Section Env-Wq 709.02 - Screening Devices: Design and Capacity(a) Each by-pass screen device shall have clear openings between bars from 1.0 to 1.75 inches wide.(b) Clearance between bars on coarse racks or screens preceding mechanically-cleaned screens may be greater than 1.75 inches.(c) Manually-cleaned screens shall be: (1) Designed and installed to accommodate being cleaned;(2) Be placed on a screen slope of 30 to 45 degrees with the horizontal if for non-emergency use; and(3) Used only in by-pass channels.(d) Clear openings for mechanical screens other than by-pass screens and preliminary screens or racks shall be ½-inch or less to maximize removal of inert material.(e) For manually-cleaned bar screens, the screen chamber shall be designed to provide a velocity through the screen of one foot per second at an average rate of flow calculated from design average daily flow.(f) For mechanically cleaned screens, maximum velocities during wet weather periods shall not exceed 2.5 feet per second, calculated based on the vertical open cross-sectional area below the flow line.(g) The screen channel invert shall be 3 to 6 inches below the invert of the incoming sewers. To prevent jetting action, the length and construction of the screen channel shall provide for a reestablished hydraulic flow pattern following the drop in elevation.(h) The capacity of all screening equipment shall be as follows:(1) If one unit is installed, the unit shall be sized to handle peak hourly design flow;(2) If multiple units are installed for flexibility of maintenance, the peak hourly design flow shall be handled by the remaining units with the largest unit out of service; and(3) To avoid excessive head loss and potential damage to very fine screens when very fine screens are installed, multiple stages of screens with progressively smaller openings shall be used.(i) Influent channels shall be equipped with gates to isolate each screening device. The channel preceding and following the screen shall be shaped to eliminate settling of solids.(j) Where a single mechanically-operated screening device is used, auxiliary manually-cleaned screens shall be provided. The design shall include provisions for automatic diversion of the entire sewage flow through the by-pass screen if the mechanical unit fails.N.H. Code Admin. R. Env-Wq 709.02
Amended byVolume XXXIV Number 46, Filed November 13, 2014, Proposed by #10693, Effective 10/15/2014, Expires10/15/2024.