INDUSTRIAL WASTE MANAGEMENT 529
information may result if sampling procedures and test
parameters are selected in a careless or naive manner.Flow MeasurementAn essential part of the wastewater sampling and monitor-
ing program is the collection of flow data. A knowledge of
flow rate, flow variability, and total flow is essential. A vari-
ety of flow measuring devices are available. the selection of
the proper measuring method or device will depend on fac-
tors such as cost, accessibility, type of flow, and character of
waste. A list of many different types of methods and devices
commonly used for wastewater flow measurement are given
in Table 2.^2SamplingIt is essential that waste stream samples be truly representa-
tive of the waste discharge. Waste conditions may vary both in
magnitude and composition over a 24-hour period. Therefore,
care should be taken in selecting the method of sampling, fre-
quency and duration, sample handling, and parameters to be
measured. All these items are briefly discussed below.Method of SamplingTwo most common methods of sampling are known as grab
samples and composite samples. Either may be obtained
manually or automatically. Grab samples are single batch
samples taken at a given time. Composite or integrated
samples are taken at constant time intervals usually over
a 24-hour period, then mixed in proportion to flow at the
time of sampling to obtain one representative sample of the
total flow for the day.
Grab samples provide valuable information at low cost.
They are recommended where: (1) condition or quality
remains relatively uniform over long periods; (2) the effect
of slug loads are desired; (3) concentration of certain constit-
uents are needed for adjustment of chemical feed; (4) flow is
intermittent; and (5) the sample requires immediate analysis
due to instability of the constituents.
Composite sampling is done to obtain the average qual-
ity data for the day. Continuous samplers that take a sample
volume in proportion to the flow are desirable. Another type of
composite sampler removes samples on the hour and deposits
it into different bottles. A volume of sample from each bottle is
mixed manually in proportion to flow to obtain the compositeTABLE 2
Types of flow measurement devices commonly used for measuring wastewater dischargesFlow Measurement Devices Principle of Flow Measurement- For pressure pipes
a. Venturi meter Differential pressure is measured
b. Flow nozzle meter Differential pressure is measured
c. Orifice meter Differential pressure is measured
d. Electromagnetic meter Magnetic field is induced and voltage is measured
e. Turbine meter Uses a velocity driven rotational element (turbine,
vane, wheel)
f. Acoustic meter Sound waves are used to measure the velocity - For open channels
a. Flumes (Parshall, Palmer-Bowlus) Critical depth is measured at the flume
b. Weirs Head is measured over a barrier (weir)
c. Depth measurement Float is used to obtain the depth of flow in the
sewer, and velocity is calculated from slope
d. Acoustic meter Uses sound waves to measure velocity and depth - Computing flow from freely discharging pipes
- Pipes flowing full
a. Nozzles and orifices Water jet data is recorded
b. Vertical open-end flow Vertical height of water jet is recorded - Pipe partly flowing full
a. Horizontal open-end pipe Dimensions of free falling water jet are obtained
b. Open flow nozzle (Kennison nozzle or
California pipe method)
Depth of flow at free falling end is determined- Miscellaneous methods
a. Dilution method A Constant flow of a dye tracer is used
b. Bucket and stopwatch A calibrated bucket is used and time to fill is
recorded
c. Pumping rate Constant pumping rate and pumping duration are
recorded
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