1. Introduction
The meat processing industry is a continuously developing field that includes the utilization of large quantities of natural resources, such as water and energy [
1]. As a result, the meat processing industry is accountable for severe environmental impacts (on air, water, and soil), which are constantly growing due to vast amounts of energy and water consumption, as well as waste production [
2]. Moreover, the management and treatment of the produced wastes requires further consumption of energy and raw materials that can further burden the environmental footprint of the specific industry due to the high organic content of both solid wastes and wastewater [
2]. However, the nature of the produced wastes provides a plethora of opportunities for treatment and valorization (water recycling and reuse, energy production, material recovery, etc.) [
3].
Among several methods for the treatment and valorization of meat processing waste aiming at energy and production water reuse, the ones that have been selected as the most appropriate due to their efficiency in wastewater treatment and renewable energy production via waste valorization are the following: membrane bioreactor, aeration treatment, chlorination, ultraviolet (UV) treatment, anaerobic digestion, hydrothermal carbonization (HTC), and composting. A membrane bioreactor is a state-of-the-art alternative method for wastewater treatment that couples the biological process with membrane filtration. Specifically, it consists of a bioreactor tank, in which the biomass is degraded, followed by membrane filtration for the removal of microorganisms from the treated water [
4]. Aeration treatment involves the addition of air into wastewater, thus allowing the biodegradation of organic compounds resulting in water decontamination [
5]. UV treatment is an effective method for the disinfection of treated water, in which water is exposed to ultraviolet light resulting in the disinfection of hazardous pathogens, such as bacteria and viruses [
6]. Chlorination is a reliable and efficient method used for water disinfection, which possesses the ability to efficiently oxidize a wide spectrum of organic and inorganic compounds as well as to eliminate any microbial hazards [
7]. Anaerobic digestion constitutes an anaerobic fermentation process for solid wastes (wet), in which organic matter is efficiently degraded by microorganisms and converted into biogas [
8]. Subsequently, the biogas is transferred into a biogas cogeneration (combined heat and transfer—CHP) unit and generates power (renewable) in the form of electricity and heat [
9]. One other advantage of anaerobic digestion is derived from the fact that the solid residue of the process (digestate) can be utilized in composting, further increasing the circularity of the solid wastes [
10]. Hydrothermal carbonization (HTC) involves the conversion of organic compounds through certain chemicals into structured solid fuels, which can subsequently be utilized for the generation of electricity and thermal energy [
11]. The combination of several of the aforementioned methods in the treatment of wastewater and solid wastes produced during meat processing has the potential not only to reduce the total waste of the industry, thus improving the environmental footprint of the sector, but also to reuse the recovered water and produced energy within the industry, increasing to a degree self-sufficiency of natural resources and reducing the operating cost [
12].
However, it is necessary to confirm the environmental benefits of the specific methods in comparison to the conventional existing ones. Life Cycle Assessment (LCA) is a verified tool, defined by the International Organization for Standardization (ISO 14040:2006) for assessing the environmental behavior of processes/products/services [
13]. LCA takes into consideration the inputs, outputs, and potential environmental effects of a product system across its life cycle, and can pinpoint hot points and recommend improvements in the production process aiming at environmental sustainability [
14]. LCA can be performed according to two different principal approaches, the attributional and the consequential methods. The first reports the environmental features of a current state system, while the latter, which is used in the present work, focuses on prognosticating the effect of changes in established procedures [
15]. Additionally, the life cycle impact indicators can be quantified by various methods, including ReCiPE, EDIP, and CML, which frequently exhibit different impact categories, classification of inventory, and model characterization [
14].
The primary aim of the present study was to evaluate the environmental sustainability of various treatment methods for wastewater and solid wastes utilized in meat processing industries. For this purpose, a conventional meat processing industrial line was first investigated to highlight the environmental impact of the specific sector and the necessity for efficient utilization of the wastes for energy production and wastewater purification. Subsequently, three different scenarios (Scenarios A, B, and C) for the treatment of wastewater and solid wastes were studied, the first consisting of conventional methods and the latter two of innovative ones, aiming at confirming the environmental benefits of the proposed methods for energy production and wastewater purification.
