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Membrane aerators and stirring systems for the operation in large and small wastewater treatment plants
~
Pergamon
Waf. Sci. Tech. Vol. 34, No. 3-4, pp. 329-338,1996. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + (}oo
PH: 50273-1223(96)00590-2
MEMBRANE AERATORS AND STIRRING SYSTEMS FOR THE OPERATION IN LARGE AND SMALL WASTEWATER TREATMENT PLANTS Marcus W. A. Hofken, Peter Huber, Marcus Schafer and Ralf Steiner INVENT Umwelt- und Verfahrenstechnik GmbH & Co. KG, Am Weichselgarten 7, 91058 Erlangen, Germany
ABSTRACT Stirring and aeration processes are the major operations needed for effective wastewater treatment. Es• pecially in activated sludge processes and the variants of this process stirring and aeration systems need to be chosen carefully since both operations are crucial for the purification power and the energy de• mand of wwfp's. The aim of the present work is to define the demands on stirring and aeration systems for activated sludge treatment plants, to give an overview about suitable systems currently available on the market including latest developments and to give advice how to choose the optimum system. A brief review of the literature and the market of stirring and aeration systems leads to a simple classifi• cation of the existing numerous systems. In order to choose the optimum system, first one has to decide, about the demands in each process step. In anaerobic and anoxic reactors the use of hyperboloid stir• ring systems or combined hyperboloid stirring and aeration systems which distribute small air bubbles throughout the entire reactor in the aeration mode (for intermittent aeration) and gently circulate bacte• ria flocks in the stirring mode is recommended. For aerobic processes like BOD-removal and nitrification highly efficient membrane aerating systems using silicone membranes are recommended. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Activated Sludge Process; Aeration; Mixing; Stirring
INTRODUCTION The activated sludge process stands for variety of continous, space and time-oriented periodic processes. The activated sludge process can be used for BOD-removal or complete nitrogen removal. Well-known variants are the preliminary nitrification/denitrification, the post-denitrification or the alternating nitrifica• tion and denitrification. Well known batch processes are e.g. the Sequencing Batch Activated Sludge Reactor (SBASR), the Cyclic Activated Sludge System (CASS) and the Intermittent Cycle Extended Aeration System (ICEAS) which are covered in detail in the literature (see e.g. Irvine, 1971; Wilderer and Schroe329
330
M. W. A. HOFKEN et al.
der, 1986 or Metcalf and Eddy, 1991). All these biological purification processes as well as the k~o.wn standard activated sludge process include two major operations: stirring and aeration. Without stIrring and aeration an effective reduction of BOD and Nitrogen is impossible. Since approximately 70% of the total energy consumption is used for these two processes (Hofken et a/., 1995) a proper choice and de• sign of such systems is indispensible. Hence, only innovative and energy-efficient equipment should be used.
STATE OF THE ART Stirring Systems A review of the stirring systems currently available on the market for wastewater treatment will reveal an impressive number of different products (Hofken, 1993). In order to somewhat classify the large variety it is possible to refer to the definitions of the basic stirring tasks as they are used in the fields of chemical and process engineering. A distinction is made in process engineering between the following basic stirring tasks:
• Homogenization • Suspension • Dispersion • Heat transfer
Compensation of differences in concentration or temperature Whirling up and suspending of. solid particles Liquid/liquid ~ Emulsions, polymerizations Liquid/gas ~ Aeration, mass transfer Intensifying of the heat transmission (cooling, heating)
This breakdown leads to a classification of all the stirring systems available on the market today that are used for wastewater treatment and enables an example of application in wastewater treatment to be found without difficulty for each stirring task shown in table 1.
Aeration Systems In the field of biological wastewater treatment, aeration systems in various technical designs are used to supply wastewater with the oxygen required for decomposition of harmful substances. All pressure aero· tion systems available on the market teday aim at distributing air in wastewater as uniformly as possible. The air entering the aeration elements normally escapes on the surface of the elements through a kind of 'membrane'. The material and the geometrical form of the membranes vary widely: Porous, solid materials. The air is released from such aeration elements via labyrinth-type ramifications of a porous material (plastic-coated quartz sand with grain diameters between 0.3 and 0.6 mm, sintered metals or plastics, ceramics). The construction of the aeration systems varies greatly; on the one hand, because round plates of porous material are sealed in a hollow body and, on the other hand, because there are constructions that consist entirely of porous material used in the form of pipes. The disadvanta• ge of these materials, however, is that water can enter the rigid porous structure with pore widths of 50 to 250 ~m during intermittent operation, which might result in clogged pores caused by suspended so• lids and dirt. In the worst case, this can prevent such systems from a new startup after a standstill and results in disruption of operation.
