09/05/2007 Council Packet - Laserfiche WebLink

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DRAFT 8 -17 -07 for maintenance. To meet the projected Lino Lakes ultimate peak demand of 8.4 MGD a sixth filter would be required. The building would be sized to allow the sixth filter to be installed when needed without expanding the structure. To meet the ultimate peak demand with one filter out of service the remaining five filters would be operated at a rate of 3.4 gpm /sf. This filter loading rate is within the filters design capacity. The proposed plant includes a small storage tank for treated water ( clearwell) beneath the filters. Due to the groundwater conditions at the proposed site, the clearwell cannot be constructed deep enough to provide the projected 1 MG of treated water storage. The Minnesota Department of Health requires a 3 foot separation between the clearwell and the seasonal high groundwater level. The groundwater at the proposed plant sites would limit the depth of the clearwell to about 4 feet. To provide 1 MG of storage, a 4 foot deep clearwell would be about 0.75 acres in area. Thus, a separate ground storage tank is proposed. A disadvantage of having a separate ground storage tank is that the treated water from the filters would have to be pumped into the storage tank. An additional small clearwell beneath the filters is required to provide a basin for the transfer pump to withdraw water. The filtered water would discharge directly into the clearwell thus reducing the amount of piping required to convey the water to the transfer pump. In order to reduce the amount of wastewater discharged to the sewer, and the sewer cost to the City, the water from backwashing the filters is captured in a tank called a reclaim tank. The solids in the backwash water are allowed to settle out and the majority of the water is pumped back to the head of the plant for treatment. The remaining water and solids in the reclaim tank are pumped to the sanitary sewer. To provide good solids settling conditions the reclaim tanks should have a minimum depth of 10 feet. The reclaim tanks are also required have 3 feet of separation from the groundwater which means they must be constructed adjacent to the plant building instead of beneath the building. To allow maximum settling time, two reclaim tanks are normally provided with each tank having a capacity for the volume from one filter backwash cycle. This allows one tank to remain undisturbed while the other tank receives backwash water. Each of the two reclaim tanks would need to hold about 140,000 gallons which would require a tank Water Treatment Plant Study 13 13843.000