Sustainable management of tannery hair waste through composting
Bovine hair, resulting from the unhairing of hides and skins, forms a major component of tannery solid wastes that is often disposed to landfill. However, following increasing environmental legislation restricting the disposal of commercial and industrial organic wastes to landfill, alternative treatment of tannery solid hair waste is necessary. The overall aim of this research was to develop conditions favourable for the decomposition of bovine hair waste in a composting environment. In this study, chemical pretreatments of bovine hair were carried out to enhance susceptibility to enzymatic digestion. Pretreatment using a combination of potassium thioglycolate and sodium hydroxide enhanced enzymatic digestibility of hair. Comparative solubilisation of black, brown and white hair samples with sodium sulfide showed that, the percentage soluble nitrogen, from white hair was 73% (w/w) as compared to 28% (w/w) and 24% (w/w) of black and brown hair, respectively; indicating that white hair was more vulnerable to solubilisation. Although chemical pretreatment was effective, it was considered non-compatible with the composting process. Consequently, enzymatic and microbial degradation were considered suitable alternatives. The results of the preliminary biochemical studies indicated that, the breakdown of the hair structure was enhanced when treated with a combined protease solution containing proteinase K and trypsin enzymes as compared with individual enzymes. Enzymatic and microbial degradation of hair was observed to be preferential, starting with the least resistant components, in this sequence: (1) cell membrane, (2) endocuticle, (3) medulla, and (4) cytoplasmic material and fibrillar components of the cortex. The cuticle was most resistant to degradation. Synergistic action of microbial-expressed enzymes was observed to be the fundamental mechanism for the enhanced hair degradation. Composting of bovine hair, mixed with dry leaves, soil, and wood chips/sawdust was performed in a rotary drum under controlled conditions. Optimum conditions, in order to sustain thermophilic microbial flora, were found to be (1) a moisture content of 55-+2% RH, (2) pH 7-8, (3) temperature of 49-+ C, and (4) a carbon to nitrogen ratio of 35:1. Structural changes in hair, assessed during composting using scanning electron microscopy (SEM), indicated that degradation occurred under these conditions. Microscopy results further indicated that the addition of an inoculant, containing a microbial crude enzyme, expressed by the bacteria of Bacillus species, improved the biodegradation of hair during the controlled composting process. The compost stability evaluated by monitoring the humification parameters and carbon to nitrogen ratio, indicated that the final product reached the following values: degree of humification (73 - 78%), humification ratio (26 - 32%), humification index (0.29 - 0.35), and a carbon to nitrogen ratio (25 - 29). The high level of humification of the end-product indicated reasonable stability, suggesting the product from the composted bovine hair may be suitable, suggesting the product from the composted bovine hair may be suitable for agronomical purposes. The use of composting as alternative technology for the treatment of tannery solid bovine hair waste, as a value added product, has been demonstrated.
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