Temperature dependent (37–15 °C) anaerobic digestion of a trichloroethylene-contaminated wastewater
Highlights
► Successful treatment of TCE at 15 °C at influent TCE concentrations of 50 mg l−1. ► Reductive dechlorination of TCE favours the production of cis-1,2 DCE. ► Temperature is a more significant driver of microbial community dynamics than the presence of TCE. ► Clones related to cold adaptation of granular biomass were only detected at 15 °C.
Introduction
The process of anaerobic digestion has been extensively tested and proven to be feasible for the treatment of a wide range of wastewater streams at low temperature (<20 °C; Enright et al., 2009; Scully et al., 2006). Adaptations of bioreactor configuration and increased knowledge of the microbial communities involved have allowed significant advances in our fundamental understanding of the process. In particular, these advances have allowed researchers to overcome the traditional complications associated with low-temperature anaerobic digestion, such as decreased substrate-utilisation, growth rates, and biogas production, and increased levels of dissolved methane in bioreactor effluent (Lettinga, 1995). Despite this, the majority of full-scale applications are still implemented in the mesophilic or thermophilic range, although most industrial wastewaters are released for treatment at temperatures below 18 °C (Lettinga et al., 2001). As such, the subsequent heating of these wastewaters prior to treatment can consumes up to 30% of the net energy produced, reducing the efficiency of the system (Del Pozo et al., 2002). Therefore, low-temperature anaerobic digestion (LTAD) presents an attractive alternative, particularly in temperate climate such as in Ireland.
LTAD is an established means for the treatment of recalcitrant, toxic compounds, with laboratory-scale studies reporting the successful treatment of phenol- (Scully et al., 2006) and toluene- (Enright et al., 2007) contaminated wastewaters. In the past decade, the treatment of the chlorinated alkene, trichloroethylene (TCE), by anaerobic digestion has been investigated, as complete conversion of TCE to ethylene can be accomplished under anaerobic conditions by reductive dechlorination, due to symbiotic interactions between phylogenetic groups (Middeldorp et al., 1999). TCE has industrial applications as a metal degreaser, and improper handling of wastewaters has resulted in extensive contamination of groundwater, including 852 of the 1430 sites on the US National Priorities List (ATSDR, 2003) and 45 out of 59 supply boreholes tested in the Birmingham (UK) Triassic Sandstone aquifer (Rivett et al., 1990). TCE is regarded as a priority pollutant, and the US National Toxicology Program 11th Report on Carcinogens listed it as ‘reasonably anticipated to be a human carcinogen’ (USNTP, 2010).
Traditionally, the process of anaerobic digestion for the treatment of such a toxicant was operated as a “black-box” entity, with little investigation into the required adaptation of the microbial communities involved. However, the development of archaeal and bacterial communities, capable of unimpeded treatment of toxicants, and concurrent successful anaerobic digestion, is crucial to the continued success of this process. In this study therefore, molecular techniques were used to investigate adaptations in the microbial community structure in response to environmental stimuli, such as temperature and toxicant addition, during laboratory scale anaerobic treatment. Data from clone library and qPCR analysis is presented to document the changes in archaeal community structure, while PCR–DGGE was employed to monitor bacterial community dynamics.
In light of the above the aim of this research was 2-fold: (1) to investigate the development of the archaeal and bacterial communities in response to variations in temperature and influent toxicant concentrations and (2) to evaluate the feasibility of LTAD (15 °C) of TCE contaminated wastewater by comparison to conventional mesophilic operation (37 °C).
Section snippets
Source of biomass
The granular, anaerobic sludge employed in this study was described by Siggins et al. (2011), and was sourced from a mesophilic (37 °C), full-scale (1500 m3) internal circulation (IC) alcohol wastewater treatment bioreactor, operated at Carbery Milk Products, Ballineen, Co. Cork, Ireland.
Design and operation of EGSB bioreactors
Four pre-inoculated glass, laboratory-scale (3.5 l) expanded granular sludge bed (EGSB) bioreactors, R1–R4 (Siggins et al., 2011), were used for the treatment of a synthetic volatile fatty acid (VFA) based
Bioreactor performance
Following a successful start-up phase (P1; Fig. 1), TCE was introduced to the influent of both R1 and R3 on day 48 (Table 1). COD removal efficiencies remained high (>75%) during the subsequent 26 days (Fig. 1a and b), although at day 94 a decline in both R1 COD removal efficiency (65%; Fig. 1a) and biogas CH4 content (57%; Fig. 1c) were noted. However, they recovered at day 103, with COD removal efficiencies and biogas CH4 content of 85% (Fig. 1a) and 70% (Fig. 1c), respectively. In addition,
Conclusions
The following conclusions can now be drawn:
- (1)
The successful treatment of TCE at 15 °C was noted, with stable bioreactor operation and 96% TCE removal noted at influent TCE concentrations of 50 mg l−1.
- (2)
Reductive dechlorination of TCE favours the production of cis-1,2 DCE, although lack of evidence of accumulation of DCE in the bioreactor indicates that further dechlorination is likely.
- (3)
Temperature appeared to have a more significant effect on both the archaeal and bacterial communities than the
Acknowledgements
The financial support of Enterprise Ireland, The Irish Environmental Protection Agency and Science Foundation Ireland is acknowledged.
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