Photochemical disinfection of effluents-pilot plant studies

A new method of photochemical disinfection of domestic effluents for crop irrigation was investigated in a pilot plant. It uses sunlight as the energy source, the oxygen dissolved in water (DO) as the oxidizing agent and methylene blue (MB) as an intermediary for the absorption and transfer of the sunlight energy (red range) to activate DO and/or to destabilize the organic matter. The pilot plant consists of a series of six identical reactors, each 488 cm in length and 35 cm in height. The reactors were constructed from welded galvanized steel plates having a trapezoidal cross-section. A height differential of about 10 cm between two adjacent reactors ensured free overflow of the treated effluent through the pilot plant. Hydraulic experiments carried out with different effluent flow rates led to the construction and mounting of hydraulic devices which improved the relative residence time of the effluent in the reactors and decreased the hydraulic short-circuiting of the flowing effluent. The lack of DO in the supplied effluent was overcome by using a new method of adding O₂ (from 0.1-0.5 to 4-6 mg l^-1), by introducing the effluent under pressure. The best microbiological results were obtained in disinfection experiments done under the following conditions: pH, 8.8-8.9 (CaO 80-90 g m^-3); DO, 4.5-5.5 mg l^-1; MB, 0.85-0.90 g m^-3; effluent flow rate, 10 m³ h^-1; effluent depth, 20 cm; average sunlight exposure, 58 min; and sunlight intensities, 700-2000 μE^-2s^-1. The decrease in the microorganism count was (logs): coliforms. 3.0 ± 0.5; fecal coliforms, 3.1 ± 0.4; enterococci, 3.76 ± 0.4; and polioviruses, 1.8 ± 0.4. These results were reproducible and could be improved by technical amelioration of the pilot plant.