ReviewPre-treatment and bioconversion of wastewater sludge to value-added products—Fate of endocrine disrupting compounds
Introduction
In 2001, the Stockholm Convention under the auspices of United Nation Environmental Program, specified a suite of persistent organic chemicals considered as potential endocrine disrupting chemicals (EDCs) in the environment (UNEP, 2001). Such concerns have heightened the need for novel and advanced bioremediation techniques to effectively remove EDCs from a variety of contaminated environmental media including water, wastewater sludge, sediments and soils (Duran and Esposito, 2000, Romantschuk et al., 2000). A number of organic pollutants, such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls and pesticides are resistant to degradation and represent an ongoing toxicological threat to both wildlife and human beings.
Furthermore, recently, wastewater sludge (WWS) has been subjected to reuse for production of value-added products (VAPs) through the route of bioconversion. Bioconversion of WWS into VAPs (biopesticides or other bio-control agents, microbial inoculants, industrial enzymes, bacterial bioplastics and other biopolymers) has been achieved with successful and encouraging results (Ben Rebah et al., 2001a, Ben Rebah et al., 2001b, Ben Rebah et al., 2002a, Ben Rebah et al., 2002b, Ben Rebah et al., 2002c, Lachhab et al., 2001, Montiel et al., 2001, Tyagi et al., 2002, Vidyarthi et al., 2002, Brar et al., 2004, Brar et al., 2005a, Brar et al., 2005b, Brar et al., 2006a, Brar et al., 2006b, Brar et al., 2007, Brar et al., 2008, Brar et al., in press, Barnabé et al., 2005, Verma et al., 2005, Verma et al., 2007a, Verma et al., 2007b, Yezza et al., 2005, Yan et al., 2006). Production strategies were developed to enhance product yield as for example the increase of WWS biodegradability through pre-treatment (Barnabé et al., 2005). VAPs as biopesticide and microbial inoculants will be field applied after preparing formulations from fermented WWS. Apart from the use of WWS as a raw material to replace synthetically used raw material to produce VAPs, WWS possesses other interesting inherent characteristics (Ben Rebah et al., 2001b, Brar et al., 2004, Brar et al., 2006a, Brar et al., 2006b) example, a) suspended solids acting as ultra-violet light shield for insecticidal proteinaceous toxins; b) buffering action; c) presence of extracellular polymeric substances; d) presence of UV absorbing chromopheric units that reduce the need for additives in formulations and preserve or improve the functionalities of the microbial derivatives.
Despite the accruing novel use of WWS for VAPs, WWS are known to contain toxic metals, organic micro pollutants and pathogens. If biopesticides and microbial inoculants derived from WWS are applied on agricultural or forest land, the contaminants in biotransformed WWS could add constraints to their application. Nevertheless, environmental risks related to toxic inorganics; organic compounds, namely dioxins, furans and pathogens in VAPs based WWS can be controlled by: (i) selecting a WWS having lower content of contaminants that respects the local legislations for land application (Yezza et al., 2005); (ii) application of decontamination process to remove toxic metals; (iii) by adopting the necessary step of sterilization for bioconversion process that eliminates pathogens. Moreover, developing contaminant free-WWS based VAPs will certainly favour technology transfer and product commercialization. The VAPs, prior to field application and commercialization are actually subject to rigorous ecotoxicological (namely teratogenicity, carcinogenicity and mutagenicity) tests during registration so that presence of these compounds could limit their use.
Many research studies reporting effect of WWS treatment on endocrine disrupting compounds (EDC) have been compiled by Scrimshaw and Lester (2003). In fact, the pre-treatment of WWS and subsequent bioconversion into VAPs is a very promising approach to dispose and reuse these residues. It allows sludge volume reduction, improvement of sludge quality and potential gains from microbial product sales. For industries, it gives a possibility to produce commercial microbial products from inexpensive and abundant raw materials (even negative cost when considering sludge volume reduction), available all year round and everywhere. Higher product yield can be achieved when the right choices are made with respect to the microbial strain to be cultivated and the technologies for pre-treatment, fermentation, harvesting and recovery (if required) to be applied (Barnabé et al., 2003, Barnabé, 2004). Having WWS based VAPs free of EDCs is important to protect the ecosystem and end users or public reticence over beneficial uses of WWS. Thus, it is important to conduct studies on the fate of EDCs during or after pre-treatment of WWS and subsequent bioconversion into VAPs to secure this innovative practice by obtaining WWS based VAPs, free of contaminants. Thus, this review is an effort to provide information on the most important and abundant EDCs in wastewater sludge. This paper also reports the literature findings on possibility of EDCs removal through WWS pre-treatment applied in various value addition processes (methane, composts, biopesticides, microbial inoculants). The microorganisms involved in value added production, their potential to remove EDCs and produce or degrade toxic intermediates are discussed. Finally, this paper discusses the concept of integrating EDCs removal methods into the scheme of development of VAPs from WWS.
