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1.11: UV Disinfection And Scada Operation

  • Page ID
    5733
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    Learning Objectives

    • Explain UV disinfection theory
    • Describe UV disinfection applications
    • Describe supervisory control and data acquisition systems (SCADA)
    • Explain the functional usage of SCADA systems

    Ultraviolet Systems

    Ultraviolet light (UV) is found just beyond the visible light spectrum. When UV light is absorbed by cells of microorganisms, it damages the genetic material in such a way that the organisms are no longer able to grow or reproduce, and ultimately, it kills them. Today with growing concern about the safety aspects of handling chlorine and the possible health effects of chlorination byproducts, UV disinfection is gaining in popularity. UV technology can also provide inactivation of Cryptosporidium and Giardia, which are resistant to common disinfectants like chlorine or ozonation.

    The combination of UV technology and chlorination allows an efficient disinfecting system by killing or inactivating a larger range of microorganisms than using only one disinfectant. The UV disinfection process is particularly adapted to water with a good quality. The efficiency of UV disinfection depends on the quality of water and on the treatment stages upstream. Raw water with low turbidity and with low levels of color favor the penetration of UV light and improves disinfection efficiency.

    Corrosive water can damage UV systems, and technological advances are being made. Several manufacturers produce UV disinfection systems for water and wastewater applications. As operating experience with installed systems increases, UV disinfection may become a practical alternative to the use of chlorination at water treatment plants.

    UV Lamp Types

    Each UV lamp assembly consists of a UV lamp enclosed in an individual quartz sleeve with the ends appropriately sealed using an O-ring and a quartz end plug. All lamps within a UV system are identical type, length, diameter, power, ad output. Three types of electrode-type lamps are used to produce UV radiation, and these types are:

    • Low-pressure, low-intensity
    • Low-pressure, high-intensity
    • Medium-pressure, high-intensity

    UV lamp technology is currently changing as manufacturers strive to improve their products and search for potential new technologies. A ballast is a type of transformer that is used to limit the current to a UV lamp. Because UV lamps are arc-discharging devices, the more current in the arc, the lower the resistance becomes. Without a ballast to limit current, the lamp would destroy itself. Therefore, matching the lamp and ballast is very important in the design of UV disinfection systems.

    UV Systems Types

    The usual source of the UV radiation for disinfection systems is from low-pressure mercury vapor UV lamps that have been made into multi-lamp assemblies. Each lamp is protected by a quartz sleeve and each has watertight electrical connections. The lamp assemblies are mounted in a rack and these racks are immersed in the flowing water. The racks may be mounted within an enclosed vessel or in an open channel. Most UV installations are of the open channel configuration.

    When UV lamps are installed in open channels, they are typically placed horizontal and parallel to the flow or vertical and perpendicular to the flow. In the horizontal and parallel-to-flow configuration, the lamps are arranged into horizontal modules of evenly spaced lamps. The number of lamps per module establishes the water depth in the channel. For example, 16 lamps could be stacked 3 inches apart to provide disinfection for water flowing through a 48-inch-deep open channel.

    Each horizontal lamp module has a stainless-steel frame. Each module is fitted with a waterproof wiring connector to the power distribution center. The connectors allow each module to be disconnected and removed from the channel separately for maintenance. The horizontal lamp modules are arranged in a support rack to form a lamp bank that covers the width of the UV channel and several such lamp banks should be placed along the channel wall. The number of UV banks per channel is determined by the required UV dosage to achieve the target effluent quality.

    When it is necessary to maintain pressure within the water transmission system, UV lamps can be installed in a closed pressure vessel.

    Another type of UV system, the thin film types, uses a chamber with many lamps spaced one-quarter inch apart. This system has been used in the water industry for a 9 MGD treatment plant.

    Operators may also encounter a Teflon tube UV disinfection system, although this design is not in common use. Water flows in a thin-walled Teflon tube past a series of UV lamps. UV light penetrates the Teflon tube and is absorbed by the fluid. The advantage of this system is that water never comes in contact with the lamps. However, scale does eventually build up on the pipe walls and must be removed, or the Teflon tube must be replaced. This type of system has generally been replaced by the quartz sleeve.

    Safety

    The light from a UV lamp can cause serious burns to one’s eyes and skin. Always take precautions to protect the eyes. Never look into the uncovered parts of the UV chamber without proper protective glasses. Do not plug a UV unit into an electrical outlet or switch a unit on without having the UV lamps properly secured in the UV water chamber and the box closed.

    UV lamps contain mercury vapor, a hazardous substance that will be released if a lamp is broken. Handle UV lamps with care and be prepared with the proper equipment to clean up any spills.

    Typical UV disinfection configuration.
    Figure \(\PageIndex{1}\): UV Disinfection in Drinking Water – Image by the EPA is in the public domain

    Operation

    The operation of UV disinfection systems requires little operator attention. To prevent short-circuiting and ensure that all microorganisms receive sufficient exposure to the UV radiation, the water level over the lamps must be maintained at the appropriate level. Water levels in channels can be controlled by weirs or automatic control gates.

