Treatment of Industrial Waste Water and Process Water With Membrane Processes and Membrane Bio Reactors

8/3/2019 Treatment of Industrial Waste Water and Process Water With Membrane Processes and Membrane Bio Reactors

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G erm an A sso ciatio n fo r W ater. W astew ater a nd W aste


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Treatm ent o f Indus tria l W astew aterand P rocess W ater w ith M em braneP rocesses and M emb rane B io reacto rTechno logyN ovem ber 2007

,DH Publisher/Market ing:D eu tsch e V ere in ig un g fU r W asse rw irtsch aft, A bw as se r u nd A bfa ll e . V .G erm an A ssociation for W ater, W astew ater and W asteT he od or-H eu ss-A lle e 1 75 37 73 H en ne f D eu tsch la ndT el.: + 4922 42872 -333 F ax: + 492 24287 2-100E-Mai l : kundenzent [email protected] . I n te rnet : www.dwa.de
mailto:[email protected]://www.dwa.de/http://www.dwa.de/mailto:[email protected]


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Treatment of Industrial Wastewater and Process Water

The German Association for Water, Wastewater and Waste, DWA (former ATV-DVWK), is the spokesmanin Germany for all universal questions on water and is involved intensely with the development of reliableand sustainable water management. As politically and economically independent organisation it operatesspecifically in the areas of water management, wastewater, waste and soil protection.In Europe the DWA is the association in-this field with the greatest number of members and, due to its spe-cialist competence it holds a special position with regard to standardisation, professional training and infor-mation of the public. The ca. 14,000 members represent the experts and executive personnel from munici-palities, universities, engineer offices, authorities and businesses.The emphasis of its activities is on the elaboration and updating of a common set of technical rules andstandards and with collaboration with the creation of technical standard specifications at the national and in-ternational levels. To this belong not only the technical-scientific subjects but also economical and legaldemands of environmental protection and protection of bodies of waters.

Translation:VERHOEVEN,Waltraud, Pegnitz-TrockauPrinting (English version):Vasen BigPrint, HennefISBN:ISBN 978-3-940173-55-3

ImprintPublisher and marketing:DWA German Association forWater, Wastewater and WasteTheodor-Heuss-Allee 1753773 Hennef, GermanyTel.: +492242872-333Fax: +492242872-100E-Mail: [email protected]: www.dwa.de Printed on 100 % recycling paper DWA Deutsche Vereinigung fOr Wasserwirtschaft, Abwasser und Abfall e. V., Hennef 2008German Association for Water, Wastewater and Waste

All rights, in particular those of translation into other languages, are reserved. No part of this publication may be reproduced in anyform - by photocopy, microfilm or any other process - or transferred into a language usable in machines, in particular data processingmachines, without the writ ten approval of the publisher.

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Treatment of Industrial Wastewater and Process WaterForewordThese topics have been prepared by the DWA Working Group IG-5.5 "Membrane Technology" in the DWATechnical Committee IG-5 "Treatment of Industrial Wastewater". The topics consist of several parts.Part 1 deals with membrane processes per se, l.e. the application of this process step in the separation ofundissolved, colloidal or dissolved matter.Part 2 describes the membrane bioreactor processes. Thereby, the emphasis is on the process unit,consisting of the biological degradation in the aeration tank and the separation of the biomass by means ofmembranes. Details are given, in particular, on the requirements and specifics of membrane bioreactors incontrast to conventional activated sludge processes.Part 3 (Membrane Processes) and Part 4 (Membrane Bioreactor Processes) present practical examples,operating experience and details on design (not part of the English version).

AuthorsThe following members belong to the DWA Working Group:

BROCKMANN,MartinBROr1,UlrichCORNEL,PeterDEBUS,OliverKIEFER,MichaelKRAUSE,StefanKUNZ, Peter M.NEUHAUS,OttoPETERS,ThomasROSENWINKEl,Karl-HeinzSCHMIDT,WinfriedTHEilEN, UlfWAGNER,Frieder

Dr.-Ing., RatingenDipl.-Biol., HertenProf. Dr.-Ing., Darmstadt (Sprecher)Dr.-Ing., HamburgDipl.-Ing., StuttgartDr.-Ing., WiesbadenProf. Dr., MannheimDr.-Ing., BergkamenDr.-Ing., Neuss Prof.Dr.-Ing., Hannover (stellvertr. Sprecher)Prof. Dr., GelsenkirchenProf. Dr.-Ing., GiefsenDr.-Ing., Heuweiler

With the collaboration of:ACHTABOWSKI,AnnetteBRINKMEYER,JorgKlEGRAF, FerdinandOlES, VolkerPAULITSCHEK,MarkusWOZNIAK,Thomas

Dipl.-Biol., BergkamenDipl.-Ing., OldenburgDipl.-Ing., A-WienDr.-Ing., RosdorfErkrathDipl.-Ing., Reutlingen

Project focal point in the DWA Head Office:GRABOWSKI,Iris Dipl.-Ing, HennefDepartment of Wastewater and Water Protection

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Treatment of Industrial Wastewater and Process WaterContents Part I: Membrane ProcessesForeword................................................................................................................................................ 3Authors 3Contents Part I: Membrane Processes................................................................................................ 4List of Figures Part 1.............................................................................................................................. 5List of Tables Part I 5Contents Part II: Membrane Bioreactor Technology.......................................................................... 5List of Figures Part 11............................................................................................................................. 61 Introduction 82 Selection of the Membrane Process................................................................................... 102.1 Separation Range....... 102.2 Membrane Material................................................................................................................. 112.3 Membrane Modules 132.4 Operating Mode of Membrane Units 153 Exemplary Applications 174 Membrane Units - Project Planning.................................................................................... 184.1 Basic Evaluation. 184.2 Project Planning and Design 184.2.1 Bench-scale Preliminary Tests................................................................................................ 184.2.2 On-site Pilot-plant Tests.......................................................................................................... 184.2.3 Plant Design............................................................................................................................ 195 Criteria for the Selection of Membrane Units 205.1 Technical Evaluation of Membrane Processes Regarding Application and Sufficiency.......... 205.1.1 Task Definition 205.1.2 Material and Mass Flow during the Operation of Membrane Units 205.1.3 Use/Disposal of Products Formed during the Process............................................................ 215.1.4 PreTreatment 215.1.5 Technique 215.1.6 Redundancies 215.1.7 References/Similar Applications 215.2 Operational Costs 225.2.1 Resources 225.2.2 Additional Resources 225.2.3 Personnel Costs...................................................................................................................... 225.2.4 Lifetimeand Replacement of Membranes.............................................................................. 225.3 Changes in the Operating Conditions of the Plant..... 235.4 Miscellaneous 235.4.1 Malfunctions.................................................................................................................. 235.4.2 Preliminary Tests 236 Questionnaire Process Data Collection 246.1 Description of the Separation Task to beSolved by the Membrane Process 246.2 Evaluation/Integration of the Membrane Process within an Overall Treatment Concept 256.3 Plant Design 266.4 Requirements onthe Implementation andConstruction of the Membrane Unit...................... 27Appendix.. 287 Further Reading.................................................................................................................... 29

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Treatment of Industrial Wastewater and Process WaterList of Figures Part IFig. 1:Fig. 2:Fig. 3:Fig. 4:Fig. 5:Fig. 6:Fig. 7:Fig. 8:Fig. 9:

Scheme showing the principle of membrane processes......................................................... 8Classification of membrane and filtration processes............................................................... 8Cross section through a phase inversion membrane, here a UFhollow fibre membrane....... 11Principle design (left) and cross section (right) of composite membranes 12Frontal view of atubular module with 5.5 mm tubular membranes (photo: X-Flow) 14Cushion module (type Rochem FM) 14Principle design of a spiral wound module.............................................................................. 15System of dependences in the selection of membranes and modules 16Material and mass flow during the operation of membrane systems...................................... 20

List of Tables Part ITab. 2.1: Membrane processes and their areas of application 10Tab. 2.2: Common membrane materials used indifferent membrane processes.................................. 12Tab. 2.3: Characteristics and areas of applications of modular designs 13

Contents Part II: Membrane Bioreactor Technology1 General Information.. 312 Constructural Set-up 322.1 Design........................................................................................................................ 322.1.1 Submerged Membrane Modules............................................................................................. 322.1.2 External Membrane Modules.................................................................................................. 332.2 Fouling Control........ 332.2.1 Fouling Control in Submerged Systems 332.2.2 Fouling Control in External Systems.. 332.2.3 General Remarks.................................................................................................................... 342.3 Cleaning Management............................................................................................................ 342.4 Constructional Details............................................................................................................. 373 Inflow Requirements............................................................................................................. 383.1 General Information 383.2 Mechanical Pretreatment 383.3 Mixing and Storage Tanks 383.4 Calcium Concentration............................................................................................................ 393.5 Iron and Aluminium Concentrations 394 Information on Membrane Biorector Design 394.1 General Information 394.2 Footprint........... 404.3 Elimination Rates.................................................................................................................... 404.4 Aeration 414.5 Hydraulics 424.5.1 Flexibility 42

