Reduced quantities of waste made possible by good housekeeping frequently result in more concentrated waste water making good treatment practices even more important though not more difficult. The definition of the problem, the first essential step in the design of an abatement program, becomes easier because more concentrated wastes are easier subjects for accurate analysis. For good definition of the problem, grab samples are not adequate as they represent only one moment in an 8-hr shift. Composite sampling proportional to flow- or time-based tend to mask the peaks and valleys of pollutant concentration which, if unknown, may result in inadequate treatment system design or one subject to frequent upsets. Continuous analysis and recording of one or more meaningful parameters over a time period considered representative of a plant's manufacturing cycle will provide valuable information for subsequent treatment method selection. Continuous analysis should be augmented by composite samples collected proportional to flow and grab samples collected automatically whenever one of the continuously monitored parameters exceeds the anticipated norm. Sampling in this manner provides: 0 continuous 24-hr records of flows and meaningful analytical parameters 0 discrete samples for off-limit conditions together with the time information on the recorded chart when the offlimit condition occurred 0 a 24-hr (or 8-hr) composite for average daily treatment plant influent quality analysis. In many cases some or all of the instrumentation required for continuous monitoring and off-limit sampling can be rented for the duration of the study. The decision to rent or purchase will depend on federal, state, and local reporting requirements for the treated effluent when the treatment plant is on-line. With stringent reporting requirements, it may well make economic sense to purchase the equipment outright as long as the ranges of concentration for the investigation period are not so high as to make the readout inaccurate for the low levels of contamination of treated effluent. Frequently, range change is only a matter of changing a plug-in module in the signal conditioner and is accomplished quickly and inexpensively. Another alternative might be to lease with option to buy. Under this arrangement all or part of the rental fee could be applied against the purchase price when the option to buy is exercised. The chart record of the monitor is of invaluable help in understanding the variations in waste water characteristics so essential for proper design of a treatment facility. I t must be augmented by continuous recording of flow so that both flow and pollution load variations are known to the design engineer. Where only one or two parameters need to be monitored to establish the concentration of the pollutant in
FEATURE
lnstrumentation for water pollution monitoring Continuous analysis and composite sampling instruments are an integral part of any waste water treatment scheqe
Christopher P. Blakeley and Theodore K. Thomas Honeywell, Inc., Fort Washington, Pa. 19034
waste water, it is generally more economical to measure these directly using either a submersible sensor nest or individual submersible sensors. Where this is not practical because of inaccessibility, individual sensors in bypass sensor holders (photo), or individual analyzers feeding either a multipoint recorder or single- or two-pen continuous recorder are suitable. Once the problem has been defined, the selection of the treatment process can begin. Treatment processes
For organic wastes such as sewage or effluents from milk plants, food processing plants, organic chemical wastes, or oil-bearing wastes, to name a few, one of the activated sludge processes will be applicable. Raw waste may be given primary sedimentation to remove the suspended solids that will readily settle before being sent to an aeration chamber. In the aeration tank the growth of certain strains of aerobic bacteria is stimulated through aeration (0.5-1.5 scf/gal) and agitation. The bacterial cultures develop on the finely suspended organic materials which tend to agglomerate or form clusters. These clusters are the activated sludge "floc." From the aerator the mixed liquor, fluid and floc combined, is piped to secondary settling where the sludge is allowed to settle. The settled sludge is normally divided into two streams, one being wasted and the other returned to the aerator influent as return activated sludge. This sludge, being rich in mature bacteria increases the bacteria-to-food-supply ratio in the aeration tank and thus increases the efficiency of this type of treatment. Ordinarily the sludge is returned in some ratio to the flow of raw or settled waste influent, although other means of controlling the sludge return rate are also used. The waste sludge normally is thickened, dewatered by vacuum filtration or centrifuging, and dispos'ed of by landfill, incineration, or other means. There are a number of modifications of the conventional activated sludge process described. High-rate plants are designed for higher BOD loadings per unit volume and may have less then the normal 4-8-hr retention time in aeration. The main difference between conventional and other sludge processes is the addition of a reaeration tank in the return sludge line. Proponents of this type of treatment claim that the reaeration step stimulates the bacteria in the sludge to greater activity thus providing more effective treatment in small aeration tanks when the sludge is finally mixed with the incoming wastes. This process is reported to be more stable than either the conventional or high-rate treatment under conditions of widely varying BOD loads. The step aeration propess adds waste water to the aeration tank at incremental points along the line of flow. The theory behind the deSign states that since the main
