GERMANATV-DVWK RULES AND STANDARDS

ADVISORY LEAFLETATV-DVWK-M 265E

Regulation of Oxygen Transferwith the Activated Sludge Process

March 2000

ADVISORY LEAFLETATV-DVWK-M 265E

Regulation of Oxygen Transferwith the Activated Sludge Process

March 2000ISBN 978-3-937758-64-0

GERMANATV-DVWK RULES AND STANDARDS

Publisher/Marketing:Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V.German Association for Water, Wastewater and WasteTheodor-Heuss-Allee 17 • 53773 Hennef • GermanyTel.: +49 2242 872-333 • Fax: +49 2242 872-100E-Mail: kundenzentrum@dwa.de • Internet: www.dwa.de

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The German Association for Water, Wastewater and Waste (DWA) is the spokesman in Germany for all universal questions on water and is involved intensely with the development of reliable and sustainable wa-ter management. As politically and economically independent organisation it operates specifically in the ar-eas of water management, wastewater, waste and soil protection.

In Europe the DWA is the association in this field with the greatest number of members and, due to its spe-cialist competence it holds a special position with regard to standardisation, professional training and infor-mation of the public. The ca. 14,000 members represent the experts and executive personnel from munici-palities, universities, engineer offices, authorities and businesses.

The emphasis of its activities is on the elaboration and updating of a common set of technical rules and standards and with collaboration with the creation of technical standard specifications at the national and in-ternational levels. To this belong not only the technical-scientific subjects but also economical and legal demands of environmental protection and protection of bodies of waters.

Imprint

Publisher and marketing: DWA German Association for Water, Wastewater and Waste Theodor-Heuss-Allee 17 D-53773 Hennef, Germany Tel.: Fax: E-Mail: Internet:

+49 2242 872-333 +49 2242 872-100 kundenzentrum@dwa.de www.dwa.de

Translation: Richard Brown, Wachtberg Printing (English version): DWA

ISBN-13: 978-3-937758-64-0 The translation was sponsored by the German Federal Environmental Foundation (DBU).

Printed on 100 % Recycling paper.

© DWA Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V., Hennef 2007 (DWA German Association of Water, Wastewater and Waste)

All rights, in particular those of translation into other languages, are reserved. No part of this Advisory Leaflet may be reproduced in any form - by photocopy, microfilm or any other process - or transferred into a language usable in machines, in particular data proc-essing machines, without the written approval of the publisher.

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Preparation This Advisory Leaflet ATV-M 265E was elaborated by the ATV Specialist Committee 2.13 “Automation of Wastewater Treatment Plants”. The following have collaborated:

Dr. rer. nat. Jan-Ulrich Arnold, Bergisch Gladbach Dr.-Ing. Peter Baumann, Stuttgart Dipl.-Ing. Ulrich Blöhm, Berlin Dr.-Ing. Peter Hartwig, Hannover Dr.-Ing. Ulrich Jumar, Barleben Dr.-Ing. Jörg Lohmann, Viersen Dipl.-Ing. Eberhard Michel, Waldbronn Dr.-Ing. Sigurd Schlegel, Essen (Chairman) Dr.-Ing. Hans-Helmut Schneider, Berlin Dipl.-Ing. Werner Worringen, Düsseldorf

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Contents Preparation............................................................................................................................................... 2

User Notes................................................................................................................................................ 5

Foreword .................................................................................................................................................. 5

1 Area of Application ................................................................................................................. 5

2 Basic Elements........................................................................................................................ 5 2.1 Process Technology.................................................................................................................. 5 2.2 Control Engineering .................................................................................................................. 7

3 Dimensioning and Design of Aeration Systems .................................................................. 11

4 Surface Aeration ..................................................................................................................... 11

5 Pressure Aeration ................................................................................................................... 13 5.1 General ..................................................................................................................................... 13 5.2 Regulation ................................................................................................................................. 17 5.2.1 Single Cycle Regulation............................................................................................................ 17 5.2.2 Constant Pressure Regulation .................................................................................................. 19 5.2.3 Cascade Regulation with Independent Sequential Circuit........................................................ 21

6 Notes on Energy Saving and on the Concept ...................................................................... 22 6.1 Optimisation of Energy Consumption ....................................................................................... 22 6.2 Measuring Point for O2 Measurement Electrodes..................................................................... 23 6.3 Design Value............................................................................................................................. 23 6.4 Substitutional Value Strategies ................................................................................................. 24 6.5 Examination of the Economic Efficiency with the Concept ....................................................... 24

7 Summary of Standard Specifications and Standards ......................................................... 25

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User Notes This Advisory Leaflet is the result of honorary, technical-scientific/economic collaboration which has been achieved in accordance with the prin-ciples applicable therefore (statutes, rules of pro-cedure of the ATV-DVWK and the Standard ATV-DVWK-A 400). For this, according to precedents, there exists an actual presumption that it is textual-ly and technically correct.

The application of this Advisory Leaflet is open to everyone. However, an obligation for application can arise from legal or administrative regulations, a contract or other legal reason.

This Advisory Leaflet is an important, however, not the sole source of information for correct solu-tions. With its application no one avoids responsi-bility for his own action or for the correct applica-tion in specific cases; this applies in particular for the correct handling of the margins described in the Advisory Leaflet.

Foreword A revision of the Guide ATV-H 265 (1991) ”Regula-tion of the Oxygen Transfer with the Activated Sludge Process” has become necessary due to the technical development which has taken place in the meanwhile, and the operating experience gai-ned since that date. In particular there were further developments with frequency regulated motors, with roller aeration for deep tanks and diffused air aeration membranes. Through the increased in-stallation of flexible aeration zones, constant pres-sure regulation has gained in significance. In addi-tion, economical factors have been taken into account with the revision.

1 Area of Application With the activated sludge process sufficient oxy-gen concentration is a prerequisite for effective treatment of wastewater. Furthermore, the oxygen requirement is subject to variations in a biological stage due to changes to the loading, the tempera-ture, the concentration of the activated sludge and other parameters. The facilities for the transfer of oxygen must therefore be designed in a way that, on one hand, demand peaks can be covered and, on the other hand, the plant is not fed too much oxygen with lower loading.