    Proper water depth must be maintained in the UV channel to ensure acceptable disinfection levels over the entire range of design flows. The UV channel water level control device must be regulated by the operator to:

    • Minimize variation of the channel’s water level
    • Maintain the channel’s water level at a defined level
    • Keep the UV lamps submerged at all times
    • Prevent excessive water layer thickness above the top lamp row

    UV Light Intensity Effectiveness

    To disinfect the water, UV light must be intense enough to penetrate the cell walls of the pathogens. The UV light intensity that reaches the pathogens is affected by the condition of the UV lamps and the quality of the water. The UV unit automatically adjusts UV dose according to the light transmission and effluent flow.

    The UV lamp condition is affected by the lamp’s age and the amount of slime on its surface. An old or dirty lamp has a reduced UV light intensity. The UV unit periodically cleans the lamps by mechanical means. The operator can adjust the frequency and the number of wiping cycles of the cleaning process. The UV lights can be cleaned manually in cleaning tanks that contain a chemical solution formulated for the purpose.

    Upstream processes affect the quality of the water, measured as turbidity and total suspended solids (TSS). High turbidity inhibits light transmission through the water; thereby, reducing the disinfecting power of the light in proportion to its distance from the light source. High TSS, besides inhibiting light transmission, shields bacteria and protects organisms from the UV radiation. Insufficient UV light intensity can initiate a chain of events leading to ineffective disinfection and noncompliance.

    Low UV light intensity will produce a low level of disinfection, and low intensity will result in high total coliform bacteria or high virus counts in the plants effluent, insufficient disinfection, and noncompliance.

    The lamp must be replaced when its output at maximum power is insufficient to disinfect.

    Minimum UV Dose Management

    The primary control function of a programmable logic controller is to manage the minimum UV dose applied to a UV channel. The actual UV dose control (dose pacing) is controlled for each UV channel and is based on maintaining a minimum dosing level.

    The maintenance of a minimum dosing level is usually done by flow pacing. The applied dose is calculated from the flow rate and the end-of-lamp-life intensity at the specified transmittance multiplied by ballast power.

    The dose calculation is based on received dose, derived from flow and input from the intensity sensors.

    The PLC (programmable logic controller) also controls the UV intensity in the UV channel. Each channel has a separate ballast control loop using the intensity set point as its target. The loop controls the intensity by calculating the lowest intensity of the banks in use. Then, it adjusts the ballast/lamp power to achieve the intensity set point. The action of the UV system PLC is summarized as:

    • Receives a minimum UV intensity from a preset value, which is field adjustable
    • Receives a target UV intensity form a preset value, which is field adjustable
    • Compares the target UV intensity with the actual UV intensity
    • Makes an adjustment to the output power of all of the ballast cards in operation accordingly

    If the actual UV intensity goes below minimum UV intensity for more than 2 minutes, the PLC should activate a low-intensity channel bank alarm and flag the bank as failed. This action will start the next assist bank, and after a preset time (10 minutes) will shut down the failed bank.

    The level of ballast output power is identical for all of the ballast cards within a UV bank.

    The UV system operates to maintain a minimum UV dose at all times, with a safety margin to accommodate operational changes and changeover procedures:

    • As flow increases or transmission reduces, the UV dose will be reduced and the PLC will increase ballast output to compensate. It will start up the assist bank when the duty bank is at 85-percent of full power or the dose is less than the design level plus the safety factor. Once the assist bank is at 100-percent power, output will be reduced on both banks to get the correct dose.
    • As flow decreases or transmission increases, the UV dose will be increased and the PLC will reduce ballast output to compensate. It will shut down the assist bank when ballast output is at 50-percent and the dose is greater than the design level plus the safety factor. Output on the duty bank will increased to 100 percent before shutting down the assist bank.

    UV Dose Calculation

    The intensity of the radiation and the contact time determine the UV dose received by the bacteria, leading to the effectiveness of the process. UV dose is the standard indicator of the UV effectiveness and is expressed as:

    • UVdose , mJ/cm2 = UVintensity , mW/cm2 x T, s where...
      • T = Retention Time, s

    Dosage is typically expressed as milli-joules per square centimeter (mJ/cm2) and intensity as milliwatt seconds per square centimeter (mW s/cm2).

    In water disinfection systems, molecules are suspended solids in the water absorb UV energy. The worst-case intensity (the farthest point from the UV source) is used in the calculation.

    Channel Volume Calculation

    The UV channel volume refers to the irradiated volume of the UV reactor. This area is the volume in which the bacteria are exposed to UV radiation. This space is a fixed value calculated as:

    • UV Channel Volume per Bank, ft3 = (UV Channel Width, ft x Top Water Level, ft x lamp Arc Length, ft) – (Volume of Quartz Sleeves, ft3)

    Retention Time Calculation

    The retention time is the amount of time that the bacteria are in contact with the UV radiation. Head loss and velocity calculations ensure that an optimal hydraulic condition exists in the channel. The retention time is calculated by dividing the UV channel volume by the flow rate within the UV channel as:

    • T(s) = Vreactor , ft3/Q, ft3/s where...
      • Vreactor = Volume of the UV Reactor
      • Flow Rate Calculation

    The total inflow rate is supplied by the flowmeter. This value is scaled by using the maximum flow, which transforms the inflow rate to tenths of MGD, then is displayed on a screen as a flow rate. A programmable logic controller calculates the rate per channel. This flow rate is for use in the control of the UV system exclusively and should not be used for any other purpose.

