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CHAPTER 1
Introduction
Rita Hochstrat, Dietmar Schlosser, Philippe Corvini and Thomas Wintgens
1.1 POLLUTANTS IN THE AQUATIC ENVIRONMENT
1.1.1 Types, occurrence and fate
The production, use, and discharge of a huge variety of different chemicals continue to pollute the environment. In addition, accidents and armed conflicts contribute to the contamination of environmental compartments. Prominent though by far incomplete examples include heavy pollution of aquatic as well as terrestrial environments with various hydrocarbons arising from crude oil spills as well as spills and leaks related to the production, storage, handling, and use of fuels, and groundwater contamination with chlorinated aliphatic hydrocarbons (CAHs) widely used as organic solvents. 'Classical' environmental mass pollutants frequently have been released into the environment in high quantities, often resulting in very high environmental concentrations and being highly persistent especially under anoxic conditions as exemplified for certain MINOTAURUS target pollutants such as benzene-ethylbenzene-toluene-xylenes (BTEX) compounds, the synthetic car fuel additive methyl tert-butyl ether (MTBE), and CAHs (under anoxic conditions the more highly chlorinated compounds are faster reductively dechlorinated than the less chlorinated ones, often resulting in a relative or even absolute enrichment of less chlorinated compounds). Groundwater concentrations of up to 38 mg/L (benzene), 75 mg/L (toluene), 830 mg/L (MTBE), and 2000 mg/L (CAHs) can be found at particularly contaminated sites (Davis et al. 1999; Heidrich et al. 2004; Rosell et al. 2006).
Point source pollutions arise from single, identifiable sources or origins of release of contaminants such as, for example, leaking fuel storage tanks (Davis et al. 1999; Hyman, 2013) or industrial effluents and wastes (Heidrich et al. 2004; Mazzeo et al. 2010). Groundwater contamination with BTEX compounds, MTBE, and multiple groundwater contamination with BTEX, CAHs, and chlorobenzenes are typical examples for point source pollution with recalcitrant mass contaminants (Davis et al. 1999; Heidrich et al. 2004; Hyman, 2013). Nonpoint source pollutions arise from diffuse sources. Major contributions to nonpoint source pollution come from runoff, impervious surfaces, spray drift, and drainage in agricultural areas. Regardless of whether pollutants arise from point or nonpoint sources, groundwater and soils are preferred sinks for such compounds once they have been released into the environment (Andreoni & Gianfreda, 2007). Accordingly, these environmental compartments were addressed by the MINOTAURUS project (Table 1.1). The possible channeling of certain environmental pollutants, especially micropollutants arising from urban and industrial activities (see below), using sewer systems offers the possibility to remove them using suitable treatment processes. Hence, wastewater treatment was another focal point of the MINOTAURUS project (Table 1.1).
Environmental mass pollutants may be distinguished from the so-called micropollutants (or trace pollutants) mainly due to the respective amounts released and the environmental concentrations resulting thereof. Advanced, mainly mass spectroscopy-based analytical techniques have increasingly enabled detection and quantification of micropollutants occurring at only very low environmental concentrations during the last two decades (which is one reason for also referring to such compounds as emerging contaminants). Accordingly, scientific, public, and legal awareness of micropollutants has increased only during the last decade(s). There is no general definition of micropollutants. Due to the continuous development of new chemicals and the discontinued use of others, a comprehensive list of micropollutants has to be dynamic and its development would be challenging. The most common characteristic of compounds referred to as micropollutants is a concentration in the aquatic environment in the ng/L to the lower µg/L range (Kümmerer, 2011; Murray et al. 2010). Emerging or micropollutants include both hydrophobic and polar compounds with quite diverse chemical structures and applications. They arise from urban, industrial, and agricultural activities, involve nonpoint source as well as point source emissions, and are frequently neither sufficiently degraded nor retained in conventional municipal wastewater treatment plants (WWTPs) not designed for their removal (Kümmerer, 2011; Lapworth et al. 2012; Silva et al. 2012). Recent classifications of emerging contaminants and micropollutants differentiate between industrials, pesticides, and pharmaceuticals as well as personal care products (PPCPs) (Murray et al. 2010). Following this classification approach, the MINOTAURUS target pollutants bisphenol A (BPA) and nonylphenol (NP) represent industrials, whereas carbamazepine (CBZ), diclofenac (DF), 17α-ethinylestradiol (EE2), sulfamethoxazole (SMX), and triclosan (TCS) represent PPCPs (TCS may be considered as a personal care product whereas all other compounds represent pharmaceuticals). Among these target pollutants, EE2, BPA, NP, and TCS exhibit endocrine activities and are therefore also referred to as endocrine disrupting chemicals (EDCs).