The necessary energy for the oxygen transfer amounts to up to two thirds of the total requirement of a wastewater treatment plant. For this, the controlled operation of the aeration units is to enable a reduction of the energy required for the oxygen transfer. In addition, too high O2 concentra-tions which, as a rule, disrupt the processes of bio-logical phosphate removal and denitrification can be prevented through regulation.

Depending on the selected process technology for the removal of carbon, nitrogen and phosphorus, various regulation strategies are recommended, which are described comprehensively for nitrogen removal in Advisory Leaflet ATV-M 268E [2]. This Advisory Leaflet is based on this and deals with the adoption of the appropriate controls and regu-lation for oxygen transfer.

Information on the dimensioning of plants for main-ly municipal wastewater is, inter alia, to be found in Standard ATV-A 131E [1].

Objective of this Advisory Leaflet is to give the user assistance with the selection of a suitable regulati-on concept for oxygen transfer and information on the mechanical engineering equipment with the ac-tivated sludge process.

The area of application of this Advisory Leaflet is limited to the activated sludge process and to aera-tion using atmospheric oxygen.

2 Basic Elements

2.1 Process Technology

In order to degrade carbon compounds and to oxi-dise nitrogen compounds, the aerobic micro-organism active in the activated sludge process require oxygen. The aerobic environment can be created via the aeration using atmospheric oxygen whereby the transfer, as a rule, takes place from the surface using mechanical surface aerators or using compressed air from the bottom of the aera-tion tank. In addition, anaerobic areas are neces-sary for biological phosphate removal and anoxic areas for denitrification. These can be spatially se-parated or can be created through an intermittent operation of the aeration facilities.

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The interrelationship between process, mechanical as well as measurement and control technical re-quirements is taken into account with a reliable and economic solution for the oxygen transfer. Such a solution assumes clear terms of reference, an agreed punctual planning and a solution mat-ching the function for each of these sub-aspects and takes into account the effects of the individual components (thickener, aeration system etc.) on the overall process.

For regulation the delay behaviour of the control system (aeration tank) is of considerable signifi-cance. It is fundamentally determined by the mi-xing in the reactor.

The following can be differentiated (see Fig. 1):

a) Completely mixed tank

Here the circulation is so intense that local diffe-rences in the concentration of the activated sludge and of the oxygen do not occur. From a technical regulation aspect this type is simple to master.

b) Cascade tank

The transfer behaviour of a cascade tank corre-sponds with various, sequentially arranged, mostly completely mixed aeration cells.

In cascade tanks the setting of only one measu-ring point for the representative recording of the oxygen concentration is involved with difficul-ties. For practical reasons this is not situated in the front part of the cascade. From this results a more or less pronounced delay response which, nevertheless, is controllable from a technical regulation aspect. The delay response, howe-ver, is improved through the distributed feed of wastewater.

Type of tank

Completely mixed tank

Cascade tank

Tank with plug flow

Step response* Process management and measuring point (•)

O2 concentration

O2 concentration

O2 concentration

Time

Time

TimeDead time

Q + Q RS

Q RS

Q + Q RS

Q + Q RS

Q

*Step response = behaviour of the output parameter (here O2 concentration) with assumption of a step-shapedchange of the input parameter (here: loading)

Fig. 1: Types of tanks with the activated sludge process for different process management with in-formation on the measuring point for the oxygen concentration (minimum requirements)

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c) Tank with plug flow

Examples for this type of reactor are large tanks with pressure aeration and addition of wastewa-ter and return activated sludge up front. Due to the considerable dead time component of the delay characteristics the regulation perform-ance of such tanks is particularly unfavourable. In general it can only be improved by carrying out a breakdown into several control cycles. The control capability also improves with in-creasing recirculation due to the resultant in-creased thorough mixing of the tank.

Oxidation ditches with integrated or downstream secondary settling can, dependent on the number of rpm m, be allocated approximately to the indi-vidual reactor types as follows:

For m > 10/h thoroughly mixed tanks

m ≈ 5/h cascade tanks

m < 1/h tanks with plug flow

All components of an aeration system basically in-fluence the possibilities for regulation. The selec-tion of the unit (surface aerator, thickener) and their possible gradation (rotary frequency con-verter, pole changing capability etc.) have a great influence on the regulation of the complete system. The planning must therefore take all factors into account early and adequately.

2.2 Control Engineering

The process control system with the regulation of the oxygen transfer is the activated sludge tank in-cluding the aeration and the measuring facility. As disturbances, the following, inter alia, can have an effect: variations of the carbon and nitrogen loads, the temperature, the concentration of activated sludge and the α-value, with controlled compres-sors, however, also the variations in the air temperature (day/night – summer/winter). For practical purposes the O2- concentration is selected as control parameter. With regard to the control concept set value control, regulation using signal control and regulation using disturbance variable compensation and their combinations are to be differentiated.

The aeration facilities are to be selected in a way that the required control range can be covered. In this connection the following characteristics of the aeration unit and the characteristic data of the aeration system are important (see Fig. 2): de-mand characteristic curves, delivery characteris-tics, dynamic characteristics, upper dynamic point, lower dynamic point, point of most frequent application.

The dynamic characteristics are determined through the demand characteristics or the demand characteristic curves of the plant, with which the minimum and maximum oxygen transfer are de-termined, as result of the process technical design limiting them, and the delivery characteristics of the aeration facility in which the performance limits of the aeration facility are realised. Moreover, plant-specific constraints such as, for example, the required minimum circulation or a minimum, can limit the dynamic characteristics.

The technical requirements on facilities for measu-ring the oxygen concentration in activated sludge tanks are listed in Advisory Leaflet ATV-M 256 [3] [Not available in English]. The overlaying of further parameters (depletion, redox potential, NH4-N, NO3-N) is possible.

