Physicochemical and Microbiological Properties and Humic Substances of Composts Produced with Food Residues

The consolidation of a wide and effective management system of solid residues, especially biodegradable ones, is one of the great challenges of current society. Composting was evaluated as an option of organic fertilization for soil enrichment, using raw food residues in substitution to bovine manure. The compost piles were built with 30% of biodegradable residues mixed with 70% of ground tree pruning material. The effects of different proportions of food residues (FR) and bovine manure (BM) as source of carbon were tested in 5 treatments (T1 = 15%BM + 15%FR, T2 = 20%BM + 10%FR, T3 = 10%BM + 20%FR, T4 = 30%BM and T5 = control, 30%FR), in randomized blocks, under open field conditions for 90 days. The pH, temperature and moisture content of the compost were measured weekly. The aged compost was evaluated for physicochemical and microbiological properties and carbon contents in the humic substances. The analyses of the results indicated that all studied composts reached the maturation stage with satisfactory contents of humic substances, macronutrients, and micronutrients, indicating that food residues can be used as a source of carbon in compost piles to produce organic fertilizers. The contents of the evaluated chemical contaminants were much lower than those established in the main legislation and current normative instructions and, in terms of contamination by pathogens, there was the absence of total coliforms, thermo tolerant coliforms, and Salmonellas.


Introduction
A number of solid residues disposed of by the population is extremely large and, when inadequately disposed, they become a problem for society, because they can cause environmental impacts, modifying the quality of soil, air and water bodies, which represents a risk to the public health (Bulcão et al., 2010).
Thus, to face the situation in the social context of the country, there arise individual (garbage collectors) and collective (associated groups, cooperatives, and companies) initiatives whose operations are based on the process of collection, separation, reuse and/or recycling of these materials.
Currently, there are two recycling associations in the city of Mossoró/RN: Associação Comunitária Reciclando para a Vida (ACREVI) and Associação dos Catadores de Material Reciclável de Mossoró (ASCAMAREM) (Cavalcanti et al., 2011). These associations work with the selective collection and collaborate to minimize the negative impacts that the residues can cause to the environment (Ferreira et al., 2012).
However, as in most recycling associations in Brazil, there are no programs for the recycling of biodegradable materials and their disposal in landfills is common, causing serious environmental problems. From the sustainability point of view, biodegradable residue must efficiently return to the economic and productive cycle, to guarantee their final disposal.
In this context, composting becomes an option for the reuse of biodegradable residues as one form of production of organic fertilization, which can be used to enrich soil, since it provides nutrients that are essential to its fertility, besides meeting sanitary, social, ecological and economic aspects (Inácio & Miller, 2009).
Nevertheless, to avoid negative environmental impacts of the use of the composting of biodegradable residues in agriculture, it is necessary to define parameters relative to physicochemical and microbiological quality, besides the possible contamination by pathogens or heavy metals. Thus, composting was evaluated as an option for an organic source of fertilization in agriculture using raw material food residue in substitution to bovine manure.

Material and Methods
The experiment was carried out in the courtyard of the Associação Comunitária Reciclando para a Vida (ACREVI), located in the Nova Vida neighborhood, municipality of Mossoró -RN (5º14′9″ S; 37º18′59″ W; 18 m asl.).
The biodegradable residues (bovine manure and food residues) used for the composts were collected in popular restaurants and households of the local urban area. Filling materials consisted of tree pruning residue collected by the urban cleaning service of the municipality. In total, approximately 7 tons of vegetal material, 1,500 kg of bovine manure and 1,500 kg of food residue were collected. All piles were built with 70% of the vegetal material (V) and 30% of bovine manure (BM) and food residues (FR), as recommended by Pereira Neto (2007).
The compost piles were built with a conical shape, 1.60 m high and 2.00 m wide, spaced and parallel to one another to facilitate the turning, passage of materials and access. The manual turning of the compost piles occurred every three days in the first week of composting and, subsequently, every ten days. The piles were daily irrigated using tap water.
During the maturation process, the piles were weekly monitored through measurements of the parameters temperature, moisture, and pH. In addition, the contents of total organic carbon (TOC), total nitrogen (TN) and C/N ratio were determined in four periods, 2 during the degradation stage of the composts (beginning and at 10 days from the building of the piles) and the others at 30 and 60 days after the beginning of the composting and during maturation.
TOC was determined through the wet oxidation method with external heating, while TN was determined through the wet digestion method in the open system, using a block digester (Tedesco et al., 1995;Santos et al., 2009). The C/N ratio was obtained by the ratio between the TOC and nitrogen, expressed in percentage.
After maturation, the composts were analyzed for humic substances (Swift, 2001), macro and micronutrients, heavy metals (Santos et al., 2009), total coliforms, thermo tolerant coliforms and Salmonella (Siqueira, 1995). For the determination of humic substances, the samples of each treatment were ground, passed through a 60-mesh sieve (0.210 mm) and subjected to the fractioning of humic substances (swift, 2001). From this fractioning, the fractions corresponding to fulvic acids, humic acids, and humins were obtained through the differential solubility in acidic and alkaline solutions.
The contents of macro-and micronutrients and chemical contaminants were determined through wet digestion in a closed system using a microwave oven as a source of heat and concentrated nitric acid to digest the compost. Then, the liquid extract obtained from this process was analyzed for the contents of Cu, Zn, Fe, Mn, Cd, Ni, Pb, Ca and Mg, through atomic absorption spectrophotometry, and P, Na and K, through calorimetry and flame spectrophotometry, respectively (Santos et al., 2009).
Total coliforms and thermo tolerant coliforms were evaluated using the most probable number (MPN) technique, also known as a multiple-tube method (Siqueira, 1995).
All analyses were performed in triplicate. The results were subjected to analysis of variance (p < 0.,05), using the statistical program ASSISTAT 7.7 (Silva & Azevedo, 2016). Tukey test was used to compare the mean values. Table 1 shows the composition and characteristics of the piles during the composting process. At the end of the composting, the results showed that there is no variation of temperature (35 °C) between the studied treatments and the observed values allow the reduction and/or elimination of pathogens. However, there were small variations of pH (7.3 to 7.8), TOC (7 to 11%) and TN (0.5 to 0.8%). Regarding the C/N ratio, the piles of the treatments with 30% of food residues (OC 5 ) showed higher C/N ratio than the other treatments, while the OC 4 compost pile showed the lowest moisture content (48%). Note. C = composition; T = temperature; U = moisture; VM = vegetal material; TOC = total organic carbon; TN = total nitrogen; C/N = nitrogen carbon ratio; BM = bovine manure; FR = food residues; St. = start; Int. = intermediary; En. = end.

