## 3 Applications and Limits of ATPs in Water and Wastewater Treatment
### ATP Efficiency Enhancement and Process Combination
One major drawback of pure graphene in water treatment applications is its hydrophobic nature; however, its oxide and reduced oxide (also known as functionalized graphene) are derived hydrophilic materials, suitable for such applications that were tested extensively in combination with AOPs. Studies have shown that graphene-based materials can improve the generation of hydroxyl and sulfate radicals in solutions treated with Fenton processes (enhancing the ferrous ion regeneration rate in conventional Fenton, hydrogen peroxide generation capacity in electro-Fenton processes, and hydroxyl radical generation in photo-Fenton processes), significantly improve the performance of catalytic ozonation, and enhance the activation of sulfate radicals from persulfate and peroxymonosulfate in ARPs [85]. Gamma radiation was observed to induce catalytic degradation of p-nitrophenol (PNP) in the presence of titanium dioxide (TiO\({}_{2}\)) nanoparticles in aqueous solution. In the presence of low TiO\({}_{2}\) doses (0-2.0 g/L), the catalytic effect of NPs on PNP decomposition was not obvious, since this compound could be removed well by irradiation alone; however, the removal of total organic carbon (TOC) and total nitrogen (TN) was significantly accelerated in the presence of TiO\({}_{2}\). TOC removal increased from 16% to 42%, and PNP mineralization was dramatically enhanced [87]. An innovative Fe/C nanomaterial fabricated with Fe-based metal organic frameworks (MOFs) was shown to catalyze \(\gamma\)-induced degradation of the antibiotics, cephalospor C (CEP-C) and sulfametethazine (SMT), in aqueous solutions [88]. An increase of the H\({}_{2}\)O\({}_{2}\) yield (at pH < 4 or > than 11) was observed in radiolysis of aerated solutions with added Al\({}_{2}\)O\({}_{3}\) NPs. It was also observed that the addition of Al\({}_{2}\)O\({}_{3}\) could inhibit scavenging of OH radical precursors by solute NO\({}_{3}\)\({}^{-}\)[89]. Growing research interest in the field of radiation-induced heterogeneous chemical transformations in solutions with nanocatalyst addition could provide significant progress to irradiation-based processes in the future. The potential of ATPs and in particular EB technology to remove toxic organic substances was widely documented, as shown in Tables 6-8. The possibility of combining these and other conventional processes to maximize efficiency and cost-effectiveness was also demonstrated. [PERSON] et al. showed that the EB irradiation and activated sludge process combination was a highly effective method for enhancing the biodegradability of textile wastewater [90]. The effect of ionizing radiation on the biodegradability and toxicity of individual drugs was also studied: Changes in the biodegradability and toxicity of aqueous solutions containing sulfamethoxazole (SMX) by ionizing radiation showed that SMX biodegradability was improved by applying small (0.4 Kgy) irradiation doses. At the 2.5 Kgy dose, full SMX conversion to biologically treatable substances was noted [91]. The combination of radiolytic treatment with conventional processes was studied in a significant number of applications to wastewaters of different origins. The application of \(\gamma\)-irradiation to landfill leachates showed considerable biodegradability improvement [92]; cellulosic wastewater showed accelerated enzymatic hydrolysis [93]; pulp mill effluents registered increased removal of adsorbable organic halogen (AOX)in biological treatment (from 50% to 96%) following \(\gamma\)-irradiation [94], and significant improvement of COD removal after EB pre-treatment [95]. The combined EB irradiation and activated sludge treatment of textile dyeing effluents achieved equal treatment levels at more than halved biological process HRTs [53]. The biodegradability of textile dyeing wastewater was reported to increase from the 0.34-0.61 range, to between 0.87 and 0.96 after irradiation [96]. Studies highlighted that other intermediate processes (i.e., coagulation prior to irradiation) could be appropriate for some (e.g., textile) wastewaters [97].