For example, the energy requirement can, if requi-red, be reduced through the coupling of the NH4-N value in the effluent of the biological reactor and, through this type of control, particularly with larger plants, can achieve an economic advantage (comp. Advisory Leaflet ATV-M 268) [2]. A summa-ry of the technical control and regulation solutions is shown in Fig: 3.

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Fig. 2: Diagrammatic representation of characteristic values for the design of aeration facilities

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Functional description Control scheme Assessment

3.1 Set value control

The deviation is determined through the comparison of design and actual oxygen concentration and from this the controller output is formed for the actuator of the aeration facility.

QRC O2

Simplest and most widely used form of control.

Set value control, with an ideally mixed tank, leads to good control results.

With cascade or plug flow, difficulties due to long delay times can occur – In particular when the oxygen measuring probe is installed at the end of the aera-tion tank. As an aid an additional oxygen concentration measurement device can be installed at the beginning of the sys-tem and the actual value for control can be derived through a (weighted) forma-tion of the mean of both concentrations.

Functional description Control scheme Assessment

3.2 Set value control with separate control circuits

Improvement of regulation quality through separate control circuits.

Division of the controlled system into several, e.g. two separate control cir-cuits, in order to balance differences in oxygen requirement or to be able to run different oxygen concentration levels in the individual tank sections.

QRC O2

QRC O2

Advantageous with long aeration tanks (plug flow).

Disturbances, e.g. through change of the wastewater load, are already recorded at the beginning of the system and can be well stabilised.

The observance of the oxygen concen-tration profile simplifies itself with an in-creasing number of control circuits. On the other hand there is the increasing expense.

Fig. 3: Summary of the technical control and regulation solutions, Part 1

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Functional description Control scheme Assessment

3.3 Regulation using signal control

With known progression of the distur-bance variables such as pollutant and ammonium load, the design value of the oxygen concentration can be specified as time-variation curve (KC), in order to achieve a matching of the oxygen trans-fer to the demand time-variation curve.

With the removal of nitrogen the phases of the preferable nitrification and denitri-fication alternate. For both phases dif-ferent design values are specified (signal control) and appropriate aeration units are switched on or off or modified in their oxygen transfer.

KC Y Y QRC

O2

aerobic aerobic

anoxic anoxic

Value should be placed on the pro-gramme being capable of being matched simply to changes of the progression of the disturbance variable. The quality of the effluent must be monitored regularly in order to control the agreement with the actual conditions. This combination of control and regulation leads to good results so far as the actual progression does not deviate too heavily from the specified characteristic curves.

Under certain circumstances different characteristic curves can be necessary for working day, holiday and weekend operation.

Relatively simple and cost-effective process.

Functional description Control scheme Assessment

3.4 Signal control taking into account further measurement parameters (NH4-N)

Basically comparable with 3.3.

The design value of the O2 concentration is altered dependent on the NH4-N con-centration in the effluent.

With rising NH4-N content the O2 design value increases and with falling NH4-N decreased. Moreover, the switching on and off of the flexible nitrification/denitri-fication zones can be regulated via the NH4-N content. NO3-N and PO4-P can be included in the regulation.

QRC

NH2-N

aerobic

anoxic anoxic

aerobic

QRCO2

(NO3-N) (PO4-P)

The influencing of the O2 design value through the NH4-N content of the effluent enables an optimisation of the specific O2 transfer. The ensuring of a sufficient circulation is required as lower limit of the lowering of the O2 content.

With too small O2 concentrations there is a danger of the formation of bulking sludge.

During the phase of the denitrification, as the aeration is switched off, suitable circulation facilities must be switched on.

Fig. 3: Summary of the technical control and regulation solutions, Part 2

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3 Dimensioning and Design of Aeration Systems

The design of aeration systems is based on the required oxygen transfer (OC in kg/h) and with the application of wastewater-specific (α-value) and system-specific characteristic data, inter alia the specific oxygen transfer OCL,h, measured in g/(m3

N · m). While the necessary oxygen transfer and the α-value have to be specified by the plan-ner, the specific characteristic data of the aeration system as a rule is to be given by the outfitter. Due to the complex relationships attention is to be paid that the actual dimensioning (αOC) takes place on the basis of a careful determination of data and includes the selection of plausible loading cases. Under or over-dimensioning of the aeration system can often not be compensated through the control technology employed.

The details from the outfitter with regard to the specific oxygen transfer and the therefrom resul-tant aerator design are to be monitored for their plausibility. Each overall larger design (compres-sor) or an underestimation of the efficiency of the selected system (aerator) leads to an overdimen-sioning. In addition to the dimensioning of the peak demand in accordance with the current guidelines, the minimum necessary oxygen transfer is to be verified. This can, for example, take place via the calculation of the endogenous respiration.

If, in this case, an amount of air is determined for the actual status which lies below that for the mi-nimum circulation, additional, separate circulation facilities can be installed. If, with ceramic aeration elements, a greater amount of air is necessary to achieve the minimum effective hourly capacity, it is to be examined in how far the number of aerator elements can be reduced in the actual condition. Only with the provision of a total system, which is matched to the actual loading, can the control of the oxygen transfer meet the target of a sufficient aeration and an economic operation.

It should be noted that, from case to case, for e-xample through a pre-treatment of industrial waste-water, in future lower loading can occur than for the actual state, so that in this case the minimum anticipated loading is relevant for dimensioning.

The thus carried out establishment of the minimum and maximum quantities of air gives the required working range of the aeration system. Through an appropriately matched mechanical equipment it is to be ensured that the oxygen consumption cha-racteristic curve, heavily influenced through local conditions, can be uninterruptedly covered at least in the ratio 5:1, with plants with nitrification and de-nitrification in individual cases greater than 10:1.

With several tanks or tank areas with different oxy-gen demands the design of the aeration system has to take place statically on the basis of an estimation of the respective oxygen demand. In cascade tanks and with tanks with plug flow a gradient in the requi-red oxygen demand will occur in the direction of flow. Through appropriate gradation of the aerator elements a matching to the gradient is possible. The arrangement of the aeration system for this case can be supported by a dynamic simulation.