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Treatment of Industrial Wastewater and Process Water4.5.2 Recirculation 424.6 Influence of Temperature 425 Membrane Bioreactors - Special Features 435.1 Sludge Characteristics 435.1.1 Sludge Characterization.......... 435.1.2 Rheological Properties 435.1.3 Abrasive Substances 445.1.4 Production of Surplus Siudge....................... 455.1.5 Sludge Treatment 455.1.6 Foaming 456 Economic Efficiency............................................................................................................. 466.1 Definition of Economic Efficiency............................................................................................ 466.2 InvestmenUCapital Costs (Cupex) 466.3 Operating Costs (Opex) 466.4 Comparison of Cost-influencing Factors 47References and Further Reading 48

List of Figures Part IIFig. 1:Fig. 2:Fig. 3:Fig. 4:Fig. 5:Fig. 6:Fig. 7:

Schematic comparison of conventional aeration systems and membrane bioreactors........... 31Design of submerged membrane modules in the aerobic part of the aeration tank................ 32Design of submerged membrane modules in a separate filtration tank 32Design of external membrane modules.................................................................................. 33Qualitative correlation of required membrane surface area, energy demand and flux.. 34Dependence of the a-factor on the total solids content............................................ 41Total solids against viscosity, at a shear rate of 40 S-1 44

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Trea tm en t o f Indus tria l W astew ate rand P rocess W ater w ith M em braneP rocesses and M em brane B io reac to rTechno logyPar t 1 : M em brane P rocessesN ovem ber 200 7

Pub li sh e r/M a r k et i n g :D eu tsc he V ere in ig un g fU r W a ss erw irts ch aft, A bw ass er u nd A bfa ll e . V .G erm an A ss oc ia tio n fo r W a te r, W a ste wa te r a nd W a steT he od or-H eu ss -A lle e 1 7 . 5 37 73 H en ne f . G erm an yTe l.: +49 2242 872-333 . Fax : +49 2242 872-100E-Mai l : [email protected] . In te rnet : www.dwa .de


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Part I: Membrane Processes1 IntroductionThe principle of membrane processes is the physical separation of substances, i. e. wastewater or processwater to be treated is separated into treated water (filtrate/permeate) and the concentrated phase (concen-trate). The driving force for the separation is the difference in trans-membrane pressure. The factor distin-guishing these pressure-driven processes is the amount of the pressure difference. Membrane processeswhich use other driving forces, e. g. electric fields or a difference in concentration, are not discussed here.In contrast to conventional f iltration technology, pressure-driven membrane processes allow the separationdown to the molecular structure.

wastewater(Feed)100% ---+I

treated wastewater(permeate, filtrate)... .------!~ e.g. 90 %embrane process

concentrated wastewateror resource(concentrate)e.g. 10 %

Fig. 1: Scheme showing the principle of membrane processes In order to ensure the success of membrane processes, there are two properties of key importance: Theselectivity of the membranes, i. e. their ability to differentiate between the components of a mixture(e. g. between oil and water or between ions and water). Thereby, the resistance of the membraneagainst transport processes differs according to the characteristics of the components.

The performance of the membranes (often called membrane flux), i. e. the permeate or filtrate flux, re-spectively (usually expressed as LI(m2h)), to be achieved under defined operating conditions.

In Fig. 2 membrane processes are classified as a function of particle or molecule size, respectively, andpressure difference.

OJ0-aoenenO J0.ErnxO J

100"i::"rnB0- 10e"C~:Jenen~0-OJCc: : 0,1~c: ::2~rn0-OJen

Fig. 2:Classification ofmembrane andfiltration processes

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Part I: Membrane ProcessesBy using membrane processes, components, such as solids, dissolved matter, colloidal matter and two-phase liquidscan be separated.Irrespective of the process and the separation task, various treatment targets of economic interest can beachieved by means of membrane technology, such as1) clean-up of water, e.g.

- to meet discharge requirements- for reuse

2) enrichment of the components, e. g.- for the recycling of resources- to reduce disposal costs

In order to comprehend the selectivity of membranes, models have been developed, which can be summa-rised in two categories of membranes, the so-called solution-diffusion membranes (reverse osmosis, nano-filtration) and porous membranes (microfiltration and ultrafiltration). The solution-diffusion membranes feature a homogenous separating layer, comparable to a gel. Inorder to pass the membrane, the compound has to dissolve in the membrane material. Hence, the selec-tivity is based on the difference in solubility and permeation velocity of the compounds to be separated.Transmembrane mass transport follows the laws of diffusion (Fick's Law). In all diffusion-controlledmembrane processes, the driving force for the dissolved compounds is the difference in the chemical orelectro-chemical potential on the two sides of the membrane. The driving force for the solvent, i. e. wa-ter, is the pressure difference. This model describes the separating effects of reverse osmosis mem-branes. In order to describe the separating effects of nanofiltration membranes, electro-chemical in-teractions with - normally negatively - charged membrane surfaces have also to be considered.

Porous membranes feature a porous structure with channels. Their selectivity is based on the sievingeffect, which is defined by the pore size distribution of the membranes. Mass transport is purely convec-tive following the laws of laminar pipe flow (Hagen-Poisseuille Law) and results from the pressure differ-ence between the two sides of the membranes. This theoretical model describes the separating effectsof microfiltration and ultrafiltration membranes. However, in practice, these processes are usuallycontrolled by surface layers. These surface layers ("secondary membranes") build up as the componentsconcentrate on the surface of the membrane.

During operation, this build-up of components which are retained by the membrane on the raw water sidewill have negative effects on the efficiency of membrane systems. They might impair the membrane sur-face, components of the membrane elements or the membrane module, respectively:scaling = deposit of inorganic water compounds, due to precipitation processes caused by oversaturation,

forming of crystals= formation of coatings by organic water compoundsouling

biofouling = formation of biofilms, caused, amongst others, by microorganismsThese effects can be avoided or at least reduced by adequate measures. The precipitation of inorganiccomponents, such as CaS04 or CaC03, can be prevented, for example, by adjusting the pH-value - andthus the limits of solubility - and/or by the addition of antiscaling agents (complexing agents, e.g. phos-phonic acid, polycarboxylic acid). The growth of biofilms can be prevented or reduced, by applying ade-quate preliminary treatment measures or rinse-and-clean schemes, which have to be adjusted to the re-spective boundary conditions.

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Part I: Membrane Processes2 Selection of the Membrane ProcessIn order to select a certain membrane process, to begin with, four basic decisions have to be made: definition of the separation range the membrane has to feature selection of membrane material selection of the type of module selection of the processThe selection criteria are described below.2.1 Separation RangeThe compounds which have to be eliminated from wastewater or process water or the required selectivity,respectively, define the type of membrane to be adopted. In Tab. 2.1 examples are presented in order toallocate different water compounds to be removed to types of membranes. Thereby, regarding porousmembranes, commonly used pore sizes are stated. The dimension units and separation ranges, given inthe table, comply with the definitions generally used by the manufacturers. The cited unit for the molecularweight (g/mol) corresponds to the unit Dalton, which is used in other publications.

Tab. 2.1: Membrane processes and their areas of application- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -particle> 0.1 IJm 0.1 - 1 IJm microfiltration (MF) 50 - 300 kPaemulsified substances [porous membranes] (0.5 - 3 bar)

colloids, macromolecules 2,000-200,000 ultrafiltration (UF) 50 - 1000 kPamolar mass> 2,000 g/mol g/mol [porous membranes] (0.5 - 10 bar)emulsified substances (0.004 - 0.1 IJm)

organic molecules, > 200 g/mol nanofiltration (NF) 500 - 4000 kPapolyvalent, retention [solution-diffusion- (5 - 40 bar)inorganic ions of MgS04 membranes containing> 90 % ionogenic groups](0.001 - 0.005 IJm)

organic molecules < 200 g/mol reverse osmosis (RO) 500 - 7000 kPaand all ions retention (5 - 70 bar)

of NaCI high-pressure-RO(HP-RO) up to 12000 kPa> 95% [solution-diffusion (up to 120 bar)

membranes]

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Part I: Membrane Processes2.2 MembraneMaterialToday, almost all applications of membrane technology in the area of water and wastewater treatment relyon synthetic polymer membranes. Below, a short overview is given. Despite high investment costs, latelyceramic membranes have established themselves in specific fields of application (high temperatures, ag-gressive media, solvents).It is known from the basic laws on mass transport (convection, diffusion) that mass flow density (permeateflux in L/(m2'h)} is inversely proportional to the length of the transport path. This is why efforts have beenmade in membrane development to provide separation layers of small thickness. All the same, mechanicalstability has to be ensured.While in microfiltration normally symmetric membranes are used, in ultrafiltration, nanofiltration as well as inreverse osmosis mainly asymmetric membranes prevail. Here, depending on the production technique, onecan distinguish between phase inversion membranes and composite membranes. With phase inversionmembranes, the active layer and the supporting layer are made from the same material, whereas withcomposite membranes, a homogenous polymer layer, as thin as possible, is applied to the supporting layer,thus enabling the separate optimization of the layers. Fig. 3 and Fig. 4 present the principle design of phaseinversion membranes and composite membranes.Membranes made from cellulose derivates, commonly used in the past, have largely been replaced by mem-branes made from fully synthetic polymers (polysulfone, polyethersulfone, polyamide, polypropylene, poly-acrylnitrile, etc.). These polymers offer, in particular, the advantage of improved stability against the involvedmedia. Tab. 2.2 gives an overview on common membrane materials used in different membrane processes.

