Introduction

Composting processes are used to remove or alter contaminants; therefore, its use is successful to rehabilitate saline and contaminated with metals soils (^{Ansari, 2008}; ^{Coutris et al., 2012}). According to ^{Sharma and Sharma (1999)}, the implementation of vermicompost process to reduce organic waste is important because the landfills are insufficient to confine rising debris. However, studies on vermicompost carry out only cost/benefit analysis that excludes environmental variables, which poses a risk of higher damages because of the fact that they ignore the resources required from the ecosystems. Because of that, it is necessary to incorporate methods or tools that analyze and compare the sustainability of the different forms of goods production on a fair and equitable basis.

Emergy is an ecological-thermodynamic method for environmental assessment based on the conversion to common units of the energy, mass, and money flows used in a socio-ecological system (^{Odum, 1996}). In addition, this method quantifies and classifies all renewable (R), non-renewable (N) and market derived (F) resources that directly or indirectly a product requires. Emergy synthesis connects, associates and display different types of energy in a simple way, using diagrams with which fluxes are calculated and indexes are determined (^{Odum, 1996}), while the emergy accounting tool provides the means to keep the balance of the economy, society and the environment into a single account of results which similarly respond like financial business analyses or individual accounts (^{Campbell et al., 2005}).

Therefore, the use of emergy analysis increases every year and is applied in Brazil, China and the US (^{Bonilla et al., 2010}; ^{Campbell and Lu, 2014}; ^{Xie et al., 2014}). However, there are few emergy studies that focus on agricultural production systems and it is, therefore, important to develop studies to assess the resources required in these production systems.

The objective of this study was to quantify the emergy it in the production of three vermicomposts from organic waste from cattle, rabbit, and pork, by quantifying the OM, P and K of each variant. Our hypothesis was that the vermicompost that uses more market resources will have a lower sustainability index.

Materials and Methods

The research was conducted at the Instituto Federal de Educación, Ciencia y Tecnología de Minas Gerais, campus Inconfidentes, Brazil, located at 19° 52’ 18.85” S, 43° 57’ 58.49’ W, at an altitude of 900 m. The research was carried out in three phases, as described below.

Phase 1. Vermicompost production

In this phase, cattle, rabbits and pigs manure, sawdust and fresh grass (Axonopus affinis) were used in various combinations because manure has different carbon-nitrogen contents and standardizing them was necessary to achieve a 25:1 to 35:1 ratio, according to ^{FAO recommendations (2013)}. The amount of each component used to make the vermicompost was measured with a PCE-HB 2000 laboratory balance (Table 1).

Table 1 Dry weight mass content in the variants 

The pre-composting process was made on a 0.7 m wide by 1.0 m long rectangular surface, and was covered with black plastic. The composting temperature was stabilized at 30 °C 20 days after initiating the process. Later, 80 kg samples from each variant were inoculated with adult Eisenia foetida earthworms contained in 3.5 kg and humidity was kept between 70 to 80 %, for 40 d (^{Morales, 2011}).

Phase 2. Determination of the emergy environmental cost due vermicompost production

The emergy methodology proposed by ^{Odum (1996)} was used in this phase. First, all the resources, goods and services directly or indirectly used in producing vermicompost and their interactions were identified. With this information a diagram of the emergy process was carried out.

In the next phase the information was organized in emergy tables. Each resource was classified according to R, N, and F nature, and the amount of material used in the elaboration of vermicompost by variant was calculated. The obtained data was converted into emergy units using the transformity values on Table 2. The emergy from each element was calculated considering a 2-month period duration of the process.

Table 2 Transformity references in emergy per unit. 

(^{†Brown y Buranakarn, 2003}; ^{¶Romitelli 2000}; ^{§Ulgiati et al., 1994}; ^{ΦPulselli et al., 2008}; ^{¤Silva, 2006}; ^{††Buenfill, 2001}; ^{¶¶Brandt-Williamsd, 2002}; ^{§§Geber y Bjorklund, 2001}; ^{¤¤Odum, 1996}).