4 Surface Aeration In plants with surface aerators the oxygen transfer can be influenced through the following control quantities:

– the number of machines operating,

– the rate of rotation (rpm),

– the submerged depth.

Actuator for the change of the number of machine groups in operation and the rate of rotation is the motor. The operation of a motor with pole changing or frequency control is technically possible without problem. With the frequency controlled drive the reduction of the efficiency in the partial load range is to be noted and a sufficient cooling of the motor is to be ensured.

The modification of the submerged depth as a rule takes place through weirs in the outlet of the aera-tion tank. The height adjustment of the surface ae-rator has not proved itself technically. With weirs the width of the overfall must be selected large e-nough so that variations of the flow amounts effect no great changes of the water level. The rate of change of the weir shutters is to be dimensioned so small that reactions on the settling behaviour of the sludge in the secondary settling tank are exc-luded. Furthermore, the connection between oxy-gen transfer and submerged depth must be suffi-ciently steep.

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It is to be noted that the circulation performance of the aerator reduces with deeper submerged depths from which, primarily the deep zones of the aeration tank are affected. As the measurement of oxygen in general takes place in the upper zone of the tank sludge deposits and undetected dad zo-nes can occur with insufficient determination of the working range. The permitted control range of the submerged depth must therefore be determined carefully taking into account the tank geometry and the other employment conditions (e.g. water level with minimum and maximum flow). For the safegu-arding of a sufficiently large control range it can additionally be practical to separate circulation and oxygen transfer by the installation of separate cir-culation facilities.

Because the influence factors in the planning stage can barely be determined reliably via the submer-ged depth with a regulation of the oxygen transfer, the submerged depth serves for the most part only for the rough setting of the oxygen transfer.

With the regulation of roller aerators in oxidation tanks it is convenient to create a zone with intensi-ve aeration and to enlarge this with increasing oxygen demand through the connection of additio-

nal units. If, instead of this, aerators are operated distributed evenly over the tank there are mixing zones with oxygen concentrations for example between 0.4 and 0.8 mg/l, in which neither nitrifica-tion nor denitrification can take place effectively. In addition to lower efficiencies with nitrogen removal one has also to reckon with a relatively unfavou-rable sludge settling behaviour (higher sludge vo-lume index).

With the employment of impeller aerators a mat-ching to the different oxygen demands through in-termediate switching, change of the submerged depth and adjustment of rpm is fundamentally pos-sible. The intermittent switching for control purposes can, however, be applied to a limited extent only as the impellers, in addition to aeration also have the task of circulation. The employment of impellers is limited through the dependence on the tank geo-metry. If advanced measures are demanded for noise, aerosol and odour reduction higher additional costs can result with the employment of impeller ae-rators. Information on current regulation facilities with surface aerators is contained in Table 1.

Table 1: Summary of performance of surface aerators

Type of aerator Regulation and/or control through Per unit

Adjustment range in %

Maximum O2 input in kg/h

Impeller aerator Intermittent switching Submerged depth RPM adjustment

0, 100 50 – 100 50 – 100

200

Roller aerator Intermittent switching Submerged depth RPM adjustment

0, 100 40 – 100 50 – 100

100 or 6 – 10 kg/(m·h)

respectively

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5 Pressure Aeration

5.1 General

The aeration system consists of the compressors, the pipelines incl. fittings and the actual input elements. A summary of performance for dynamic and controlled compressors and information on current regulation facilities with pressure aeration are given in Table 2.

The characteristic diagram of a compressed air fa-cility must be determined through combination of the oxygen transfer curves of the planned input e-lements with the delivery characteristic diagram of the compressor.

The lower limit of the working characteristic diagram is determined through the demand for a thorough sludge circulation, when this is not guaranteed through ancillary facilities. For the ancillary facilities it applies that with an optimally designed circulation facility and more favourable tank geometry, a per-formance density of 1 – 2 W/m3 suffices.

Table 2: Summary of performance of compressors

Type of compressor

Working range in % of the maximum de-livery flow

Delivery flow per machine m3

N/h

Efficiency %

Remarks

Controlled compressors

Sliding vane ro-tary compressor

ca. 20 – 100 Range of speed con-trol 32 – 100 %

Up to 20,000 60 – 80 Robust machine, small requirements on op-eration and mainte-nance

Dynamic compressors

Screw-type com-pressor

ca. 20 – 100 Range of speed con-trol 30 – 100 %

Up to 15,000 75 – 80 For overpressures > 0.75 bar

Radial flow com-pressor with regulating diffuser

ca. 45 – 100 3,000 – 120,000 75 – 85 Precise design necessary, note surge limit

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=

a) Characteristic diagram for n = constant and temperature T = constant

Fig. 4: Delivery characteristic diagram of a dynamic compressor, Part 1

b) Characteristic diagram for n = constant and different temperatures

Fig. 4: Delivery characteristic diagram of a dynamic compressor, Part 2

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Controlled compressors, due to their construction principle, produce an rpm proportional volume flow. In opposition to this, dynamic compressors deliver a certain mass flow; the air volume de-pends also on the condition parameters pressure and temperature.

In practice the suction pressure varies only to a very small degree. Changes to the inlet temperature, however, occur to a considerable degree. Thus, for example, the inlet temperature can change from winter to summer from -15 °C to +35 °C. Accor-dingly the mass flow of the air, which is relevant for the oxygen transfer, changes by 26 %! The limiting value for the outside temperature must therefore be taken into account with the establishment of the deli-very characteristic diagram (see Fig. 4 b).

With a fine bubble pressure aeration it should be taken into account when determining the demand characteristic curve that, in addition, the required total pressure, depending on the submerged depth, can increase by up to 100 mbar through contamination of the aerator.

In plants with pressure aeration the oxygen trans-fer is influenced by changes to the airflow. Suitable concepts are:

– change of the number of machine sets in opera-tion through switching in and out, whereby the permitted frequency of switching is to be ob-served

– adjustment of rpm

– guide vane adjustment on the suction side (pre-rotation control)

– guide vane adjustment on the pressure side (regulating diffuser) of dynamic compressors.

