Metering & monitoring.
The first eight chapters of this manual's water supply text discuss equipment, supplies, and procedures for producing potable water and then distributing the same. This chapter covers the adjunct functions of keeping track of the system's performance and evaluating the information obtained. Measuring customer water use with meters provides the basis for determining an equitable cost per customer of system operation.
Mainline metering determines water use for other community needs, such as fire fighting, main flushing, and street cleaning. By carefully analyzing all records, a theoretical water balance can be established that will indicate how efficiently the system functions - whether there are major leaks, water losses, or metering defects. Determining flow and pressure on a section by section basis makes possible evaluating pipe conditions and whether maintenance measures are warranted.
Another form of evaluation procedure is discussed here - determination by laboratory analysis the quality of the product delivered. This is not only required by federal and sometimes state statutes, but is important to proper system management. Establishing quality verifies that treatment procedures are effective and that a safe and palatable water is being produced and delivered. It also indicates whether treatment modifications are needed.
The tremendous advances in electronics and data acquisition technology has allowed pumping stations and treatment plants to become increasingly automated. Even water meters can be remotely read and data records placed in electronically retrievable storage. Despite the sophistication of the new devices, however, much still depends on the basic primary equipment, the meters and other measuring devices - their construction and inherent accuracy. In many communities computer operation of the distribution system, customer metering and billing, and treatment facilities is standard practice. Accurate meters can help promote efficient water use and help secure proper revenues.
Effluent must be continually monitored to evaluate the effectiveness of the treatment procedures and to make adjustments as required. Coagulation and sedimentation will depend on turbidity, alkalinity, pH, and in well-equipped plants, zeta potential measurement. Softening plants will add hardness determinations to the list. Where iron and manganese are removed, routine iron and manganese concentrations have to be monitored closely. Communities that are fluoridating will regularly need to check fluorides; and desalting plants, total solids and salinity.
The AWWA publishes a manual on water meter selection, installation, testing, and maintenance, designated AWWA M6; and a companion volume for estimating service requirements, entitled "Sizing Water Service Lines and Meters." The latter carries the designation AWWA M22.
The water meters used in municipal systems to measure flow to individual consumers are classified as service meters. Mainline meters measure the flow to or in the distribution system.
Important characteristics of service meters are accuracy and sensitivity, both when new and after use; durability; low pressure loss; cost of purchase and installation; and ease and cost of maintenance. For general application, the importance of these characteristics is about in the order named, but the order may vary for different classes of service.
Since the quantities to be measured may vary from that used by a small residence to the large amount required by an industry, various sizes and designs are necessary. Few meters can properly handle the demands of these wide flow variations. Where there is a very wide range of consumption, two meters are generally used; a small-capacity meter to detect and measure the low rates, combined with a large capacity meter for higher flows.
With the development of reliable thermoplastic materials, some meter manufacturers use plastic parts for internal components and meter bodies. Many of these components are long-wearing and relatively corrosion resistant.
Disc Meters. These are used primarily for services supplied through pipes with a diameter 2 in. and smaller, although they are made in larger sizes. A disc meter incorporates a circular disc plate with a ball at its center, both of which are closely fitted to a measuring chamber. Water moving through the meter displaces the disc within the chamber in a precise nodding or nutating motion, which measures a known volume of water for each cycle. The nutating disc assemblies are made of either hard rubber or special synthetic polymers. The measuring chambers are made of bronze or synthetic polymers.
Most manufacturers offer two types of meter assemblies, one for use where freezing is possible and the other for warm climates. When designed for warm climates the bottom plate is manufactured of bronze or synthetic polymer. For cold climates where freezing is possible the bottom cap is designed to break under frost pressure to protect the meter from undue distortion. The bottom cap can be easily replaced when ruptured by freezing.
Frost proof meters are offered in sizes 5/8 in. through 1-1/4 in. Meters for larger pipe sizes are of the "split-case" design and do not incorporate the frost protective bottom. Split-case meters in sizes above 1-1/2 in. pipe size are designed for easy serviceability without removal from the pipe line.
Oscillating Piston Meters. These meters contain a cylindrical measuring piston closely fitted into a cylindrical measuring chamber. Similar to disc meters, the oscillating piston displaces a specific volume of water with each oscillation cycle. Oscillating pistons are made of hard rubber or synthetic polymer and the measuring chambers are made of bronze or synthetic polymer.
Oscillating piston meters are used for residential service in sizes up to 1 in. and for larger flows in sizes up to 3 in. pipe size. All displacement meters, whether disc or piston type, utilize magnetic couplings to drive the register mechanism, which is dry and sealed against entry of dirt or moisture that would impair reading visibility and accuracy.
Both these types of displacement meters are covered by AWWA Standard C700.
Vortex Flow Meters. A type of meter that can be used for liquid, steam, and gas processes is the vortex shedding meter. In this type meter the material flows into a "bluff" object that produces vortices or pressure pulses on alternating sides of the object. The flow rate is proportional to the rate at which the vortices are "shed." A sensor in the object converts the pulses into a frequency signal that can then be read as an analog output.