source of solids-consuming bacteria is the return activated sludge, the food should be introduced to the bacteria gradually rather than all at one time. Generally speaking, step aeration is more efficient although the initial and operating costs may be somewhat higher due to extra piping, valves, meters, and other equipment. The Kraus interchange process can be applied to any process in which a digester is included or, planned for routine sludge treatment. In the Kraus modification, part of the return activated sludge is mixed with digester supernatant and aerated before being recombined with the remainder of the return sludge and added to the aeration tanks. This process, originally developed as a means of disposing of digester supernatant, has been shown to improve the ability of the process to withstand sudden changes in BOD loadings. Further, there is some evidence that the Kraus process can reduce sludge "bulking." This is a condition peculiar to the activated sludge process in which particles tend to agglomerate into masses that have large volume but comparatively small surface area. In the Kraus process, digester supernatant, lacking in oxygen, effectively decreases the aerobic level of the return sludge thus adjusting the bacterial count to alleviate or prevent bulking. Bulking sludge is frequently associated with wastes from food processing plants. All these processes require an excess of oxygen and adequate holding time for bacterial action. The bacterial floc is kept in suspension by the aeration process. Control parameters
Flow measurements (Figure 1) and dissolved oxygen content are the most important control parameters. The magnetic flowmeter is probably the most commonly used device for this purpose. The operating principle of magnetic flowmeters is based upon Faraday's law of electromagnetic induction: "The voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor." The metered fluid is a conductor moving at a velocity through a uniform magnetic field. The voltage measured across the electrodes is proportional to the velocity of the fluid. Dissolved oxygen measurement is essential to control the amount of air applied either through diffusion or by surface aeration or both. The polarographic dissolved oxygen sensor, probably the most prevalent, is an electrolytic cell consisting .of an anode and a cathode immersed in an electrolyte. A voltage is applied across the electrodes, and the resultant current flow is measured by measuring the voltage across the precision resistor placed in series with the electrodes. The electrolytic cell, then, is used to analyze for a particular constituent--Le., oxygen, which is identified by a "wave." The voltage at the center of the rising part of the "wave"-the "half-
wave"-specifically identifies the material being reduced. The height of the current p!ateau with a wave is proportional to the concentration of the element present. With a fixed potential applied continuously, the amount of current flow between electrodes, then, is directly proportional to the amount of constituent reduced at the cathode. The dissolved oxygen sensor then selectively analyzes the amount of oxygen dissolved in water without interference from other ions. The only entrance for oxygen is through a permeable membrane covering the gold or platinum cathode. The amount of oxygen reduced at the cathode depends upon the amount of pressure exerted on the membrane by the oxygen in the water. Small aeration basins may well be controlled on the basis of dissolved oxygen content positioning a weir which, in turn, controls the effluent quantity and thus the residence time in the aeration tank. The weir position governs aerator efficiency. In larger basins using multiple surface aerators, control by dissolved oxygen analysis may be exercised from monitor switches. Dissolved oxygen measurement also is used for blower control. A s the sensor calls for more or less oxygen, the blower butterfly inlet valve or inlet guide vanes are positioned using pneumatic control. To prevent the blower from surging, an independent control system is used consisting of temperature and flow sensors, a multiplying relay, control station, and vent valve. The setpoint will be a preset value based on the constant speed of the electric motor and the surge characteristics of the compressor, while the dissolved oxygen-based control positions the guide vanes at the inlet to the blower to raise or lower header pressure and thus control oxygen entering the aeration tank.