Section snippets
EDCs in wastewater sludge
Municipal wastewater contains a complex mixture of EDCs originating from personal care products, pharmaceuticals, excreted hormones, household and industrial chemicals etc. Main sources are households, institutions, the industry (Eriksson et al., 2003), and rain that collects pollutants from air and surfaces before entering the sewer (Eriksson et al., 2002). Current wastewater treatment plants are normally designed for carbon, nitrogen, and phosphorus removal but a partial EDCs removal is often
Toxic intermediates generated during degradation of EDCs
Biological and chemical degradation and volatilization are major removal pathways for organic pollutants during sludge treatment and dewatering processes (Scrimshaw and Lester, 2003). Some EDCs, namely DEHP, BPA and LAS can be easily degraded by WWS microbial flora generating toxic intermediates (often recalcitrant or having endocrine disrupter properties). In fact, WWS comprises different types of microflora which can degrade various EDCs either to mineralized forms or metabolize them to
Sludge pre-treatment and EDCs removal
One of the limiting steps for the use of WWS as a raw material for the production of VAPs and/or during conventional treatment of WWS by aerobic or anaerobic processes is the availability of WWS biodegradable organic matter (about 30 to 40% of initial total solids present in wastewater sludge) (Verma et al., 2007c). Consequently, many research efforts have been made in the recent past to increase the WWS biodegradability by adopting various types of pre-treatments (chemical, thermal, mechanical
EDCs removal in wastewater sludge during production of VAPs
Biogas and compost production from WWS demonstrate the long standing potential of sludge to support microbial growth (Yan et al., 2007) resulting in the formation of primary and secondary metabolites. From a commercial point of view, WWS rich in carbon, nitrogen and phosphorus and micronutrients could be a very interesting raw material for industrial fermentation. Cultivation of industrial strains in WWS using conventional industrial fermentation procedures (media sterilization, inoculum
Capacity of VAPs producing microorganisms to degrade EDC in raw or pre-treated WWS
The indigenous microbial flora of WWS can produce panoply of enzymes such as fungal or bacterial peroxidases, tyrosinases and laccases, which form a group of phenol-oxidizing enzymes that degrade or detoxify organic pollutants (determined in our laboratory) (Verma et al., 2007b, Subramanian et al., 2008). Enzymatic treatment of wastewaters using oxidative or hydrolytic enzymes has been extensively reviewed (Duran and Esposito, 2000, Tanaka et al., 2000, Torres et al., 2003, Ikehata et al., 2004
Fate of EDCs in finished VAPs during post-application
Committee on the use of treated municipal wastewater effluents and sludge in the production of crops for human consumption, National Research Council report published in 1996 stated that, “wastewater treatment processes also remove many toxic organic chemicals in the wastewater stream through volatilization and degradation. Those that remain in the final effluent may volatilize or decompose when reclaimed water is added to soil. Consequently, only negligible quantities of toxic organic
Future challenges
To date, essentially most of the research on EDCs has been driven by effects (or purported effects), which have caused public concerns. In fact, it would be more logical to start with a chemical, and make an assessment of what effects, if any, it will induce. USA is in the process of screening a very large number of chemicals (probably 50,000!) through a tier of assays for endocrine disrupting activity. This program, termed the Endocrine Disrupter Screening program, has recently begun and will
Conclusions
Determining the fate of EDCs during WWS pre-treatment and subsequent bioconversion into VAPs is primordial for minimizing environmental risks following the application of WWS based VAPs in environment. It can help to improve the WWS quality for beneficial uses. Generally, pre-treatments involve physico-chemical reactions to increase sludge biodegradability. Priority pollutants in WWS will affect their beneficial uses. Thus, studies need to be performed to determine the fate of priority organic
Acknowledgements
The authors are sincerely thankful to the Natural Sciences and Engineering Research Council of Canada (Discovery Grants A4984 and 355254, STP235071, Canada Research Chair) and INRS-ETE for financial support. The views or opinions expressed in this article are those of the authors and should not be construed as opinions of the U.S. Environmental Protection Agency.
References (171)
- et al.
Priority organic pollutant assessment of sludges for agricultural purposes
Chemosphere
(2005) - et al.
Fate of polycyclic aromatic hydrocarbons during composting of lagooning sewage sludge
Chemosphere
(2005) - et al.
Endocrine disrupting compounds removal from wastewater, a new challenge
Process Biochem
(2006) Enzymatic treatment effects on dewaterability of anerobically digested biosolids-1. Performance evaluations
Process Biochem
(2005)- et al.
Combining anaerobic digestion and ozonation to remove PAH from urban sludge
Process Biochem
(2005) - et al.
Ozone pre-treatment as improver of PAH removal during anaerobic digestion of urban sludge
Chemosphere
(2007) - et al.
Impact of Tween 80 during Bacillus thuringiensis fermentation of wastewater sludges
Process Biochem
(2005) - et al.
Sludge based Bacillus thuringiensis biopesticides: viscosity impacts
Water Res
(2005) - et al.
Efficient centrifugal recovery of Bacillus thuringiensis biopesticides from fermented wastewater and wastewater sludge
Water Res
(2006) - et al.
Bacillus thuringiensis fermentation of hydrolyzed sludge — rheology and formulation studies
Chemosphere
(2007)