Introduction

Brazil is the fourth higher producer and exporter of swine meat, with a herd of 41.1 million pig heads and swine meat production equal to 3.75 million tons [1]. Actually, one of the difficulties faced by swine producers is finding the proper destination of waste generated in large volumes throughout swine production; approximately 75 million cubic meter of swine waste were generated per year, considering the pig Brazilian herd aforementioned. This waste presents high contaminant load, thus, the environmental impacts from an improper destination and agricultural swine waste use have been discussed due to the high organic load and pollutants present that may contaminate soil, surface water, groundwater, and agricultural product.

The swine waste is from hygienic-sanitary management adopted for most pig farms in Brazil, which perform pig stall cleaning with a pressure washer as a sanitary measure to control vectors that lay eggs in organic material remaining on the farm. Its physico-chemical, chemical, and microbiological characteristics depends on the animal life cycle, type of feed provided to animals, and pig stall cleaning type [2,3,4]. The improper disposal of agricultural wastes has been discussed in the Brazilian Environmental Director Plan, to propose solutions for sustainable agricultural development [5].

The raw swine waste agricultural use can cause contamination of soil–water–air–plant systems [6]. The high contents of ammonium nitrogen could be lost to the atmosphere by ammonia volatilization [7,8,9], in addition to the leaching of nitrate nitrogen from the nitrification process [10, 11]. Furthermore, other contamination can occur due to pathogenic organism presence as enteric bacteria [12], leaching, or accumulation of sodium, potassium, copper, and zinc [13,14,15,16]. Thus, there has been searching for treatment systems able to reduce the swine effluent pollutant potential, aiming sustainable agricultural use.

Swine have a monogastric digestion system and low digestion of raw materials, such as soy and maize grains that are used for pig feeding. Thus, due to this low feed conversion, swine excreta present high contents of nutrients and/or contaminants that are provided to animals in rations. The zinc can be present in swine rations in concentrations equal to 3000 mg kg−1 [17], which is used to control diarrhea. The copper can be present in concentrations equal to 250 mg kg−1 and is used as animal growth promoter [17, 18].

The sodium is supplied to increase the rations palatability and water retention in animals prior to slaughter [19]. Lima et al. [20] found discrepant values between the Brazilian and American swine nutrition tables used for nutrients recommendation in swine feed. In accordance with these authors, the sodium amount recommended by Brazilian nutritional guide is 50% higher than American recommendation.

Swine waste treatments systems should be established in accordance with waste characteristics, considering the management and disposal impact of swine waste [21]. Mainly due to the high organic load and contaminant present in high levels, the liquid swine waste should be treated by treatment systems with different phases. Initially, the coarse and suspended materials are removed by sieve or sedimentation tanks. Afterwards, the effluent organic loads are reduced by systems, such as anaerobic digester, remaining soluble organic material, nitrate, phosphorus, copper, zinc, and pathogenic organisms. The remaining organic load and contaminants should be removed by aerobic and physico-chemical systems.

In accordance to Bertoncini [22], medium and large Brazilian swine farmers have used the anaerobic digesters. However, anaerobic digestion has been conducted improperly, resulting in a low treatment efficiency due to absence of a previous solids removing. These wastes present suspended solids around of 6%, which need be removed for increase the anaerobic digestion process efficiency and digester longevity, since recalcitrant solids presence can contribute for reduction in microorganisms activity and, in turn, reducing the treatment process efficiency. Furthermore, this amount of suspended solids could decant together at to anaerobic sludge at the digester floor, reducing the storage capacity of the anaerobic digester that will need frequent maintenance for cleaning and, thus, the anaerobic biodigester could present a lower longevity.

These facts represent an increase in the operating cost of the treatment system, due to its lower treatment efficiency to reduce the load of pollutants present in waste, when compared to systems with previous removal of suspended solids, in addition to the costs of a greater number of operations for the system maintenance. It is emphasized that the suspended solids removed previously may be sent to composting, generating organic fertilizer and representing a source of income to the producer. Through the anaerobic digestion process, microorganisms degrade the organic material in the absence of oxygen, generating methane, carbon dioxide, and other products from reactions involved in this process. Several microbial types are involved in this process that present four phases: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [23]. The knowledge of specificity of each phase is fundamental to improving the organic load and contaminant removal; for this, some factors should be considered, such as temperature, pH value, waste characteristics, hydraulic retention time, and presence of components that may be toxic to microorganisms involved in the process, among other factors [24].

