Could EB irradiation be the simplest solution for removing emerging contaminants from water and wastewater?

This review paper addresses an innovative, highly promising water treatment solution, aimed at tackling the pressing problem of Emerging Contaminants in natural and drinking waters, and wastewater discharges. Contaminants of emerging concern (CECs), include pharmaceuticals, personal care products, and may have endocrine disrupting effects. They are chemicals without regulatory status so far, and which impact on environment and human health are poorly understood. These compounds are increasingly being detected at low levels in surface waters, and there is concern that they may have adverse impact on aquatic life, and interfere with natural hormones and endocrine systems of living organisms (human and animal). CECs may demonstrate low acute toxicity, but cause significant reproductive effects even at very low levels of exposure. In addition, the effects of such exposure during the early stages of an aquatic organism's life may not be observed until adulthood. It is feared that health effects on exposed individuals could extend long after exposure has stopped, and that exposure in the womb may have life-long effects, and even bear consequences for subsequent generations.

‘Conventional’ water pollutants have been described for decades and their impact on human health and the environment are well-known; effective technologies for their removal are also well established. This is not the case for most emerging contaminants: no effective removal technologies have been discovered up to date, to simultaneously remove all of the concerned contaminants, even though some techniques have been demonstrated to remove some contaminants to a certain extent. It is a shared opinion among researchers that investigation on new technologies for removal of emerging contaminants will be top priority in the future years (Shi et al. 2012 Radiation processing using electron beam (EB) accelerators or gamma irradiators has shown promising in water-related applications. Radiation processing is an additive-free process using short-lived reactive species formed during the radiolysis of water to carry out the decomposition of pollutants (IAEA 2008). Isolated studies have demonstrated the effectiveness of radiation, alone or in combination with other treatments, in the decomposition of refractory organic compounds in aqueous solutions and in the removal or inactivation, of microorganisms and parasites. Application of EB processing promises a cost-efficient, by-products-less and ultimately effective process taking advantage of characteristics not available in other treatment technologies, such as: total absence of chemical additives, capability of generating at the same time both strong oxidants and reducers (oxydril and hydroxil radicals, hydrogen atoms and solvated electrons), capability of processing aqueous, colloidal and opaque solid solutions, compatibility with existing technologies.

The review paper on this technology summarizes results of applications in drinking water, waste water (urban and industrial) and groundwater, and discusses its possible future application.

Since they are so numerous, diverse and ubiquitous, CECs are frequently lumped into categories that describe their purpose, use or other characteristic. Common categories used are: pharmaceuticals (prescription and over-the-counter), personal care products, plasticizers, flame retardants, and pesticides. Other categories describe their nature: surfactants (used in detergents to aid grease removal, in cosmetics as emulsifiers), synthetic hormones (mimicking the action of natural hormones). These categories can overlap, leading to confusion, as there is no standardized set of categories used in studies on CECs. Some of the most common terms used for categorization are listed in Table 1 (WRRC 2013).

Although not all CECs have endocrine disrupting effects, concern with the endocrine disrupting properties of CECs is so common that often the terms ‘Endocrine Disrupting Compound (EDC)’ and the more general term ‘Contaminant of Emerging Concern’ are used interchangeably. From a total of 564 chemicals that have been suggested by various organisations, or listed in published papers or reports, as being suspected EDs, 147 were considered likely to be either persistent in the environment, or produced in high quantities. Of these, however, clear evidence of endocrine disrupting activity was noted in a first scientific assessment for only 66 compounds (assigned Category 1, using the criteria adopted in the study) (EC 2015). From these, a list of 12 compounds (the ‘Dirty Dozen’), such as: Bpa, Dioxin, Atrazine, Phthalates, Perchlorate, Fire retardants, Lead, Arsenic, Mercury, Perfluorinated chemicals, Organophosphate pesticides, Glycol Ethers, has been indicated as the most probably threatening to human health (EWG 2015).

Many CECs have chemical properties making them resistant to natural environmental degradation processes, and even those compounds that undergo transformation or degradation may generate other chemicals that could potentially be more problematic than the original ones. A comprehensive understanding of the sources and pathways of exposure to emerging contaminants is necessary to fully comprehend and evaluate the health risks to humans and ecosystems. The main pathway of CEC exposure begins with the discharge of municipal wastewater effluents (Figure 1). Conventional wastewater treatment typically removes organics, converting them to common gases and water, and pathogens, but it is not designed to remove all CECs. Well-operated facilities can reduce the concentrations of many CECs, and studies have shown that with process enhancement, they could further improve their removal, although the fate of these substances may not always be clear (they might accumulate in the biological sludge or they could transform in other compounds). Some drugs, on the other hand, appear to degrade very little or none when treated with conventional activated sludge. Complete degradation (mineralization) of these compounds would be the preferred choice should a suitable process be available that guarantees such a result.

