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INTRODUCTION
Anaerobic digestion (AD) has proven to be an economically feasible technology for the cyclic use of large-scale and small-scales organic waste (Huang et al., 2016; Alkhalidi et al., 2019). It is a widely used to convert organic material into energy-rich biogas and the residual sludge known digestate (Alburquerque et al., 2012). Digestate can be used as a biofertilizer for arable land, enabling recirculation of plant nutrients, and thus reducing the need for fossil fuel-dependent inorganic fertilizers (Alkhalidi et al., 2019). The rapid development of biogas plants in European countries has increased the recycling of waste for energy generation, while producing large quantities of digestate (Franchino et al., 2016). There has been a tendency in the last decades of increased emphasis on improved sustainability in agriculture and preservation of natural resources, thus changing the focus of digestate processing from nutrient removal and disposal, to integrated nutrient recovery and recycling (Drosg et al., 2015). This contributes to promoting the circular economy in the agroindustrial sector, relevant aspect at present (Suárez et al., 2019). The use of digestate has great importance, especially in developing countries, not only because inorganic fertilizers are often too expensive, but also because recycling nutrients from organic sources is essential.
The concentration variability of macronutrients, micronutrients and matter in digestate depends on substrate type, the anaerobic digestion performance and the digestate postreatment like type of solid-liquid separation (Makádi et al., 2012; Akhiar et al., 2017). When digestate is obtained from different manures, the nutrient composition is greatly dependent on several factors such as digestion type (omnivore, ruminant), sex, species, age and diet of the animals, as well as the geographical and climatic conditions of the regions (Lukehurst et al., 2010; Risberg et al., 2017).
In Cuba, researchers have been mainly focused on the direct application of digestate on beans (Negrin Brito & Jiménez Peña 2012), tomatoes (Utria-Borges et al., 2008; López Dávila et al., 2017a), carrot, radish (Hernández et al., 2008) and onion (López Dávila et al., 2017b). These authors focused on digestate application to crops and its effect on plants. In all cases, they obtain benefits in terms of crop yield. However, very few information was found on the characteristics of the digestate applied (considering the type of biodigester and the substrate used for AD) in the Cuban context; which leads to excessive application of nutrients in crops becoming toxic for plants. The characterization of Cuban digestate in terms of nutrient and matter contents can enable to assess it as biofertilizer and use it safely, with possible positive impacts on crop yields and reduction in the use of inorganic fertilizer. Therefore, the aim of this work is to characterize the digestate obtained in the province of Sancti Spíritus, Cuba, in terms of nutrient and matter content, considering the type of biodigester and the substrate used during AD processes. To this end, the objectives were (i) to classify the digestate in terms of liquid and solid fraction, (ii) to determine the content of matter and nutrients using classical and instrumental analytical techniques, as well as (iii) to evaluate the liquid fraction as irrigation water. The characterization of the digestate can determinate its biofertilizer potential, which has positive environmental impact due to the recirculation of nutrients from substrates considered waste and the decrease in the demand of inorganic fertilizer. Which help reduces the exploitation of natural reserves and also the high pollution that is generated in the industrial manufacture of these inorganic fertilizer.
MATERIALS AND METHODS
The methods applied to fulfill the objective of this research are described in this section. Firstly, overviews of the existing biodigester technologies in Sancti Spíritus (21°56′02″N 79°26′38″W), Cuba are briefly described. Later, the methods for the sampling procedure, the physical-chemical analysis and the characterization as irrigation water are also explained.
Biodigester technologies. The most widespread biodigesters in Cuba are: fixed dome biodigester, floating drum biodigester and tubular biodigester (Suárez-Hernández et al., 2018).
Fixed dome biodigesters. Consist of a closed system, usually built of masonry and under ground level. These biodigesters have a fixed dome-shaped cover that contains the biogas inside, an inlet to feed the substrate, and a digestate outlet that allow solid and liquid phases separation to produce digestate in both states, without mechanical process. The lack of agitation in this technology allows the solids to form flocs and precipitate to the bottom of the reactor (increasing the solid retention time), while in the compensation tank the liquid digestate comes out by overflow. The sludge, together with other non-degraded materials that precipitate, is extracted every 15 or 30 days and forms the solid digestate.
Floating drum biodigesters. are formed by a masonry cylinder in its lower part (with a stop to support the bell), and a floating bell storing the gas in the upper part. Inside the reactor, the substrate flows uniformly allowing a better contact of the microorganism and the substrate, and a higher degradation of the organic matter with respect to fixed dome biodigesters. The digestate is collected as sludge.
