Biostimulation and bioaugmentation enhances aerobic biodegradation of dichloroethenes
Introduction
The frequently observed accumulation of the isomers of dichloroethenes (DCEs) as the dominant products of microbial reductive dechlorination activity in soil and water under anaerobic conditions and the lack of understanding of the factors controlling biodegradation of DCEs in situ represent significant obstacles to the application of bioremediation as a remedial option in many contaminated systems (Pavlostathis and Zhuang, 1993, Gibson et al., 1994, Bradley and Chapelle, 1997). Therefore, pump-and–treat systems using air stripping and adsorption onto granular activated carbon are the primary technologies used for removing volatile organic compounds (VOC), including DCEs. A decade of performance data, however, has demonstrated that these systems are not as efficient as once thought (Haley et al., 1991). In situ bioremediation has therefore been proposed as an alternative method in order to reduce time and cost for site restoration of systems contaminated with this group of compounds (Skeen et al., 1993, Saaty et al., 1995).
Along with deciding whether or not a site should be remediated using on site treatment or in situ treatment, it is necessary to decide the bioremediation options that needed to be employed. Three types of bioremediation are predominant in the industry today: natural attenuation, biostimulation, and bioaugmentation. The simplest method of bioremediation to implement is natural attenuation, where contaminated sites are only monitored for contaminant concentration to assure regulators that natural processes of contaminant degradation are active (Kaplan and Kitts, 2004). Biostimulation requires adjustments to the site (contaminated soil or water) in order to provide bacterial communities with a favourable environment in which they can effectively degrade contaminants. This includes the addition of nitrogen, phosphorus, and trace minerals while also making appropriate pH adjustments for the proliferation of indigenous microorganisms, hence speeding up the bioremediation process (Venosa et al., 1996, Salanitro et al., 1997). In cases where natural communities of degrading bacteria are present in low numbers or even absent, bioaugmentation, i.e., the addition of contaminant-degrading organisms, can speed up the degradation process (Al-Awadhi et al., 1996, Van Limbergen et al., 1998).
Partial dehalogenation of chloroethenes under anaerobic conditions has led to the speculation that a subsequent aerobic treatment may be necessary. Efforts have therefore been tailored towards investigating the biodegradation of DCEs under aerobic conditions (Malachowsky et al., 1994, Hopkins and McCarty, 1995). Several recent investigations indicate that microorganisms can oxidize DCEs to CO2 in the absence of apparent alternative substrates in microcosms (Bradley et al., 1998, Bradley and Chapelle, 1998a, Bradley and Chapelle, 1998b, Klier et al., 1999). Significant aerobic oxidation of DCE was demonstrated for an organic-rich stream bed sediment (Bradley and Chapelle, 1998a, Bradley and Chapelle, 1998b), organic-rich surface soils (Klier et al., 1999), and organic-poor aquifer sediments (Bradley et al., 1998, Bradley and Chapelle, 1998a, Klier et al., 1999). However, to the best of our knowledge, no study has been conducted to investigate the potential for aerobic biodegradation of DCEs in microcosms incorporating soil and water from contaminated sites in Africa, with the view of providing baseline information for the possible natural attenuation of these compounds in contaminated sites in the African continent.
In this paper, we report the aerobic biodegradation of cis- and trans-DCE by indigenous microorganisms in soil and water collected from some contaminated sites in South Africa as well as the effects of biostimulation and/or bioaugmentation on the biodegradation of the compounds in soil and water microcosm settings under aerobic conditions pursuant to providing necessary information for the possible in situ bioremediation of these compounds in contaminated sites in South Africa.
Section snippets
Sample collection
The soil samples used in this study were collected from separate locations to represent different soil types (clay and loam soil), while the wastewater samples were collected from the clarification tanks of the Northern and New Germany wastewater treatment plants in Durban, KwaZulu-Natal, South Africa. The soil samples were sieved using a 1.7 mm laboratory test sieve and transported immediately to the laboratory after collection and stored at 4 °C prior to the construction of the microcosms.
Bacterial cultures
The
Soil and wastewater characterization
The physicochemical properties of the soil and wastewater samples used for the microcosm experiments are listed in Table 1, Table 2, respectively. The soil sample AE was characterized as sandy clay soil, while soil sample BF was classified as a sandy soil based on textural analysis (Table 1). The pH of the soil and the water samples ranged between 4.92–5.02 and 6.94–6.98, respectively. Sodium concentration was found to be 10-fold higher, potassium threefold higher, calcium about one hundred and
Discussion
Some chlorinated compounds act as sole carbon source for some microorganisms under aerobic conditions and several microorganisms have been isolated that can use a number of chlorinated compounds as growth substrates (Stucki et al., 1983, Janssen et al., 1985). However, there is very little information on the potential for aerobic biodegradation with dichloroethenes as a sole carbon source, despite the observed accumulation of these compounds as products of reductive dechlorination of higher
Acknowledgements
This study was supported by the National Research Foundation (NRF) of South Africa, Water Research Commission of South Africa and the South African Consortium for Development and Environment—Industry and Urban Areas (SACUDE—I & UA), Denmark. The authors wish to express their appreciation to Dr. D.H. Pienaar of the Department of Chemistry, University of KwaZulu-Natal (Westville Campus) for assistance with gas chromatography. The first author is grateful to the management of Obafemi Awolowo
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