Despite the higher pollutant potential of swine waste, the agricultural use of treated swine effluents can represent a plant nutrient source [25, 26] and organic matter, causing increases in productivity and quality of harvested product [27], as well as improvements in soil fertility, promoting of rational water use, and economy in chemical fertilizers. Additionally, this waste can represent the only nutrient source available in little farms.

In Brazil, there are no defined legislations about the agricultural use of swine effluents, as well as no contaminant limits or criteria for its agricultural use. The Resolution nº 54 of the National Council of Water Resources [28] encourages the lower quality water reuse; however, contaminant limits present in these wastewaters, and agricultural use criteria are not yet established by this legislation. In São Paulo State, there are legislations specifically for agricultural use of vinasse [29], effluents from citrus industry [30], and sanitary effluents [31]. Limiting factors for agricultural use, such as sodium, nitrate and heavy metal contents, pathogens, and electrical conductivity values, are established for these legislations of São Paulo State.

Thus, this study's aim was to evaluate the potential pollutant reduction of raw swine waste through a pilot treatment system composed of a sieve and anaerobic digester, aiming for the agricultural use potential of the treated swine effluents to be in accordance with current environmental and agricultural legislations.

Materials and methods

The study was carried out in a Research and Development Unit of the Secretariat of Agriculture and Food Supply of São Paulo State, in the municipality of Piracicaba, São Paulo, Brazil (22º 43′ 31″ S, 47º 38′ 57″ W; altitude of 547 m). This unit contains an experimental pig farm with capacity for 180 animals in finishing phase, with frequent cleaning of pig stalls with high-pressure washer. The volumes of washing water used and those liquid swine waste generated were the same during the study period, since there was no changes in phase of swine production (finishing phase) and in hygienic-sanitary management of pig stalls. It is important to mention the phase of swine production, since the feed provided to animals in each phase (nursery, growth and finishing) is different, changing characteristics of the manure generated.

Swine raw waste (SRW) from pig stall cleaning was sieved in a static sieve with capacity for 10 m3 h−1 and 0.65 mm of holes diameter, to remove suspended solids, such as soy and maize grain wastes badly digested by animals (Fig. 1). Afterwards, the liquid fraction was treated in a commercial tubular anaerobic digester, which is also known as plug-flow or Canadian digester that are long, horizontal and trapezoidal cross section. The dimension of this anaerobic digester was 3 m in diameter and 12 m in length. Its installation was carried out in a ditch with 2 m of depth, 3 m in width upper, 2 m in width bottom, and 12 in length, in soil impermeabilized with polyvinyl chloride (PVC). The plastic (PVC) covered this ditch to contain the biogas generated, presenting a digester total storage capacity equal to 90 m3, with 60 m3 of effluent and 30 m3 of biogas (Fig. 2).

Fig. 1
figure 1

Diagram flow of swine waste anaerobic treatment pilot system and dilution of treated swine effluent

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Fig. 2
figure 2

Schematic diagram of commercial tubular anaerobic digester operated in batch, represented in trapezoidal cross section

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The anaerobic digester was operated in batch and the hydraulic retention time (HRT) was equal to at least 20 days, in accordance to efficiency test performed by the manufacturer. However, in this study, the effluents remained for a longer time in the anaerobic digester, due to periods of sanitary emptiness, which were periods with no animals in the stalls for cleaning and sanitization in compliance with sanitary practices adopted at experimental farm. Thus, it was possible evaluated the effects of different HRT about the characteristics of treated swine effluent generated..