Tubular biodigesters. are formed by a bag resistant to environmental conditions. The substrates are fed by the inlet pipe and occupy the bottom of the bag while the upper part serves as a container for the biogas generated during the operation. Once the fed substrate has been digested, the digestate leaves the bag through the outlet pipe, coming out in the form of sludge, similar to the floating drum biodigester.
Digestate collection
The number of samples was calculated using the methodology established by Vivanco (2005) through equation 1:
Where: n: number of digesters to be sampled
p: proportion of digesters in operation (p = 0.8)
q: proportion of digesters out of operation (1- p) (q = 0.2)
N: size of the population (number of digesters)
Z: deviation from the mean value accepted to achieve the desired level of confidence, based on the 95% confidence level, with Z = 1.96.
d: level of absolute precision, referring to the amplitude of the confidence interval desired in the determination of the average value of the variable under study. This value was taken as 0.16.
Taking into account the existence of several types of digesters in the province of Sancti Spíritus and the different types of substrates, it is necessary to carry out a random stratified sampling procedure (descriptive analysis), to ensure that each technology and substrate are proportionally represented in the final sample. For that aim, equation 2 was used (Vivanco, M, 2005; Angelidaki, 2003 ):
Where:
fh: fraction of the stratum; n: sample size; N: population size.
The following operating parameters: temperature range, hydraulic retention time, reactor volume, feeding substrate and methane productivity were considered in the descriptive analysis and evaluation in the equation 1 and 2.
The digestate was taken in all cases at the biodigesters outlet. The collection of samples in tubular and floating drum biodigesters was carried out by using a valve located in the lower part of the biodigesters. In the fixed dome biodigesters, the liquid and solid fractions were collected separately as the technology allows that. The liquid fraction was taken from the compensation tank, prior to the exit of the pos-treatment system (lagoons); while the solid fraction was collected from the sludge outlet at the biodigester bottom. All samples were stored in sterile glass bottles and kept in freezing at -20˚C to avoid biodegradation until their analysis.
Physical - chemical analyses. As the liquid digestate is highly colored and turbid due to the presence of suspended matter, all digestate were converted to ashes. The ashes were used for determining sodium, potassium, sulfate, phosphate, calcium and magnesium, matters content, electrical conductivity and ammonium nitrogen were determinate in fresh samples.
For the physical-chemical characterization of digestate, The Standard Methods for the Examination of Water and Wastewater (APHA, 2012) were used. The content of dry matter (DM, %) (section 2540-B), fixed matter (ash, %) (section 2540-E) and organic matter (OM, %) (section 2540-E), were determined by gravimetric methods. Calcium (Ca2+) and magnesium (Mg2+) were determined by complexometric titration (sections 3500-Ca-B and 3500-Mg-B, respectively); Orthophosphate (section 4500-PO4 3--C) and sulfate (section 4500-SO4 2--E), by spectrophotometry; and ammonium nitrogen by (Kjendalh) (section 4500-NH4 +-C). In addition, it was determined Potassium K2O-K (section 3500-K-B) and sodium Na2O-Na (section 3500-Na-B), by flame photometry, whereas the electrical conductivity (EC) were determined by conductimetry (section 2520-Salinity-B).
Characterization as irrigation water. To assess the quality of the liquid fraction of digestate as water for irrigation, the sodium adsorption ratio (SAR) (Del Valle, 1992) and the Kelly ratio (KR) (Quispe Mamani, 2016) were used (Equations 3 and 4, respectively). In addition, the relationship of both parameters with the hardness and electrical conductivity was taken into account to classify the quality of digestate as irrigation water.
Where c(Na) is the sodium concentration (meq L-1), c(Ca) is the calcium concentration (meq L-1) and c(Mg) is the magnesium concentration (meq L-1).
Statistical analysis. All statistical descriptive analysis (biodigestor technology and substrate used frequency as well as predominant nutrient and matter contents per substrate) performed in the selection and evaluation of technologies and samples were done with SPSS software, version 23,0 (SPSS, 2013). The analyses were performed in triplicate, the central tendency and dispersion statistics (mean, standard error of the mean, and ranges) were calculated as shown in the figures and tables of the study.
