Abstract
This paper deals with the study of bacteriological quality of effluents that have undergone consecutively different macrofiltration system (pressure sand filter or disc filter used as a secondary treatment) and UV254 irradiation process (used as a tertiary treatment). These two successive systems of treatment were evaluated to determine their possible application as commonly alternatives to the conventional system of wastewater treatment and disinfection before wastewater reuse. They both combined systems of wastewater treatment released effluent of excellent bacteriological quality, with almost total absence of feacal coliforms, of E. coli and of P. aeruginosa). However, if the bacteriological quality of the effluent remained constant in the case of macrofiltration system (disc filter or pressure sand filter); the UV disinfection process showed to deeply depend on the quality of effluent, particularly with regard to UV transmittance. The daily bacteriological monitoring of the secondary effluent at the exit of the pressure sand filter by UV reactor and by using a dose of 96 mJ/cm2, corresponding to an exposure of 16 min, showed an average 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 ml for feacal coliform and E. coli. For P. aeruginosa, the remaining number is less than 100 bacteria/100 ml. These two last values 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 bacterial indicators. In contrast, our study revealed that feacal coliforms and E. coli were more UV light resistant than P. aeruginosa. This finding could be explained by the fact that E. coli and feacal coliform forms aggregates in the treated effluent, while P. aeruginosa exists either as discrete cells or as cell pairs.
Highlights
Reclaim and reuse of urban wastewater have increased in recent years, largely due to lack of water resources and inadequate economic structures, in arid and semi-arid countries [1]
The efficacy of UV disinfection of freshwater and wastewater depends on the UV dose
The wide divergence noted for E. coli could be explained by the poor quality of the influent or the clogging phenomena occurred in the pressure sand filter
Summary
Reclaim and reuse of urban wastewater have increased in recent years, largely due to lack of water resources and inadequate economic structures, in arid and semi-arid countries [1]. The contamination of urban wastewater with pathogenic microorganisms represents a significant risk to public health due to the possible presence of human enteric pathogens. The usage of treated effluent is considered as an important alternative water resource. I.e. primary treatment, disc filter and pressure sand filter (macrofiltration systems) well-known as secondary treatment and recognized to remove up to 99% of microorganisms, were not sufficient to achieve requirements for wastewater discharge and wastewater reuse [3]. The disinfection treatment is considered as the primary mechanism for the inactivation or destruction of pathogenic organisms, to prevent the spread of waterborne disease to downstream users and the environment
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