Benchmarking has been defined as the between indicators of plant performance and a certain number of existing plants. It is a way to start the procedure of energy optimization in WWTP. Austria has developed benchmarking activities since the late nineties, as discussed by Wett et al. (2007), while Molinos-Senante et al. (2014) have developed an entire analysis on the advantages of benchmarking activates in WWTPs. Comparative analysis allows to identify pros and cons of each WWTP and helps to save costs: recently a significant number of publications have focused on producing performance indicators for wastewater services using benchmarking procedures. The Enerwater EU project aims to “develop, validate and disseminate an innovative standard methodology for continuously assessing, labeling and improving the overall energy performance of WWTPs” (Enerwater EU project official website). The evaluation of the WWTPs economic performance is the starting point for identifying potential savings. Available benchmarking methodologies, from the work of Molinos-Senante et al. (2014), are:

Benchmarking activities can be also classified into metric benchmarking and process benchmarking. Metric benchmarking is a quantitative comparative study that can be used between companies which are comparable and evaluation parameters are quality, environmental efforts, and costs for a defined time. Process benchmarking, on the other side, compares only specific processes inside a single facility (Jonasson, 2007).

Benchmarking activities are also performed by the Task Group of IWA (International Water Association): the work is currently led by the IWA Task Group for benchmarking of control strategies. A benchmark simulation model has been developed by the Task Group and is used for control strategies development for organic and nitrogen removal (Jonasson, 2007; IWA Task Group on Benchmarking of Control Strategies for Wastewater Treatment Plants, 2014). Furthermore, different benchmarking activities have been developed and are under development worldwide: from the Water and Sanitation Program-South Asia (WSP-SA), the Bavarian water conservation authority and the Swedish Water & Wastewater Association (SWWA).

Sustainability indicators for WWTP have been deeply discussed by Muga and Mihelcic (2008), where traditional economic, environmental and societal indicators are discussed. The work compares traditional mechanical WWTP with lagoon and terrestrial water treatments. Among the three options, mechanical WWTP are the most expensive and energy intensive but are also able to reach higher outlet water quality and require a lower area.

A typical result from benchmarking studies in WWTPs is the trend of electrical energy consumption as function of the entering load (as will be shown in Figure 5). From the aggregation of available data on existing plants, a set of points can be produced and a trend can be drawn. Other WWTPs can place their data (entering load and energy consumption) inside the graph and discover if their performance are below or above the trend for a certain country or geographical area. This trend could be also used to set limits and guidelines in national or international regulations. The electrical consumption can be analyzed not only as a unique value for the entire plant but can also include the share of consumptions the different plant processes: in this way, a more detailed plant analysis can be done.

An alternative to benchmarking activities is the development of energy manuals. Both Swiss ministry and North Rhine Westphalia developed best practice manuals for WWTP, in 1994 and 1999 respectively, related to energy efficiency, reduction in operational cost and GHG emissions. They include a manual in which energy consumption at traditional WWTPs is described and a clear-structured strategy for the implementation of energy optimization is provided (Wett et al., 2007). The process suggested for the WWTP optimization includes two stages: first, a screening phase, to understand the loads and the plant structure, followed by a short-term optimization measures, usually related to plant management, which do not require any investment. The second phase is a plan for medium and long-term plant optimization measures for energy optimization and includes a detailed techno-economic analysis of every single stage of the process. Thanks to these manuals and guidelines, different plants in EU have reached near self-sufficiency or complete self-sufficiency in the last decade. Paragraph 4.2.4 will analyze existing optimized plants.

The UK environment agency has released a report called “Transforming wastewater treatment to reduce carbon emissions,” outlining five key strategies: “Source control, Least-carbon end-of-pipe/process addition, Greater operational efficiencies, Redeveloping existing treatment processes and Renewable energy generation” (Sadler et al., 2009; Belloir et al., 2015).

The US Water Environment & Reuse Foundation has published different guidelines to reach net-zero energy water facilities (WERF). From their models and plant analysis, WERF researches concluded that 40% lower energy consumption can be reached only by the consistent use of best practices. Furthermore, they underlined the importance of savings in the blowers electricity consumption thanks to improved operation and maintenance procedures and exploitation of biogas in high efficiency CHP systems.