Elastic, perforated materials. In contrast to the porous materials, aeration elements consisting of these materials are often characterised by a circular, disc or key-shaped construction with an elastic membrane attached to the upper side. The materials used for the membrane include elastic materials made of rub• ber (EPDM), synthetic rubber and polyurethane, which are subsequently perforated by punching holes or cutting slits. The smooth surface and elasticity of these materials prevent deposition and clogging during intermittent operation or enable such effects to be eliminated to a high degree when the plant is started
Membrane aerators and stirring systems
331
Table 1: Design of stirrers and their application Sti rrer type
Schematic
Propeller stirrers (slow-sti rri ng)
Homogenization,
Hyperboloid stirrers
A , "
C-·,
Surface aerators
"-
U
Helical aerators
n
cation
Homogenization, Biological phopsphate Suspension, Dis- elimination, denitrifipersion, Heat cation, nitrification, intermittent processes, sludge treatment, combined with aerator for nitrification
Suspension, Disper- Nitrification
sion
A
!L ~
U ""
phosphate denitrifi-
\J
~
Digested sludge mixers
Biological
Dis- elimination,
r-,
LJ
~
Homogenization,
Suspension, persion
transfer
C_-,
=
tanks,
(higher-viscosity fluids)
~\C /
Storm water sludge tanks
Suspension
ciT r-.:.;J·-h
Submerged aerators
Homogenization,
I~
Applications biological phosphate elimination, denitrification, combined with aerator for nitrification
Suspension
r"
Propeller stirrers (fast-stirring)
Blade stirrer
Stirring task(s)
Suspension, Disper- Nitrification, treatment
sludge
Suspension, Disper- Nitrification, treatment sion
sludge
sion
Homogenization
Sludge digesters
'7"
( )
Note: The stirring tasks marked in bold type should preferably be performed using the stirring system assigned to them in the above table. In rare cases this may be inconsistent with normal appli• cations in sewage treatment
up again. Here, the type of material and perforation has a decisive effect on the characteristics of the aerator and on the bubble pattern produced. The right choice often helps to prevent water entering the aerator unit. If a conically shaped hole in a perforated elastic membrane arches up during operation, the hole diameter extends on the upper side and the gas is free to escape. If the gas flow is interrupted, the column of water presses on the membrane from above, which then closes the orifice again. With the commonly used type of perforation in the form ofs lits, the sealing effect is achieved by the cutting edges produced when making the holes forming additional lip seals. A detailed summary of the aeration sy• stems commonly used can be found under Popel and Wagner (1991), and Bischof (1994).
M. W. A. H0FKEN et al.
332
DESIGN OF STIRRING AND AERATION SYSTEMS FOR WWTP'S Design of stirring systems Denitrification and biological phosphate elimination, which need stirring are anaerobic processes, i.e. oxygen-transfer capacity via the free surface is undesirable and disrupts the biological processes. A cer• tain degree of turbulence promotes the work of bacteria since the flock remains limited in size and as a result of a larger surface area in addition with the occurrence of periodic stress conditions a larger amount of substrate is provided. The aim to distribute the activated sludge flock as evenly as possible and to avoid dead flow zones in the tank remains. These preliminary observations raise the question of the optimum flow conditions in such reactors. In order to prevent the activated sludge flock from settling on the tank bottom, it is necessary to produce the maximum possible flow rates together with a certain degree of turbulence. For this reason the energy intake should take place preferably at the bottom of the tank. Taking the macro conditions of the flow into consideration it is sufficient here to stir the tank vol• ume uniformly in order to achieve even distribution; too much turbulence in this case would have a negative effect on the efficiency. It is especially important to avoid surface turbulence, since this would increase oxygen-transfer via the surface. One way to compare the various methods of performing the stirring task at hand - suspension in an acti• vated sludge tank - is to examine the tanks according to their geometries. Since the majority of wwtp's are built with circular or rectangular tanks, the flow in this type of tank was examined in more detail by H6fken (1993). It has been deduced that the most favourable energy conditions are obtained for sus• pension using tank bottom stirring systems arranged centrally in the tank. Clarification is needed on the question of the minimum flow velocity required at the bottom of the tank. A practical method for calcu• lating the minimum tank bottom velocities needs to consider initially the critical point for deposits in the reactor. It can be shown that the critical area is around the edge of the tank at the point where the hori• zontal and vertical flows meet (H6fken, 1994). Here the bottom boundary layer reaches its full length and normally increases to such an extent that a greater number of particles can penetrate the viscous sublayer due to the zones of lower flow rates present close to the wall. It is also necessary to create the equivalent of the weight of the particles in addition to matching their roll resistance, resulting from the flow forces acting on the particles. The full method for calculating minimum bottom velocities is Minimum Bottom Velocity [m/s] 0,2
0,15
0,1
0,05
~~ ....
~-
---
~
-
~ d"60E-6m •
o
Dichle
o
2
3
4
5
6
~ d"200E-6m
1. 50kg/cbrr
7
d-l00E-6m
8
Developing Length of Boundary Layer [m]
Fig. 1: Minimum bottom velocity as a function of tank size
9
10
333
Membrane aerators and stirring systems
described in detail by Hofken (1994). Figure 1 shows the minimum bottom velocities forvarious particle diameters as a function of length 1 of boundary layer=f (tank size, method used to create the flow) for a typical reactor. For centrally mounted stirrers the length I (horizontal axis) stands for half the diameter or length of the reactor. It can be seen that bottom flow velocities of 15 cm/s are adequate enough to keep activated sludge suspended and to stir it up. The recommended design particle size is d p = 100 Il m (middle curve) and the real value for d p=60 Ilm as depicted in the bottom curve. Normally bottom ve• locities of 10cm/s are therefore also sufficient. This statement does not contradict the recommendation of the ATVl to maintain bottom velocities of 10-30 cm/s, but clearly shows that a demand for minimum bottom velocities greater than 30 cm/s is unjustifiable. Even maximum particle sizes do not demand bottom velocities greater than 20 cm/s. Taking possible density fluctuations into account, the application of bottom velocities greater than 15 cm/s is therefore fully adequate. The considerations also show that it is not recommendable to apply particular power inputs, since these cannot guarantee particular bot• tom velocities. The power input required dependens on the stirring system and on the type of flow gen• eration as described above. Following the above considerations one stirring system shows unique advantages over the others given ir table 1: the hyperboloid stirring and aeration system, better known as HYPOCLASSIC®stirring and aera• tion system. This system fulfills all demands on a stirring system in an ideal way: Through its special sha• pe, a flow is induced that remains attached to the upper surface of the stirrer body. Hence, separation of the flow and the resulting energy losses are avoided. Referring to figure 2, the transport ribs located on the upper side of the stirrer body serve to transport fluid away from the stirrer and to thoroughly circulate the reactor contents. The tangential flow resulting from the stirrer rotation generates a sufficiently high turbulence intensity so that, for example, biological organisms can be provided with sufficient substrate but are not damaged. This process can be controlled by the stirrer speed. At the same time, only gentle movement of the fluid at the surface of the tank occurs, whereby the additional input of air during anaerobic processes and the emission of aerosols are avoided (figure 2). air pipe
shaft
Hyperboloid stirring and aeration system micro vortices transport ribs
shear ribs
Fig. 2: HYPOCLASSIC®stirring and aeration system For intensive mixing or oxygenation purposes, the rotational speed can be increased above that required for stirring. This can be accomplished, for example, with a two-speed., g~ared mo~or or, for ~etter con• trollability, with a frequency oscillator. Furthermore, air, other gases, liqUids,. or solids can be Introduced into the system through an additional pipe beneath the stirrer b~dy. The deSign ~f the outlet depends ~n the gas or liquid which is to be introduced. The shear ribs (see figure 2) on t~e. CIrcumference of t~e stir• rer body can produce small gas bubbles or induce intensive microscale mixing between two different
