Advances in Water Treatment and Enviromental Management

GREATER LYON EMERGENCY WATER INSTALLATIONS 59THE DECISION TO CONSTRUCT EMERGENCY WATER INSTALLATIONSFor this reason, in 1984 the Greater Lyons Council decided to set up emergency resources,which would make it possible to interrupt pumping operations in the catchment area duringthe passage downriver of the pollution slick. The capacity of the emergency water installations(150,000 cubic metres/day) was defined following studies carried out on the risks of pollutionand the corresponding durations of interruptions to water gathering operations, it beingunderstood, that low-priority water consumation (for the purposes of watering, road washingetc…) will be suspended during the passage downriver of the pollution.Amongst the different solutions examined, that involving treatment of water from Lake Miribel-Jonage quickly emerged as the best choice, from both a technical and economic standpoint.This lake, recently created, is a result of careffully-coordinated extraction of alluvial depositsas part of a project for the development of a site for leisure activities. Located to the North-East of Lyons, it features a capacity of 7 000 000 cubic metres (which will eventually beincreased to 12 000 000 cubic metres) of water with highly satisfactory bacteriological andchemical characteristics.Given the geographical location of the lake, the logic behind the project is quite straightforward:Untreated water is drawn from the South-West side of the lake, and sent by a pumpingstationalong a pipe 1,10 metres in diameter to the treatment-plant, which is located inRillieux, next to the Miribel canal. Treated water is returned to the catchment field pumpingnetwork, which means that the primary pumping installations of the conurbation can beused.The development of this project was entrusted to the “Compagnie Générale des Eaux”, aspart of a concession agreement for construction and operation of the project.TREATMENT PROCESSThe treatment process is of course adapted to suit characteristics of lake water.The Rillieux plant is thus the first installation of its size to use the ozoflotation techniquedeveloped by OTV, which is particularly suited to eliminating algae and other suspendedparticles: although Lake Miribel does not for the time being give rise to any worries on thisaccount, algae development remains a constant threat to lake water.This process, which is carried out at the start of treatment operations, comes after flocculationwith ferric chloride followed by flash mixing. The principle is as follows (see diagram below).Water overflow into the ozonation compartment: porous plates diffusing ozone are sweptalong by a stream of water, causing tiny bubbles to be created. This tiny bubbles are draggedinto the second compartment (flotation compartment), trapping a very hight proportion offloating particles and ensuring optimum ozone transfer. The treated water exits from base ofthe flotation compartment and rejoins the outlet channel.Periodic evacuation of impurities is carried out by raising the level of the water and using atransversal stream of water to sweep away the foam.The water is subsequently filtred by a battery of 9 twin-layer “anthracitesand” filters, afterwhich it undergoes a final ozonation process before entering the two 3 000 cubic metre tanksfor treated water. These tanks constitue the buffer storage of the installation and are used forbash washing the filters as well as for immediate start-up of the treatment plant.Sludge coming from the ozoflotation process and from the filters is treated with the aid of arevolving blade thickener/clarifier followed by a belt-filter; this process makes it possible toreach a dry content value of over 25 %.One aspect related to the production of ozone is worthy of note: the ozone used here isproduced not from air but from pure oxygen, via a battery of two ozonizing devices.