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Integration of membrane filtration into the activated sludge process in municipal wastewater treatment
~
Wat. Sci. Tech. Vol. 38, No. 4-S, pp. 429-436.1998.
Pergamon
IAWQ C 1998 Published byElsevier Science Ltd.
Printed inGreat Britain. All rights reserved
PIT: S0273-1223(98)OO542-3
0273-1223/98 $19'00 + 0'00
INTEGRATION OF MEMBRANE FILTRATION INTO THE ACTIVATED SLUDGE PROCESS IN MUNICIPAL
WASTEWATER TREATMENT N. Engelhardt, W. Firk and W. Warnken Erft River Association, Paffendorfer Weg42. D-50/26 Bergheim, Germany
ABSTRACf Energy-efficient membrane modules for microfiltration are available to realize a new processengineering in municipal wastewater treatment The microfiltration membrane ensuresthat all microorganisms are retained in the aeration tank. A contentof mixed-liquor suspended solidsof e.g. IS gMLSSII can easily be achievedin a large scale plant. Thus the aeration tank is considerably reducedin size. A secondary clarifier is no longer needed. A filtration and a disinfection can be dismissed. A pilot plant gives first knowledge on the application of the activated sludge process with submersed membrane filtration. Based on the tests' results and the knowledge gainedduringthe operation of the pilotplant,a WWTPwithmembrane filtration for 3000 inhabitants is designed. The costs of investment and operation are estimated. ~ 1998 Published by Elsevier ScienceLtd. All rightsreserved
KEYWORDS Activated sludge process; disinfection; hollow fiber membrane; membrane filtration; microfiltration; submersed filtration.
REASON For the treatment of municipal wastewater normally the activated sludge process is used. In the aeration tanks conditions are created that stimulate the growth of various species of microorganisms. so that the pollutants are taken up and converted into cell material. The mixture of activated sludge and water is a suspension which subsequently is separated into cleaned wastewater and thickened activated sludge. As the following process step a secondary settling tank is commonly used. This downstream-placed settling tank is the determining or limiting factor for the operation of the activated sludge process because the settling processes in secondary treatment mainly depend on the biomass concentration in the aeration tank. The experiences gained during many years with the activated sludge process are compiled in the design principles of the ATV worksheet 13I. To achieve a safe separation of activated sludge and cleaned wastewater, the average biomass concentration in the activated sludge tank. measured as mixed-liquor suspended solids (MLSS), should be 3 - 4 g MLSSII and should not exceed 5 g MLSSII. At present light sludges from mineralization processes, bulking sludge. autolysis are undesired states of operation of an activated sludge plant. 429
N.ENGELHARDTrtm.
430
INTEGRATION OF MEMBRANE FILTERS INfO THE ACTIVATED SLUDGE PROCESS The above disadvantages of the process combination aeration tank-secondary clarifier are no longer of importance, if complete separation of the cleaned wastewater from the activated sludge is possible alternatively to sedimentation. Energy-efficient membrane modules for microfiltration with a pore size of less than 0.1 um have been developed, which are directlyinserted into the aeration tank.Thus a procedure is now available by which the activated sludge including all microorganisms can be retained completely in the aeration tank. Starting from these considerations the use of a membrane for the separation of biologically treated wastewater and activated sludgeresults in modified performance data for the activated sludgeprocess: Operation witha mixed-liquor suspended solidscontentof up to 30 g MLSSII is possible. The occurrence of autolysis and hydrolysis in low-loaded activated sludge processes, up to now undesired, no longer impairs the process control becauseof the independence of the sedimentation. Now these phenomena are even desired because at this operating point the production of excess sludgeis minimized. Due to the defined pore diameter the microfiltration membrane ensures that nitrobacteria and other microorganisms are retained in the aeration tank. Displacement of activated sludge into the secondary clarifieras a result of hydraulic peak loads and scouring of activated sludge into the effluentof the secondary clarifier are no longer possible. The problems with bulkingsludgeare also overcome. With a separation limitof e.g, 0.1 um the membrane retains all relevantparticles. Compared to the conventional technique of a wastewater treatment plant (WWfP), which includes the components screen, grit chamber, primary settling, aeration tank, secondary settling, filtration (if necessary) and disinfection, only the foIlowing parts are still necessary, if microfiltration is used to separate the activated sludge: screen witha slit widthof 2.0-4.0mm grit chamber aeration tank (reducedin size by the raisingfactorfor the raisedbiomass concentration) microfiltration units withsubmersed membranes. In Figure ) the process engineering scheme of conventional advanced wastewater treatment is compared with the combination of aerationtank and membrane filtration. It is striking that the components are rather small-sized, and the wholeprocessshowsless stages,so that a space-saving conception is possible. screen
I
grit chamber
aerationtank
secondarysettling
filtration
I
screen
grit chamber
I
7
Figure I. Comparison of conventional advanced wastewater treatment and the combination of activated sludge process/membrane filtTalion.
