wastewater automation

7/28/2019 Wastewater Automation

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Instrumentation for the automation of wastewater treatment plants

Dr. Richard Furness CEng FInstMC ISA FellowMeasurement Consultant

JDF + Associates LtdGloucester, UK

Introduction

Water Authorities and municipalities are governed by national or local regulations on the quality of waterthey discharge, so effluent from industrial and domestic use must be cleaned and treated prior to releaseto the environment. The treatment plant should therefore remove any materials that are likely to beharmful to the environment, ranging from large objects (cans, wood and debris) to suspended solids,floating material such as fat and even dissolved substances. Consequently a range of treatment isrequired to remove all these contaminants. In order to operate the plant correctly and efficiently,measurements of the main parameters are required. These usually include:

* Flow and level * pH, conductivity and turbidity* Pressure and temperature * water analysis parameters (DO, P, NH4, NO3 etc.)

A typical treatment process is shown in figure 1. The dotted lines show discrete parts of the overallprocess, so that a generic 4-step process can be listed for both the water side and solids handling as:

Module A: The influent line and associated processesModule B: First stage mechanical separation primary clarificationModule C Effluent discharge chemical treatment and disinfection processesModule D: Biological degradation nitrification/denitrification breakdown of influent solidsModule E: Sludge blending and thickening mixing of raw and waste activated solidsModule F: Digestion processes final breakdown of solids by bacterial meansModule G; Dewatering process removal of excess fluid and solids optimisationModule H: Solids disposal

Figure 1 General arrangement of a wastewater treatment plant

In the upper half of the process diagram (figure 1), the first stage consists of the influent lines, pumpstation and plant intake (section A), where coarse screens remove large objects. Sometimes smallerscreens (1-2cm) are used to remove floating fat, paper and smaller items such as fruit, rind or waste food

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lumps. The liquid then passes into the primary clarifiers (Section B) where suspended solids and floatingsubstances are removed. The raw liquid passes to the biological treatment tanks (section D) and theprimary sludge goes to the thickener (section E) for blending.

In the aeration tanks, bacteria feed on the organic matter and nitrify harmful ammonia. Artificialoxygenation promotes this process so the bacteria multiply and accelerate the water cleaning process.The partly-cleaned liquor is fed to the secondary clarifier where further separation occurs. The activated

sludge in the bottom of the secondary tanks is passed to the thickening tank (section E) for blending withthe sludge from the primary clarifiers. The relatively clean liquid from the top of the secondary clarifier isthen chlorinated and de-chlorinated prior to discharge (section C). In some more advanced plants, achemical cleaning process is added to the biological process . These may use precipitants such as ferricchloride or aluminum sulphate to remove organic carbon, phosphorus and some of the nitrogen basedcompounds. This may occur in either the primary or secondary clarifiers or in the aeration tank.

The lower half of figure 1 shows solids handling and disposal. Sludge from the primary and secondaryclarifiers is mixed. The thickened sludge passes to the digester (section E) where further biologicalprocess generates methane gas, which is often used as an energy source. The digested sludge from thebase of the digester then passes to the dewatering stage, (section G) where a variety of techniques areused to remove most of the water and produce an energy-rich residue. This residue is pumped to thedisposal area (section H) to be used for incineration, for fertilizer production or for landfill, depending onthe chemical composition of the treated residue.

Inlet Measurements

Process control maps have been developed for each section and each variant as detailed in figure 1.Section A1 is the channel network prior to the plant inlet. This may contain industrial wastewater whichhas been pH balanced prior to discharge into the network (section A7). In large plants there may bestorm water basin to help reduce the variations in plant inlet load during periods of heavy rain andsurface run-off (section A2). Following this is the pump inlet station (section A3) where the influentstream is pumped to the primary clarifier. In some countries a fecal station may be present (section A4),where septic materials can be loaded into the treatment process. After this, the influent passes to thebar screens (section A5) and sometimes to a sand and grit removal tank (section A6).

Measurements at the plant inlet are usually mandatory help monitor the incoming pollution load and tosatisfy the design parameters of the plant are not exceeded. The most important measurements are flowquantity, pH, temperature, conductivity and the biological oxygen demand (BOD). Measurement of levelis used extensively in this part of the plant for optimum control reasons. Because of the nature of the inletflow, Venturi type flowmeters are more commonly used in the USA than magnetic meters. This isparticularly the case when high organic, fat or grease quantities are present. (These are insulatingmaterials and can induce false readings). In the rest of the world magnetic meters are used in preferenceto Venturi tubes. In large open channels weirs and flumes find extensive use worldwide. One variant, aKhafagi-Venturi is not affected by the presence of large incoming objects or deposits. Accurate levelsensors can give flow readings within 2% using this type of primary.

