Inhibition of heavy metals on fermentative hydrogen production by granular sludge
Introduction
Heavy metals are present in significant concentrations in some industrial wastewaters and municipal sludge, and are often the leading cause for the upset of the wastewater treatment process (Lester et al., 1983, Stronach et al., 1986, Fang and Chan, 1997). Heavy metals can be stimulatory, inhibitory, or even toxic in biochemical reactions depending on their concentrations. A trace level of many metals is required for activation or function of many enzymes and co-enzymes. Excessive amounts, however, can lead to inhibition or toxicity. This is mostly due to the chemical binding of heavy metals to the enzymes, resulting in the disruption of enzyme structure and activities (Vallee and Ulmer, 1972).
Sludge and concentrated wastewater are conventionally treated by anaerobic processes leading to the production of methane (Gallert et al., 2003). More recently, a new anaerobic technology has been developed to convert organic wastes into H2 (Li and Fang, 2007). Recent research has shown that the H2-producing sludge may agglutinate into granules, similar to methane-producing sludge, in upflow reactor to facilitate H2 production with high biomass concentration (Li et al., 2006). Although the effects of heavy metals on the anaerobic methane-producing process have been widely studied (Fang and Hui, 1994, Fang and Chan, 1997, Lin and Chen, 1999), little is known on their effects on fermentative H2 production.
A series of batch experiments were conducted in this study to investigate the inhibition effect of heavy metals on the production of H2 from a sucrose-containing wastewater by granular sludge. The six heavy metals selected were those commonly found in electroplating effluents, including Cd, Cr, Cu, Ni, Zn, and Pb.
Section snippets
Inoculums
H2-producing granular sludge was sampled from a packed-bed upflow reactor which has been operated at 26 °C treating sucrose-containing wastewater for over 500 d. This sludge had a H2 production yield and rate of 1.22 mol-H2 mol−1-sucrose and 6.17 l-H2 l−1 d−1, respectively. Detailed characteristics of this granular sludge have been reported in a previous study (Li et al., 2006).
Batch tests
Six series of batch tests were conducted in duplicate for Cd, Cr, Cu, Ni, Zn and Pb, respectively, in 250 ml glass serum
Results and discussion
In all experiments, the biogas produced contained only H2 (35–56%), CO2 (44–65%) and residual N2 from the initial purging. The biogas was free of methane due to the lack of methanogenic activity in the sludge. The VFA and alcohol residues in the mixed liquor were mainly acetate, butyrate, ethanol and propanol, plus small amounts of butanol, valerate and caproate.
Inhibition of individual metal to H2 production was represented by concentration CI,50, at which the H2-producing activity of the
Conclusion
Inhibition of six heavy metals commonly found in electroplating effluent inhibited the bioactivity of H2-producing sludge in the following order: Cu (most toxic) ≫ Ni ∼ Zn > Cr > Cd > Pb (least toxic). The CI,50 values for individual heavy metals were Cu 30 mg l−1, Ni and Zn 1600 mg l−1, Cr 3000 mg l−1, Cd 3500 mg l−1, and Pb >5000 mg l−1. H2-producing sludge exhibited in general higher resistance to metal toxicity than methanogenic granular sludge.
Acknowledgements
The authors wish to thank the Hong Kong Research Grant Council for the financial support of this project (HKU7007/02E), and Chenlin Li wishes to thank HKU for the postgraduate studentship.
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