| (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.
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