A particular challenge with respect to the biodegradation and bioremediation of micropollutants is that due to their very low environmental concentrations they represent only poor growth substrates for microbes, which is not in favour of the evolution of productive microbial degradation pathways typically found in bacteria. Accordingly, microbes capable of utilizing micropollutants as growth substrates seem rare, and the quest for them is challenging. Moreover, micropollutants usually occur in mixture, whereas pollutant-degrading microbes such as bacteria are often more or less compoundspecific (Harms et al. 2011). An overview of the MINOTAURUS target pollutants comprising 'classical' mass pollutants as well as micropollutants, and related important characteristics is given in Table 1.1.
1.1.2 Regulatory frameworks
In line with the distinctions made in section 1.1.1 between classical and emerging pollutants, the regulatory frames established for both types of contaminants differ considerably. They could also be distinguished as regulated and unregulated pollutants.
Sites contaminated with classical pollutants often show higher concentrations of substances which are (acute) toxic and as such dangerous. This certainty about the potential hazard drove the establishment of regulatory frames requesting remediation action and compliance with limit values. Pollution mostly occurs in soil and groundwater and can often be related to a current or past industrial activity. The polluter is often known, as for example, the owner of a site, which allows assigning liability and consequently responsibility for remediation actions.
The situation is quite different for the so-called micropollutants. Substances subsumed under this term are occurring in low concentrations in aquatic compartments and there is still uncertainty about their effects. Stemming from medical and pharmaceutical applications they are not per se hazardous but might exert detrimental effects on nontarget organisms being permanently exposed to low concentration and mixtures of pollutants. Most of them enter the environment through municipal wastewater treatment plants and are discharged by households and citizens. The question whether and how the occurrence of these substances should be tackled using existing or new regulatory instruments is controversially discussed.
1.1.2.1 EU level legislation
In the European Union the protection of surface water and groundwater is governed by European legislation. The community framework for protecting and managing water has been established through the Water Framework Directive (2000/60/EC, WFD). Its final objective is to achieve good status of surface and groundwater bodies. With respect to chemical status, this particularly includes the definition of certain (hazardous) substances and measures to control their discharge into the aquatic environment.
The most relevant daughter directives are:
• The Directive 2008/115/EC on environmental quality standards in the field of water policy (Priority Substances Directive) as amended by Directive 2013/39/EU.
• Directive 2006/118/EC on the protection of groundwater against pollution and deterioration (Groundwater Directive).
These pieces of legislation define further details, such as, compounds, limit values and types of measures. The list of priority substances, for example, includes nonylphenol. Next to environmental quality standards, that is, concentration limits for substances, a so-called watch list of substances has been established. For these 'union-wide monitoring data are to be gathered for the purpose of supporting future prioritisation exercises'. 'Diclofenac (CAS 15307-79-6), 17-betaestradiol (E2) (CAS 50-28-2) and 17-alpha-ethinylestradiol (EE2) (CAS 57-63-6) shall be included in the first watch list, in order to gather monitoring data for the purpose of facilitating the determination of appropriate measures to address the risk posed by those substances'. The directive directly dealing with discharge of treated wastewater into receiving waters (91/271/EEC – Urban wastewater treatment Directive) does not specify any requirements for micropollutants either and thus does not create a need to take action.
Based on the Groundwater Directive (2006/118/EC) Member States are requested to identify pollutants causing risks to the status of groundwater bodies and consequently establish groundwater quality standards at the most appropriate level (national, river basin district or groundwater body) taking into account local or regional conditions. Furthermore Member States are required to set these standards at least for eight substances (including tri- and tetrachloroethylene) as defined by Part B of Annex II of the GWD.
Throughout these directives it is acknowledged that a combined approach is required to effectively control both, point and diffuse sources. It is understood that the range of actions can also comprise clean-up activities and techniques for discharge minimisation, next to preventive measures.
1.1.2.2 National legislation
Whilst most countries have established rules under which they request and perform groundwater remediation, there are almost not legally binding approaches in place to minimize the discharge of certain micropollutants with treated wastewater. Just recently Switzerland has adopted a change in its Water Protection Act to introduce requirements for advanced wastewater treatment including targets for micropollutants removal (FOEN, 2014). The technology to achieve this is not prescribed. To date adsorption to powdered activated carbon and oxidation by ozonation have been tested successfully.
1.2 ENVIRONMENTAL BIOTECHNOLOGY OPTIONS
Established natural attenuation and engineered processes such as activated sludge treatment do not always eliminate organic pollutants. The absence of microorganisms possessing the biocatalytic equipment enabling the biodegradation of the pollutant is one of the more obvious explanations among the multiple possible reasons for the persistence of these compounds. The addition of biocatalysts, that is, whole cells of exogenous microorganisms with degradative capacities or enzymes thereof has been proposed.
