Toward chemical-free reclamation of biologically pretreated greywater: solar photocatalytic oxidation with powdered activated carbon

Heterogeneous photocatalytic oxidation is a water reclamation technology which avoids chemical consumption and can be powered by solar radiation. Because this generally sustainable process is of limited efficiency for the treatment of biologically pretreated greywater, it was combined with activated carbon adsorption. The effluent of a constructed wetland for treatment of separately collected greywater was subjected to photocatalytic oxidation using the photocatalyst titanium dioxide (TiO2) “P25” in both the absence and the presence of powdered activated carbon (PAC). Photocatalytic oxidation alone with UV fluences of about 10 Wh L−1 was not capable of reducing total organic carbon (TOC) from an initial concentration of 5.5 mg L−1 safely below 2 mg L−1 as a prerequisite for high-quality water reuse purposes. However, when PAC was added, TOC concentrations subsequent to photocatalytic oxidation were less than 2 mg L−1 even after reusing the TiO2/PAC mixture 10 times. PAC addition is estimated to reduce the insolation area necessary to achieve this target by solar photocatalytic oxidation of biologically treated greywater by a factor 7. This combination process represents an innovative chemical-free technology within wastewater reuse schemes.