A measurement of pH at several points in the inlet area is required to ensure efficient plant operation and

to monitor the effect of the influent water on the concrete structures and channels. This measurement isoften made alongside a temperature sensor for similar reasons. A low pH value often induces damage inpipes, valves and other metallic components. Conductivity is often not measured at the inlet but is usefulto help monitor salt content and other materials that affect pH and temperature measurement. Somechemical addition at inlet may be required to keep the influent within the range pH 6 - 8.5. The BODfigure is used to guage the amount of oxygen required by the microbacteria to break down organics andother waste present. A high BOD means that large amounts of air or oxygen will be needed to affect thesolids breakdown. Sometimes sampling systems are also used the help determine the plant loadingand report on removal efficiency when compared to effluent samples.

Level measurements are common in the pumping station and in the coarse screens to ensure that theplant operates within capacity and that the bar screens do not become clogged or damaged. One of themost important measurements on the entire plant is the accurate measurement of flow at the main pump

station inlet. This signal is fed to other parts of the plant as part of the control signal. Errors in meteringhere may affect the entire efficient operation of the plant. For this reason, a large proportion of the worldhas gone to the use of magnetic meters wherever possible in closed pipe inlets. In open collection



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channels, fully compensated ultrasonic systems installed with standard flumes are common. Overalluncertainty here would be 3-5% depending on the instrumentation installed.

Primary Clarifier

Heavy sediments or fine grit are encouraged to settle in the primary clarifier. This is usually a largecircular vessel or long rectangular structure with a retention time of a few hours. In large plant there maybe up to 20 primary clarifiers, each taking a percentage of the inlet load. Two possible process variationsare possible, the basic process being just a simple inlet to each clarifier basin and the more enhancedvariation of flocculent addition in the clarifier inlet line. Whatever the process variant, the vessel bottom isusually extended to allow sludge to collect for efficient draw-off. It is important that sludge level andquantity of the suspended solids is measured. Both these parameters act as control inputs to the clarifierdump valve that allows the primary sludge to be pumped to the thickener. Turbidity and flowmeasurement of the outlet liquor to the biological treatment tanks are important and sometimessamplers are additionally used to ensure a rich liquor for treatment. The purpose of flocculent dosing isused at the clarifier inlet to separate light and heavy solids particles. If this is performed, a suspendedsolids monitor in the outlet pipe can be used to pace and control the flocculent dosing flowmeter as wellas efficiently removing sludge from the unit into the blender/thickener. Figure 2 below shows a typical

instrumentation arrangement for the clarifier. (The numbers in the diagram refer to E+H models fromtheir extensive range). It is important to control the quality and quantity discharge of the primary sludgefor a both technical and financial reasons.

M

Figure 2 Instrumentation of the primary clarifier

The key to good clarifier control is the simultaneous use of an ultrasonic sludge level sensor (CUS 70)with an optical solids monitor (CUS 41) mounted in a retractable holder (CUA 461) for regular cleaning.The output from the Liquisys S control unit (or the level output reading) can open the discharge valve atthe base of the clarifier when required sludge volumes have been reached. The user benefits are:

Ensures the correct sludge concentration into the thickening stage, optimising pumping needs Increases the hyrdraulic efficiency and capacity of thickener

Will increase gas yeild in the digester further downstream

Decreases the energy input required in the digester, thereby lowetring operating costs Reduces the polymer usage in the dewatering process

Biolog ical Treatment Processes

After first stage mechanical cleaning by the screens and the primary clarifier, biological processes areused to purify the wastewater. There are two basic types of bacteria, aerobic and anaerobic, eachhaving advantages and disadvantages. Aerobic bacteria have higher operating costs and require more

space, but generate lower sludge loadings and lower by-product volumes. With anaerobic bacteria thesludge loading is high. The aim is to convert the rich waste material into lower energy material withwater and carbon dioxide as the by-products. Domestic wastewater is often far cleaner than industrialeffluent and aerobic bacteria is more commonly used for such purification. Anaerobic bacteria is



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preferred in industrial treatment, where more concentrated effluents with added chemicals are found,examples being paper or sugar production plants. These latter processes are much more sensitive towastewater quality and therefore require more chemical measurement for control and operatingpurposes. The growth of bacteria is influenced by several factors that include temperature, nutrientconcentration, oxygen supply and pH. BOD measurement is a good indicator of microbacterial activityand is dependent ontemperature. The aeration tank is the basis of this process. In this part of the plant,air (or oxygen) is blown into the base of the tank. A secondary clarifier is almost always associated with

this part of the process.