Integration of membrane filtration
431
Separation of solids by a membrane replaces the sedimentation process and thus forms the basis for the realization of the new process engineering. Microfiltration has already been used for a long time for special applications in industry. In non-European countries some municipal WWTP are also equipped with this technique. The systems "Kubota" (Japan) and "Zenon" (Canada) can be cited here by way of example. CHOICE OF THE MEMBRANE MODULES The membranes used for microfiltration have a pore size of c. 0.1 11m. To force the water through the membrane, a transmembrane pressure difference is necessary. The energy requirement for this process is a decisive criterion for the choice of the membrane and the construction of the modules. In municipal wastewater treatment it has to be ensured that the permeate flow through the membrane be as high and constant as possible, because the wastewater flows continuously into the WWTP. Thus the permeate flow through the membrane becomes the determining factor for the design of the WWTP. The pore diameter and the coating that develops during operation on the membrane surface are decisive for the necessary transmembrane pressure difference and, with this. for the energy demand. Operatin~ procedures of membrane filters
In microfiltration technique one distinguishes between two operating procedures : dynamic operation of "cross-flow filtration" and static operation of "dead-end filtration". In cross-flow filtration a flow (the so-called feed) is built up with high pressure in parallel to the membrane. The feed has to control and to limit the development of the coating on the membrane. and thus represents an important design feature. The adsorbed solids are washed away with the flow velocity of the feed by the shearing forces. At the same time the medium is forced through the membrane by the pressure to be applied of e.g . 6 bar (transmembrane pressure difference). From this results the high energy demand of the crossflow filtration process. It amounts to e.g. 7 kWhlm 3 permeate output with a flow velocity of 6 mls. Static operation or dead-end operation of a membrane has no feed. A negative pressure of 0.2-0.6 bar is built up at the membrane. The permeate is sucked through the membrane. The retained particles settle on the membrane. In order not to cause a break down of the permeate flow, the coating has to be removed from the membrane surface. The feed flow velocity controlled fouling prevention. known from cross-flow filtration. is replaced by other cleaning methods. Membrane fjlters wjth dead-end operatjon The two module systems relevant in wastewater treatment, constructed by the firms Kubota (Japan) and Zenon (Canada), are described in the following . The plate module of the firm Kubota consists of several filtration plates which are assembled to a module. The individual plates are made of a support ing plate with a membrane sheet fixed in front of it. The fabric in between leads the permeate. which has been sucked through the membrane. to the discharge manifold . Between the individual filtration plates of the module air is injected from below in medium-sized to coarse bubbles. The mixture of air and water rises and forms a flow on the vertical membrane surface, which is comparable to the feed in cross-flow filtration. A continuous cleaning effect is obtained. When the coating is irreversible, the whole module has to be removed and cleaned. This is normally done by spraying with pressurized water and chemicals. if necessary.
432
N. ENGELHARDT et aI.
Between these cleaning intervals the permeate flow is continuous. The negative pressure rises until removal and manual cleaning of the module become necessary. The classical dead-end cleaning method of backwashing (reverse of the permeate flow) is possible only to a rather limited extent. This is due to the special construction of the module (plate) with the membrane sheet fixed in front of the supporting plate. Cleaning is realized by the upward flow of the air-water mixture vertically to the membrane surface. To support the cleaning effect, filtration is interrupted periodically for a short time (Kraft, 1995). The construction of the Zeeweed module of the finn Zenon is completely different. It consists of hollow fiber capillary membranes. The membrane is a flexible tube which is stable in itself. It resembles a thin macaroni noodle the inner duct of which serves to discharge the permeate. Filtration is realized from the outside to the inside. Thus the external surface of the hollow fiber is responsible for the filtering effect. The single membranes, i.e, the "tubes", are fitted vertically between two blocks of synthetic resin. The hollow fibers are open at the top and at the bottom and combined at both places to a collecting main. The permeate is discharged by these mains at the top and at the bottom. The outer membrane surface is continuously cleaned by injecting air in coarse to medium-sized bubbles. The air main is fitted into the bottom of the cast block of the membranes. The rising flow of the air-water mixture produces a cleaning effect along the membrane surface. At the same time the threadlike hollow fiber membranes are set in motion so that they move like seaweed (= ZeeWeed) within the suspension and come into contact with each other. This leads to further mechanical cleaning effects. As a result of the flexible construction of the hollow fiber capillary membrane, which in parallel is stable in itself, backwashing with reversed permeate flow is possible. The rinsing process can be realized by feeding both ends of the hollow fiber membranes so that the filtration process is reversed, and the pores are rinsed from the inside to the outside. Moreover, a rinsing of the hollow fiber duct by exclusive feeding from the inferior end of the hol1ow fiber is provided. Rinsing with permeate is done at fixed intervals. With this the filtration process takes a quasi-continuous course. Backwashing of the single modules is possible in a staggered way. If necessary, the hol1ow fiber membranes can be cleaned chemically e.g. with hydrogen hypochlorite. The solution of chemicals is added to the backwash water.
