MICROFILTRATION USED AS A MEANS OF DISINFECTION DOWNSTREAM - A BACTERIAL TREATMENT STAGE ON FIXED-BED BACTERIA

Conventional methods of disinfecting sewage are all highly sensitive t o variations in the effluent quality upstream of the disinfection stag e. Microfiltration was tested as a tertiary treatment downstream of bi ofiltration, simulating very poor biofilter efficiency, to test the li mits of the microfiltration process. Despite the poor quality of the s econdary effluent (COD between 100 and 400 mgO2/l, BOD5 between 30 and 150 mgO2/l and suspended solid concentrations between 15 and 90 mg/l) , the microfiltration process (filtration level : 0.2 mum) eliminated all faecal germs and tenia and ascaris eggs. The total elimination of free amoeba cysts still needs to be confirmed. With the COD and BOD5 r educed by an average of 60 and 70% respectively, the effluent quality is equivalent to level e after microfiltration (COD: 90 mgO2/l and BOD 5 = 30 mgO2/l on samples averaged over 24 hours). Turbidity, measured instead of suspended solids at outlet from the microfilter, was 99 % e liminated. The colour remained between 50 and 150 mg Pt.Co/l. Under th e operating conditions applied, the minimum filtration cycle was 72 ho urs for a minimum permeate flow of 80 l/h/m2 of membrane. When microfi ltration process was used to treat a secondary effluent of good qualit y (COD between 14 and 40 mg/l ; turbidity between 1.4 and 5.1 NTU) the germ elimination remained the same ; the COD removal (31 % average el imination) allowed us to get an effluent with an average COD concentra tion of 24 mg O2/l. The filtration cycle was much longer (300 hours).