Aeration Tank

In normal practice, oxygen is measured and regulated in the aeration tanks, but control of the returnsludge or return water flow less common. The essential measurements therefore in the aeration tank aredissolved oxygen content and pH. This part of the process can more effcienttly be control using acombination of an ammonia monitor and a DO sensor. A basic control loop is shown in figure 3. Figure4 alongside shows the simultaneous control of the suspended solids concentration in the aeration tank.

Figure 3 Biological process aeration control Figure 4 Biological process suspended solids control

As the nitrification proceeds, ammonia concentration falls and free oxygen concentration will rise. Thusless aeration is required at later stages of the process. In figure 3, an oxygen sensor (COS 4) and an ionselective ammonia sensor (CA70AM) controls the aeration valve to ensure an efficient process It is alsoimportant that the biomass in the system is controlled to ensure the correct solids degradation rate. Anoptical turbidity probe (CUS 41) is therefore used to control the ratio of the feedback flow to that leavingthe aeration tank and passing into the secondary clarifier. The benefits from these two control loops are:

Amount and rate of the aeration decreases energy consumption and plant running costs

Nutrients avaailable to the bacteria is more tightly controlled

Better control of the effect of changing plant loading on the aeration process Correct biomass ensures efficient pollution removal effective bacteria

Ensures no floating sludge in the secondary clarifier (an indication of poor process management)

Reduces the need for expensive and repetative laboratory testing

Provides real time process information for smoother plant control

Allows rapid identification of process abnormalities and fault conditions

Ensures the secondary clarifier discharge conditions meet the environmental effluent regulations

Secondary Clarifier

Overflow water from the aeration tank goes into the secondary clarifiers. The purpose of this item of plant

is to complete the separation of liquid and solid phases before the water passes for final treatment anddischarge. It is important to keep a fixed level of sludge in the base of the clarifier to ensure theavailability of biomass in the system for recirculation to the aeration tank and maintain its concentrationwithin acceptable limits. Sludge discharge is either continuously or intermittently made from the base of



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the clarifier, passing into the pre-thickener and sludge blender. Suspended solids level (and evenconcentration profile) measurement ensures an effective separation takes place in the clarifier. In orderto monitor the quality of water in the overflow channels, turbidity should be monitored. The discharge ofthe water activated sludge is correctly controlled by the output of the solid level monitor. Figure 5 showsbasic control of the return sludge from secondary clarifier. About 20-25% by mass is kept inrecirculation, the exact fraction dependant on the inlet plant loading.

Figure 5 Control of the activated sludgeThe return activated sludge is discharhe back into the aeration basin, ensuring the bacterial count ismaintained. The quality of this return sludge is one input used to control the discharge of activatedsludge to the solids handling processes. An optical sludge detector (CUC 101) is used to measure theblanket at the base of the tank. The operator benefits are:

Early detection of sludge overflow than with a turbidity sensor in the main effluent discharge

Sludge blnaket level optimizes the WAS discharge to the thickener

Ensures that low concentration sludge is not pumped into the solids handing stages

Ensures bacterial population is always sufficient to meet changing plant conditions

Biology Process variations

To reduce the amount of nutrients fed in the discharge into underground or surface waters, as much

nitrogen as possible should be removed from the wastewater during the cleaning process. This process(denitrification) is done biologically with an enzyme that reduces nitrates to simple nitrogen. In thisreaction H+ions are eliminated and raise the pH that fell during the nitrification process. Thus a moreefficient biological cleaning includes both nitrification and denitrification. There are many process layoutvariations with respect to the biological part of the process. The common practice places thedentrification first followed by nitrification, (as in example 3 of the process maps) to insure that there isenough organic matter for the denitrifying bacteria. (In the plant overview diagram nitrification is listed asmodule D2 and denitrification as module D5). However it is possible to see the following variations inwater treatment plants:

Nitrification first followed by de-nitrification

Simultaneous nitrification/denitrification in the same basin

Alternating nitrification/denitrification

Intermittant denitrification (in this process oxygen is frequently added) Cascading denitrification (with sludge recycling between the cascades)