334
M. W. A. HOFKEN et a/.
phases. The macromotion induced by the transport ribs distributes the bubbles or the mixed ~olume throughout the entire reactor as shown in figure 3 and a high mass transfer rates between .the different phases results. This are the reasons why the hyperboloid stirring and aeration system is the Ideal system for use in anaerobic and anoxic reactors. Since it is bottom mounted also different water levels do not effect the different operation modes.
Fig. 3: HYPOCLASSIC®stirring and aeration system in aeration mode
Design of Aeration systems Independent of the different technical designs, an assessment of the 'quality' or performance capacity of aeration systems in use is generally based on the yield, i.e. the quotient of oxygen quantity supplied and the herefore required energy. The oxygen quantity supplied depends on several parameters: the range of bubble sizes generated, the oxygen content in the bubble during generation, the oxygen content in the water, the content of other gases (e.g. CO 2 ) in the water, water depth and temperature, and the extent to which the water is contaminated with surfactants. PE, the energy supplied to the aeration system per unit of time can be divided into an amount required for generation of bubbles and, if necessary, for stirring of the water:
1
PE
with 11 K the efficiency of the blower,
Vtot
.
= - . V,ot 11K
. !1Ptot
+ Pst
the volume flow of air and the total pressure loss
(1) !1Ptot
being in
practice a sum of several portions:
!1p tot i.e. the pressure loss in the air supply pipe
= !1p pipe + !1p hyd + !1p ex
!1P pipe'
(2)
the pressure loss determined by the water depth in the
tank L'lPhyd and a pressure loss component occurring as the excess pressure L'lPex in the actual aeration
335
Membrane aerators and stirring systems
devic~. This shows that, with regard to the oxygen transfer efficiency applied for quality assessment, aeration systems can only differ in their range of bubble size generated and in their expenditure of ener• gy for bubble generation. This facilitates uniform theoretical considerations applicable to all aeration systems used in biological wastewater treatment. As it is shown by Bischof (1994), the technical differences of existing aeration sy• stems are irrelevant to theoretical considerations to determine the oxygen yield; only the range of bubble sizes generated by the aeration system and the energy used for bubble generation are of interest. If these parameters are given, the oxygen transfer efficiency of the system can be calculated and compared to the theoretical maximum value. This comparison allows a statement on the quality of the aeration system in question by simulating to what extent the technically determined transfer efficiency differs from the maximum possible value. These considerations are summarized in Bischof et a!. (1994). It was shown that: •
Maximum oxygen input is achieved with gas bubbles of 1.6 mm to 1.9 mm. Hence, this size range can be defined as effective aeration with fine bubbles. With regard to the energy for bubble production, it can be said that, within this range, the preferable size is 1.6 mm. If we ex• tend the above range to achieve greater bubble size distribution, we can see that generally very high input values between 1.3 mm and 2.3 mm (see figure 4) can be achieved.
_ ~
2. 0
Minimum Gas Input Maximum Gas Input
3. 0
4.00
Bubble diameter [mm]
~.oo
Fig. 4: Oxygen input with gas bubbles of uniform size {Bischof et a!.
(1994)
According to this theoretical considerations and taking fluid mechanical and mechanical aspects into account, a novel membrane aeration system which is based on silicone membranes has been developed by INVENT. This so-called SILIKOFLEX® aeration system offers unique characteristics concerning oxygen transfer and yield alongside with outstanding material characteristics. Due to the design of the module, which consists of several porous, free swinging silicone hoses bubbles with optimum bubble sizes in the range of 1.3 and 2.3mm are produced. The achievable bubble separation frequencies are higher than those of conventional rigid aerators and coalescence of the bubbles is prevented to a very high degree due to the gentle oszillation of the hoses. The porous silicone-membrane which is produced without any additives like plastificizers and further filler materials known from conventional EPDM-membranes was developed for heavy-duty and guarantees the excellent characteristics over the whole life-time. Figure 5 shows the design of the SILIKOFLEX®-membrane aeration system schematically. Both ends of the sili• cone-based aeration pipes are attached to air distributors, which are mounted to the bottom of the ba• sin. A SILIKOFLEX®-membrane aeration system consists of several individual pipes. To prevent excessive
336
M. W. A. HOFKEN ef al.
ball valve ~
'" ~I-~L-----------------~'-----i 0..
air distributor
quick joint
flexible pipe
air distributor with air supply
clamping bar
Fig. 5: Design of SILIKOFLEX®-membrane aeration system curving up of the aeration pipes, the latter are held down by clamping bars. The air supply via down• pipes can be turned off for each aeration pipe separately. If the aeration pipes are charged with air in this way, they curve up and, at low opening pressure, the pores open and the air escapes in small bub• bles as shown in figure 6.
