Removal of diclofenac from wastewater using sludge from Chiang Mai university wastewater treatment plant: role of biodegradation and sorption

Abstract

A major source of environmental contamination by pharmaceuticals is the effluent from wastewater treatment plants (WWTPs) since they were not designed to treat these emerging pollutants. One of the prevalent pharmaceutical contaminants is the non-steroidal anti-inflammatory drug (NSAID) diclofenac (DCF), of which concentrations in nanogram to microgram per liter have been detected in the effluent. This research aimed to study the role of the two major mechanisms of DCF removal, i.e., biodegradation and adsorption, in wastewater using activated sludge (AS) from Suan Dok WWTP (CMU-WWTP). Wastewater obtained from the facility was spiked with 1.0 and 10 mg L-1 of DCF salts, and the removal was observed in batch experiments conducted in the laboratory. DCF concentrations in the samples were determined by HPLC. The DCF removal efficiencies of 90.89 and 89.7% were observed in 48 hours for 1.0 and 10 mg L-1 of DCF, respectively. To observe the role of biodegradation, DNA was extracted from the fresh AS for 16S rRNA gene sequencing to identify the sludge bacterial community. The AS comprised 183 species of bacteria belonging to 22 phyla, in which the most abundant phyla were Planctomycetota (29.44%), Actinobacteria (17.4%), Proteobacteria (16.58%), Chlorofexi (12.22%), Acidobacteria (8.03%), and Bacteriodota (7.5%), accounting for more than 90% of all the AS bacterial microbiome. Several genera belonged to the families were identified in literatures as DCF biodegraders including Acidobacteriaceae, Sphingomonadaceae, Phyllobactriaceae, Polyangiaceae, Caulobacteraceae, Xanthobacteraceae and Solirubrobacteraceae. Role of adsorption was observed by adsorption experiments with dried biomass of the AS at the same DCF concentrations. The adsorption process was found to be pH and sludge dosage dependent. Sludge had the highest adsorption capacity in the DCF solution at the neutral pH 7 (14.4 x 10-2 mg g-1), followed by pH 10 at 12.3 x 10-2, then pH 4 (4.0 x 10-2 mg g-1). The adsorption capacity reduced as the sludge dose increased whilst the removal percentage increased as the sludge dosage increased from 35.08 to 53.5, and 71.3% for 1, 2, and 4 g of sludge, respectively. The adsorption was better fitted to a pseudo-second-order kinetic model with R2 values > 0.98 and was dominated by the chemisorption process. Biodegradation should be the dominant mechanism of DCF removal by sludge in wastewater, followed by the adsorption process. The findings of this study update the understanding of the role of AS in the removal of DCF from wastewater in the AS treatment system. Further research is needed to study the intermediates of DCF biodegradation since they were not identified in this study and may play a role in effluent ecotoxicology.