After the anaerobic digester had been installed in December of 2014, 30 m3 of sieved swine raw waste were added to the anaerobic digester, to start the anaerobic treatment process, which was inflated initiating its activity. The first recharge in the anaerobic digester was performed with 18 m3 of sieved swine raw waste in February of 2015. After this period, there was a second recharge with 18 m3 of sieved swine raw waste in June of 2015. These two recharges were done with 18 m3 of SRW to replace the amount of treated effluent used in some agricultural areas of this research unit during the study period, maintaining the microbiological activity inside of digester.

During the study, there was always an organic charge maintained from previous recharges, which was enough to maintain the biological activity of the anaerobic digester. After June of 2015 and until the study end, there were no other recharges. The effluents remained in the anaerobic digester without an agitation process and external inoculum addition, with temperature of effluent inside anaerobic digester ranging from 40 to 45 ºC, under influence of external temperature that ranged from 15.3 to 36.7 ºC during the study period (May of 2015 until January of 2016).

Through this study, the treatment system efficiency of SRW and the agricultural use potential of treated swine effluent from anaerobic digester (SEB) were evaluated. Furthermore, considering the lack of study in Brazilian conditions about efficiency of swine waste treatment systems and characteristics of treated swine effluents for purposes of agricultural reuse or disposal in water bodies, the dilution of treated effluent (SED) (1:50, v:v) was also done. For this, the treated swine effluent from anaerobic digester was pumped for a tank and mixed with water, simulating characteristics of effluents from complementary phases of swine treatment systems, which could promote higher reducing in pollutant charges.

The effluents were characterized during May, June, July, December, and January, these samplings referred to 100, 130, 30, 180, and 210 days of HRT, respectively. For HRT calculation, the time between the charging and sampling was considered. Thus, two sampling were done to 100 and 130 days after the first recharge of 18 m3 performed in February of 2015. Afterwards, three sampling were done to 30, 180 and 210 days after of second recharge with 18 m3 performed in June of 2015.

Immediately after effluent sampling, the pH, electrical conductivity (EC) values, and total solids (TS), fixed solids (FS), volatile solids (VS) were determined and then, the biochemical oxygen demand (BOD) for the 5-day period at 20 °C, and the chemical oxygen demand (COD) was also determined [32].

In addition, the nitrogen content (N-Kjeldahl) by the Kjeldahl method (N-organic + N-NH4+), ammonium nitrogen (N-NH4+) [33], and nitrate nitrogen (N-NO3−) [34] were determined. After nitric acid digestion of the effluent samples [32], the pseudo-total contents of potassium (K) and sodium (Na) were determined by flame photometry. Phosphorus (P) was determined by colorimetry, and the elements calcium (Ca), copper (Cu), zinc (Zn), magnesium (Mg), iron (Fe), and manganese (Mn) were determined by plasma emission spectrometry (ICP-AES).

After 100 days of HRT, microbiological analyses were performed to assess the presence of Salmonella spp. and thermophilic and fecal coliforms density [35] in SRW, SEB, SED, and in water used for dilution.

Results and discussion

Characterization of swine waste and effluents from anaerobic treatment system

Table 1 shows the characterizations of SRW, SEB, and SED sampled from May 2015 until January 2016. There was a reduction equal to 90% in BOD of SEB and SED in comparison to SRW, which is higher than the 80% that is the standard established by São Paulo State legislation [36] and provides limits for effluent release into water bodies, after wastewater treatment. After 10 months of anaerobic treatment of swine waste, Vivan et al. [37] observed a reduction of 84% in COD values of swine effluent from a treatment system comprising an anaerobic digester and facultative lagoons, showing the high organic matter degradability present in this waste. There was reduction higher than 90% in volatile solids amounts after the anaerobic treatment performed in this study (Table 1), reflecting the fact mentioned by these authors.

Table 1 Characterization of swine raw waste (SRW), swine effluent from biodigester (SEB), diluted swine effluent (SED) (1:50, v:v)
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Smaller reductions were observed for fixed solids, equal to 78, 86, and 70%, for the months of May, June, and July, respectively. The lowest reduction in fixed solids content observed in July was related to the effluent inflow into anaerobic digester at the end of June. Thus, the significant reductions observed in the BOD, COD, and solid series, due to the swine manure anaerobic decomposition, indicated high process efficiency in the anaerobic digester.

