Agriculture in Tunisia faces acute problems of water quality and quantity caused by limited conventional water resources. One possibility to cope with low water resources is to purify wastewater for reuse. Among different methods for the disinfection of wastewater, the inactivation of fecal coliforms using a combination of a photosensitiser (Rose Bengal, Methylene Blue, cationic porphyrin) with sunlight was determined on a small scale. In parallel the sensitizer photobleaching was also followed under the same conditions. The results described in this paper show that the meso-substituted cationic porphyrin is more efficient and more photostable than Methylene Blue and Rose Bengal in wastewater. A lower cationic porphyrin concentration, 1 microM, resulted in very little cell death. Higher concentrations, 5 microM or 10 microM, produced more cell death. Nevertheless there was a small difference between concentrations 5 and 10 microM. By increasing the duration of irradiation we can improve the log reduction in bacteria and compensate for a low concentration of sensitizer or for a less efficient type of sensitizer. The same log reduction in fecal bacteria was obtained with 5 and 10 microM of cationic porphyrin during the fourth hour of treatment.
Helminth eggs and protozoan cysts were enumerated in raw and treated wastewater in Tunisia in order to determine their removal by wastewater treatment and to provide quantitative data for developing regulations for wastewater quality that are currently lacking. Raw and treated wastewater samples were collected from 17 plants in Tunisia during 2006-2007 and analyzed for parasites using the modified Bailenger method. Two groups of parasites, namely, Ascaris sp., Entamoeba coli, Enterobius vermicularis, and Taenia sp. (group 1) and Entamoeba histolytica/dispar, Giardia sp., and Taenia sp. (group 2) could statistically be distinguished according to their removal by wastewater treatment. Group 1 parasites were removed by 1.1 log(10) (92.4%) and group 2 parasites by 0.61 log(10) (76%). The ubiquitous presence of parasitic protozoa in Tunisian wastewater and ineffective wastewater treatment lead to their proliferation in surface waters with a high probability of exposure of human and animals to these parasites and consequent adverse health effects, as is apparent from epidemiologic data as well. This study provides a quantitative basis for risk assessment studies and development of mitigation strategies, such as improving wastewater treatment efficiency.
Tunisian untreated wastewater exhibits an average of 30 human helminth eggs per litre. After treatment, the concentration decreases to one egg per litre, or more in some cases. The percentage removal cited for wastewater processes provides no real indication of the destruction of the organisms, but merely of their transfer to another medium. In this study, we report the use of an environmentally friendly photoactive compound for wastewater disinfection. Photosensitization involves the generation of very toxic short-lived species on absorption of light by porphyrin. Microorganism photosensitization is potentially useful for sterilization and for the treatment of certain bacterial diseases. Gram-positive bacteria can be photoinactivated by a range of photosensitizers, but Gram-negative bacteria are not usually susceptible to photosensitized destruction. Our findings clearly demonstrate that the cationic meso-substituted porphyrin, tetra-(4-N-methylpyridyl) porphin tetra-tosylate (T4MPYP), is an efficient photosensitizer of helminth eggs on visible light illumination. The microscopic observation of helminth eggs shows that many types of ultrastructural alterations are potentially associated with exposure to T4MPYP and an adequate intensity of light: morphological changes without breakage; small alterations of eggshells; complete destruction. The degree of egg alteration increases with both increasing T4MPYP concentration and irradiation time. Moreover, the dissolved oxygen concentration, water quality and the type of eggs can influence the sensitivity of helminth eggs to photosensitization. Indeed, suspended solids (turbidity) were the most influential solution parameter on the efficiency of the photochemical process.
Methylene blue (MB) has been shown to photoinduce the direct inactivation of Gram-positive bacteria Staphylococcus aureus (S. aureus) in water. We have investigated the mechanism of S. aureus photoinactivation conducting firstly sodium azide (reactive ion N3-), as a good physical singlet oxygen quencher, then the amino acids tryptophan (Trp), as a non-specific singlet oxygen quencher and the mannitol, as an hydroxyl free radical scavenger. Inactivation of MB photosensitization is the antioxidants type dependent. When the bacteria was treated with MB (20 microM) under light during 10 min of exposure, it was found that survival fraction had decreased dramatically to about 31.27+/-5.39%. The presence of sodium azide and Trp failed to shown any protection from the MB photodynamic activity. In the presence of mannitol, S. aureus could be protected, reaching a protection level of about 27%. It is possible that the photodynamic activity of MB occurred in part, via a Type I mechanism in which *OH was produced.The interactions between MB and S. aureus were studied spectrophotometrically. This demonstrated that a metachromatic reaction took place between MB and S. aureus bacteria. Furthermore, S. aureus bacteria induced additional dimerization of MB.
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