1. Introduction
Circular economy is a new development strategy aimed at the environment protection, pollution prevention, and sustainable development [
1]. It is based on a conception that is either restorative or regenerative by intention and design, and whose only objective is to maximize resource efficiency and minimize waste production within the framework of economic and social sustainability [
2,
3,
4].
In the circular economy model, waste is turned into resources, which are also called technological nutrients [
5,
6] and reintroduced into the production processes [
7,
8]. In this time of great transformations, the objective is to go from lineal economy, where society is based on waste production, consumption, and disposal, to the new model provided by circular economy, which is focused on the principles of the “3 Rs”: reduction, reutilization, and recycling of as much waste as possible. Such waste is derived from the production and consumption processes, implementing processes with circular flows of matter and energy where the utilization of raw material and energy is carried out in numerous stages [
9,
10]. Consequently, under this concept of circular economy, the consumption of matter and energy, as well as the environmental degradation are minimized without limiting social or economic growth and technological progress [
11].
In this regard, the water industry, and more specifically the wastewater one, has been quite revolutionary in its approach towards the circular economy model due to the importance of water in relation to human life and the energy and matter it contains [
12,
13]. In this way, wastewater is not considered a waste but a valuable non-conventional resource, since it contains water and nutrients, such as nitrogen, phosphorus, and organic matter [
14]. The recovery of wastewater components for their reuse has economic and environmental benefits [
15].
One of the most important challenges that the circular economy has to develop, which is also indicated by European Union, is the necessity of designing indicators that allow for assessing the advance obtained regarding the efficiency in terms of reduction, reutilization, and recycling of waste generated in the linear economy model. The indicators of circular economy are useful in order to determine the degree of approximation of any specific process to the circular economy model.
The impact of pig manure on the environment is one of the main challenges faced by the pig farming industry. More specifically, pig population in Spain rose to 28.3 million head in 2015. Consequently, Spain became the country with the highest pig population in the European Union for the first time, with an average pig manure production of 7 L head
−1 day
−1, and a pig manure generation of 200,000 m
3 day
−1 [
16]. It can make us understand the magnitude of the problem. The high amount of pig manure coming from the pig industry complicates the management of this type of effluents, leading to a serious impact on the environment, since it pollutes the soil, the water, and the air. It also poses a risk for the human being and the local wildlife [
17]. Thus, sustainable development plays an important role in pig farming industry [
18].
Denmark, France, Germany, Holland, Poland, and Spain represent more than two-thirds of the total pig production in Europe (Eurostat, December 2008 survey), which directly implies high volumes of pig manure generated as a result [
19]. With proper management, pig manure can be used as fertilizer in order to provide nutrients to crops and improve soil properties by means of the addition of organic matter [
20].
Regarding air pollution, methane and carbon dioxide are emitted, both being greenhouse gases contributing to global warming. In addition, foul odors are produced and there is an excessive emission of ammonia into the atmosphere by means of losses through volatilization [
21,
22].
In addition to an excess of nitrogen, it is also possible to find an excess of assimilable forms of phosphorus and potassium, accumulation of heavy metals (Cu and Zn), and salinization [
19] in the soil.
Pig manure also pollutes surface and underground waters, as its content in nitrates and phosphates can contribute to some phenomena, such as water eutrophication, which causes a decline in its quality [
19,
23,
24,
25].
Regarding hygienic-sanitary risks, gas emissions (sulfhydric, mercaptans, ammonia...) and the presence of pathogens could cause significant impacts on human and animal health due to direct toxicity and smelling aggression [
19,
26,
27,
28,
29].
Consequently, it is necessary to develop and introduce systems of pig manure treatment before its spillage [
30]. In this regard, anaerobic digestion stands as an appropriate process that has been widely used in pig manure treatment. This process is carried out by means of a biochemical process comprising of several stages and different types of microorganisms [
31]. During such treatment, the complex organic matter contained in pig manure is hydrolyzed, fermented, and reduced to methane and carbon dioxide [
32,
33]. It is necessary to control different factors in this process, such as the influent characterization, microbial biomass, temperature, presence of inhibitors, mixing conditions, pH levels, and trace elements [
34]. Thus, it should be pointed out its ability to produce renewable energy sources, such as biogas, with the subsequent reduction in the emissions of organic pollutants and greenhouse gases (GHG) [
35]. In light of this, the obtaining of biogas contributes to the achievement of sustainable energy systems [
6,
36]. Moreover, biogas plays an important role in the decarbonization of the energy sector in Europe, which promotes that European Biogas Association (EBA) is developing a platform to register the biogas at national level, and to remove the boundary barriers, thus encouraging the domestic market.
In this regard, the principles of Life Cycle Assessment (LCA) methodology have been followed in the literature as a basis for the environmental assessment of the impact caused by different pork production systems in Europe and outside its boundaries [
37,
38]. All of the environmental analyses aim at assessing GHG emissions in order to reduce the climate change related impacts as one of the main requirements for sustainable production [
37,
39,
40]. In this sense, there is a growing need to meet the European Union Climate Action targets, which aim to reduce GHG emissions to 40% of the 1990 levels by 2030 [
41]. In addition to this, the environmental evaluation within the framework of this research area also includes carbon and water footprints, which are selected as indicators of primary interest for companies in the sector [
37,
42].
Therefore, this study is focused on the development and evaluation of indicators of circular economy as applied to pig manure management from an environmental point of view, and as a first case of implementation in pig production, a sector of particular importance in Spain. This research could supply information of interest to companies in the sector that contributes to the development of LCA methodology and carbon and water footprints.
4. Conclusions
The present article provides the first indicators for circular economy efficiency, as well as tools for technological nutrient performance for the pig farming industry, allowing the quantification of the degree of approximation to a circular economy model.
This study explains that pig manure can be considered as a technological nutrient that can also be reintroduced into the production system, enabling the recovery of those resources present during such processes, such as water, nutrients, and energy, from a treatment process whose central nucleus is anaerobic digestion. Such resources are used in the process itself, as in the case of water, reducing water consumption by 47.01%, biogas, reducing gas natural consumption by 5.33%, and some other products resulting directly from this type of industrial activity, as in the case of the biofertilizer. In addition, the treatment process allows the recovery of 0.97 m3 water m−3 pig manure, 49.40 kg biofertilizer m−3 pig manure, and 5.33 m3 biogas m−3 pig manure. In this way, the proposed indicators can be taken into account by the production plant managers in order to improve the efficiency of the use of resources and the minimization of waste generation.
Consequently, this research provides the pig farming industry with new ideas and measuring tools so that it becomes a reference in sustainable production and contributes to reduce the negative polluting effects, in environmental terms, of pig manure generation. This article provides relevant details so that pig industry can develop strategies for pig manure treatment in compliance with the new requirements of the European Union in relation to waste generation in 2020 and renewable energy consumption in 2030 (27%), and, in this way, being able to comply with this regulation, which highlights the importance of the circular economy. In addition, it is also a way of incorporating innovations in its processes to make it more sustainable from the environmental point of view, contributing to a higher effectiveness in terms of a decrease of negative externalities.