The P and Ca contents of SEB and SED were reduced around of 90% after SRW treatment. Probably, these reductions were related to a precipitation process that could have occurred due to an increase in the pH values of SEB, ranging from 7.7 to 8.7(Table 1), reducing the chemical soluble forms after anaerobic treatment. This effect agrees with reductions observed by Vivan et al. [37] and Viancelli et al. [38] in P and Ca contents present in swine effluents treated through anaerobic treatment systems, which are factors also attributed to the chemical precipitation, such as that of calcium phosphate. Fernandes et al. [39] suggested the presence of calcium–manganese phosphate in sludge from a swine manure treatment plant, such as hydroxyapatite and carbonate-hydroxyapatite, which are amorphous P compounds and present more availability as a plant nutrient in comparison to crystalline compounds.

For the magnesium contents, there was a reduction around of 80% throughout the evaluation periods, ranging from 23.5 to 33.0 mg L−1 in SEB (Table 1). These elements' removal may also be related to the salt formation, which was adhered to the sludge on the lagoon floor. Duda and Oliveira [40] observed the same fact after swine manure treatment in a system composed of anaerobic reactors (Upflow Anaerobic Sludge Blanket—UASB) and biological filter. The authors performed electron microscopy analysis and X-ray dispersive energy microanalysis, noting that salt formation was composed mainly of P, Mg, and Ca minerals adhered to biological filter support material.

Removal efficiencies equal to 80% for Fe and Mn and 90% for Cu and Zn were observed (Table 1). Amaral et al. [41] obtained lower efficiency in Cu and Zn removal throughout 17.86 days of raw swine waste treatment in an upflow anaerobic digester, equal to 30%, due to dragging of solids during this low HRT, reducing the treatment efficiency and precipitation of elements, such as sulfide. Szögi and Vanotti [42] only obtained a satisfactory Cu and Zn removal, equal to 80%, when solid separation and N contents reduction by biologic treatment were performed previously the lagoon columns treatment over 15 months, simulating 2.0 m depth of an anaerobic lagoon.

These reductions observed could be related to increasing in pH values of swine effluents throughout anaerobic decomposition process (Table 1), which were above of 7 due to carbonates and ammonia formation [43], promoting the precipitation of these elements in sludge formed in anaerobic digester floor. Furthermore, several other process could occur during anaerobic digestion, promoting reductions in contents of these elements as sorption to solid material, complexation with organic compounds and linking formation with sulfides [41, 44].

There were no changes in the N, Na, and K contents present in the SEB in comparison to SRW. Additionally, the contents of these elements present in the SED were reduced due to the dilution effect (1:50, v:v) (Table 1), and only for sodium and zinc was there no correlation with dilution rate performed. Considering that Na and K form extremely soluble compounds in water, these elements remained in the swine effluent after anaerobic treatment.

Furthermore, most of N remained as ammonium, and no changes in nitrogen levels were observed. The nitrogen inorganic form was predominant in swine effluents, preferably as N-NH4+, as expected for predominantly organic effluents from anaerobic systems (Fig. 3). In accordance with the National Research Council [45], excretion of nitrogen ranges from 40 to 70% of the total provided to swine, being excreted in feces and urine. In addition, the major part is excreted as ammonium or urea that is rapidly converted to ammonium after 72 h, comprising about 90% of total nitrogen present in swine waste [46].

Fig. 3
figure 3

Changes in mineral (N-min) and organic (N-org) nitrogen forms present in swine raw waste (SRW), swine effluent from anaerobic digester (SEB) and diluted effluent (SED). (a)Recharge performed with 18 m3 of SRW sieved at February 2015. (b)Recharge performed with 18 m3 of SRW sieved at end of June 2015

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Throughout the treatment process, the mineral forms represented 80% of nitrogen content (N-Kjeldhal), while the organic forms of nitrogen represented 20%. Possibly, this was due to the immobilizing of N-mineral for more stable organic compounds, which were formed during organic material decomposition present in SRW. Although the SED had presented lower N levels in relation to SEB (Table 1) due to dilution effect, in percentage terms, the same tendency for the N-mineral and N-organic forms present in SED was observed, indicating that the dilution process had no effect about element transformations.