The delivery characteristic diagram of dynamic compressors is additionally limited by the surge limit. It forms the “separation line” between the stable and instable areas. The operating point – intersection of the delivery characteristic and the demand characteristic curve – must always lie in the stable area of the compressor (see Fig. 4). In order to cover the differing high oxygen demand, dynamic compressors offer relatively simple pos-sibilities of continuously influencing the family of delivery characteristics in a wide range and also to operate in the partial performance area with an energetically favourable efficiency. The family of delivery characteristics can be modified using

prerotation control and regulating diffuser. The achievable final pressure depends on air intake temperature. Therefore the characteristic curves for summer and winter operation also differ from each other significantly (see Fig. 4 b). Therefore a combined surge limit protective quantity control with temperature compensation is required for the maintenance of the surge limit. Furthermore a start-up load alleviation with appropriate precauti-onary measures to prevent noise emissions is to be provided.

With controlled compressors such as, for example sliding vane rotary compressors, there is no limita-tion of the working range through a surge limit. They are considerably less sensitive to changes of back pressure. The modification of the delivery flow can, however, only be sensibly undertaken through changes to rpm. As long as the maximum value, which is determined by temperature and motor capacity, increasing back pressure effects an increased power consumption (see Fig. 5).

The volume-related efficiency of forced flow compressors is determined by the gap leakages which, on their part are dependent on back pressu-re and also on the operating rpm. Forced flow compressors therefore have their most satisfactory efficiency at their nominal rpm. With regard to effi-ciency such units should be operated only with rpm upwards from ca. 40 % of the nominal rpm.

For the run-up of sliding vane rotary compressors a possibility for blow through (start up relief) is to be provided for the protection of the motor with unregu-lated and pole changing motors. With this attention is to be paid to a limitation of noise emissions.

Sliding vane rotary compressors are robust, main-tenance-friendly machines which have proved themselves particularly in small and medium-sized wastewater treatment plants. Unfavourable is the ca. 10 – 20 % smaller overall efficiency compared with dynamic compressors, particularly in the part- performance range. The more favourable invest-ment costs are to be compared with the higher o-perating costs caused by this.

As a rule, an economic comparison, which takes into account the point of most frequent employ-ment, is required for the decision for dynamic or sliding vane rotary compressor.

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Fig. 5: Delivery characteristic diagram of a controlled compressor

Mainly ceramic or membrane aerators are em-ployed as aeration elements.

With the structural design of pressure aerator sys-tems it is absolutely necessary to take into account the hydraulic conditions caused by the aeration. Attention is drawn particularly to two aspects:

– it is not practical to distribute the aeration ele-ments evenly over the complete tank area as, through this uncontrollable flow conditions would result. To be preferred is an arrangement with which a small strip is kept free of aeration elements in the longitudinal axis of the flow path. Through this there results clearly deline-ated aeration rollers through which the retention time behaviour is positively influenced and short-circuit flows are avoided.

– particularly problematic are activated sludge tanks with circulating flow which are provided with pressure aeration and horizontal acting cir-culation elements, above all if their arrangement does not take sufficient consideration of the hy-draulic characteristics. Here, the danger of short circuits is particularly large as the energy con-sumption caused by the aeration is several times the energy density effected by the circula-

tion propeller. In areas where no aerators are mounted such as, for example, at the ends of the tank, flows are then possible which run op-posite to the flow direction sought and, depend-ing of the position of the inlet and outlet, can lead to serious short-circuiting.

The short-circuit flows caused through such hy-draulic shortcomings can be the reason that, in particular, loading peaks affect the outflow and endanger the maintenance of the specified limit-ing values. Therefore everything should be done constructively to avoid short-circuit flows.

If, in a longitudinally flowed tank, there are both aerated as well as unaerated zones which are not separated, there is a danger that bulking sludge collects. Here help can be provided by the creation of a directed surface flow or the permanent me-chanical break-up of the bulking sludge blanket.

With plants with small redundancy it can be practi-cal to design the aeration facility in a way that this can be raised for maintenance and repair tasks without having to empty the tank.

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5.2 Regulation

5.2.1 Single Cycle Regulation

The direct control of the aeration machine has the advantage that the pressure loss which would re-sult from a throttle control is avoided. The machine is used as actuator.

The compressors as a rule are brought together locally centrally and the compressed air fed via a distributor network to the individual users. If indi-vidual tank sections, individual tanks or tank groups are be operated alternatively aerobic, an-oxic or also anaerobic and regulated independently of each other, then at least one own compressor with its own distributor network is necessary per unit. Due to the thus associated costs, the break-down into several user control circuits with the di-rect control of compressors, is mainly carried out only in larger plants.

Otherwise, due to operational experience, the air distribution within a tank or in several parallel tanks is set manually. Deviations of the planned distribu-tion can be detected and corrected through regular control measurements.

Control of the compressor takes place either con-tinuously

– with forced flow compressors through rpm ad-justment via frequency converter

– with dynamic compressors through regulating diffuser and prerotation

or in steps

– with forced flow compressors through pole changing or intermittent switching.

The following example (see Fig. 6) describes the direct control of sliding vane rotary compressors with which the compressed air can be continuously modified in the delivery range of ca. 7 : 1 through a combination of rpm regulation and pole changing. For this at least three sliding vane rotary compres-sors are required of which at least one is rpm regu-lated through a frequency converter and the others through pole changing. A fourth compressor is held as reserve. For the even allocation of running times, Compressors 1 and 2 or 3 and 4 respec-tively can be exchanged in the sequence selection switching. Therefore Compressor 2 is also fitted with a frequency converter.

