Greywater treatment using modified solar photocatalyst- degradation, kinetics, pathway and toxicity analysis

Abstract

Solar photocatalytic treatment of greywater, collected from a residential apartment, was performed in the study using nitrogen-doped TiO2 (NP-TiO2) coated on gravels. XRD pattern displayed dominance of the anatase phase of NP-TiO2. Morphological analysis by SEM images indicated the improved porous structure of NP-TiO2 with an average particle size of 109.11 nm with a size range of 91–120 nm. Elemental analysis confirmed the formation of NP-TiO2 by the doping of N into the P-25 structure, and UV–visible spectroscopy results showed improvement in visible light absorption. The efficacy of greywater treatment was evaluated in terms of the degradation of organics, nutrients, surfactants and benzophenone (BP). Solar photocatalytic degradation resulted in significant removal of organics (total organic carbon, TOC removal of 93.7%), with a degradation rate of 0.393 h−1. A maximum of 50% TKN removal efficiency was achieved with photocatalytic oxidation to nitrate. Furthermore, the nitrate removal efficiency of 43% was observed due to the photocatalytic reduction. The maximum removal efficiency of phosphate, surfactant and BP was 55, 75, and 98.5%, respectively. Besides, toxicity in terms of bioluminescence inhibition (for 30 min incubation) showed a reduction from 13.6 to 4% for the treated greywater. The non-specific hydroxyl radical attack, demethylation, and bond cleavage between the carbonyl group and benzene ring carrying hydroxyl and methoxy groups were the dominant mechanism of BP degradation. The catalyst reusability test showed ~0.7% and ~1.5% decrease in TOC and BP removal efficiencies, respectively, after three consecutive cycles.