5.8. SR 12-20-2004 - Laserfiche WebLink

Home Browse Search

been considered. Manufactured vortex type grit units are available in a standard 7.0 MGD size, and that size has been assumed for design. SCREENING IMPROVEMENTS The objective of screening is to remove rags and coarse to fine solids from the influent wastewater stream. Mechanically cleaned bar screens remove the larger solid materials from municipal sewage and wastewater such as cloth, paper, kitchen refuse, pieces of wood, cork and fiber. A bar screen is composed of vertical or inclined bars spaced at equal intervals across a channel through which wastewater flows. Coarse solids are retained on the bars. The use of bar screens tends to reduce labor costs, provide better flow conditions, and reduce equipment wear downstream. The existing manual and mechanical bar screen are proposed to be demolished. The primary screen channel, bypass piping, manual screen channel, and concrete floor should also be demolished. Two new channels and a new concrete floor are proposed for the west half of the screening room (Figure 2). The existing channel, which contains the mechanical screen, is too narrow for design flow capacity, and it is recommended that the channel be rebuilt. The channel is proposed to be rebuilt between the west wall and the Parshall Flume. To accommodate future peak flow, the channel should be widened to 2'-6". Velocity through the channel, based on a channel configuration of 2' -6" wide by 3 ' -0" deep (with a flow depth of 1'-2") and a flow of 3.5 MGD (A WW flow), would be 1.7 FPS. This meets Ten State Standards requirement for a flow velocity greater than 1.25 FPS. Currently, wastewater is piped into and out of the manual screen. A new channel must be constructed to accommodate the redundant mechanical screen. The new channel would be constructed parallel to the primary flow channel. It would have inlet and outlet channels that connect to the primary channel. Slide gates would be provided to allow for isolation of the channels. Two identical, coarse mechanical bar screens, which accommodate flow rates of 7.0 MGD each, are proposed at the inlet of the existing Pretreatment Building ahead of other pretreatment processes. Raw wastewater would enter the channel and flow through one of the two screens. The second screen would provide redundancy. In order to maintain gravity flow through the plant, limiting the headloss through the screens is critical to screening equipment selection. The new bar screens would be cleaned with a climber rake mounted on the screen. The rake would be set to run automatically in specified intervals (using a timer) or when water upstream in the channel rises above a preset level. Disposal of screenings can be facilitated several ways, which are described later. Bar screens with clear openings of 3/4 inch are recommended. Bar screens with 1/4-inch openings were evaluated, but headloss through the fmer screens was considered unacceptable. Alternative models for mechanical screens are shown in Table 1 along with a manufacturer cost comparison. 5