Fig. 3: Cross section through a phase inversion membrane, here a UF hollow fibre membrane

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Part I: Membrane Processes

activejayer upto2500 A = 1/4000 mmthickness

~ micro-porous ~supporting layer porous polyester netupto 50 pm thickness up to 125jJm thickness

COMPOSITE-MEMBRANE

Fig. 4: Principle design (left) and cross section (right) of composite membranes

Tab. 2.2: Common membrane materials used in different membrane processes

microfiltration

ultrafiltration

nanofiltration

polypropylene (PP),polyvinylidenfluoride (PVDF),

polysulfone (PSU),a-aluminium oxidestainless steel,titanium dioxidezirconium oxide

polysulfone (PSU),cellulose,

polyacrylnitrile (PAN),polyethersulfone (PES),

titanium dioxidezirconium oxide

polyvinylidenfluoride (PVDF)polyamide (PA),

(zirconium oxide), PES,cellulose acetate (CA)

reverse osmosis

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polyamide (95 %),cellulose acetate (5 %)

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Part I: Membrane Processes2.3 MembraneModulesThe module is the core piece of the membrane unit, in which the membrane layers are arranged to form anoperational unit. The ideal module should meet the following requirements: good, even flow across the membranes without producing dead water zones mechanical, chemical and thermal stability low pressure drops high packing density cost-efficient production easy to clean easy to exchange membranes low tendency towards cloggingThere is no module which fulfils all aspects, as they are - to some extent - contradictory in themselves.Therefore, different modular designs have been developed, some for special areas of application. Themodules can be classified, irrespective of constructional details, into two groups:

modules with tubular membranes: - tubular module- capillary module- hollow fibre module

modules with plate membranes: - plate module- spiral wound module- cushion module- rotary module

The size and therewith the investment and operational costs of a membrane unit strongly depend on the spe-cific permeate flux and the corresponding membrane surface area to be installed. The arrangement of themembranes should be as efficient and, thus, as compact as possible, without endangering operational safety.In the case of wastewater treatment, type and concentration of the solids fed into the membrane unit, andthe solids formed during the process, respectively, playa major role in the selection of module systems.Tab. 2.3 presents an overview on the characteristics and areas of applications of modular designs.Tab. 2.3: Characteristics and areas of applications of modular designs

rotary module 10 - 50 +++ MF and UFtubular module 20 - 300 ++ MF, UF, NF, ROplate/cushion 100 - 250 ++ MF, UF, NF, ROmodulespiral wound module 600 -1200 0 + MF, UF, NFcapillary module 700 - 1000 0 RO,NF,UFhollow fibre module > 1000 ++ RO,UF- negative; 0 average; + positive



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Part I: Membrane ProcessesFigs. 5 - 7 present examples of module designs.

Fig. 5: Frontal view of a tubular module with 5.5 mm tubular membranes (photo: X-Flow)

Fig. 6: Cushion module (type Rochem FM)



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feed spacer

is wound

feed spacer

drain-off the permeate af terpassing the membrane

Fig. 7: Principle design of a spiral wound module2.4 Operating Mode of Membrane UnitsAs with the selection of the type of module, the operating mode of membrane systems is determined by theloading/concentration, of the dissolved and/or undissolved components of the respective wastewater orprocess water. Via the specific energy demand, the operating mode influences the economic efficiency,and, thus, the feasibility of the respective application.Normally, membrane processes are run in "cross-flow" configuration, also called tangential flow filtration.Here, the incoming flow is tangential to the retentate/concentrate side of the membrane surface, thus minimiz-ing the formation of coatings on the membrane and keeping the permeate flux on the highest possible level.The contrast is the "dead-end" configuration. Here, as with classic, cake forming static filtration, the filtrationefficiency decreases with increasing thickness of the coatings. Combined with efficient backwash procedures,this configuration is successfully realized in respective applications of microfiltration and ultrafiltration.Processes combining characteristics of both configurations are called semi-cross flow processes.Regarding the energy demand, dead end and cross-flow configurations differ substantially.The energy demand of pure cross-flow processes is in the range of 0.3 to 5 kWh/m3 (MF and UF) and1 to 10 kWh/m3 (NF and RO), while pure dead-end processes (only MF and UF) get by with an energyinput of 0.1 kWh/m3 to 0.3 kWh/m3Ultimately, the task of the engineer in selecting the adequate technology, is to select membranes, modulesand operating modes, which offer an optimum in the required investment and operating costs while, at thesame time, ensure sufficient operational safety.Exemplary approaches for optimizing the process are: optimization of preliminary treatment processes, thereby the application of more compact (and thus moreefficient) modules is feasible

if applicable, two-step design of the membrane unit, e. g. starting with microfiltration as preliminarytreatment with high permeate flux, followed by a compact reverse osmosis unit as competitive maintreatment step.



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Part I: Membrane ProcessesFig. 8 shows the areas of conflict for the engineer.

packing densitydead-end-modeFig. 8: System of dependences in the selection of membranes and modules



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Part I: Membrane Processes3 Exemplary ApplicationsAreas of application, where pressure-driven membrane processes have proved of value are listed below.This list is not exhaustive. filtration of etching acids (MF, UF) treatment of wastewater from chemical treatment units in the plating industry (RO) cleaning of leachate from landfill sites (NF, RO) extension of the life time of degreasing baths (MF, RO) recovery of resources from paint wastewater (UF) cleaning of paint wastewater - textile, lacquer, flexo paints (MF) cleaning of wastewater from spark eroder production (MF) treatment of fermentation waste liquor (UF, RO) cleaning of wastewater from vibratory finishing (MF) treatment of grey water on ships (RO) recycling of solids-containing wastewater from the production of semiconductor components(CMP, grinding, sawing) (UF)

reuse of water in the semiconductor industry (RO) removal of oil from compressor condensates (MF) enrichment of cooling lubricants (UF) treatment of alkali liquors (MF, UF, NF) water reuse in the food industry (RO) treatment of rinsing water from degreasing processes in metal industry (RO) cleaning of circulation water in non-ferrous metal processing (UF) removal of biologically persistent organic compounds (NF) cleaning of wastewater from flue gas scrubbers (MF) treatment of acids (NF) treatment of sludge water (filter backwash) (MF, UF) treatment of sludge water from swimming pool operation (RO) recirculation of swimming pool water (UF, RO) enrichment prior to thermal processes (RO) pretreatment of wastewater for further treatment in reverse osmosis (MF, UF) closure of water cycles (UF, NF, RO) recovery of resources from water-based lacquers (UF) reduction of bacteria in discharge water from wastewater treatment plants (MF, UF),see part II of the topics

sludge retention in membrane bioreactors (MF, UF), see part II of the topics



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Part I: Membrane Processes4 Membrane Units - Project Planning4.1 Basic EvaluationIn order to provide the fundamentals for the conceptual design and planning of membrane systems, allrelevant data on the respective wastewater or process water have to be collected and documented over aperiod as long as possible. These data should include volume flow or batch size, respectively, chemi-cal/physical parameters as well as other process-specific dissolved and undissolved water components(see Chapter 6 "Questionnaire Survey of Process Data").Beside the wastewater situation during the current operating status, the water balance of the entire planthas to be determined, in order to obtain fundamental data for the specification of the designated systemand the approximate evaluation of the impacts of a new process concept on the operating procedure. Inaddition to the estimation of cost savings and the expected results, partial flows and changes due to up-stream process steps should be examined.4.2 Project Planning and DesignAfter the basic evaluation has shown the feasibility of a specific membrane process, the following procedurefor the development of such plants has proved of practical value. Normally, only the described proceeding canensure successful operation.4.2.1 Bench-scale Preliminary TestsThe preliminary tests serve as first orientation and are normally carried out under the following aspects: selection of membranes and modules and evaluation of the general suitability of the selected membraneprocess for the treatment of the respective medium

preselection of preliminary treatment measures, where necessary, including the assessment of scaling,fouling and biofoulung risks as well as preliminary tests on membrane cleaning

approximate determination of the principal process parameters, such as pressure, temperature, flowvelocity and achievable results

