Retention of bacteria and DNA by microfiltration membranes in wastewater reclamation

Abstract

16S rRNA gene copy numbers and bacterial counts in the permeate from different microfiltration membranes installed in membrane bioreactors (MBRs) were measured in order to provide information on the selection of suitable membrane for wastewater reclamation and reuse. 16S rRNA gene copy numbers in the MBR effluent (105 - 107 gene copies / mL) were 1/5 to 1/500 of those in the centrifugal supernatant of the mixed liquor. High bacterial counts and high gene copies were observed in the permeate of the membranes at the beginning of the filtration stage. The permeate contained bacteria without exceptions by using a 0.4 μm or larger pore size membrane, although the passage of bacteria was sporadic by the use of a membrane with cylindrical pores of 0.2 μm diameter. Acinetobacter junii and Microbacterium fluvii were identified in the permeate of the 0.2 μm pore size membrane based on the closest match of 16S rRNA gene sequence. The possibility of selective passage of small size bacteria in the mixed liquor of activated sludge through the membrane was suggested.