Level monitoring and control is a fundamental requirement in any wastewater treatment process. You will find level instrumentation installed at even the simplest treatment plants for pump control, chemical storage tanks and process level controls or alarms. Operators have only limited or emergency control over treatment plant influent, so level controls and instrumentation play an important role in managing the wastewater treatment process.
By careful planning of process elevations, engineers can design treatment systems where gravity manages wastewater levels—and therefore flow—through most of the treatment process. The type of plant and topography of the site are big factors, but the requirement for pumps, controls, instruments and operator intervention can be minimized by good treatment plant design.
Wastewater treatment plant operators overwhelmingly endorse simple plant designs and instrumentation. “We try to keep things simple. More complicated equals more expensive.” comments Susan Salvetti, Instrument Technician at the Albany County Sewer District. Corrosive atmospheres in wastewater treatment plants attack everything from instrumentation to the mechanical structures and fixtures at a treatment plant. To be reliable, instruments must be designed to withstand the harsh operating environment.
36.1 LEVEL CONTROL AND ENVIRONMENTAL REGULATIONS
Environmental standards for wastewater treatment plants are becoming increasingly stringent. Although enforcement varies from region to region, both public and private treatment plant operators may now be held legally responsible for proper operation of their wastewater treatment facilities. New regulations are constantly being introduced with higher and higher standards of safety and effectiveness of the treatment process. Tank overfill protection and combined sewer overflow monitoring are current examples where regulators are imposing new requirements for level controls and instrumentation.
36.2 COMMON TERMS USED IN WASTEWATER LEVEL INSTRUMENTATION
SENSOR - any device that directly measures a physical condition. A Float is an example of a Sensor.
TRANSDUCER - a device which contains a sensor and converts its signal into an electrical signal for communication with other devices. A hydrostatic pressure sensor is an example of a Transducer.
INSTRUMENT – is the combination of sensors or transducers into a communication device that may transmit a signal, display a variable and provide control functions. An ultrasonic level indicating transmitter is an example of an Instrument.
TRANSMITTER – an instrument that transmits a continuous process signal to a remote device. A 4-20mA analog signal is the most common where each Transmitter uses a pair of wires (“twisted pair”) to deliver an analog current proportional to the measured variable. Modern serial, or digital transmitters are rapidly entering the market where a number of “multidrop” transmitters can share the same wire by taking turns, or by transmitting data only when “polled” by a remote controller.
ACCURACY – How closely an instrument measures the actual value of a material being sensed.
RESOLUTION – The smallest increment of change that the instrument can measure.
36.3 CONSIDERATIONS IN THE SELECTION OF LEVEL INSTRUMENTS
36.4 LEVEL INSTRUMENT TECHNOLOGIES USED IN WASTEWATER TREATMENT
36.41 Ultrasonic Level
Common Level Applications
- Pump Control in wet wells and sumps
- Chemical storage tank inventory
- Aeration basin level
- Aerobic Digester level
- Chlorine contact tank level
- Sludge tank level
- Bar screen differential level
The operating principle of ultrasonic level instruments is quite simple. They use a transducer with combined transmit and receive capability. The instrument measures the time it takes for a sound pulse to travel from the transducer to a target and then for the echo to return. Because we know the speed of sound in air (1086 ft/sec or 331 m/sec), the distance to the target can be accurately calculated.