Due to the strict regulations and reuse policies that gov wastewater’s use as an irrigation water resource for agricultural purposes, especially in dry climates, optimization of the disinfection process is of the utmost importance. The effects of solar radiation along with Titanium dioxide (TiO2) nanoparticles applied to optimization of the photolysis and photocatalysis processes for inactivating heterotrophic bacteria were investi-gated. Temperature, pH, and dissolved oxygen fluctuations in the dairy wastewater effluent treated by activated sludge were examined. In addition, different dosages of TiO2 were tested in the solar photocatalysis (ph-C S) and concentrated solar photocatalysis (ph-C CS) processes. The results show that the disinfection efficiencies of the solar photolysis (ph-L S) and concentrated solar photolysis (ph-L CS) processes after 30 min were about 10.5％ and 68.9％, respectively, and that the ph-C S and ph-C CS processes inactivated 41％ and 97％ of the heterotrophic bacteria after 30 min, respectively. The pH variation in these processes was negligible. Using the ph-L CS and ph-C CS processes, the synergistic effect between the optical and thermal inactivation caused complete disinfection after three hours. However, disinfection was faster in the ph-C CS process than in the ph-L CS process. Significant correlations were found between the disinfection efficiency and the variation of the dissolved oxygen con-centration in the ph-C S and ph-C CS processes, while the correlations between the disinfection efficiency and temperature variation were not significant in the ph-L S and ph-C S processes. Moreover, the oxygen consumption rate was greatest (3.2 mg?L-1) in the ph-C CS process. Hence, it could be concluded that the ph-C CS process is an efficient photocatalysis process for disinfection of dairy wastewater effluent.