Conventional methods of disinfecting sewage are all highly sensitive to variations in the effluent quality upstream of the disinfection stage. Microfiltration was tested as a tertiary treatment downstream of biofiltration, simulating very poor biofilter efficiency, to test the limits of the microfiltration process. Despite the poor quality of the secondary 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 µm) 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 reduced by an average of 60 and 70% respectively, the effluent quality is equivalent to level e after microfiltration (COD: 90 mgO2/l and BOD5 = 30 mgO2/l on samples averaged over 24 hours). Turbidity, measured instead of suspended solids at outlet from the microfilter, was 99 % eliminated. The colour remained between 50 and 150 mg Pt.Co/l. Under the operating conditions applied, the minimum filtration cycle was 72 hours for a minimum permeate flow of 80 1/h/m2 of membrane.
When microfiltration process was used to treat a secondary effluent of good quality (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 elimination) allowed us to get an effluent with an average COD concentration of 24 mgO2/l. The filtration cycle was much longer (300 hours).