Nutrients Cycle within Swine Production: Generation, Characteristics, Treatment and Revaluation

The swine production generates slurries nutrients rich, which could be revaluated in cereal crops used for its food and energy generation (biogas) for use on the farm. However, the revaluation requires to know their physical–chemical and biological characteristics, which allow giving an adequate transformation (treatment). On the one hand, swine production and consumption market reveal the superiority of emergent countries on meat/cereal (feed) production and swine meat consumption (concentrated population). The food composition and growth phase will influence the swine slurries composition, which is rich in organic matter, macronutrients (N, P) and micronutrients (Cu and Zn). These characteristics will generate odors (organic matter, macronutrients) and ecotoxicology effects (macro/ micronutrients) if they are not treated. Moreover, the swine slurries treatment allows revaluated them in agriculture and obtaining energy. Anaerobic technologies (anaerobic lagoon, mixed complete reactors, UASB, among others) are the most used/cost-effective to organic matter removal from swine slurries, obtaining from 0.28 to 0.83 m3 biogas/kg organic matter. Meanwhile, passive technologies (constructed wetlands) are the most used technologies to nutrients and metals removal. Treated swine slurries from constructed wetlands have agronomic properties. Therefore, the nutrients cycle within swine production would favor concepts of revaluation in origin.


Introduction
The economic and demographic growths are the factors that are activating the current world meat production [1]. Thus, developing countries where 75% of the population is located also concentrate 52% (41.5 million ton/year) of swine production. Moreover, in the last 40 years the growth of emergent economies are concentrating both meat and cereal production in one place. China is an emergent economy in where is 46% of the world's swine production, and 30% of cereal production.
On the other hand, demand is the main trigger factor of the current "livestock revolution" [2]. Thus, the meat consumption acquires connotation different depending on the consumer type. In developed countries, higher purchasing power and demanding lifestyles have promoted a higher swine meat demand (25 kg/per capita year). Meanwhile, developing countries consume only 8 kg/per capita year, condition related mainly with subsistence habits. However, developing countries (75% worldwide population) concentrates 64% (43.3 million ton/year) of the swine meat total consumption; while that, developed countries concentrate 36% (34.4 million ton/year) remaining. However, emergency economies (Brazil and China) are increasing about 21% the swine meat consumption per capita of their population, concentrating swine meat consumption in these countries [3].

Swine slurry: generation and characteristics 2.1 Origin of swine slurry
The swine slurry generation is related to the growth phase and its water/food requirements. Thus, lactation and reproduction phases have a great water supply (12.2-41.1 L water/animal d), mainly related to hydration improvement or fertility [4,5]. Meanwhile, fattening and weaning have a greater food intake (1.9 kg food/ animal d), exclusively given by increased weight or age [5,6].
The swine diet is based on proteins, carbohydrates and starch concentrates, which exceed 50% of food total composition [7]. Indeed, fattening tends to prioritize the protein intake (>30%) [6]; while, reproduction and weaning consume mainly fiber (10%) to avoid overweight [8]. In both cases, the low food digestibility could generate nitrogen excretion (<30%) and phosphorus (<10%) [9]. This factor can be observed in the swine conversion index, which can vary from 2.0 to 4.3 and 0.9 to 2.3 kg feed/kg weight gained for fattening and previous phases, respectively [6,[10][11][12]. Indeed, digestibility can vary depending on the food and animal type, being slightly lowers in the final growth phases (fattening ~ 25%) than initial phases (weaning >50%).
Finally, the water/food ratios allow indirectly evaluating the swine slurry quantity and its composition. Thereby, initial phases (weaning) generate the water/feed ratios from 4.4 to 5.1 L water/kg food [8]; while, final phases (fattening) reaches values from 1.6 to 2.5 L water/kg food [11]. This indicator is indirectly related with nitrogen concentrations from urine/feces values. Indeed, greater urine/feces values are obtained by fattening (6.6-9.1) than weaning (5.5-6.3) [13]. At this point, it's important to note that within the swine excreta around 75% of the nitrogen is in urine, so a higher water intake will increase the nitrogen generation in the slurries [14] ( Table 1).