In the Netherlands, the foundation for applied water research has published in 2010 a roadmap for the WWTP of 2030 (STOWA Foundation for applied water research, 2010). The document focuses on the concept of “NEW” plants, which should move from the waste approach to the resource approach, by managing the three available basic resources: Nutrients, Energy and Water (NEW). The roadmap is developed by assumptions on the features of future WWTPs, in relation to demographic, economic, social, technological, ecological and environmental policy directions and impacts on the WWTP sector. Effluent quality, costs and energy neutrality have been detected as the factors with the highest priority.

This section present a general overview on energy consumption in WWTP and the role of the high energy intensive sections of the process (e.g., secondary treatment/ biological process). After discussing current and target plant consumptions, a list of possible ways to improve the plant energy efficiency and reach self-sufficiency is presented, with a review on the available literature works. The last section is about real examples of best practices in terms of energy efficiency in WWTPs.

Electricity is the main energy source required in WWTPs, accounting for around 25–50% of the operating costs of traditional activated sludge plants (Foladori et al., 2015). This data is confirmed in the work of Panepinto et al. (2016), which mentions that in a typical WWTP 25–40% of operating costs is due to electricity consumption. This value varies in the range of approximately 0.3–2.1 kWh/m³ of treated wastewater. EPRI report also mentions a similar value: energy consumption ranges from 0.413 to 0.87 kWh/m³ depending on the plant size (Electric Power Research Institute and Water Research Foundation, 2013). The share of energy into operating costs is also shown in the work of Husmann (2009), where energy is the higher slice in the OPEX yearly value, accounting for around 35%, followed by chemicals, disposal, maintenance and personnel. The report pointed out that most impacting elements in the energy consumption of a conventional WWTP are the aeration of mixed liquor (55–70%), primary and secondary settling with sludge pumping (15.6%) and sludge dewatering (7%). These data are also confirmed by WERF report (Crawford and Sandino, 2010). Shen et al. (2015), while analyzing the US WWTP sector, underlined that electric energy accounts for over than 30% of the total operation and maintenance cost and up to 80% of the GHG emission. The EPRI report suggest again that 52% of WWTP energy consumption is used for aeration, 12% for pumping and 30% for bio-solids processing, thus confirming previous data (Electric Power Research Institute and Water Research Foundation, 2013).

When mentioning energy consumption, the present work is referring to the overall electrical energy consumption. In WWTPs, not only electrical energy is consumed but also chemical (fuel) energy, labor energy and thermal energy. On the other hand, as discussed by Belloir et al. (2015), the largest portion of the overall annual primary energy consumption is electrical.

Thermal energy can be also not negligible, especially for non-optimized plants. But a large portion of thermal energy is trapped inside the inlet wastewater to the plant, and is usually unexploited. Especially in cold climates, this temperature difference could be used as a heat source for a heat pump in order to recover extra thermal energy (WERF, 2014b, 2016). Furthermore, digested sludge exiting from anaerobic digester is usually found at 36°C (mesophilic digester). This could be another available heat source, but it is commonly not included in the sludge dewatering process. In many available WERF reports it is mentioned that wastewater contains nearly 5–10 times the amount of energy needed for the WWT process (1.93 kWh/m³ energy content with 500 mg/l COD, while typical energy consumptions ranges from 0.3 to 0.8 kWh/m³, Hao et al., 2015). In particular, 90% of this energy is thermal (due to the water heat capacity), 20% chemical (express by the COD) and less than 1% hydraulic (potential and kinetic) (WERF, 2014b, 2016). This means that, if properly recovered, this energy could eliminate WWTP net consumption and generate an extra production. Anyway, the comparison between the energy contained in the inlet waste water and the plant energy consumption should also consider the conversion efficiency between the different energy forms: the amount of inlet energy which could be recovered in terms of useful electrical (or thermal) energy is high but currently unquantifiable and reliant on the energy conversion processes.

WERF estimated that US could make the 100 largest WWTP facilities (which are 17% of the total WWTP energy consumption) energy neutral by investing around 4.8 billion $ but saving 41 million barrels of oil and avoiding 18 million tons of CO₂.

Chemical energy contained in WWTP can be partially recovered by means of sludge anaerobic digestion, where biogas is produced and can be exploited for electricity and/or heat production. Thermal energy recovery is more challenging instead because of the low-grade heat available in the inlet wastewater, which is difficult to be captured due to poor efficiency at the low temperatures and the risk for inlet wastewater to foul heat exchangers. For what concerning hydraulic energy, although the low energy content in wastewater, the technology to convert this into electrical energy (via turbines or kinetic devices) is well known, with an efficiency up to 90%.