In relation to swine effluents pathogenicity, reduction in total coliforms values and elimination of Escherichia coli and Salmonella spp. were observed (Table 2) when the SEB and SED were compared to SRW. It should be observed that coliforms found in SRW belonged to the E. coli, as occurs for sanitary effluents. After 100 days of treatment in a anaerobic digester (May 2015), total coliform values decreased from 1.6 × 107 MPN/100 ml to 5.3 × 105 MPN/100 ml.

Table 2 Microbiological characterization of swine raw waste (SRW), swine wastewater treated in biodigester during 100 days (SEB), swine wastewater diluted (SED) (1:50, v:v) and water used in dilution
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Thus, the treatment resulted in E. coli absence, indicating its elimination and the proliferation of other types of coliforms. Therefore, these coliforms species identifications are needed to determine the importance from sanitary and environmental viewpoint. From this, it will be possible to evaluate if only the quantification of E. coli species is enough to guarantee the treated swine wastewater quality for its agricultural use.

Orrico-Júnior et al. [47] observed similar coliforms removal efficiency, from 3.6 × 109 MPN/100 ml to 3.6 × 102 MPN/100 ml, for both total coliform and fecal coliform, after 36 days of swine effluent treatment in an anaerobic digester. It is emphasized that retention time of waste in a biodigester, as well as the waste characteristics and other factors related to anaerobic treatment, could influence the coliforms removal efficiency [48].

The thermotolerant coliforms quantification substitution by E. coli quantification is accepted by international legislation about wastewater microbiological quality for agricultural use [49] and São Paulo State legislation about sanitary wastewater agricultural use [31]. These legislations consider E. coli such as thermotolerant coliforms species more abundant, which can indicate fecal contamination. However, Brazilian legislation on the water bodies classification and their uses [50] establish that bacteria counts should be in accordance to limits stated by an environmental agency.

The standards used for regulatory agencies in the United States are presented in USEPA [51] guidelines; California has one of strictest standards for crop irrigation with wastewater, establishing < 2.2 total coliforms/100 mL for irrigation of food crops and < 23 total coliforms/100 mL for irrigation of pasture and landscape areas. In accordance to World Health Organization (WHO) [49], the United States agencies do not consider epidemiological studies for establishing water quality standards, which could be important for some countries due to consideration of local epidemiological and economic factors.

In a study performed by São Paulo State environmental agency [52], the bacterial presence of Klebsiella genus was identified in water collected from different São Paulo State regions, representing 67.5% of thermotolerant coliforms quantified, and the main species was K. pneumoniae, another enteric bacteria species that can also be indicative of fecal contamination. Considering this and other factors, the environmental agency established criteria for use of E. coli as a parameter for water microbiological quality evaluation.

Bilotta and Kunz [53] mentioned that anaerobic treatment systems promote lower pathogen reductions compared to aerobic treatments, due to the mesophilic temperature range, in which they operate. However, in this study, there was the sanitization of the swine wastewater studied that included the fecal coliforms and Salmonella spp., although there was a proliferation of other groups of coliforms bacteria.

This fact could be related to higher treatment efficiency promoted by previous swine waste sieving, which resulted in removing of recalcitrant coarse material such as soy and corn straw that present slow degradation, increasing the microbiological activity during organic matter degradation. Orrico-Junior et al. [47] obtained a total and fecal coliforms reduction from 3.6 × 109 MPN/100 mL to 3.6 × 102 MPN/100 mL after 36 days of treatment of swine waste sieved and treated in anaerobic digester, while a reduction from 7.3 × 109 MPN/100 mL to 1.5 × 104 MPN/100 mL in total and coliform account was obtained for the effluent treated in anaerobic digester without previous sieving.