4.2.2 On-site Pilot-plant TestsThe pilot-plant tests which are designed according to the results of the bench-scale tests serve the deter-mination of the actual design basis, the cleaning management and other boundary conditions with regard toprocess engineering and plant specifics.Thereby, the pilot plant has to be designed in such a way, that the hydraulic boundary conditions (overflowconditions of the membrane, module wiring) of the membrane elements and the modules, respectively, canbe transferred to the projected plant. Only this way, upscaling is possible without causing problems.The pilot-plant tests should be carried out under the following conditions: continuous operation of the pilot-plant scale unit on-site and under real conditions, and recording of alldesign-relevant data, including preliminary treatment steps

determination of the permeate flux as a function of process length and time securing the cleanability of the membranes, determination of the cleaning intervals, optimization of thecleaning management, evaluation of chemicals demand

Optimal chemical cleaning of the membranes is essential for their long-lasting functioning. Depending onthe type of, different chemicals have to be applied. The cleaning procedures have be worked out in detail



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Part I: Membrane Processesfor each individual case in order to achieve optimal results, however, the principially used chemicals for dif-ferent types of contamination soiling are listed below:Membrane contamination by scaling:membrane cleaning with acids, e. g. citric acid, hydrochloric acid and, if applicable, complexing agentsMembran contamination by fouling:membrane cleaning with oxidants, e. g. hydrogen peroxide, peracetic acid, sodium hypochloriteMembrane contamination by biofouling:membrane cleaning with oxidants or alkalis, e. g. sodium hydroxideIn order to avoid interaction between the used chemicals, the plant has to be rinsed with water after eachcleaning step.For example, the following cleaning procedures can be applied:1. removal of the process water from the plant2. cleaning with citric acid/hydrochloric acid at a pH-value of 3, in order to remove inorganic layers,

duration approx. 1 to 4 hours, at slightly increased temperatures3. intermediate rinsing with water, thus removing the previous cleaning solution4. treatment with NaOCIINaOH at a pH-value of 11, in order to oxidize bacteria growth, duration approx.

5 to 8 hours, at slightly increased temperatures5. final rinsing with water in order to completely remove all chemicals from the system6. restart of the plant with process waterDepending on the degree of contamination, the time needed for the single steps varies, and it might benecessary to repeat some of the steps.4.2.3 Plant DesignBased on the results of the pilot-plant tests, the actual design of the plant can be started. The following pro-cedures have to be carried out: evaluation of test data including the determination of the principal parameter, i. e. membrane permeabil-ity (defined as LI(m2'h'bar)) at the end of its expected life time, at each point of the membrane process

plant design taking into account the operating conditions of the selected membranes and modules. integration of the membrane process into the overall process.



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Part I: Membrane Processes5 Criteria for the Selection of Membrane UnitsAfter having completed the described procedure and prior to the implementation, the selection processshould be evaluated once more.The following checklist was compiled to support the work of future plant operators and to assist in theevaluation of tenders.5.1 Technical Evaluation of Membrane Processes Regarding Application

and Sufficiency5.1.1 Task Definition Have the components of the wastewater been specified and documented sufficiently? Have variations in wastewater quality and quantity been defined? Does the listing of wastewater data include the occurrence of rare, cyclic conditions during the produc-tion process?

5.1.2 Material and Mass Flow during the Operation of Membrane Units Are the described material and mass flows conclusive, i. e. are they balanced?(sum of input = sum of output!); see Fig. 9

incoming wastewater

0)c ::~Q)a.o. . . ..EtilIiio'EQ).

0)c: :'c!1lQ)(3. . . ..EtilIiio'EQ)s:o

concentrate

Ee-0). . . . c : :Q) .-- c : :!1l !1l~_gJ t>til!1l: ; ; :

treated wastewater(permeate/filtrate)

membran unit

Fig. 9: Material and mass flow during the operation of membrane systems



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Part I: Membrane Processes Have the necessary measuring tools for the evaluation of the plant's operation been included? Is it possible to bridge the time gap of chemical cleaning procedures via in-plant measures or by buffertanks?

Does the plant design consider excess loads of wastewater - occurring during the cleaning procedures -to be handled in a reasonable period of time? Is waste disposal (replaced membranes, prefilters) problematic? If so, what are the costs?5.1.3 Use/Disposal of Products Formed during the Process Does the treated water (permeate/filtrate) meet the requirements for the discharge into the sewer systemor the receiving water body or the quality requirements for reuse, respectively?

Is the quality controlled continuously? Is it possible to use the produced concentrate within the plant or does the concentrate meet the require-ments for the discharge into the sewer system or the receiving water body, respectively?

Does wastewater recycling influence the present wastewater discharge conditions or the present waste-water treatment plant (increase in concentration)?

What about the disposal of the produced wastewater from chemical cleaning procedures?5.1.4 Preliminary Treatment Do the implemented membrane modules and membranes need preliminary treatment? Do the technical design and financial calculation include these pretreatment steps? What happens, in case the preliminary treatment unit fails(emergency measures for membrane protection)? Have safety measures (metrology, safety filter) been established for cases of insufficient preliminarytreatment?

5.1.5 Technique What is the automation level of the plant? Is the automation level sufficient for the operational needs of the plant? Does the plant concept include adequate handling of expected variations in the wastewater?5.1.6 Redundancies What are the operational consequences in case of a total breakdown over a period of severalhours/several days?(disposal costs/disposal guarantee/loss of production/follow-up costs)

Have redundancies been sufficiently accounted for in the plant's concept with regard to the potentialcosts due to plant failures?

5.1.7 References/Similar Applications Do plants exist already with comparable concepts? Does the supplier hold references for the selected membrane technology, if so, in similar applications?



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Part I: Membrane Processes5.2 Operational Costs5.2.1 Resources What is the connected load/electric energy consumption? What is the consumption of/are the costs of compressed air? What is the water demand for additional measures?

(e. g. for chemical cleaning, backwash)? Is sufficient water available to carry out these measures and, if so, does it meet the requirements

regarding quality and quantity? Which chemicals are needed for normal plant operation? Are these chemicals available in thefacilities? How are they provided?

Which chemicals are needed for chemical cleaning, if need be? Are these chemicals availablein the facilities? How are they provided?

What is the consumption of these chemicals? Annual costs? What are the annual costs for analyses?5.2.2 Additional Resources Lifetime/annual costs of prefilters, if necessary (does a tender exist for wear parts?).5.2.3 Personnel Costs What are the daily/weekly personnel costs of plant operation?5.2.4 Lifetime and Replacement of Membranes What is the lifetime of the membranes? Expected lifetime/guaranteed lifetime.(guaranteed value at least 2/3 of the expected value)

Does a long-term offer exist for replacement membranes? Does the offer include the labour input for membrane replacement?



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5.3 Changes in the Operating Conditions of the PlantIn addition to the assessment of membrane processes regarding sufficiency, costs and process safety,several boundary conditions have to be kept during plant operation, which, in case of neglect, can lead tooperational problems or damages. Below, case studies from existing plants are listed: corrosion of individual plant components due to changes in cleaning chemicals changes in the ingredients of cleaning chemicals, e. g. due to a change in supplier changes in the sequence of cleaning steps changes in temperature during cleaning changes in the hydraulics of the inflow or within the plant, e. g. due to reduced pump capacity changes in the inflow quality, e. g. due to changes in the load or additional components(employment of process chemicals from different suppliers or changes in the production process)

membrane replacement (changing to a different membrane supplier) insufficient maintenance of the measurement and analysis instrumentation (e.g. pH electrode)This listing of past observations of operational problems clearly shows that the maintenance of membranesystems is of crucial importance.Certainly, changes in the inflow quality of the wastewater to be treated cannot be avoided, as changes inproduction processes are always possible. Usually, small changes will not have any influence on the treat-ment plant.However, in case of substantial changes, it is strongly recommended to keep close contact to the plantmanufacturer, as even significant changes in the inflow can often be compensated by only small modifica-tions in the membrane unit.Thereby it is also required that the plant operator is given due notice of these changes via internal measures.5.4 Miscellaneous5.4.1 Malfunctions In case of necessity, are there sufficient replacement membranes/membrane modules on short notice? Is this delay of time acceptable (possible shutdown of the plant during this period)?5.4.2 Preliminary Tests Have case-specific preliminary tests been carried out? If so, do the data of the tender agree with the data of the preliminary tests?



