(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.