Improving granular sludge stability via stimulation of extracellular polymeric substance production by adding layered double hydroxides

Abstract

One of the main challenges for aerobic granular sludge in applications is its stability. In this study, the interaction between layered double hydroxide and extracellular polymeric substance was investigated and the mechanism for enhancing stability of aerobic granular sludge was also analyzed. Three sequencing batch reactors with nothing added (reactor 1), with layered double hydroxide added (reactor 2) and with metal ion solution added (reactor 3) were set up to investigate sludge developing characteristics and reactor performances, respectively. It was found that the mixed liquid suspended solids, sludge volume index and chemical oxygen demand removal efficiency of reactor 2 were better than those of reactors 1 and 3. At the same time, the concentrations of protein and polysaccharides extracted from the sludge in reactor 2 were also higher than those in reactors 1 and 3. This is possibly because with layered double hydroxide added, continuously and controllably released metal ions stimulate more extracellular polymeric substance production than the others and increase the hydrophobicity of the sludge. Fourier transform infrared spectroscopy characterization showed that the functional groups could combine with metal ions. A stable structure such as metal ions-extracellular polymeric substance–metal ion network may be formed and enhanced the granular sludge stability. This investigation provides a possibility for further application of layered double hydroxide on acceleration of sludge granulation and improvement in granule stability.