Anaerobic digestion of tannery waste: Semi-continuous and anaerobic sequencing batch reactor processes
Introduction
The European Union is among the leading leather producers. A survey conducted in 2001 revealed that in 1998 there were around 3000 tanneries in Europe, with Italy, the major leather producer, producing 190 million m2 of leather per year (EC, 2001). It has been estimated, that between 400,000 and 900,000 tonnes of sludge (per fresh weight basis), the majority of which is deposited in landfills, is generated annually in the EU from leather processing (EC, 2001). Dhayalan et al. (2007) reported that in addition, some 170,000 tonnes of tanned leather waste are generated annually. Disposal of waste generated in leather production is therefore a serious problem and the importance of technological measures to combat environmental challenges from leather processing activities is now increasingly recognized (Thanikaivelan et al., 2005).
The high chromium content in tannery waste prevents its use as a fertilizer and conventional handling of tannery waste involves landfills and incineration. However, these two technologies are discouraged as they fail to resolve the solid waste disposal problem in an ecologically acceptable manner (Dhayalan et al., 2007). Consequently, new alternative re-uses of tannery waste, such as biodiesel production (Özgünay et al., 2007) and remanufacture into bone flour (Montoneri et al., 1994) are being developed. In light of recent developments in the renewable energy market and the increasing cost of waste treatment the on-site anaerobic treatment of tannery waste to produce biogas has become an attractive option for the tannery industry.
Although considerable work has been done in treatment of tannery wastewater (Song et al., 2003, Song et al., 2004, Lefebvre et al., 2006), anaerobic treatment of tannery waste has received less attention in recent years. There are some older reports on anaerobic digestion of tannery waste (Cenni et al., 1982, Tunick et al., 1985, Lalitha et al., 1994, Urbaniak, 2006), and reports on hydrolysis of tannery waste (Raju et al., 1997) but few papers deal with the technology of anaerobic digestion of tannery waste.
Tannery waste consists of wastewater, and solid waste fleshings and waste skin trimmings, the former two being composed mostly of lipids and proteins. Gaseous efficiency from fats is estimated to be higher than those of carbohydrates and proteins, therefore lipid-rich waste can be regarded as a large potential renewable energy source (Cirne et al., 2007). For example, 1250 L (68% CH4, 31% CO2, 1% other) of biogas was estimated to be produced from 1 kg (dry solids) of fat, while 790 L and 704 L of biogas was produced from the same amount of carbohydrates and proteins, respectively (Urbaniak, 2006). Indeed high gaseous efficiency was previously found for tannery waste, varying between 950–1120 L from 1 kg organic dry solids, which was predetermined by high content of fat (Urbaniak, 2006).
Anaerobic biomethane formation is a complex process, in which organic compounds are mineralised to biogas. It consists of several phases, such as hydrolysis, acidogenesis, acetogenesis and methanation, carried out by different groups of microorganisms, which partly stand in syntrophic interrelation and depend on different requirements in the environment (Deublein and Steinhauser, 2008). For instance, lipids are first hydrolyzed by acidogenic bacteria to glycerol and free long-chain fatty acids; furthermore glycerol is converted to acetate, while fatty acids convert to acetate, propionate and hydrogen. Finally, methanogenic bacteria which utilize methanol, acetate or hydrogen and carbon dioxide, produce methane (Cirne et al., 2007, Deublein and Steinhauser, 2008).
The aim of this study was to investigate the potential for anaerobic digestion of different types of tannery waste, including fleshings, skin trimmings and tannery wastewater sludge. We focused on two techniques for anaerobic digestion, conventional semi-continuous and more recent, ASBR (Anaerobic Sequencing Batch Reactor) process. The ASBR operates in a cyclic batch mode with four distinct phases per cycle. The four phases are: filling, reacting, settling and release. The advantage of ASBR process according to reports in the literature (Lee et al., 2001, Wang et al., 2002) is better TCOD removal as well as higher biogas production compared to conventional digestion systems. ASBR systems are also popular largely due to possible elimination of equalization tanks and secondary clarifiers as well as relatively simple operations (Zhang and Dugba, 2000, Ioannis and Bagley, 2002, Zupančič et al., 2007). The suitability of these two techniques will be evaluated and the beneficial impact of anaerobic treatment of tannery waste will be discussed.
Section snippets
Characterisation of tannery waste
Leather processing is characterized by large amounts of solid and liquid waste. Pretanning processes are those in which most of the solid waste is produced. Leather production produces vast amounts of wastewater, which is usually treated on-site and also produces excess sludge, which is the main component in the mass balance of tannery waste in our case amounting to 50% of the entire organic load. The other two viable substrates for biogas production produced in tannery industry are waste
Characteristics of tannery waste
The basic characteristics of tannery waste treated in this study are shown in Table 1. The composition of waste fleshings and skin trimmings was constant during our collection, while waste sludge varied substantially. Waste fleshings were predominantly lipid substrate containing 90–92% of fat and up to 7% protein in dry matter. Total nitrogen content was 1.0–1.3% of dry matter. Skin trimmings were predominantly protein substrate with 70–80% of protein and up to 20% fat in dry matter.
Conclusions
The anaerobic digestion potential of different tannery wastes, including fleshings, skin trimmings and tannery wastewater sludge was evaluated. Conventional semi-continuous and ASBR experiments have shown that OLR of 4.0 kg m−3 d−1 of VSS (corresponding to 8.5 kg m−3 d−1 of TCOD) represents the limits of successful and economic operation. In the ASBR process with OLR of 3.96 kg m−3 d−1 we determined a SMP of 0.596 m3 kg−1, and a VSS removal of 71.4%. The anaerobic digestion of selected substrates was
Acknowledgements
Authors wish to thank Mr. Gregor Grom, Mr. Matjaž Omerzel, Mr. Gasan Osojnik and Mrs Liljana Piščanec for help in the presented study.
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