Harnessing electrons – seizing the potential of transferable technology

Suresh Pillai, Andrzej Chmielewski and Jianlong Wang describe how ionising radiation technologies, routinely used for medical disinfection, could be transformative for the water sector. 

Traditional wastewater treatment focuses on the treatment of sewage to prevent the transmission of disease and to avoid pollution. However, increases in population, industrial expansion, the necessity for greater food production and acute water shortages have encouraged utilities to take a more pragmatic approach to wastewater management, which sees human waste as an opportunity to recover water, energy substrates and nutrients. This aim is driving innovation and the adoption of new technologies that are globally applicable, economically sustainable, and highly scalable. 

Transferable technology 

One potential technology that could benefit wastewater treatment has, to date, been associated with the pharmaceutical industry. Ionising radiation technologies apply gamma, electron beam (eBeam) or X-ray techniques to create matrices that can treat microorganisms and pollutants, such as polychlorinated biphenyls (PCBs), per- and polyfluorinated substances (PFAS), and trichlorothylene (TCE), effectively treating even trace levels of pollutants (parts per million or parts per billion). Ionisation is so efficient that organic molecules that were once considered recalcitrant are now susceptible to even very low doses of ionisation.  

Why have ionising technologies been overlooked? 

Use of gamma irradiation for environmental applications is not new. Scientific publications dating back to the 1950s have highlighted the value of this technology for various wastewater streams (Lowe et al., 1956; Praveen et al., 2013; Chmielewski and Han, 2017; Wang et al., 2022). It has been tried at laboratory and pilot scale in several countries, including the USA, Germany, Japan, Poland, Canada, India, China, Austria, and the former Czechoslovakia. Table 1 lists pilot-scale and full-scale gamma and eBeam projects.  

One of the reasons the environmental industry has overlooked this powerful technology was probably because the original iteration of it was the use of radioactive cobalt-60 for generating gamma radiation as the source of ionising radiation. However, times have changed. Today, when the world refers to ionising technology, many are referring primarily to eBeam technology. There have been several advancements in this technology in terms of robustness and dependability, and it is now used routinely for the sterilisation of medical devices. 

Cobalt-60 has been used for more than 65 years, saving millions of lives and bringing economic benefits to sectors such as medicine, agriculture and industry. However, security concerns over using a radioactive isotope in the context of a wastewater treatment plant, and the economics of acquiring radioisotope cobalt-60, makes this technology unsustainable for environmental industries in much of the world. 

Several international bodies have funded research into ionising technology for the destruction of organic pollutants, such as microbial pathogens and man-made organic pollutants. In the USA, the Water Environment Research Foundation (WERF), the Strategic Environmental Research and Development Program (SERDP) and the Environmental Protection Agency (EPA) have funded extensive research to demonstrate the utility of eBeam technology for the destruction of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) in solid matrices and liquid media, including soils, effluent, sewage sludge and groundwater. Several publications highlight mechanisms of degradation and the usefulness of eBeam technology to degrade PFAS, including a comprehensive workshop report published by the US Department of Energy, detailing the technical specifications of the equipment required to address the needs of the environmental sector.  

Electron beams can be produced by commercial off-the-shelf capital equipment known as electron accelerators (see box). There are several types of accelerators – such as Dynamitrons, Rhodotrons, and linear accelerators – and there are North American, European and Asian manufacturers of this equipment. The different equipment is characterised by differences in the energy of the electrons that they can generate, as well as differences in the power of the system. The electron energy (measured in million electron volts) will dictate the depth of water/sludge that the electron beam will penetrate, while the beam power (kilowatt) determines the throughput of the system, and the application parameter dose (measured in kilograys) indicates the energy deposited. There are several good reference texts that discuss these features in greater detail. Electron beam technology is not new. It is quickly becoming the go-to technology for the sterilisation of medical devices, the pasteurisation of foods, and the phytosanitary treatment of agricultural products when crossing international borders.  