    Monitoring Lamp Output Intensity

    Lamp output declines with use so the operator must monitor the output intensity and replace lamps that no longer meet design standards, as well as any lamps the simply burn out. Lamp intensity monitors can be installed to assist the operator in monitoring the level of light output. Lamp failure indicators connected to the main UV control panel will alert the operator when a lamp burns out and requires replacement. In addition, computerized systems are available to monitor and record the age (burn time) of each lamp.

    Monitoring Influent and Effluent Characteristics

    Care must be taken not to exceed the maximum design turbidity levels and flow velocities when using this types of equipment. Suspended particles will shield microorganisms from the UV light and protect them from its destructive effects. Flows should be somewhat turbulent to ensure complete exposure of all organisms to the UV light, but flow velocity must be controlled so that the water is exposed to UV radiation long enough for the desired level of disinfection to occur.

    Because ultraviolet rays leave no chemical residual like chlorine does, bacteriological tests must be made frequently to ensure that adequate disinfection is being achieved by the ultraviolet system. In addition, the lack of residual disinfectant means that no protection is provided against recontamination after the treated water has left the disinfection facility. When the treated water is exposed to visible light, the microorganism can be reactivated. Microorganisms that have not been killed have the ability to heal when exposed to sunlight. The solution to this problem is to design UV systems with a high efficiency for killing microorganisms.

    Emergence Alarms

    UV systems require extensive alarm systems to ensure continuous complete disinfection of the water being treated. Typical emergency alarms on UV systems include:

    • Inlet level high
    • Inlet turbidity high
    • Sample flow low
    • Inlet gate motorized gate control failure
    • Inlet channel high transmittance
    • Inlet channel low transmittance
    • Diversion gate failure
    • Isolation gate failure
    • Inlet gate failure
    • System flow rates
    • Unit power failure
    • Channels inlet gates failure
    • Channel level sensor low/high
    • Channel level sensor low/low
    • Low dose

    Operators need to inspect these instruments and be sure they are performing as intended.

    Maintenance

    A UV system is capable of continuous use if a simple maintenance routine is performed at regular intervals. By checking the following items regularly, the operator of a UV system can determine when maintenance is needed.

    • Check the UV monitor for significant reduction in lamp output.
    • Monitor the process for major changes in normal flow conditions such as incoming water quality.
    • Check for fouling of the quartz sleeves and the UV intensity monitor probes.
    • Check the indicator light display to ensure that all of the UV lamps are energized.
    • Monitor the elapsed time meter, microbiological results, and lamp log sheet to determine when UV lamps require replacement.
    • Check the quartz sleeves for discoloration. This effect of UV radiation on the quartz is called solarization. Excessive solarization is an indication that a sleeve is close to the end of its useful service life. Solarization reduces the ability of the sleeves to transmit the necessary amount of UV radiation to the process.

    Maintenance on UV systems requires two tasks: cleaning the quartz sleeves and changing the lamps.

    Algae and other attached biological growths may form on the walls and floor of the UV channel. This slime can slough off, potentially hindering the disinfection process. If this condition occurs, the UV channel should be dewatered and hosed out to remove accumulated algae and slimes.

    Quartz Sleeve Cleaning

    Fouling of the quartz sleeves occurs when cations such as calcium, iron, or aluminum ions attach to protein and colloidal matter that crystallizes on the quartz sleeves. As this coating builds up on the sleeves, the intensity of the UV light decreases to the point where the buildup has to be removed for the system to remain effective. The rate at which fouling of the quartz sleeves occurs depends on several factors, including:

    • Types of treatment processes before UV disinfection.
    • Quality of water being treated.
    • Chemicals used in the treatment processes.
    • Length of time that the lamps are submerged.
    • Velocity of the water flowing through the UV system. Very low or stagnant flows are especially likely to permit the settling of solids and the resulting fouling problems.

    How often quartz sleeves need to be cleaned depends on the quality of the water being treated and the treatment chemicals used before disinfection. Dipping the UV modules for 5 minutes in a suitable cleaning solution will remove scale that has deposited on the quartz sleeves. Cleaning is best done using an inorganic acid solution with a pH between 2 and 3. The two most suitable cleaning solutions are nitric acid in strengths to approximately 50 percent concentration and a 5 percent or 10 percent solution of phosphoric acid. To clean the system while still continuing to disinfect normal flows, single modules can be removed from the channel, cleaned, and reinstalled. The other modules remaining online while one is being cleaned should be able to provide for continuous disinfection.

    In-channel cleaning of UV lamps is another option, but it has some disadvantages. A backup channel is required and a much greater volume of acid solution is needed. Also, additional equipment and storage tanks for chemicals are required. Precautions must be taken to prevent damage to concrete channels from the acid cleaning solution. Epoxy coatings normally used to protect concrete from acid attack are not used in UV disinfection systems because the epoxy tends to break down under high UV-light intensities.