Multi-jet meters have been used widely in Europe for many years, and their use has increased here. They differ from displacement meters in that the amount of water passing through the meter is not actually measured in volumetric segments, but rather is inferred relative to its velocity by a paddle wheel element. For this reason multi-jet meters are known as velocity meters. The accuracy of this type meter is directly related to the paddle wheel's rotation speed relative to the flow velocity. These meters are commercially available in sizes from 5/8 in. and larger and are covered by AWWA Standard C708.
To measure flow rates greater than the capacity of 2-in. disc meters, many manufacturers make "current" meters. Current meters are made in two distinct types, propeller-driven and turbine-driven.
Turbine type meters are covered by AWWA Standard C701-78, which further sub-divides turbine meters into two categories, namely Class I and Class II.
Class I turbine meters are vertical shaft and low velocity horizontal meters with a maximum head loss of 15 psi at maximum capacity. Class Il turbines are in-line, high velocity meters having a maximum head loss of 7 psi at maximum capacity. Class II turbines have higher maximum continuous duty ratings and wider measuring range than Class I types.
Propeller driven current type meters are covered by AWWA Standard C704 and are primarily used for both water measurement where accuracy is less stringent than turbine meters, and where measurement of lower flow rates is not required. Both the propeller and turbine meters are current or velocity inferential meters. Both are proportional to a linear relationship of the velocity of the stream over the operating range of the meters. The basic operating difference is in the construction of propeller and turbine meters. Turbine meters are more sensitive at lower rates of flow and are designed to be accurate over relatively wide flow ranges. Propeller meters are used where generally flow rate variation is not nearly as great and thus are generally most advantageously used for narrow flow rate ranges predominantly at the mid-range operating flows.
Compound meters contain, within a single casing, a displacement meter section for measuring low flows, a turbine meter section for measuring high flows, and an automatic compounding valve. Low flow rates pass through the disc or piston meter section only, until the maximum capacity is reached, at which time the valve opens and the higher flow rates are measured through the turbine section.
Compound meters, used where wide flow rate variations are encountered, total flow on two registers or on a combined single register. This is designated in AWWA C702.
Updating Old Meters
Nearly all leading meter manufacturers offer a factory method of updating old mechanical drive meters to modern magnetic drive units. The old types were designed to mechanically drive registers through gear trains. The newer meters employ magnetic couplings for this purpose, eliminating many mechanical parts.
Meter Testing & Repair
Meters should be tested periodically to determine and verify their accuracy. A meter can be tested without removal from its permanent setting by using an additional special meter. These devices permit passing through a known quantity of water at different rates and these flows are checked with the meter reading.
The amount passed is measured by either weighing or gauging the volume. In the latter, the volume may be measured before it enters or after it leaves the meter. In large cities, convenience and time savings in installing and removing the meters is important. Where many meters are to be tested, a number can be set together in a series and all tested at once.
When testing a meter that has been in service, it is desirable to restore it to good condition by cleaning all parts and replacing worn ones. Dilute acid has been used for cleaning metal parts; the manufacturer can probably suggest the most appropriate means for cleaning.
To increase meter reading efficiency and eliminate missed readings from lockouts or inaccessible meters, several manufacturers offer remote meter reading equipment. This equipment is divided into two categories; generator remotes that provide a visual display of the quantity registered on a remote counter, and encoder systems that read the meter electronically and transfer the data for storage and interrogation to an acquisition device.
Generator remotes utilize a self-powered pulse generator/register at the meter, a two wire conductor, and a remote visual totalizer. The remote totalizer is read visually and the reading is manually recorded on route books, electronic recorders, or other means.
Encoder-type remote systems utilize a remote receptacle directly connected to the encoded meter reading at the register. Readings are obtained by inserting a probe that either displays the meter reading visually and/or records the meter reading and meter identification number on magnetic tape for automatic data processing and bill preparation.
The encoder register and receptacle are electrically passive and are powered by batteries in the reading equipment only during the time the reading gun is inserted in the receptacle. Manufacturers of encoder remote systems offer the complete system including tape records for data acquisition and terminals for data transmission to billing computers.
An automated remote wireless water meter reader that sends signals to stationary receivers that then relays the data over phone lines to a central microcomputer at the utility offices is also available.
Plate-type strainers are recommended to protect the metering elements from damage and to improve the velocity profile of the flow stream entering the meter.
When meters are set outside, they should be enclosed in a box, which should keep out dust; should protect them from freezing in cold climates; should make it easy for the meter reader to read them; and difficult for vandals to reach. They should be set in the box so removal is easy to facilitate repair, cleaning, etc.
Meter boxes generally extend to a depth several inches greater than the deepest frost penetration and should be large enough to give plenty of air between the riser pipes and the side of the drum. For very cold climates, two covers, one a few inches above the other, can be used, giving dead air insulation. Manufacturers provide meter models with frost plugs, frangible meter bottoms or tops, which minimize damage to the internal measuring element when freeze-ups occur.
The meter can rest on the ground in the bottom of the box. But unless it is a very shallow box it is better to place it above the soil (and possibly mud) and near the surface for reading. This is effected by bringing the service pipe up with two vertical risers and fastening the meter between their tops.
This procedure is best done by using a yoke - a metal frame that connects rigidly and permanently the inlet and outlet pipe at the point where the meter is connected to them - at the top of the risers.