SHORTDESCRIPTION OF THE Pn.,OT PLANT To study the relevant operational parameters for the membrane filtration and for the appropriate design of the activated sludge process, the Erft River Association has run a pilot plant on semi-industrial scale, which was situated at the site of a small municipal WWTP of the association. The main component of the pilot plant was an aerated bioreactor with two integrated membrane filtration modules. It was completed by an upstream-placed tank for denitrification, an extraction pump, a blower and a cabinet with the equipment for the control of the microprocessors. Figure 2 shows the scheme of the pilot plant. influent rotary screen
equalizing tank
CXCC" sludge
denitrification tank
nitrification tank
+-----'---K.
Figure2. Schematic representation of the pilotplant.
effluent I permeate
Integrationof membrane filtration
433
The influent of the pilot plant was extracted by a submersible motor-driven pump from the effluent of the existing grit channel of the WWTP. The inflow rate was about 10 m3/d. Mechanical primary treatment of the wastewater was realized by a self-cleaning curved sieve with a mesh width of 1.5 mm. Sampling of the influent of the pilot plant was done behind the sieve. Here an equalizing tank was arranged in parallel to the influent main which was continuously passed. It served to equalize the influent to the pilot plant during the night, when it was not possible to branch off a sufficient volume flow from the influent to the WWTP. Thus a constant inflow to the pilot plant was always ensured. The upstream-placed denitrification tank (V = 500 I) was fed by an additional pump. A mechanical agitator served to prevent the sludge from settling and ensured continuous mixing. At times an additional denitrification tank (V = I 000 I) was operated which had been added subsequently to the pilot plant. From the denitrification tank the mixed liquor was led by a pipeline into the aerated bioreactor. Two membrane filtration modules. from which the cleaned wastewater was withdrawn. were integrated into this bioreactor. The membrane filtration modules consisted of a large number of hollow fiber membranes arranged in parallel with a filtering surface of a total of 27.8 m 2• through which the permeate was sucked under negative pressure out of the bioreactor, The individual hollow fiber membranes were constantly washed around by an air-water mixture and set in motion so that adsorbed solid particles could be removed continuously by peeling and overflowing. For this purpose concentrated air was injected from below the membrane modules. This air also served to supply the biomass in the reactor with oxygen . For oxygen supply a supplementary aeration facility was installed in the reactor. which could be connected if necessary. Both aeration facilities were supplied by a blower. The air volume was adjusted by a manual control device. The mixed liquor was fed by a pump from the bioreactor back to the denitrification tank. For recirculation and extraction of the return sludge a common main was sufficient From this main a branch led to a valve for the extraction of the excess sludge. By another branch it was possible to spray water upon the bioreactor surface to prevent foaming. For reasons of construction the mixed liquor, too. had to be led via the surface to the denitrification tank. Recirculation and withdrawal of excess sludge as well as the influent pump were controlled manually. A microprocessor control device automatically controlled the extraction of the permeate.The extraction was interrupted by periodic backwash procedures. They were started automatically by a time control device (every 300 s for 35 s). The flow direction of the permeate was reversed. and the permeate stored intermediately in a container was pumped back through the hollow fiber membrane into the bioreactor. This effected a rinsing of the pore openings.
If the negative pressure used for the extraction of the permeate exceeded a certain value. a longer backwashing process was automatically started. Moreover. an intense backwashing process was started at regular intervals. In this case the hollow fibers were rinsed additionally upwards in a vertical direction. The cleaned wastewater (permeate) was recycled into the WWTP. TEST RESULTS To get first knowledge on the application of the activated sludge process with integrated membrane filtration in municipal wastewater treatment, the Erft River Association has carried out tests with a pilot plant on semi-industrial scale from May to October 1996. Aims of the tests were to create a safe basis for design and construction of a large-scale plant and to gain sufficient experience concerning operational safety and the mode of operation, but also to get information on the achievable purification efficiency . During the whole test duration the parameters NH4-N, N0X-N and P0 4-P were measured on-line in the effluent. The parameters COD, NH4-N, NOrN, NOz-N. Norg• P04-P and P tot were examined with the help of 24 h- and 2 h composite samples. The physical parameters such as influent, temperature and MLSS concentration were daily recorded.
434
N. ENGELHARDT et al.
The semi-industrial tests were divided into the following phases : running-in phase operation at dry weather flow operation at combined water flow The bioreactor was filled with activated sludge from the activated sludge tank with circulating flow of the neighbouring WWTP. For the running -in phase the biomass concentration of the pilot plant was adjusted to values between 12 g and 20 g. When precipitants were added . the MLSS concentration temporarily amounted up to 30 gil. During the test phase "simulation of dry weather flow" the plant was fed with a characteristic dry weather load. The daily influent was about 10 m3/d. In this case especially the behaviour of the membranes with various biomass concentrations was observed. The membrane flux varied between 20 and 25 1/(m2.h) with a reduced pressure at the permeate side of 0.25 bar. Raising the negative pressure to c. 0.4 bar it was possible to increase the membrane flux for a short period (two days) to more than 30 V(m2.h). The following effluent concentrations were reached: Chemical oxygen demand (COD): With mean influent concentrations of c. 800 mgll COD the effluent values were normally below 20 mgll COD .
Nitrification: With an influent concentration of 35 mgll TKN effluent values of less than I mgll NH 4-N were observed nearly throughout. Sporadically higher effluent values could be explained by the provisional equipment of the plant which had no automatic oxygen control.