    The complexity of the cleaning system depends on the size of the system and the required cleaning frequency.

    Lamp Maintenance

    The lamps are the only components that have to be changed on a regular basis. Their service life can be from 7,500 hours to 20,000 hours. This variation can be attributed to three factors:

    • The level of suspended solids in the water to be disinfected and the fecal coliform level to be achieved affects the life of the lamp. Better-quality effluents or less-stringent fecal coliform standards require smaller UV doses. Since lamps lose intensity with age, the smaller the UV dose required, the greater the drop on lamp output that can be tolerated.
    • The frequency of ON/OFF cycles to a maximum of 4 per 24 hours can considerably prolong lamp life.
    • The operating temperature of the lamp electrodes affects lamp service life. System temperatures usually depend on system conditions. Systems with lamp electrodes operating at the same temperature operate up to three times longer than systems where the two electrodes operate at different temperatures. This operating difference can occur in systems with lamps protruding through the bulkhead where only one electrode is immersed in the water and the other electrode is surrounded by air if the air temperature is routinely higher than the water temperature.

    The largest drop in lamp output occurs during the first 7,500 hours. This decrease is between 30 and 40-percent. Thereafter, the annual decrease in lamp output (5 to 10-percent) is caused by a decreased volume of gases within the lamps and by a compositional change in the quartz (solarization), which makes it more opaque to UV light.

    Operators should contact the appropriate regulatory agency to determine the proper way to dispose of used UV lamps. Do not throw used lamps in a garbage can to get rid of them because of hazardous mercury in the lamps.

    Troubleshooting

    System Hydraulics

    Erratic or reduced inactivation performance is often caused by poor system hydraulics. System short-circuiting, poor entry and exit flow conditions, and dead spaces or dead zones in the reactor can be sources of poor performance.

    Biofilms on UV Channel Walls and Equipment

    Biofilms are typically fungal and filamentous bacteria that develop on exposed surfaces and are especially troublesome on areas exposed to light. Biofilms can contain and shield bacteria. When biofilms break away from surfaces, they protect the bacteria in the clumps as they pass through the UV disinfection system. Operators should periodically remove biofilms using a hypochlorite disinfecting solution.

    Particles Shielding Bacteria

    Particles can shield bacteria and reduce the effectiveness of the UV disinfection process. These particles should be removed by upstream treatment processes such as improve clarifier performance or some type of filtration.

    Supervisory Control and Data Acquisition System (SCADA)

    A supervisory control and data acquisition system (SCADA) is a computer-monitored alarm, response, control, and data acquisition process that is used to monitor and adjust treatment processes and operate treatment facilities. SCADA systems have become essential to operating water and wastewater facilities. A SCADA system is a collection of monitoring and communication equipment with a computer interface running the SCADA software package. It is designed to help operators monitor and control treatment processes.

    Primitive SCADA systems began with oversight and monitoring of a variety of industrial systems, including power companies, major utilities, building environments, manufacturing processes, and mass transportation systems. In addition to process control, SCADA systems perform automated monitoring, data logging, alarm, and diagnostic functions that allow treatment facilities to be run safely and efficiently using a relatively small staff. SCADA systems collect real-time data from the plant, make adjustments based on plant conditions, and regulate processes to prevent costly failures.

    SCADA systems can be as basic as a single personal computer connected to a small laboratory or manufacturing process through simple input/output (I/O) interfaces. This simple system can be more than sufficient for a small plant, and could easily be configured with assistance from in-house staff. Improved staff efficiency and system reliability are seen even at this basic level.

    Midsize plants may use one or more programmable logic controllers (PLCs) networked together with distributed I/O subsystems and multiple operator interfaces. Some configurations could be handled by in-house staff, or a SCADA system provider could work with staff to set the appropriate monitoring and alarm functions that would operate across the system. Midsize systems may monitor several thousand I/O points.

    The most complex SCADA systems include networks of remote telemetry (terminal) units, or RTUs, that may cover entire plant complexes or pipeline distribution systems. Such systems use data concentrators and I/O subsystems to communicate with one another across telecommunication media. For such large, complex systems, a SCADA system override is likely to be necessary. Designing, installing, and setting up operations for these complex SCADA systems is a specialized field. O&M staff need to coordinate with the SCADA system override to ensure that alarms and monitoring functions are properly designed.

    Controllers

    Programmable logic controllers (PLCs) are control devices that act as replacements for hardwired relay panels that were used in the early years of process automation. PLCs are used in many industries, including manufacturing, assembly lines, and lighting applications. They originally used simple ladder logic, but programming languages and environments have evolved over the years. PLCs can be used in complex sequential relays, process controls, distributed control systems, and networking. A programming standard, produced by the International Electrotechnical Commission (IEC), has been established to program PLCs; however, a variety of logic languages can be used. The dramatic increase in processing power, networking capabilities, and program memory storage capacities has helped PLCs to become more practical and cost-effective for many process control applications, from industrial treatment plants to small facilities.