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In all these cases a secondary clarifier is usually an important part of the biological cleaning and is thereason why it has been added to each variation in this part of the whole process. In parts of the USA, atrickling filter is located in front of the aeration basin. The purpose here is to inject air from the basewhich intermingles with primary clarifier overflow liquor to raise oxygen level prior to full aeration.Another enhancement is the use of a precipitant to promote better separation of liquids and solids. In some plants where denitrification follows nitrification, organic carbon may be added to aid the process.Measurement of ammonia in the aeration tank and nitrate in the denitrification tank allows full biological

control of the entire process and can reduce operating costs substantially. Enhancement example 2.1from the complete set of maps (figure 6 below) shows the instrumentation for this configuration. Noticethe interconnections between the various instruments in order to automate the control process.

2001 Endress+Hauser, Inc.

fromPrimaryClarifier

D 5 DenitrificationBasin

D 2 Nitrification Basin(Aeration)

D 6 Simultan.Precipitant

Dosage

D 7 OrganicCarbon

Dosage

Return Activated Sludge (RAS)

Circulating Sludge

DenitrificationBasin

NitrificationBasin

Q IPHTI

Q ISS

Q IRSS Q IRDO

D2-07D2-04

D2-10

D5-01 D5-02

D5-03

D2-05

FI

A6-05

D2-07

D6-03

from

SecondaryClarifier

to SecondaryClarifier

D2-09

D2-08

D6-02

D6-01

D7-01

FIA6-05

D2-09

Q IATOC

FI

FIC

D5-05

D5-06

D5-04

Figure 6 PID for complete biological cleaning process in wastewater

This figure is one of 25 detailed maps developed for each stage of the wastewater treatment process.

Effluent lines

Control of the volume and quality of the effluent is more important as that of the influent. Regulationsgoverning the quality of the discharged water are rigidly enforced in many developed countries followingsome environmental accidents, where river and lake wildlife has been destroyed. Important effluentdischarge parameters are the volume discharged (usually a magnetic flowmeter) the pH and liquidtemperature, and several liquid analytical measurements. These include the liquid turbidity, dissolvedoxygen content, and the phosphate/ammonia concentrations. An additional desirable would include a fullliquid sampler and nitrate. In some plants this latter is inferred by using Redox measurements to revealthe nitrite levels. Prior to final discharge the effluent is first chlorinated to kill any bacteria carried overfrom the aeration tanks and then dechlorinated using sulphur dioxide or simliar chemicals becauseclhlorine itself is harmful to aquatic life. The process map for final chlorination has also been developed.The key measurement is that of outlet free chlorine, with the output signal of this measurement beingused to control the sulfite addition in the dechlorinator.

The process maps developed show a basic discharge system. and one where a filter system is used togive a final polish to the discharging liquid. In those plants where filters are used in the outlet stage, aflocculent is often used to dose into the line from the secondary clarifier. The control map for this section



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is shown in figure 7, and several instruments form the basis for automatic control for this part of the plant.An additional differential pressure measurement across the filter bed can be used to activate thebackwash flows automatically. Turbidity measurements at several points can aid in the production of aclean discharge flow and in the correct dosage of flocculent.

Figure 7 Instrumentation for effluent filtration system

The benefits to be realised are:

Saves energy through control of the backwash flows and air blown into the filters

Allow quicker filter cycle time because all the fines are not discharge

Flocculant addition will be correct, based in accurate flow measurement

Allows far better control of the filtered water

A phosphate sensor in the discharge line can used to pace the precipitant addition prior to the finalsedimentation tank. All these maps developed show the importance of analytical measurements at theoutlet of the treatment plant. This will help safeguard operators from regulatory infringments.

Sludge Blending

Sludge blending is the beginning of the solids waste treatment process. Measurements in this half of theoverall map tend to be more difficult than in the water treatment section. In the sludge blender, primarysludge from the first stage clarifier is mixed with activated sludge from the second stage clarification afterbiological treatment. This activated sludge still contains bacteria, an essential ingredient for this part of

the cleaning process. Three process maps have been developed (listed in appendix). The first showsbasic blending and thickening with flocculent dosing to promote separation of the liquid and solidsphases in the thickening tanks. Level measurement is an inportant parameter in this process.Ultrasonic instruments are commonly used for liquid level measurement and sludge level is alsomeasured, both usually with associated indication, recording, control and alarm functions. Pumps at theinlet and outlet control the level in the tank and are activated by signals from the level sensors.