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Part I: Membrane Processes6 Questionnaire Process Data Collection6.1 Description of the Separation Task to be Achieved by the Membrane Process

What kind of wastewater has to be treated? Origin? Composition? Physical and chemical analysis data(temperature, pH-value, electrical conductivity, hardness, tendency towards fouling, content of solids, typeof solids)

What kind of components have to be removed from the medium?

What are the requirements on the permeate quality?

Does the medium contain components, which might damage the membrane?(for orientation see list of substances in the appendix)If so, which components?

Does the medium show microbiological peculiarit ies?



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Part I: Membrane Processes6.2 Evaluationllntegration of the Membrane Process within

an Overall Treatment ConceptAre there recycling options for permeate/filtrate and concentrate?(further use within the plant, resource recovery)

In case the concentrate has to be treated, what kind of options is there?What about disposal paths and disposal costs?

What are the potential cost savings due to the implementation of a membrane process?(chemicals, water and wastewater fees, etc.)

What kind of processes competing with the membrane process have to be considered?



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Part I: Membrane Processes6.3 Plant DesignWhat is the expected treatment volume? What is the feeding mode? What about mixing and buffer tanks?Future development? (time-variation curves of the volumes and components concentrations, data expressedas m3/h, m3/d and m3/a)

What is the minimum acceptable yield (= % permeate per treatment volume)?

Is it possible to manipulate the process temperature?



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6.4 Requirements on the Implementation and Construction of the Membrane UnitWhat are the requirements on the materials to be employed?Are there materials which cannot or should not be used?

Are there further requirements on the membrane unit? (e.g. explosion protection, sanitary application,operation in compliance with food industry standards, feasibility to use CIP)"

What is the available floor space for the membrane unit?



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Part I: Membrane ProcessesAppendixDepending on the concentration and the operating mode, the following substances may have an impact onthe efficiency of the overall system and, therefore, should be examined in detail: oxidants (e.g. chlorine, peroxide, chromium VI) cationic surfactants flocculants anti-foaming agents polymers silicones organic solvents silicates calcium barium strontium iron/manganese tin acids/bases (pH-value) gypsum lime abrasive substances fibres, hair, roping material



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Part I: Membrane Processes7 Further ReadingBAUMGARTEN,G.: Behandlung von Deponiesickerwasser mit Membranverfahren - Umkehrosmose, Nanofil-tration - Ver6ffentlichungen des Institutes fOr Siedlungswasserwirtschaft und Abfalltechnik der UniversitatHannover, Heft 99 (1998)

DIVERSEAUTOREN:Membrantechnik in der Wasseraufbereitung und Abwasserbehandlung, Beqleitbucherzu den Aachener Tagungen Siedlungswasserwirtschaft und Verfahrenstechnik, IVT der RWTH Aachen(jahrlich seit 1997)

DIVERSEAUTOREN:Preprints zu den Aachener Mernbran Kolloquien, GVT, VDI-GVC Dusseldorf und IVT,Aachen

DWA (2007): Aufbereitung von Industrieabwasser und Prozessabwasser mit Membranverfahren und Mem-branbelebungsverfahren. DWA-Themen. Hennef: Deutsche Vereinigung fOr Wasserwirtschaft, Abwasserund Abfall. ISBN: 978-3-940173-28-7 [Editors's note: The German version includes fields reports on ap-plications in various industries in Part 3 and 4]

EBERT,K.; OHLROGGE,K.: Grundlagen, Verfahren und Industrielle Anwendungen, Wiley VCH Verlag GmbHFLEMMING,H.-C.: Biofouling bei Membranprozessen, Springer-Verlag Berlin Heidelberg 1995GROSCHEL,A.: Umkehrosmose organisch/wassriger Systeme - Stofftransport in Membranen und Ver-fahrensentwicklung; Dissertation RWTH Aachen (1991)

IRMLER,H. W.: Dynamische Filtration mit keramischen Membranen. Vulkan-Verlag Essen 2001MULDERM.: Basic principels of membrane technology (second edition). Verlag Kluwer Academic PublishersPETERS,Th.: Wasseraufbereitung mit Membranfiltrations-Verfahren. Umwelt Bd. 28 (1998), Nr. 4, S. 34-39PETERS,Th.: M6glichkeiten und Grenzen der Membranverfahren aufgezeigt an internationalen Beispielen.Handbuch Fachveranstaltung .Membranverfahren in der industriellen und kommunalen Abwassertech-nik", Haus der Technik, 22.-23. November 2001, Berlin

RAUTENBACHR.: Membranverfahren - Grundlagen der Modul- und Anlagenauslegung. Springer-Verlag Ber-lin Heidelberg 1997



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Trea tm en t o f Indus tria l W astew ate rand P rocess W ater w ith M em braneP rocesses and M em brane B io reac to rTechno logyPar t 2: M em brane B io reacto r P rocessesN ovem ber 2007

,DWA Publisher/Marketing:Deutsche Vereinigung fur Wasserwirtschaft, Abwasser und Abfall e. V.German Association for Water, Wastewater and WasteTheodor-Heuss-Allee 17 . 53773 Hennef GermanyTel.: +49 2242 872-333 . Fax: +492242872-100E-Mail: [email protected] . Internet: www.dwa.de


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Part II: Membrane Bioreactor Processes1 GeneralInformationThese topics are addressed to plant operators, planners and plant manufacturers who deal with thetreatment of industrial wastewater. It is the first revision of the second part of the progress report "Treat-ment of industrial wastewater and process water by membrane processes and membrane bioreactorprocesses" published 2002 [ATV-DVWK, 2002]. It builds upon the progress report "Membrane bioreactorprocesses" for municipal plants which have been compiled by the Technical Committee KA-7. and con-tain the fundamentals and definitions [DWA, 2005].In membrane bioreactors, final sedimentation is replaced by microfiltration or ultrafiltration, respectively.Thereby, two major advantages arise solid-free effluent non-dependence of the biomass concentration on the sedimentation behaviour.Thus, membrane bioreactor processes achieve significantly higher total solids concentrations than conven-tional activated sludge systems. The tank volume can be reduced by the same factor, as long as the samedesign approach us used.Fig. 1.1 schematically compares membrane bioreactors and conventional activated sludge systems.

inflow discharge inflow membrane discharge

Fig. 1: Schematic comparison of conventional activated sludge systems and membrane bioreactors

These topics are structured as follows. In Chapter 2 the constructive set-up of membrane bioreactors is de-scribed, while in Chapter 3 the requirements on the inflow are specified. In Chapter 4 information on designissues is given, and in Chapter 5 some specifics of membrane bioreactors in comparison with conventionalaeration systems are described. Chapter 6 presents details on the economic efficiency of membrane biore-actors. Field reports on applications in various industries can be found in Parts 3 and 4 [DWA, 2007], [Notpart of the English version]



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Part II: Membrane Bioreactor Processes2 Constructional Set-upMembrane bioreactors consist of the aeration tank, in which the biological treatment of the wastewater viaactivated sludge is carried out, and the filtration unit. In the latter the activated sludge is retained by mem-branes (installed in modules), thus achieving a solid-free effluent.2.1 DesignThe market offers a series of different membranes and membrane modules which differ in module con-struction, separation range (microfiltration or ultrafiltration), membrane material, membrane structure(plate, tubular and capillary membranes), filtration direction (from inside to outside or vice-versa), location(external or submerged systems) and the operating mode. Due to the high solids content of the activatedsludge, the filtration unit of membrane bioreactors is normally operated in cross-flow mode, i. e. thesludge-water mixture is directed tangentially across the membrane surface. Thus, a partial flow perme-ates the membrane and is removed as filtrate. By varying the overflow conditions, the forming of coatingscan be influenced.Compared to external membrane modules, the specific energy demand for generating the cross flow insubmerged systems is lower, however, larger membrane surface areas are needed, as the flux (1/(m2h)) islower, too.2.1.1 Submerged Membrane ModulesSubmerged membrane modules are installed in the aerobic part of the aeration tank or in a separate "filtrationtank" (Figs, 2 and 3). The latter option facilitates cleaning procedures and is therefore preferred in industrialapplications. Aeration systems installed beneath the membranes and/or mechanical agitation generates thenecessary cross flow. The filtrate is removed by applying negative pressure of approx. 0.05 to 0.6 bar (asslight as possible).

Fig. 2: Design of submerged membrane modules in the aerobic part of the aeration tank

Fig. 3: Design of submerged membrane modules in a separate filtration tank



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Part II: Membrane Bioreactor Processes2.1.2 External Membrane ModulesIn external membrane module applications, the sludge-water mixture is withdrawn from the aeration tankand pumped through the module. The cross flow is generated by a pump. Due to the loss of flow pressure,the energy demand of external modules is higher than in submerged systems. However, effective foulingcontrol leads to significantly higher specific flow. Fig. 4 shows a scheme of the design.