EBeam technology is not only an efficient disinfection and environmental pollutant remediation technology, but studies suggest it can be a disruptive technology to several commonly used technologies in the water and wastewater industry, such as UV disinfection, ozone disinfection and, possibly, thermal hydrolysis of sewage sludge. Studies performed in South Korea demonstrate that, even at a very low dose of six kilograys, methane generation was at 231 m3/day with only a 10-day residence time. The resulting sludge had reduced viscosity and dewatering was potentially improved. Preliminary studies suggest that the total cost per dry ton of a 100 million gallons/day treatment plant, using a 100-kilowatt eBeam system inclusive of mesophilic aerobic digester (MAD), was approximately $293. Studies in Poland have also demonstrated the economic value of eBeam technology for sludge hygienisation and co-generation of methane. Detailed comparison studies are needed to truly determine the cost-effectiveness of eBeam technology compared with thermal hydrolysis.  

Scaling up 

Though there is a significant amount of empirical data to demonstrate the economic value and the technological effectiveness of eBeam technology, no mega-scale treatment facilities employ it. Other than in China, where there are operating eBeam treatment plants for treating industry and hospital wastewater, there are no functioning eBeam facilities in the West. There are several possible reasons for this, the most important being that the environmental community rarely deploys a technology with which it is not familiar. This lack of familiarity could be attributed to the lack of exposure of this technology in undergraduate and graduate curricula. There needs to be a concerted effort to socialise this technology among the environmental engineering community. The global emphasis on sustainable technologies to address the Sustainable Development Goals will require the environmental community to better understand how this technology can be used to harness electrons and clean and power the world of the future. •

More information 

Chmielewski, A.G., Han, B., Electron Beam Technology for Environmental Pollution Control, 2016, Top Curr Chem (Z) 374, 68. doi.org/10.1007/s41061-016-0069-4 

Lowe, H. N., Lacy, W. J., Surkiewicz, B. F., Jaeger, R. F., Destruction of microorganisms in water, sewage, and sewage sludge by ionizing radiations, 1956, Journal (American Water Works Association), 48(11), 1363-1372. 

Praveen, C., Jesudhasan, P. R., Reimers, R. S., Pillai, S. D., Electron beam inactivation of selected microbial pathogens and indicator organisms in aerobically and anaerobically digested sewage sludge. Bioresource Technology, 2013,144, 652-657. 

Wang, J., Wang, S., Chen, C., Hu, J., He, S., Zhou, Y., Lin, J., Treatment of hospital wastewater by electron beam technology: Removal of COD, pathogenic bacteria and viruses, 2022, Chemosphere, 308, 136265. 

Suresh D Pillai is professor of molecular microbiology at the Department of Food Science & Technology and director of the National Center for Electron Beam Research, Texas A&M University, USA; Andrzej G Chmielewski is director of the Institute of Nuclear Chemistry and Technology, Warsaw, Poland; and Jianlong Wang is deputy director of the Institute of Nuclear and New Energy Technology (INET), Tsinghua University, China. 

Electron beam technology – the science 

The primary difference between eBeam and gamma technology is that eBeam technology does not rely on radioactive isotopes. Instead, it uses electricity to energise electrons in industrial-scale ‘electron accelerators’ to as much as 10 million electron volts (10 MeV). These energetic electrons are extremely efficient for radiolysis of water molecules. The water radical cation (H2O•+) and pre-hydrated electrons (e-aqueous) formation is the primary process of radiolysis of water. Both reactions take place at a speed of approximately 10-16 seconds. The radical cation quickly (in approximately 10-14 seconds) loses a proton to the neighbouring water molecules forming the hydroxyl radical as follows H2O•+ + H2O g•OH + H3O. The hydroxyl radical (•OH) is extremely reactive and is responsible for many of the beneficial attributes of this technology. The relative amounts of •OH and the e-aqueous depends on the pH of the aqueous medium. Under basic conditions e-aqueous tend to predominate, while •OH predominate under neutral or acidic conditions. In totality the radiolysis of water can be expressed as e- (electrons) + H2O g[2.6] eaq-  +[0.55]H•+[2.7] H3 O+ +[0.7] H2O2+[2.6]HO•+[0.55] H2 where the values in parenthesis represent G values (number of species produced by 100 kV energy absorbed).  

EBeam technology is paradigm shifting because it uses regular electricity and simultaneously creates both reduction and oxidation processes without the addition of any chemicals. These powerful oxidation-reduction reactions occur almost instantaneously and can be characterised as an advanced oxidation-reduction process. Besides the reactive species, energetic electrons can also cause direct damage to organic molecules, such as the DNA, RNA, and organic pollutant molecules. Therefore, both the direct and indirect mechanisms make this technology extremely effective and time efficient.  

Table 1: Summarised list of ionising technologies used in the wastewater industry