RESULTS AND DISCUSSION
Digestates were collected from 20 biodigesters (according to the results from equation 1), located 11 in Cabaiguán municipality (22°05′02″N 79°29′43″W) and 9 in Banao town (21°82′86″N 79°56′86″W). To ensure representative samples, three strata were formed in accordance with the type of biodigester (Table 1), which consists of 13 fixed dome, 1 floating drum and 6 tubular biodigesters, (according to equation 2). In addition, in accordance with the type of substrates, three strata were also considered consisting of 13 swine manure (SM), 2 cow manure (CM) and 5 codigestion of swine and cow manure (SCM) (equation 2).
Table 1 shows, the operating parameters: temperature range (mesophilic) and hydraulic retention time (HRT) (15-50 days), were equal for all plants; while reactor volume (1.8-50 m3) and feeding (2-4 kg of volatile solid/day/m3) were different. The methane productivity of all plants was 0.5 m3 reactor/day).
Table 1. Operating parameters and methane productivity for small-scale biodigesters which produced the digestates studied.
| Biodigesters types | Reactor Volume (m3) | Temperature range | Feeding (kg/day/m3) | HRT (days) | Methane productivity (m3 reactor/day) |
|---|---|---|---|---|---|
| Fixed dome | 20-50 | Mesophilic | 2 | 15-50 | 0.5 |
| Floating drum | 1,5 | Mesophilic | 2 | 15-50 | 0.5 |
| Tubular | 3 | Mesophilic | 2-4 | 15-50 | 0.5 |
HRT: hydraulic retention time.
As 50% of the digestate produced in Sancti Spíritus, corresponds to fixed dome biodigesters treating SM, they were the higher strata (13 biodigesters). This can be attributed to the decentralized pig production in Cuba, the necessity for SM treatment in situ and the high dissemination of fixed dome biodigesters (Sosa et al., 2014). For that reason, this substrate and this technology were chosen for a more detailed analysis in the present study.
Digestate characterization
Dry matter. The content of dry matter (DM), organic matter (OM) and ash for SM digestate (solid and liquid fractions) obtained from fixed dome biodigesters is shown in Figure 1.
Figure 1. Contents of dry matter (DM), organic matter (OM) and ash in swine manure (SM) digestate (solid and liquid fractions) obtained from fixed dome biodigesters. The bars indicate the standard deviation.
The solid fraction of the digestate showed a DM content of 17.77 ± 3.30% (Figure 1), being considered as a solid digestate, following the approach of Makádi et al. (2012), who suggested this classification for DM content in digestate above 15%. For that reason, SM digestate (solid fraction) obtained from fixed dome biodigesters can be considered as biofertilizer rich in stable and fibrous material which should have a positive effect on soils by minimizing erosion and improving water-holding capacity and soil structure, especially when they are used on sandy soils (Bermejo et al., 2010). The liquid fraction of this digestate presented a very low DM content (average 0.48 ± 0.03%), due to the large dilution of the SM occurring when more than 40 L of water per animal head are used during the cleaning of piggeries (Sosa et al., 2014).
Organic matter. The efficiency of OM conversion through mesophilic (i.e. 35°C) or thermophilic (i.e. 55°C) AD is generally in the range of 13-65%, and depends on the type of substrate fed to the biodigester, as well as on anaerobic reactor parameters, such as the Organic Loading Rate (OLR) and the HRT (Menardo et al., 2011). In the solid and liquid fractions of the SM digestate, OM represented more than 50% of DM, agreeing with the values reported by Marcato et al. (2008). These high OM values can be attributed to several aspects: 1) the high OLR and short HRT applied to fixed dome biodigesters in piggeries; as the number of pigs increases during the time for the same biodigester volume; and 2) the low removal efficiency (˂ 50%) of organic matter reported for fixed dome biodigesters due to its cylindrical shape that does not ensure a uniform path of the substrate inside the biodigester (Montalvo and Guerrero, 2003).
These aspects induce a rise of the more recalcitrant molecules in digestates, such as lignin, cutin, humic acids, steroids, complex proteins. These non-degraded stable carbon compounds are beneficial for the soil, being potential humus precursors with high biological stability (Kalakodio et al., 2017; Diacono et al., 2019). This allows increase fertility, functionality, microbial activity, aeration, and water storage capacity in the soil (Kalakodio et al., 2017).
Ash. The ash content in the solid and liquid fractions was 39% and 50%, respectively. During AD, organic compounds are broken down by bacteria resulting in the production of methane and carbon dioxide. Thus, as a result of the digestion process, a number of changes in the solid content can be expected; including a substantial reduction (up to 25%) in the total solids and a consequent increase in the ash content (as %DM), due to the conservation of minerals and the organic matter removal (Fageria and Moreira, 2011).
