To make wastewater reuse secure for crop irrigation, a multi-barrier procedure to wastewater treatment is required [37] [38] [39]. Such barriers have to involve standard techniques for municipal wastewater treatment (that is, primary mechanical pre-treatment, possible primary settling, biological treatment, etc.) and advanced treatments. Conceivable choices of TTs presenting diverse effluent qualities are shown in Figure 1.

The lowest treatment scheme for safe reuse must involve a traditional depth filtration downstream of a biological process (or an ultrafiltration (UF) membrane [40] as in case of membranebiological reactor (MBR), Figure 1(b)), pursued via a disinfectionstage with UV radiation (Figure 1(a)). This TT has to efficiently permit to handle standard factors (like biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspendedsolids (TSS), E. coli [41] [42] [43], etc.) put in wastewater reuse regulation and guidelines [1].

Chemical disinfection (especially via chlorine [44] [45] [46] [47] [48] ) (Figure 1(c)) is cheapercontrasted to other disinfection choices; however, the generation of disinfection by-products (DBPs) [49] [50] [51] [52] must be taken into account, and the TT may become costly contrasted to other solutions if DBPs are eliminated before reuse [1] [53].

It has to be mentioned that, chemical disinfectants (like chlorine), as well as an MBR with UF membrane and UV radiation, are deficiently efficacious in dealing with CECs [1].

As long as, if (i) the corresponding limit for bacterial indicators is so stringent that UV disinfection is not enough and/or (ii) CECs pollution has to be efficiently decreased, other more efficacious treatment techniques require to be adopted (Figures 1(d)-(g)) [1].

In the middle of AOPs, ozonation and photochemical techniques presented good performances in reducing CECs and ARB. Especially, in the short term, ozonation and UV/H₂O₂ methods are more interesting choices (Figure 1(d)) contrasted to other photo-driven AOPs to remove CECs as well as to efficiently demobilize pathogens [16] since:

1. Their performance has been established via numerous researches found in

publications. Nevertheless, ozonation requires much less energy contrasted to UV/H₂O₂ remediation for the identical CEC decrease degree and illustrates full-scale implementation;

2. Other homogeneous photocatalytic techniques (like photo-Fenton) may need extra expenses (such as pH adjustment, chelating agents’ addition) and/or have not yet been thoroughly studied (like UV/free chlorine, sulfate radical based AOPs, etc.);

3. Heterogeneous photocatalytic techniques until now have critical practical hindrances for full-scale utilization [1].

It should be mentioned that ozonation and AOPs basically necessitate biological post-treatment (i.e. biological sand or activated carbon filtration) to reduce biodegradable oxidation by-products and transformation products (Figure 1(d)). Rapid depth filtration or otherwisea dissolved air flotation treatment may be employed as a pretreatment technique just prior to AOP in the case that residual suspended solids should interfere with subsequent processes [1].

Adsorption on granular activated carbon (GAC) in packed reactors pursued by UV disinfection (in contrast to O₃ and UV/H₂O₂, adsorption is not a disinfection technology) is one more choice to enhance the quality of effluent wastewater prior to reuse (Figure 1(e)). To prohibit GAC packed reactors from a rapid blocking and elevate back flushing periods, cloth or rapid sand filtration may be utilized to eliminate suspended solids prior to the adsorption method [1].

Provided that PAC adsorption is employed in integration with the biological method (via introducing PAC into the biological setup) or as a distinct component subsequently, either depth filtration and/or MF/UF membrane processes have to be utilized to eliminate residual PAC particles prior to discharge (Figure 1(f)). Like in GAC remediation, a UV disinfection must be put [1].

Finally, membrane filtration with NF or RO pursued by UV disinfectionis one more conceivable choice for advanced treatment of wastewater prior to reuse (Figure 1(g)). Pre-treatment via sand filtration may be implemented to reduce suspended solids to dominate membrane fouling, even if it is more frequent to filter settled effluent directly using MF or UF membranes. Further, MF and UF membranes give appropriate pre-treatment for the NF or RO stage (in such a situation ultimate disinfection via UV radiation is not required for crop irrigation). Most importantly, RO treatment would be also useful for crop irrigation thanks to the elimination of salts from the effluent. Nevertheless, for membrane techniques to be potential, there is a necessity of a thorough investigation of the appropriate treatment and/or recycling of concentrates on a case by case basis. Employing efficient concentrate treatment possesses the capacity to improve treatment performance, move to a near zero-liquid discharge and evade undesirable discharge of CEC [1].

This debate aims to propose the “best available technologies (BATs) able to minimize the release of micro-contaminants including ARB & ARGs, and biological risk, and fulfill requirements for safe reuse for crop irrigation” [1]. For all the TTs presented above, the main problems are listed in Table 1. As a result, and taking into account that no detailed comparative investigations handling CECs and ARB & ARGs reduction via advanced treatment techniques are published [16], a comparative economic evaluation would be controversial. Especially, advanced treatment techniques have been compared in terms of either CECs elimination, costs, disinfection performance, ARB & ARGs reduction, generation of DBPs and oxidation reaction products, and final toxicity; however, the total effect on nature via the simultaneous assessment of all these problems has not been examined [16].