Smith et al. [54] attributed to several factors the pathogens removal efficiency by anaerobic digestion process, as temperature, retention period, reactor configuration, microbiological competition, and chemical interactions. Massé et al. [55] observed a reduction in Salmonella spp. counts from 103 CFU g−1 to no detectable levels, after psychrophilic anaerobic digestion (20 ºC) of swine waste, over 2 weeks in anaerobic digesters arranged in batch, showing the Salmonella spp. removal efficiency of this process, despite low temperatures. Thus, the treatment process studied complied with the required variables for Salmonella spp. removal (Table 2), such as temperature and hydraulic retention time.

Therefore, the contamination risks of soil and those exposed to treated swine wastewater were reduced, considering fecal coliforms and Salmonella spp. presence. Furthermore, other research may be developed from this study that evaluates the effects of a lower hydraulic retention time on pathogen presence and quantifying of other enteric microorganism groups, such as helminth eggs and protozoa cysts. Additionally, bacterial regrowth should be studied during a long period of anaerobic digester activity and agricultural reuse of treated swine wastewater, since studies have shown the recontamination and regrowth of fecal coliforms after successful reduction through anaerobic digestion of organic wastes [56,57,58].

Agricultural potential use and the current environmental legislation

In Brazil, there is no legislation about limits for swine wastewater agricultural use. Some southern states in Brazil present laws established by state environmental agencies, guiding swine wastewater use. The CNRH Resolution nº 54 [28] encourages the agricultural reuse practice without limits or criteria established for this use. In São Paulo State, the Directory Decision from the State Environmental Agency [59] established the need for legislation review or creation in cases whose effluent use is not predicted by current regulations.

Comparing EC and Na values of swine effluents found in this study with limits of current legislation for other effluents, [30, 31], it was observed that these parameters were higher than 2.9 mS m−1 and 69 mg L−1, respectively. The Na contents in SEB were five times higher (Table 1) than limit established by this legislation. Furthermore, the water used for pig stall cleaning presented Na content equal to 96.2 mg L−1, which was higher than standard for agriculture irrigation, contributing to Na contents increase in treated swine effluents.

São Paulo State Environmental Legislations determine conclusive reports submission by an official research institution, to evaluate the agricultural use viability of effluents with Na contents higher than 69 mg L−1. Furthermore, area monitoring should be periodically performed to evaluate the changes in specifically soil parameters aiming at maintaining environmental quality. Therefore, Na contents in swine effluents may be a limiting factor for its agricultural use.

Sodium Adsorption Ratio (SAR) is one of parameters used by Food and Agriculture Organization of the United Nations (FAO) [60] to classify water quality for irrigation. Thus, using the results obtained for swine effluents characterization, this parameter was calculated according to following equation:

$$ {\text{SAR } = \text{ }}\frac{{{\text{Na}}}}{{\sqrt {\frac{{\text{Ca } + \text{ Mg}}}{{2}}} }} $$

wherein: SAR = Sodium Adsorption Ratio, Na = sodium content present in swine effluent, mmolc L−1, Ca = calcium content present in swine effluent, mmolc L−1, Mg = magnesium content present in swine effluent, mmolc L−1.

In accordance with FAO guide [60], none, medium, and severe, are classification for the water infiltration reduction risk in soil; depending on the SAR values associated with the EC values of irrigation water. These parameters should be considered to avoid sodium ions predominance in relation to the calcium and magnesium ions present in the soil ion exchange complex. Otherwise, the sodium could occupy the soil charges, and the clays expansion would occur due to the higher sodium hydration radius, reducing the soil permeability and drainage [61, 62].

The Ca and Mg removal throughout treatment of SRW resulted in the SAR value increase of SEB and SED (Fig. 4). For sampling performed in July 2015, higher SAR values for SED (30.8) were observed, due to maintenance of Ca and Mg contents associated with the increase in Na content present in SED (Table 1), reflecting the recharge performed at end of June 2015 and Na contents present in dilution water.

