Mechanisms and kinetics of greywater treatment using biologically active granular activated carbon

The high adsorption capacity of granular activated carbon (GAC) makes it an effective biofilter media for greywater (wastewater generated in households and office buildings from streams without fecal contamination). This study investigates the mechanisms and kinetics of greywater treatment using biologically active GAC (BAC). This was achieved by assessing the role of each of the sorption and biodegradation mechanisms to the overall treatment process, characterizing and modelling the adsorption capacity of the media, and analysing and modelling the kinetics of adsorption. The biodegradation mechanism was found to contribute less than 26% to the overall treatment process with biomass density of 178.71 ± 14.12 mg g^-1 BAC, whereas sorption mechanisms were responsible for the remaining greywater treatment. The Freundlich isotherm was found to best-represent the equilibrium adsorption data with Freundlich constant and intensity parameter of 1.48 × 10^-5 L g^-1 and 0.39, respectively. Pseudo-second order and intraparticle diffusion models were created to fit the kinetics adsorption with rate constant values of 0.12 g mg^-1 h^-1 and 1.91 mg g^-1 h^-0.5 during the first 2 h of the experiment and 0.08 g mg^-1 h^-1 and 0.50 mg g^-1 h^-0.5 onwards, respectively. Intraparticle pore diffusion was determined to be the rate limiting step of the greywater treatment; some mass transfer resistance was observed due to external film diffusion at lower substrate gradients during greywater treatment. This study improves our understanding of the behaviour of GAC biofilters through understanding their treatment mechanisms and kinetics, leading to more efficient greywater treatment.