    Control Functions

    SCADA systems allow operators to control virtually the entire advanced water treatment facility from a computer. The operator can control tank levels, pump sequences, pump speeds, DO residuals, chemical dosing, membrane functions, advanced oxidation, UV dosing, and diverting flows from one treatment train to another. SCADA systems also allow operators to monitor and control advance water treatment plants with mobile devices such as smartphones and computing tablets.

    Monitoring Functions

    SCADA systems provide the operator an effective visual interface. Most SCADA systems provide animated graphic depictions of the processes combined with individual process values. Real-time data can be obtained and analyzed as a trend to observe any process changes. These tools provide the operator with the ability to monitor the treatment plant systems effectively and to catch process upsets, often before they occur.

    Data Logging Functions

    As the PLC receives information from the equipment and advanced water treatment processes and subsystems, the data is transferred to the SCADA system. The SCADA system electronically archives selected data to be able to recall and review it as needed. These electronic records can be exported from the SCADA system in a variety of forms, including a comma-separated variable (.CSV) file. These files can be used in other applications for further analysis or formatting into reports. Hard copies of status and alarm data can be printed and retained for plant recordkeeping. These records include date, time, and changes made. Changes to the processes can be tracked through an archive of set-point changes, alarms, and equipment adjustments. This historical data assists operators with investigating process upsets and equipment failures, and it provides well-documented data for reporting purposes.

    Alarm Functions

    SCADA alarm functions are important tools for operators. The alarm functions are integrated into the SCADA system, alerting operators to process upsets by pinpointing the precise area where the upset occurs. Operators can respond quickly and accurately, reducing the chance that a process upset will result in a violation of state or federally issued waste discharge requirements.

    Some systems include the ability to automatically contact operators who are on call. These systems can sometimes be set to communicate with mobile devices such as smartphones and computing tablets, providing operators with more flexibility.

    Diagnostic Functions

    SCADA systems can be set up to incorporate statistical online analyses of process data. This tool can be used to assist operators with avoiding equipment failures, process upsets, or false instrument readings. The system can detect random changes that may occur when instrument calibration drifts occur or when control components are at risk of failure. The operator is provided with the information in order to respond proactively, before failures or upsets occur.

    Remote Telemetry Units

    SCADA systems are capable of monitoring sprawling systems by using remote (or radio) telemetry units (RTUs). These units handle inputs, generate control outputs, and concentrate data for transmission back to the host computer. RTUs are used when monitoring devices need to be placed at isolated equipment sites, pump stations, wells or intake structures, or along a stretch of pipeline. Because RTUs usually communicate using radio waves in the same range as commercial radio frequencies, they may require Federal Communication Commission licensing. In some instances, the remote monitoring devise may use traditional electrical cabling. These units are considered remote terminal units (also RTUs).

    Operator Interface

    Operator interfaces (OIs) are alternately available as MMIs (man machine interfaces), HMIs (human machine interface), VDUs (video display units), VDTs (video display terminals), and probably a few more configurations as well. Operator interfaces allow the operator to view the entirety of the process on one (or several) screen(s). Operator interfaces vary in size and design, from rudimentary terminals displaying basic process information to large, full-color touchscreens with animated graphics. Often, a combination of display types is used to allow operators to interface at a number of key process points. Small displays may be mounted directly on process equipment to allow process changes to be made in the field, while large displays are better suited to a central control room or office where the entirety of the process can be overseen. All the display types are intended to inform the operator of the status of the process being monitored, including alerting the operator to any problems in the system and providing the means to make any necessary adjustments to the process.

    Process Computer Control Systems

    The computer control system is a computer-monitor alarm, response, control, and data acquisition system used by operators to monitor and adjust their treatment processes and facilities. Computer control systems used for process control can be classified as distributed control systems (DSC) and supervisory control and data acquisition systems (SCADA). The DCS and the SCADA systems preform the same functions in different settings. The distributed control systems (DSC) are typically used to control and monitor processes in treatment plants. The supervisory control and data acquisition systems (SCADA) are most commonly used to control and monitor distribution system facilities that are widely separated geographically. At larger water facilities DCS and SCADA system are used to provide treatment process control and distribution system controls. Smaller utilities often combine all of the controls necessary into a SCADA system. In large and small utilities, the operator interface to each system and plant processes is provided in a single control room.

    The computer control system collects, stores, and analyzes information concerning all aspects of operation and maintenance, transmits alarm signals, and allows fingertip control of alarms, equipment, and processes. The computer control system provides the informant that operators need to resolve minor problems before they become major incidents. As the nerve center at the treatment plant, the system allows operators to enhance the efficiency of their facility by keeping them fully informed and fully in control.

    The five components of a computer control system are:

    • Process instrumentation and control devices that sense process variables in the field and actuate equipment.
    • The input/output (I/O) interface sends and receives data with the process instrumentation and control devices.
    • The central processing unit (CPU) is the system component that contains the program instructions for the control system. These instructions are programmed to react based on a control strategy. The CPU gathers data from the various interfaces and sends commands to field devices to operator the plant processes.
    • The communication interfaces provide the means for the computer control system to send data to and from outside computer systems, business systems, other process control systems, and equipment.
    • The human machine interface is commonly a computer workstation that is running the computer control system software that provides the plant data to the operator on the workstation screen.

