S-rGO/ZnS nanocomposite-mediated photocatalytic pretreatment of dairy wastewater to enhance aerobic digestion

Abstract

The treatment of real dairy wastewater by aerobic digestion is a time taking process due to the complex nature of organics present in dairy effluent. Herein, solar light-mediated photocatalytic pretreatment of the dairy wastewater was performed to decompose the complexed organic into shorter chair organics. The sulfur-doped reduced graphene oxide/zinc sulfide (S-rGO/ZnS) nanocomposite was applied as an efficient photocatalyst to solubilize and decompose the organic components in dairy wastewater under natural solar light. The results showed that the photocatalytic treatment enhanced the solubilized chemical oxygen demand (SCOD) by 113% after 6 h of sunlight exposure as compared to 28.1% of SCOD under the photolysis conditions. The aerobic digestion of the pretreated dairy wastewater showed 94% removal of total chemical oxygen demand (TCOD) after 36 days. TCOD of pretreated and untreated dairy wastewater was decreased to the level 71mg/L and 257mg/L after the aerobic digestion, indicating the effectiveness of the pretreatment process. A water quality analysis of photocatalysis-aerobic treatment showed that the values for various parameters such as COD, total solids, nitrogen, alkalinity, oil and grease content, electrical conductivity and pH were at acceptable limits for environmental discharge of dairy effluent. This study reveals that photocatalytic pretreatment of dairy wastewater is an effective method for solubilization of the complex organic components of dairy effluents which can be easily decomposed by the microbes during the aerobic digestion process.