Sight Flow Indicators. These devices provide a rather inexpensive way of monitoring flows through a process line. They are installed directly on a line and are activated by the fluid flow. The operator can see the flow directly through the viewing lens; and when several are installed, the flow can easily be checked at differently locations throughout the plant.
Risers, Yokes, Couplings, Setters. The effect of combined risers and yokes is obtained by devices known as "copper setters" and "resetters," available with or without valves. The coppersetter connects directly into the service line with pack joints, flared copper tube, or double purpose coupling for copper, iron, or plastic lines.
The resetter inlet and outlet connections are spaced and sized to fit in place of a standard 5/8-in., 5/8-in. by 3/4-in., or 1-in. meter, and repositions the meter in a higher, more accessible position.
For indoor settings, the meter may be set in a horizontal run of service pipe; or in a vertical run by a device somewhat similar to a yoke, which brings the meter in the horizontal position in front of the vertical pipe.
Differential Pressure Meters. A differential pressure flow meter bases flow measurement upon the hydraulic fact that, when the area of a conduit is restricted and velocity thereby increased, the hydrostatic pressure is decreased. These are called "primary sensing devices" for indication and recording of flow rate. Since the flow rate is related to the differential pressure produced, it can be measured in terms of the latter. These devices include venturi tubes, and orifices and nozzles placed within a pipe section.
Pressure differentials can be detected and transmitted mechanically, pneumatically, electrically, or electronically by sensing devices and transducers.
Propeller and Turbine Meters. To determine flow velocities in a main or branch line, these meters use a propeller placed inside the pipe and revolved by water flowing past it, the rate of revolution related to the velocity of flow. Meters that are designed to employ this principle are made for use in pipes 2-in. in diameter or larger.
Magnetic Flow Meters. In this meter the flowing liquid, moving in a magnetic field, forms the moving conductor necessary to generate a voltage. Linear flow readings are produced proportional to the average flow velocity.
Ultrasonic Meters. Ultrasonic transducers receive and transmit information generated by a liquid flow pattern in a selected pipe section. The flowmeter incorporates circuitry in which the output signal is linearly proportional to flow. Some options with the device regarding output signal generations are 0 to 20 pulses per second for digital instrumentation or 4 to 20 milliamperes for analog type instruments. Transducers are mounted externally on the pipe section to avoid head loss.
Other Types. The discharge from the end of a pipe (as into a reservoir) can be measured by a Kennison flow nozzle, "open flow nozzle," or parabolic flume. If the discharge is to a confined channel equipped with a weir or measuring flume, it can be indicated or recorded with a float or level sensor coupled with an integrated flow rate indicator and totalizer as well as a chart recorder.
Ultrasonic Flow Measurement in Open Channels. Ultrasonic technology is also very useful to determine open channel flow. Ultrasonic waves generated by a transducer are reflected off the liquid surface being measured and the travel time of the wave is used to determine the height of the liquid and consequently the flow rate.
Gauges, either indicating, recording, or both, can be mounted at any convenient place in a plant or pump station, or several can be ganged on a central control panel. These can show at all times such important factors as: water elevation in a filter bed or a distant reservoir or tank; discharge rate of pumps or flow through filters; pressures in mains at any distant point; rainfall and stream flow through watersheds; purification plant influent and effluent clarity; amount of residual chlorine, pH, hardness, turbidity, and nearly any constituent that may be measured electrolytically, photometrically, or colorimetrically; flow rate; and variation in flow or pressure. Also, devices can be installed that make continuous records of any or all these elements.
Gauges are operated by pressure on bellows and springs; on a diaphragm; a Bourdon tube; a helix; by a float in a manometer; a solenoid-operated ratchet wheel; by a multiple contact switch; or electronic signal. These may be equipped for pneumatic and electric transmission for control applications on a set-point basis.
Indicating-recording equipment is usually part of the instrument package in data acquisition systems, with such equipment as necessary for overview of system operations mounted on a control panel.
Demand Metering. Long used in the power industry, the subject of demand metering - determining peak rates of water usage by industries or other commercial establishments and residential complexes - is relatively new. It is very useful for many purposes, such as future planning, sizing services, reallocation of charges in rate structures, and even in leakage or unaccounted-for-water surveys. A flow monitor that operates continually, recording instantaneous rates as well as those for short and relatively long intervals, could be classed as a demand meter as long as some provision is made to interpret recorded data.
Types of Systems
Sitting at a central control console and being able to determine the status of any part of the system at any time without having to go into the field is an ideal situation. With the availability of microcomputers, modems, remote terminal units (RTU), programmable logic controllers (PLC), and other components of a supervisory control and data acquisition (SCADA) system, such an ideal situation can be realized. Further, remote control of field functions (pumps, motor controls, valves, etc.) can be accomplished by this telemetering process. By definition, a telemeter is the complete measuring, transmitting, and receiving apparatus for indicating, recording, or integrating, at a distance, the value of a quantity.
Data loggers can be used to measure and record a wide range of parameters. Some of these include water flow, water height, water quality, rainfall, temperature, energy use, weather conditions, pressures, and other physical measurements. These measurements are converted into an electrical signal such as voltage, current, or frequency - as the value of the measurement changes so does the signal. This can be converted into digital number the result of which is stored in the logger. Information from the logger can then be transmitted to a central control station for processing. Combined with a computer and the appropriate software, the sensing devices and data loggers can be very powerful tools in the efficient operation of treatment facilities.