Fig. 6: SILIKOFLEX®-membrane aeration system in operation
-Membrane aerators and stirring systems
337
The upward flow that develops between the aeration pipes causes an oscillating movement which results in higher bubble separation frequencies and prevents depositions forming under the aeration modules. When not in use, the pipes collapse and remain water-tight. Intermittent operation as needed in discon• tinous processes like the SBR-process is therefore possible without any problems. Looking at the application of stirring and aeration systems such as the hyperboloid-stirring and aeration system and the SILIKOFLEX®-membrane aeration system in wwtp's two general recommendations can be given: 1. In anaerobic and anoxic reactors, the HYPOCLASSIC® stirring system or the ®stirring and aeration system should be used if additional intermittent aeration is needed. It is easy to install and to operate. The investment costs are affordable also for small budgets. During the stirring mode (denitrification) the power consumption of the HYPOCLASSIC®-stirrer is approx. 1-2 W/m 3 in the aeration mode (nitrification) oxygen yields of 2.0 to 2.9 kg0 2/kWh can be achieved depending on oxygen demand and reactor geometry (Hofken et a/., (1995). 2. For aeraobic reactors and if operational costs shall be optimised, the SILIKOFLEX®-membrane aera• tion system is the favourable solution because of the higher achievable oxygen yields of the membra• ne aeration system (between 4 to 6 kg0 2/kWh). In some cases oxygen yields up to 7 kg0 2/kWh were measured in pure water (Schafer and Hofken, 1996).
CONCLUSIONS To achieve optimum purification power in wwtp's, advanced and efficient stirring and aeration systems have to be used because stirring and aeration are the major processes needed in the biological treat• ment of wastewater. Approximately 70% of the total energy demand of a wwtp is needed for these processes. A study of systems avaible on the market showed that numerous systems are offered which can be classi• fied and compared using some simple considerations. It was shown that efficient stirring systems should be bottom mounted and fluid mechanically optimised. Minimum bottom velocities of approx. 15 cm/s are sufficient in most of the applications. The power demand of the best systems available was reduced to approx. 1 to 2 W/m 3 . Generally there are two types of aeration systems available: porous, solid materials and elastic, perfora• ted materials. Solid materials are not recommendable for wwtp's using discontinous processes because they cannot be shut down without being flooded with water. Intermittent operation is therefore not pos• sible. For optimum oxygen yield, bubble sizes between 1.3 and 2.3 mm should be produced by the aeration system. It is most favourable to use advanced membrane materials e.g. silicone-membranes which are free of plasticizers and filler materials to keep high oxygenation efficiencies over the whole lifetime of the aeration system. Two recommendations for the use of stirring and aeration systems in wwtp's were given. For anaerobic and anoxic reactors a hyperboloid stirring system or a combined hyperboloid stirring and aeration sy• stem, respectively if intermittent aeration is needed are the best choice. For aerobic reactors membrane aeration using special silicone hoses is recommended.
338
M. W. A. HOFKEN et al.
REFERENCES
Bischof, F. (1994). Untersuchung der Blasenbildung und des Stoffaustausches unter dem EinfluB ober. flachenaktiver Substanzen und geloster Gase. Ph.D.-thesis completed at the Lehrstuhl fur Stromungsmechanik (LSTM) of the Friedrich-Alexander-University Erlangen-Nurnberg. Bischof, F., Durst, F.,Hofken, M. and Sommerfeld, M. (1994). Theoretical Considerations about the De· velopment of Efficient Aeration Systems for Activated Sludge Treatment. Aeration Technology, FED. Vol. 187, ASME Summer Annual Meeting, Lake Tahoe, USA. Hofken, M. (1993). Die Bedeutung der ROhrtechnik in der Abwasserreinigung. Abwassereinigung Alte
Probleme - Neue Losungen, LSTM-Seminar 23./24. June 1993 in Sulzbach-Rosenberg, Oberpfolz, Germany.
Hofken, M. (1994). Moderne experimentelle Methoden for die Untersuchung von Stromungen in ROhr• behaltern und fOr ROhrwerksoptimierungen. Ph.D.-thesis completed at the Lehrstuhl fur Stromungs• mechanik (LSTM) of the Friedrich-Alexander-University Erlangen-Nurnberg. Hofken, M., Bischof, F. and Durst, F. (1995). Energy Savings in the Biological Treatment of Sewage. The Future of the Baltic Sea, Metropolis Verlag. Irvine, R. L. and Davis, W.B. (1971). Proc. 26th. Ann. Ind. Waste Conf., Purdue Univ., Ann Arbor Sc.,
Michigan.
Metcalf and Eddy (1991). Wastewater Engineering - Treatment, Disposal and Reuse. McGraw Hill Inc. Popel, H. J. and Wagner, M. (1991). Grundlagen von BelOftung und Sauerstoffeintrag. Schriftenreihe WAR, Vol. 54, BelUftungssysteme in der Abwassertechnik, Darmstadt, Germany. Schafer, M. and Hofken, M. (1996). Internal Report. INVENT Umwelt- und Verfahrenstechnik GmbH & Co.KG, Erlangen, Germany. Popel, H. J. and Wagner, M. (1996). Sauerstoffzufuhrvermogen und Sauerstoffertrag des SILIKOFlEX®• Begasungssystems. Expert Opinion, Technical University of Darmstadt, Germany.
HYPOCLASSIC® and SILIKOFLEX® are registered trademarks of INVENT Umwelt- und Verfahrenstechnik GmbH & Co.KG.