    These components are the means by which the control system gathers and distributes information for the human operator and the process instrumentation and other equipment.

    The computer control system may be used in various capacities, from data collection and storage only, to total data analysis, interpretation, and process control.

    Computer control systems monitor levels, pressures, and flows and operate pumps, valves, and alarms. They monitor temperatures, speeds, motor currents, pH, turbidity, and other operating parameters. They also provide control, as necessary. Computer control systems provide a log of historical data for events, analog signal trends, and equipment operating time for maintenance purposes. The information collected may be read by an operator on computer screen readouts or analyzed and plotted by the computer as trend charts.

    Computer control systems provide a picture of the plant’s overall status on a computer screen. In addition, detailed pictures of specific portions of the system can be examined by the operator through the computer workstation. The graphical displays on the computer screens can include current operating information, which the operator can use to determine if the guidelines are within acceptable operating ranges or if any adjustments are necessary.

    Computer control systems are capable of analyzing data and providing operating, maintenance, regulatory, and annual reports. Operation and maintenance personnel rely on a computer control system to help them prepare daily, weekly, and monthly maintenance schedules, monitor the spare parts and inventory status, order additional spare parts, print out work orders, and record completed work assignments.

    Computer control systems can also be used to enhance energy conservation programs. Operators can develop energy management control strategies that allow for maximum energy savings and maximum treatment flow before peak flow periods. In this type of system, power meters are used to accurately measure and record power consumption. The information can be reviewed by operators to watch for changes that may indicate equipment problems.

    Emergency response procedures can also be programmed into a computer control system. Operator responses can be provided for different operational scenarios that might be encountered as a result of adverse weather changes, fires, earthquakes, or other emergency situations.

    Typical Computer Control System Functions

    Computer control systems for water treatment plants and distribution systems are usually operated together, with the controls located at the treatment plant. Information that historically was recorded on paper strip charts is now being recorded and stored by computers. This information can be retrieved and reviewed easily by the operator. Therefore, computer control systems are more efficient in providing operators with the information that they need to make informed and timely decisions.

    Computer control systems give the treatment plant operator the tools to optimize plant processes based on current and historical operating information. The treatment plant influent and effluent are monitored continuously for many process variables, such as flow, turbidity, pH, ammonium, chlorine, and nitrogen. If these indicators change significantly or exceed predetermined levels, the computer control system alerts the operator or changes the process based on a preprogrammed control strategy defined by the operator.

    SCADA Control Monitor
    Figure \(\PageIndex{2}\): SCADA Control Monitor – Image by hhdgomez is licensed under CC BY-SA 4.0

    Historical operating data stored in a computer control system is readily available at any time. The computer control system can be queried to identify, for instance, when peak plant influent flows were greater than a set normal flows. Plant performance under these conditions can be recalled using the computer control system, analyzed be the operators, and the results used to operate the plant accordingly.

    Electrical energy consumption can be optimized by the use of computer control systems. Computer controls can be programmed with a control strategy to reduce energy costs by automatically operating equipment when demands for power are low. Most power companies are eager to help operators save money by structuring their rates to encourage electrical energy consumption when demands for power are low and to discourage consumption when demands for power are high. Computer controls can be programmed with a control strategy to reduce energy costs by automatically operating equipment when demands for power are low.

    Computer control systems are being continually improved to help operator do a better job. Operators can create display screens the show graphics and whatever operating characteristics that are wished to be displayed. The main screen could be a flow diagram from influent to effluent showing the main treatment and auxiliary process areas. Critical operating information could be displayed for the main treatment flow path and process area, with navigation capabilities to easily access detailed screens for each piece of equipment.

    Information on the screen should be color-coded to indicate if a pump is running, ready, unavailable, or failed, of if a valve is open, closed, moving, unavailable, or failed. The computer uses a failed signal to inform the operator that something is wrong with the information or the signal it is receiving or is being instructed to display. The computer senses and reports information that is not consistent with the rest of the information available.

    The operator can request a computer to display a summary of all alarm conditions in a plant, a particular plant area, or a process system. A blinking alarm signal indicates that the alarm condition has not yet been acknowledged by the operator. A steady alarm signal, one that is not blinking, indicates that the alarm has been acknowledged but the condition causing it has not yet been fixed. Also, the screen could be set up to automatically designate certain alarm conditions as priority alarms, requiring immediate operator attention.

    With proper security implementation, computer control systems allow operators to have remote access to plant controls from anywhere using a laptop or remote workstation. This option provides the flexibility for off-duty staff to help on-duty operators solve operational problems. Computer networking systems allow operators at terminals in offices, in plants, and in the field to work together and use the same information or whatever information they need from one central computer database.

    A drawback of some computer control systems is that when the system goes down due to a power failure, the numbers displayed will be the numbers that were registered immediately before the failure, not the current numbers. The operator may therefore experience a period of time where accurate, current information about the system is not immediately available.

    Customer satisfaction with the performance of a water utility can be enhanced by the use of an effective computer control system. Coordination of the treatment facility control and the distribution system control is used to avoid water shortages and low pressures.

