Wastewater treatment by means of advanced oxidation processes at basic pH conditions: A review

- Hydroxyl and sulfate radical related AOPs at basic pH were reviewed.
- Studies of model and real wastewater were discussed, with emphasis to real effluents.
- The studies of degradation pathways and by-products were reviewed.
- Economical evaluation of the processes was discussed.
- Good practice for planning the research is described.

Advanced oxidation processes (AOPs) have been used as an alternative and effective option for treatment of industrial wastewater, especially in the case of the non-biodegradable compounds. Despite of several well developed AOPs, the majority of them are effective only at acidic or neutral pH, namely Fenton related processes, making the list of available effective advanced oxidation technologies strongly limited. In many cases, industrial effluents are formed at basic pH conditions. This paper reviews the state of the art of AOPs exclusively at alkaline pH, the type of compounds and effluents effectively degraded, the influence of pH on the efficiency of the processes, economic evaluation and degradation pathways. Spent caustic, polyester and acetate fiber dye effluents, phenol, acidic and sulfur based compounds, specific dyes and drugs were effectively degraded at basic pH. Factors like point of zero charge, pKa of the compounds, amount of radicals produced and activation of the oxidants are crucial factors that affect the treatment efficiency of AOPs at basic pH. H2O2 and O3 were the cheapest processes while peroxone was the most expensive. The treatment costs were in general too expensive and unrealistic using the methodology used in the literature. The alternative methodology proposed in this paper reduced the treatment costs by three orders of magnitude to values more realistic and economical feasible. Literature regarding treatment of real effluents using AOPs at basic pH is scarce and further research is needed to perform complete analysis. It is important to present some alternatives regarding the treatment of alkaline effluents without pH correction using such technologies.

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