Denitrification: In the beginning of the test phase the denitrification efficiency of the pilot plant was insufficient. Denitrification was hindered by a too low denitrification volume and uncontrolled carry-over of oxygen. After installation of an additional denitrification tank the denitrification efficiency was clearly improved. By further optimization (e.g. by reducing the oxygen input. modification of the influent distribution. increase of the recirculation efficiency) it could be proved that effluent values of less than 10 mgll N totare achievable. Phosphorus removal: As expected. no reinforced phosphorus elimination was observed before the chemical phosphorus elimination facility had been put into operation. Starting up the chemical precipitation the P0 4 effluent values were clearly reduced. With an influent concentration of c. 9 mgll effluent values of less than I mg/l were achieved. Sodium aluminate and iron chloride sulphate were used as precipitants. Reduction of germs: The test operation was accompanied by regular sampling carried out by the Hygiene Institute of the University of Bonn. The retaining efficiency of the membranes towards pathogenic agents and other microorganisms was examined. None of the examined fecal indicators (E. coli. coliform germs. fecal streptococcus. coliphages) were detected in the filtered wastewater (permeate) in 100 ml samples. These microorganisms were always found in high concentrations in the inflowing wastewater. According to the test results the calculated retaining efficiency of the membranes always was more than 4 19degrees. as a rule up to 7 19degrees. so that in any case a disinfection of the wastewater can be assumed (Schoenen, 1997). Sludge development: When the pilot plant was operated without phosphorus precipitation. the sludge growth rate was about 1.11.5 leg dry solid matter/d. Related to the daily BOD load. the calculated excess sludge volume is about 0.4-
Integration of membrane filtration
43S
0.6 kg per kg BODS' These values are nearly identicalwith the results of similar pilot tests carried out by the English water associationWessex Water with the "Kubota"system. DESIGNOF A LARGE-SCALE PLANT (3000 INHAB.) Based on the test results and the knowledge gained with the operation of the above pilot plant the Erft River Association has planned a large-scale WWTP with membrane filtration. Since the cleaned wastewater is discharged in a water body with very reduced own flow, the demands on the wastewatertreatment procedure are extremely high. Neither is it reasonableunder economic aspects to transfer the wastewaterto an efficient WWTP nor is it justifiable for reasons of water ecology to renounce the discharge into the receiving water. Therefore the Erft River Association has decided to extend the WWTP at the existing site. This extension will be realized with help of the process combination activated sludge process/membrane filtration because of the space-saving and future-oriented process engineering and the excellent test results obtained with the pilot plant. The concept is presented in Figure 3.
new plant
Figure3. Site planof the Rlidingen WWTP(3000inhab.).
The plant is designed for the following data: Populationequivalents (PE): Annual amount of wastewater(Qa): Daily amount of wastewater (Qd): Dry weather flow (Qt): Combined water flow (Qm):
3000 inhab. 300,000 m3/a 650m3/d 65 m3/h 135 m3/h
The design for the WWTP with the combination of aeration tank/membranefiltration contains the following process stages: pumping station coarse screen aerated grit chamber nitrification reactor with a volume of 200 m3 denitrification reactor with a volume of 200 m3 membrane filtration with a total membranesurface of 4500 m2
436
N. ENGELHARDT tt al.
The process conceptprovidesthat the nitrification- and denitrification tanks may be operated simultaneously or as pre-denitrification. The membrane modules will be installed in separate containers so that both variants can be tested. For sludge storage the existing capacities will be used. COSTESTIMAnON The investment costs for the extension of the large-scale plant with 3 000 PE and the process stages mentioned in chapter6 are estimated at 6 million DM. The operating costs are estimated in the following. As a result of the tests the energy demand of the membrane filtration is assessed at 0.3 kWhlm 3• According to the experience gained by Wessex Water in a two-year test operation. the sludge amount is determined at c. 0.5 kg dry solid matterlkg BODS' For largescale application an approach of 0.65 kg dry solid matterlkg BODs (added) is recommended (Churchhouse, 1996). Personnel costs: Energy costs: Costs for precipitants: Costs for maintenance including replacement of membranes: Costsfor the disposal of residues: Wastewater tax:
145000DMla 65000DMla IOQQQDMla
Total operating costs:
375000 DMla
70000DMla 75 000 DMla 10000 DMla
The total operating costs correspond to specific operating costs of 1.25 DMlm3. On the assumption of 9% capital costs for depreciation and payment of interest the total costs are 915 000 DMla. This calculation leads to specifictreatment costs of 3.05 DMlm3 wastewater. LITERATURE Churchhouse, S. (1996).Personal informationon the investigations of Wessex Water(unpublished). Krafr, A. and Mende. U. (l99S). Niedrigencrgie-Membranverfahren zum Biomassenruckhalt in Abwasscrreinigungsanlagen. F & S Filtrieren und Separieren.Jg. 9. H. 6. Moslang. H. (1997). Neue Perspektiven in der KUlrtechnik. Lecture on the occasion of the Aachen Membrane Colloquium. 3-' March 1997. Schoenen, D. (1997). Optimierung der Abwasserreinigung durch Membranliltration. Report to the Erft River Association (unpublished).