Finally, the emergy for R, N and F resources of each vermicompost was divided between the amount of humus retrieved from each variant, to quantify the results per unit of output. With this information and the use of equations 1, 2, 3 and 4 the four emergy indicators described below were calculated.

Emergy yield ratio (EYR) is the total emergy used per emergy unit inverted. The relationship is used to understand how much an investment enables a process to exploit local resources to contribute to the economy (^{Odum, 1996}). ^{Voora and Thrift (2010)} indicated that the systems with an EYR lower than one are unsustainable. This relationship is expressed as:

(1)

The environmental load index (ELR) is the relation between the sums of the inputs of the economy and the non-renewable and the renewable resources. The ELR is an indicator of the pressure of a transformation process on the environment (^{Odum, 1996}). According to ^{Brown and Ugliati (2004)} values lower than 2 indicate a low impact on the environment, and values between 2 and 10 represent systems that cause a moderate impact. Values greater than 10 indicate that the system will cause stress in the environment. Its formula is:

(2)

The emergy sustainability index (ESI) is the ratio of EYR to ELR. A system is considered to be sustainable when it has a low ELR and a high EYR (^{Ulgiati and Brown, 1998}). An ESI lower than 1 is not sustainable in the long term, between 1 and 5 is sustainable over the medium term, and greater than 5 is sustainable in the long term (^{Brown and Ulgiati, 2002}). The relationship between these indicators is expressed as:

(3)

where Y is total emery (seJ months^-1), F: emery from the market resources (seJ months^-1), R: emery from the renewable resources (seJ months^-1), N: emery from non-renewable resources (seJ months^-1).

The resources percentage (% R) is the relationship of renewable emery and the total emery use. On the long run, only processes with a high % R are sustainable (^{Odum, 1996}). The formula is:

(4)

Results and Discussion

Figure 1 shows the amount of humus of the vermicomposting process. The production of bovine, rabbit and pig humus production was of 50.2, 72.4, and 43.9 kg. Bovine and porcine vermicompost data coincide with those reported by ^{Ravera and De Sanzo (2003)}. The rabbit variant surpassed the humus production of to the bovine and porcine by 30.66 and 39.77 %.

Figure 1 Production of three vermicompost variants 

Figure 2 shows the input of all the necessary resources for vermicompost production. The output of the system is presented as eroded soil, which is an energy loss (due to entropy). The final system products are humus and worms.

^†Diesel was only used in the bovine vermicomposting process.

Figure 2 General vermicomposting diagram 

Table 3 shows the resources used to build and operate vermicomposting sites, the resource classification, energy values, transformities and emergy for a two months period.

Table 3 Calculation of the emergy in the elaboration of vermicompost with bovine, rabbit and pork manure. 

^†Emergy unit value.

In the three vermicompost elaboration processes, the manure represents the highest percentage of used renewable resources: 67, 88 and 78 % respectively. From the used resources, labor was 11 % for bovine, 5 % for the rabbit and 9 % for the swine manure. Only in the bovine vermicompost diesel was used (7 %), because it was necessary to transport the manure to the composting facilities.

Table 4 presents the integration of R, N, F resources and the total emergy per production unit. Bovine vermicompost had the lowest emergy cost, although diesel was used as a market resource on a higher proportion with respect to the other two variants.

Table 4 Resources quantification of vermicompost production. 

^†Renewable resources; ^¶non-renowable resouces; ^§market resoures; ^¤emergy.

Table 5 shows the emergy indicators of from the three vermicomposts. The EYR was superior in the rabbit variant with a value of 8.69, followed by the porcine and bovine. ^{Gianetti et al. (2011)}, ^{Giannetti et al. (2016)}, ^{Bonilla et al. (2010)} and ^{Del Pozo et al. (2014)} calculated this same indicator in mango, organic coffee, bamboo, and banana production in agroforestry systems. These had values of 1.90, 1.13, 1.36, and 3.16, respectively. According to ^{Voora and Thrift (2010)}, the emergy in the three composting processes in this research are sustainable, because they present greater than one emergy values.

