Pollutant limits in the industrial user’s permit are often expressed in terms of mass loadings to an industrial facility or a POTW (e.g. categorical standards for conventional pollutants, organics, metals, microorganisms, etc.). To determine a mass loading, it is necessary to obtain and record accurate flow data. This section briefly describes the types of devices and procedures used to measure flow. In situations where flow‐measuring devices such as those described in the following sections are not available, the facility may need to rely on the use of water consumption records at the facility. However, when a mass loading needs to be determined for assessing compliance, the facility should have the ability to assess directly the flow at the facility during the sampling event.
Open Channel Flow
Open channel flow, where the flow occurs in conduits that are not full of liquid, is the most prevalent type of flow at industrial user discharge points regulated by the pretreatment program. Partially full pipes that are not under pressure are classified as open channels as well. Open channel flow is measured using both primary and secondary devices (as described below).
Primary Devices
Primary devices are calibrated, hydraulic structures installed in the channel so flow measurements can be obtained by measuring the depth of liquid at a specific point in relationship to the primary device. Weirs and flumes are examples of primary devices.
The most common type of weir consists of a thin, vertical plate with a sharp crest that is placed in a stream, channel, or partially filled pipe. Figure 4.2 shows a profile of a sharp‐crested weir and indicates the appropriate nomenclature. Four common types of weirs (rectangular, trapezoidal, sharp‐crested, and v‐notch or triangular) are shown in Figure 4.3. The crest is the upper edge of the weir to which water must rise before passing over the structure. The vertical distance from the crest of the weir to the top of the water surface is termed the “head.” To determine flow rate, the inspector must measure the hydraulic head. The rate of flow over a weir is directly related to the height of water (hydraulic head) above the crest. To measure the hydraulic head, a measuring device is placed upstream of the weir at a distance of at least four times the head. To approximate the head, the inspector can measure at the weir plate. However, this value will provide only a rough estimate of flow.
Assuming negligible upstream velocity, the basic uncorrected weir equation is
where C w is the weir coefficient, b is the width, and H is the weir height.
EXAMPLE 4.4
Water flows over a broad‐crested weir with a flow rate of 3 cfs when the weir head is 0.5 ft. Determine the flow rate expected when the head is increased to 0.8 ft. Assume that the weir coefficient remains the same for each case.
SOLUTION
From Eq. (4.5), 
, H = 0.5 ft
Thus, C w b = 2.24 ft; hence, with H = 0.8 ft,
The flume is an artificial channel constructed so the wastestream flows through it. The wastestreams flow is proportional to the depth of water in the flume and is calculated by measuring the head. A flume is composed of three sections: (i) a converging upstream section; (ii) a throat or contracted section; and (iii) a diverging or dropping downstream section. The two principal types of flumes are the Parshall flume and the Palmer‐Bowlus flume.
Figure 4.4 presents a sketch of the Parshall flume, identifying its different parts. In the Parshall flume, the floor level of the converging section is higher than the floor of the throat and diverging section. The flume operates on the principle that when water flows through a constriction in the channel, a hydraulic head is produced that is proportional to the flow. Flumes are good for measuring open channel waste flow because they are self‐cleaning. Sand and SS are unlikely to affect the device’s operation (Benefield and Judkins 1984).
Under free‐flowing (unsubmerged) conditions, the Parshall flume is a critical‐depth meter that establishes a mathematical relationship between a steady height, h, and discharge, Q. For a flume throat width (w) of at least 1 ft (0.305 m), but less than 8 ft (2.44 m), flow can be calculated using the following equation:
where
- Q = flow (cfs)
- W = throat with (ft)
- H = upper head (ft)
EXAMPLE 4.5
What is the calculated wastewater flow through a Parshall flume with a throat width of 2.0 ft at the maximum free‐flow head of 2.5 ft?
SOLUTION
Using Eq. (4.6),
A Palmer‐Bowlus flume, which may or may not have a constriction, has a level floor in the throat section and is placed in a pipe for approximately the length of the pipe’s diameter. The depth of water above the raised step in the throat is related to the discharge rate. The head should be measured at a distance (d/2) upstream of the throat, where (d) is the width of the flume. The height of the step is usually unknown until the manufacturer’s data are consulted, and it is difficult to measure manually the height of water above the step at an upstream point. The dimensions of each Palmer‐Bowlus flume are different. Therefore, the manufacturer’s data must be consulted to establish a relationship between the head and the discharge rate. Figure 4.5 contains a sketch of a free‐flowing Palmer‐Bowlus flume.
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