Different works are available in literature on energy audit, energy benchmarking and energy analysis of WWTPs. Results are summarized in Table 1 and discussed below. In the table, energy consumption is expressed in kWh per PE or kWh per cubic meter. The conversion among the values is not performed since this requires the knowledge of the organic load in the entering water and the PE definition according to the area of the world. For this reason, results are provided in their current form as found in literature.

Chinese WWTPs generally have a lower energy consumption (~0.3 kWh/m³/y, see Table 1) with respect to EU and US plants: this is related, when consumption is expressed per cubic meter of waste water, to the low COD of Chinese wastewater, usually around 300 mg/L against 500–800 mg/L of other countries. The low organic content means a low electrical consumption but also a more difficult path to reach the carbon neutrality (Hao et al., 2015).

Presented values for Sweden, Austria, and Germany (between 23 and 42 kWh/PE/y from Jonasson, 2007, see Table 1) are related to large size systems (>50,000 PE). Tonawanda WWTP (67 kWh/PE/y) is also a medium large plant and serves around 200,000 PE (calculated from the data on the entering load and a PE definition of 60 g BOD per day according to Directive 91/271/EEC).

Foladori et al. (2015), on the other side, have analyzed the energy consumption of five small WWTPs, with an entering load <10,000 PE, from 582 to 9,727 PE. Results are analyzed after an energy audit period of two years in the five plants: the processes to which is related the higher energy consumption are oxidation/ aeration, recirculation of settled sludge and aerobic stabilization. Inefficiencies related to blowers were solved by moving from a continuous to an intermittent operating regime, which enable the plant to obtain the same useful effect while strongly reducing energy consumption.

Data referred to the UK WWTPS (Table 1), are two values derived by the analysis of two sites (Belloir et al., 2015). This work showed how sites with similar energy consumption rate and similar shares, can require different optimization approaches.

Thanks to the WERF foundation, many data are also available for US WWTPs. One of the available report mentions that more than half of energy consumption is linked to aeration process (54.1%), confirming data shown in Table 1 (Crawford and Sandino, 2010). Furthermore, 14.3% of energy consumption is related to pumping systems and another 14.2% to anaerobic digestion mixing.

In Italy, a detailed energy balance has been performed for the Castiglione WWTP (Turin, IT). The same plant will be the one considered for the case study (section Co-digestion Case Study) of the presented work. The plant is serving 2.7 million PE and shows an energy request of 66.78 GWh/y of electricity, of which about 50% from aeration in oxidation tanks, and 49.15 GWh/t of thermal energy, of which 93% for the sludge line (Table 1).

Based on the macroscale analysis, the EPRI report for the US approximates that a realistic achievable potential for water and wastewater industry by 2030 is approximately 8% of baseline (Electric Power Research Institute and Water Research Foundation, 2013). The data is confirmed by Longo et al, where the overall energy savings from WWTP operational optimization and technology improvements is estimated between 5 and 30% (Longo et al., 2016).

In Europe, an ongoing Horizon 2020 project named Enerwater, is trying to develop a standard approach to evaluate WWTPs (Enerwater EU project official website²; Longo et al., 2016). Starting from an initial sample of 610 WWTPs, the project has then focused on 388 plants between US, EU and Asia, which have been analyzed and compared. As described above, energy consumption can be expressed as function of the influent flow rate, person equivalent and COD removed. In the work of Longo et al. (2016), Enerwater members have listed all available benchmarking analyses on WWTPS, underlining advantages and criticalities of each study. The final output is a trend able to correlate the PE served by the WWTP and the energy consumption in terms of kWh per kg of COD removed. The trend is clear: energy consumption is exponentially reducing when moving from small to larger plants, as discussed before, and an asymptotic value of around 0.5 kWh/kgCOD removed seems to be detectable. Data are also grouped by country and type of technology, but there is not a visible trend based on those indexes. From the analysis of the influence of different plant section, it is confirmed that generally for medium to large plants, the treatment sections characterized by higher energy consumption are biological oxidation, lifts (pumping and sludge recirculation) and generally mechanical dewatering of sludge and/or aerobic sludge digestion if present.