Matter content of the remaining digestates. A summary of DM, OM and Ash for the remaining digestates available in Sancti Spíritus is provided in Appendix A. The DM content of the digestate solid fraction obtained from fixed dome biodigester during the treatment of SCM, rendered similar values (17.50 ± 1.89%) with respect to fixed dome biodigester using SM only. However, a lower DM content was obtained in tubular and floating drum biodigesters (7.08±0.10% and 4.42±0.20%, respectively) treating CM. Tubular biodigesters using SM and SCM reported a DM content of 1.02±0.03% and 6.10±0.30%, respectively. Organic matter and ash represented from 40-50% and from 20-40% of DM, respectively. These values make the digestates available in Sancti Spiritus, a suitable soil amendment, based on its matter content.
Nutrient content. Figure 2 shows the nutrient content of SM digestate in term of nitrogen (NH4 +-N), phosphorous (PO4 3--P), sulfates (SO4 2--S), calcium (Ca2+-Ca), magnesium (Mg2+-Mg) and potassium (K+-K) concentrations (g kg-1 DM).
Figure 2. Nutrient content expressed in g per kg of dry matter (DM), in the swine manure (SM) digestate (solid and liquid fractions). The bars indicate the standard deviation.
Nitrogen. The NH4 +-N concentration in biofertilizers is of great importance as it is immediately available to the plant. The concentrations of NH4 +-N in the liquid and solid fractions were 0.06 ± 0.00 and 0.01 ± 0.00g NH4 + kg−1 DM, respectively. The N content of the digestate was a consequence of its preservation during AD (Tambone et al., 2010; Drosg et al., 2015). During organic matter degradation, part of the organically bound nitrogen is reduced to the NH4 form, (Kuusik et al., 2017). The NH4 +-N is concentrated in the digestate by the degradation of the proteins available in the fed substrate (Kryvoruchko et al., 2009). Typically, pigs include a high protein content in their diet (e.g., animal feed, food waste and slaughterhouse waste), as part of these proteins are not assimilated by the pigs, being expelled in the excreta; they are degraded to NH4 + during the AD process and released in the liquid digestate mainly.
For that reason, values between 1.00 and 1.80g of NH4 + kg−1 DM has been considered as typical for the liquid digestate obtained from biogas plants treating SM (Rossi and Mantovi, 2012; Tigini et al., 2016). However, in this study the NH4 +-N concentration in the liquid fraction was 0.06 g of NH4 + kg−1 DM, being 60% of the typical values reported (Rossi and Mantovi, 2012; Tigini et al., 2016). The higher concentration of N for the liquid fraction with respect to the solid fraction, agreed with Tampio, Marttinen and Rintala (2016), who stated that solid-liquid separation is increasing in application during digestate treatment for the extraction of nitrogen from the liquid fraction.
Phosphorous. The phosphorus content of digestate is expressed as total phosphorus or as phosphate equivalents. The AD process does not affect the content of phosphate in digestate, which is mostly dependent on the content in the substrate (Drosg et al., 2015). The concentration of PO4 3--P obtained in this study for the liquid fraction digestate, was 14.63±1.20g kg−1 DM, agreeing with the values reported in Alburquerque et al. (2012), being 7 times higher than the values (1 to 2 g PO4 3- kg−1 DM) reported by Rossi and Mantovi (2012).
The concentration of PO4 3--P for the solid fraction digestate was 40.14±1.93g kg−1 DM, showing that phosphorous is mainly accumulated in the solid fraction of digestates, which agree with the values reported by Tampio et al. (2016) (Figure 2). Phosphates are a limited non-renewable resource, which is as an essential plant nutrient that cannot be replaced by other substances (Shepherd et al., 2016). Therefore, the use of this digestate for the Cuban agriculture can generate economic and environmental benefits, by substituting P-rich commercial biofertilizers; which is of prime importance in the recycling of limiting nutrients like phosphorous.
Potassium. Potassium concentrations of 0.09±0.00g kg−1 and 3.06±0.17g kg−1 DM were obtained for the solid and liquid fractions of the digestate, respectively. Alburquerque et al. (2012) reported values from 50 to 1000 times higher for the liquid and solid fractions, respectively. As potassium degradation during AD is negligible, the differences found in potassium concentration can be attributed to the pig diet in the different regions. For example, pigs Cuban farms eats cereals but also home-made food such as cassava and sweet potato yogurt (Almaguel et al., 2016).