The three forms of telemetering most used in the public utility fields today are electric, pneumatic, and electronic. Some treatment operations may employ all three methods, depending on equipment locations. In the first, transmission is by electrical circuits with limitations only by length of wire connections and circuit reliability. Pneumatic transmission has distance limitations because air lines and air pressure are involved. Electronic transmission is subject to any limitations involving transmission of radio waves or telephone signals in a particular area. For remote stations, cellular telephones and transmission via satellite are other possibilities.
Electric Telemetering. A telemetering system uses a sensing device, a transmitter, and a receiver. The sensing element measures the value of a quantity, such as the flow rate in a pipe. The transmitter translates this value into a signal and "sends" it to the receiver, where the signal is translated or decoded back to the original quantity, which is indicated or recorded. In electric telemetering, the signal may take the form of a "pulse." This is an on-and-off signal that may vary in frequency (the number of impulses in a given time interval), which behavior is termed the "pulse rate" or "pulse frequency" method.
Here, of course, the pulse rate is proportional to the quantity measured, e.g., the greater the flow in a pipe, the faster the impulse. In another method, the impulse may vary in duration, the length of time the impulse is "on." These impulses are part of a cycle of definite or fixed interval, such as 10 or 15 seconds and are of duration proportional to the quantity measured.
In a third method of electric telemetering, the signal takes the form of a varying voltage induced by a conductor passing through an electromagnetic field, the magnitude of the voltage being proportional to the value of the quantity measured. Instead of using an induced voltage, the electrical transmission may involve a directly measured voltage or current such as by means of electrodes.
Pneumatic Telemetering. In a pneumatic telemetering circuit, the transmitter and receiver are connected by small diameter tubing containing clean, dry air under pressure. The signal transmitted from the sensing device becomes increments of air pressure, the value measured corresponding to a definite pressure. The Instrument Society of America has established a standard range of 3 to 15 psi to be used by instrument manufacturers. The measured value is translated into air pressure of a magnitude between these limits, proportional to the range. In other words, a minimum differential would be represented by air pressure of 3 psi and a maximum of 15 psi. Pneumatic transmission has its greatest field of application in treatment plants where distances are not excessive.
Electronic Telemetering. This involves use of electronic circuits to amplify signals, to generate signals as audio tones, or to transmit microwave signals and multiplex them. Formerly, vacuum tubes were used, but modern devices are usually all solid-state, following the general rapid advance of this field. The latter seldom need replacement or servicing.
Possibly its most general application in water works telemetering systems is in generating, transmitting, and receiving signals as tones in audio frequency ranges. By using audio frequencies, transmission may be by telephone wires. In this system, the transmitter converts the measured value into a tone of fixed frequency, amplifies it, modulates a carrier frequency, and transmits it over the communications circuit.
The receiver is equipped with electronic circuits that will detect that particular frequency and through interposing relays will translate the signal into the proper control function. The tones may be transmitted in on-and-off cycles, with a varying pulse duration as in electric telemetering.
Microwave transmission has application for telemetering over relatively great distances and is economically justified where private or leased wires are not practical. It uses frequency modulated radio waves in definite frequency channels of 300 megacycles or higher. Of course, transmitters and receivers require radio circuits. Cellular phone technology and satellites can also be used for remote stations.
Combinations. As indicated, each form of telemetering has its specific advantages, depending on the equipment location. Some instrument manufacturers offer equipment used in all types of transmission and can offer advice in selecting the most advantageous system.
Telephone lines can also be utilized for telemetering, with equipment adapted as required or desired.
To activate automatic equipment a sensing device is used. The controlling factor may be rising and falling water in a filter or water tank; pressure in a water main; flow rate in a main; a clock or timing mechanism; conductivity; pH; oxidation-reduction potential; or even from the turbidity acting through a photoelectric cell.
Rate of Flow. Liquid or gas flow rate sensors require a "primary device" that responds to flow in a manner that can be measured. Flow meters, as discussed above in this chapter of this manual, are actuated by such devices. The sensor detects the response and relays it to the transmitter. Manufacturers of primary devices usually include the sensors and transducers appropriate for the instrument and may also furnish transmission and reception equipment as desired.
There are modifications. For example, rate of flow may also be measured by a differential pressure electronic cell, which when converted to a signal is transmitted to a pulse duration controller. The controller may be used with a reversible motor to drive a control valve, damper, or rheostat.
Water Level. Liquid levels can be sensed by floats, electrodes, temperature sensitive probes, ultrasonic sensors, or compressed air systems. In systems using compressed air sensing, air supplied by a compressor is piped to a submerged point in the tank or reservoir, and is released as bubbles or else forms an air pocket in a pipe or bell. Back pressure in the air line caused by level increase is sensed by bellows that trigger pump switches.
Because water conducts electric current, electrodes may be employed for pump control, one at each level to limit the range desired. The upper will stop a pump and the lower will start it. They may be used in protective circuit devices to protect pumps from damaged caused by low water levels. The switches are electromagnetic induction relay types and are essentially trouble-free. Ice-free electrode units are also offered for outdoor applications.