Table 5 Emergetic indicators for bovine, rabbit and pig manure composting process. 

^†Emergy yield; ^¶environmental load index; ^§sutainability index; ^Φresources percentaje.

For the ELR indicator the bovine variant had the highest rate (0.315), followed by the porcine and cunicular (Table 5). This is because more labor is used in the process (11 %) and diesel was used for the transportation of the manure (Table 3). In the three composting processes the ELR indicator values were less than 2, which represent a low impact on the environment. These results are consistent with those from ^{Del Pozo et al. (2014)} and ^{Giannetti et al (2016)} who report 0.46 and 1.4 for this indicator. Nevertheless, ^{Bonilla et al. (2010)} and ^{Gianetti et al. (2011)} obtained values of 2.75 and 4.13.

The ESI indicator for the cuniculus variant showed the highest value, 66.80. The bovine variant showed the lowest mainly due to the human labor and diesel. The ESI in the three processes was greater than 6. This indicates that they are sustainable in the long term, and it coincides with the 14 value reported by ^{Giannetti et al. (2016)}. However, ^{Bonilla et al. (2010)}, ^{Gianetti et al. (2011)} and ^{Del Pozo et al. (2014)} showed values of 0.05, 1.30 and 0.30.

With regard to the renewable emergy and the total emergy use relationship (% R), the rabbit manure variant used 88.42 %, followed by the porcine and bovine manure variants. This indicates that the composting processes are sustainable, by using a higher percentage of renewable resources, which is in accordance with ^{Odum (1996)}. ^{Del Pozo et al. (2014)} and ^{Giannetti et al. (2016)} reported values of 41.9 and 68.33 % for this indicator.

When comparing the EYR, ELR, ESI and %R emergetic indexes in the evaluated vermicompost elaboration process, the rabbit vermicompost was the most long term sustainable process. Yet, the three variants are long term sustainable and with low environmental impact.

These results correspond to our hypothesis, which states that the process in which most market resources are used will have a lower sustainability index.

Table 6 shows the results of the OM, P and K content for each variant and its standard deviation, as well as the Tukey’s test results. According to the data, there are significant differences between the three vermicompost processes. The cunicular variant exceeded the OM, P and K contents levels respect to the bovine and porcine vermicompost. In the three variants, the OM content complied with the quality standards set in the ^{NMX-FF-109-SCFI-2007} norm, which suggests values between 20 and 40, and coincided with the results reported by ^{Castro et al. (2009)}.

Table 6 Nutrient content in three vermicompost variants 

Means with a different letter in a column are statistically different (Tukey, p≤0.05); +/- standard deviation.

^{Duran and Henríquez (2007)} indicated higher P and K values in the bovine manure vermicompost respect to those obtained in our study, which is attributed to the time for the vermicomposting process, as the earthworms were 60 d in the substrates. According to ^{Garv and Gupta (2010)}, the chemical differences in the vermicompost composition due to the earthworm’s action can be seen at 105 d of the vermicomposting process. Also, ^{Tognetti et al. (2005)} showed that vermicompost chemical composition depends on the type of food provided to the earthworms and the handling of the production system.

Conclusions

The emergetic indicators show that the vermicompost processes with bovine, cunicular and porcine manure are long term sustainable with low environmental impact. In addition, cunicular manure vermicompost presents the highest sustainability index, contains and provides more OM, P and K compared to the bovine and porcine manure vermicompost.

The methodology for emergy determination is recommended for agricultural production systems purposes to assess the required resources and thus optimize their emergy synthesis, achieve efficient use of the resources and reduce its environmental impact