Treatment of printing and dyeing wastewater in biological contact oxidation reactors comprising basalt fibers and combination fillers as bio-carriers: Elucidation of bacterial communities and underlying mechanisms

Highlights: • N removal and sludge reduction were studied in different bioreactors. • Sludge reduction was the highest in R-BF but N removal was sufficient in all reactors. • Conventional nitrification and denitrification were likely dominant routes in R-A/O. • Heterotrophic nitrification and aerobic denitrification were prominent in R-BF and R-CF. • S cycle was observed in R-BF and R-CF only, thus R-A/O was not ideal to remove sulfur. This study investigates the nitrogen removal and sludge reduction efficiency of biological contact oxidation reactors in the presence of two bio-carriers namely basalt fibers (R-BF) and combination fillers (R-CF). The results were also compared with a conventional anoxic-oxic reactor (R-A/O). The reactors were operated at pilot-scale (550 L) and pollutant removal mechanisms were studied based on physicochemical parameters, bio-carrier surface morphology, microbial community structures, and activated sludge characteristics. It was found that chemical oxygen demand (COD) removals for R-BF, R-CF, and R-A/O were comparable in all reactor types, whereas R-A/O performed better in terms of ammonium-nitrogen (NH₄⁺-N) and total nitrogen (TN) removal followed by R-BF and R-CF. The sludge reduction in R-BF was 40.4% higher than that in R-CF and 61.9% higher than that in R-A/O. This observation was further correlated with the abundance of protozoa (Vorticella and Epistylis) and metazoan (Aeolosoma hemprichi) communities, diverse and rich bacterial communities, stronger dehydrogenase activity, and low specific oxygen uptake rate in R-BF. The results of 16S amplicon sequencing suggested that microbiological conditions in R-BF were more favorable for heterotrophic nitrification and aerobic denitrification, sulfur cycle, hydrolysis and fermentation compared to R-CF. In R-A/O, however, sulfur oxidizing and reducing bacteria were completely absent which also indicated its poor efficiency for sulfur removal. This study concludes that R-BF was a better choice towards sludge reduction whereas conventional R-A/O could be a favorable approach for TN removal.