A float may operate a valve (or its pilot valve) directly, and is connected with it by a lever or cord passing over a pulley. In one device, a stationary vertical tube inside the tank is connected at the top to a bellows, and air compressed in the tube by the rising water expands the bellows and causes an electric contact. In another, an ultrasonic sensor with electronic analog or digital readout is used.
Instead of a float, the pressure in a pipe connected to the tank or reservoir may be used as the sensor. In some cases the pressure used is taken from the delivery pipe at a distance from the tank, an allowance being made for pipe friction.
Pressure. Sensing elements for pressure include some of the same devices used for rate of flow, but they are placed directly on a line without an intervening primary device. They are bellows, diaphragms, pistons, Bourdon tubes, pressure switches, mercury wells, and others. The applications are found in measurement of line pressure, in automatic booster pump control and in pressure regulating and reducing valves.
The sensing elements, transmitters, and relays are often included in a single package for booster station control and for automatic valves, and the telemetering is used only for indication and recording at a control center with provision for overriding manual control.
Acoustical. Vibrations and high frequency sounds are endemic to the machinery used in treatment processes. By monitoring the high frequency sounds, variations from "normal" levels can be detected. These variations, called stress waves, are produced by friction, fluid turbulence, impacts, cavitation, and other factors that can have damaging effects on equipment.
Replacing damaged equipment is more costly and disruptive than if problems could be detected in advance and preventive measures taken. The stress waves, however, spread in all directions throughout the device and thus can be detected externally using non-invasive sensors. The sound wave is converted into an electrical signal that can used to trigger alarms, record data, and assist preventive maintenance.
Transmission & Reception
Provision should be made for the measurement to be indicated somewhat at least near the position of the sensor, even though the control point is some distance away. For that reason, transmitters frequently incorporate some way to indicate or record.
Compressed air lines employed in pneumatic-transmission systems are subject to fouling by moisture and oil vapors. Line dryers, filters, traps, and combination filters and dryers for compressed air systems are recommended.
Electronic equipment for this purpose transmit and receive analog (linearly variable) or digital (pulses) signals via independent wire systems, telephone lines, or microwave.
When remotely operated stations are subject to transmission of numerous control functions, those functions can be handled from a central panel on which receivers are mounted to indicate, record, and transmit signals with report-back features. This is supervisory control. The transmission medium becomes a problem because of the multitude of signals being employed. It is obviously not desirable to use a separate pair of wires for each signal transmission.
One method of solving the problem of signal overload is to reduce the transmitted signals to tones in the audio frequency range with electronic tone generators and receivers. Where the tones are generated and received on a preset frequency basis, many simultaneous transmissions can be handled with a single pair of wires.
With electrical transmission of impulses, scanning switches driven by synchronous motors can be employed, one at the dispatching station and the other at the remote station. Transmitters and receivers are connected to corresponding contact points of the scanning commutators. One positions the other to carry out the command. For variable measurement, the scanning mechanism at each station is stopped long enough for at least two impulse signals to be received, assuring the measurable variable has not changed.
Through other contact points controls can be operated. In addition, each control function can be checked at frequent intervals with signals to the dispatcher station. Numerous control and advisory functions can be handled with a single pair of wires. Various combinations of tone and electric transmissions are possible, greatly multiplying the total points supervised and functions transmitted.
For compact assembly of recording and indicating mechanisms, switches and alarms at a convenient central location, control panels may be obtained factory assembled and wired according to specific needs. They can be organized to show flow diagrams and signal lights to indicate use of related units.
A synthesized speech circuit board can be installed as part of a warning system, remote monitoring, normal systems operations, or other process applications.
Automatic logging of data of plant operation is possible with remote transmission facilities, even conversion of information to typewritten form. Instrumentation firms are making available machines and computer techniques able to handle data according to a preestablished program to work out solutions to problems and to "instruct" the proper controlling devices, thus completing the automation cycle for water system operation.
To express it in simple technology, data reduction can be a cathode ray tube (CRT) display on a TV type screen on command or sequentially controlled; a teletypewriter for recorded display; or both. The data are "stored" on magnetic tape, disks, or other current medium with command access to any or all memory systems. Essentially, then, data reduction systems permit acquisition of stored data processed by computer in recorded or indicated form.
Usually the CRTs and teletypewriters are part of a control center, in an administrative or laboratory office of a treatment plant, main pumping station, or similar convenient point to oversee system operation.
Through the application of microprocessors, conventional analog indicators and recorders can be replaced by direct digital readout on all plant and system functions as well as digital control.
Where data processing is relied upon for system operation, especially when equipment is housed in rooms subject to flooding or other water damage, a warning signal is needed to indicate a problem situation.
Manufacturers of supervisory control equipment can supply data reduction equipment as part of an instrument package, either of their own manufacture or from a subcontractor making compatible equipment.
With the development of microprocessors, appliance computers are adaptable to process control. Instrument manufacturers can supply software for programming pumping stations, for example. Control circuit cards can be plugged into a compatible base for expanding the computer hardware as necessary. Autocon Industries, Inc. furnishes operation and maintenance management software.
It is also possible to obtain computerized utility management systems either through a contract arrangement or purchase/lease of equipment with software and hardware packages. Programming and maintenance services can be furnished.
