Abstract
This study was carried out for gathering qualitative information about the potential of photocatalytic oxidation for the removal of trace organics (analysed by gas chromatography coupled to mass spectrometry, GC/MS) from biologically pretreated greywater to make it suitable for high quality reuse applications like groundwater recharge. Additionally, fractions of bulk organics (humic substances, building blocks, and low molecular weight organic acids) were quantified by liquid chromatography with organic carbon detection. Biologically pretreated greywater was subjected to photocatalytic oxidation in open stirred vessel reactors with UV lamps positioned over the reactors. UV doses of 0, 5, and 15 Wh·L-1 and TiO2 P25 photocatalyst concentrations of 1, 5, 10, and 20 g·L-1 were investigated. Photocatalysis experiments with a 15 Wh·L-1 UV dose were also conducted in the presence of 1 g·L-1 powdered activated carbon. Subsequent to mere contact of the photocatalyst to biologically pretreated greywater without UV, GC/MS did not indicate a substantial removal of trace organics, while humic substances were increasingly adsorbed by increasing photocatalyst concentration. A UV dose of 15 Wh·L-1 and TiO2 concentrations > 5 g·L-1 were favorable conditions for photocatalytic oxidation resulting in the removal of most of the trace organics, especially chlorinated phosphate flame retardants. Also humic substances were efficiently removed under these conditions. Photocatalytic oxidation is thus a promising process for advanced greywater treatment prior to groundwater recharge. Addition of powdered activated carbon did not improve trace and bulk organics removal by photocatalysis with a UV dose of 15 Wh·L-1 and with photocatalyst concentrations > 5 g·L-1.
Highlights
Compared to municipal wastewater, greywater is a more suitable resource for high quality reuse applications such as groundwater recharge, because it is safely segregated from industrial wastewaters and less polluted with nutrients and pathogens
A UV dose of 15 Wh·L−1 which was achieved within 3 days irradiation under the experimental conditions, seems to be sufficient for nearly complete mineralization of organics contained in biologically pretreated greywater
In a previous study on advanced treatment of biologically pretreated greywater by photocatalytic oxidation (PCO) combined with powdered activated carbon (PAC) dosage [22], it was shown that PAC addition contributed very well to nonpurgeable dissolved organic carbon (np-DOC) removal when the UV dose was only about 10 Wh·L−1
Summary
Greywater is a more suitable resource for high quality reuse applications such as groundwater recharge, because it is safely segregated from industrial wastewaters (potentially containing hazardous contaminants) and less polluted with nutrients and pathogens. Organics can be removed from greywater by aerobic biological treatment. TOC concentrations were reported to be in the range of 12 - 48 mg·L−1 when bathroom greywater was treated by multistage rotating biological contactors [1]. For higher quality reuse applications, biological treatment of greywater might not suffice. It has been suggested that for groundwater recharge the TOC of the infiltrated wastewater should be reduced to a level similar to the natural background of the local aquifer [4], i.e. to a concentration of 3 mg·L−1 or even less. In California, the TOC guideline for direct groundwater recharge is as low as 0.5 mg·L−1 unless the reclaimed wastewater is diluted with clean water [5]. When treated wastewaters have to be chlorinated before reuse, the TOC should be lower than 2
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