Wat. Sci. Tech. Vol. 38, No. 4-S, pp. 429-436.1998.
Pergamon
IAWQ C 1998 Published byElsevier Science Ltd.
Printed inGreat Britain. All rights reserved
PIT: S0273-1223(98)OO542-3
0273-1223/98 $19'00 + 0'00
INTEGRATION OF MEMBRANE FILTRATION INTO THE ACTIVATED SLUDGE PROCESS IN MUNICIPAL
WASTEWATER TREATMENT N. Engelhardt, W. Firk and W. Warnken Erft River Association, Paffendorfer Weg42. D-50/26 Bergheim, Germany
ABSTRACf Energy-efficient membrane modules for microfiltration are available to realize a new processengineering in municipal wastewater treatment The microfiltration membrane ensuresthat all microorganisms are retained in the aeration tank. A contentof mixed-liquor suspended solidsof e.g. IS gMLSSII can easily be achievedin a large scale plant. Thus the aeration tank is considerably reducedin size. A secondary clarifier is no longer needed. A filtration and a disinfection can be dismissed. A pilot plant gives first knowledge on the application of the activated sludge process with submersed membrane filtration. Based on the tests' results and the knowledge gainedduringthe operation of the pilotplant,a WWTPwithmembrane filtration for 3000 inhabitants is designed. The costs of investment and operation are estimated. ~ 1998 Published by Elsevier ScienceLtd. All rightsreserved
KEYWORDS Activated sludge process; disinfection; hollow fiber membrane; membrane filtration; microfiltration; submersed filtration.
REASON For the treatment of municipal wastewater normally the activated sludge process is used. In the aeration tanks conditions are created that stimulate the growth of various species of microorganisms. so that the pollutants are taken up and converted into cell material. The mixture of activated sludge and water is a suspension which subsequently is separated into cleaned wastewater and thickened activated sludge. As the following process step a secondary settling tank is commonly used. This downstream-placed settling tank is the determining or limiting factor for the operation of the activated sludge process because the settling processes in secondary treatment mainly depend on the biomass concentration in the aeration tank. The experiences gained during many years with the activated sludge process are compiled in the design principles of the ATV worksheet 13I. To achieve a safe separation of activated sludge and cleaned wastewater, the average biomass concentration in the activated sludge tank. measured as mixed-liquor suspended solids (MLSS), should be 3 - 4 g MLSSII and should not exceed 5 g MLSSII. At present light sludges from mineralization processes, bulking sludge. autolysis are undesired states of operation of an activated sludge plant. 429
N.ENGELHARDTrtm.
430
INTEGRATION OF MEMBRANE FILTERS INfO THE ACTIVATED SLUDGE PROCESS The above disadvantages of the process combination aeration tank-secondary clarifier are no longer of importance, if complete separation of the cleaned wastewater from the activated sludge is possible alternatively to sedimentation. Energy-efficient membrane modules for microfiltration with a pore size of less than 0.1 um have been developed, which are directlyinserted into the aeration tank.Thus a procedure is now available by which the activated sludge including all microorganisms can be retained completely in the aeration tank. Starting from these considerations the use of a membrane for the separation of biologically treated wastewater and activated sludgeresults in modified performance data for the activated sludgeprocess: Operation witha mixed-liquor suspended solidscontentof up to 30 g MLSSII is possible. The occurrence of autolysis and hydrolysis in low-loaded activated sludge processes, up to now undesired, no longer impairs the process control becauseof the independence of the sedimentation. Now these phenomena are even desired because at this operating point the production of excess sludgeis minimized. Due to the defined pore diameter the microfiltration membrane ensures that nitrobacteria and other microorganisms are retained in the aeration tank. Displacement of activated sludge into the secondary clarifieras a result of hydraulic peak loads and scouring of activated sludge into the effluentof the secondary clarifier are no longer possible. The problems with bulkingsludgeare also overcome. With a separation limitof e.g, 0.1 um the membrane retains all relevantparticles. Compared to the conventional technique of a wastewater treatment plant (WWfP), which includes the components screen, grit chamber, primary settling, aeration tank, secondary settling, filtration (if necessary) and disinfection, only the foIlowing parts are still necessary, if microfiltration is used to separate the activated sludge: screen witha slit widthof 2.0-4.0mm grit chamber aeration tank (reducedin size by the raisingfactorfor the raisedbiomass concentration) microfiltration units withsubmersed membranes. In Figure ) the process engineering scheme of conventional advanced wastewater treatment is compared with the combination of aerationtank and membrane filtration. It is striking that the components are rather small-sized, and the wholeprocessshowsless stages,so that a space-saving conception is possible. screen
I
grit chamber
aerationtank
secondarysettling
filtration
I
screen
grit chamber
I
7
Figure I. Comparison of conventional advanced wastewater treatment and the combination of activated sludge process/membrane filtTalion.
