Semi-continuous anaerobic digestion of solid poultry slaughterhouse waste: effect of hydraulic retention time and loading
Introduction
In the past few decades, poultry products have been gaining popularity and presently constitute a significant part of all meat consumption. As a result, poultry slaughterhouses are producing increasing amounts of organic residues and wastes, which again are used as a protein source for animal feed. Because of legal restrictions, rising treatment costs, and environmentally conscious consumers, the treatment of some solid residues and wastes and particularly residues from wastewater treatment processes has emerged as a major concern not only in poultry processing but also in meat industries in general. Diseases such as bovine spongiform encephalopathy (BSE) in cattle has made the industries increasingly aware of the need for hygiene regulations, tighter process control, and the prohibition of further utilization of some animal by-products. In addition, legislation has restricted the disposal of organic residues and wastes in landfill, while disposal costs have been constantly rising, reflecting the legislators’ “polluter pays” principle.
Anaerobic digestion has become an established and proven technology as a means of managing solid organic waste [1]. Besides generating biogas for energy use, the process also destroys pathogens and produces stabilized material to be used as fertilizer in land applications. However, relatively little research has been done on the treatment of solid slaughterhouse wastes by means of anaerobic digestion, relatively few anaerobic digestion plants have been built to treat solid slaughterhouse wastes, and many of these are no longer in operation [2], [3], [4]. Slaughterhouse wastes are generally regarded as a difficult substrate for anaerobic digestion, mainly because of their typically high protein and lipid content [2], [3], [4], [5]. Protein degradation produces ammonia, the unionized form of which is inhibitory to anaerobic microorganisms in high concentrations [6], [7], [8]. Lipids, on the other hand, may cause problems in anaerobic digestion because of their tendency to form floating scum and accumulated long-chain fatty acids (LCFA) [5], [9], [10], [11]). LCFA degradation (β-oxidation) is considered a limiting step in the anaerobic degradation of complex organic substrates [5], [9], [10], [12], because LCFA oxidizing bacteria are slow growers (growth rate 0.1–0.3 d−1) [13] and because as syntrophic substrates, like volatile fatty acids (VFA), their anaerobic microbial degradation is limited by high hydrogen (H2) partial pressure [13], [14]. H2 is produced in several steps in the anaerobic degradation of complex organic substrates and removed from the process mainly by hydrogen-consuming methanogens and some acetogenic bacteria [14]. Furthermore, in high concentrations LCFA [10], [15], [16], [17], [18], [19] and unionized VFA [20], [21] are inhibitory to anaerobic microorganisms. Consequently, to successfully prevent LCFA and VFA from accumulating in the anaerobic digestion of slaughterhouse wastes, the effect of two parameters in particular need to be determined, loading and hydraulic retention time (HRT). Because of the importance of these parameters as regards lipid-rich wastes and because of the scanty information available on the subject, we investigated the effect of the above parameters on anaerobic digestion of solid poultry slaughterhouse wastes in mesophilic semi-continuously fed digesters.
Section snippets
Waste characteristics
The digester studies utilized, as substrate, poultry slaughterhouse waste (Atria Ltd., Nurmo, Finland), which contained minced and mixed fractions of bone and trimmings (42.1% by weight), blood (15.8% by weight), offal (31.6% by weight), and feather (autoclaved at 120°C for 5 min, 10.5% by weight). The waste mixture (pH 6–7, total solids (TS) 31.2%, volatile solids (VS) 26.0%, Kjeldahl-N 24.3 g N/l, ammonia 380 mg N/l, proteins 15.2% of TS, and lipids 10.0% of TS) was prepared and then stored at
Results
We studied anaerobic digestion of the waste in semi-continuously fed, laboratory-scale digesters at 31°C, particularly the effect of HRT and loading on process performance. Fig. 1, Fig. 2 show the loading and performance of the digesters, Fig. 3 the VFA concentrations, Fig. 4 the LCFA concentrations, and Table 2 a summary of the effects of loading and HRT on the characteristics of the digested material.
Discussion
As far as we are aware this is the first study to report the VFA and LCFA concentrations and the methane yields in the long-term continuous operation of anaerobic digesters treating solid poultry slaughterhouse wastes. The process appeared stable with loadings of up to 0.8 kg VS/m3 d and a HRT of 50–100 days, producing methane at a specific rate of as high as 0.52–0.55 m3/kg VSadded. The methane yield here agrees well with the methane production potential of the waste mixture in batch assays,
Conclusions
We found very significant the effect of HRT and loading on the anaerobic digestion of poultry slaughterhouse wastes in semi-continuously fed, laboratory-scale digesters at 31°C, for the process worked well with a loading of up to 0.8 kg VS/m3 d and an HRT of 50–100 days, producing methane at a high rate of 0.52–0.55 m3/kg VSadded. A higher loading and a shorter HRT, however, inhibited and/or overloaded the process, as shown by the accumulated VFA and LCFA and the partially depressed methane yield.
Acknowledgments
We thank Juha Einola, Anni Sirviö, and Miira Riipinen for their assistance and gratefully acknowledge the financial support from the Academy of Finland (Grant No. 38044).
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