Fig. 4
figure 4

Sodium absorption ratio evolution (SAR) of swine raw waste (SRW), water used in pig stall cleaning, swine effluent from anaerobic digester (SEB) and diluted effluent (SED) (1:50, v:v). (a)Recharge performed with 18 m3 SRW sieved at February 2015. (b)Recharge performed with 18 m3 of SRW sieved at end of June 2015

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The EC values should be higher than 1.9 mS m−1 when SAR values range from 6 to 12 and higher than 1.2 mS m−1 for SAR values among 3 to 6 [60], in order for there to be no infiltration reduction of water in soil. Considering this, it was verified that the SEB could not cause water infiltration reduction in soil, since there was correlation between SAR (Fig. 4) and EC values (Table 1). Nonetheless, the EC values of SEB were lower than 1.9 mS m−1 and SAR values changed from 5.7 to 30.8, presenting moderate risks, such as water infiltration reduction in soil. This fact reflected the Na content present in water used for pig stall cleaning, equal to 96.2 mg L−1.

Comparing the SRW to SEB, it was observed that there were increases in EC of SEB (Table 1) due to salts solubilization during swine raw waste degradation, presenting soil salinization risk in accordance with an EC limit equal to 3.0 mS m−1 established by FAO [60] and São Paulo State environmental legislation [31]. However, the EC values of SEB (Table 1) were lower than the value of 25.2 mS cm−1 obtained by Moral et al. [19] for swine raw waste samples from 36 pig farms with animals in grow-termination phase. Furthermore, the SEB presented SAR values lower than the limit equal to 12 established by São Paulo State legislation [31], excluding the sampling of SEB performed on December 2015 that presented an SAR value equal to 13 (Fig. 4) due to lower Ca contents (Table 1).

However, the FAO guide [60] advises that these interpretations should not be used as a unique criterion to guide the agricultural use management of low quality water and encourages scientific research to identify potential risks. The São Paulo State environmental legislation about sanitary effluent agricultural use [31] also establishes that an official research institution should perform studies to verify the agricultural use effects of sanitary effluents with sodium contents higher than the adequate limit.

A few studies were performed in Brazilian conditions to evaluate the agricultural use effects of treated swine effluents, particularly the sodium contents in a soil–water–plant system. Most studies have evaluated the use effects of effluents in stabilization lagoons without previous treatment and, consequently, with higher solids and salts content, promoting water infiltration reduction in soil caused by high sodium content present in swine raw waste [62].

Thus, studies should be conducted to verify the effects of successive use of effluents with sodium content higher then limits stated by current legislation, evaluating different soils and cultures. Concomitantly, animal nutrition studies should be performed to evaluate the reduction in sodium amount provided to animals, since the sodium removal present in swine effluent requires sophisticated and costly methods. Furthermore, high sodium contents in effluents increases the risks of soil salinization and sodification, reducing the soil permeability and water infiltration, with low water absorption by plants [63, 64].

In accordance to CONAMA Resolution nº 357 [50], water class 3 could be used for agricultural irrigation and should present ammonium nitrogen contents (N-NH4+) equal to 5.6 mg L−1 when the pH range is 7 to 8. Additionally, São Paulo State legislations establish that citric or sanitary effluents should present N-NH4+ contents up to 20 mg L−1. Furthermore, scientific studies should be performed to prove the protection water bodies in cases where sanitary or citric effluents present higher ammonium nitrogen contents. In this study, the medium N-NH4+ contents were equal to 1,775.96 mg L−1 for SEB and 37.72 mg L−1 for SED, which is higher than the content accepted by the aforementioned legislation. Nitrate contents were lower than 10 mg L−1 (Table 1), which is the limit established by the studied legislations.

Considering that the medium nitrogen content in SEB and culture nitrogen needs equal 200 kg ha−1, it would be needed to apply 118 m3 of swine effluent from an anaerobic digester for one crop. Thus, the amount and frequency of effluent generation should be considered during the elaboration of a treatment station project, to predict previous alternatives for the destination and management of treated swine effluents; the nutrient content present in effluent and crop nutritional needs will be limited to the agricultural area available for sustainable application of swine effluent.

Treatment systems for nitrogen removal should be evaluated, as well as the effects on nitrogen transformations in soils amended with effluents from different treatment systems. An anaerobic treatment system generates effluents with ammonium nitrogen predominance, whereas systems composed by both anaerobic and aerobic processes present nitrate nitrogen predominance, causing different effects in the soil–water–plant system.