1-----------------------------
_
ATV: German Association for Water and Pollution Control
Pergamon
Waf. Sci. Tech. Vol. 34, No. 3-4, pp. 329-338,1996. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + (}oo
PH: 50273-1223(96)00590-2
MEMBRANE AERATORS AND STIRRING SYSTEMS FOR THE OPERATION IN LARGE AND SMALL WASTEWATER TREATMENT PLANTS Marcus W. A. Hofken, Peter Huber, Marcus Schafer and Ralf Steiner INVENT Umwelt- und Verfahrenstechnik GmbH & Co. KG, Am Weichselgarten 7, 91058 Erlangen, Germany
ABSTRACT Stirring and aeration processes are the major operations needed for effective wastewater treatment. Es• pecially in activated sludge processes and the variants of this process stirring and aeration systems need to be chosen carefully since both operations are crucial for the purification power and the energy de• mand of wwfp's. The aim of the present work is to define the demands on stirring and aeration systems for activated sludge treatment plants, to give an overview about suitable systems currently available on the market including latest developments and to give advice how to choose the optimum system. A brief review of the literature and the market of stirring and aeration systems leads to a simple classifi• cation of the existing numerous systems. In order to choose the optimum system, first one has to decide, about the demands in each process step. In anaerobic and anoxic reactors the use of hyperboloid stir• ring systems or combined hyperboloid stirring and aeration systems which distribute small air bubbles throughout the entire reactor in the aeration mode (for intermittent aeration) and gently circulate bacte• ria flocks in the stirring mode is recommended. For aerobic processes like BOD-removal and nitrification highly efficient membrane aerating systems using silicone membranes are recommended. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Activated Sludge Process; Aeration; Mixing; Stirring
INTRODUCTION The activated sludge process stands for variety of continous, space and time-oriented periodic processes. The activated sludge process can be used for BOD-removal or complete nitrogen removal. Well-known variants are the preliminary nitrification/denitrification, the post-denitrification or the alternating nitrifica• tion and denitrification. Well known batch processes are e.g. the Sequencing Batch Activated Sludge Reactor (SBASR), the Cyclic Activated Sludge System (CASS) and the Intermittent Cycle Extended Aeration System (ICEAS) which are covered in detail in the literature (see e.g. Irvine, 1971; Wilderer and Schroe329
330
M. W. A. HOFKEN et al.
der, 1986 or Metcalf and Eddy, 1991). All these biological purification processes as well as the k~o.wn standard activated sludge process include two major operations: stirring and aeration. Without stIrring and aeration an effective reduction of BOD and Nitrogen is impossible. Since approximately 70% of the total energy consumption is used for these two processes (Hofken et a/., 1995) a proper choice and de• sign of such systems is indispensible. Hence, only innovative and energy-efficient equipment should be used.
STATE OF THE ART Stirring Systems A review of the stirring systems currently available on the market for wastewater treatment will reveal an impressive number of different products (Hofken, 1993). In order to somewhat classify the large variety it is possible to refer to the definitions of the basic stirring tasks as they are used in the fields of chemical and process engineering. A distinction is made in process engineering between the following basic stirring tasks:
• Homogenization • Suspension • Dispersion • Heat transfer
Compensation of differences in concentration or temperature Whirling up and suspending of. solid particles Liquid/liquid ~ Emulsions, polymerizations Liquid/gas ~ Aeration, mass transfer Intensifying of the heat transmission (cooling, heating)
This breakdown leads to a classification of all the stirring systems available on the market today that are used for wastewater treatment and enables an example of application in wastewater treatment to be found without difficulty for each stirring task shown in table 1.
Aeration Systems In the field of biological wastewater treatment, aeration systems in various technical designs are used to supply wastewater with the oxygen required for decomposition of harmful substances. All pressure aero· tion systems available on the market teday aim at distributing air in wastewater as uniformly as possible. The air entering the aeration elements normally escapes on the surface of the elements through a kind of 'membrane'. The material and the geometrical form of the membranes vary widely: Porous, solid materials. The air is released from such aeration elements via labyrinth-type ramifications of a porous material (plastic-coated quartz sand with grain diameters between 0.3 and 0.6 mm, sintered metals or plastics, ceramics). The construction of the aeration systems varies greatly; on the one hand, because round plates of porous material are sealed in a hollow body and, on the other hand, because there are constructions that consist entirely of porous material used in the form of pipes. The disadvanta• ge of these materials, however, is that water can enter the rigid porous structure with pore widths of 50 to 250 ~m during intermittent operation, which might result in clogged pores caused by suspended so• lids and dirt. In the worst case, this can prevent such systems from a new startup after a standstill and results in disruption of operation.
