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

Home

7.8.5 7.9 7.9,1 operating conditions of the filters. Control panels (fixed or mobile) will be provided to allow the operators to manually initiate backwashes from the upper revel. Clearwell A clearwell is located adjacent to the gravity filter treatment facility. The clearwell provides storage and operational flexibility (the box shown on Figure lA in Appendix F represents 1 million gallons of storage). This storage is necessary to (1) maintain a volume of water for backwashing filter cells, (2) to provide the flexibility to treat water at a rate different than the raw water pumping rate, and (3) to provide additional storage for the distribution system. To provide operational flexibility and to supplement system storage, a 1 million gallon clearwell is the minimum size recommended. Treatment Alternative 2 — Pressure Filter Layout General In an iron and manganese pressure filtration system, water to be filtered is pumped directly to, and through, the facility's components under pressure. Oxidation occurs inside the pipelines and filter vessels upstream of the filter media. Following the oxidation process, the water flows through the filter vessels from top to bottom. As the water passes through the filter media, the insoluble particles of iron and manganese are removed from the flow. 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 pressure facility, this restriction to flow, called head, is measured in pounds per square inch (psi). As the solids accumulate, the headloss, or difference in pressures between the top and bottom sides of the filter media increases. Due to the design and construction of pressure filters, headless can be driven as high as 15 psi. although 5 to 6 psi is the preferred upper limit to ensure water quality. The advantages to pressure filtration systems are: 1. The facilities tend to have less capital cost than gravity facilities. 2. Plants are typically constructed on one floor level. 3. Provide for greater available headloss than gravity faci[ities which can result in longer filter run times than a comparably sized gravity facility. Longer filter run times require less backwashing which keeps a filter in service longer. The disadvantages lo pressure filtration systems are: 1. Pressure filter systems have less ability to add additional treatment processes (aeration, detention) if regulations or water quality changes. 2. Closed from view and difficult to access internally. This prevents observation of the systems operation. Condition of the filter media and flow distribution during a backwash cycle cannot be readily monitored. 3. Pressure filters are constructed out of steel and require periodic blasting and painting. 4. Inspection of the pressure filters requires entry into a confined space which is a safety hazard. FEASIBILITY STUDY -DRAFT RAMSY 154354 Page 30