Instrumentation & Control System for Waste Water Treatment
Instrumentation & Control Flow Measurement Devices
Wastewater treatment processes are characterized by continuous disturbances and variations that cannot be detected by manual flow measurement. It has to be done with the precision and within the time span necessary for maintaining proper operation of the facility of water treatment. Typical process disturbances include process inputs and conditions such as variable flow rates, chemical and biological composition, temperature and density.
Instrumentation and automation control allow continuous monitoring of process variables, rapid transfer of data to the operator or manager. It also facilitates immediate automation of execution of corrective flow measurement when needed. The use of instrumentation and automation control for water treatment is growing nowadays. This is owing to the multitude of benefits that they confer in terms of process improvement, equipment and performance for water treatment.
A typical instrumentation and control system comprises of the following components:
- Signal transmitting device
- Computer and central control room
Measuring devices, referred to as sensors, include instruments that sense, flow measurement or compute the process variables. These variables fall into three categories: physical (flow, pressure, water level, temperature, etc.), chemical (pH, oxidation-reduction potential, turbidity, specific conductance, dissolved oxygen, chlorine residual, and so on) and biological (oxygen consumption rate, TOC reduction rate, sludge growth rate, etc.). Sensor devices can flow measurement variables directly, indirectly or by inferential means. Moreover, flow measurement may be performed on-line or off-line, continuously or intermittently.
Instrumentation Signal Transmitting Devices
The function of a signal-transmitting device is to transmit a process variable signal from a sensor to areadout device or controller for water treatment. The signal may be transmitted mechanically, by means of the movement of apen, indicator, float or cable, pneumatically. By means of a detector or an amplifier, or electronically bymeans of voltage and current, pulse duration, or tone. In voltage and current transmission, signals aretransmitted by milliamp direct current or by voltage signals. In pulse duration or time-pulse transmission,the length of time the voltage is transmitted is in proportion to the flow measurement data.
In tone transmission,standard telephone lines are normally used to transmit signals.Radio or microwave transmission has recently been developed and put into practice. This transmissionmethod is particularly advantageous where the gathering points are scattered over a large area for water treatment. And wheretelephone lines are either not available or prohibitively expensive. Electronic and radio/microwave controlsystems are becoming more attractive for a number of reasons.
Instrumentation Data Display Read-out
Readout devices display the transmitted operation in a configuration that is usable by the operator for water treatment. The most common types of readout devices are indicators, recorders, and totalizers on panels or computer screens. The data display is placed either locally, close to the equipment site, or at a central operating room for the whole facility.
Instrumentation Control Systems
Time control systems used in waste-water engineering fall into three categories:
- Digital control
- Analog control
- Automatic control
Digital control systems have two positions (on/off, open/close, alarm/normal). The transmitted signal, originating from a position, limit, float or pressure switch, indicates a status change.
Analog control systems, in contrast, transmit data as a range of values measuring flow rate, concentration and water level. Analog data may be reused and transmitted unchanged, converted to digital form, or transmitted as a combination of the two.
Automatic control systems may be discrete or continuous. In discrete control, the status of equipment and status changes (digital flow measurement) are correlated with a preset value or program of events. The operation may be initiated manually by the operator, using a push button, or automatically by an internal process-generated event.
Continuous control, on the other hand, requires analog flow measurement for its input and manipulates a final control element as its output. The control element may be feedback and feed forward control loops and control systems or controllers. The devices automatically regulate the control variable.
Instrumentation Data Acquisition Systems
Data acquisition systems effectively accumulate, format, record, and display data transmitted from sensors. Modern data acquisition systems, commonly referred to as Supervisory Control and Data Acquisition (SCADA) systems, can provide accurate, impartial documentation of process flow measurement and operator actions. In addition to data accumulation and processing, SCADA systems can produce the necessary process corrections such as chemical solutions, air supply, pump scheduling, and so on.
Data acquisition systems are located in a central control room, displaying treatment information, important events and alarms in a centralized location. Automatic or manual actuation of final control elements is also performed at the central control room, with the result that fewer personnel are required to operate a large treatment facility.
Instrumentation & Control Applications in Wastewater Treatment
There are many factors that determine the need for instrumentation and control elements in wastewater systems. These factors include the size of the facility, hours of manned operation, complexity of the process, reliability requirements, and availability of instrumentation maintenance personnel. Ultimately, most of the resultant decisions are made on an economic basis. The decision to use instrumentation, automation, and control in wastewater treatment systems should be made early in the conceptual design phase of a facility.
The instrumentation influences the design of the entire system; the size and configuration of existing vessels, tanks, channels, pipes and mechanical equipment will frequently have to be substantially altered to accommodate good instrumentation and control practices.
Scope for Instrumentation in Wastewater Treatment
As new and more sophisticated instrumentation is developed, waste-water characterization is likely to improve in the years to come. With devices that can flow measurement values of micrograms and even nanograms per liter, contaminants that are present only in trace amounts will be accurately detected. This means that a broader range of compounds will be monitored, and that stricter limits imposed on waste-water discharges will be met.
Improved characterization of waste-water, made possible by more sensitive detection methods and advanced analytical techniques, will also yield more knowledge about the behavior of waste-water constituents and their relationship to process performance. This is especially true for biological treatment processes, where microbiological techniques, including RNA and DNA typing, help optimize process performance. As process modeling becomes more accurate, the design and operation of wastewater treatment facilities will be greatly enhanced.
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