Weather Warnings. To provide storm warnings for water control operations, operators may contract with an established weather service, providing both short- and long-range forecasts. A radio that receives U.S. Weather Bureau broadcasts can be very helpful.
Because of the presence of certain soluble materials, water may exhibit different degrees of alkalinity, hardness, acidity, and salinity. Alkalinity and acidity are related to the concentrations of hydrogen ion in the solution known more commonly as the "pH." In general, pH refers to the intensity of the alkalinity or acidity produced by varying portions of dissolved mineral of bicarbonates, carbonates, and hydroxides of calcium, magnesium, sodium, and potassium. Hardness is a measure of the soap consuming power and is determined by the amount of calcium and magnesium. Salinity can refer to the total dissolved salts in broad terminology, but is usually measured by the amount of chlorides present.
Until the passage of the Safe Drinking Water Act, the test for the above plus those for turbidity, color, taste and odor, fluoride, iron, manganese, and chlorine residual - where applicable - were considered routine for daily process control. Other common ions have been determined periodically. However, with the implementation of mandatory new standards for chemical contaminants such as heavy metals, nitrate, organic chemicals, and pesticides the picture has changed drastically. The EPA and state drinking water authorities should be consulted as to the frequency with which such contaminant levels must be monitored
In those areas that have indicated high levels of contaminants, testing would be more frequent and intensive. Indiscriminate industrial discharges, chemical spills, and illegal disposal of hazardous wastes have in many areas encroached upon and damaged potable water supplies. Groundwater systems are very difficult to model and contaminated aquifers can be extremely difficult to restore.
Although great strides have been made with new treatment methods and technology, damage to some aquifers may be virtually irreversible. At the very least, mitigation costs can be extremely high. The resulting loss of an area's most important resource can lead to health, economic, and environmental hardships.
To test for the contaminants and minerals described above, basic equipment such as flasks, beakers, burettes, pipettes, bottles and other glassware, comparators with appropriate standards, a gas or alcohol burner, balance, indicators, reagents, hot plates and drying ovens, and related apparatus will be required. A mechanical stirrer and a turbidimeter are also desirable.
Among the most important items in the laboratory is a copy of the book, Standard Methods for the Examination of Water and Wastewater. The testing techniques in this important work are indispensable to the proper analysis of water and the search for contaminants. More information on this book can be obtained from the American Public Health Association, American Water Works Association, or the Water Environment Federation.
Chemical reagents are also supplied by distributors of laboratory equipment and supplies. Glass, polypropylene, and polyethylene bottles for sampling and laboratory storage use are all used in laboratory work.
Plastic materials are now widely used in laboratory ware. Sample bottles, utility flasks, funnels, Erlenmeyer flasks, beakers, graduates, petri dishes, and even evaporating dishes are now made from plastics. In addition to their being unbreakable, they are lightweight, non-slippery, and somewhat chemically inert, although long storage of reagents, etc. in them is not recommended. Materials used are polyethylene, polypropylene, polystyrene, and Teflon.
Distilled water or water very low in dissolved ion concentration is used for glassware rinsing, quantitative analysis, dilution water for reagents, etc. An alternative to installing a still is a deionization unit.
Visual Colorimetric Tests
Analyses can be made for chlorine, pH, iron, manganese, fluoride, and many other substances by colorimetric methods. Such tests are based on the colors produced when indicating solutions are added to the sample. based on the range of the pH or concentration of substances present in the sample, the intensity of the color will vary; hence, the concentration may be determined by comparison with known color standards. These tests may be performed by using solutions prepared in the laboratory, but it is generally more convenient to use a commercially available comparators.
Colorimetric analytical kits can be purchased for maximum convenience and minimum capital outlay, with sample vials, premeasured reagent powders, or solution reagents for drop-by-drop titration, for a broad spectrum of parameters.
Automated potentiometric titration systems can ease some of the testing work. A colorimetric titrator determines the endpoint color change using a fiberoptic probe.
Results of visual color comparison may be considerably in error due to the lack of sensitivity of the eye to subtle color differences and to the possibility of personal bias. Through the use of instruments employing photocells or phototubes, much greater accuracy is possible. Such instruments measure the percent of transmission of a monochromatic light through a sample to be tested. Readings are made on a galvanometer, and calibration curves.
Photometric instruments will be required in routine analysis to determine heavy metals, nitrates, and fluorides under the Safe Drinking Water Act and its amendments.
Filter Photometers. These analyzers use color filters to obtain a light source in a narrow wavelength range.
Spectrophotometers. These are arranged to produce the entire spectrum from a white light source, using a monochromator that is either a diffraction grating or a prism. A slit, which may be fixed or variable, isolates the wave length band desired for a particular analysis. A photodetective device is necessary to measure the radiant energy passing through the sample in the light's path.
Spectrophotometers are made for the portion of the radiation spectrum that is visible, for the ultraviolet range, or for infrared. The atomic absorption spectrophotometer, which utilizes atomization of samples, is almost necessary for proper quantification of trace amounts of heavy metals.
Greater accuracy in determining the pH can be achieved by using a meter instead of the standard colorimetric test.
