Removal of CODB by combination of biological treatments especially slow sand filtration in drinking water treatment plan
The city of Paris has decided to improve the Ivry/Seine plant which participates to the water supply of the city. The flow of this plant is 300 000 m3/day and SAGEP has imagined a new fine of treatment which takes in account the biodegredation at all the treatment steps. The line of treatment is divided into 4 steps : preozonation, contact coagulation, coagulation on filter, slow sand filtration and ozonation, filtration on CAC, then final disinfection with chlorine. This new line of treatment has been tested on a pilot plant and we have first verified the efficiency of this plant to remove BDOC. The material used for the contact coagulation is biolite and we have observed a good removal of BDOC on this stop. A process of bioflocculation has been shown on this stop. Nevertheless, whatever the concentration of BDOC in the raw water, the concentration of BDOC of the effluent water of slow sand filters is always very low (0,3 mg. l-1). We have measured also others parameters concerning organic matter. We observed a good removal of UV absorption and fluorescence along the whole water treatment line but the ratio of the fluorescence compared to the dissolved organic carbon increases after slow sand filtration. These observations confirm what was shown previously. Secondly, we have tested the influence of preozonation on the efficiency of slow sand filtration. Preozonation has a different efficiency depending on the nature of dissolved organic carbon in the raw water. Preozonation improves efficiency of slow sand filtration but we have observed an optimal ratio of 0,12 mg 03/mg TOC. If the dose of ozone is over this ratio, we observed a higher concentration of BROC in the water flowing out the slow sand filters. In conclusion, we can say that origin of BDOC is different in the influent and effluent of slow sand filtration and we think that it is necessary to privilege a step of adsorption on GAC after slow sand filtration.