## 3 Applications and Limits of ATPs in Water and Wastewater Treatment
### Energy Demand and Cost-Effectiveness of ATPs
Available cost figures from 2012 [53] suggested that the specific price tag (US$/W) of electron accelerators (at 1 MeV electron energy) decreases with the installed power according to the approximate relationship:  
\[c_{EB}=220\ P^{0.665}, \tag{13}\]  
that would make the cost of this technology more affordable with increasing treatment capacity (both in treated flow and irradiation dose).  
This is confirmed by preliminary comparative calculations estimating the total cost of _3-log_ disinfection of municipal effluents for irrigation water reuse, summarized in Figure 5[102]. It can be seen that at increasing capacity, the irradiation technology-specific cost becomes similar or lower than that of conventional AOP processes, save for chlorination. Since all these processes (except chlorination) are based on energy conversion (electric to radiation, or to oxidants), it is safe to assume that similar economies of scale would be seen also in uses different from simple disinfection. From the OPEX point of view, it was noted that, generally, EB is energetically more efficient than other AOPs, such as ozonation and UV irradiation [50].  
O\({}_{3}\) production from liquid oxygen generally requires 8-9 kWh/kg (may decrease to 7 kWh/kg in large facilities with capacity > 25 kg/h). Production from ambient air requires the circulation of higher air flows (about fivefold, due to O\({}_{2}\) air content); therefore, the overall specific consumption is higher in this case. UV irradiation normally absorbs about one third of the total installed nominal power. It has been shown that process efficiency does not depend solely on the conversion from electric current to UV-C radiation but also on the system configuration: Even if last-generation horizontal lamps are individually more efficient (42% electric to UV-C conversion) than inclined ones (35% conversion), due to the more advantageous installation geometry, the latter result in a lower overall specific energy demand. In this respect, an important feature of EB equipment is the high conversion efficiency of electric power to irradiation, in excess of 95% for modern accelerators. This results in high radicals' generation efficiency of over 1.0 M\({}_{\rm{OH}}\)/kW.  
Figure 5: Specific total disinfection costs (US$/m\({}^{3}\) treated) of urban effluents for irrigation reuse according to different technologies (data from [102]). The energy demand and efficiency of water treatment facilities is a currently an issue of great relevance, associated with process sustainability and their environmental impact [103]. A method to estimate the energy efficiency of different ATPs was suggested as the determination of the electrical energy per order (EE/O) of a process for a specific contaminant [104]. Defined as the amount of kWhs required to reduce a pollutant concentration by an order of magnitude in 1 m\({}^{3}\) of solution, it is easily applicable to any energy-driven process, and could also include the embedded energy contained in chemical additives or catalysts (i.e., the energy required for their production and supply to the process). Although EE/O is not directly related to the total process costs (it only reflects operational costs, it could provide an indication about the feasibility of alternative process approaches, with rapid determination of their operating costs, known as the expected reduction in contaminant concentration (in orders of magnitude) and the local cost of electricity. Of course, there are other factors (chemical needs, operation/maintenance costs, capital amortization, etc.) that go into a complete cost analysis. EE/O values may vary widely (even more than one order of magnitude) when considering common ATPs, and largely depend on a process efficiency in radicals' generation but are also affected by matrix interference (scavengers), specific process equipment (e.g., lamp efficiency for UV systems), and operating conditions.