Electrically operated titrimeters or pH meters are available. The same standard solutions are used as with manual-visual methods, but the endpoints are determined by meter readings rather than colorimetric indicators. The performance of pH tests by comparing color reactions of the sample with standard colors is simple and inexpensive, but electrically operated pH meters offer many advantages, and avoid color, operator bias, turbidity, and salinity interference problems.
Electrometric determinations of pH are made by measuring with apotentiometer the voltage developed by two electrodes, one of which is fixed and known while the other varies with the pH of the sample. Normally, in laboratory instruments, the fixed electrode is calomel and the indicating electrode is glass.
The instruments are now usually of solid-state components; they may be battery powered, line operated, or both. Portable meters can be equipped with rechargeable batteries. Read-out can be by a scale calibrated in pH units, though digital read-out meters are usually employed only in process control instrumentation.
The basis for turbidity measurement was originally by the Jackson turbidimeter and is the depth of suspension required to make a standard candle flame disappear. In the application of this method, turbidity standards for comparison with samples may be prepared from suspension of fuller's earth, although these must be renewed frequently. Instrumental methods have largely replaced the candle turbidimeter for control of treatment; however, instruments may still be calibrated in units based on the Jackson Turbidity Units (JTU) or Jackson Candle Units (JCU).
Some instruments are standardized using suspensions of formazin, and results are expressed as formazin turbidity units, or FTU.
Turbidity measurement under the provisions of the Safe Drinking Water Act must be performed using the nephelometric technique. A nephelometer measures light scatter by suspended particles and employs a photocell for detection and quantification. When such a method is used, results are expressed in nephelometric turbidity units (NTU). It is important that this equipment be calibrated - after a year or so of operation the turbidimeter's light bulb can have "aged," producing light wavelengths that are different from those specified.
Under the EPA "Guide to Requirements for Public Water Systems Using Surface Water Sources," it is noted that particle size analysis can be used to indicate the removal efficiency of the treatment system. The systems can also be used to track particular particle sizes (such as those associated with parasitic cysts as Giardia and Cryptosporidium), the growth of coagulant particles, and more. These devices usually use a "light blockage" system, typically a laser beam, for measurement.
The presence of dissolved ions in solution makes possible their estimation by applying an electromotive force and measuring resistance to current flow. This is referred to as conductivity, the unit for which is reciprocal ohms, or mhos. In most dilute solutions, such as drinking water, the solutions are almost completely ionized, making conductivity roughly equal to total dissolved solids. Therefore, some conductivity meters are calibrated in terms of mg/L of dissolved solids.
Electronic laboratory instruments used in routine analytical work include dissolved oxygen meters, residual chlorine monitors, organic carbon analyzers, and gas or liquid chromatographs. The latter are especially useful in estimating the presence of organics in source samples.
Selective ion electrodes determine fluorides and chlorides and can be used with the same apparatus as used for conductivity. The EPA's lead and copper rules make detection of these elements very important. Lead in particular, is suspected of causing or promoting numerous health and neurological problems, especially in infants and children. Where nitrates are a problem it may be necessary to monitor them very often, if not on a continuous basis.
Temperature control and readout is important in many water and wastewater monitoring situations.
A fiberscope using glass fibers for the transmission of both image and light. The flexible device is useful for inspecting pipe interiors, turbine blades, and other difficult to access equipment and components.
The device, depending on configuration, can utilize a wide-angle viewing mode and can be combined as part of a video imaging system for a useful video record of the inspection.
The multitrade of legislation and regulation that now encompasses the treatment industry has elevated the laboratory to new prominence. No longer enough to check and see if the water is "safe" and "palatable," the laboratory must ensure that the production water has attained exact and specified levels of "purity." Failure to meet the parameters can create a host of problems with regulatory agencies, consumer notification requirements, and more.
Careful thought must be given to the design, layout, and space requirements of the modern laboratory. It may be necessary to comply with certain regulations for the lab to be certified (check with the appropriate state agency). Provisions should be made to allow for the installation of new equipment and expansion as might be necessary because of new testing requirements. The design should be such that equipment can be serviced and changed with ease and without interfering with other operations. Proper disposal of spent regents and hazardous materials must be considered. Different aspects of laboratory design are discussed in various publications including Public Works.
A laboratory should have ample window space, extending to the ceiling. For artificial lighting, overhead fluorescent lights are desirable - a minimum of 100 foot candles should be used. Concrete floors are generally covered with battleship type linoleum; tiles of asphalt, cork, rubber; or other suitable and corrosion- and slip-resistant material. For making color comparisons, chemical laboratories should have north light available.
Each laboratory room should have hot and cold water; electrical and gas outlets; piped distilled water; and vacuum and air pressure, all easily accessible. Each working room should have an acid-proof sink of alberene or porcelain; the chemical laboratories should have porcelain sinks at least 15 in. square set flush with the work space. The actual dimensions, layout configurations, equipment placement, and the such will be controlled by the availability of space, economic, and perhaps certification considerations. However, some recommendations are as follows: benches 30 in. wide; a central table 48 in. to 60 in. wide; passage ways 4 to 5 ft wide; benches 36 in. high for bacteriological studies; corner cabinets 26 in. high; table for analytical balance 20 in. by 48 in., 30 in. to 32 in. high, with low stool for use by the chemist while weighing; reagent cabinet for chemicals having opaque doors (light deteriorates some chemicals) with shelves 4 in. to 6 in. deep; open fume hoods, preferably with forced draft ventilation; a centrally located stoneware sink, with drains of acid-proof materials; and electric or gas refrigeration.
