Anaerobic biological treatment of phenol at 9.5–15 °C in an expanded granular sludge bed (EGSB)-based bioreactor
Introduction
Anaerobic treatment of phenol-containing wastewater in upflow anaerobic sludge bed (UASB) and expanded granular sludge bed (EGSB)-based bioreactors has been well documented (e.g. Chang et al., 1995; Fang et al., 1996). However, virtually all laboratory- and full-scale reactor trials were carried out under mesophilic (>25 °C) operating conditions. Despite this, most wastewaters in the northern hemisphere (e.g. in EU member states) are released for disposal/treatment at ambient or sub-ambient temperatures (<18 °C; Lettinga et al., 2001), which requires that the wastewater be heated prior to treatment, which can consume up to 30% of the energy produced. The feasibility of psychrophilic, or low-temperature (<20 °C), anaerobic digestion (PAD) has been demonstrated at laboratory-scale for the treatment of various wastewater categories (Rebac et al., 1995; Lettinga et al., 1999; Collins et al., 2003; Enright et al., 2005), including phenolic wastewater (Collins et al., 2005b, Collins et al., 2005a). Studies have shown anaerobic degradation of organic matter as low as 2 °C is possible; however, reaction rates are reduced and require more energy to proceed at lower temperatures (Lettinga et al., 2001). Decreased substrate-utilisation and growth rates, and biogas production have been recorded under psychrophilic conditions, as well as increased levels of dissolved (unavailable) methane in bioreactor effluents (Switzenbaum and Jewell, 1978; De Mann et al., 1988; Lettinga, 1995). Nevertheless, the application of the expanded granular sludge bed (EGSB) bioreactor design, which facilitates effluent re-circulation throughout the reactor system, and, thus, promotes mixing and substrate-biomass contact, can alleviate some of these process difficulties.
The maximum phenol loading rate (PLR) and minimum temperature tested was 1.2 kg phenol m−3 d−1 and 15 °C respectively (Collins et al., 2005a). In order to maximise the potential for PAD applications for full-scale treatment, higher PLRs and lower operational temperatures should now be explored. In addition, the potential economic benefits of PAD of phenol have not yet been accurately evaluated (e.g. to include a determination of volumetric CH4 production and daily reactor yields). This research is imperative, as recommended by Enright et al. (2005) and Connaughton et al. (2006), in order to assess the potential of PAD for full-scale application. Positive data in this regard could provide the justification and impetus required for initial capital investments in PAD technologies.
In the above contexts, the aim of this experiment was to establish the upper limit of phenol loading applicable in PAD EGSB-based bioreactors at 9.5–15 °C. PLRs of up to 2 kg phenol m−3 d−1 were applied, and daily methane yields were recorded to assess the economics of the process. Specific methanogenic activity, toxicity and biodegradability batch assays were used to assess the temporal physiological activity of reactor biomass and the toxicity of phenol.
Section snippets
Source of biomass
A granular, anaerobic sludge was obtained from a mesophilic (37 °C), full-scale internal circulation (IC) alcohol wastewater treatment bioreactor at Carbery Milk Products, Ballineen, County Cork, Ireland. The granules were of a regular, spherical shape (∅, c. 0.5–1 mm) and a grey-green colour. The volatile suspended solids (VSS) concentration of the sludge was 75 g l−1.
Reactor set up and operation
Two 3.5 l glass laboratory-scale expanded granular sludge bed-anaerobic filter (EGSB-AF) bioreactors, R1 and R2, which were of the
Bioreactor performance: periods 1–2
A rapid start-up was observed in both bioreactors during the first operational period (P1) with >90% removal noted from day 64 onwards. Upon the supplementation of phenol to R2 influent at the beginning of P2 (day 93) at the concentration of 500 mg l−1 (PLR 1 kg m−3 d−1), reduced treatment efficiency was observed, and by day 106, the R2 COD removal rate was 59%. HPLC analysis of R2 effluent samples taken indicated the accumulation of phenol to a level of 253 mg l−1 (Fig. 1). Improved COD removal
Discussion
This study indicates that phenol can be successfully removed from wastewater in anaerobic bioreactors at 15 °C and at applied PLRs of up to 2 kg phenol m−3 d−1, which represents a 66% increase on previously reported treatment efficiencies (Collins et al., 2005b, Collins et al., 2005a). Our data also compare favourably with those from previous studies carried out under mesophilic and thermophilic operating conditions; for example, Fang et al. (1996) and Fang et al. (2006) operated UASB bioreactors at
Conclusions
The following conclusions can be inferred from this study: (1) the treatment of phenol appears possible in EGSB-based bioreactors at 15 °C and at applied PLRs of up to 2 kg m−3 d−1; (2) phenol-induced toxicity at 15 °C in the EGSB-AF reactor was reversible, thus also indicating the robust nature of the R2 biomass; (3) PAD treatment of both synthetic VFA (R1) and phenolic (R2) wastewaters is feasible at temperatures below 10 °C; (4) the bioenergy yield from phenolic wastewater at 9.5 °C compared
Acknowledgements
The financial support of Science Foundation Ireland (SFI Investigator Programme), through the Environmental Change Institute (ECI), NUI, Galway, and a Laois County Council Research Scholarship to C.S. are gratefully acknowledged. Dr. Gay Keaveney, Galway-Mayo Institute of Technology, Galway, is thanked for his gracious assistance with HPLC analysis.
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