The properties of the sludge affect the choice of instruments. Sometimes heavy foams may be presenton the surface or the associated settling tanks. These can alter the echos from the ultrasonic leveldevices and also absorb the transmitted sound. It is also important to position the transducers correctly.Usually they are mounted on a robust frame near the centre of the tank so the sludge can be measuredat the deepest point. The blocking distance (the minimum distance to the highest surface level) of theliquid level sensor also needs careful consideration. It is also important to guard against sunlight andother external influences or erroneous measurement will result. A thick sludge can also raise theposition of a sludge level sensor in the tank. In these cases, a stilling well is sometimes used to insulatethe measurement from the influence of rapid level changes and turbulence in the tank when pumps are



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activated. In some plants, the sensor floats on top of the sludge. It is important to note that rapidchanges in sludge density may also cause bias effects in the measurement.

Other measurements include suspended solids concentration when sludge is removed from the base ofthe tanks and into the digester. Flowrate is also important here. The clearer liquid overspill is frequentlyrecycled back to the inlet for dilution purposes. Other variants in the plant may include a sludge storagevessel to help spread and equalize the load going to the digester. In a large number of plants in theUSA, a centrifuge is used in place of static thickeners to speed the thickening process. This produces aliquid centrate, where a turbidity measurement is required. This helps dose the correct amount offlocullent prior to the centrifuge. Other measurements are as descibed above.

Finally in some parts of the world a dissolved air flotation method is used to separate liquid and solids.Here air attaches itself to the solid particles and they float on the liquid surface to be skimmed off byboards or rotating brushes. Measurement of turbidity in the water phase again paces the flocculentdosage, and suspended solids level continuously senses the floating layer. This is used to correctlyactivate the dump valve from water drawoff or the speed of the skimmers for solids drawoff. Again solidsstorage may be optional. Figure 8 shows the control map for this type of process.

Figure 8 Control and optimization of a flotation plant (DAF)

The incoming waste activated sludge is control via a magnetic meter and a suspended solids probe. An

adjustable optical sludge level sensor (CUC 101) measures the sludge interface position and the outputis used to control the exit valve position to balance the flotation tank. Chemical and polymer feed ratesare controlled using flowmeters, paced from the activated sludge inlet flow. Operator benefits include:

Ensures clear water quality does not exceed process or regulatory limits

Ensures sludge concentration is not too thin thereby reducing excessive dewatering later

Optimizes expensive chemicals addition

Allows the plant to be controlled on sludge quality and not just inlet flowrate

Sludge digester

The digester is by far the most complex for measurement and control in the entire process. This isbecause hazardous area measurements are needed because of the methane gas produced in theprocess. Both aerobic and anaerobic process are used. Aerobic digestion is well suited to industrialsludge treatment and in small sludge-activated municipal plants. It has lower capital cost and is moresimple to control, with a more bilogically more stable final product. Anaerobic processes are more



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effective with high organic sludge loading and produce less biomass, but generally the digester runs athigher temperatures to maintain microbial activity rates.

Measurements that are common include the following:

Inlet flow and suspended solids concentration from the thickener

Differential pressure and temperature across the heat exchanger (where used)

Temperature variations within the digester (to control bacteria effectiveness)

Pressure in the gas discharge tank

Waste gas flowrates and pressures prior to usage Discharge flowrate of sludge from the base of the digester into dewatering.

Level control within the digester, to control sludge withdrawal to the dewatering stage.

Figure 9 below shows a control map for a typical wastewater digester, using anaerobic bacteria.

2001 Endress+Hauser, Inc.

F1-12F1-08

Figure 9 Control map for wastewater plant digester

Again the key is control of the quality of the sludge at the inlet and in the sludge thickener, sometimesmounted downstream of the digester itself.

Temperature is a very important parameter in this part of the process, as bacterial digestion depends onthe maintenance of a nearly constant temperature within the digestion tank. Temperature is measuredwith Pt-100 probes installed in protective pipes in the tanks walls. In the case of large digesters, severalprobes may be employed to give a temperature profile to ensure an efficient process. The length of theprobe is chosen so the sensitive tip penetrates the sludge by some 50cm. A measurement range up to50

oC is usually sufficient for these units. There is also usually one additional unit installed at the top of

the tank to ensure the highest temperatures are recorded. The process maps also give an indication ofwhich measurements are alarmed for automatic control purposes.

The aerobic process also usually involves the measurement of pH from the base of the digester to givean indication of the digestion process. Hydrostatic level within the the unit is also usually measured witha pressure guage mounted in the base of the tank. It is normally installed through a pipe in the wall andequipeed with a non-return valve. Sometimes flushing devices are also used.