Generation and physicochemical characteristics
The physicochemical composition from swine slurries will be influenced by the low digestibility of nitrogen (<33%), phosphorus (<32%) and micronutrients (<3%) [9,17], as well as the animal growth phase [18]. Initial phases (maternity and lactation) produce more slurries (10.0-41.1 L/animal d) than weaning-fattening (3.5-9.10 L/animal d) [4][5][6]. This could be related with dilution by more water intake. Table 2 summarizes the physicochemical characteristics from swine slurries according to the growth phase. The main differences are related to a higher content of organic matter (1.8-2.4 times), nitrogen (1.3-2.8 times) and phosphorus (2.1-4.6 times) from fattening with respect to maternity and weaning. This condition is related to a higher food intake (1.7 times), which is made up of carbohydrates and proteins [14]. Meanwhile, micronutrients, such as zinc are excreted (2.7-13.0 times) in the initial phases (weaning). In a different proportion, copper is excreted by swine slurries, increasing during fattening (1.5-7.2 times). Differences in the presence of copper and zinc excreted is related to the use of both metals as growth promoters and specifically copper that is used additionally for therapeutic purposes (fighting diarrhea) in the most vulnerable swine population (weaning) [17]. Chloride, ammonium and potassium salts may also be present within swine slurries. The feeding has influenced a greater salts excretion (1.4-2.3 times) during weaningfattening. These characteristics are related to the fact that the salts in the swine feeding, favor the liquids retention increasing weight during the growth [18].

Olfatometric characteristics
The odoriferous characteristics within swine slurries are evaluated using analytical methods (compounds) and sensory methods (odor) [46]. Olfactometric characteristics measured as odor concentration (OC/m 3 ) are influenced by the type/time of slurries storage. [47]. Indeed, fattening phase (2.1-5.6 times) has more odor than maternity/weaning. On the other hand, fresh swine slurries (1.5-5.4 times) generate more odor than stored slurries (2 months). These results have also shown the odor relationship with volatile compounds presence, which from by diet or slurries management. Thereby, sulfur, ammonia, phenolic and volatile fatty acid compounds from swine slurries influence the odor generation, which show different correlation degree (R 2 = 0.66-0.89) with the generated odor [46]. Particularly, odor precursor compounds from the swine production will be given by the protein's presence from food, generating sulfurous, indole, phenolic and long-chain fatty acids [47][48][49]. Meanwhile, carbohydrates generate short-chain or volatile fatty acids (less than 6 carbons) [40]. Thus, biological and chemical conditions determine the odor compounds formation. It has been possible to establish the presence of some autochthonous microorganisms (Streptococcus, Peptostreptococcus, Eubacterium, Lactobacilli, Escherichia, Clostridium, Propionibacterium, Bacteroides, Megasphaera, among others). Incomplete anaerobic digestion from these microorganisms produce and/or reduce macromolecules to odor compounds, so intermediates (e.g. volatile fatty acids) and finals (e.g. sulfides) [50,51]. However, not only microorganisms can condition the odorant compounds formation, but also environmental conditions (pH and temperature), which active biological and chemical (e.g. ammonium/ammonia) processes [52,53]. Temperature affects the microbial growth rate; while, pH influences the buffer capacity, favoring volatile fatty acids generation [54] (Table 5).

