Agenda - Council Work Session - 10/20/2020 - Special

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7.8 7.8.1 7.8.2 Treatment Alternative 1 - Gravity Filter Layout General In an iron and manganese gravity filtration system; water to be filtered is pumped, under low pressure, to the treatment facility where it flows by gravity through the various treatment processes. Following the oxidation process, the water flows through the fitter cells from top to bottom. As the water passes through the filter media, the insoluble particles of iron and manganese are removed_ As more and more water is filtered, the restriction to flow, created by the accumulation of iron and manganese solids on the media, steadily increases_ In a gravity facility, this restriction to flow, called head, is measured in feet of water depth in the filter cells. As the solids accumulate, the depth of water in the filter cells increases_ Due to the physical nature of a gravity filter, when the depth of water in a cell reaches its maximum designed head (high water level) backwashing is required. Failure to backwash at the proper time could result in the filter overflowing or poor effluent water quality being produced. Gravity filters are typically constructed of concrete or steel. Steel filters are generally found in smaller water systems. Because of the Large size of the filters required for Ramsey, steel filters are not being considered. The advantages to gravity filtration systems are: 1. Gravity filters provide for more treatrnent options including aeration and detention without requiring another pumping step. If regulations change or the water becomes contaminated, additional treatment steps can be added to gravity filters. 2. Water from the gravity filters does not go immediately into the distribution system. If problems with the filters occur or if sodium permanganate is overfed (causing pink water), operators have time to react and correct the problem. 3. Gravity filters are open to view and access. This is advantageous in that it enhances the observation, operation and maintenance of the filter functions and components_ 4. Gravity filtration systems have a greater amount of flexibility with less disruption during normal maintenance procedures. The disadvantages to gravity filtration systems are: 1. The facilities tend to have more capital cost than pressure type facilities. 2. Typically requires the facility lo be constructed on two floor levels. 3. Provides for less available headloss than pressure facilities which can result in shorter filter run times. Shorter filter run times result in more backwashing which takes a filter out of service. Building Layout/General Sequence Gravity filter building layouts are included in Appendix F. The chemical rooms are located on the east side of the building, with exterior doors accessible for 'deliveries_ The electrical, mechanical, high service pump room, and generator rooms are located in close proximity to each other to allow for short conduit runs to motor controls. The gravity filter layout occurs on two levels to allow for filter height to provide head for the filtering process. The raw water enters the building through the high service pump room where chlorine and potassium permanganate are added. The water travels through the filters by gravity FEASIBILITY STUDY -DRAFT RAMSY 154354 Page 28