1. Introduction
Every year, in Italy, around 30 million tons of municipal solid waste (MSW) are generated, and about half of this amount is represented by organic biodegradable waste, also known as the organic fraction of MSW (OFMSW) [
1]. OFMSW is mainly composed of food waste (FW) from domestic kitchens and yard waste (YW) from garden maintenance. Due to a high carbon and volatile solid (VS) content, the first is considered as a potential energy source [
2], and the latter comprises green waste, such as grass, leaves, and branches, characterized by high lignin content [
3]. Currently, the main OFMSW treatment systems are composting and anaerobic digestion (AD). Composting is a biological treatment that aerobically decomposes organic waste, producing compost with a release of heat, humidity, and carbon dioxide (CO
2). Compost is a nutrient-rich soil that can be used as an agricultural amendment. AD is an anaerobic biological treatment for different types of substrates that converts organic matter to biogas and digestate [
4]. Biogas consists of 50–70% methane (CH
4), 25–45% CO
2, and traces of hydrogen sulfide (H
2S), humidity, and other gases, thus, it can be used as a source for renewable electric and thermal energy production through a combined heat and power (CHP) system [
5]. Digestate is rich in nutrients, but, due to a lower waste biodegradation of the AD treatment than composting, it doesn’t achieve good biological stability, and requires further treatments [
6]. AD treatment is receiving increasing attention due to its high valorization value for waste; FW presents an excellent organics degradation rate and methane productivity, and YW contains a compact structure that impedes the attack of anaerobes, thus often resulting in a long digestion period and low biogas yield [
7]. Hence, by balancing the characteristics of FW and YW substrates in OFMSW, anaerobic co-digestion (co-AD) of OFMSW could be considered a significant strategy for the management of these wastes in terms of circular economy and performance.
Generally, waste management is a difficult practice, and often includes elevated costs. In Italy, the costs of waste collection and disposal services covers the main part of the funding for waste management [
8]. Other than the environmental pressure exerted by waste treatment plants, the effects of waste transportation from the collection points are not negligible, compared to those generated by other segments of the waste management system [
9]. Concerning the waste management through AD, Lamnatou et al. [
10] showed that the transportation of organic waste impacts from 44% to over 70% of the whole environment. Hence, the introduction of a transfer station or small waste treatment plant with a view to a decentralized system, instead of a bigger one, could decrease environmental impacts, such as energy demand and emissions, especially in medium and large cities [
11]. Despite the economic cost required by new infrastructures, the introduction of an AD plant at a local site could reduce the cost of waste transportation and generate energy from biogas combustion. Energy could be delivered to the urban electricity network or on-site to compensate for the energy consumption of the AD plant [
12]. Similar plants exist around the world, and sparse proof-of-concepts can be found in the literature. Walker et al. [
13] have presented an urban micro-scale AD plant located in London (UK). The plant was built in 2013 and continues to operate to-date, processing urban food waste and generating biogas for use in a community café.
While co-AD is technically and scientifically widely reported in the literature, most of the studies focused on OFMSW treatment and management at a local site are still few in number, and results are often in conflict. The deployment of a medium-sized biogas power plant from OFMSW was promoted by Di Matteo et al. [
14]. Three urban models were identified and related with a typical OFMSW matrix for each urban area. The OFMSW types had a significant effect on the energy production, and source separated OFMSW gave the best results. Different scenarios based on AD treatment of OFMSW were assessed through a life cycle assessment by Grosso et al. [
15], and the results were compared to the real metropolitan situation, where food waste ended up mixed with the residual waste in a waste-to-energy plant. All new scenarios based on AD attained similar or better results compared with the reference one for almost all of the impact indicators. Mainardis et al. [
16] evaluated the techno-economic feasibility of AD implementation in small breweries located in northern Italy. Due to good biodegradability and high-energy content of feedstock, energy balance revealed that AD implementation could be a sustainable solution for providing most of the energy needed. From an economic point of view, the feasibility of commercial-scale AD and composting systems that received both YW and liquid AD effluent has been evaluated by Lin et al. [
17]. The results suggest that AD would be more economically favorable than composting when the plant size increases. Therefore, AD and composting may be favored for centralized and de-centralized treatment, respectively. Furthermore, micro-composting of OFMSW in residential areas was investigated by Chanakya et at. [
18]. The suitability of these substrates for micro-composting in plastic bins was evaluated by tracking the decomposition pattern and physical changes. All of the feedstock analyzed was found to have good biological methane potential, and showed promise for conversion to biogas under a mixed feed operation. Thus, AD appears to be a more suitable micro treatment option.
