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The use of a novel electrochemical oxidation system is investigated for in situ generation of hydrogen peroxide, which constitutes the major reactant for hydroxyl radical (OH•) production via the Fenton reaction. The novel electro-Fenton (EF) “filter” is comprised of a stack of carbon anodic and cathodic electrode pairs, for operation in continuous mode, with potential applications in elimination of toxic organic substances (e.g., pesticides, pharmaceuticals) from drinking and similar water sources. Experiments are performed to assess the performance of three types of electrodes (made of woven carbon fibers, loose carbon fibers, and powdered carbon) in the synthesis of hydrogen peroxide by supplying to the system a low voltage direct current. The efficiency of H2O2 electro-generation as a function of various process parameters (i.e., electrode potential, solution pH, ionic strength) is studied for the most promising carbon material (i.e., loose carbon fibers). The results indicate that the optimal cathodic potential for H2O2 generation is 1.3 V vs Ag/AgCl reference electrode at pH 3, with initial mean dissolved oxygen concentration 8.5 mg L–1. Under these conditions, the average current density and average current efficiency are 5.2 A·m–2 and 70%, respectively. Reduced electrolyte (Na2SO4) concentration significantly affects the H2O2 electrogeneration rate, whereas increased solution pH leaves the current efficiency unaffected. Research is ongoing regarding optimization of the EF “filter” and the effective impregnation (in the porous cathodic electrodes) of iron nanoparticles, which mediate the continuous degradation of organic substances.

The multi-criteria analysis gives the opportunity to researchers, designers and decision-makers to examine decision options in a multi-dimensional fashion. On this basis, four tertiary wastewater treatment (WWT) technologies were assessed regarding their sustainability performance in producing recycled wastewater, considering a 'triple bottom line' approach (i.e. economic, environmental, and social). These are powdered activated carbon adsorption coupled with ultrafiltration membrane separation (PAC-UF), reverse osmosis, ozone/ultraviolet-light oxidation and heterogeneous photo-catalysis coupled with low-pressure membrane separation (photocatalytic membrane reactor, PMR). The participatory method called simple multi-attribute rating technique exploiting ranks was employed for assigning weights to selected sustainability indicators. This sustainability assessment approach resulted in the development of a composite index as a final metric, for each WWT technology evaluated. The PAC-UF technology appears to be the most appropriate technology, attaining the highest composite value regarding the sustainability performance. A scenario analysis confirmed the results of the original scenario in five out of seven cases. In parallel, the PMR was highlighted as the technology with the least variability in its performance. Nevertheless, additional actions and approaches are proposed to strengthen the objectivity of the final results.

This manuscript summarizes the successful start-up and operation of a hybrid eco-engineered water treatment system, at pilot scale. The pilot unit, with 100-L capacity, has been devised for the efficient electrocatalytic production of H2O2 at an air-diffusion cathode, triggering the formation of • OH from Fenton's reaction with added Fe 2+ catalyst. These radicals, in combination with those formed at a powerful boron-doped diamond (BDD) anode in an undivided cell, are used to degrade a mixture of model pesticides. The capability of the plant to produce Η2Ο2 on site was initially optimized using an experimental design based on central composite design (CCD) coupled with response surface methodology (RSM). This aimed to evaluate the effect of key process parameters like current density (j) and solution pH. The influence of electrolyte concentration as well as liquid and air flow rates on H2O2 electrogeneration and current efficiency at optimized j and pH was also assessed. The best operation conditions resulted in H2O2 mass production rate of 64.9 mg min -1 , 89.3% of current efficiency and 0.4 kWh m -3 of energy consumption at short electrolysis time.Performance tests at optimum conditions were carried out with 75 L of a mixture of pesticides (pyrimethanil and methomyl) as a first step towards the elimination of organic contaminants by solar photoelectro-Fenton (SPEF) process. The combined action of homogeneous ( • OH) and heterogeneous (BDD( • OH)) catalysis along with photocatalysis (UV photons collected at a solar CPC photoreactor) allowed the removal of more than 50% of both pesticides in 5 min, confirming the fast regeneration of Fe 2+ catalyst through cathodic reduction and photo-Fenton reaction.

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