Anaerobic digestion of tannery waste: Semi-continuous and anaerobic sequencing batch reactor processes

https://doi.org/10.1016/j.biortech.2009.07.028Get rights and content

Abstract

Disposal of the vast amounts of tannery waste that are currently generated is a significant problem. Anaerobic treatment of different types of tannery waste (fleshings, skin trimmings and wastewater sludge) was investigated. The biochemical methane potential is the same at 37 °C or 55 °C and an assay of this was shown to be an appropriate screening tool with which to estimate the susceptibility of a substrate to anaerobic digestion. The start-up procedure of a tannery waste thermophilic anaerobic digestion in 100 days using seed from mesophilic digester processing municipal sludge is presented. The specific methane production potential at 55 °C is estimated to be 0.617 m3 kg−1 of volatile suspended solids for tannery waste sludge, 0.377 m3 kg−1 for tannery waste trimmings and 0.649 m3 kg−1 for tannery waste fleshings. Additional concerns such as chromium content, salinity and temperature fluctuations were also addressed. Chromium content and salinity showed no adverse effects; however a reactor temperature reduction of 4.4 °C led to a drop in biogas production of 25%, indicating a requirement to keep the temperature constant at 55 °C.

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.

References (29)

  • G.D. Zupančič et al.

    Treatment of brewery slurry in thermophilic anaerobic sequencing batch reactor

    Bioresource Technology

    (2007)
  • G.D. Zupančič et al.

    Heat and energy requirements in thermophilic anaerobic sludge digestion

    Renewable Energy

    (2003)
  • APHA-AWA-WEF, 2005. Standard Methods for the Examination of Waters and Wastewaters, 21st ed. Washington,...
  • BIPM-IEC-IFCC-ISO-IUPAC-IUPAP-OIML, 1995. Guide to the Expression of Uncertainty in Measurement. International...
  • Cited by (97)

    • Enhancement of biogas production by anaerobic co-digestion of leather waste with raw and pretreated wheat straw

      2022, Energy
      Citation Excerpt :

      Another great advantage of AcoD system is that it can treat more than one waste simultaneously [15,17]. The main points that must be considered to evaluate the co-digestion with leather waste are: (i) they are not as bioavailable to micro-organisms due to the processing of leather [18]; (ii) recycle techniques, such as incorporation in other materials or their direct use as fertilizer, are restrictive because of the large number of inhibiting compounds such as tanning agents and chlorides in the wastes [19]; (iii) the main management technique of these wastes is their disposal in landfills [11], which is highly detrimental to groundwater, soil and atmosphere due to the uncontrolled release of methane [18]. Therefore, besides producing biogas, the biodegradation of these wastes also aims to reduce their organic load and volume since the remaining waste still needs to be managed, and a more mineralized waste is environmentally inert for disposal [20].

    View all citing articles on Scopus
    View full text