For these reasons, in most Countries, there is now an environmental priority and ongoing regulatory effort to achieve removal of ED compounds from drinking waters and discharge streams, as stated, in the 2013 Berlaymont Declaration on Endocrine Disruptors (EurActive 2015). The most commonly used processes, however, such as biodegradation and Advanced Oxidation Processes (AOPs), although partly effective, may result in formation of numerous degradation/transformation by-products. These, in turn, are not well studied, as typical screening approaches for known compounds (typically, low resolution mass spectrometry utilising triple quadrupoles technology) are not capable of their identification. Also, as biotransformation pathways are not often known (Wu et al. 2012), and very few standards are available for these products (Helbling et al. 2010).

State-of-the-art process technologies for the destruction of CECs in water include physicochemical treatments such as coagulation–flocculation, with or without addition of PAC, oxidation by chlorination and ozonation, UV irradiation, or combinations thereof.

Among the ‘conventional’ processes present in state-of-the-art wastewater treatment facilities (Bolong et al. 2009; Liu et al. 2009), physicochemical techniques such as coagulation–flocculation were generally found unable to obtain significant CECs removal, except when conjunctly using addition of Powdered activated carbon (PAC), oxidation by chlorination and/or ozonation, and/or UV irradiation (Westerhoff et al. 2005). CECs (carbadox, sulfadimehoxine, trimethoprim) were not removed by metal salt coagulants (Adams et al. 2002), nor were diclofenac and carbamazepine (Petrovic et al. 2003; Vieno et al. 2006). In the end, PAC appears to be the most effective adsorbent especially for those substances containing refractory organic, non-biodegradable compounds (Abu-Zeid et al. 1995), however, addition requires subsequent separation of the spent material and its further processing.

Oxidation is a promising removal mechanism for some compounds, especially when using chlorine or ozone as agents. However, care is needed as oxidation of these chemicals was found to generate by-products, the effects of which are mostly unknown. Ozone oxidizes substrates either directly or by producing hydroxyl radicals that react with other entities. Both are strongly reactive and have been proposed by some (e.g.: Huber et al. 2005) as promising options for the removal of CECs.

Biodegradation delivers mixed results, as not all the compounds (such as steroid oestrogens) are completely broken down or converted to biomass. Even with Best Available Technologies (BAT) adopted, biological treatment removes only a small part of the wide range of emerging contaminants. In a study concerning activated sludge removal of three oestrogens, two ED's and pharmaceuticals, it was found that, at neutral pH, these compounds appear as ions and remain undegraded in the water phase, and are not removed or adsorbed into the activated sludge mass (Urase & Kikuta 2005).

Advanced treatments for removing CECs include ultra-violet (UV) photolysis, ion exchange, and membrane filtration, often in combination with chemical oxidizers (e.g.: O₃, H₂O₂) and/or external catalysts (e.g., TiO₂) addition.

Bisphenol-A (BPA) removal by photocatalysis under simulated solar light (Xenon arc lamp of 450 W power) was experimented by Mboula et al. (2013): BPA conversion up to 99% was achieved with exposure times of up to 140 min. However, the conversion corresponded to a mineralization ratio of just 40% (implying that the remaining 59% had been transformed into unknown intermediate by-products). By reducing exposure times to 20 min., the conversion was limited to 35% (with just 10% mineralization). While UV and ion-exchange do improve the removal of CECs, they are insufficient to be considered as feasible removal options, as shown in the study by Adams et al. (2002), where UV photolysis removed 50–80% of the target compounds, but required an absorbed dose a hundred times higher than that required of a typical disinfection.

A combination of photocatalysis by means of a TiO₂ metal oxide semiconductor, and chemical irradiation by near-UV light (λ < 385 nm), originating the formation of free hydroxyl radicals for removing CECs from urban wastewater was reported (Belgiorno et al. 2007). Removal of some CECs ranged from 12.5 to 99%, with reaction times between 30 minutes and 8 hours, and catalyst concentration in the reaction volume between 0.2 and 2 g TiO₂/L. Degradation kinetics depended also on the presence of alternative additives, such as H₂O₂, KBrO₃ and (NH₄)2S₂O₈, besides free molecular oxygen.