Sulfates. A high sulfate concentration of the solid and liquid fractions was observed, with average concentrations of 41.89±4.92 and 30.61±3.82g kg-1 DM, respectively (Figure 2). These values were similar to those reported by Chen et al. (2010), who states that this is also related to the proteins consumption in the pigs diet.
Calcium and Magnesium. In the solid and liquid fractions, the Ca:Mg ratio was around 1:1; being these values lower than those reported by (Negrin Brito and Jiménez Peña, 2012) for a digestate obtained from the AD of agricultural waste and manure (88 Ca% and 12 Mg%DM ). A further discussion will be provided in the next section about Ca and Mg concentrations in the liquid fraction of the SM digestate.
In general, AD enables the attainment of a final product (digestate) with good fertilizing properties, having a nutrient ratio (NH4 +:PO4 3-: K+: SO4 2-:Mg2+: Ca2+) in the liquid (0.002:0.80:0.10:1.00:0.89:0.93) and solid (0.0003:0.96:0.002:1.00:0.52:0.50) fractions, that would contribute to return macronutrients to the soil after AD of SM. A summary of the nutrient content for the digestates available in Sancti Spíritus is provided in Table 2.
Table 2. Nutrient and matter content of the digestate taken from fixed dome, tubular and floating drum digesters when using different substrates (n=3).
| Characteristic | Fixed dome | Tubular | Floating drum | ||||||
|---|---|---|---|---|---|---|---|---|---|
| SM | SCM | SM | SCM | CM | CM | ||||
| Liquid | Solid | Liquid | Solid | Liquid | |||||
| DM (%) | 0.48 ± 0.03 | 17.77 ± 3.30 | 0.55 ± 0.01 | 17.50 ± 1.89 | 1.02 ± 0.03 | 6.10 ± 0.30 | 7.08 ± 0.01 | 4.42 ± 0.02 | |
| OM (%) | 0.24 ± 0.04 | 10.78 ± 1.72 | 0.17 ± 0.01 | 11.69 ± 0.82 | 0.58 ± 0.07 | 2.39 ± 0.17 | 4.99 ± 0.76 | 3.05 ± 0.42 | |
| Ash (%) | 0.24 ± 0.03 | 6.96 ± 1.84 | 0.18 ± 0.01 | 5.48 ± 0.57 | 0.43 ± 0.05 | 1.20 ± 0.19 | 2.09 ± 0.23 | 1.37 ± 0.01 | |
| NH4 +- N (g kg-1 DM) | 0.06 ± 0.00 | 0.01 ± 0.00 | 0.04 ± 0.00 | 0.003 ± 0.00 | 0.04 ± 0.00 | 0.002 ± 0.00 | 0.003 ± 0.00 | 0.001 ± 0.00 | |
| PO4 3-P (g kg-1 DM) | 24.63 ± 1.20 | 40.14 ± 1.93 | 11.93 ± 0.81 | 17.03 ± 0.34 | 13.29 ± 1.32 | 7.23 ± 0.22 | 6.64 ± 0.02 | 12 ± 0.06 | |
| K+- K (g kg-1 DM) | 3.06 ± 0.17 | 0.09 ± 0.00 | 2.65 ± 0.31 | 0.06 ± 0.02 | 1.45 ± 0.27 | 0.25 ± 0.02 | 0.16 ± 0.01 | 0.36 ± 0.16 | |
| SO4 2-S (g kg-1 DM) | 30.61 ± 3.82 | 41.89 ± 4.92 | 18.67 ± 1.51 | 45.00 ± 1.29 | 29.92 ± 1.55 | 68.13 ± 4.80 | 12.82 ± 0.02 | 56.32 ± 0.23 | |
| Ca2+- Ca (g kg-1 DM) | 28.47 ± 2.06 | 21.09 ± 1.94 | 18.50 ± 1.70 | 31.54 ± 2.83 | 15.82 ± 1.68 | 4.47 ± 0.30 | 12.23 ± 0.15 | 7.80 ± 0.26 | |
| Mg2+- Mg (g kg-1 DM) | 27.24 ± 2.10 | 21.87 ± 4.42 | 20.27 ± 1.00 | 7.79 ± 1.08 | 10.74 ± 0.93 | 1.27 ± 0.38 | 5.51 ± 0.17 | 4.48 ± 0.21 | |
| pH | 7.30 ± 0.02 | 7.34 ± 0.03 | 7.24 ± 0.21 | 7.49 ± 0.29 | 7.20 ± 0.09 | 7.11 ± 0.08 | 7.18 ± 0.08 | 7.09 ± 0.01 | |
| EC (μS cm-1) | 1.04 ± 0.01 | 1.31 ± 0.02 | 1.15 ± 0.04 | 1.05 ± 0.04 | 1.37 ± 0.09 | 1.46 ± 0.11 | 1.33 ± 0.01 | 1. 47 ± 0.01 | |
SM: swine manure, CM: cow manure, SCM: Co-digestion of swine and cow manure, DM: Dry matter, OM: Organic matter, EC: Electrical conductivity.
