Sunlight disinfection of domestic effluents for agricultural use

The photochemical method of disinfecting domestic effluents planned for use as irrigation water for edible crops was developed further, reaching a stage at which it can be used on an industrial scale. This disinfection method uses sunlight as the activation energy source; the oxygen dissolved in water (DO) as the oxidizing agent; and a dye-sensitizer (methylene blue) as an intermediary for the absorption and transfer of the sunlight energy to activate DO and/or to destabilize the organic matter molecules and the microorganisms as the oxidation target. The study was carried out in an experimental pilot-scale plant, capable of treating up to 50 m³/h of effluent supplied by an activated sludge sewage treatment plant located in the Tel-Aviv area. The plant consists of a series of 10 identical photoreactors (6 × 2 × 0.3 m³), installed in series on an unpaved road with a 2% slope which ensures free overflow of the treated effluent through the pilot plant. Preceding the photoreactors there is a mixing reactor (2 × 2 × 1 m³) which supplies the effluent with DO (> 6 g O₂/m³), MB (0.7 ± 0.1 g/m³) and calcium hydroxide (33 ± 3 g/m³) for pH correction (8.7-8.9). Operating the pilot plant at an effluent flow rate of 33 ± 3 m³/h (effluent detention time: 35 ± 2 min), sunlight intensities 700-2600 μEm^-2 s^-1, the following decreases in microbial counts were observed (log counts): coliforms -3.2 ± 0.3; fecal coli -3.12 ± 0.2; fecal streptococci -3.9 ± 0.3; poliovirus -1.9 ± 0.25. The treated effluents did not show regrowth of these microorganisms during 7 days storage in photoreactors, and did not form an impermeable crust when infiltrated into sandy soils. The effluent disinfection cost in a "sunlight disinfection plant" producing 200 m³ disinfected effluent per hour is estimated to be U.S.$ 3.95 per 100 m³.