Slurries reduction
Swine has low digestibility (<30%) of nutrients and micronutrients, being necessary mechanisms of digestibility improvement, which could improve the physicochemical characteristics from slurries (feces + urine) [9]. Indeed, about 78% of N and P from swine food (proteins) is not assimilated, excreting concentrated urine and feces [14]. Therefore, farm managements are focused on the improvement of diet type and food quantity during each phase growth. On the one hand, raw protein is substituted by fiber, reducing until 8% (10 g RP/kg food) of nitrogen in the urine [7]. Other strategies are related to vary crude protein concentration (155, 145 and 135 g RP/kg) in the food, achieving the decrease of NH 4 + (20.3-28.4%) in the excreta [13]. Studies have evaluated the replacement of crude protein by digestible or ileal amino acids (lysine, threonine, methionine, tryptophan, isoleucine and valine), finding that they can reduce the ammonium excretion in the urine from 40 to 50% [69]. Meanwhile, other techniques use feeding multi-phases, which improve the protein digestibility, reducing between 20 and 42% the nitrogen excretion [59,60]. On the other hand, introducing phytases in the swine diet, it is possible to reduce 18% of phosphorus in the feces. Metals (Cu, Zn) used as growth promoters have been decreased (100-250 ppm Cu, 2000-3000 ppm Zn) by antibiotics (3-220 g/ ton food) [61,62]. Indeed, sulfonamides, tetracyclines and β-lactams increase the index conversion rate between 3 and 4%, improving the protein assimilation [62]. However, antibiotics also are excreted up to 10%, not being a good strategy because they are emerging contaminants [63]. The implementation of efficient water drinking reduces the slurry generation. The dozers incorporation and excreta handling techniques (e.g. hot beds) could reduce the floor washing, reusing waste organics (rice husk, straw) [64]. These strategies have reached reduce water requirements from 5 to 80% [4].

Treatment and re-evaluation of by-products
The swine slurry treatment is the most used tool management within intensive farms by environmental pressures (legislation), which regulates its discharge on water bodies or soil revaluation. The slurries management requires a balance between the environmental/social and economic requirements in the farms. Ideally, this management starts with the excreta fractionation (slurry = urine + feces/feces) by physical/chemical separation. Some techniques, such as: polymers, filter press, flotation, sedimentation, screw press, among others are used to remove sedimentable/suspended material, reducing mainly organic matter (62-84%) and phosphorus (70-89%) [65]. The solid fraction corresponding to non-mineralized organic matter is subjected to composting (aerobic/anaerobic), which stabilizes giving it agronomic properties (C/N < 20). The liquid fraction (slurries) with a C/N ratio about 10 is subjected to biological (aerobic/anaerobic) removal processes of organic matter, nutrients and other microcontaminants (metals, emergent) [66]. Several technologies are grouped within the aerobic biological processes (aerated lagoons, activated sludge, among others) and anaerobic (anaerobic lagoons, fixed bed reactors, SBR or Sequencing Batch Reactor, UASB or Upflow Anaerobic Sludge Blanket, among others) [67]. Thus, the removal of dissolved and colloidal organic matter is usually carried out by anaerobic lagoons (Environmentally Superior Technologies), which reduce 50% of organic matter. Meanwhile, anaerobic reactors (Manure-based biogas plants) remove more than 80% of organic matter. The by-products obtained from this stage are usually stabilized effluents (C/N < 10) used as soil stabilizer and biogas [65,68]. In this last point, specific temperature conditions (psychrophilic, mesophilic and thermophilic) have allowed the biogas (60-70% CH 4 , 40-30% CO 2 ) production between 0.03 (anaerobic lagoons) and 650 (anaerobic reactors) m 3 /d. The main biogas uses are related with thermal energy (0.02-390 m 3 gas) and/or electrical (0.07-1560 kWh) within farms [65,69]. Additionally, anaerobic treatment under optimum conditions (35°C) reduces odors (1.9 units depending on the hedonic tone) [52]. This anaerobically treated effluent can be subjected to biological treatment (nitrification/ denitrification, SBR or constructed wetlands) [70][71][72] or physical-chemical (stripping, vacuum evaporation, precipitation) [71,73,74] to nutrients removal (nitrogen and phosphorus). The nitrogen removal efficiencies vary from 40% (constructed wetlands) to 97% (denitrification-denitrification) and 100% (stripping) [71,73]. Meanwhile, phosphorus is removed between 44% (constructed wetlands) and 80% (chemical precipitation) [71,[75][76][77]. In very few cases, have been reported metals removal (Cu, Zn), mainly due to their low concentrations (<1 g/L) [26]. However, the metals removal has allowed to obtain removal efficiencies between 75% (precipitation) and 92% (constructed wetlands) [78,79]. The by-products obtained in this stage can vary from crystallized ammonium salts for agronomic use [73] to treated effluents with a C/N ratio <5 usable in irrigation [71].