Integration of membrane filtration
431
Separation of solids by a membrane replaces the sedimentation process and thus forms the basis for the realization of the new process engineering. Microfiltration has already been used for a long time for special applications in industry. In non-European countries some municipal WWTP are also equipped with this technique. The systems "Kubota" (Japan) and "Zenon" (Canada) can be cited here by way of example. CHOICE OF THE MEMBRANE MODULES The membranes used for microfiltration have a pore size of c. 0.1 11m. To force the water through the membrane, a transmembrane pressure difference is necessary. The energy requirement for this process is a decisive criterion for the choice of the membrane and the construction of the modules. In municipal wastewater treatment it has to be ensured that the permeate flow through the membrane be as high and constant as possible, because the wastewater flows continuously into the WWTP. Thus the permeate flow through the membrane becomes the determining factor for the design of the WWTP. The pore diameter and the coating that develops during operation on the membrane surface are decisive for the necessary transmembrane pressure difference and, with this. for the energy demand. Operatin~ procedures of membrane filters
In microfiltration technique one distinguishes between two operating procedures : dynamic operation of "cross-flow filtration" and static operation of "dead-end filtration". In cross-flow filtration a flow (the so-called feed) is built up with high pressure in parallel to the membrane. The feed has to control and to limit the development of the coating on the membrane. and thus represents an important design feature. The adsorbed solids are washed away with the flow velocity of the feed by the shearing forces. At the same time the medium is forced through the membrane by the pressure to be applied of e.g . 6 bar (transmembrane pressure difference). From this results the high energy demand of the crossflow filtration process. It amounts to e.g. 7 kWhlm 3 permeate output with a flow velocity of 6 mls. Static operation or dead-end operation of a membrane has no feed. A negative pressure of 0.2-0.6 bar is built up at the membrane. The permeate is sucked through the membrane. The retained particles settle on the membrane. In order not to cause a break down of the permeate flow, the coating has to be removed from the membrane surface. The feed flow velocity controlled fouling prevention. known from cross-flow filtration. is replaced by other cleaning methods. Membrane fjlters wjth dead-end operatjon The two module systems relevant in wastewater treatment, constructed by the firms Kubota (Japan) and Zenon (Canada), are described in the following . The plate module of the firm Kubota consists of several filtration plates which are assembled to a module. The individual plates are made of a support ing plate with a membrane sheet fixed in front of it. The fabric in between leads the permeate. which has been sucked through the membrane. to the discharge manifold . Between the individual filtration plates of the module air is injected from below in medium-sized to coarse bubbles. The mixture of air and water rises and forms a flow on the vertical membrane surface, which is comparable to the feed in cross-flow filtration. A continuous cleaning effect is obtained. When the coating is irreversible, the whole module has to be removed and cleaned. This is normally done by spraying with pressurized water and chemicals. if necessary.
432
N. ENGELHARDT et aI.
Between these cleaning intervals the permeate flow is continuous. The negative pressure rises until removal and manual cleaning of the module become necessary. The classical dead-end cleaning method of backwashing (reverse of the permeate flow) is possible only to a rather limited extent. This is due to the special construction of the module (plate) with the membrane sheet fixed in front of the supporting plate. Cleaning is realized by the upward flow of the air-water mixture vertically to the membrane surface. To support the cleaning effect, filtration is interrupted periodically for a short time (Kraft, 1995). The construction of the Zeeweed module of the finn Zenon is completely different. It consists of hollow fiber capillary membranes. The membrane is a flexible tube which is stable in itself. It resembles a thin macaroni noodle the inner duct of which serves to discharge the permeate. Filtration is realized from the outside to the inside. Thus the external surface of the hollow fiber is responsible for the filtering effect. The single membranes, i.e, the "tubes", are fitted vertically between two blocks of synthetic resin. The hollow fibers are open at the top and at the bottom and combined at both places to a collecting main. The permeate is discharged by these mains at the top and at the bottom. The outer membrane surface is continuously cleaned by injecting air in coarse to medium-sized bubbles. The air main is fitted into the bottom of the cast block of the membranes. The rising flow of the air-water mixture produces a cleaning effect along the membrane surface. At the same time the threadlike hollow fiber membranes are set in motion so that they move like seaweed (= ZeeWeed) within the suspension and come into contact with each other. This leads to further mechanical cleaning effects. As a result of the flexible construction of the hollow fiber capillary membrane, which in parallel is stable in itself, backwashing with reversed permeate flow is possible. The rinsing process can be realized by feeding both ends of the hollow fiber membranes so that the filtration process is reversed, and the pores are rinsed from the inside to the outside. Moreover, a rinsing of the hollow fiber duct by exclusive feeding from the inferior end of the hol1ow fiber is provided. Rinsing with permeate is done at fixed intervals. With this the filtration process takes a quasi-continuous course. Backwashing of the single modules is possible in a staggered way. If necessary, the hol1ow fiber membranes can be cleaned chemically e.g. with hydrogen hypochlorite. The solution of chemicals is added to the backwash water.
