**5. Bactericidal effect**

Until a recent past, the radiation effects on the bacterial load and removal of noxious chemical compounds had only been performed in small samples of sludge irradiated under laboratory conditions and mainly address either only the microbiological or the chemical effect of radiation in a sample of sludge [16–20]. Processing and disposal of wastewater sludge is a critical problem worldwide [18] and especially in large metropolitan such as Washington DC in the USA and Tokyo in Japan. Therefore, new technologies to solve the problem of safely disposing of sewage sludge are constantly being sought. An attractive solution for the disposal of wastewater sludge is its utilization in crop fields and landscaping as a fertilizer due to its high content in natural nitrogen compounds. However, in order to be used as such, it must be converted into a class A biosolid [21] which is a form of treated sludge that is deemed safe for humans and animals. In recent years, electron beam technology have shown to be an economically alternative that could be used to meet these regulations by considerably reducing the number of potentially harmful bacteria such as the fecal coliform bacteria and helminth ova such as A*scaris ova* which are both ubiquitous contaminants present in the sewage sludge. The fecal coliform bacteria are a group of bacteria that are released in the environment through the fecal excrement of humans, wildlife, and livestock. They typically occur in the digestive tract of humans and warm-blooded animals. When present in sewage sludge, they are indicative of the presence of human and animal pathogens including some strains of *Escherichia coli*, *Shigella flexneri*, and *Salmonella typhimurium*. Among the fecal coliforms, the *E. coli O157:H7* strain which was responsible for many outbreaks has retain attention of media in the past several years. Indeed, *E. coli O157:H7* was found contaminating drinking water and vegetables, causing cases of stomach cramping, vomiting, and bloody diarrhea in patients. However, in most cases infections with the *E. coli O157:H7* strain were mild or with no symptoms. Most people infected with *S. typhimurium* develop diarrhea, stomach cramping, and fever which could cause the patient to experience fatigues and dehydration. An infection with *S. flexneri* is much more serious with severe stomach pain, dehydration, severe diarrhea, and fever. Patients suffering for shigellosis feel very sick and stay in bed at the pick of the infection. In farming areas where livestock is grown, beside occurrence of fecal coliforms, ascaris ova that when ingested by a person or an animal would hatch and develop into adult worms, *Ascaris lumbricoides* cause stomach pain, nausea, vomiting, and fatigue. In patients who are heavily infested with ascaris worms, the parasites could be expelled through feces and/or vomit.

Additionally, the electron beam technology could be used to reduce the concentration of volatile organic sulfides and other volatile organic compounds (VOCs) responsible for the unpleasant odor in sewage sludge. Moreover, electron beam treatment of sludge is not an energy intensive process which means that it has a small footprint and the processing times are usually short. The main effect of radiation on sewage sludge is in the radiolysis of water producing OH\* and H\* radicals and hydrated electrons, highly reactive chemical species which rapidly react with organic compounds in the sludge. The main intermediates in this process are:

$$\mathrm{H\_2O} \rightarrow \mathrm{[2.7]} \mathrm{OH^\*} + \mathrm{[2.6]} e\_{aq}^- + \mathrm{[0.6]} H^\* + \mathrm{[2.6]} H\_3 \mathrm{O^\*} + \mathrm{[0.7]} H\_2 \mathrm{O\_2} + \mathrm{[0.45]} H\_2 \tag{1}$$

The effect of electron beam irradiation on the microbial reduction in sludge is accomplished by a two-fold effect of the irradiation on the sludge [22]. The first one is the direct effect of radiation on the microorganisms disrupting the structural integrity of DNA molecule and eventually killing them. The other one is the indirect effect caused by the radiolysis of water described above. The chemical active species which are produced in water because of the irradiation will cause oxidative damage on nucleic acids, proteins, and lipids in microorganisms leading to their death. Typically, at a given electron beam radiation dose, the killing of microorganisms occurs at a constant rate over time. Similarly, the effectiveness of the killing of the microbial populations including that of bacteria increases with the radiation dose. The effectiveness of electron beam radiation on the killing of microorganisms is better represented by the so called *D10* value which is the dose of radiation required to kill 90% of the population of microorganisms present in the sludge sample. Because of the difference in structural complexity of microorganisms, *D10* values may vary more or less significantly from one microorganism to another. Thus, for *Ascaris ova,* that *D10* value was determined to be in the order of 0.39 kGy [23]. For other microbial contaminants of sludge including *S. typhimurium* and *E. coli*, these *D10* values were determined to be 0.3 and 0.34 kGy respectively [4]. Recent studies performed by Engohang-Ndong and his research collaborators have shown that the electron beam technology could be used at industrial scale to eliminate potential microbial pathogens from sewage sludge [9]. The US-based research team showed that a dose of 25.7 kGy was enough to eliminate *Ascaris ova* to a level that is not detectable to available technique used to count the helminth eggs in sewage sludge including the use of Sedgwick Rafter cells to count the detectable ascaris ova. Thus, at that electron beam radiation dose, it was possible to achieve a class A sludge. According to the US Environmental Protection Agency (EPA) standards, to be considered class A biosolids, the sewage sludge must contain less than one *Ascaris* ovum per four gram of sludge dry weight [21]. The dose needed to eliminate fecal coliforms to the norm set by the US EPA to convert sewage sludge to class A biosolids is much lower. The experimental dose determined by Engohang-Ndong and collaborators is 6.7 kGy. In other words, when risks of contamination of sewage sludge by helminth eggs is reduced such as in heavily urbanized areas, the doses needed to convert sewage sludge to class A biosolids that in turn could be used to enrich the soil for landscaping and agricultural purposes is very beneficial and requires low consumption of energy.

### **6. Economic aspects**

An important aspect in the implementation of a new technology such as an electron accelerator in a wastewater treatment plant is to anticipate its impact on the operation costs of the facility and on the environment. With respect to the irradiation of sewage sludge several authors have addressed this issue. A team in Florida reported a cost of \$2.50 per 1000 gallons of sludge for a 1.5 MeV electron irradiation facility running at 160 gallons per minute [24], while another group compared gamma and electron beam irradiations for a sample of activated sludge and obtained treatment costs of \$4.20/m3 for gamma irradiation and \$2.10/m3 for electron beam irradiation, which are lower compared to \$4.85–\$5.19 when using conventional technology at the Central District Wastewater Treatment Facility in Miami Dade County [25]. Furthermore, a comparison was made between irradiation at a dose 6 kGy and incineration of sludge samples and showed a cost of \$60.87/ m3 for this latter compared to \$3.12/m3 when using gamma radiation. Similar results have been obtained by Engohang-Ndong et al. using a 3 MeV electron accelerator. In this case these authors reported that the cost to irradiate one cubic meter of sludge to a dose of 25.7 kGy will be \$1.26 [9]. That comparative cost analysis tends to show that electron beam irradiation of sludge consumes less energy than other technologies. Furthermore, electron beam irradiation requires processing times,

and very importantly more environmentally friendly technology compared to other technologies such as gamma radiation and incineration.
