Low maintenance gravity-driven membrane filtration using hollow fibers: Effect of reducing space for biofilm growth and control strategies on permeate flux

Abstract

The implementation of centralized drinking water treatment systems necessitates lower operational costs and improved biopolymer removal during ultrafiltration (UF), which can be afforded by gravity-driven membrane (GDM) filtration. However, prior to implementing GDM filtration in centralized systems, biofilm growth in compacted membrane configurations, such as inside-out hollow fiber (HF), and its impact on permeate flux need to be investigated. To this end, we operated modules with distinct limits on available space for biofilm growth: (1) outside-in 1.5 mm 7-capillary HF (non-limited), (2) inside-out 1.5 mm 7-capillary HF (limited), and (3) inside-out 0.9 mm 7-capillary HF (very limited). Here, we observed that the lower the space available for biofilm growth, the lower the permeate flux. To improve GDM performance with inside-out HF, we applied daily shear stress to the biofilm surface with forward flush (FF) or combined relaxation and forward flush (R+FF). We showed that applying shear stress to the biofilm surface was insufficient for controlling flux loss due to low available space for biofilm growth. At the experimental endpoint, we backwashed with a stepwise transmembrane pressure (TMP) increase or a single TMP on all inside-out HF modules, which removed the biofilm from its base. Afterwards, higher fluxes were yielded. We also showed that all modules exhibited a gradual increase in biopolymer removal followed by stabilization between 70 and 90%. Additionally, control of biofilm growth with surface shear stress did not affect biopolymer removal. In summary, the implementation of inside-out HF with GDM filtration is challenged by low available space for biofilm growth, but may be remedied with a regular backwash to remove biofilm from its base. We showed that a wider range of GDM applications are available; making GDM potentially compatible with implementation in centralized systems, if space limitation is taken into consideration for operation optimization.

Keywords: Available space for biofilm growth; Backwash; Gravity-driven membrane filtration; Hollow fiber membranes.