Table 2 Advantages, disadvantages and measures to overcome drawbacks [in brackets] of different AOPs considering practical, environmental and economic aspects
From: Advanced oxidation processes for the removal of cyanobacterial toxins from drinking water
AOPs | Advantages | Disadvantages |
|---|---|---|
Ozonation and O3-based AOPs | Ozonation widely employed in water treatment O3 is more selective than ∙OH and less affected by NOM O3 decomposes to ∙OH in water | Toxicity of gaseous O3 [Needs O3-destructor and sufficient ventilation for safe working environmenta] O3 production is expensiveb Dissolved O3 stability is affected by pH and alkalinity |
Photolysis | UV often used for disinfection | Pollutant degradation requires high UV doses [UV-LEDs are mercury-free, have longer lifetime, low energy demand and operational costsc,d] Turbidity reduces penetration depth and attenuates light [requires removal of turbidity prior to treatment; NOM can act as photosensitizer and improve degradation] |
Photolysis in combination with oxidants | UV often used for disinfection UV/H2O2 forms two ∙OH due to homolytic cleavage H2O2 is easy to handle and environmentally sound Chlorine more readily activated by UV and cheaper compared to H2O2 | Turbidity reduces penetration depth and attenuates light [requires removal of turbidity prior to treatment] Gas discharge-based UV lamps have relatively short lifetime and high energy demand [UV-LEDs are mercury-free, have longer lifetime, low energy demand and operational costs] Chlorine may yield toxic halogenated byproducts |
Photocatalysis | UV often used for disinfection Most often used catalyst TiO2 is non-toxic and cheape Pollutant degradation directly by catalyst and indirectly by reactive species | Catalysts may be released into water or deposited into sludge [requires catalyst removal after treatment; heterogeneous or magnetic catalysts simplify removal; immobilized catalysts are reusable which reduces costs] TiO2 requires UV activation [doping with other elements allows use of visible and solar light; UV-LEDs are mercury-free, have longer lifetime, low energy demand and operational costs] Turbidity reduces penetration depth and attenuates light [requires removal of turbidity prior to treatment] |
Fenton oxidation | Iron is highly abundant and non-toxic, H2O2 is easy to handle and environmentally sound Relatively inexpensive reagents and no energy demand Can use also, e.g., ferric iron or other transition metals Potential incorporation of Fenton into iron-based coagulation by addition of H2Of,g2 Photo- and sono-Fenton increase efficacy and reduce costs, especially for solar photo-Fenton | Requires acidic conditions with optimum pH ≈ 3 reported [heterogeneous or immobilized catalysts may extend pH range] Release of iron (or other transition metals) into water or deposition into sludge [requires sludge removal and its treatment; use of heterogeneous or immobilized catalyst reduces environmental release] |
Non-thermal plasma | Reagent-free treatment Continuous production of reactive species, electrons and photons Degrades pollutants persistent to other AOPsh Residual oxidative and disinfective effect of plasma-treated water, which may reduce energy demands and costs; alternative to post-chlorination avoiding chlorine taste and odor | Residual oxidative and disinfective effect of plasma treated water may generate undesired, e.g., toxic or acidic products Requires energy input |
Sulfate radical-based AOPs | Various ways of PMS and PS activation, where activation by redox reactions reduces costsi Continuous production of reactive species in electric discharges and electrochemical processes High reactivity across broad pH spectrum SO4−∙ can degrade pollutants resistant to ∙OHj Lower energy for cleavage of peroxide bond in PS SO4−∙ less affected by NOM and alkalinity than ∙OH | Sulfate has noticeable taste at 250–500 mg L−1, laxative effects at 1000–1200 mg L−1, contributes to corrosionk Possible acidification due to dissociation of HSO4− when PMS is usedl,m |
Electrochemical oxidation | Large number of commercial electrolytes and electrodes Pollutant degradation directly at electrodes and indirectly by reactive species Continuous production of reactive species | Catalytic electrodes (Ti, Ir, Pt, BDD) have high efficiency, but are more expensive than metallic (Cu, Fe, Zn) which can produce secondary contaminationn [Cheap and environmentally sound alternatives proposed—e.g., Ti-coated C-electrode made from pencils or carbon sticks from recycled batteriesn] Cl-based electrolytes may lead to formation of halogenated byproducts N- and P-based electrolytes increase N and P in water (eutrophication) |
Sonolysis | Reagent-free and clean technology Pollutant removal by chemical reactions, thermo- and pyrolysis, and shockwaves and shear forces Easy scale-up of hydrodynamic cavitation processeso | Higher energy inputs increasing the treatment costs [optimization toward lower frequencies may reduce energy demandp; combination with other AOPs reduces costs; hydrodynamic cavitation avoids noise, energy demand and costs] Treatment of large volumes requires greater number of ultrasound transducerso Cavitation is a violent process destructive to materials and requires periodical maintenanceo |
Radiolysis | Homogeneous system Use of scavengers produce specific or single reactive species valuable for studying degradation mechanisms | High capital costs and safety measuresq Rather unsuitable for large-scale drinking water treatment |