[Show abstract][Hide abstract] ABSTRACT: The toxicity of the by-products found in water disinfected by chlorination methods prompted the regulatory agencies to insist on the use of alternative, ecologically friendly methods. This paper presents new disinfection techniques developed in our laboratory, which use sunlight or artificial UV radiation to promote photochemical disinfection processes. The sunlight was either used as global irradiation or concentrated by mirrors via an intermediary photosensitizer dissolved in water. Under these conditions the sunlight produces oxidative species in water which kill the microorganisms and oxidize organic materials. The disinfection efficiency of these methods has been proven in two experimental pilot plants operating in a continuous process with outputs of 50 and 0.15 m3h−1, and retention times of 35 min and 3 s, respectively. The former process is economically competitive and is ready for practical use, especially in countries with high solar radiation flux densities. Data about a potential industrial application of concentrated solar radiation for detoxification of industrial wastewater polluted by a pesticide (bromacil) are presented. A new technique was used for UV (254 nm) water disinfection; it employs custom-designed elliptical UV reflectors which concentrate the radiation on a UV-transparent pipe through which the wastewater flows. The 5 m3h−1 laboratory installation provided efficient disinfection of wastewater with turbidities up to 20 NTU. For large UV water disinfection plants (> 100 m3h−1), a new design is proposed, which could replace the present gravitational systems.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.