Conditioning of materials and composting

The experiments were carried out using local biosolids, derived from an activated sludge process. A thin layer of biosolids (BS) was placed on a polyethylene film to dehydrate them by solar radiation to obtain 35 ± 5 % moisture (Hbh) and then grind with a hammer mill. Clay soil (SA) came from clay mines located in the southern area of Valle de Toluca, Estado de México. BS and CS were sifted using a 5.0 mm mesh to have a uniform particle size. Equine degraded manure (DM) was stored for more than three months on mounds. Each substrate was obtained from a single lot to form the initial mixture of treatments.

The piles were placed in open field. Four treatments (ratios of substrates) and one control treatment (100 % biosolids) were established; experiments were performed in quintuplicate. The ratios of BS:CS:DM in piles of 250 kg were 70:30:00, 65:30:05, 60:30:10, 50:30:20 (w/w, dry base) and were identified as PA, PB, PC and PD, respectively. Subsequently, the proper ratio of the three substrates in the piles of 250 kg was used in the piles of 500 and 2 000 kg, identified as PE and PF, respectively.

The process variables were measured in morning hours. The temperature (T), humidity and pH were determined every 24 h; turns and irrigation were conducted every 48 h. These variables were monitored throughout the composting process, which included the following phases: mesophilic (T < 45 °C), thermophilic (T ≥ 45 °C) and second mesophilic or cooling phase. When temperature matched the room temperature, the composting process was concluded. The temperature was measured using a digital thermometer (HANNA® Model HI740024, Italy) on the inner and outer layers of the piles at nine random points. Moisture was determined with an OHAUS® moisture analyzer (model MB45, USA) and pH was measured with a CONDUCTRONIC® potentiometer (model PC18, Mexico) in a solution of compost:water (w/v) with a ratio of 1:5 at 25 °C.

Compost characteristics

Sampling and analysis of compost were carried out under the guidelines of the Mexican regulation NOM-004-SEMARNAT-2002 (^{SEMARNAT, 2003}). Analyzes were carried out in triplicate, determining the following quality parameters: OM by the method of ^{Walkley and Black (1934)}, TKN by ^{Kjeldahl digestion-distillation (1982)}, K and Na available by the Mehlich-3 reagent extraction method and atomic absorption spectrophotometry (Perkin-Elmer, model A100, Mexico), and P available by Mehlich-3 extraction and colorimetry (spectrophotometer Vis Thermo Scientific ™ GENESYS 10S, Germany).

The heavy metals Cd, Cr and Pb were determined by atomic absorption spectrophotometry, prior digestion with concentrated HNO₃ and HCl in a ratio of 1:3 v/v. The contents of fecal coliforms and Salmonella spp. were determined by the multi-tube fermentation method or most probable number method and the viable helminth egg content by the modified Balinger method (^{SEMARNAT, 2003}).

The experiment was completely randomized and the data generated were subjected to an analysis of variance (ANOVA). All values were expressed as statistical means. The Tukey test (P < 0.05) was applied to determine statistically significant differences, using the statistical package SPSS 16.0 (^{SPSS, 2007}).

Selection of the optimum ratio of BS:CS:DM in compost piles of 250 kg

After 24 h of initiating the process, the temperatures were higher than 45 °C, where the thermophilic phase lasted from 9 to 14 days. At this stage, the PB treatment (65:30:05) had the highest temperature (63.8 °C) in contrast to the control pile (48.7 °C) (Figure 1). According to the results of the microbiological analysis, reported in Table 1, the temperatures reached ensure the elimination of pathogenic microorganisms. These were similar to those obtained by ^{Ho, Yuan, Jien, and Hseu (2010)}, who incorporated residues of activated clay and rice husk as bulking agents to biosolids.

On the other hand, the addition of substrates influenced the availability of metals since a higher concentration was observed in P0 with respect to the rest of the treatments (Table 1). However, the concentration was lower than that established by Mexican legislation (^{SEMARNAT, 2003}) and the United State Environmental Protection Agency (^{USEPA, 1999}), which is important when considering its incorporation to the soil due to phytotoxicity and low tolerance of certain plant species.

Figure 1 Temperature and pH profile in the composting process of biosolids: clay soil: degraded equine manure in different ratios (P0 = control, PA = 70:30:00, PB = 65:30:05, PC = 60:30:10, PD = 50:30:20). 