Fig. 4: Design of external membrane modules

2.2 Fouling Control2.2.1 Fouling Control in Submerged SystemsSubmerged systems are installed either directly in the aeration tank or in a separate filtration tank (cf. Figs.2 and 3). Medium to large bubble sized air is introduced near the bottom floor of the modules. The rising airbubbles generate a flow within the module as well as countless small turbulences on the membrane sur-face. The pressure differences within the eddies remove adsorbed particles from the membrane surface.Due to the upward flow, the concentrated sludge-water mixture is removed from the module.In hollow fibre membranes, extensive movements of the hollow fibres to each other are generated, due tothe great number of eddies. By applying intermittent aeration, these movements can be intensified, thusproducing a pumping effect which induces a cross flow within the fibre bundles.In flat sheet membranes, flow across the membrane can be enforced by fitting the membrane into a frameor via mechanical movement. In some systems, e. g. plate and frame membranes, the resulting airlift willonly lift the activated sludge efficiently, in case a free uplift channel for accelerating the air-sludge-watermixture is fitted beneath the membrane.

2.2.2 Fouling Control in External SystemsWith external systems, filtration in membrane bioreactors is operated in "cross-flow" mode. By directing theflow tangentially across the membrane surface on the solids containing side, the formation of fouling by de-posit of retained particles is limited. Thus, a constant, high level permeate flux is achieved. The more per-meable, i. e. normally the thinner, the surface layer on the membrane, the higher is the flux. In order toachieve efficient fouling control, a multiple of the filtrate volume flow is recirculated. Common velocities ofcross-flow filtration are approx. 1 mls up to 4 mis, depending on the module structure,



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Part II: Membrane Bioreactor Processes2.2.3 General RemarksThe generation of the "cross flow" takes up a large share in the specific energy demand of membrane filtra-tion. The objective of many processes developments is to minimize the energy consumption by reducingthe cross flow and, at the same time, sustain a high flux. This way, the required membrane surface areaand, thereby, the investment and membrane replacement costs are minimized.In Fig. 5 the theoretical correlation of membrane surface area, energy demand and flux of membranebioreactor processes is presented.

-.c.E--..J-

energy demand (kWh/m::l)submerged external

membrane modulsFig. 5: Qualitative correlation of required membrane surface area, energy demand and flux

2.3 Cleaning ManagementCleaning procedures present necessary preventive measures against a decrease of the flux (more accu-rately: of the permeability, expressed as LI(m2.hbar)) or to enable the recovery of the permeability after adecrease in the flux. Possible causes for the loss of permeability are: deposit of (colloidal) organic and inorganic particles deposit and precipitation of salts (see scaling) deposit of organic macromolecules biofouling, i. e. penetration and growth of microorganisms and/or their excrements, such as enzymes,EPS (extracellular polymeric substances), in and on the membrane

poor filtration properties of the activated sludge decrease in temperatureCorresponding to the variation of the causes, the cleaning management has to be adjusted to the wastewa-ter composition, the operating mode of the activated sludge unit, and the applied membranes and mem-brane modules.Thereby, one has to differentiate between process-driven integrated backwash, periodical in-situ mainte-nance cleaning, e. g. by chemically enhanced backwash, and irregular, intensive cleaning with chemicalsduring shut down of the filtration unit. During shut down periods the membranes can "relax" (relaxation).



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Part II: Membrane Bioreactor ProcessesBy increasing the temperature, cleaning procedures can be shortened. In addition, the chemicals concen-tration can be reduced, thus allowing a more gentle cleaning procedure, while - at the same time - achiev-ing the same cleaning effect. For organic membranes there is practical experience in large-scale plants fortemperature ranges of approx. 35C - 40 C.As the cleaning procedures strongly depend on the type of membranes and modules applied, and theirdevelopment is rather fast, only general, exemplary details can be given here.Rinsing/BackwashBackwash is defined as the intermittent, short-term reversal of the flow direction within the membranes withthe objective to remove "particles" (surface layer) which have deposited during the filtration process. Nor-mally, backwash is carried out by means of filtrate water.A common procedure for commercially available hollow fibre modules is a filtration period of 5 - 8 minutes,followed by a backwash interval of 30 - 40 seconds.With external membrane modules, rinsing is defined as the - normally short-term - operation with clear wa-ter on the retentate side, without reversing the permeation direction, with the objective to wash away andremove the coatings.Maintenance cleaning/chemically enhanced backwashIn (maintenance) cleaning, chemicals (e. g. citric acid) or oxidizing chemicals (e. g. hypochlorite) are addedto the (back) wash water. The cleaning is carried out in-situ, i. e, the membranes normally stay in contactwith the sludge-water mixture.The intervals between two cleaning procedures, the type of chemicals and their concentration depend onthe respective application. Typical intervals, e. g. with sodium hypochlorite (NaOCI) or acid, are a few daysup to several weeks. In order to minimize the chemicals load, the cleaning is carried out with very littlebackwash flow - often as a cycle of: short-term rinse - reaction time - rinse. This procedure is normally re-peated several times.It is also possible to drain the sludge-water mixture prior to the cleaning procedures to such an extent that themembranes are exposed to the open air. Thereby, one refers to "on-air" maintenance cleaning. The advan-tage of this mode of cleaning, which is only applied in submerged processes, is that the cleaning solutionstays on the membranes without being diluted and can display its optimal cleaning effect. Thus, a procedureof intensive cleaning with low chemical demand is accomplished. However, one has to make sure that thebackwash pressure during the cleaning process is sufficient. Only this way it is ensured that the cleaning solu-tion covers the overall length of the membrane and not only the bottom area, where the hydrostatic pressureis the highest. A wetting of only parts of the membrane leaves the cleaning incomplete.The outlook on membrane development clearly points towards higher permeability. Thereby, the amount ofbackwash water needed for the generation of the required backwash pressure will increase. The increasedconsumption will obliterate the difference between "on-air" maintenance cleaning and intensive cleaning.Intensive cleaningDepending on the application, it might be necessary to carry out intensive cleaning procedures, generallyonce per month down to once per year. Therefore, either the membrane modules are transferred to a sepa-rate cleaning tank, or the sludge-water mixture is drained off and replaced by cleaning solutions.



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Part II: Membrane Bioreactor ProcessesNaturally, the selection of cleaning chemicals depends on the application. Often, it is advisable to use warmwater for cleaning. A typical cleaning procedure includes, for example, the following steps: rinsing of the membranes with water treatment with acid, e. g. citric acid (250 - 2,000 ppm), and - for adjusting the pH-value at 2 - 3 - hydro-chloric acid or sulphuric acid, in order to remove inorganic coatings, reaction time 0.5 h to approx. 4 h,alternating backwash and reaction time

intermediate rinsing (neutralization)for safety reasons and in order to avoid salt deposits and heat development in the membrane (neutrali-zation heat) in case the cleaning is proceeded at alkaline pH-values

cleaning with oxidative chemicals in order to oxidize organic and bacteria deposits, e. g. 0.05% NaOCIsolution (weight percentage) (= 500 ppm (500 mg/I) active chlorine) in a slightly alkaline medium, orapprox. 0.4% H202 solution (= 4,000 ppm NaOCI), both methods using a reaction time of 5 to 20 h, at in-creased temperatures, where applicable.

final rinsing with waterAfter cleaning in alkaline medium has been finished, water of low hardness should be used for removingresidues of chemicals. Besides safeguarding the required permeability, the scaling potential is reduced.

Generally, one has to bear in mind that - depending on the degree of contamination - the cleaning solutionhas to be conditioned or replaced, respectively.Cleaning details and experienceDue to the complexity of applications and the variety in membranes and modules available, concepts of andexperience in cleaning procedures are not randomly transferable. However, generally, one has to considerthe following: Cleaning chemicals may - particularly in concentrated form - corrode membranes, supporting tissues ofmembranes, components of membrane modules as well as casing walls and fittings. This is the caseespecially with low (high) pH-values. Thereby, due to chemical reactions, etc., locally there might besignificantly higher pH-values than measurements/calculation for the overall reactor show (e. g. increasein pH-value due to the oxidization of organic compounds with sodium hypochlorite).

Oxidizing chemicals corrode organic membranes and result in accelerated deterioration. The more aggres-sive the chemical/the higher the concentration/the longer the reaction time, the more intensive is mem-brane damaging. Several membrane manufacturers name chemicals-specific maximum levels for theirproducts, calculated by multiplyil"lg the concentration of the chemical and the reaction time. Example:250,000 ppmh of free chlorine signifies 500 h with 500 ppm solution or 100 h with 2,500 ppm solution.