One critical issue in the analysis of the energy consumption is related to the size of the system and the effluent requirements in terms of purification. The unit power consumption decreases asymptotically as the plant size increases, and increases as effluent requirements becomes more stringent. In the work of Awe et al. (2016), it is mentioned that a 4,000 m³/d WWTP consumes 0.591 kWh/m³, while a 378,500 m³/d plant consumes 0.272 kWh/m³. Furthermore, if advanced nitrification is performed in the smallest site, energy consumption increases from 0.591 to 0.780 kWh/m³. This is confirmed also by Table 1, where data referred to the Italian small WWTPs show a higher specific energy consumption. Panepinto et al. (2016) have also underlined this criticality, even underlining that the effect of the size is not linear. As can be seen in Table 2, quite low consumption rates (20–25%) are related to large systems, and are already quite close to the optimal values suggested by the guidelines available (see the work from Wett et al., 2007). On the other hand, energy consumption increases from 25 to more than 60% in other analyzed plants, not always “small” in terms of size.

The analysis of small WWTPs is justified by the high number of existing micro and small plants: in EU, according to the EEA maps on existing urban WWTPs (European Environment Agency, 2015), the largest group of plants, in almost all countries, is below 50,000 PE and the average entering load per plant in EU is 21,966 PE. However, as mentioned above, small size plants can show higher energy costs and different share among the processes (Foladori et al., 2015).

The link generally seen between nominal capacity (PE) and specific electrical consumption is also confirmed by Figure 4, where state-of-the-art electrical consumption is shown as function of the plant capacity and decrease from more than 60 kWh/PE/y for small WWTPs (<10,000 PE) to less than 45 kWh/PE/y for larger systems (>100,000 PE). In the Enerwater project, link between PE/ inlet flow rate and specific energy consumption has been analyzed for hundreds of EU plants (see Figure 5): small plants seem to have a very different behavior in terms of energy consumption, while the trend reach asymptotically a stable value when the size reaches values around 100,000 PE.

This is also confirmed by a similar study by Goldstein and Smith (2002) where inlet water flow rate is shown in relation to specific electrical consumption. Despite the water treatment process analyzed (trickling filters, activated sludge, advanced treatments), the electrical consumption is always reducing when plant size is increasing.

Nevertheless, in all the cases WWTPs show room for plant optimization and reduction in energy consumption, despite their size.

Another criticality which could lead to wrong discussions in the comparison among different plant is the PE definition used for the calculation of the specific energy consumption. Most of the EU WWTPs are oversized and the plant capacity (in PE or entering load/water) can differ from served PE. In the work from Foladori et al. (2015), design capacity for the five analyzed small plants was around the double with respect to the served population. On a EU basis, according to the EEA database values (European Environment Agency, 2015), the average ratio among served PE and design capacity is higher than 80%. Furthermore, when WWTPs are operating at a entering load lower than their actual capacity, the specific energy consumption starts to increase, as shown in Deines (2015).

Wett et al. (2007) report target values for WWTP optimization in terms of electricity consumption (20–25 kWh/PE/y for the whole process and 16–21 kWh/PE/y for the biological process): a specific biogas yield (>475 l/kg volatile dry solid entering in the anaerobic digester) and a self-sufficiency rate (electric energy 90%, thermal energy 99%). This optimal view is not 100% self-sufficiency but it is close to the final goal. In this work, target and ideal energy consumption values are also reported. Table 1 shows, in the last rows, target energy consumption data from guidelines, Enerwater project and models.

Opportunities for efficiency improvement in WWTP exist in numerous categories as upgrading to more efficient apparatus, improving energy and plant management and energy generation on-site. On the contrary, barriers are found in terms of capital costs and reluctance to change well-known practices (Copeland, 2014).

The energy content of wastewater is includes the heat and the organic constituents found in the inlet wastewater (Tchobanoglous et al., 2009). The chemical energy content of wastewater is around 13 MJ/kg COD, while heat energy is 41.9 MJ/10°C m³ and is usually 2–4 times higher than the plant energy request, data confirmed also by WERF (2014b).

With traditional existing WWTPs, not optimized, around 25–50% of the energy consumption can be covered by internal energy generation (Mccarty et al., 2011). Further improvements are indeed required to reach self-sufficiency.

As already mentioned, energy efficiency of WWTPs can be also increased by means of thermal recovery, untapped in the inlet wastewater (influent) and in sludge exiting from the anaerobic digester. In Stockholm (SW), for example, a WWTP with a maximum capacity of 450,000 m³/d generates about 597,000 MWh of low-temperature heat using 199,000 MWh electricity through heat pumps ((ESMAP), 2012).