3. Results and Discussion
The environmental effects of the typical meat processing industry, along with the environmental effects of each individual process, are presented in
Figure 5.
According to the obtained results, the meat processing industry can be classified as an energy-intensive sector that produces large amounts of solid wastes and wastewater and exhibits severe environmental impact on various categories. Generally, the most energy-intensive, water-demanding, and environmentally harmful processes of the studied industry are the processing of meat after the removal of the inedible parts and the boiler, which is necessary for water heating and steam production and can be attributed to the amount of consumed electricity and fossil fuels. More specifically, based on the collected data and taking into account the assumptions and limitations that may lead to a certain level of uncertainty in the studied indices, approximately 0.141 kg CO
2 eq. and 0.001 kg 1,4-DB eq. are produced during the processing of meat per 1 kg of meat products, while freshwater consumption rises up to 0.005 m
3/kg of meat product. The obtained results are similar to those already existing in the literature regarding LCA in meat processing industries. According to a study conducted on pork production in Denmark, climate change was evaluated as equal to 0.1 kg CO
2 eq./kg of pork products [
31], while research studying poultry production indicated that 0.16 kg CO
2 eq. are emitted per 1 kg of chicken final products [
32]. Furthermore, notable environmental effects were observed for all the other studied indicators, including fossil and metal depletion (circa 0.07 kg oil eq./kg of meat product and 0.0004 kg Cu eq./kg of meat product) and marine ecotoxicity (0.001 kg 1,4-DB eq./kg of meat product). Therefore, in the context of environmental protection, sustainability, and circular economy, it is deemed necessary to incorporate appropriate methods of water purification and waste utilization for energy production within the meat processing industry to improve its environmental footprint. Based on the aforementioned, three different scenarios were selected for this work: the first hypothesizes that the meat processing industry is not directly involved in the treatment and valorization of its waste (Scenario A), while in the latter two scenarios, wastewater and solid wastes are treated on-site within the boundaries of the industry (Scenarios B and C). More specifically, in Scenario B, wastewater is treated using a membrane bioreactor and UV radiation, and solid wastes are valorized for the production of biogas, via anaerobic digestion. Whereas, in Scenario C, wastewater is subjected to aeration treatment and disinfection with sodium hypochlorite, and the valorization of solid wastes for the generation of electricity and thermal energy is achieved via HTC.
Figure 6,
Figure 7 and
Figure 8 depict the environmental effects of Scenarios A, B, and C, respectively. In
Figure 7 and
Figure 8, the total environmental effect of the slaughter plant is not included in order to highlight the effect of each method on water purification and solid waste valorization. The total values are presented in
Table 5.
Based on the attained results of the LCA study of the three distinct scenarios, it can be observed that Scenarios B and C exhibit substantially better environmental footprints compared with Scenario A. The disposal of solid waste on the soil as a landfill, as hypothesized in Scenario A, results in a sharp increase in the emissions of greenhouse gases, measured as climate change and expressed in kg CO
2 eq. Moreover, this specific method of solid waste handling leads to a further increase in metal and fossil depletion and in fine particulate matter formation. This can be attributed to a necessary possible pretreatment of the solids prior to their disposal and their subsequent treatment in the biodegradation site [
33].