Elastic, perforated materials. In contrast to the porous materials, aeration elements consisting of these materials are often characterised by a circular, disc or key-shaped construction with an elastic membrane attached to the upper side. The materials used for the membrane include elastic materials made of rub• ber (EPDM), synthetic rubber and polyurethane, which are subsequently perforated by punching holes or cutting slits. The smooth surface and elasticity of these materials prevent deposition and clogging during intermittent operation or enable such effects to be eliminated to a high degree when the plant is started
Membrane aerators and stirring systems
331
Table 1: Design of stirrers and their application Sti rrer type
Schematic
Propeller stirrers (slow-sti rri ng)
Homogenization,
Hyperboloid stirrers
A , "
C-·,
Surface aerators
"-
U
Helical aerators
n
cation
Homogenization, Biological phopsphate Suspension, Dis- elimination, denitrifipersion, Heat cation, nitrification, intermittent processes, sludge treatment, combined with aerator for nitrification
Suspension, Disper- Nitrification
sion
A
!L ~
U ""
phosphate denitrifi-
\J
~
Digested sludge mixers
Biological
Dis- elimination,
r-,
LJ
~
Homogenization,
Suspension, persion
transfer
C_-,
=
tanks,
(higher-viscosity fluids)
~\C /
Storm water sludge tanks
Suspension
ciT r-.:.;J·-h
Submerged aerators
Homogenization,
I~
Applications biological phosphate elimination, denitrification, combined with aerator for nitrification
Suspension
r"
Propeller stirrers (fast-stirring)
Blade stirrer
Stirring task(s)
Suspension, Disper- Nitrification, treatment
sludge
Suspension, Disper- Nitrification, treatment sion
sludge
sion
Homogenization
Sludge digesters
'7"
( )
Note: The stirring tasks marked in bold type should preferably be performed using the stirring system assigned to them in the above table. In rare cases this may be inconsistent with normal appli• cations in sewage treatment
up again. Here, the type of material and perforation has a decisive effect on the characteristics of the aerator and on the bubble pattern produced. The right choice often helps to prevent water entering the aerator unit. If a conically shaped hole in a perforated elastic membrane arches up during operation, the hole diameter extends on the upper side and the gas is free to escape. If the gas flow is interrupted, the column of water presses on the membrane from above, which then closes the orifice again. With the commonly used type of perforation in the form ofs lits, the sealing effect is achieved by the cutting edges produced when making the holes forming additional lip seals. A detailed summary of the aeration sy• stems commonly used can be found under Popel and Wagner (1991), and Bischof (1994).
M. W. A. H0FKEN et al.
332
DESIGN OF STIRRING AND AERATION SYSTEMS FOR WWTP'S Design of stirring systems Denitrification and biological phosphate elimination, which need stirring are anaerobic processes, i.e. oxygen-transfer capacity via the free surface is undesirable and disrupts the biological processes. A cer• tain degree of turbulence promotes the work of bacteria since the flock remains limited in size and as a result of a larger surface area in addition with the occurrence of periodic stress conditions a larger amount of substrate is provided. The aim to distribute the activated sludge flock as evenly as possible and to avoid dead flow zones in the tank remains. These preliminary observations raise the question of the optimum flow conditions in such reactors. In order to prevent the activated sludge flock from settling on the tank bottom, it is necessary to produce the maximum possible flow rates together with a certain degree of turbulence. For this reason the energy intake should take place preferably at the bottom of the tank. Taking the macro conditions of the flow into consideration it is sufficient here to stir the tank vol• ume uniformly in order to achieve even distribution; too much turbulence in this case would have a negative effect on the efficiency. It is especially important to avoid surface turbulence, since this would increase oxygen-transfer via the surface. One way to compare the various methods of performing the stirring task at hand - suspension in an acti• vated sludge tank - is to examine the tanks according to their geometries. Since the majority of wwtp's are built with circular or rectangular tanks, the flow in this type of tank was examined in more detail by H6fken (1993). It has been deduced that the most favourable energy conditions are obtained for sus• pension using tank bottom stirring systems arranged centrally in the tank. Clarification is needed on the question of the minimum flow velocity required at the bottom of the tank. A practical method for calcu• lating the minimum tank bottom velocities needs to consider initially the critical point for deposits in the reactor. It can be shown that the critical area is around the edge of the tank at the point where the hori• zontal and vertical flows meet (H6fken, 1994). Here the bottom boundary layer reaches its full length and normally increases to such an extent that a greater number of particles can penetrate the viscous sublayer due to the zones of lower flow rates present close to the wall. It is also necessary to create the equivalent of the weight of the particles in addition to matching their roll resistance, resulting from the flow forces acting on the particles. The full method for calculating minimum bottom velocities is Minimum Bottom Velocity [m/s] 0,2
0,15
0,1
0,05
~~ ....
~-
---
~
-
~ d"60E-6m •
o
Dichle
o
2
3
4
5
6
~ d"200E-6m
1. 50kg/cbrr
7
d-l00E-6m
8
Developing Length of Boundary Layer [m]
Fig. 1: Minimum bottom velocity as a function of tank size
9
10
333
Membrane aerators and stirring systems
described in detail by Hofken (1994). Figure 1 shows the minimum bottom velocities forvarious particle diameters as a function of length 1 of boundary layer=f (tank size, method used to create the flow) for a typical reactor. For centrally mounted stirrers the length I (horizontal axis) stands for half the diameter or length of the reactor. It can be seen that bottom flow velocities of 15 cm/s are adequate enough to keep activated sludge suspended and to stir it up. The recommended design particle size is d p = 100 Il m (middle curve) and the real value for d p=60 Ilm as depicted in the bottom curve. Normally bottom ve• locities of 10cm/s are therefore also sufficient. This statement does not contradict the recommendation of the ATVl to maintain bottom velocities of 10-30 cm/s, but clearly shows that a demand for minimum bottom velocities greater than 30 cm/s is unjustifiable. Even maximum particle sizes do not demand bottom velocities greater than 20 cm/s. Taking possible density fluctuations into account, the application of bottom velocities greater than 15 cm/s is therefore fully adequate. The considerations also show that it is not recommendable to apply particular power inputs, since these cannot guarantee particular bot• tom velocities. The power input required dependens on the stirring system and on the type of flow gen• eration as described above. Following the above considerations one stirring system shows unique advantages over the others given ir table 1: the hyperboloid stirring and aeration system, better known as HYPOCLASSIC®stirring and aera• tion system. This system fulfills all demands on a stirring system in an ideal way: Through its special sha• pe, a flow is induced that remains attached to the upper surface of the stirrer body. Hence, separation of the flow and the resulting energy losses are avoided. Referring to figure 2, the transport ribs located on the upper side of the stirrer body serve to transport fluid away from the stirrer and to thoroughly circulate the reactor contents. The tangential flow resulting from the stirrer rotation generates a sufficiently high turbulence intensity so that, for example, biological organisms can be provided with sufficient substrate but are not damaged. This process can be controlled by the stirrer speed. At the same time, only gentle movement of the fluid at the surface of the tank occurs, whereby the additional input of air during anaerobic processes and the emission of aerosols are avoided (figure 2). air pipe
shaft
Hyperboloid stirring and aeration system micro vortices transport ribs
shear ribs
Fig. 2: HYPOCLASSIC®stirring and aeration system For intensive mixing or oxygenation purposes, the rotational speed can be increased above that required for stirring. This can be accomplished, for example, with a two-speed., g~ared mo~or or, for ~etter con• trollability, with a frequency oscillator. Furthermore, air, other gases, liqUids,. or solids can be Introduced into the system through an additional pipe beneath the stirrer b~dy. The deSign ~f the outlet depends ~n the gas or liquid which is to be introduced. The shear ribs (see figure 2) on t~e. CIrcumference of t~e stir• rer body can produce small gas bubbles or induce intensive microscale mixing between two different