Compressor 1 or 2, rpm controlled

Compressor 1 or 2, rpm controlled

Compressor 1 or 2, rpm controlled

Compressor 1 or 2, rpm controlled

Compressor 1 or 2, rpm controlled

Compressor 3 rpm n2

Compressor 3 rpm n2

Compressor 3 rpm n2

Compressor 3 rpm n2

Total volume flow in %

Total volume flow in %

100

90

80

70

60

50

40

30

20

10

0

100

90

80

70

60

50

40

30

20

10

0

0 1 2 3 4 5 6 7 8 9 10 0 50 100 in %

Switching steps rpm / switching steps

80.0 % 60.3 % 46.7 % 30.0 % 13.3 %

100 %

Compressor 4 n2 ON 83.3 %

Compressor 4 n1 ON 66.6 %

Compressor 3 n2 ON 50.0 %

Compressor 3 n1 ON 33.3 %

Compressor 1 or 2 (reserve)

Compressor 4 rpm n1

Compressor 4 rpm n2

Fig. 6: Example of switching of sliding vane rotary compressors for a single-circuit control or constant pressure control Sliding vane rotary Compressors 1 and 2 using frequency controlled drive, 3 and 4 using pole changing drive

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Here, for reasons of simplified maintenance, four identical compressors are employed. The delivery range of the compressors, depending on the drive control of the rpm, is limited to 40 % or 50 % of the nominal rpm respectively.

The compressors are switched in cascades, for example in the following switching steps (see Ta-ble 3 and Fig. 7):

Switching step 1: Compressor 1 regulated from Qmin to Qmax

Switching step 2: Compressor 1 regulated to Qmin, Compressor 3 switched to n1 (Qmin)

Switching step 3: Compressor 1 regulated from Qmin to Qmax, Compressor 3 de-livers with n1 (Qmin)

Switching step 4: Compressor 1 regulated to Qmin, Compressor 3 switches to n2 (Qmax)

etc.

Table 3: Example for switching steps of slid-ing vane rotary compressors with a single-circuit or constant pressure control

No. of the compres-sor

1 2 (Reserve) 3 4

Control- ability

Frequency controlled Delivery qty min. to max.

Pole changeable rpm n1, n2

Switching step 1

Min. – Max. 0 0 0

2 Min. 0 n1 0

3 Min. – Max. 0 n1 0

4 Min. 0 n2 0

5 Min. – Max. 0 n2 0

6 Min. 0 n2 n1

7 Min. – Max. 0 n2 n1

8 Min. 0 n2 n2

9 Min. – Max. 0 n2 n2

CONTROL INPUT

CONSTANT PRESSURE OR DISSOLVED OXYGEN

TO THE AERATION TANKS

10 – 100 %

40- 100 %

600- 1.500 min-1

600- 1.500 min-1

40- 100 %

750- 1.500 min-1

50- 100 %

750- 1.500 min-1

50- 100 %

1 2 M 3~

M 3~

M 3~

M 3~ 3 4

n1 N2 n1 N2

Sliding vane rotary compressors 1 and 2 with frequency-regulated drive

Fig. 7: Example for switching steps and delivery volume flow of sliding vane rotary compressors with single circuit or constant pressure control

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During the switching procedures control is bridged using time function elements.

In the example, in simplified form, the change of the volumetric efficiency (gap leakages) in the delivery range between 40 and 100 % are assu-med to be negligible (see also Fig. 5) which, within the scope of the mechanical design for the delivery range, has to be checked in detail. The delivery characteristic diagram of the compressor group must cover the working range. In addition, the delivery range must overlap the individual compressors and the switching procedure provi-ded with a hysteretic, so that with a change of the switching step no instable condition results and the frequent switching ON and OFF of a further compressor is avoided.

5.2.2 Constant Pressure Regulation

Often several consumers in the activated sludge plant have to be provided with compressed air in-dependent of each other. The supply takes place normally via a central network in which the air is provided at constant pressure. The local control circuits take the respectively required quantities, independent of each other, from this network. The feeding of the compressors into the network is regulated via the pressure in the network (see Fig. 7). Through this the individual oxygen controls are decoupled from the machine control. This has the advantage that the frequency of switching off the machines is reduced and, in principle, any number of desired oxygen control circuits can be connected to the network. Here, however, it must be noted that the permitted maximum charging of the aerators is not exceeded as with a too high quantity of air damage can occur in the aerators.

In addition, the lower delivery limit of the compres-sors adjusts itself according to the lower working point of the complete plant and no longer accor-

ding to the respective control circuit. Through this the number of required compressors is reduced and an essentially bigger delivery range compared with individual circuit control is achieved.

For constant pressure control both easily regula-ted dynamic compressors and also controlled compressors can be employed insofar as these are equipped with infinitely variable drives. Controlled compressors, due to their very steep characteristic curve, under certain circumstan-ces, require special measures in order to achieve a stable overall control, e.g. control procedures in the local consumer control circuits can take place with staggered times.

Even if a run-up relief is provided switch over pro-cedures with the individual compressors lead to variations in pressure in the pipe network. They can be bridged using appropriate time function e-lements if the local user controls react too sensiti-vely to network pressure variations. In many cases the control parameters of the local regulation can be set accordingly.

In order to avoid instability the consumer control circuit – for example the local throttle control – has always to be more sluggish than the compressor control, because this change over procedure – de-pending on the size of the machine and on the sys-tem – is certainly frequent and demands an ap-propriate amount of time. This is generally possible as a change to the oxygen demand takes place comparatively slowly.

The following example (see Fig. 8) describes a constant pressure control using dynamic compres-sors and the control of the local oxygen through actuating fittings – as a rule diaphragm control val-ves or annular piston valves.

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TC TC TC TC

C C C C B B B B

PC PCPCPC

PC

1 2 3 4

A

M M M QRC QRC QRC O2 O2 O2

Wastewater

Fig. 8: Example of the control of pressure aeration with constant pressure control with employ-ment of dynamic compressors

As in the previous example the feeding of air is controllable in the range of ca. 7:1, whereby each of the three compressors can be regulated down to 40 % of the nominal load through prerotation con-trol and regulating diffuser.

The fourth compressor serves a reserve For rea-sons of maintenance all compressors have the same build. Through this their switching sequence is infinitely exchangeable.

