Microorganism repair after UV-disinfection of secondary-level effluent for agricultural irrigation

A pilot study of microorganism repair after UV disinfection was performed for agricultural reuse of secondary-level effluent in paddy rice fields in Korea. Effluent from the bio-filter of a 16-unit apartment was used in a flow-through type UV-disinfection system. The average concentration of suspended solids (SS) and biochemical oxygen demand (BOD) were 3.4 and 5.9 mg L⁻¹, respectively. The mean total coliform level was in the range of 1.5 × 10⁴ MPN 100 mL⁻¹. Photoreactivation and dark repair were apparent at a low UV dose (6 mW s cm⁻²). In low-dose UV disinfection, microorganisms increased within 12 h by approximately 5 and 1% due to photoreactivation and dark repair, respectively. This increase was not significant at a high UV dose (16 mW s cm⁻²). The repaired microorganisms were further inactivated, rather than reactivated, by solar irradiation, and numbers decreased to non-detectible levels after 4 h of exposure to solar irradiation. Based on UV disinfection and repair studies, a UV dose of 30 mW s cm⁻² is recommended as sufficient to produce reclaimed water virtually free of pathogens and may be adequate for disinfection of secondary effluent for agricultural irrigation in paddy rice culture.