Cleaning chemicals themselves may induce interferences via chemical reactions or form harmfulsubstances. Examples:

saponification caused by the reaction of NaOH with fats and -+ils intensive foamingremedy: intensive rinsing with watersalt deposits within the membrane caused by neutralization reactionsremedy: sufficient intermediate rising with water

- formation of AOX caused by the reaction of OCI- with organic componentsremedy: application of drinking water (instead of filtrate) for preparing the cleaning solution; applica-tion of halogen-free oxidants (e.g.H202' peracetic acid) which, in dependence of the application,have to be tested for their long-term cleaning efficiency.

Cleaning agents have to be stored according to the rules and can be conditioned and reused severaltimes, where applicable. All safety regulations regarding the handling of chemicals have to be followedstringently. One should bear in mind that the effectiveness of cleaning solutions weakens with time. So-dium hypochlorite, particularly, should be stored cool and dark, and the solution should not be older thanthree months when being used. In addition, one should pay attention to precipitations which might formduring storage and which might impair the cleaning efficiency.



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Part II: Membrane Bioreactor Processes Used cleaning solution have to be treated according to the rules. Normally, it is possible to feed them inthe activated sludge tank, after preliminary treatment, where necessary. (Separate disposal has to bereviewed regarding regulatory approval!)

Already in the planning phase, the decision has to be made whether required intensive cleaning proce-dures should be carried out in-situ or in external cleaning tanks. Based on this selection, there are eco-nomic consequences regarding choice of material (e.g. tank linings, fittings), disposal of chemicals andlogistics of removal and transport of membranes. Furthermore, the feasibility of cleaning at increasedtemperatures, flexibility of chemicals selection, etc, have to be considered.

Precise information on the concentration of cleaning solutions is essential. With the use of hypochlorite,in particular, it is necessary to specify whether the quoted concentration is based on NaOCI, HOCI, ocr,calculated as CI or free (active) chlorine. When converting from litre to kg, weight-% or ppm, the density ofthe commercial NaOCI solution (sodium hypochlorite) of (p = 1.2 kg/L) has to be considered. The solution'smaximum content is 12 % to 13 % of effective (free, active) chlorine.

At the market, there are commercial cleaning solutions of often unknown composition. Prior to their use, theapplication should be coordinated with the membrane manufacturer.

Details on the various cleaning alternatives should be written down in the operation manual of the re-spective plant. Changes in boundary conditions and an increase in operational experience may requireadaptations of the cleaning procedures. In this case, it is recommended to discuss the new cleaning pro-cedures in detail with the membrane supplier or the membrane manufacturer and get his written confir-mation. Generally, the performance of cleaning procedures should be recorded in detail, not least towatch the long-term efficiency of the individual steps.

2.4 Constructional DetailsWith high trans-membrane pressures, in particular, outgassing from the filtrate might occur during mem-brane passage. Thereby, small bubbles accumulate forming a gas cushion in the permeate collector. Thepiping design should include this eventuality, allowing the continuous cotransport of small air volumes ortheir systematic collection in a suitable location. Furthermore, the input of gas during backwash or chemicalbackwash (CIP cleaning in place) should be prevented. Gas inclusions on the filtrate side allow false sig-nals (pressure, flow) and - for example with hollow fibre membranes - lead to reduced backwash efficiency(areas, in which an air cushion is embedded).Therefore, in order to discover excessive occurrence of gas bubbles during day-to-day operation and toallow for leak tests, if necessary, it is recommended to install a sight glass into the fitrate piping after themembrane stage. As gas accumulation within the permeate piping can not be prevented totally, controlledand automated ventilation should be arranged for, which, in most cases, is combined with backwash proce-dures. Regarding the arrangement and operation of the valve, dynamic pressure conditions during differentoperating conditions have to be accounted for sufficiently. Else, the exhaust valve might quickly turn into anair inlet valve.



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Part II: Membrane Bioreactor Processes3 Inflow Requirements3.1 General InformationPrior to feeding wastewater into membrane bioreactors, inferring substances, such as long-fibre, thread-likeor highly abrasive, sharp-edged material, which, depending on the structure, might clog the module or me-chanically destroy the membrane, have to be removed.In addition, one should take into account that dissolved wastewater components, too, can damage themembranes. As the case may be, membrane specifications (limit values) have to be followed. This does notonly apply to (non-degradable) organic solvents in the wastewater, but also to substances which have beenadded during wastewater treatment, such as anti-foaming agents (have to be silicone-free!) and organicpolymers. The experience of manufacturers and suppliers should be utilized for respective applications.In order to detain "foreign" material (e.g. branches, leafs, etc.) tanks should be covered, if necessary.

3.2 Mechanical PretreatmentSieves and ScreensIn all applications of membrane bioreactors, roping, sharp-edged and clogging material have to be held back.There are large differences in the wastewater composition of the various industries. Only in few branches,fibre loads comparable to municipal wastewater, arise. In general, sieves and screens with 1 mm to 2 mmhole or mesh size, or 0.5 mm - 1.0 mm slit width, respectively, are sufficient. Mesh sieves reach higher elimi-nation rates in comparison with slotted screens [Frechen und Schier, 2006]. The screening plant has to beoperated redundantly without by-pass, thus baring unsieved wastewater from entering the plant at any time.Prior to the initial operation of the plant, intensive cleaning should be carried out, in order to remove residuesleft from assembly (e.g. cable straps which might damage the membranes).Oil/Grease SeparationUndissolved, persistent fats and oils have to be removed from the wastewater prior to feeding the mem-brane bioreactor, as they might affect the filtrate efficiency. Easily degradable fats and light liquids normallydo not constitute a problem.Preliminary SedimentationIn case of high concentrations of solids, preliminary sedimentation should be accounted for, as to not re-duce the percentage of the activated sludge in such a way that the biological treatment efficiency isimpaired.3.3 Mixing and Storage TanksHydraulic balancing is very important in the operation of membrane bioreactors. The membrane surfacearea has to be dimensioned - equivalent to the final sedimentation - according to the maximum water load.As the provision of large membrane surface areas is cost intensive and an increase in operational capacityin case of high loads of wastewater is only feasible to a limited extent, it is advisable to keep the wastewa-ter flow to the membranes constant. Here, storage tanks can be used for volume balancing, independent onthe matter of balancing concentrations.In case the wastewater load shows only slight variations, the necessary storage volume is supplied by thetank itself, as water can be impounded in the activated sludge tank to a limited degree. In addition, the mem-brane flux can be adjusted to a certain extent and therefore can be increased for short periods of time.



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Part II: Membrane Bioreactor Processes3.4 Calcium ConcentrationHigh Ca2+ concentrations (> 200 mg/ l ) near the membranes, resulting from either the application in produc-tion, upstream precipitation measures or neutralization of acidic wastewaters with lime water, may causeproblems. Due to the high input of air, which allows for sufficient turbulent flow at submerged membranes, ahigh percentage of CO2 which is being formed in the activated sludge tank is driven out. Hereby, the pH-value increases (keywords: carbonic acid equilibrium, solubility product) and might cause - just as the po-tential release of CO2 during membrane passage - the precipitation of CaC03 (= scaling) on the membrane(in case of high pressure loss across the membrane). The result is a drop in filtration efficiency, and thusthe necessity of shorter cleaning intervals (with acid). One alternative to reduce precipitation is to adjust thepH-value continuously by adding acids.3.5 Iron and Aluminium ConcentrationsSome partial streams of industrial wastewater contain iron or aluminium salts. Often, these are used asprecipitants in preliminary sedimentation steps, but they may also be part of the partial stream itself. Oxi-dized compounds and particulate matter do not impair membranes and filtration characteristics. Dissolvedcompounds, however, which are oxidized in the aerobic medium of the activated sludge tank, may causethe formation of clogging layers. Partly, the oxidation occurs directly on the membrane, through which theprecipitate adsorbs on the surface. If necessary, the resulting visual coloration can be removed by selectiveacidification.

