3.6. Regrowth ability test
As we mentioned, E. coli K-12 was completely inactivated after 8 h photocatalytic disinfection by NS under white LED with the intensity of 200 mW cm−2 at pH 8 ( Fig. 3b). After that, the reaction mixture was sampled and undergone a 96 h recovery period for bacteria re-growth. The results (data not shown) indicated that no detectable bacterial colony was observed after 96 h incubation period. It means that 8 h photocatalytic disinfection for E. coli K-12 with the NS is an effective disinfection time ( Rincón and Pulgarin, 2004b), which is defined as the time required for complete bacterial inactivation without regrowth in a subsequent dark period for 96 h. This result also suggests that photocatalytic disinfection by the NS under VL led to irreversible damage to E. coli K-12, or brought the damaged bacteria to a state called “viable but non-culturable” state, which meant the bacteria could no longer grow on conventional media unless favorable condition was provided ( Dunlop et al., 2002). This result illustrates that the photocatalytic disinfection system using the NS under white LED irradiation with the intensity of 200 mW cm−2 will not result in the bacterial recontamination, thus it can be concluded that photocatalysis is a safe and effective disinfection method in the real wastewater treatment.