Carbon Contents in Humic Substances
According to the ANOVA, the variable carbon fractions of the humic acids (C-HA), fulvic acids (C-FA) and humin (C-HUM) were significantly influenced (p < 0.01) by the proportions of food leftovers of the compost piles (Table 2). Note. ns; *; ** not significant, significant at the 0.05 and 0.01 level of significance according to the F test; CV = coefficient of variation.
In general, among the fractions of humic substances, there was higher C content in humins (C-HUM), followed by humic acid (C-HA) and fulvic acid (C-FA) ( Figure 1). This high C content in C-HUM is possibly due to the lower solubility of this fraction associated with the high polymerization and, consequently, higher degree of stabilization.  composts. These reductions were approximately of 85, 48, 74, and 63% referring to OC 2 , OC 3 , OC 4 ,and OC 5 , respectively.
In general, the organic composts OC 1 and OC 4 stood out in the quantification of the macronutrients and OC 3 in the evaluation of micronutrients. However, despite these differences in the quantification of nutrients in the different organic composts, it should be pointed out that there are no specifications of standard limits for the contents of nutrients analyzed in organic composts, because both macro and micronutrients are related to the type of matter used in the production of the compost or in the composition of the compost piles. Since these contents do not exist, studies conducted in this research line are adopted as reference, as well as concentrations considered as ideal for agriculture.
The values obtained in the present study indicate that the analyzed organic composts had good quality, since, according to Malavolta (1980), they have macro and micronutrients in adequate concentrations for agriculture.

Heavy Metals
According to the summary of the ANOVA, only nickel (Ni) showed significant effect (p < 0.01) of the proportions of food residues in the composting (Table 4). Note. ns; ** not significant and 0.01 level of significance according to the F test; CV = coefficient of variation.
As in the quantification of nutrients, there is not a specific legislation containing the acceptable concentrations of the metals that are considered as chemical contaminants to soil and/or plant; instead, there are proposals of resolution referring to the quality parameters for the organic compost (CONAMA resolution proposal 02/2016). This proposal presents the quality contents that the organic compost must have for any allowed application as fertilizers and soil conditioners, including agricultural, gardening and landscaping applications.
Thus, the values referenced in the present research were based on the proposals of resolution, studies and acceptable contents established by some countries of Europe and the United States (Table 5). Therefore, the results ensure the stability of the produced composts, since there was a complete microbiological decomposition, free from pathogens and toxicity.

Conclusions
All the produced composts reached the maturation stage with satisfactory contents of humic substances, macronutrients and micronutrients, indicating that the food residues can be used as source of carbon in compost piles.
The concentrations of heavy metals in the organic composts were lower than those established in the main European and American legislations, as well as in the NI n o 27/2006 of the Secretariat of Agriculture and Livestock Defense (SDA) and in the resolution proposal 02/2016 (CONAMA process 02000.001228/2015-37).
The composts suffered a complete microbial decomposition, which led to the reduction and/or removal of contaminant microbiological agents, showing absence of total coliforms, thermotolerant coliforms and Salmonella.
The composting of biodegradable residues does not have factors of risk to the public health, emerging as an initiative and means of recycling of food leftovers, avoiding negative environmental impacts.