334
M. W. A. HOFKEN et a/.
phases. The macromotion induced by the transport ribs distributes the bubbles or the mixed ~olume throughout the entire reactor as shown in figure 3 and a high mass transfer rates between .the different phases results. This are the reasons why the hyperboloid stirring and aeration system is the Ideal system for use in anaerobic and anoxic reactors. Since it is bottom mounted also different water levels do not effect the different operation modes.
Fig. 3: HYPOCLASSIC®stirring and aeration system in aeration mode
Design of Aeration systems Independent of the different technical designs, an assessment of the 'quality' or performance capacity of aeration systems in use is generally based on the yield, i.e. the quotient of oxygen quantity supplied and the herefore required energy. The oxygen quantity supplied depends on several parameters: the range of bubble sizes generated, the oxygen content in the bubble during generation, the oxygen content in the water, the content of other gases (e.g. CO 2 ) in the water, water depth and temperature, and the extent to which the water is contaminated with surfactants. PE, the energy supplied to the aeration system per unit of time can be divided into an amount required for generation of bubbles and, if necessary, for stirring of the water:
1
PE
with 11 K the efficiency of the blower,
Vtot
.
= - . V,ot 11K
. !1Ptot
+ Pst
the volume flow of air and the total pressure loss
(1) !1Ptot
being in
practice a sum of several portions:
!1p tot i.e. the pressure loss in the air supply pipe
= !1p pipe + !1p hyd + !1p ex
!1P pipe'
(2)
the pressure loss determined by the water depth in the
tank L'lPhyd and a pressure loss component occurring as the excess pressure L'lPex in the actual aeration
335
Membrane aerators and stirring systems
devic~. This shows that, with regard to the oxygen transfer efficiency applied for quality assessment, aeration systems can only differ in their range of bubble size generated and in their expenditure of ener• gy for bubble generation. This facilitates uniform theoretical considerations applicable to all aeration systems used in biological wastewater treatment. As it is shown by Bischof (1994), the technical differences of existing aeration sy• stems are irrelevant to theoretical considerations to determine the oxygen yield; only the range of bubble sizes generated by the aeration system and the energy used for bubble generation are of interest. If these parameters are given, the oxygen transfer efficiency of the system can be calculated and compared to the theoretical maximum value. This comparison allows a statement on the quality of the aeration system in question by simulating to what extent the technically determined transfer efficiency differs from the maximum possible value. These considerations are summarized in Bischof et a!. (1994). It was shown that: •
Maximum oxygen input is achieved with gas bubbles of 1.6 mm to 1.9 mm. Hence, this size range can be defined as effective aeration with fine bubbles. With regard to the energy for bubble production, it can be said that, within this range, the preferable size is 1.6 mm. If we ex• tend the above range to achieve greater bubble size distribution, we can see that generally very high input values between 1.3 mm and 2.3 mm (see figure 4) can be achieved.
_ ~
2. 0
Minimum Gas Input Maximum Gas Input
3. 0
4.00
Bubble diameter [mm]
~.oo
Fig. 4: Oxygen input with gas bubbles of uniform size {Bischof et a!.
(1994)
According to this theoretical considerations and taking fluid mechanical and mechanical aspects into account, a novel membrane aeration system which is based on silicone membranes has been developed by INVENT. This so-called SILIKOFLEX® aeration system offers unique characteristics concerning oxygen transfer and yield alongside with outstanding material characteristics. Due to the design of the module, which consists of several porous, free swinging silicone hoses bubbles with optimum bubble sizes in the range of 1.3 and 2.3mm are produced. The achievable bubble separation frequencies are higher than those of conventional rigid aerators and coalescence of the bubbles is prevented to a very high degree due to the gentle oszillation of the hoses. The porous silicone-membrane which is produced without any additives like plastificizers and further filler materials known from conventional EPDM-membranes was developed for heavy-duty and guarantees the excellent characteristics over the whole life-time. Figure 5 shows the design of the SILIKOFLEX®-membrane aeration system schematically. Both ends of the sili• cone-based aeration pipes are attached to air distributors, which are mounted to the bottom of the ba• sin. A SILIKOFLEX®-membrane aeration system consists of several individual pipes. To prevent excessive
336
M. W. A. HOFKEN ef al.
ball valve ~
'" ~I-~L-----------------~'-----i 0..
air distributor
quick joint
flexible pipe
air distributor with air supply
clamping bar
Fig. 5: Design of SILIKOFLEX®-membrane aeration system curving up of the aeration pipes, the latter are held down by clamping bars. The air supply via down• pipes can be turned off for each aeration pipe separately. If the aeration pipes are charged with air in this way, they curve up and, at low opening pressure, the pores open and the air escapes in small bub• bles as shown in figure 6.