Field work. Bypass mounted p H 'ORP sensor holders can be used in areas inaccessible to other types of instruments
inorganic wastes
The treatment of inorganic wastes usually employs one or more of the following procedures: oxidation, reduction, precipitation, or neutralization. While oxidation may take various forms, the two-step destruction of cyanide-bearing wastes is a typical example. In the first step pH is raised to no less than pH 8 . 5 but no higher than pH 10.0 with chlorine added to the elevated pH water to oxidize cyanide to cyanate. Oxidation-reduction potential measurement (ORP) is used to determine when the reaction NaCN f 2NaOH f CI2 = NaCNO 2NaCI f H20 is completed. In the second step pH and ORP are again used to oxidize the cyanate to form carbon dioxide, nitrogen, and water. In this case pH range should be 8.5-9.0 for the completion of the reaction with chlorine as follows: 2NaCNO 4NaOH 3c12 = 6NaCI f CO2 f N2 2H20. The system is most easily visualized using a two-
+
+
+
Meter. The magnetic flow meter is most commonly used io determine dissolved oxygen content and flow measurement
IF
+
FIGURE 1
Points of flow measurement in activated sludge treatment of waste water PRIMARY
Oil, fat, and grease
SECONDARY
Effluent
sludge to disposal @ Points of flow measurement
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Environmental Science & Technology
tank approach, although frequently, a batch method may be used where both steps are practiced sequentially in one tank. In the two-tank method, level switches operate the fill valves and energize the pH control system which starts an alkali feed pump to raise the waste pH into the desired range of 8.5 to 10. When the pH setpoint is reached, alkali feed is stopped, and the ORP control system is activated, and chlorine feed begins. When the ORP setpoint is reached, chlorine feed is stopped, and a timer is energized. After a predetermined time delay, the tank drain system is activated. The second step in the second tank is quite similar with acid addition, if needed, to bring the pH into the recommended range of 8.5 to 9. The ORP controller then again activates chlorine feed to reach and maintain the desired ORP end point. When this has been reached, a timer provides time delay at the correct ORP at which time the drain valve is opened and the treated waste discharged. The reduction of hexavalent chrome waste utilizes al? most identical instrumentation as that for the oxidation of cyanide waste. In this case, again, it is a two-step procedure with the reduction of the hexavalent chrome to trivalent chrome taking place in the first step usually by using sulfuric acid to bring the pH into the range of 2.0-2.5 and the addition of sulfur dioxide, sodium bisulfite, or sodium metabisulfite to the desired ORP end point. The second step differs from that of cyanide in that it is a precipitation of the trivalent compounds formed in the first step. Precipitation is achieved by raising the pH of 8 to 8.5 which forms the chromic hydroxide and calcium sulfate precipitates. In addition to the batch process for precipitation described for the second chrome destruction step, continuous precipitation of metals in solution is also practiced. In a conventional design, the three steps of precipitation include a rapid mix of chemicals with influent waste water, a slow mix where reaction goes to completion, and followed finally by a quiescent zone where the precipitate is allowed to settle. These three steps are frequently combined in upflow-type clarifiers. pH adjustment materials, such as lime and/or soda ash or caustic soda, are fed proportional to incoming flow and pH measurement in the reactor. A metal coagulant, if needed, or a coagulant aid is fed proportional to flow. Since the amount of sludge is largely a function of lime feed, it is possible to automate the sludge blowoff as a ratio to lime feed (although it is more usual to provide a timer to operate a sludge blowoff valve). Both batch and continuous neutralization are practiced in industry. The batch process is relatively simple in that a tank is filled until a level probe shuts the influent valve. The pH is measured by a submersible probe which activates the pump or valve adding neutralizing chemical until the desired end point is reached. Good agitation is essential. Depending on the titration curve of the waste material it may be desirable to slow down reagent feed as the end point is approached. The most complex system would be one of continuous neutralization of variable flow and variable pH waste water. In all pipeline pH control applications sufficient distance must be provided between point of chemical addition and point of pH measurement to ensure complete mixing and reaction of the chemicals added. Consideration must also be given to the possibility of heat development due to chemical reaction. Flow is measured by a primary sensing element converted to an electric signal by a differential pressure to current transmitter, linearized, and received by a recorder/controller. The controller varies the speed of the chemical feed pumps for alkali and acid through SCR speed controllers. pH is sensed, and the signal is amplified and recorded. The rather complex subject of industrial waste water
treatment and control has now been broken down into its multipurpose unit processes. Figure 2 shows a cyanidebearing waste to be oxidized, a chrome-bearing waste to be reduced, and a flow of either acid or alkaline waste water to be neutralized. The objective is to provide a treated, neutralized effluent suitable for partial or total reuse or discharge in compliance with current legislation. In this composite plant, cyanide is oxidized to cyanate on a continuous basis using the alkaline chlorination process. Hexavalent chrome is reduced to trivalent chrome by means of sulfuric acid and sulfur dioxide feed. The high pH cyanate and low pH trivalent chrome effluents are combined with the acid/alkali wastes and by addition of alkali or acid cyanate are further oxidized in the presence of chlorine while trivalent chrome is precipitated. Similarly, other metal-bearing wastes could be added at this point for precipitation of metals. The reacted waste water, including precipitate, is then coagulated with adFIGURE 2
Control system for continuous plating waste treatment A J l e c t r o n i K Ill recorders
