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1 <br />MIXING AND REACTION_CHAMBER This unit was designed to accomplish the <br />mixing of chemicals with the raw water and also to allow sufficient retention <br />time for the "floc" to form prior to entry into the settling basins. With <br />average waters this time of retention should be 30 to 50 minutes and the <br />velocity through the unit should be slow enough to avoid a breaking up of <br />the "floc ", yet fast enough to prevent premature settling. Each water varies <br />slightly at this stage of treatment so it is necessary to run laboratory <br />coagulation and floculation tests to determine exactly what capacity such a <br />unit possesses. In this case, the mining and reaction chamber has a reten- <br />tion time of 18 minutes when operating at a flow capacity of 8 MGD. <br />Laboratory tests on the raw water sample taken from the grit basin on <br />April 10, indicate that it is possible to form a good floc with 15 minutes <br />retention time if a rapid -mix operation of one minute duration precedes the <br />reaction area. On the basis of these findings, we are recommending the <br />installation of a flash -mixer at the point of chemical addition. <br />Figure 6 shows the chemical -feed building with the reaction chamber to <br />the left. Figure 7 is a view of the present chemical feeders. <br />- 31 - <br />Figure 7 <br />