Bacteriological quality of effluent submitted consecutively to a macrofiltration and ultraviolet light systems in the Tunisian conditions

This paper deals with the study of bacteriological quality of effluents that have undergone consecutively differentmacrofiltration system (pressure sand filter or disc filter used as a secondary treatment) and uv254 irradiationprocess (used as a tertiary treatment). these two successive systems of treatment were evaluated to determine theirpossible application as commonly alternatives to the conventional system of wastewater treatment and disinfectionbefore wastewater reuse. they both combined systems of wastewater treatment released effluent of excellentbacteriological quality, with almost total absence of feacal coliforms, of e. coli and of p. aeruginosa). however, if thebacteriological quality of the effluent remained constant in the case of macrofiltration system (disc filter or pressuresand filter); the uv disinfection process showed to deeply depend on the quality of effluent, particularly with regardto uv transmittance. the daily bacteriological monitoring of the secondary effluent at the exit of the pressure sandfilter by uv reactor and by using a dose of 96 mj/cm2, corresponding to an exposure of 16 min, showed anaverage rate of inactivation of around 3 u-log, for feacal coliforms, e. coli and p. aeruginosa, respectively. therefore,the average bacterial concentration remaining in the water at the exit of the uv reactor is less than 1000 cfu/100 mlfor feacal coliform and e. coli. for p. aeruginosa, the remaining number is less than 100 bacteria/100 ml. these two lastvalues coincide substantially with the range recommended by several standardized international guidelines. therefore,numerous authors reported that p. aeruginosa is very resistant to uv irradiation compared to the other bacterialindicators. in contrast, our study revealed that feacal coliforms and e. coli were more uv light resistant thanp. aeruginosa. this finding could be explained by the fact that e. coli and feacal coliform forms aggregates in thetreated effluent, while p. aeruginosa exists either as discrete cells or as cell pairs.