Pre-depositing PAC-birnessite cake layer on gravity driven ceramic membrane (GDCM) reactor for manganese removal: The significance of stable flux and biofilm

Water laden with dissolved manganese ions presents a challenge to water treatment processes and drinking water quality. To address manganese removal, a gravity driven ceramic membrane filtration system was developed; the membrane was pre-modified by depositing freshly formed birnessite type manganese oxide (MnOx) attached on powdered activated carbon (PAC) on the membrane. The result indicated that PAC-MnOx coupled with ceramic membrane rejection enabled the enrichment of manganese oxide bacterial (MnOB) (10³ MPN/mL) on the membrane, facilitating biogenetic manganese formation. The dominant MnOB (i.e., Hyphomicrobium) was identified using microbial community analysis. Intermittent dosage of PAC-MnOx at start-up period was likely to generate a cake layer effectively contributing to autocatalytic oxidation; The flux stablised at 41 L/(m²·h) after only 15-d operation, and the permeate of manganese was 0.089 mg/L. After 30-d operation, laser scattering particle analyzer (LASP) revealed that the particles (70.9 μm) within biofilm of gravity driven ceramic membrane (GDCM) grew larger compared with those present initially (48.5 μm) due to the newly formed biogenic MnOx attached to the carrier PAC, indicated by Raman analysis and X-ray Diffraction (XRD) analysis. Mn(III) as the predominant valence in MnOx conferred its outstanding catalytic oxidative capacity, as evidenced by X-ray photoelectron spectroscopy (XPS) analysis. SEM-EDS mapping demonstrated that a uniform flower-like birnessite structure was formed on the PAC surface, facilitating the transformation of MnOx to newly-formed birnessite on the vicinity of GDCM. Taking advantage of PAC and birnessite, the gravity driven filtration system developed in this study is anticipated to be an effective water treatment technique, particularly suitable for small footprint decentralized water supply sanitation.