SHORTDESCRIPTION OF THE Pn.,OT PLANT To study the relevant operational parameters for the membrane filtration and for the appropriate design of the activated sludge process, the Erft River Association has run a pilot plant on semi-industrial scale, which was situated at the site of a small municipal WWTP of the association. The main component of the pilot plant was an aerated bioreactor with two integrated membrane filtration modules. It was completed by an upstream-placed tank for denitrification, an extraction pump, a blower and a cabinet with the equipment for the control of the microprocessors. Figure 2 shows the scheme of the pilot plant. influent rotary screen
equalizing tank
CXCC" sludge
denitrification tank
nitrification tank
+-----'---K.
Figure2. Schematic representation of the pilotplant.
effluent I permeate
Integrationof membrane filtration
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The influent of the pilot plant was extracted by a submersible motor-driven pump from the effluent of the existing grit channel of the WWTP. The inflow rate was about 10 m3/d. Mechanical primary treatment of the wastewater was realized by a self-cleaning curved sieve with a mesh width of 1.5 mm. Sampling of the influent of the pilot plant was done behind the sieve. Here an equalizing tank was arranged in parallel to the influent main which was continuously passed. It served to equalize the influent to the pilot plant during the night, when it was not possible to branch off a sufficient volume flow from the influent to the WWTP. Thus a constant inflow to the pilot plant was always ensured. The upstream-placed denitrification tank (V = 500 I) was fed by an additional pump. A mechanical agitator served to prevent the sludge from settling and ensured continuous mixing. At times an additional denitrification tank (V = I 000 I) was operated which had been added subsequently to the pilot plant. From the denitrification tank the mixed liquor was led by a pipeline into the aerated bioreactor. Two membrane filtration modules. from which the cleaned wastewater was withdrawn. were integrated into this bioreactor. The membrane filtration modules consisted of a large number of hollow fiber membranes arranged in parallel with a filtering surface of a total of 27.8 m 2• through which the permeate was sucked under negative pressure out of the bioreactor, The individual hollow fiber membranes were constantly washed around by an air-water mixture and set in motion so that adsorbed solid particles could be removed continuously by peeling and overflowing. For this purpose concentrated air was injected from below the membrane modules. This air also served to supply the biomass in the reactor with oxygen . For oxygen supply a supplementary aeration facility was installed in the reactor. which could be connected if necessary. Both aeration facilities were supplied by a blower. The air volume was adjusted by a manual control device. The mixed liquor was fed by a pump from the bioreactor back to the denitrification tank. For recirculation and extraction of the return sludge a common main was sufficient From this main a branch led to a valve for the extraction of the excess sludge. By another branch it was possible to spray water upon the bioreactor surface to prevent foaming. For reasons of construction the mixed liquor, too. had to be led via the surface to the denitrification tank. Recirculation and withdrawal of excess sludge as well as the influent pump were controlled manually. A microprocessor control device automatically controlled the extraction of the permeate.The extraction was interrupted by periodic backwash procedures. They were started automatically by a time control device (every 300 s for 35 s). The flow direction of the permeate was reversed. and the permeate stored intermediately in a container was pumped back through the hollow fiber membrane into the bioreactor. This effected a rinsing of the pore openings.
If the negative pressure used for the extraction of the permeate exceeded a certain value. a longer backwashing process was automatically started. Moreover. an intense backwashing process was started at regular intervals. In this case the hollow fibers were rinsed additionally upwards in a vertical direction. The cleaned wastewater (permeate) was recycled into the WWTP. TEST RESULTS To get first knowledge on the application of the activated sludge process with integrated membrane filtration in municipal wastewater treatment, the Erft River Association has carried out tests with a pilot plant on semi-industrial scale from May to October 1996. Aims of the tests were to create a safe basis for design and construction of a large-scale plant and to gain sufficient experience concerning operational safety and the mode of operation, but also to get information on the achievable purification efficiency . During the whole test duration the parameters NH4-N, N0X-N and P0 4-P were measured on-line in the effluent. The parameters COD, NH4-N, NOrN, NOz-N. Norg• P04-P and P tot were examined with the help of 24 h- and 2 h composite samples. The physical parameters such as influent, temperature and MLSS concentration were daily recorded.
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The semi-industrial tests were divided into the following phases : running-in phase operation at dry weather flow operation at combined water flow The bioreactor was filled with activated sludge from the activated sludge tank with circulating flow of the neighbouring WWTP. For the running -in phase the biomass concentration of the pilot plant was adjusted to values between 12 g and 20 g. When precipitants were added . the MLSS concentration temporarily amounted up to 30 gil. During the test phase "simulation of dry weather flow" the plant was fed with a characteristic dry weather load. The daily influent was about 10 m3/d. In this case especially the behaviour of the membranes with various biomass concentrations was observed. The membrane flux varied between 20 and 25 1/(m2.h) with a reduced pressure at the permeate side of 0.25 bar. Raising the negative pressure to c. 0.4 bar it was possible to increase the membrane flux for a short period (two days) to more than 30 V(m2.h). The following effluent concentrations were reached: Chemical oxygen demand (COD): With mean influent concentrations of c. 800 mgll COD the effluent values were normally below 20 mgll COD .
Nitrification: With an influent concentration of 35 mgll TKN effluent values of less than I mgll NH 4-N were observed nearly throughout. Sporadically higher effluent values could be explained by the provisional equipment of the plant which had no automatic oxygen control.
