Adsorptive and photocatalytic remediation of greywater in wastewater: a review

Bathroom, and laundry greywater (GW) components are considered significant urban wastewater and are classified as hazardous substances that contaminate groundwater resources. Thus, achieving permitted levels for GW before discharging into the environment requires the removal or reduction, which has become a challenge. Various techniques have been developed to decontaminate GW from wastewater, comprising biological, chemical, filtration, adsorption, membrane separation, and photocatalytic degradation. Due to the simplicity, cost-effectiveness, abundance of materials, and capacity for facile scaling-up for remediation purposes, adsorption and photocatalysis technologies have been widely utilized in GW wastewater treatment. This review thus first explains the sources of GW and components found within this particular wastewater, which are critical for removal. The second part reviews various adsorbents or photocatalysts, including materials of macro, micro, and nanosize utilized for GW treatment. The review highlights the significance of activated carbon among all adsorbents under adsorption technology reviewed with the highest removal rate of chemical oxygen demand (COD), and biochemical oxygen demand BOD in GW. Moreover, the doped titanium dioxide photocatalyst also presented significant removal of COD, and BOD in GW within a shorter space of time. The impact of surface area and chemical functionalities of the adsorbent, and whilst aspect of nanostructure and absorptivity of photocatalyst in the visible region of the solar spectrum on the expedited removal of GW was also highlighted. Furthermore, this review emphasizes photocatalyst nanomaterial achieving a complete mineralization of different components present in GW, into mineral products.