Photocatalysis supplemented by ultrasonic irradiation, which chemical degradation effects derive from acoustic cavitation, i.e., the formation, growth, and implosive collapse of cavitation bubbles in the liquid was also experimented. Under these conditions, extreme temperatures of several thousand degrees, and pressures of several hundred atmospheres are developed locally within the bubbles, serving as ‘hot spot’ micro-reactors in an otherwise cold liquid. Destruction of chemicals can be achieved through a combination of pyrolytic reactions occurring inside or near the bubble and hydroxyl radical-mediated reactions occurring in the liquid bulk (Papadaki et al. 2004).

Ultrasonic irradiation at power levels up to 125 W was applied in addition to UV irradiation. The extent of degradation was around 80% after 120 min of sonication, however, degradation intermediates proved difficult to oxidise (only 20–25% of initial carbon content was transformed into carbon dioxide). A review on the use of AOP's for CEC removal in effluents was also presented by Esplugas et al. (2007).

Membrane filtration technology (reverse osmosis, RO) and nanofiltration (NF) demonstrated themselves as promising alternatives for eliminating micropollutants (Yoon et al. 2006; Esplugas et al. 2007; Bolong et al. 2009). Comparatively, a NF membrane is ‘looser’ than RO, therefore, RO will give almost complete removal, but the higher energy consumption makes it more unfavourable. Mass transport during NF originates from different mechanisms, namely convection, diffusion (sieving) and charge effects. Convection occurs due to the applied pressure difference over the membrane, whereas diffusion mechanism happens due to concentration gradient across the membrane. A third mechanism is the charge effects due to electrostatic repulsion between charged membrane and charged organic compound. In filtration technology, the composition of feed water and its operating parameters have major effects on the rejection on each, or combinations, of the compounds present (Braeken et al. 2006).

Results similar to those above reported were also summarized in a paper by Huerta-Fontela et al. (2011), where incomplete degradation or removal of these compounds through a potabilization process were observed, and further studies indicated as needed, in order to evaluate complete elimination of pharmaceuticals, and their transformation into by-products with potential toxic effects.

All of these process technologies have some more or less significant drawbacks: almost all require extended contact times for the removal or destruction of contaminants to occur (in the order of few- to-several hours to achieve transformations up to about 90% of the original contaminant mass); furthermore, even when such transformation occurs, it does not necessarily lead to full mineralization, as the oxidation of these chemicals was often found to generate by-products, the effects of which are mostly unknown, and may also be harmful (Huerta-Fontela et al. 2011). Destruction of CECs with these technologies was shown to be heavily dependent on environmental and process conditions, with possible cross-interferences among various involved factors.

To enhance their effectiveness, most of these processes require addition of PAC, nanomaterials, catalysts, which constitute extra costs, both as process coadiuvants, and as process residues (e.g.: spent PAC and nanoparticles, process sludges) that should be properly disposed of. When strong oxidants are used, furthermore, process and operator's safety is always an issue. Some of these additives, when used improperly, or by accident, could themself cause the generation of new pollutants (i.e. disinfection by-products, DBPs, when using chlorine, that themselves can become precursors of carcinogenic compounds), or induce undesired environmental contamination when released into the environment (Westerhoff et al. 2005; Chang et al. 2009; Liu et al. 2009; Wu et al. 2012).

As previously mentioned, ED compounds' complete destruction in water/wastewater requires treatments far more advanced than those currently available, even in modern design facilities. EB irradiation might be the key process that can completely eliminate all organically-based CECs (and, potentially, many other organic contaminants, more efficiently and economically than with current technologies). This paper will illustrate the existing EB irradiation technology, used nowadays for many different purposes, seldom, but successfully applied to the field of water treatment (Chmielewski 2011).

Electron beam (or EB irradiation) is a process which involves the use of electrons with high internal energy, to treat an object or medium for a variety of purposes. The effect of the EB irradiation causes the degradation of molecules, breaking their internal chains and therefore reducing their molecular weight. The process in used in the most diverse application areas, from optimization and improvement of industrial productions (production of new films, packagings, cable insulators, medical hydrogels and lubricants; process of surface coatings and adhesives, grafting of new membrane materials, sterilization of pharmaceuticals and of disposable medical devices), to food distribution and safety (irradiation of fruits and vegetables, chilled meats and fish for optimized, additive-free storage, and of agricultural seeds for improved conservation), to flue gas emissions and contaminated soils treatment (IAEA 2008).

Some preliminary applications in water treatment have shown very promising results for emerging pollutants and ED's in recent laboratory tests (IAEA 2014). It is in these applications that EB could represent a true ‘Columbus' Egg’, since radiation processing can be an additive-free, residual-free and, potentially, by-products-free technology exploiting short-lived reactive species formed during the radiolysis of water (solvent) for the efficient decomposition of pollutants contained therein (solutes). For practical purposes, therefore, these processes can be considered highly innovative, alternative AOP's with none of the potential shortcomings related to the latter.