Complete laboratories designed specially for use in plants and factory assembled and equipped are furnished by various suppliers of such equipment. The basic unit is fitted with necessary utility fixtures and outlets. Additional matching units may be added. A periodic inventory of chemicals and supplies should be taken.
Expired chemicals and those no longer used should be discarded - keep on hand only what is needed. Equipment that is no longer needed or obsolete can be disposed of or donated. Spills should be cleaned up promptly and broken glassware and equipment should be removed.
First aid supplies, eye wash stations, and other appropriate equipment should be provided in the laboratories and other areas of the treatment buildings as may be required. Adequate supplies of first aid supplies should be maintained and personnel should have basic first aid training.
The testing laboratory must be equipped to detect microorganisms that could be potentially dangerous to the consumer. To be prepared for making bacteriological tests, a water works laboratory should have a supply of culture tubes, culture media and petri dishes, a sterilizer, and an incubator, in addition to most basic laboratory equipment required to make chemical analysis. An autoclave and a small refrigerator are also desirable.
The membrane filter detection technique offers several advantages over older methods of bacteriological examination of water. The method uses filters of uniform porosity of between 80 to 90 percent, through which water is removed by a vacuum pump, leaving the microorganisms on the film's surface. Retained microorganisms may be cultured directly on the surface of the filter sheet by placing it in wet contact with liquid nutrients.
To defer incubation of inoculated membranes, a holding medium is available, which effectively preserves the viability of coliform organisms for as long as 72 hours and minimizes growth.
Thus, considerable latitude is provided for time elapsed in shipment without serious detriment to the validity of the sample evaluation. When the culture assembly is received at the laboratory, the inoculated membrane is transferred to an appropriate coliform differentiating medium. Also available is a presterilized filter that can be dipped in water for coliform detection.
When the Safe Drinking Water Act was passed in 1974, its amendments, and subsequent regulations issued by the EPA, complicated water treatment operational procedures. The need to monitor water distributed for public use, using techniques beyond the capability of many local technicians, encouraged municipalities to rely on field service laboratories.
In some cases treatment procedures and regulations, required to comply with the law, have created another industry as in the wastewater treatment field: operation and maintenance by contract. Under this arrangement, the contractor assumes responsibility for plant operation in compliance with the law, either using local personnel with outside management or hiring and training suitable personnel.
Service Laboratories. There are many of these and practicability of using them depends on location, because samples have to be shipped or otherwise transported. Some have regional branches.
PROCESS CONTROL INSTRUMENTATION
General Purpose Monitors
Certain chemical characteristics significant in water plant operation can be measured automatically with electrodes placed in the line of flow, or by intermittent sampling. Since pH, oxidation-reduction potential, and conductivity can be determined electrolytically by a potentiometer circuit, these determinations can readily be translated into meter readings and chart records. With color photometers, any test that may depend on color comparisons or densities can be read automatically and recorded.
Through electronic circuitry, and in some cases pneumatic transmission, the measurements can be related to limits to control pumps and valves. The general hardware required for process control is discussed earlier in this chapter of this manual.
pH Recorders. Electronically operated pH recorders can be adapted with suitable sensors, transducers, and servomotor systems to control feeders, mixers, and other equipment.
pH Monitoring Electrodes. Electrodes for in-line pH monitoring are offered.
Colorimetric Analyzers. Continuous flow colorimetric analyzers can continually sample and record results of analysis in situations where colorimetric procedures apply, such as chlorine residual, fluoride, hardness, iron, pH, phosphate, chloride, and sulfite. Some use electrolytic sensors as well as colorimetric and provide digital readout; they can be equipped with alarm systems and other control equipment.
Electrolytic and Physical Sensors. Water quality monitors use electrolytic sensors for chemical characteristics such as conductivity, pH, oxidation-reduction potential, dissolved oxygen, and chlorides - as well as sensors for physical characteristics such as turbidity, color, temperature, and solar radiation.
Zeta Potential. For control of coagulant dosages, a device known as a zeta meter measures the electrophoretic mobility of colloidal particles. Apparatus of this nature can be fully automated to make important electrokinetic measurements, including electrophoretic mobility of the particles, specific conductance, and pH.
Streaming Current. Detectors based on streaming current flow can also be used. A streaming current is an electrokinetic phenomenon of ionic and colloidal surface charges being physically moved in a liquid and quantitatively measured by a pair of electrodes. A small sample of the treated water or filtrate flows into a test cell where ions are sheared from particles in the water and moved across the electrodes. The flow of ions or "streaming current," reflects the neutralizing effect of treatment chemicals on the colloidal particles in the water.
The signal generated from this flow can be transmitted to a controller, and in response to changes in the streaming current chemical dosages from the feeder can be increased or decreased. This method allows quick responses for changes in raw water turbidity, flow, color, pH, and other conditions.
Carbon Detection. Total organic carbon monitors are used to check for carbon levels.
Radiation Detection. Radioactivity monitoring equipment is available.
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|Title Annotation:||measuring water supply|
|Date:||Apr 15, 1998|
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