Several studies have reported that bioaugmentation of contaminated milieus through the addition of pure strains or enriched consortia to groundwater and wastewater as well as soil is a promising technology (Cirja et al. 2009). Like the hydrolytic treatment of sewage sludge using lipases and esterases to increase the anaerobic digestion of these organic wastes (Dauber & Boehnke, 1993, Knezevic et al. 1995, Lin et al. 1997), the addition of enzymes catalyzing degradative reactions has been proposed to improve the abatement of emerging micropollutants, including EDCs (Husain, 2006, Haritash & Kaushik, 2009).
1.2.1 Challenges for the implementation of bioaugmentation
Despite the catalytic advantage of introducing exogenous biocatalysts various reasons are causing bioaugmentation strategies to be inefficient so far (El Fantroussi & Agathos, 2005):
• The concentration of the pollutants may be too low if serving as growth substrate for degrading microorganisms.
• For cometabolic degradation activity, primary growth substrates need to be provided.
• The growth of added microorganisms can be inhibited by other substances contained in the media to be treated or by the operational conditions (e.g., temperature).
• The competition with autochthonous microorganisms may also affect the process.
• The degradation of target substances requires sometimes long acclimatization period before the onset of efficient degradation.
• The bioreactor configuration and the operation in continuous mode of treatment systems are sometimes not appropriate for bioaugmentation.
Combined together, these reasons often lead to the early washing out of the added microorganisms. This is reflected by the poor performance of activated sludge treatment in removing micropollutants (Cirja et al. 2008). The application of long sludge retention time (SRT) was proven efficient for the removal of many xenobiotics and has been widely recommended. Under these conditions slowly growing microorganisms which can use pollutants as carbon source can multiply to a larger extent counterbalancing their removal within excess sludge (Clara et al. 2005).
1.2.2 Challenges for the use of enzymes
Many remediation applications using enzymes are rather limited to soluble forms of the latter. Similarly to the bioaugmentation, the addition of free enzymes in water and soil media is not trivial. Enzyme processes are often limited by the harsh environmental conditions of the polluted milieu. The enzymes mainly undergo proteolytic reactions catalyzed by exoproteases (Gianfreda & Bollag, 1994; Modaressi et al. 2005). Further factors such as non-optimal pH, unfavorable ionic strength, non-ideal temperature, and presence of inhibitors of the enzymatic reactions represent severe drawbacks for the implementation of such technologies (Brady & Jordan, 2009). The application of soluble enzymes in continuous systems such as WWTPs is limited due to premature washout of the biocatalysts under hydraulic retention time conditions, which are usually less than ten hours (Lopez et al. 2002; Cirja et al. 2008). Due to these difficulties high amount of enzymes are required and cost of treatments increase.
1.2.3 Immobilization of whole cells and enzymes as a solution of choice to circumvent bioremediation difficulties
The formation of microbial biofilms and the sorption of enzymes represent natural processes, which increase their persistence in several systems. The almost ubiquitous colonization of soils and roots by microbial biofilms, the difficulties in decontaminating surgical materials and in cleaning surfaces from proteins are only few examples where immobilization plays a key role (Monteiro et al. 2009). The advantages of the adhesion of microorganisms or enzymes thereof on support materials are numerous (Stewart & Franklin, 2008). Their attachment represents a protection against for example, washing out caused by circulating fluids and associated turbulences and shear forces. Biofilms constitute physical barriers limiting diffusion of inhibitors or competing microorganisms and represent favorable micro-environments for the growth of microorganisms (commensalism, cell to cell communication, etc.) while immobilized enzymes have increased stability and catalytic activity (Brady & Jordaan, 2009). The modern biotechnology makes use yet of immobilization supports to intensify industrial biotransformation reactions based on the use of whole cells of microorganisms and enzymes. Environmental technologies for wastewater and air treatment take advantage of immobilization of microorganisms, for example, biofilms of rotating discs or percolating filters (Pons et al. 2009) or cell aggregates in the activated sludge process, where the recycling of microorganisms relies on the capability of microorganisms to form flocs and to sediment. (Liwarska-Bizukojc, 2005). The immobilization/aggregation or retention (e.g., in MBR) mechanisms can help to operate stable wastewater treatment.
1.3 RECENT RESEARCH – THE MINOTAURUS PROJECT
In addressing the previously mentioned challenges, the MINOTAURUS project has been put together as a collaborative research and development activity. The acronym stands for 'Microorganism and enzyme Immobilization: NOvel Techniques and Approaches for Upgraded Remediation of Underground-, wastewater and Soil'.
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Excerpted from "Immobilized Biocatalysts for Bioremediation of Groundwater and Wastewater"
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