A
I
' A
-. I ', Cyanate & I
I i
~
1
trivalent chrome
FAS regulated filtered a i r supply
izhrome waste treatment
,-stment of pH to satisfy the requirements of coagulation and neutralization. Sludge is thickened and dewatered, and in both cases the supernatant waste is recycled to clarification. I f desired, the clarified effluent can be filtered if filtration makes the water reusable in part or in its entirety by one or more of the production processes. In addition, all or part of the filtered water might be further processed by ion exchange, reverse osmosis, carbon adsorption columns, or other treatment methods to make it suitable for specific applications. Instrumentation The instrumentation needed for these processes is readily available and, in most cases, has been used for many years Tn these and similar applications. For example, the flow measurement signal from the magnetic flowmeter covered earlier is delivered to a signal convertVolume 7, Number 11, November 1973
1009
transmit the low-level signals of electrochemical sensors measuring pH, ORP, dissolved oxygen, and conductivity. This equipment is now available and has proved its reliability in many industrial applications under varying ambient conditions. T'he analyzer-transmitters are optionally equipped with auxiliary switches to initiate alarm or control action as the system calls for it. Many municipal and state water pollution authorities require some manner of written documentation to assure that plant effluents meet pollution standards of pH, suspended solids, free oils, and other characteristics. The most accurate and economical way to produce these records is with continuous recording instruments. Recorders vary in size from the miniature strip chart type with a 4 - i n wide chart through the intermediate 6-in. chart size up to the conventional 12-in. chart width. Circular chart models are also available. Different chart drive speeds are available with these instruments which enable the treatment plant operator to obtain continuous records over a period varying from several minutes to several days. Multipoint strip chart models can produce up to 24 separate records on a single chart by means of a sequential printing mechanism. Intermediate and larger-size instruments combine the functions of indicating, recording, and control within the case. Miniature recorders function separately from the control loop as "trend-recorders,'' while the control functions are housed in indicating control stations. Final control elements Modern. Electronic monitoring and control from a central point within a manufacturing facility is a current trend
er which transduces it to either a pneumatic (3-15 psi) or electrical [4-20 mA dc (milliamps direct current)] value depending upon the requirements of the control system. The signal can then be fed to conventional indicating or recording instrumentation which can be located at a point remote from the magnetic flowmeter location. If the system requirement is for automatic control, the instrument's control unit delivers a signal to a valve operator to control the flow at the desired setpoint. Again, the control signal can be pneumatic or electric depending upon the application. Another type of flow sensor widely used in waste treatment applications is the flow tube in which the difference in pressure across the neck or restriction is measured and transmitted by a differential pressure transmitter. The electric model uses a diffused silicon chip as a pressure measuring element, thus avoiding the problems of friction and hysteresis or lack of repeatability common to electromechanical types. The same technological principle of the differentialpressure transmitter can be used to measure the level of liquid in a tank. The pressure difference proportional to level is sensed and measured as static head pressure against either a diaphragm device in the side of the tank or in an air "bubbler" pipe mounted vertically within the tank. Indication, recording, and control of level can be accomplished by the instrumentation on the receiving end of the transmitter's signal. The versatile differential-pressure transmitter can also be used to sense and measure the level difference between the upstream and downstream faces of traveling screens. The difference in level is a measure of the amount of material being screened out of the waste water influent stream. In a further application the differential-pressure transmitter can sense and measure the "loss-of-head'' pressure across a filter. The requirement that industrial waste water treatment be as automatic as possible motivated the industry to design and manufacture reliable equipment to amplify and 1010
Environmental Science & Technology
The final control element is the device which the controller adjusts or manipulates to maintain the variable at the intended setpoint value. Control elements utilized in waste water treatment applications include valves, chemical feed pumps, dry material feeders, chlorinators, and other gas feeders. All of these devices can be operated -some by means of transducers-by the standard electric and pneumatic control signals now on the market. The current trend of instrumentation technology in the water pollution control field is toward electronic monitoring and control from a centrally located point within the manufacturing process facility (photo) where a single operator can easily monitor and control the system. The availability of broad lines of miniature pneumatic and electric instrumentation has assisted designers in concentrating a maximum of control system capability within a minimum of panel space. This results in a small control room area at a lower overall space cost. Additional reading
"Pollution Abatement and Smart Water Management," Blakeley, C. P., Instrumentation, 23, No. 2, 1970.
Christopher P. Blakeley is market manager for water management at Honeywell, Industrial Diw'sion. He has held key assignments for many years, both as consultanf',and marketing executive in the fields of municipal and industrial water and waste water treatment. Address inquiries to Mr. Blakeley. Theodore K. Thomas is marketing communications manager for water management and CPI at Honeywell, lndustrial Division. Mr. Thomas has held assignments as both sales engineer and communications editor in the field of industrial process instrumentation.