    When operators decide to initiate or expand a computer control system for their plant process system, the first step is to decide what the computer control system should do to make the operators’ jobs easier, more efficient, and safer, and to make their facilities’ performance more reliable and cost-effective. Costs savings associated with the use of a computer control system frequently include reduced labor cost for operation, maintenance, and monitoring functions that were formerly performed manually. Precise control of chemical feed rates by a computer control system eliminates wasteful overdosing. Preventive maintenance monitoring can save on equipment and repair costs, and energy saving may result from off-peak electrical power rates. Operators should visit facilities with computer control systems and talk with the operators about what they find beneficial and detrimental with regard to computer control systems and how the systems contribute to their performance as operators.

    The greatest challenge for operators using computer control systems is to realize that just because a computer system says something does not mean that the computer is always correct. Also when the system fails due to a power failure or for any other reason, operators will be required to operate the plant manually and without critical information.

    Operators will always be needed to question and analyze the results from computer control systems. They will be needed to see if the effluent looks as it should, to listen to a pump to be sure it sounds right, and to smell the process and the equipment to determine if unexpected or unidentified changes are occurring. Treatment plants and distribution systems will always need alert, knowledgeable, and experienced operators who have a feel for their plants and their distribution systems.

    Operation

    Usually, instrumentation systems are remarkably reliable year after year, assuming proper application, setup, operation, and maintenance. Reliable measurement systems even though outdated are found in regular service at some plants up to 50 years after installation. Good design and application account for such long service. Most important is the careful operation and regular maintenance of the instruments’ components. The key to proper operation and maintenance is the operator’s practical understanding of the system. Operators must know how to recognize malfunctioning instruments so as to prevent prolonged damaging operations, shut down and prepare devices for seasonal or other long-term nonoperation, and perform preventive maintenance tasks to ensure proper operation in the long term. A sensitive instrumentation system can be ruined in short order with neglect in any one of these three areas.

    Operators should be familiar with the technical manual of each piece of equipment and instrument encountered in a plant. Each manual will have a section devoted to the operation of a certain component of a complete measuring or control system. Detailed descriptions of the maintenance tasks and operating checks will usually be found in the manual. Depending on the general type of instrument and the suggested frequency of the operation and maintenance/checking tasks can range from none to monthly. From an operations standpoint, these tasks include learning, and constant attention to, what constitutes normal function. From a maintenance standpoint, they include ensuring proper and continuing protection and care of each component of the instrument.

    Indication of Proper Function

    The usual pattern of day-to-day operation of measuring and control systems in a plant should become so familiar to operators that they almost unconsciously sense any significant change. This reality is especially evident and true for systems with recorders where the pen trace is visible. An operator should watch indicators and controllers for their characteristic actions and pay close attention to trend records. Using the trend capability of the computer control system provides a method to analyze the reaction of one process variable to a change in another or other process variables.

    Two of the surest indications of serious electrical problems in instruments or power circuits are smoke or a burning odor. Such signs of a problem should never be ignored. Smoke/odor means heat, and no device can operate long at unduly high temperatures. Any electrical equipment that begins to show signs of excessive heat must be shut down immediately, regardless of how critical it is to plant operation. Overheated equipment will very likely fail very soon anyway, with the damage being aggravated by continued usage. Fuses and circuit breakers do not always de-energize circuits before damage occurs, so they cannot be relied on to do so.

    Operators frequently forget to reset an individual alarm. This failure is especially prevalent when an annunciator panel is allowed to operate day after day with lit-up alarm indicators and one light is not easily noticeable. Also when a plant operator must be away from the main duty station, the system may be set so the audible part of the alarm system is temporarily squelched. When the operator returns, the audible alarm system may inadvertently not be reactivated. In these instances, the consequences of inattention can be serious. Therefore, develop the habit of checking annunciator systems often.

    Preventive Maintenance (PM)

    Preventive maintenance (PM) means that attention is given periodically to equipment in order to prevent future malfunctions. Corrective maintenance involves actual, significant repairs. Routine operational checks are part of the preventive maintenance program in that a potential problem may be discovered and corrected before it becomes serious.

    Preventive maintenance duties for instrumentation should be included in the plant’s general PM program. If the plant has no formal routine PM program, it should have one. Such a program must be organized, set up, and recorded. Operators should have reminders, guides, and a record of PM tasks. Without explicit measures, experience shows that preventive maintenance is put off indefinitely. Eventually, the press of critical corrective maintenance and even equipment replacement projects eliminate preventive maintenance forever. The fact that instrumentation is usually very reliable may keep it running long after pumps and other equipment have failed. Instrumentation maintenance duties require proper attention periodically to maximize the instruments effective life. Actually, PM tasks and checks on modern instrument systems are quite minimal so no valid reason exists for failing to perform these tasks.

    The technical manual for each item of instrumentation in the plant should be available so the operator can refer to it for O&M purposes. When a manual cannot be located, contact the manufacturer of the unit. Be sure to give all relevant serial/model numbers in a request for a manual. All equipment manuals should be kept in one protected location and signed out as needed. Become familiar with the sections of the manual related to O&M, and follow their procedures and recommendations closely.