Denitrification: In the beginning of the test phase the denitrification efficiency of the pilot plant was insufficient. Denitrification was hindered by a too low denitrification volume and uncontrolled carry-over of oxygen. After installation of an additional denitrification tank the denitrification efficiency was clearly improved. By further optimization (e.g. by reducing the oxygen input. modification of the influent distribution. increase of the recirculation efficiency) it could be proved that effluent values of less than 10 mgll N totare achievable. Phosphorus removal: As expected. no reinforced phosphorus elimination was observed before the chemical phosphorus elimination facility had been put into operation. Starting up the chemical precipitation the P0 4 effluent values were clearly reduced. With an influent concentration of c. 9 mgll effluent values of less than I mg/l were achieved. Sodium aluminate and iron chloride sulphate were used as precipitants. Reduction of germs: The test operation was accompanied by regular sampling carried out by the Hygiene Institute of the University of Bonn. The retaining efficiency of the membranes towards pathogenic agents and other microorganisms was examined. None of the examined fecal indicators (E. coli. coliform germs. fecal streptococcus. coliphages) were detected in the filtered wastewater (permeate) in 100 ml samples. These microorganisms were always found in high concentrations in the inflowing wastewater. According to the test results the calculated retaining efficiency of the membranes always was more than 4 19degrees. as a rule up to 7 19degrees. so that in any case a disinfection of the wastewater can be assumed (Schoenen, 1997). Sludge development: When the pilot plant was operated without phosphorus precipitation. the sludge growth rate was about 1.11.5 leg dry solid matter/d. Related to the daily BOD load. the calculated excess sludge volume is about 0.4-
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0.6 kg per kg BODS' These values are nearly identicalwith the results of similar pilot tests carried out by the English water associationWessex Water with the "Kubota"system. DESIGNOF A LARGE-SCALE PLANT (3000 INHAB.) Based on the test results and the knowledge gained with the operation of the above pilot plant the Erft River Association has planned a large-scale WWTP with membrane filtration. Since the cleaned wastewater is discharged in a water body with very reduced own flow, the demands on the wastewatertreatment procedure are extremely high. Neither is it reasonableunder economic aspects to transfer the wastewaterto an efficient WWTP nor is it justifiable for reasons of water ecology to renounce the discharge into the receiving water. Therefore the Erft River Association has decided to extend the WWTP at the existing site. This extension will be realized with help of the process combination activated sludge process/membrane filtration because of the space-saving and future-oriented process engineering and the excellent test results obtained with the pilot plant. The concept is presented in Figure 3.
new plant
Figure3. Site planof the Rlidingen WWTP(3000inhab.).
The plant is designed for the following data: Populationequivalents (PE): Annual amount of wastewater(Qa): Daily amount of wastewater (Qd): Dry weather flow (Qt): Combined water flow (Qm):
3000 inhab. 300,000 m3/a 650m3/d 65 m3/h 135 m3/h
The design for the WWTP with the combination of aeration tank/membranefiltration contains the following process stages: pumping station coarse screen aerated grit chamber nitrification reactor with a volume of 200 m3 denitrification reactor with a volume of 200 m3 membrane filtration with a total membranesurface of 4500 m2
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The process conceptprovidesthat the nitrification- and denitrification tanks may be operated simultaneously or as pre-denitrification. The membrane modules will be installed in separate containers so that both variants can be tested. For sludge storage the existing capacities will be used. COSTESTIMAnON The investment costs for the extension of the large-scale plant with 3 000 PE and the process stages mentioned in chapter6 are estimated at 6 million DM. The operating costs are estimated in the following. As a result of the tests the energy demand of the membrane filtration is assessed at 0.3 kWhlm 3• According to the experience gained by Wessex Water in a two-year test operation. the sludge amount is determined at c. 0.5 kg dry solid matterlkg BODS' For largescale application an approach of 0.65 kg dry solid matterlkg BODs (added) is recommended (Churchhouse, 1996). Personnel costs: Energy costs: Costs for precipitants: Costs for maintenance including replacement of membranes: Costsfor the disposal of residues: Wastewater tax:
145000DMla 65000DMla IOQQQDMla
Total operating costs:
375000 DMla
70000DMla 75 000 DMla 10000 DMla
The total operating costs correspond to specific operating costs of 1.25 DMlm3. On the assumption of 9% capital costs for depreciation and payment of interest the total costs are 915 000 DMla. This calculation leads to specifictreatment costs of 3.05 DMlm3 wastewater. LITERATURE Churchhouse, S. (1996).Personal informationon the investigations of Wessex Water(unpublished). Krafr, A. and Mende. U. (l99S). Niedrigencrgie-Membranverfahren zum Biomassenruckhalt in Abwasscrreinigungsanlagen. F & S Filtrieren und Separieren.Jg. 9. H. 6. Moslang. H. (1997). Neue Perspektiven in der KUlrtechnik. Lecture on the occasion of the Aachen Membrane Colloquium. 3-' March 1997. Schoenen, D. (1997). Optimierung der Abwasserreinigung durch Membranliltration. Report to the Erft River Association (unpublished).