With specification of the delivery quantities for the individual compressor, the overriding computer (A) controls the load distribution and thus the pressure in the network. These specifications are converted from a special mechanical control (B) taking into account the suction and pressure side status pa-rameters (suction temperature TC, delivery diffe-rence pressure PC) in an as far as possible low-loss setting of the coarse guide screen before and after the compressor impeller. At the same time this computer takes over the technical safety moni-toring of the pump limit.

The automatic running up and down of the com-pressor, the switching of the auxiliary drive and locking are realised by the local control (C). The compressors in a cascade are switched similarly

as in the previous example (see Fig. 7), whereby the sequence is arbitrarily changeable. The compressors are, for example, switched in the following switching steps (see Table 4):

Switching step 1: Compressor regulated from Qmin to Qmax

Switching step 2: Compressor 1 regulated to Qmin, Compressor 2 switches with Qmin

Switching step 3: Compressor 1 regulated from Qmin to Qmax, Compressor 2 de-livers with Qmin

Switching step 4: Compressor 1 regulated to Qmax, Compressor 2 regulated from Qmin to Qmax

etc.

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Table 4: Example for switching steps of dynamic compressors with a con-stant pressure control

No. of the compressor

1 2 3 4 (Reserve)

Switching stage 1

Min – Max

0 0 0

2 Min Min 0 0

3 Min – Max

Min 0 0

4 Max Min – Max

0 0

5 Max Min Min 0

6 Max Min – Max

Min 0

7 Max Max Min – Max

0

5.2.3 Cascade Regulation with Independent Sequential Circuit

For the following example a similar delivery cha-racteristic diagram as in the example in Fig. 7 has been assumed. The note there for the necessary overlapping of the individual delivery characteristic diagrams also applies.

With unfavourable laying of the collection lines, for example a spur line in place of a closed circular pi-peline, the pressure at the end of the pipeline can be influenced through the previous consumer. In order, nevertheless, to guarantee a high control accuracy, it can be practical to lay the actuator

signal not immediately on to the actuator but rather as command parameter on a volume flow control (sequential control circuit of a control cascade). The sequential controller balances out pressure differences in the supply pipe train and, without de-lays, ensures the required air feed (see Fig. 9).

Cascade regulation is a good control concept if several oxygen control circuits are to be opera-ted. The system is independent of the number of connected control circuits and can be easily ex-panded. The individual oxygen control circuits can achieve a higher control quality because they are fundamentally independent of the compres-sor regulation.

Compared with the individual circuit control, a lar-ger delivery range with the same number of ma-chines or, if several consumer control circuits are to be operated, a smaller number of machines with the same delivery range of the circuit control are possible.

The expense for the mechanical regulation and control is comparatively high. Of advantage, how-ever, is the good overall efficiency which also re-mains in the partical performance range, through the setting possibilities, extensively sustained even in the partial performance range. Energy cost sa-vings resulting from this are of significance particu-larly for large wastewater treatment plants (consi-deration of economic efficiency).

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TC

PC

PC

1

QC O2

Wastewater

A

TC TC TC

2 3 4 B C B C B C B C

PC PC PC

QC O2

FC FC FC FC

Fig. 9: Cascade control with sequential control circuit for the balancing of pressure differences in the air supply line

6 Notes on Energy Saving and on the Concept

6.1 Optimisation of Energy Consumption

As the energy requirement for the aeration of the activated sludge plant has a considerable part of the overall requirement considerable energy can be saved via a regulation of the oxygen transfer.

On one hand, the control strategy can make a con-tribution to the energy optimisation, in particular in that an aeration over and above the amount re-quired for the desired treatment performance is avoided [2]. On the other hand, energy savings can be achieved through the minimisation of pres-

sure losses in that, for example, the throttle flaps in the air distribution system operate as wide open as possible (“Aperture tendency”) and suitable back-pressure valves are employed.

With staggering of compressors of different capaci-ty a load-dependent, intelligent strategy for the switching of the units has particular significance. It is energetically favourable if the compressors are respectively operated near to the operating point with the highest efficiency.

The pollution of the aeration elements and the thus accompanying pressure loss in the system can be countered through a regular flushing of the aerati-on units using formic acid. Flushing connections for this are, in any case, to be provided. The pres-sure losses in the aerators can be monitored via a differential pressure measurement.

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In addition, attention is to be paid that both the condensed water which is produced through the cooling of the compressed air and also possibly wastewater that has penetrated can be removed from the system via drainage pipelines. The drai-nage pipelines here are to be arranged at the end of each aeration screen. If possible the drainage it-self should, following commissioning, be carried out daily.

The selection of circulation facilities should take place with consideration of the tank geometry and the arrangement of the aerators in order that an as small as possible power consumption can be a-chieved. From experience, with stirring devices with low rpm, a power density of 2 W/m3 or less suffices for a sufficient circulation. A circulation via the aeration system, on the other hand, requires a power density of at least 8 W/m3.

Further, the tank geometry has an influence on the economic efficiency of the aeration system.

6.2 Measuring Point for O2 Measurement Electrodes

Fundamentally it applies that a representative re-cording of the concentration of dissolved oxygen in the tank is necessary. An unfavourable selection of the measuring point can lead to the regulation of the oxygen transfer not functioning as planned.

With the evaluation of the O2 measured values with compressed air and surface aerator systems, the different gradient of the oxygen concentration over the depth is to be taken into account. With surface aerator systems the oxygen probe can also be in-stalled in the upper zone of the tank. Here, howe-ver, the representative O2 content should be calcu-lated through a measurement of the O2 concentration over the tank depth and the control value appropriately tailored to this.

Recommendations for the selection of the suitable measurement point are to be taken from Fig.1. With this, one is concerned with a minimum re-commendation. The installation of further measu-rement devices, depending on the size of the plant, can be sensible or absolutely necessary.

While the measurement of the oxygen concentrati-on at only one point in the aeration tank as a rule is sufficient with completely mixed tanks, with casca-de reactors the installation of oxygen electrodes in

each loading sector has proved itself. The control of several parallel tanks through a comparison tank (control stretch) is less useful, as it cannot be en-sured that the wastewater feed and oxygen trans-fer into the individual reactors are equally high.