Wett et al. (2007) presented an interesting overview on the concept of self-sufficiency in WWTP systems. The work analyzed Central EU initiatives for operational optimizations, which lead to an average potential for energy saving of 30–50% for existing utilities. The well-known Strass WWTP (Austria), the first plant able to reach self-sufficiency (and currently shows a +8% electricity production, sold to the grid) is also discussed. Among the different available pathways to reach the goal of self-sufficiency, the authors mention two-stage biological WWT, on-line control of intermittent aeration for enhanced nitrogen removal, CHP plants and de-ammonification.

Most common energy intensive processes in WWTP are:

This two processes will be discussed below.

Energy consumption for aeration process is generally between 0.18 and 0.8 kWh/m³. As mentioned by Longo et al. (2016), “aeration is an essential process in the majority of WWTPs and accounts for the largest fraction of plant energy costs, ranging from 45 to 75% of the plant energy expenditure.”

Jonasson (2007) mentioned that the biological process normally accounts for 50–80% of the total electrical energy consumption. According to this study, only 9% of the used electrical energy in WWTP in Sweden was produced by the plant itself. The same document also analyzed Austrian WWTPs: here, almost 70% of energy consumption is related to secondary treatment (mechanical and biological). Table 1, together with many literature sources (Awe et al., 2016) confirms the high share of the biological process in the plant energy consumption. The high share of activated sludge process aeration in energy consumption is also confirmed by WERF reports, where 60% is mentioned (WERF, 2016).

Panepinto et al. (2016) analyzed the SMAT Castiglione WWTP, the fifth largest WWTP in Europe with an installed capacity of 2,700,000 PE. The plant current energy consumption is not tremendously high, especially if compared to other plants (see Table 2), but the authors have identified many opportunities for energy saving. About 25% of energy saving could be reached thanks to optimization of primary settling efficiency by means of coagulants, 20–36% with aeration in oxidation tanks controlled with automatized control of Dissolved Oxygen (DO) and Sludge Retention Time (SRT) and 64% with Optimization of dissolved air flotation in to solids thickening.

The aeration system requires more energy than any other process, but it also shows the highest potential for energy savings. To decrease the aerobic treatment of biological sludge, COD reduction needs to be as high as possible, while retention time must be as low as possible. Because of the variable inlet wastewater, oxygen addition in the aeration process must be controlled and adjusted with feedbacks from on-line measurements. Replacement of old aeration systems with new and more efficient aeration machines is another key element for energy saving.

Aeration blower and filter blower modifications were suggested as key actions for WWTPs self-sufficiency already in 1998 by NYSERDA in their analysis of six existing WWTPs (The New York State Energy Research and Development Authority, 1998). Blowers are usually operated continuously 24/7 and relief valves are often opened when air is not required. A partial and intermittent operation of blowers would guarantee the same effect with a reduced energy consumption. Such modifications have not a high impact in terms of treatment process, efficiency, and costs.

A dynamic model for the optimization of the aeration process is also presented by Fikar et al. (2005). The author built a model able to determine the optimal aeration strategy for small-size activated sludge WWTPs, which could then be used in existing WWTP control system to minimize energy consumption.

The NETL report on strategies for WWTPs mentioned that aeration and oxidation basins, which accounts for 77.9% of energy consumption, represent the plant sections in which the highest saving could be obtained. In particular, 90% saving can be generated by turning off mixers when aerators operate (Daw et al., 2012).

Other important consumers in WWTPs are pumps (18.9%) (Daw et al., 2012). From the NYSERDA report, 40.18% of the Tonawanda WWTP (US) is related to secondary treatment, 11.32% to heating and ventilation, 9.19% to pumping and the rest to other processes (The New York State Energy Research and Development Authority, 2005). Pumps elevation differences because of different geographical location could also generate higher electricity consumption.

Pumps are large energy consumers in WWTPs and the differences in terms of energy efficiency between pumps are important (Awe et al., 2016). Awe et al. (2016) mentioned that “Variable Frequency Drives (VFDs) can be used to vary the speed of pump to match the flow conditions and affinity laws for centrifugal pumps suggest that even a small reduction in motor speed can reduce pump energy by as much as 50%.”

Other WWTP sections where energy consumption could be reduced thanks to new processes and components upgrading are:

We assume that together with the currently available sewage sludge (SS) (from primary and biological treatments), the OFMSW collected in municipality of Torino is also available for co-digestion with sludge (Figure 6). Both achieved and feasible SS:OFMSW ratios have been then compared to other literature works to validate our initial assumption.

From the local authority for the protection and research on Environment (ISPRA), data are available on the separated waste collection (ISPRA, 2015). Especially, the organic fraction collected in the province of Torino is reviewed.