Figure 1 shows that from day 2, the pH reached a value of 9.0, due to the CO₂ and NH₃ released into the atmosphere during aeration of the piles. Emissions were higher in the control (P0), which had the highest pH levels. These differences show that the absence of CS and DM favored greater dissociation of the ammonium and rapid decomposition of nitrogenous compounds, causing losses of nitrogen by volatilization. The final pH (8.0 to 8.5 units) indicated the stability of the organic matter (^{Hachicha et al., 2009}). These results are similar to those reported by other researchers such as ^{Malinska, Zabochnicka-Swiatek, and Dach (2014)}, who used biosolids, wood shavings and charcoal, and ^{Scoton, Battistelle, Bezerra, and Akutsu (2016)} who used sugar cane and coffee.

Table 2 shows the initial and final measurements of the composting process. In this study, initial moisture varied from 34 to 45 %, because the addition of CS and DM increased water retention and made it difficult to mix the substrates during the aeration process. It was observed that this did not limit the microbial activity, as high temperatures were reached in response to the heat released by the degradation of OM. Variation and loss of moisture were attributed to the generation of water by biodegradation, evaporation, aeration rate and substrate capacity to maintain moisture in the piles (^{Jolanun & Towprayoon, 2010}). Other researchers such as ^{Barrena, Font, Gabarrell, and Sánchez (2014)} worked with values lower than 40 %, while ^{Ho et al. (2010)} reported humidity from 45 to 65 %.

With respect to OM, we found significant differences between treatments (P < 0.05). The control treatment (P0) had the highest percentage (Table 2), which is attributed to the chemical and microbiological oxidation of OM caused by the substrates in the rest of the treatments. The reduction of OM in treatments was 51.14 ± 5.13 %, as recommended by ^{Antil, Raj, Narwal, and Singh (2011)}, who mention that the loss of OM should be greater than 42 %. As shown in Table 2, the substrates and the proportion used influenced the degradation of OM. Other researchers such as ^{Bustamante et al. (2008)} made mixtures between wine residues and cattle and poultry manure, observing a faster mineralization of OM. In the study of ^{Jolanun and Towprayoon (2010)}, when 15 % of granulated clay was added, an improvement in the thermophilic phase of the composting was tested, as well as an increase in the degradation of OM.

Table 2 also shows that the C/N ratio is strongly influenced by the initial substrates. The C/N ratio of the compost obtained does not represent an environmental risk when applied to soil nor does it alter the microbiological balance when it is lower than 15 (typical C/N ratio in soils), therefore, it can be assumed that the compost has maturity for use (^{Antil et al., 2011}; ^{Bernal, Navarro, Roig, Cegarra, & García, 1996}). The C/N values were similar to those reported by ^{Yañez, Alonso, and Díaz (2009)} in the composting of biosolids with mimosa (Acacia dealbata Link).

The duration of the composting process in the treatments was 21 to 29 days (Figure 1), in contrast to the control that failed to stabilize in this period. The use of the substrates in the piles influenced the processing time due to the characteristics of the components, since the DM is rich in degrading microorganisms and CS favors the conditions for their growth.

The P content was higher and statistically different (P < 0.05) in PB and PD compared to the rest of the treatments, and the K/Na ratio was higher in PA and PB, those characteristics denote a product of agronomic quality.

The treatment PB (65:30:5) was chosen for the piles of 500 (PE) and 2 000 kg (PF) taking into account the results of temperature, pH, Hwb, physicochemical and microbiological variables and composting time.

Composting process in piles of 250, 500 and 2 000 kg

The thermophilic stage had a duration of 10, 14 and 16 days for PB (250 kg), PE (500 kg) and PF (2 000 kg) with maximum temperatures of 63.8, 57.9 and 56.0 °C, respectively. The composting time was prolonged in PF at 32 days (Figure 2). The pH was lower in PE and PF with maxima of 9.1 and 9.2, and minimum of 8.2 and 8.8, respectively. The moisture content of 34.3 to 43.5 % ensured the proper activity of the microbial consortia.

The parameters of quality showed significant differences with respect to the pile weight (Table 2). The most affected variables were OM and K/Na ratio. The PF treatment had the lowest concentrations of OM, P and K/Na ratio, with temperature and pH lower than the rest of the treatments; in addition, the composting time increased to 32 days (Figure 2). In contrast, composting time with PB was only 21 days; although the duration of the thermophilic phase was lower, the temperatures reached were higher than in the treatments performed at greater weight. On the other hand, according to the microbiological analysis, the population of fecal coliforms, Salmonella spp. and viable helminth eggs were removed during composting.

The initial and final weight of each pile was compared and we observed a reduction of 74.82 ± 5.63 %, indicating the efficiency of composting and showing that the use of substrates is an alternative in the treatment and reduction of biosolids.

Figure 2 Temperature and pH profile in the composting process of biosolids: clay soil: equine degraded manure in a ratio of 65:30:5 with different pile weight.