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The gas generated is used to heat the sewage plant or may be used in cogeneration systems thatproduce heat and electricity. Separate measurements may show the gas balance and if produced gas issupplied to third party users. In additional, gas detectors may be used to check for any leaks or escapinggas. Such instruments must conform to the local regulations for gas production and distribution.For level measurements in the wet gasometer tank, a capacitive probe is often used. The probe is boltedto the tank roof and floats up and down as gas is used. A rope connects sensor ball to the transmitter.The capacitance changes inversely to the proportion of the gas volume. In dry gasometers, ultrasonicmethods replace the capacitive ones and sense the positionn of the floating section within the gas tank.

Sludge is also recirculated from the digester to be mixed with the incoming thickened sludge. The flowmeasurement is best performed using magnetic flowmeters, with the output signals used to open andclose valves. Meters lined with Teflon are used, but care should be exercized because the sludge maycontain air which gases at low flowrates. This frequently induces erroneous signals at the electrodes ofthe meter. Sticky deposits within the sludge may also coat the electrodes, so one way to overcome thisis to periodically wash the pipes through with water.

Sludge Dewatering

Three basic processes are employed to dewater the sludge that is pumped from the base of the digester.Two of these are continuous and one non-continuous. All process sometimes use flocculent dosage topromote separation. When a chamber filter press is used, inlet pressure is often measured and in thisnon-continuous process acid is dosed into the press to help caking action. The filtrate liquid is returnedto the inlet of the entire process. Whichever process is used, the measurements made are virtuallyindentical. These include:

Inlet flowrate, inlet suspended concentration

Liquid centrate turbidity

Centrate tank level (if such a tank is part of the process)

Flocculent dosing rate (if used)

In the continuous centrifuge or belt filter press processes, the only difference is an aearation tank used inthe first option to eliminate gas bubles before the meaurement of the concentration of suspended solidsin the centrate. Measurements in both process are the same, with figure 10 below showing the automaticcontrol of a de-watering ecntrifuge. A streaming current detector (SCM) can provide additionalinformation for the optimization of the process and control of polymer feed rates.

Figure 10 Automatic sludge de-watering control



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In this control diagram, a magnetic meter and a pair of turbidity sensors (CUS 41) are used to control thespeed of the centriguge and the conditioning of the sludge. The liquid removed is usually returned to theplant inlet. A small multi-input control unit can take the place of a more complex control system at afraction of the cost. The benefits of optimizing the centrifuge are:

Optimizes solids output quality to allow a dry mass (flow x suspended solids) control strategy

Produces sludge with higher solids content

Provides much more efficient polymer usage, thereby reducing costs

Sludge incineration and disposal

The sludge cake produced from the outlet of the dewatering stage can be put to many uses. Frequentlyit is metered using magmeters prior to burning in the plants own incinerators. Sometimes it is passed totrucks and transported to landfill sites. In those cases where it has a high phosphate content it may beused as a fertilizer in agricultural applications. The solids concentration is frequently between 25 and30% by weight.

Advanced Measurement and Control

All the process maps that have been developed include the logic signals from the various processmeasurements which are fed to valves, pumps or the starters of other pieces of equipment when moreautomated plants are designed and built. The maps also indicate which signals are used for alarmfunctions to activate safety features and equipment on the plant. Common communications protocol(such as Profibus) ensure capatibility of signals and equipment throughout the plant, and minimizeproblems from instrument interconnectability.

Closure

In such limited space, the full extent of the control strategies cannot be fully documented or appreciated.Only 10 figures from the 33 developed process diagrams have been included in the paper to givereaders a feeling for the benefits that plant automation can bring. The appendix lists all the instrumentsthat can be used each section of the treatment process. The numbers down the left hand side refer toeach of the process maps available. Notice the importance of flow and turbidity (suspended solids)measurement throughout the entire process. The ideal plant is one where all pollutants and suspendedmaterials are removed.

The major benefits from instrumentation and automation are very clear. The two most important areenergy saving and chemicals usage. In a world world where pollution and energy usage are growing,automation can bring environmental and financial rewards on a large scale.

Acknowledgements

The paper is published with permission from Endress +Hauser (Holding) Ltd. It was developed by theSIG Environmental team, of which the Author was a member. The maps are the property and copyrightof Endress +Hauser. They are published with the permission of E+H Consult, Reinach Switzerland.



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