4 Information on Membrane Bioreactor Design4.1 General InformationBasically, biological degradation of organic compounds in membrane bioreactors does not differ from con-ventional treatment plants, L e. they are oxidized to CO2 while energy is released, and are used for the con-struction of cells. The resulting biomass forms - together with the non-degradable and undissolved particu-late matter - the surplus sludge (see Chapter 5.1.3).Membrane bioreactors, however, particularly differ from conventional treatment plants by high MLSS(= Mixed Liquor Suspended Solids) concentrations in their activated sludge tanks (usually 10 g I L to 15 g I L ,with tube modules as high as 30 g I L , compared to 3 g I L to 5 g I L in conventional activated sludge processes)and the resulting changes in sludge characteristics.While activated sludge tanks are designed according to the load to be degraded and the sludge age, themembrane surface area is determined by the hydraulic capacity, correspondingly to the final sedimentation.Due to the high cost relevance of filtration units, hydraulic design should be accomplished with special accu-racy (ct. Chapter 6). With regard to hydraulic design one has to differentiate between gross flux (flow duringthe filtration process) and net flux, i.e, the flux which is achieved during an entire sequence of filtration andbackwash including the backwash volume.The comparison of operating results from membrane bioreactors with theoretical models shows that thesludge yield can be represented relatively accurately, in particular with detailed models, such as the Acti-vated Sludge Model of the International Water Association. Simpler approaches can also prove of value, aslong as the specific boundary conditions of the respective industrial wastewater are taken into account ade-quately. With high sludge age, a calculation should be carried out based on the steady-state metabolism[Wichern und Rosenwinkel, 2002].All in all, references show that today's design and engineering which derive from the results of pilot-plantpreliminary (pilot) tests are the basis for many efficient plants.



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Part II: Membrane Bioreactor Processes4.2 FootprintIn industrial wastewater treatment, the footprint required for the treatment plant often is a decisive factor.Normally, no problems arise with greenfield developments, however with industrial plants, which havegrown over decades, on-site wastewater treatment is often difficult or not feasible at all.The footprint required for wastewater treatment plants can be greatly reduced by reducing the size of therequired activated sludge tanks to approx. 1/2 to 1/3 in comparison with conventional activated sludgeplants and the omission of the final sedimentation. Often, for industrial plants this is of great advantage or,sometimes, allows on-site wastewater treatment in the first place.4.3 Elimination RatesWith operating modes of low surplus sludge loads, the large increase in oxygen consumption, the dischargequality and the potential accumulation of pollutants and inhibitors in the activated sludge have to be takeninto account.The specific oxygen consumption increases with decreasing sludge loading, i. e. with decreasing specificsurplus sludge load, as the organic matter has to be oxidized to CO2 almost entirely [Cornel, 2000].In individual cases one has to decide - in comparison with the conventional activated sludge process - be-tween two borderline cases, potentially provided by membrane bioreactors, due to increased total solidsconcentrations. These are: smaller volumes of activated sludge tanks, with constant sludge loading and constant surplus sludgeload, or constant volumes of activated sludqetanks and lower sludge loading, with low surplus sludge, howeverwith the result of significantly higher energy costs.It is not feasible to realize all "positive" attitudes - low energy consumption, low surplus sludge and low-volume activated sludge tanks -, as, to some extent, they are mutually exclusive.In individual cases, the influence of operating modes on the discharge quality ("refractory" compounds) hasto be evaluated and depends on the components and the operating conditions (sludge age). Ultrafiltrationand microfiltration membranes practically do not hold back low-molecular compounds, however, increaseddegradation of very slowly-degradable substances at high sludge ages and the retention of macromole-cules by the surface layer ("secondary membrane") are possible. Components bonded to particles, e. g.adsorbed AOX compounds, are detained by the membrane unit.Compared to conventional activated sludge processes, intermittent peak loads have a larger impact on dis-charge concentrations, due to less dilution - caused by the smaller volume of the activated sludge tank -and the missing final sedimentation tank and, at the same time, constant sludge loading.Some industries observe the retardation of degradation processes caused by the accumulation of heavymetals in the activated sludge. With increased concentrations of heavy metals in wastewater, one in par-ticular has to pay attention to the control of the sludge age, in order to prevent too large an accumulation.Phosphorus removal in membrane bioreactors can easily be carried out by applying simultaneous precipitationwith three-valent metal salts. Improved discharge quality can be achieved, as ultrafiltration ensures the reten-tion of even very small f1ocs. Thereby, the molar ratio can be adapted even more favourable. The deposit ofprecipitant residues on membrane surfaces can be counteracted by adjusting cleaning procedures and clean-ing agents accordingly. Normally, membrane filtration itself is not affected by routine dosages of precipitants.Appropriate design allows biological phosphorus elimination even in case the sludge age within the stabili-zation range.Membrane filtration plants generate wastewater which is free of solids and almost entirely germfree. Con-ventional, mechanical-biological processes cannot produce comparable discharge quality, even with down-stream conventional fi ltration, e. g. sand filter.



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Part II: Membrane Bioreactor Processes4.4 AerationThe mass transfer of oxygen from the bubble into the liquid phase is influenced, among others, by thefollowing parameters: salt concentration, viscosity of the medium, surface-active compounds, surface ten-sion, solids content, turbulence and pressure (water depth). For industrial (membrane) bioreactors, it is notpossible to determine a general reference value for oxygen transfer and a-factors. Caused by the differentwastewater components and, in particular, by high salt concentrations (> 5 g IL) , a-factors> 1 can be foundin industrial wastewater treatment plants with conventional technology. They positively influence oxygentransfer by reducing coalescence processes. However, as the oxygen transfer rate depends on the dy-namic viscosity and the latter in turn also depends on the sludge concentration, with higher total solids con-centration one has to expect lower a-factors than in conventional activated sludge plants, whereas with in-creased salt concentrations, one has to expect relatively higher a-factors than in municipal processes.

1 , 00 , 90 , 80 , 70 , 6

:I: 0 , 5-0. . . . . .o 0 , 4J !!I~0 , 30 , 20 , 1

Rosenberger [ 2 0 0 2 ] alpha = e"O,049oTSI . . . - - - ------- Krause [ 2 0 0 5 ] alpha = e"O,056oTS: : - - - ::--'C l . - - - - " ~GOnder [ 1 9 9 9 ] alpha = e"O,07HS., ~0~rampe [ 2 0 0 1 ] alpha = e"O,OBBoTS

~. . r>


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Part II: Membrane Bioreactor Processesbubble aeration systems, the transferability to large-scale plants was proved for a depth of submergence aslow as 1.30 m. Oxygen transfer measurements in test plants should be performed with the wastewater tobe treated and the target sludge concentration, preferably with the same tank geometry and the samedepth of submergence as the planned large-scale plant.4.5 Hydraulics4.5.1 FlexibilityThe modular concept of membrane bioreactors allows for adaptations in case of load variations, by turningon and off individual modules of submerged systems or by reducing the overflow velocity in external mod-ules, respectively. With submerged systems, in particular, the turning off of single modules in case of lowvolumes is normally more cost efficient than the turnover of the flow across the entire membrane surfacearea. In addition, the membrane recovers during the break. Generally, one should consider that the net flow(after deducting rest periods, backwash periods and backwash volume flows) is decisive.In case the wastewater amount increases long-term, the modular concept allows for easy retrofitting andadaptation according to the new requirements. One should pay special attention to allow the feeding of themodules as uniformly as possible, during planning as well as during operation.4.5.2 RecirculationDue to the removal of the filtrate, the activated sludge thickens at the membrane. In case submerged mem-branes are installed directly into the activated sludge tank, it is sufficient to supply systematic flow duct (viaa mixer) in order to balance the concentrations.In case submerged membranes are installed in separate tanks, sufficient recirculation has to be guaran-teed. In external modules, sufficient recirculation is given by the necessary overflow velocity during opera-tion. Generally, the recirculation volume should reach at least 4 - 5 times the actually removed amount of fil-trate. Herewith, too high a concentration of the sludge-water mixture - caused by the removal of the filtrate- is prevented. In order to avoid short-circuit flow in filtration tanks with submerged modules, the outletshould be located opposite to the inlet.With submerged membranes, the recirculation flow is enriched with oxygen, a phenomenon to be consid-ered in case this stream is to be recirculated directly into the denitrification tank.4.6 Influence of TemperatureResulting from organic degradation, during wastewater treatment the temperature increases approx. 2 "C upto 3 "C per degraded g I L COD. In case highly polluted industrial wastewater is treated, this increase in tem-perature has to be accounted for; in case of low-temperature wastewater there should not be a problem. Withexternal systems one has to examine the risk potential of increased temperatures in the activated sludge tankcaused by the input of energy via electric aggregates, e.g. pumps for generating the cross flow.With respect to the flow characteristics, increased temperatures prove quite favourable when using mem-brane technology. With activated sludge plants, the temperature has to be limited to 37 "C up to 39 "C.Generally, the required membrane surface area is dimensioned according to the lowest possible temperature,as - due to the increasing viscosity of the water - the permeability decreases with decreasing temperatures[e. g. GOnder, 1999; Krampe, 2001]. The change in permeability can roughly be determined to be 2 % per "C.



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Part II: Membrane Bioreactor Processes5 Membrane Bioreactors - Special Features5.1 Sludge Characteristics5.1.1 Sludge CharacterizationDue to the increased solids content, the sludge characteristics significantly differ from conventional acti-vated sludge. High shear stress, as is found in cros

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Treatment of Industrial Waste Water and Process Water With Membrane Processes and Membrane Bio Reactors