The Technical Regulation P4.231 [29] establish K contents as a limiting factor for agricultural use of vinasse, considering the K present in vinasse and soil beyond the nutrient amount required by sugarcane (185 kg ha−1). There were no changes in K contents after swine raw waste treatment, due to its high solubility. The medium K contents were equal to 1,157.2 mg L−1 and 21.9 mg L−1, respectively, for SEB and SED, which is lower than the pseudo-total (6.2 g L−1) and available (2.6 g L−1) contents of K present in a natural vinasse characterized by Possignolo et al. [65]. This element was less limiting in relation to Na and N, considering that the current Brazilian legislation for effluent agricultural use from other activities.

São Paulo State legislation establishes Cu and Zn limits equal to 0.2 mg L−1 and 2.0 mg L−1, respectively, for agricultural use of sanitary effluent [31] and effluent from citrus industry [30]. These are metals that are most limiting for swine wastewater agricultural use due to their supply in animal diet as supplements and medicines, which are excreted in high contents by animals. Cu and Zn contents present in SRW were higher than these limits, equal to 13.6 and 44 mg L−1 (Table 1), respectively. Moreover, only the longest hydraulic retention times (higher than 100 days) were able to provide treated swine effluents with Cu and Zn contents in accordance to previously mentioned legislations [30, 31].

For pathogenic microorganisms, the São Paulo State legislation [31] about effluent agricultural use considers the limits established by World Health Organization. Fecal coliforms or Escherichia coli count should be lower than 105 MPN/100 ml in cases where there is sprinkler irrigation with exposure of workers and nearby communities, excluding children lower than 15 years. However, if the irrigation is performed by furrow or flood, fecal coliforms count should be lower than 103 MPN/100 ml, considering the same exposition group and children lower than 15 years.

In accordance with this legislation, it was observed that SEB presented an E. coli count lower than aforementioned limit (Table 2), representing the potential for agricultural use considering this parameter. However, other thermotolerant microorganism species could have grown due to temperatures higher than 40 ºC and anaerobic conditions, considering that the total coliforms count was equal to 105 MPN/100 mL. These species should be identified to assess possible sanitary and environmental risks related to treated swine effluent agricultural use.

Thus, the main restriction for agricultural use of effluent from an anaerobic digester were the high N and Na contents, which could be leached or carried with particles in the function of soil characteristics. Animal nutrition studies should be performed to increase the sodium use efficiency by animals, establishing an adequate cost–benefit ratio based on zootechnical and environmental parameters. This is because the higher sodium content in animal excreta results in higher treatment costs due to the technology needed to remove this element and, in some cases, impedes the swine waste use in agricultural soils.

The legislations discussed in this study were developed for other types effluents. Furthermore, the Environmental Agency of São Paulo [59] provides that rules should be elaborated on for effluents that are not attended to by the current legislation. Thus, studies about swine waste treatment and its agricultural use effects are important, considering the absence of criteria established through agricultural and environmental legislations regarding the sustainable agricultural use of swine effluents.

Conclusions

The SRW should not be used in agriculture, since there was a pathogens presence and contents of N, Na Cu, and Zn were higher than the limits established by current legislations on the agricultural use of effluents from other industrial and agricultural activities.

The treatment composed of static sieve and anaerobic digester generated treated swine effluents with lower pollutant potential and agricultural potential use, due to reductions in values of BOD, COD, volatile solids, and contents of P, Ca, Mg, Cu, Zn, Fe, and Mn. However, the effluent from the anaerobic digester presented values of EC and Na higher than limits established by current environmental and agricultural legislations.

After 100 days of retention in an anaerobic digester, fecal coliforms and Salmonella spp. present in treated swine effluents were in accordance with limits established by international and Brazilian legislations about wastewater agricultural use. However, there was growing of other coliform bacteria during anaerobic digestion.

The treated swine effluents presented agricultural use potential due to contents of N, P, and organic matter. However, use criteria should be adopted and established by future legislation. Furthermore, this study showed that the main elements that may limit these use criteria could be the Na and N.