste from municipal origin is essentially food waste or food related waste, thus, the term food waste can be broadly used to refer to municipal organic waste. In this study, food waste materials include both avoidable (such as food scraps, unsaleable produces from supermarket, unusable beverage, and waste milk) and unavoidable streams (such as food processing waste, coffee ground, tea leaves, dairy processing waste, and fat oil and grease (FOG)).
Based on visits of full-scale facilities and collaboration between authors and plant operators, this paper discusses the current state of wastewater sludge and food waste co-digestion. Operation and design experience at four co-digestion plants using wastewater sludge and food waste in Italy and Germany are described in detail to reveal potential lessons for future projects. Key bottlenecks in the integration of co-digestion to WWTPs are identified and delineated. This paper provides useful background information for the formulation of a systematic roadmap for future development of co-digestion using experiences from existing resource recovery facilities promoting the concept of a circular economy.
Section snippets
Key drivers of co-digestion
Anaerobic co-digestion of food waste and wastewater sludge provides a range of economic and environmental benefits (Fig. 1). These include the diversion of putrescible waste from landfills and the ability to recover essential resources in a circular economy. Both of these are key environmental and social drivers of co-digestion. There are also financial benefits in the form of gate fee and revenue through renewable energy production. While financial consideration is essential to justify the
Co-digestion of wastewater sludge and organic wastes
The utilisation of available OLR capacity in WWTPs to co-digest food waste with wastewater sludge has gained significant momentum in the last decade. Examples of co-digestion at WWTPs can be readily found in many parts of the world (Table 2). The fundamental science behind co-digestion can be directly adapted from the agricultural sector. Indeed, in the agricultural sector, co-digestion (mostly between manure and crop residual) has been a standard practice for several decades [36], [37], [38].
Available feedstock
The potential for co-digestion at WWTPs is immense. A broad range of co-substrates is available with sufficient quantity and supply security. Examples of these include food waste from urban areas (e.g., canteens and restaurants), organic waste from food processing, expired food from supermarkets, market waste, dairy waste, and FOG. Of a particular note, energy recovery through co-digestion is arguably being underutilised even in Germany, which is a global leader in biogas technology with
Conclusion
Drawing from the authors’ research experience and collaborations with full-scale facilities, this paper describes the current status of wastewater sludge and food waste co-digestion for simultaneous waste management and renewable energy production. Co-digestion of food waste with wastewater sludge is advantageous where spare digestion capacity of WWTPs is available to deliver significant environmental benefits. However, not all of these environmental benefits can be readily monetarised into
Acknowledgements
This work was funded by the Australia-Germany joint research co-operation scheme for Long Nghiem (project ID: 121183) and Konrad Koch (project ID: 57214290). Long Nghiem gratefully acknowledges the financial support from the Institute of Advanced Studies through the Excellence Initiative as an August-Wilhelm Scheer Visiting Professor at the Technical University of Munich. Collaborators at the Rovereto, Camposampiero, Treviso, Moosburg, Oberding, Kurobe, and Asahi plant are thanked for their
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