From the literature review, it was revealed that only a few publications have reported the analysis of an AD plant at the local scale; furthermore, no study has investigated the application of a co-AD plant in the urban districts of large cities for the management of domestic substrates (as OFMSW) as a renewable resource of energy for urban need.
Hence, the aim of this work was to evaluate the technical and economic feasibility of a co-AD plant at the local scale for energy production. For this purpose, the co-AD process was studied considering as a substrate OFMSW for Scenario 1, and a mixture of OFMSW and sewage sludge (SS) from a municipal wastewater treatment plant (MWWTP) for Scenario 2. Five different levels of district extension in terms of the number of buildings were considered. Methane yield of the mixture, energy consumption by the plant, and net energy produced were calculated through a simplified scheme. The levelized cost of waste (LCOW) index was estimated in order to evaluate the best district extension in economic terms. Each scenario result was compared with the reference scenario (Scenario 0) based on organic waste collection and composting.
4. Discussion
Starting from the W
in properties and characterization, the results from this study were in line with previous works. The W
in stream composed only of OFMSW (Scenario 1) had TS and VS values near to 39% and 92% TS, respectively, of a YW and FW mixture in a 1:3 proportion [
7]. Scenario 1 values of W
in density, BMP, and methane content, respectively, were also confirmed by other works [
12,
20,
21]. Instead, for Scenario 2, the SS BMP value was near 170 mL CH4/gVS, according to Shin et al. [
38]. In addition, the W
in properties for Scenario 2 were similar to the ones achieved by a mixture of YW, FW, and SS (in a 3:9:4 proportion), characterized by TS, VS, methane content, and BMP values of 35.9%, 82.8% TS, 64.4%, and 314.9 mL CH4/gVS, respectively [
12]. By focusing on Scenario 2, the W
in properties changed after 15 Nc as a result of the variation of SS characteristics and amounts. It is important to note that, despite the fact that the BMP assumed a lower value when Nc exceeded 15 units, the methane content remained unchanged.
From a technical point of view, the TS content of Scenario 1 was near the limit value of the ultra-dry process for AD. The introduction of SS (Scenario 2) led to a decrease in this value to a more comfortable one. As shown in
Table 8, Scenario 2 was characterized by a higher W
in stream for each district extension. Despite this condition, in Scenario 2, V was always lower than in Scenario 1, due to a higher density value. On the other hand, the lower VS and BMP values of Scenario 2 led to a minor B
out stream. Thus, this study confirmed that the introduction of SS in OFMSW co-AD leads to decreased methane production compared with mono-digestion of OFMSW single substrates [
12,
42,
46].
