Low-temperature anaerobic biological treatment of solvent-containing pharmaceutical wastewater
Introduction
Anaerobic digestion (AD) is a proven and established wastewater treatment option, which is applied to a variety of wastewater streams from the food and beverage, agricultural and fine chemicals sectors (McHugh et al., 2003). The application of this naturally occurring methanogenic process offers several advantages over aerobic wastewater treatment options, such as lower operating costs, less land requirements and, primarily, the production of biogas (>50% methane), a renewable energy source, which can be used for generation of heat and electricity for industrial or domestic use.
The increased application of AD to a broader range of wastewaters, including more recalcitrant streams such as those from pharmaceutical production, would provide significant environmental and economic benefits to these sectors. This is of particular significance to, for example, countries like Ireland, where many of the world's largest pharmaceutical producers are active and the sector accounts for significant exports. Technologies conferring environmental and competitive advantages on this industry are consequently strategically relevant. Pharmaceutical wastewater can contain a variety of solvents (rarely contain a single pollutant) at varying concentrations generally too low for viable, cost-effective recovery. Organic solvents—though common in these wastewater types—can prove toxic to biological treatment systems (Inanc et al., 2002).
The majority of AD plants worldwide are operated under mesophilic conditions (approximately 37 °C); however, most industrial wastewaters are released for treatment at temperatures below 18 °C (Lettinga et al., 2001). Therefore, many wastewaters are heated prior to treatment, thus consuming up to 30% of the energy produced. However, low-temperature, or psychrophilic (<20 °C), anaerobic digestion (PAD) has recently been proven feasible for the treatment of a range of wastewater categories (Collins et al., 2003, Collins et al., 2004, Collins et al., 2005a, Collins et al., 2005b, Collins et al., 2005c, Collins et al., 2005d; McHugh et al., 2004, McHugh et al., 2005), representing a technological breakthrough for environmental management. PAD presents an attractive alternative to conventional AD as energy is not re-absorbed back into the initial process through heating the bioreactor, thus increasing the net energy gain.
In light of the above, the 2-fold aim of this paper may be summarised as follows: (1) anaerobic granular sludge sources vary with respect to microbial composition; thus, the methanogenic activity and toxicity thresholds, etc. of different sludge samples may also vary. To this end, we investigated the metabolic capabilities of different sludge samples from full-scale anaerobic reactors as the basis for the selection of a suitable potential inoculum for PAD of solvent-containing wastewater; (2) through this screening process, one sludge source was chosen to inoculate expanded granular sludge bed-anaerobic filter (EGSB-AF) reactors and to evaluate the feasibility of anaerobic biological treatment of solvent-contaminated wastewater under low-temperature conditions.
Section snippets
Source of biomass
Three anaerobic sludges, A–C, were used in this study: Sludge A was a granular sludge obtained from a full-scale (1500 m3) internal circulation (IC) bioreactor, operated at 37 °C, at the Archer Daniels Midland (ADM) citric acid production plant, Ringaskiddy, Co. Cork, Ireland treating citric acid production wastewater; Sludge B was a granular sludge obtained from a full-scale, 37 °C IC anaerobic bioreactor at Carbery Milk Products, Ballineen, Co. Cork, Ireland, used to treat industrial alcohol
Methanogenic activity profiles of candidate sludge sources
SMA results indicated the mesophilic nature of the three sludge samples, with each exhibiting higher activity at 37 °C than at 15 °C, for all substrates tested (Table 1). Sludge A displayed a substantially higher SMA at 37 °C with respect to acetate; ethanol and hydrogen conversion than the other two biomass sources (Table 1). Nevertheless, some activity was detected under psychrophilic conditions and sludge A appeared more psychrotolerant than the other sludges (Table 1). Sludge C, which
Discussion
The screening of candidate sludges, by a combination of activity and toxicity testing, could provide a practical, time saving and cost-effective strategy for selecting seed inocula to start up full-scale bioreactors, not only for psychrophilic but also for mesophilic operation. Here, one of the three candidate biomass samples was chosen, based on the results of metabolic screening, to initiate a long-term, duplicated reactor trial (due to financial and time constraints all three sludge samples
Conclusions
The following conclusions can now be drawn: (1) insights into the methanogenic capabilities of mesophilic anaerobic sludges provided the basis for selection of a suitable inoculum for a psychrophilic reactor trial for the treatment of solvent-containing wastewater; (2) the screening of candidate sludges in this manner represents an effective method for the selection of seed inocula for full-scale plants; (3) PAD of pharmaceutical-like solvent-containing wastewater is a feasible treatment
Acknowledgements
The receipt of financial support from Enterprise Ireland and the Embark Initiative of the Irish Council for Science, Engineering and Technology (IRCSET) is gratefully acknowledged. A research scholarship to A.-M.E. from Limerick County Council is also acknowledged.
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Present address: Environmental Microbiology Research Unit, Department of Microbiology, National University of Ireland, Galway, University Road, Galway, Ireland.