Operation and Maintenance of Full-Scale Municipal Membrane Biological Reactors: A Detailed Overview on a Case Study

This paper deals with the long-term operation of a full-scale municipal membrane bioreactor, focusing mainly on the membrane section. The ultrafiltration chamber, equipped with 12 130 m(2) of submerged hollow-fiber membranes, was installed after an existing alternating oxic/anoxic bioreactor. Attention is paid to a number of practical details, from the effectiveness of different sieves to the impact of the operating parameters on the membrane performances; finally, data of the energy consumptions and items of the operating costs are presented. Starting from the pretreatments, while the wedge wire sieve (openings = 1 mm) involved the visible accumulation of fibers and trash in the filtration chamber, the punched holes (holes diameter = 1.5 mm) involved less than 100 milligrams of dry trash per liter of mixed liquor, demonstrating that they suited the following membrane system. Moving on to the off-line equalization basin, a volume of one-fifth of the bioreactor was sufficient to cope with the normal fluctuations of the municipal inloadings. As for the membrane section, a net flux of 26 LMH and chemical maintenance cleaning in place once a week with only hypochlorite (around 300 mg(Cl) L-1) was a sustainable practice to keep the long-term permeability (adjusted at 20 degrees C stable in the range 220-240 LMH bar(-1). Under this operating protocol, increases of specific aeration for membrane scouring from 0. 12 to 0. 19 Nm(3) m(-2) h(-1) did not lead to permeability gains that might justify the increased power requirements. This evidence allows us to conclude that 0. 12 Nrn(3) m(-2) h(-1) was sustainable for the normal operation of the hollow-fiber membranes. Irreversible membrane fouling (permeability decrease up to 150-160 LMH bar(-1)) was observed because of irregular discharges of municipal landfill leachate, which, from one side, caused a drastic deflocculation of the activated sludge (sludge volume index (SVI) increased from 110 up to 250 mL gMLSS) and, from the other side, involved a probable incoming of recalcitrant compounds that might have acted as foulants. The power requirements of the whole treatment facility were always lower than 0.6 kWh m(-3) thanks to the general good utilization of the air supplied. The specific operating costs related to the energy consumptions and chemicals purchase were in the range 0.06-0.08 euro m(-3)