Starting from the Italian production of OFMSW from separated collection (6,071,500 tons OFMSW), and the number of Italian inhabitants, a specific production has been derived, equal to 99.86 kg OFMSW/capita/y. This datum (range 80–100 kg OFMSW/capita/y) is also found in the literature (Giacetti and Venturi, 2007).

Considering that municipal waste (MSW) generation per capita in Italy is around 500 kg/capita/y (Eurostat, 2017), OFMSW accounts then for around 20%. If only separated collection is considered, OFMSW share is between 30 and 40%.

Starting from the specific OFMSW production, the amount generated by the province of Turin can be determined, i.e., 230,477 tons of OFMSW are produced per year, with an average solid content (ST) of 8.8% (Bolzonella et al., 2005). The final solid content of the entering biomass (mixed sludge and OFMSW) is lower than 8.8% and is thus able to guarantee a wet anaerobic digestion with a continuous operation (as is performed currently). Higher solid contents would lead to dry digestion, which is usually performed in batch mode.

The Castiglione WWTP would be thus fed by a mixed feed of OFMSW (previously treated, mixed and homogenized) and sewage sludge. Biogas yield (in cubic meters of biogas produced per ton of VS entering the digester) has been derived from available data on WWTP for the current scenario (~400 m³/ton VS), while it has been taken from literature for the co-digestion scenario. In particular, different studies have been analyzed. The SS:OFMSW ratio is not the only parameter influencing the final production rate: temperature, volume of the digester, type of waste, dilution rate, and other parameters are strongly influencing the final productivity of the plant. Nonetheless, a direct relationship between ratio and performance is not always observed (see Table 8). An average and conservative value of 550 m³/ton VS has been chosen for this work.

For all the calculations, the VS/TS ratio has been assumed equal to 0.7 (taken from SMAT available data). Furthermore, a 10% reduction in the entering OFMSW has been accounted because of material losses in the pre-treatment stage which should be installed onsite (Nghiem et al., 2017). Mass balances at the inlet of the digester before and after the addition of co-digestion are presented in Table 9.

The resulting SS:OFMSW ratio in the co-digestion scenario is:

This data is in line with literature works on co-digestion, where a 50:50 VS/TS ratio is usually proposed (Mattioli et al., 2014; Koch et al., 2015; Kim et al., 2016; Arnò et al., 2017).

The biogas production increases by 151% thanks to co-digestion. Depending on the anaerobic digestion parameters and OFMSW composition, biogas production could vary (respect to the values obtained here and seasonally). However, literature works confirmed that biogas production can be doubled thanks to co-digestion (see Section Sludge co-digestion with food waste). Furthermore, co-digestion is usually performed as a best practice in self-sufficient WWTP (Table 6).

The advantage of co-digestion is enhanced if biogas is exploited in high efficiency CHP systems, such as the SOFC. The SOFC electrical efficiency reaches 53–55% from clean biogas to AC power (Convion Fuel Cell Systems, 2016). Electrical efficiencies up to 60% and beyond have been also demonstrated in commercial systems (Lanzini et al., 2017). Furthermore, near zero emission of atmospheric pollutants is achieved. A comparison on the energy balance and plant load coverages with co-digestion is presented in Figure 7 and Table 10, with the option of biogas exploitation either in the SOFC or the ICE. Calculations have been performed with the same assumptions on CF, methane content and PV electrical production previously shown in Table 7.

Thanks to the use of co-digestion and biogas exploitation in the SOFC, both electrical and thermal load could be covered, thus reaching a 100% self-sufficiency from the electrical point of view and a near-self-sufficiency from the thermal point of view.

The electrical production is indeed increased by 228% compared to the current scenario (due to the positive effect of co-digestion and SOFC). The overall thermal production is increased only by 75% since co-digestion provides a larger amount of biogas (positive effect) but the SOFC thermal efficiency is more than 10% lower compared to ICE (negative effect).

The goal of 100% self-sufficiency from the thermal point of view could be achieved by implementing a common practice in WWTPs, i.e., sludge pre-thickening. The thermal load is currently dominated by the production of hot water for sludge pre-heating (the sludge is indeed heated prior to entering the anaerobic digester). The high thermal load is due to the low TS content (2%) of the sludge. By pre-thickening (e.g., by means of a centrifugal system), the solid content can be easily doubled thus halving the thermal load. In the pre-thickening scenario, a thermal surplus is generated with respect to the current scenario.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.