The irrigation (slurries) or soil stabilization (solid) are the most used re-valuation techniques. The treated swine slurries have nutritional value (N:P:K: 1:0.6:0.4-1:0.3:1) to be used in cereals irrigation for swine consumption (1.2:0.2:1 a 1.6:0.3:1) [23,80]. Under optimal irrigation conditions (150-200 kg N/ha year) some soil characteristics with agronomic importance (organic matter content and moisture retention) could be improved [81,82]. The slurries re-valorization in irrigation has decreased the chemical fertilizers use, being in some countries (New Zealand) valued economically (21 million USD/year) [82]. However, the livestock production intensification vs. land availability (Europe) has carried out to optimize the nutrients recovery. Thus, it is necessary to consider within slurries the balance of macro (N:P: K) and micronutrients (metals), as well as other contaminants (pathogens, emergent). Moreover, this balance must consider soil nutritional requirements and crop type, according to the international revaluation legislation [83,84]. These tools would allow the cadasters establishment, which could facilitate the communication between nutritional requirements sites (agricultural soils) and nutrients-generating sites (animal farms) [85]. Preliminary Chilean studies have reported opportunities of livestock slurries revaluation in agriculture relating nutrients recovery sites with adequate agriculture sites [86].
The biogas production from anaerobic digestion of swine slurries is another revaluation alternative. In Europe, technologies based on biogas generation have been favored by state subsidies (10-30% investment cost) at farms level. These initiatives have allowed the building the more than 5000 anaerobic digestion projects [65,87]. Swine slurries have potential capacity of methane generation between 0.25 and 0.30 m 3 CH 4 /kg VS [88]. Meanwhile, co-digestion with lignocellulosic materials (e.g. crop waste) could increase more than 16% produced methane [89]. Thus, centralized plants of anaerobic co-digestion are the current trend within European agricultural sector. Successful experiences have been reported in Germany and Denmark, where more than 32 plants have been built with a capacity between 16,000 and 200,000 tons/year of waste and with a production between 0.7 and 5.7 million m 3 biogas/year [90,91]. Preliminary Chilean studies have allowed to establish the potential generation of bioenergy (biogas) from anaerobic co-digestion (livestock/crop wastes) at regional level [92]. Figure 1 describes excretes management in the swine production.

Organic matter removal
Anaerobic treatment technologies are widely used in this field, because they allow to remove organic matter, pathogens, odors, generating by-products (biogas, bio-fertilizer) [93]. There several anaerobic technologies, such as: biodigesters, anaerobic lagoon, complete mixing anaerobic reactors, anaerobic filters, UASB or Upflow Anaerobic Sludge Blanket, among other [94]. Worldwide, anaerobic technology based on conventional anaerobic mesophilic reactors (4935 plants) and thermophilic reactors (321 plants) treat around 49 million ton/year slurries, which have been built in Europe, mainly in Germany (more than 70%) [87]. State subsides, innovation technologic and centralization of biogas plants using agricultural wastes (livestock, crop) have favored their implementation [90,91]. Meanwhile, more than 7000 technologies based on anaerobic lagoon have been built, but with only 60% operationally actives in the United States [63]. The conventional anaerobic reactors (complete mixture, piston flow) have been limited by its costs, remaining in disuse or only in building project around 77% [94]. However, currently these technologies are being replaced by more efficient technologies in terms of removal of nutrients, odors and pathogens [68,71]. Technologies such as tubular biodigesters are the most used in countries where the livestock production in non-intensive.
