Effect of calcium addition on sludge properties and membrane fouling potential of the membrane-coupled expanded granular sludge bed process

The expanded granular sludge bed reactor (EGSB) is considered a promising technology for anaerobic wastewater treatment, and the effluent quality can be further improved by membrane treatments. To mitigate membrane fouling of the membrane-coupled anaerobic process, 2.5mM CaCl₂ was added in a laboratory-scale EGSB reactor (3L in volume). The effluent was subjected to short-term dead-end ultrafiltration (UF) tests to evaluate the membrane fouling potential. The results showed that Ca²⁺ addition to EGSB efficiently alleviated the fouling of the subsequent UF treatment compared with the control. The fouling potential was not diminished when Ca²⁺ was directly added to the effluent of the control EGSB. Further investigations revealed that the addition of Ca²⁺ to the EGSB resulted in an increase in the content of extracellular polymeric substances (EPS) of the granule by 24.1%. The addition of Ca²⁺ also reduced the concentration of the soluble extracellular polymeric substance (sEPS) (or soluble microbial product, SMP) in the effluent by 47.7-60.7% compared with the control system. Analysis of the fouling model revealed that Ca²⁺ addition reduced the cake resistance (R_c) by 42.8% and delayed the transition from the standard pore blocking model to the cake filtration model. The results indicate that fouling-relevant compounds, such as sEPS (aromatic proteins, tryptophan proteins and polysaccharides) and fulvic acids are included in the sludge when Ca²⁺ is added to the reactor, thus preventing them from reaching the membrane surface.