As the most favourable location for oxygen measu-rement electrodes can first be determined through sample measurements in operation, the measure-ment devices should be so installed that a relocati-on is possible without large conversion measures. Therefore several possibilities for connection for the oxygen electrodes or appropriately long con-nection cables should be provided between probe and measuring transducers.

In case results of a technical flow investigation, for example a flow simulation, are available these can be used for the determination of convenient mea-surement points.

6.3 Design Value

The design value for the oxygen transfer, in additi-on to the plant-specific characteristics, is also de-pendent on the sludge loading. As with the specifi-cation of the design value not only the quality of treatment but also the amount of energy is influen-ced, one should operate with a sufficient but as low as possible oxygen design value.

With regard to the necessary expenditure of ener-gy the saturation deficit over the factor δ

CS δ =

CS – CX

with CS = oxygen saturation concentration, temperature dependent

CX = oxygen concentration in the reactor

is to be taken into account.

The influence of the oxygen concentration Co in the reactor on the energy requirement for the aeration is made clear in the following example:

assumed is a reactor temperature of 20 °C and thus Cs,o = 8.84 mg/l O2.

for Co = 1.0 mg/l O2 there results δ = 1.128

for Co = 2.0 mg/l O2 there results δ = 1.292

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In comparison to the operation of the reactor with Co = 1.0 mg/l O2, the operation with Co = 2.0 mg/l O2 requires a 1.292/1.128 · 100 = 14.5 % higher power consumption.

With plants with complete nitrification, oxygen con-centrations below 2 mg/l can often be realised without problem, whereby the circulation of the ac-tivated sludge is assured at all times. With plants with deliberate phosphate removal unaerated, an-aerobic zones in the area of the aeration tank out-flow, however, are to be avoided by all means available due to the associated danger of an un-wanted resolution of phosphate.

6.4 Substitution Value Strategies

With measurement devices for the processing of measured values, for example, in stored program-mable control systems disruptions to the oxygen transfer can occur which have to be brought under control through corresponding substitution value strategies, in order to counteract a slump in per-formance of the biological wastewater treatment due to a lack of oxygen. The following substitution value strategies are possible:

– arrangement of fixed settings (rpm, switching steps)

– acceptance of measured values from parallel trains

– selection of other parameters as reference quantities (e.g. oxygen content instead of con-tinuously measured NH4-N content)

– specification of characteristic curves

6.5 Examination of the Economic Efficiency with the Concept

Before selection of the aeration systems their eco-nomic efficiency is to be examined whereby, in ad-dition to the investment costs, above all also the operating costs are to be taken into account. From the overall costs of a wastewater treatment plant only a relatively small share is used for oxygen transfer. The share of operating costs in compari-son is, on the other hand, in particular for energy, very high. For this reason the assessment of the economic efficiency of the oxygen transfer is a par-ticularly important point with the selection of the aeration system. The planner should therefore present a variant comparison. As is shown below here, in addition, the different requirements which result from process technical and economic as-pects must be considered.

With a pressure aeration ceramic or membrane ae-rators are employed. Ceramic aerators, in compa-rison with membrane aerators are, as a rule, more energetically advantageous. While ceramic aera-tors in normal operation require a minimum char-ging in order to prevent penetration of water into the air line and a blocking of the aeration pores through activated sludge, membrane aerators are considered as secure against blockage even with a switching off of the air supply. For this reason membrane aerators are employed with operation of non-permanently aerated tanks as is the case with alternating or intermittent nitrificati-on/denitrification. A combination of membrane and ceramic aerators in one aeration tank requires an appropriate regulation of the air supply taking into account the different pressure losses.

A performance verification of the installed aeration system can be carried out wit the aid of oxygen transfer tests (Advisory Leaflet ATV-M 209E) [4].

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7 Summary of Stan-dard Specifications and Standards

[Translator’s note: References available in English are shown as such. For those references with no known official translation a courtesy translation is provided in square brackets]

[1] ATV (1999): Standard ATV-A 131E Dimensioning of Single Stage Activated Sludge Plants1)

[2] ATV (1997): Advisory Leaflet ATV-M 268E ”Control and Regulation of N-Elimination using the Activated Sludge Process”1)

1) Author`s afternote: The German editions of these DWA

Rules and Standards have in the meantime been with-drawn and replaced by the following Standards and Advi-sory Leaflets:

- Standard ATV-DVWK-A 131 “Dimensioning of Single Stage Activated Sludge Plants”. Date of issue: April 2000;

- Advisory Leaflet DWA-M 209 “Measurement of the Oxygen Transfer in Activated Slugde Aeration Tanks with Clean Wa-ter and in Mixed Liquor”. Date of issue: April 2007.

- Advisory Leaflet ATV-DVWK-M 256 “Employment of Operating Measurement Facilities in Wastewater Treatment Plants”. Date of issue: February 2001;

- Advisory Leaflet DWA-M 268 “Control and Regualtion of N-Elimination using the Activated Sludge Process”. Date of issue: Juni 2006.

Currently there are no translations in English of Advisory Leaflet ATV-DVWK-M 256, DWA-M 209 and DWA-M 268 available.

[3] ATV (1988): ATV-Regelwerk Abwasser-Ab- fall: Merkblatt ATV-M 2561): Einsatz von Betriebsmessein- richtungen auf Kläranlagen Vorblatt: Allgemeine Anforderun-gen, Blatt 1: Anforderungen an Sauerstoffmesseinrichtungen

[ATV Standards Wastewater-Waste: Advisory Leaflet ATV-M 256 “Employment of Op-erating Measurement Facilities in Wastewater Treatment Plants Sheet 1: Requirements on Oxy-gen Measuring Facilities”]

[4] ATV (1996): ATV Standards Wastewater-Waste: Advisory Leaflet ATV-M 209E “Measurement of the Oxygen Transfer in Activated Sludge Aeration Tanks with Clean Water and in Mixed Liq-uor”1)