TM 5-814-3/AFM 88-11, Volume III
(1) Venturi meters. Venturi meters are not to be used for measuring wastewater or sludge flow unless
sufficient hydraulic head is available , or unless the Venturi tube is so constructed as to prevent solids
accumulation at the upstream side of the throat. Clogging of the pressure tubes is avoided by providing
cleanout taps and discharging a stream of fresh water through them into the sewer. Positive separation of
potable water supply from this connection must be assured.
(2) Weirs. Weirs will be located in a channel so that the flow will not be disturbed by turbulence and in
such a manner that the depth of flow over the weir can be observed and recorded. When continuous
recording is required, the float will be installed in a chamber separated from the main channel of flow but
connected thereto by piping.
(3) Parshall flumes. The dimensions of a standard Parshall flume and a table of discharge rates are given
in appendix C. This device has many advantages: the loss of head is minimal; it is self-cleaning; flow
measurement can be made in open-channel flow; and it has no moving parts to malfunction. The downstream
water-surface elevation above the flume approach floor must not exceed 65 percent of water elevation
upstream of the flume. The flume will be designed with the narrowest throat practicable for the conditions
under consideration. This is particularly important where a Parshall flume is utilized to control the velocity
through a grit chamber.
(4) Magnetic flow meters. Magnetic flow meters can be used for flow measurement in wastewater treat-
ment plants. There are many types of magnetic flow meters, however, and direct contact with the
manufacturers is the quickest and generally most practical way to determine their application to specific
(5) Ultrasonic meters. Ultrasonic devices are being used to measure levels in Parshall flumes. A pulsing
signal is bounced to the receiver where the level is related to the time elapsed. Since no components are in
contact with the liquid, this device is applicable to many types of wastes and situations.
18-3. Monitoring equipment for process control.
Monitoring equipment will be used to indicate and/or record flow quantities and, if justified, pressure,
temperature, liquid levels, velocities; and various quality parameters such as dissolved oxygen, biochemical
oxygen demand, total suspended solids, ammonia, nitrate, and pH.
a. Monitoring at pumping stations. In sewage pumping stations, flow measurement is necessary to
control periodic pump operation. Watt-hour meters and pump-time meters will be used to ensure uniform
pump wear in multiple-pump installations.
b. Monitoring of primary treatment. Monitoring primary treatment processes will require only flow
measurement and recording and perhaps grit level monitoring. When digestion of the primary sludge follows,
raw sludge flow rates must be monitored and controlled. In digestion, gas flow rates, tank pressures and
sludge temperatures will be monitored, and digester operation adjusted accordingly.
c. Monitoring of biological treatment. Trickling filter monitoring
will include flow measurement
influent, effluent and recirculation lines, and also volume of sludge pumped to or from the digesters. These
parameters are used in determining and controlling hydraulic and organic loading as well as in controlling
settling tank efficiencies. Activated sludge treatment will require the same monitoring with the addition of
mixed-liquor, volatile suspended solids and air supply monitoring.
b. Monitoring of sludge handling. In sludge elutriation, sludge and elutriant flows will be measured in
order to determine required sludge conditioner quantities. Sludge filtration will require measurements and
control of sludge and sludge-cake flows and chemical feed rates. All chemical feed lines will be monitored
and controlled, whatever their function. Sludge incineration and drying processes will require temperature
monitoring at various points, pressure gauges, and sludge weighing equipment. Fuel flow rates, whether
waste gas or auxiliary fuel, must be measured and controlled.