The anaerobic lagoons are extensive and conventional typology, where organic matter is biodegraded without hydraulic or thermal control (environmental conditions) [95]. In the livestock sector, these systems offer some advantages related to their storage capacity and operation easy [96]. However, they can also cause odors, requiring spaces far spaces from neighboring population [94,96]. Anaerobic lagoons obtain organic matter removal efficiencies between 26 and 79%, working under environmental conditions (5-32°C) and with residence times from 90 to 232 days [97,98]. The lagoon design must consider thermal vertical stratification, suggesting depths between 0.8 and 4.0 m to maintain facultative conditions upper and anaerobic zone bottom [69,99]. Its longer residence time also favors the macro (organic matter, nutrients) and micro (metals) nutrients precipitation. Thus, has been observed that anaerobic lagoon accumulates more than 50% COD, TN, P and Cu in the bottom, decreasing pH from 6.5-7.2 throughout its depth (>1.5 m) [99]. Another advantage is related with their disinfection capacity, due to its prolonged time exposure to solar radiation, causing cell lysis of pathogenic microorganisms [100]. However, this time exposure generates greenhouse gas emission (0.02-0.5 m 3 biogas/m 2 d) [69] and odors (168-262 OC/m 3 , 101 μg NH 3 /m 2 s, 5.7 μg H 2 S/m 2 s) [101]. Operational improvements mainly related to the emission of gases from anaerobic lagoons have been made covering them.
The conventional full mix reactors or CSRT (Continuous Stirred Reactor Tank) are controlled systems, where the hydraulic retention time (residence time) is equal to the cell retention time [102]. The complete mixture is achieved through the recirculation from 25 to 40% of biogas generated [103]. It can operate under psychrophilic, mesophilic, and thermophilic conditions, which are carried out in two stages (acidogenic and methanogenic reactors) [93,103]. This technology has been widely used mainly in Europe, obtaining organic matter removal efficiencies from 25 to 74%, but with organic loading between 5 and 40 times greater than anaerobic lagoon [87,93]. CSRT reactor have been improved with the recirculation [93].
Other conventional technologies as anaerobic filters or AF generate biofilm around of the material support surface, while the flow goes up throughout the filter [102]. Inert (nylon meshes, polyurethane foams, polypropylene rings) and organic (blocks, wood chips) support material have been used [93]. The main advantage of this technology is that can operate at organic loading between 69 and 142 times greater than anaerobic lagoons; but have clogging problems [93].
The most advance technology has been developed to improve operational problems of conventional technologies. On the one hand, AFBR systems (Anaerobic Fluidized Bed Reactor) are technologies studied mainly at laboratory scale. They use support material (clay, wood and PVC), which is suspended due to the recirculation of the flow [102]. These characteristics partially avoid clogging [103]. On the other hand, UASB technology has been applied at the laboratory and pilot level. These treatment units generate biomass granulated, which sediments (4 m/h) improving the cellular retention time [102]. The flow goes up, favoring the washing of biomass non-granulated the granules are dense, harboring multi-species and diverse microbial communities. However, the granulation processes require longer periods of formation (2-8 months). Other innovative technologies from UASB systems are the EGSB (Expanded Granular Sludge Bed). This last technology is hydraulically improved respect to UASB, because it operates at greater flow velocity (>4 m/h) than UASB [104]. In general, UASB reactors are a viable alternative, since they are considered high load systems, operating at organic loadings between 2 and 162 times higher than conventional systems (anaerobic lagoons, CSRT). The organic matter removal efficiencies reach ranges between 19 and 86% [93,94,103,105]. In addition, they offer other operational advantages related to their volume (0.006-0.5 times less volume than lagoons and CSRT) and sludge production (granular from UASB vs. suspended from lagoons/CSRT) [104]. Moreover, UASB generates higher biogas production (0.28-4.05 m 3 /m 3 d) than conventional systems (0.02-1.69 m 3 /m 3 d), thanks to the fact that they operate at higher organic loading (1-8.1 kg COD/m 3 d) [69,93]. Table 6 and Figure 2 describe the operational characteristics of anaerobic technologies applied on swine slurries.