The basic components of a typical EB device are schematized in Figure 2: in a sealed container kept under high vacuum, a heated emitter (cathode) releases electrons that are accelerated by a passage through a grid under high voltage power supply (DC) or radiofrequency (in a more compact designs). Electrostatic and/or magnetic fields are used for controlling (by focusing and deflecting) the way in which the high energy beam is directed on the exit window. Electrons emerging from the window carry an energy dependent on the voltage applied to the anode, while their number depends on the cathode current. By adjusting these parameters it is possible to control, respectively, beam penetration and the dose (absorbed) rate.

The use of EB in water treatment has capability of generating at the same time both strong oxidants and reducers from the H₂O molecule itself (superoxides, a.k.a. hyperoxides, O₂⁻, hydroxyl radicals, •HO, hydrogen atoms, H, and solvated electrons, e⁻_(aq)) that subsequently carry out the degradation reactions of the pollutants. These species are extremely reactive and very short-lived, with half-life in the order of 10 μs at 10⁻⁴ M concentration (Figure 3), and have an even stronger oxidation potential than O₃ (von Gunten 2003).

The measurement of the oxidation potentials of these species is complex, and quite different values are reported by different Authors (Rao & Hayon 1975; Wood 1988). The following Table 2 (Lenntech 2015) shows some oxidation potential values, determined according to a uniform procedure, for some oxidant species. Of importance, in specific, are not the absolute values themselves, but their relative difference compared to commonly used reactive molecules (O₂, O₃, Cl₂). The reactions that occur with EB irradiation are quite similar to those occurring in AOPs, where oxidation is largely brought about by OH-radicals, but much more intense and rapid due to greater density of different radical species.

The effect of irradiation causes the degradation of molecules by the reactive species, breaking their internal chains and therefore reducing the molecular weight (Figure 4). By controlling the irradiation dose (and thus the density of reactive species), organics degradation can be controlled to obtain partial (shorter, more easily biodegradable molecules) or complete decomposition (mineralization).

Processing of liquid streams with EB technology occurs at irradiation doses dose in the order of 1–4 kGy (the Grey –Gy- is defined as the absorption of one joule of radiation energy by one kilogram of matter), and average beam current of 20 mA. These are relatively low values, in the lower range of those commonly used, for example, in the food distribution industry to irradiate fresh vegetables for fungi and bacteria control, in order to increase shelf life (Cleland 2005), an application of quite common diffusion. Irradiation occurs by creating a thin film flow (about 1 mm thick) under the irradiation window, at a water velocity of about 3 m/s, corresponding to a water transit time of about 1/100th of a second.

Since degradative processes are purely based on the high reactivity of short-lived radical species formed by water irradiation, all the contaminant degradation reactions are practically instantaneous. Excess radicals that do not react with contaminant molecules revert back quickly (in the order of few milliseconds) to the original water state, therefore no residues or radioactivity are left in the water.

There are few, but significative successful application attempts of this technology to wastewater treatment reported in literature.

Installation of the first full scale EB in Daegu (S. Korea), to treat 10,000 m³/day of textile wastewater has demonstrated that the process is a cost effective technology, compared to conventional treatment. The wastewater chemical composition showed presence of dissolved organic compounds, organic dyes, surfactants and other compounds. In the organic compounds, Terephtalic acid (TPA) and ethylene glycol (EG) were the major components, with smaller concentrations of hexane, carboxyl-methyl and hydroxyl-methyl cellulose, phenols, starch, waxes, etc. Inorganic compounds present were mainly represented by sulphate anions, sodium cations and small amounts of chlorides and carbonates.

The irradiation facility consists of a high-power electron accelerator (1 MeV, 400 kW) that has been operated continuously since 2005. A suitable and cost-effective irradiation dose was determined by pilot experiments as 1–2 kGy for the required flow rate. EB irradiation in this case is not designed to achieve complete mineralization of all wastewater-contained organic molecules, but only to their degradation to more readily biodegradable forms without the use of chemical additives.

The installation of radiation treatment immediately before biological treatment resulted in a dramatic reduction in chemical reagents consumption and retention times, with increased efficiency in COD (influent value 900 mg/l) and BOD₅ (influent value 2,000 mg/l) removal by up to 40% compared to the previous (non-EB) configuration, in a reduction of necessary biological treatment time, and in an increase in the limiting flow rate of existing facilities by 30–40%. The most significant improvements resulted in decolorizing and destructive oxidation of organic impurities in the wastewater.