    A good practice is to have on-hand any supplies and spare parts that are or may be necessary for instrument operation or service. Some technical manuals contain a list of recommended spare parts.

    Since PM measures can be so diverse for different types, brands, and ages of instrumentation only a few general considerations are applicable:

    • Protect all instrumentation from moisture, vibration, mechanical shock, vandalism, and unauthorized access.
    • Keep instrument components clean on the outside and closed/sealed against inside contamination.
    • Do not presume to lubricate, tweak, fix calibrate, free up, or modify any component of a system arbitrarily.
    • Do keep recorder pens and charts functioning as designed by frequent checking, servicing, bleed pneumatic systems regularly, ensure continuity of power for electrical devices, and do not neglect routine analytical instrument cleanings and standardizing duties as required.

    It is a good idea to get to know and cooperate fully with the plant instrument service person. Good communication between this person and the operating staff can result in better all-around operation. Or it may be a good idea to enter into an instrumentation service contact with an established company or with the manufacturer. Usually, general maintenance personnel are not qualified to perform extensive maintenance on modern instrumentation.

    Operational Checks

    Computer control systems provide the best tools for observing the operating functions of plant process systems. The operator interface provides the ability to view all areas of plant operation. Most systems provide the ability to display trends of multiple process variables on the same graphical display. These trends are tools that operators use extensively to monitor and control facility processes.

    Operational checks are most efficiently made by observing each system for its continuing sign of normal operation. However, some measuring systems may be cycled within their range of action as a check on the responsiveness of components. Whenever an operator or a technician disturbs normal operation during checking or for any reason, plant process operating personnel must be informed. Ideally, it should be before the disturbance takes place. If a recorder trace is altered from its usual pattern in the process, the person causing the upset should initial the chart appropriately and note the time. Some plants require operators to mark or date each chart at midnight of each day for easy reference and filing.

    Generally, any extensive operating check of instrumentation should be performed by the instrument technician during routine PM program activities.

    Review Questions

    1. Explain UV disinfection theory.
    2. Describe UV disinfection applications.
    3. Describe supervisory control and data acquisition systems (SCADA).
    4. Explain the functional usage of SCADA systems.

    Test Questions

    1. When UV light is absorbed by cells of microorganisms, it damages the _____ in such a way that the organisms are no longer able to grow or reproduce, and ultimately, it kills them.
      1. Cell wall
      2. Organelles
      3. Plasma membrane
      4. Genetic material
    2. The usual source of the UV radiation for disinfection systems is from _______ that have been made into multi-lamp assemblies. Each lamp is protected by a quartz sleeve and each has watertight electrical connections. The lamp assemblies are mounted in a rack and these racks are immersed in the flowing water.
      1. Low-pressure mercury vapor UV lamps
      2. Low-pressure, low-intensity lamps
      3. Low-pressure, high-intensity lamps
      4. Medium-pressure, high-intensity lamps
    3. ________ must be maintained in the UV channel to ensure acceptable disinfection levels over the entire range of design flows.
      1. Proper chlorine residual
      2. Proper water depth
      3. Proper TDS removal
      4. None of these are correct
    4. To disinfect the water, UV light must be ________ of the pathogens. The UV light intensity that reaches the pathogens is affected by the condition of the UV lamps and the quality of the water.
      1. At a very high wave length to penetrate the cell walls
      2. At a very low wave length to penetrate the cell walls
      3. Intense enough to penetrate the cell walls
      4. Of short duration in order to penetrate the cell walls
    5. The largest drop in lamp output occurs during the first _______. This decrease is between 30 and 40-percent. Thereafter, the annual decrease in lamp output (5 to 10-percent) is caused by a decreased volume of gases within the lamps and by a compositional change in the quartz (solarization), which makes it more opaque to UV light.
      1. 5,000 hours of use
      2. 5,500 hours of use
      3. 7,000 hours of use
      4. 7,500 hours of use
    6. Erratic or reduced inactivation performance of coliforms is often caused by _______.
      1. Poor system hydraulics
      2. System short-circuiting
      3. Poor entry and exit flow conditions
      4. All of the above
    7. _______ can shield bacteria and reduce the effectiveness of the UV disinfection process. They should be removed by upstream treatment processes such as improve clarifier performance or some type of filtration.
      1. Excessive water
      2. Turbidity
      3. Excessive alkalinity
      4. Low chlorine residuals
    8. ________ are most commonly used to control and monitor system facilities that are widely separated.
      1. DSC systems
      2. PLC systems
      3. SCADA systems
      4. Pneumatic systems
    9. The greatest challenge for operators using computer control systems is to realize that just because a computer system says something does not mean that the computer is ______. Also when the system fails due to a power failure or for any other reason, operators are required to operate the plant manually and without critical information.
      1. Wrong
      2. Always correct
      3. Malfunctioning
      4. Failing
    10. Computer control systems give the treatment plant operator the tools to ______ plant processes based on current and historical operating information.
      1. Optimize
      2. Diminish
      3. Ignore
      4. Degrade

    This page titled 1.11: UV Disinfection And Scada Operation is shared under a CC BY license and was authored, remixed, and/or curated by John Rowe (ZTC Textbooks) .

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