The treatment of carwash wastewater using an integrated chemical coagulation and adsorption process

Abstract

Clean water is a valuable and scarce resource in any society today. The water scarcity being faced globally will deteriorate unless water consumption is reduced, and water reuse is implemented globally. Affordable wastewater treatment technologies have the potential to alleviate this crisis being faced today. Greywater is an excellent source for water reuse and includes domestic, carwash, laundry and hospital water. Greywater contains different pollutants such as organic and inorganic components making it challenging to treat and potentially harm the environment. Chemical coagulation is an excellent treatment process for removing turbidity, grease and COD but struggles to remove specific compounds such as cleaning agents. Therefore, introducing a secondary treatment process like adsorption offers a viable solution for removing harmful pollutants from greywater. In this study a lab scale integrated treatment process was used to investigate carwash wastewater’s successful treatment. Chemical coagulation and adsorption were the treatment steps used to remove COD, FOG and anionic surfactants from the carwash wastewater for reuse application. The chemical coagulation process with polyferric sulphate (PFS) was applied to treat industrial carwash wastewater collected from a service station in Cape Town, South Africa. Polyferric sulphate concentration was tested to determine the optimum dosage concentration for the removal of pollutants. After that, adsorption using a commercial powdered activated carbon was used as a secondary treatment step. Operating conditions such as temperature, adsorbent dosage and pH were investigated to determine the effect on removing COD, FOG, and anionic surfactants (AS). The commercial activated carbon was characterized using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). Design Expert 10 was used to generate a predictive model using the Box-Behnken Design (BBD) approach to describe the effect of operating conditions on COD and Anionic Surfactant (AS) removal. Origin 2021 professional software package was used to fit adsorption (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich) and kinetic models (Pseudo-First Order (PFO), Pseudo-Second Order (PSO), Intra-Particle Diffusion (IPD) and Elovich) to the experimental data collected for Anionic Surfactants (AS).

The removal percentage of COD, FOG and AS were found to be 79.5%, 96.25 and 44.82 respectively after chemical coagulation at a PFS concentration of 120mg/l. This was deemed a sufficient coagulant dosage. The best percentage removal after adsorption for FOG, COD and AS was found to be 100, 94 and 98% respectively at operating conditions of pH: 6, Temperature: 500C and dosage: 300mg/l powdered activated carbon (PAC). The best overall removal for COD, FOG and AS was found to be 98.5, 100 and 98% respectively. This shows that the treated effluent is in line with national standards for safe disposal or reuse. The SEM images revealed a porous structure suitable for the adsorption of COD, FOG and AS. The FTIR revealed the PAC contained bonds that are advantageous for the adsorption of AS. The adsorption kinetic data for AS was shown to follow pseudo-second order (PSO) reaction kinetics the best and follow the Freundlich and Temkin adsorption isotherms the closest. The adsorption thermodynamics showed the adsorption of AS onto commercial PAC being an endothermic process. It was observed that the predictive model successfully described the optimal operating conditions for the removal of COD and AS within the design space of the model.