Composting treatment of fur waste originating from tannery

Introduction

Modern society has to address a broad suite of sustainability challenges in urban¹ and rural areas^2,3. The tanning process converts the skins of bovine animals, sheep, and pigs into leather, ready to be used for various purposes. Tannery operations are divided into four processes: riverbank, tanning, post-tanning, and finishing. From an environmental standpoint (cleaner production), the first two processes are important for the volume and pollutant load of effluents, while the last two processes account for the amount of solid wastes and solvent emissions generated in different operations that yield finished leather⁴. Of the total weight of the skins entering the tannery, 60% is eliminated⁵. In Tungurahua province, Ecuador, it is estimated that out of the 920,000 cattle headcount, 500,000 skins are treated in tanneries yearly⁶.

This study proposes, through a desulfurization process, to reduce the sulfide concentration in these residues, and then take advantage of their nutrients (organic matter and nitrogen) to carry out a composting process. As a result, a stabilized product, compost, will be obtained. In order to improve the C/N ratio, the pellet residues will be mixed with pruning remnants. Specifically, this study addresses the following objectives: (i) characterize the fur waste, (ii) build and test a desulfurization system for the fur waste, (iii) identify the appropriate percentage of fur and pruning remnants, (iv) establish the appropriate composting system, and (v) determine the quality of the compost.

Methods

A total of 576 kg of fur residues was obtained from the tannery “EL ALCE leathers” located in Guano canton, Chimborazo province.

Desulfurization consisted of catalytic oxidation of sulfide containing residues with compressed air and magnesium sulfate (ALMACEN EL AGRO, Riobamba-Ecuador), 0.4 g/(kg of pellet of fur). Sulfur determination was carried out according to APHA 4500-S2F protocol⁷. Duration of the operation was 5 h in accordance with the protocol⁷. With this process, the removal of sulfur was accomplished by converting it into sulfuric acid and reducing sulfur dioxide emissions.

Physical-chemical analysis enabled the evaluation of chemical oxygen demand (COD) and sulfide concentration in fur residues (meat, grease, and hair) and of fur water. We determined COD according to APHA 5220 D protocol⁸.

Previously desulfurized fur residues were dried and crushed (to a size ranging from 1 cm to 5 cm) and mixed with crushed pruning remnants according to CCIA 2013⁹. pH and electrical conductivity were measured for solid/water ratio of 1:10 (w/w) at each turning using glass electrode (Model Seven2Go Advanced Single-Channel Portable pH Meter, Mettler, Toledo). Organic matter was determined by calcination at 430°C, according to the method described in Navarro et al.¹⁰. Humidity was measured gravimetrically in an oven at 105 °C for two hours to constant weight. Phosphorus was determined by spectrophotometer (UV5, Mettler Toledo, USA) at 440 nm according to AOAC 958.02/960.02 protocol¹¹. Potassium was measured by flame photometry according to AOAC 965.09/945.04 protocol¹². Total nitrogen was determined according to AOAC 955.04 protocol¹³.

In order to accelerate the composting process, an inoculum was prepared with a mixture of mature compost (2.5 lb.), chicken residue (2.5 lb.), pork slurry (10 L), and molasses (2.5 L) coming from collection center of the Escuela Superior Politécnica de Chimborazo. With the conditioned waste, a 2.5 m × 2.5 m × 1.5 m pile was assembled. Three parts of fur were used for one part of pruning remnants; the homogeneous mixture was placed into a 100-liter tank for fermentation for 24 hours.

In the initial and final samples (after 2 months of maturing), in addition to the parameters listed above, germination index (GI) were determined according to Moreno and Moral 2011¹⁴, using radish seeds (Raphanus sativus) (supplier: El Agro Riobamba-Ecuador) to establish the maturity of the obtained compost; briefly: GI is calculated from the ratio of germinated seeds and the root length of radish seeds in an aqueous compost solution¹⁴.

Results and discussion

Sulfides were reduced by 23.7% (Table 1). This percentage could increase by extending the duration of the desulfurization process.

The waste degradation process lasted for 141 days, followed by a maturity of 64 days. The stack showed a reduction in volume, size, and odor, indicative of the decomposition of the materials used, including residual hair, as previously observed by Numpaque and Viteri¹⁵. One important parameter to highlight is the rapid increase in temperature, reaching maximum values of 69.7°C (Figure 1), which ensured a good disinfection process¹⁶.

Figure 1. Temperature variables.

Orange: average temperature (°C) in the compost. Blue: average temperature (°C) of environment.

The moisture content varied from 45 to 65%, which is considered as optimal¹⁷. We found slightly alkaline medium, with an average final pH 7.94. The amount of organic matter (Figure 2) decreased to 42%, and this data is consistent with the Spanish Standard Royal Decree 506/2013¹⁸ on fertilizer products. Contents of total N (3.62%), P (0.31%), and K (0.39%) of the matured compost were higher than those obtained in composting processes of waste materials with wastewater from the tannery industry, which present values of 1.36%, 0.001%, and 0.23%, respectively¹⁹. Reported electrical conductivity (EC) values of 0.27 dS/cm were below the Spanish Standard¹⁸; this is possibly due to the salts washing off over irrigation periods. It is preferable for the compost to have low EC values, since the presence of salts in high concentrations inhibits plant growth²⁰.

Figure 2. Organic matter % in the compost.

To establish the maturity of the compost, the GI value was determined using Radish seeds (Raphanus sativus). In the final compost, GI was 43.55%, a value that indicates the presence of phytotoxic substances. Nevertheless, if we start from the assumption that GI of the pile at zero time has been 10.73%, a clear decrease of these substances is evident. The increase in GI is an indication of the degree of maturity of the compost²¹.

Conclusions

The quantity and type of nutrients in this compost allowed the rapid growth of microorganisms, while recording thermophilic temperatures close to 70°C, which guaranteed an adequate hygiene of the product. Based on the contents of organic matter, in accordance with the Spanish Standard Royal Decree 506/2013¹⁸, it was assessed that the obtained compost belonged to Group 6. Organic amendments, No. 2, Annex 4 ‘Waste of Leather Industries, of the Skin and Textile’⁵, indicates that with an adequate control of the composting process, parameters of an acceptable product can be obtained, in relation to the pollutant load. Considering that the initial germination index was 10.73% and the final 43.55%, it can be concluded that there was a partial elimination of phytotoxic substances. The challenge is to improve this parameter by looking for new mixtures and inoculating efficient microorganisms that allow these substances to be degraded. At present, the compost obtained could be used in restoration projects of deteriorated and soils poor in organic matter, or as cover material for sanitary landfills.

Data availability