Nutrients and metals removal
Constructed wetlands are used as a cost-effective alternative for the nutrients removal within the livestock sector [70]. In Europe, there are around 60 livestock farms, which treat 17,000 tons/year using constructed wetlands [87]. In the United States, about 33% (~ 70 farms) of constructed wetlands are used within livestock sector, being mainly (~ 83%) surface flow constructed wetlands (SF-CW) [70].
Operationally, there is experience in the use of different types of constructed wetlands within swine sector. However, SF-CW are the most used technology, mainly to avoid clogging [106]. Generally, SF-CW are used after the anaerobic lagoon, operating at nutrient loading between 5 and 36 kg N/ha and between 1 and 6 kg P/ha. The nitrogen and phosphorous removal efficiencies obtained reach values from 50 and 90 to and 25-66%, respectively [106][107][108][109]. Moreover, horizontal subsurface flow constructed wetlands HSS-CW have been studied at laboratory and pilot scales, operating at nutrient loading between 69 and 252 kg N/ha d and      between 15 and 47 kg P/ha d. The nitrogen and phosphorus removal efficiencies vary from 10 to 24 and 47 to 59%, respectively [110,111]. There are also experiences hybrid systems (SF/HSS/VSS) operating with plant species emergent and floating, which have achieved nitrogen and phosphorus removal efficiencies higher than 50 and 42%, respectively [112][113][114]. In general, constructed wetland systems will be operationally work with any previous technology. Thus, activated sludge has been used prior to constructed wetland increasing from 2 to 20 times the nutrients removal than anaerobic lagoons [109,113]. The metals removal efficiencies have been reported in SF-CW, which have operated at rates from 0.09 to 0.25 kg Cu/ha and 0.58 to 1.58 kg Zn/ha d, obtaining removal efficiencies of up to 83 and 92%, respectively [78,79,111]. Currently, there are some innovations related to the constructed wetland treatment [106]. Likewise, the partial recirculation of pre-nitrified slurry has allowed to increase the nitrogen removal via denitrification up to 4 times, decreasing the ammonium volatilization [112,115]. Other design concepts are based on the use of "marsh-pond-marsh" [109]. Constructed wetland technologies, could be improved operationally using intermittent hydraulic rate, which favors the oxygenation improving the nitrification [116]. Table 7 details the constructed wetlands operational characteristics of constructed wetlands used in the pig sector.

Future perspective and conclusions
Currently, the swine production should be looking to set the "new zoo technical order" with improvements in the life quality of the animals. Some reasons are given by environmental and health concerns given by the presence of emerging pollutants in meat and animal excreta. Indeed, swine meat has been reported as one of the sources of staphylococcus microbial resistance in humans [117]. Moreover, studies evidence the consumption of about 63,000 ton/year antibiotics in the livestock production (veterinary/promoters), being the main source of emerging pollutants in swine excreta/slurry [118]. Both water bodies and soil can be affected when these wastes are discharged or revaluated, since current treatment technologies are not designed to remove them. These two factors are further enhanced by the greenhouse gasses emissions responsibility from livestock production. Thus, swine production generates about 24 kg CO 2 eq/kg protein, which is mainly attributed to the mismanagement of their excreta/slurries [119]. In this last aspect, it is that the closing of the cycle the generation, treatment and revaluation of swine excreta fulfills a fundamental role. Studies report that reductions of up to 30% in greenhouse emissions could be achieved by a comprehensive management of resources (slurry, excreta, crops remains) between livestock and crop production [120]. Thus, livestock production through appropriate technology and management practices can be a source of nutrients for crops that provide food to animals. Agricultural production would support the energy generation inside farms by anaerobic. In the future, the livestock production could be supported from integral improvement from animal production to treatment and revaluation of wastes.