Disinfection effects of ultraviolet on supernatants of flocculated digestate from swine farm

Abstract: There are a large number of microorganisms in the liquid digestate from a swine farm. The liquid digestate should be disinfected prior to discharge or utilization for sanitary and environmental safety, but there is a lack of literature on the disinfection of liquid digestate from animal feeding operations. In view of the high turbidity and chromaticity, the transmittance (T254) of liquid digestate from swine farm is nearly zero, and liquid digestate must be treated prior to ultraviolet disinfection. Flocculation was applied to pre-treat liquid digestate in this study in order to obtain supernatants with different transmittances (T254) for the following disinfecting trials. In terms of total bacteria count, total coliforms, and fecal coliforms, experiments were conducted to investigate the disinfecting feasibility and the effectiveness of ultraviolet on the supernatants of flocculated digestate under 60 experimental conditions, which were formed by 4×3×5 factorial combinations of transmittances (T254) (0.01%, 0.69%, 3.78% and 8.54%), water depth (WD) (1, 2 and 3 cm), and hydraulic retention times (HRT) (1, 5, 10, 15 and 20 min). Results showed that T254, WD, and HRT all significantly affected (P＜0.01) the disinfection effects of ultraviolet on the supernatants of flocculated digestate, and significant interactions (P＜0.01) were also detected among the experimental factors. The disinfection rates of ultraviolet on total bacteria count, total coliforms, and fecal coliforms were (99.99±1.20)%, (99.99±1.43)%, and (100.00±0.01)% respectively, under the condition of 0.69% T254, 2 cm WD, and 15 min HRT. In such cases, the fecal coliforms in the supernatant of flocculated digestate decreased from 3.9×106 count/L to less than 3 count/L. Therefore, ultraviolet could be applied for the disinfection of the supernatants of flocculated digestate from swine farms, and the disinfected effluent could meet the sanitary requirements of the current national standards. It is recommended that the disinfected supernatant of digestate be reused for floor-flushing in swine farms, if the farms use tap water for production. and have no land in surrounding area for digestate application.