(a) A wastewater treatment facility must include a means of effluent flow measurement with less than 10% error, or the manufacturer's error tolerance, whichever is less, throughout the expected range of flows.     
      (b) An effluent flow-measuring device must have an open channel to allow for easy inspection, calibration, and cleaning.     
      (c) Flow measurement must use a combination of primary and secondary flow measuring devices.      
        (1) Primary flow measuring devices.                  (A) A primary flow measuring device must include a weir or a flume.                   (B) A primary flow measuring device must have a non-corrosive ruler (staff gauge) that is graduated in no greater than 1/4 inch increments that are clearly visible.                   (C) A primary flow measuring device must allow for manual measurement of water depth.         
        (2) Weirs.                  (A) A channel approach section to a weir must be straight for a length at least 20 times the maximum expected head height on a weir at peak flow. An owner may use manufacturer's recommendations in lieu of this requirement if approved in writing by the executive director.                   (B) The minimum distance between a channel bottom and a weir crest must be at least twice the maximum head height on the weir at peak flow, or 1.0 foot, whichever is greater.                   (C) The upstream edge of a weir must not be corroded.                   (D) The crest of a weir must be exactly level to ensure a uniform depth of flow.                   (E) The upstream face of a weir must be smooth and perpendicular to the axis of the channel in both the horizontal and vertical directions.                   (F) A secondary flow measuring device must be installed:                      (i) upstream of the weir at a distance of three times the maximum head height on a weir at peak flow; or                       (ii) at the location and distance recommended by the equipment manufacturer.           
        (3) Flumes.                  (A) A flume must be located in a straight section of an open channel.                   (B) A flume must be installed in accordance with the manufacturer's recommendations.                   (C) A flume must distribute the approaching flow evenly across a flow channel to preclude turbulence and waves.         
        (4) Secondary flow measuring devices.                  (A) A wastewater treatment facility must use a totalizing meter as a secondary flow measuring device, which must measure the liquid level discharging through the primary flow measuring device and convert this liquid level into a flow rate that is integrated to a totalized flow.                   (B) A secondary flow measuring device must be installed in accordance with the manufacturer's recommendations and in a manner that reduces turbulence and promotes laminar flow.                   (C) A secondary flow measuring device must include a display of the instantaneous flow rate and a means of reading the totalized flow.                   (D) A secondary flow measuring device must be designed to allow recalibration.                   (E) A secondary flow measuring device must not interfere with the accuracy of the primary flow measuring device.         
     Source Note:        The provisions of this §217.33 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254