The increase in removal efficiency is due to radiolytical transformation of poorly biodegradable or refractory compounds to more readily biodigestible forms. An estimate of the comprehensive actual operation cost of the EB plant, including interest and depreciation of investment, resulted in about 0.3 US$/m³ of treated wastewater (Kuk et al. 2011).

Sparse studies in recent years have demonstrated the effectiveness of ionizing radiation such as, gamma rays and EB alone, or in combination with other treatments, in the decomposition of refractory organic compounds in aqueous solutions, and in the effective removal or inactivation of various microorganisms and parasites (IAEA 2008). EB processing of wastewater was shown to be the most cost effective process, compared to UV and ozonation, for the control of E. coli at flows greater than 5,000 m³/day, requiring irradiation dosages as low as 0.2 kGy (Han et al. 2008). An additional advantage of EB processes in water disinfection is that they would also completely eliminate the possibility of DBPs formation.

Selected dyes and pesticides were efficiently removed using radiation processing; its combination with oxidants, such as ozone or hydrogen peroxide, may further improve removal efficiencies. It has also been shown that the addition of TiO2 prior to irradiation improved the destruction efficiency of pesticides (Emmi et al. 2008; Solpan 2008a, 2008b; Takács et al. 2008; Trojanowicz et al. 2008).

Radiation processing of WWTP effluents showed that the destruction of organic compounds, elimination of estrogenic activity and efficient reduction in the number of microorganisms occurs simultaneously; also, it was shown to be very efficient at removing extremely low concentrations of estrogens (Gehringer et al. 2008; Ahn & Jung 2013).

In a demonstration study on the decomposition of antibiotics and endocrine disruptors, carried out with a mobile, 600 KeV EB, self-shielded accelerator constructed by the Korean Atomic Energy Research Institute (KAERI), a synthetic solution containing lincomycin, tetracycline and bisphenol-A was mixed with real wastewater, obtaining a final concentration of each compound after mixing of 0.5 mg/l. The solution was irradiated at a dose of 1.5 kGy and average beam current of 20 mA for about 1/100th of a second. Lincomycin removal resulted to be 99.8%, tetracycline's 98.8%, and Bisphenol-A 99.1%. In addition, it was observed sterilization of about 98.3% of the microorganisms (coliforms) present in the wastewater, and the toxicity was reduced from an initial value of 15 to just 1.8 TU after irradiation (Lee et al. 2012).

A recent study on the removal of dioxins from water by EB irradiation showed that complete mineralization of dioxins could be achieved at a cost of US$ 0.17/m³ of treated water (Kimura et al. 2012).

An advantage of electron accelerators versus other irradiating processes, such as gamma irradiation, is that they can be switched off whenever desired, contrary to those sources which continuously emit radiation.

The process does not introduce any possibility whatsoever of an eventual secondary environmental contamination, since no residual radiation content will remain in the treated medium, nor the electron accelerator will contain residual radioactivity in its components. The radicals in fact revert back in the order of few milliseconds to the original water state, if they do not react immediately with pollutant molecules, and no residues or radioactivity are left in the water.

Since contaminant degradation reactions are practically instantaneous, the process does not require large storage/reaction vessels to sustain prolonged exposition/reaction times, as traditional AOP's do, working with process contact times in the order of hours.

A few cost estimates concerning the operating financial aspects of this process show that the monetary layout necessary to implement this process is not different from that of possible alternative processes. This should also be balanced with savings (reagents) and environmental advantages (e.g. lack of by-products generation) that the introduction of this technology could induce.

Although capable of complete mineralization of contaminants, EB processes could also be used, if this turns out to be more ergonomical, in combination with existing traditional technologies to achieve the best possible treatment goals at the lowest cost possible.

Existing, though spare, literature on the subject strongly supports the adoption of EB technology in the treatment of specific pollutants, and especially CECs, in water and wastewater. Sparse but encouragively positive testing has been conducted in this area in recent years.

EB technology recent diffusion in many different application areas has caused the cost of irradiation equipment to diminish considerably in the latter years, and the economics of implemented project have been, so far, not distant, if not even competitive, from those of traditional technologies. Non-technological factors that seem to hinder acceptance of the technology are more related to popular belief (radiation being a ‘scary’ word) than facts, since this very same technology is used to process food that we eat and its packaging.

Given the current trend of introducing mandatory treatment of CECs and EDCs from waters, EB irradiation could soon become a winning and final strategy for the solution of this generalized contamination problem.