From an energetic point of view, the increasing in the W
in stream that characterized Scenario 2 could lead to a more major self-consumption need than Scenario 1. The electrical energy production per m
3 of the biogas achieved a maximum value of about 2.1 for both Scenario 1 and 2, according to previous works [
5,
33,
52]. Nh and N
CHP remained unchanged in both Scenario 1 and 2. The value of 200 kW per CHP unit was referred to a great capacity if compared to the typical literature value; almost 30% of Italian AD plants have a capacity between 100 and 500 kW [
51]. Thus, the co-AD plant under examination could be compared to a medium-large industrial AD plant. The introduction of a biogas storage unit could permit the adoption of a lower CHP unit. Finally, the LCOW analysis revealed better results for district extensions of 15 and 30 Nc. The choice of 15 Nc could reduce the district extension, leading to an easier independent OFMSW delivery by people. The LCOW of Scenario 0 achieved a value of 121 and 117 for the scheme of Scenario 1 and 2, respectively. This showed that the introduction of SS in the inlet waste for composting could have a positive impact from an economic point of view. For Scenarios 1 and 2, the PBT for 15 Nc was near 4, referring to an AD plant with a 500 kW capacity [
51]. A lower PBT could be obtained with a lower treatment cost of digestate [
52]. It was important to note that the economic analysis did not involve the revenues paid by cities or urban hygiene service administrators served by the plant for organic waste management. These compensations depend on the contract between plant owners and the affiliates [
56], and they are difficult to estimate. In addition, no revenues from user waste management taxes were considered. This kind of revenue could be significant in an economic analysis, and could lead to reduced costs until reaching a net gain. It is important to note that composting treatment was preferred to disposal in a landfill. Despite the fact that the inlet waste tariff of the landfill was lower than the composting one, the Italian government has introduced the “green tax” that the local authorities have to pay for waste disposal [
57]. This value could increase the cost of landfilling to a higher value than the composting one.
From an economic point of view, governments of different countries have been promoting biomass utilization projects, such as biogas to electricity systems, since the early 2000’s. However, many projects were not able to sustain themselves because of the imbalance of demand and supply and cost and benefit. Financial incentives were crucial to the economic feasibility of current AD systems that utilize biomass, especially investment tax credits and federal grants [
52]. In Italy, the use of financial incentives related to the production of electricity from renewable sources is not the highest, but has the fastest access [
51].
These cost and benefit calculations were made based on well-defined input parameters from the literature or experimental results. Therefore, changes to the input parameters, such as the type of feedstock used, may affect the analysis.
5. Conclusions
This study aimed to evaluate the technical and economic feasibility of a co-AD plant as a decentralized solution for energy production. The behavior of two different types of feedstock has been evaluated: OFMSW made of FW and YW (Scenario 1) and a mixture of OFMSW and SS (Scenario 2). Five different levels of district extensions have been considered. Technical and economic parameters have been evaluated and compared to the current scenario, based on collection and composting of organic waste (Scenario 0). The obtained results show how all of the proposed scenarios are better than the reference scenario. With the rising of the district extension (as expressed by Nc), the co-AD plant increased the input and output streams. The introduction of SS in OFMSW produced a Win stream characterized by a lower methane yield and VS content, but lower than 40% of TS. For both Scenario 1 and 2, the technical feasibility was ensured for all district cases. Despite the little amount of SS compared to the OFMSW one, changes to the AD process and waste properties were not negligible. The lack of biogas storage led to the adoption of CHP units of high-capacity values with long working time. Globally, Scenario 1 resulted in being the preferred path for organic waste. The introduction of SS in OFMSW increased costs, PBT, and the LCOW, other than generating a higher input and output waste amount and lower biogas yield. The feasibility of mono- and co-AD plants seemed to be effective in a medium district extension (about 15 condominiums) or larger (over 30 condominiums). For both 15 and 30 Nc, the economic analysis showed optimal results in terms of cumulated net costs, PBT, and the LCOW. The adoption of a mono-AD plant in a medium-scale district context was preferred. Indeed, this solution could lead the user to independently deliver the organic waste to the plant more easily, reducing the economic and environmental costs referred to the collection and transportation. Capital cost strongly affected the economic analysis, but revenue from the city for the management operation of the organic waste could significantly decrease costs. Financial incentives from governments could additionally help the growth of a local AD plant. The adoption of mono-AD plants in medium-sized urban districts of large cities could be a potentially excellent solution for OFMSW management in technical and economic terms. Further studies on the differences in the type of feedstock or environmental analysis through the life cycle assessment approach are considered necessary.