Liquid fraction of digestate as irrigation water. The liquid fraction generated from SM was assessed as water for irrigation based on its content of interchangeable ions, EC and hardness. Table 3 shows the experimental measurements, the calculations of these parameters and their acceptance criteria.
Table 3. Evaluation of swine manure liquid fraction digestate as water irrigation.
| Parameters | EC (µS*cm-1) | Hardness (°F) | SAR | KR (%) |
|---|---|---|---|---|
| Liquid digestate | 1320.00 | 78.50 | 0.03 | 28.22 |
| Acceptance requirements | ˂ 750 Excellent 750 - 3000 Good >3000 Unacceptable | 0-22 Sweet 32-54 Hard ˃ 54 Very hard | ˂ 10 Optimum | ˃ 35 Optimum |
SAR: Sodium Adsorption Ratio, KR: Kelly´s ratio, EC: Electrical conductivity.
SAR and KR were calculated according to equations described by Del Valle (1992) and Quispe Mamani (2016), respectively. The SAR value was 0.03 for the liquid digestate, indicating its low sodification power. In general, the higher the SAR, the less suitable the water is for irrigation (Aboukarima et al., 2018). That is either diminish the soil permeability, reducing the formation of crusts that can modify the physical-chemical properties of the soil with the consequent deterioration of the crop yield (Lesch and Suarez, 2009). Moreover, the KR value for the liquid digestate (˂28%) was lower than the optimum value (higher than 35%) for plant growing (Quispe Mamani, 2016), because of the higher proportion of Mg with respect to Ca, and the low contribution of Na concentration (see Eq. 4). In terms of hardness, the digestate was considered as very hard water (78.5ºF) because its values were higher than the acceptance criterion of 54ºF.
According to the diagram for the interpretation of the irrigation water value (Riverside Standards) (Olías et al., 2005), digestate was classified as C3S1. That is, digestate has highly saline waters (EC 1320 μS cm-1); therefore, there must be good drainage conditions, salinity must be controlled and only plants that are resistant to salinity should be cultivated. On the other hand, it presents a low alkalization hazard (SAR=0.03), so it can be used without serious damage to plant development.
Besides, according to the Wilcox Standard (Olías et al., 2005), which takes into account the percentage of Na over the other cations (Ca, Mg and K) and the EC, it was obtained that the digestate of Sancti Spiritus is in the category from “good” to “admissible”, representing a potential alternative to increase agriculture sustainability in the Cuban context. Further studies are needed on the appropriate dosage for the different crops.
CONCLUSION
The digestates obtained in biodigesters in the province of Sancti Spíritus, Cuba were characterized in terms of nutrient and matter content. Most of the digestate samples (50%) were taken from fixed dome biodigesters treating SM because they were widely available in Sancti Spíritus.
The highest dry matter content (DM) was obtained for the solid digestate sampled from fixed dome biodigesters, with values above 17%, being considered as stable and fibrous material which should have a positive effect on soils, by minimizing erosion and improving water-holding capacity and soil structure.
The nutrient content of both digestates showed good fertilizing properties, having a nutrient ratio (NH4 +:PO4 3-: K+: SO4 2-: Mg2+: Ca2+) in the liquid (0.002:0.80:0.10:1.00: 0.89:0.93) and solid (0.0003:0.96:0.002:1.00:0.52:0.50) fractions, that would contribute to the return of organic matter and nutrients to the soil in a short-term period.
Swain manure treated in fixe dome biodigester was the one that produced the most nutrient-rich digestate, therefore, it was the one with the highest biofertilizing potential.
The quality of the liquid fraction as irrigation water was assessed as good, according to the relationship between the concentration of the nutrients (Ca, Mg, Na and K) and hardness, agreeing with the water classification for irrigation of the Wilcox and Riverside standards.