On the other hand, the treatment of wastewater and solid wastes in the industry via the implementation of appropriate methods leads to an enhancement in the environmental footprint of the studied case. The efficient purification of wastewater and its safe disposal in the aquatic environment leads to a notable decrease in freshwater consumption in both Scenarios B (screening, MBR, and UV treatment) and C (screening, aeration treatment, and chlorination) [
34]. The burden on the environment observed due to anaerobic digestion, depicted in the quantity of GHG emissions and produced kg of 1,4-DB (
Figure 7a and
Figure 7b, respectively), is successfully compensated by the production of energy and heat via cogeneration (electricity credit), resulting in a positive overall sign of waste treatment in terms of sustainability and environmental safety in Scenario B [
35]. However, the generation of large amounts of thermal energy in Scenario C (approximately 1130 kJ/kg of meat products) results in a sharper decrease in the emissions of greenhouse gases compared with Scenario B, as presented in
Figure 7a and
Figure 8a and
Table 6. Finally, it must be noted that the implementation of Scenarios B and C results in negative values for various studied indices (i.e., freshwater consumption for both scenarios, human toxicity for Scenario B, and fossil depletion for Scenario C), thus further validating the positive environmental effect of wastewater treatment and waste valorization. The negative value of freshwater consumption is attributed to the disposal of cleaned water, following the UV treatment, back to the water environment, while the difference in the obtained value of this specific category is due to the transport of the derived hydrolysates, from Scenario C, to a municipal wastewater treatment plant for further treatment. In addition, the negative values of metal depletion for Scenario B, linked with the composting process, are due to the credits from the replacement of conventional fertilizers [
36]. A direct comparison of Scenarios A, B, and C is depicted in
Figure 9, and the overall reduction in the environmental footprint is presented in
Table 5. Moreover, the energy balances (gains and losses of electricity and thermal energy) in the wastewater and solid waste treatment for scenarios B and C are presented in
Table 6.
The direct comparison of the three studied scenarios highlights the enhancement in the environmental footprint of the meat processing industry in all studied categories, achieved by the utilization of novel methods aiming at wastewater purification and solid waste valorization. The utilization of innovative technologies led to a significant reduction in the amount of produced greenhouse gas, in freshwater consumption, and in metal depletion in both Scenarios B and C. Specifically, the notable decrease in greenhouse gas emissions and freshwater consumption (61.63% and 80.52%, respectively, for Scenario B and 84.22% and 49.35%, respectively, for Scenario C) is a strong indication that sustainability and preservation of the environment and the ecosystems can be achieved via waste utilization and adaptation of innovative and environment-friendly treatment methods. Finally, it must be noted that the valorization of solid wastes and the treatment of wastewater in Scenario B results in a surplus in the balance of electrical energy, due to the cogeneration of biogas, and a deficit in the balance of thermal energy, which can be attributed to the large amounts of thermal energy required in anaerobic digestion. On the other hand, in Scenario C, a surplus of thermal energy is attained due to the burning of pelletized fuels, while the deficit in energy (1.73 kJ) can be considered negligible.
4. Conclusions
Three different scenarios of wastewater and solid waste treatment produced during meat processing were studied in order to evaluate their environmental effects, via LCA analysis. The first scenario consisted of conventional waste treatment techniques, with the solid wastes being disposed of on a landfill and the wastewater transferred to a municipal wastewater treatment plant, while in the latter two, waste treatment technologies aiming at energy production and wastewater purification were used within the industry. In general, the incorporation of waste treatment technologies leads to the generation of substantial quantities of energy and a significant improvement in environmental footprint. Among the studied technologies in Scenario B, anaerobic digestion exhibited the best environmental performance due to the produced electricity and heat during CHP, while the burning of the obtained pelletized fuels in Scenario C resulted in the generation of large amounts of thermal energy. Despite the fact that thermal energy is necessary for the heating of biomass during the anaerobic digestion, this energy is considerably decreased due to the utilization of the thermal energy produced in the CHP, and the observed deficit in electricity in Scenario C is negligible, thus both studied scenarios can be efficiently applied. Furthermore, the purified water from Scenarios B and C is environmentally safe and of high quality, and thus, it can be either reused reducing further the footprint of the industry, or used for other purposes, including aquatic discharge or agricultural purposes. Results derived from the present work suggest that the proposed technologies could be used for moving toward sustainable meat production. Finally, the approach proposed in this work can be broadly extended to numerous other food systems to analyze their environmental footprints, highlight the main areas that require significant improvement, and consequently propose appropriate methodologies for energy production via solid waste valorization and wastewater treatment.