Fig. 6: SILIKOFLEX®-membrane aeration system in operation
-Membrane aerators and stirring systems
337
The upward flow that develops between the aeration pipes causes an oscillating movement which results in higher bubble separation frequencies and prevents depositions forming under the aeration modules. When not in use, the pipes collapse and remain water-tight. Intermittent operation as needed in discon• tinous processes like the SBR-process is therefore possible without any problems. Looking at the application of stirring and aeration systems such as the hyperboloid-stirring and aeration system and the SILIKOFLEX®-membrane aeration system in wwtp's two general recommendations can be given: 1. In anaerobic and anoxic reactors, the HYPOCLASSIC® stirring system or the ®stirring and aeration system should be used if additional intermittent aeration is needed. It is easy to install and to operate. The investment costs are affordable also for small budgets. During the stirring mode (denitrification) the power consumption of the HYPOCLASSIC®-stirrer is approx. 1-2 W/m 3 in the aeration mode (nitrification) oxygen yields of 2.0 to 2.9 kg0 2/kWh can be achieved depending on oxygen demand and reactor geometry (Hofken et a/., (1995). 2. For aeraobic reactors and if operational costs shall be optimised, the SILIKOFLEX®-membrane aera• tion system is the favourable solution because of the higher achievable oxygen yields of the membra• ne aeration system (between 4 to 6 kg0 2/kWh). In some cases oxygen yields up to 7 kg0 2/kWh were measured in pure water (Schafer and Hofken, 1996).
CONCLUSIONS To achieve optimum purification power in wwtp's, advanced and efficient stirring and aeration systems have to be used because stirring and aeration are the major processes needed in the biological treat• ment of wastewater. Approximately 70% of the total energy demand of a wwtp is needed for these processes. A study of systems avaible on the market showed that numerous systems are offered which can be classi• fied and compared using some simple considerations. It was shown that efficient stirring systems should be bottom mounted and fluid mechanically optimised. Minimum bottom velocities of approx. 15 cm/s are sufficient in most of the applications. The power demand of the best systems available was reduced to approx. 1 to 2 W/m 3 . Generally there are two types of aeration systems available: porous, solid materials and elastic, perfora• ted materials. Solid materials are not recommendable for wwtp's using discontinous processes because they cannot be shut down without being flooded with water. Intermittent operation is therefore not pos• sible. For optimum oxygen yield, bubble sizes between 1.3 and 2.3 mm should be produced by the aeration system. It is most favourable to use advanced membrane materials e.g. silicone-membranes which are free of plasticizers and filler materials to keep high oxygenation efficiencies over the whole lifetime of the aeration system. Two recommendations for the use of stirring and aeration systems in wwtp's were given. For anaerobic and anoxic reactors a hyperboloid stirring system or a combined hyperboloid stirring and aeration sy• stem, respectively if intermittent aeration is needed are the best choice. For aerobic reactors membrane aeration using special silicone hoses is recommended.
338
M. W. A. HOFKEN et al.
REFERENCES
Bischof, F. (1994). Untersuchung der Blasenbildung und des Stoffaustausches unter dem EinfluB ober. flachenaktiver Substanzen und geloster Gase. Ph.D.-thesis completed at the Lehrstuhl fur Stromungsmechanik (LSTM) of the Friedrich-Alexander-University Erlangen-Nurnberg. Bischof, F., Durst, F.,Hofken, M. and Sommerfeld, M. (1994). Theoretical Considerations about the De· velopment of Efficient Aeration Systems for Activated Sludge Treatment. Aeration Technology, FED. Vol. 187, ASME Summer Annual Meeting, Lake Tahoe, USA. Hofken, M. (1993). Die Bedeutung der ROhrtechnik in der Abwasserreinigung. Abwassereinigung Alte
Probleme - Neue Losungen, LSTM-Seminar 23./24. June 1993 in Sulzbach-Rosenberg, Oberpfolz, Germany.
Hofken, M. (1994). Moderne experimentelle Methoden for die Untersuchung von Stromungen in ROhr• behaltern und fOr ROhrwerksoptimierungen. Ph.D.-thesis completed at the Lehrstuhl fur Stromungs• mechanik (LSTM) of the Friedrich-Alexander-University Erlangen-Nurnberg. Hofken, M., Bischof, F. and Durst, F. (1995). Energy Savings in the Biological Treatment of Sewage. The Future of the Baltic Sea, Metropolis Verlag. Irvine, R. L. and Davis, W.B. (1971). Proc. 26th. Ann. Ind. Waste Conf., Purdue Univ., Ann Arbor Sc.,
Michigan.
Metcalf and Eddy (1991). Wastewater Engineering - Treatment, Disposal and Reuse. McGraw Hill Inc. Popel, H. J. and Wagner, M. (1991). Grundlagen von BelOftung und Sauerstoffeintrag. Schriftenreihe WAR, Vol. 54, BelUftungssysteme in der Abwassertechnik, Darmstadt, Germany. Schafer, M. and Hofken, M. (1996). Internal Report. INVENT Umwelt- und Verfahrenstechnik GmbH & Co.KG, Erlangen, Germany. Popel, H. J. and Wagner, M. (1996). Sauerstoffzufuhrvermogen und Sauerstoffertrag des SILIKOFlEX®• Begasungssystems. Expert Opinion, Technical University of Darmstadt, Germany.
HYPOCLASSIC® and SILIKOFLEX® are registered trademarks of INVENT Umwelt- und Verfahrenstechnik GmbH & Co.KG.
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ATV: German Association for Water and Pollution Control