SIST-TP CEN/TR 15897:2019

SLOVENSKI STANDARD
01-januar-2019
1DGRPHãþD
SIST CWA 15897:2013
Tehnologija potopnega membranskega bioreaktorja (MBR)
Submerged Membrane Bioreactor (MBR) Technology
Getauchte Membranbelebungsreaktor (MBR) Technologie
Technologie MBR - Bioréacteurs à membrane immergée
Ta slovenski standard je istoveten z: CEN/TR 15897:2018
ICS:
13.060.30 Odpadna voda Sewage water
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR 15897
TECHNICAL REPORT
RAPPORT TECHNIQUE
November 2018
TECHNISCHER BERICHT
ICS 13.060.30 Supersedes CWA 15897:2008
English Version
Submerged Membrane Bioreactor (MBR) technology
Technologie MBR - Bioréacteurs à membrane Getauchte Membranbelebungsreaktor (MBR)
immergée Technologie
This Technical Report was approved by CEN on 4 April 2016. It has been drawn up by the Technical Committee CEN/TC 165.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2018 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 15897:2018 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 General system — requirements. 12
4.1 Basic considerations . 12
4.2 Pre-treatment and internal sieving . 14
4.3 Characteristics of biological systems used in MBR plants . 15
4.3.1 General. 15
4.3.2 Mixed liquor suspended solids (MLSS) . 15
4.3.3 Hydraulic retention time (HRT (or detention time)) . 15
4.3.4 Sludge age (or sludge retention time (SRT)) . 15
4.3.5 Chemical phosphorus removal . 15
4.3.6 Aeration . 15
4.4 Membrane filtration system . 15
4.5 Mixed liquor recirculation. 17
4.6 Permeate extraction system . 17
4.7 Desired effluent system . 17
5 Material characteristics . 18
5.1 General. 18
5.2 Porous membranes . 18
6 Configuration . 18
6.1 Flat Sheet . 18
6.2 Hollow fibre . 19
7 Design and operating parameters . 19
7.1 General. 19
7.2 Influent characteristics . 19
7.3 Fouling . 20
7.4 Transmembrane pressure . 20
7.5 Permeability . 21
7.6 Integrity . 21
8 Acceptance, commissioning and monitoring tests . 21
9 Information and documentation . 22
10 Interchangeability . 22
10.1 Principle . 22
10.2 General. 22
10.3 Process flow diagram (PFD) . 23
10.4 Scope of supply . 25
10.5 Interchangeability aspects . 25
10.5.1 General . 25
10.5.2 Membrane type. 26
10.5.3 Layout . 26
10.5.4 Tank . 26
10.5.5 Draining and flushing . 28
10.5.6 Integrity check . 29
10.5.7 Accessibility and maintainability . 29
10.5.8 Chemical cleaning . 29
10.5.9 Process control system (PLC) . 30
Annex A (normative) Information and documentation . 34
Annex B (informative) Example for clean water permeability test . 38
B.1 Abstract . 38
B.2 Materials and methods . 38
B.2.1 Measuring apparatus . 38
B.2.2 Measuring procedure . 39
Annex C (informative) Example for vacuum leak test . 40
C.1 Abstract . 40
C.2 Materials and methods . 40
C.2.1 Measuring apparatus . 40
C.2.2 Measuring procedure . 41
Annex D (informative) Example for pore diameter measurement . 42
D.1 Abstract . 42
D.2 Materials and methods . 42
D.2.1 Latex solution . 42
D.2.2 Measuring apparatus . 43
D.2.3 Measuring procedure . 43
Annex E (informative) Paper filtration measurement . 45
E.1 Objective . 45
E.2 Measuring method . 45
Annex F (informative) Impact of pore size distribution on membrane fouling . 47
Bibliography . 48

European foreword
This document (CEN/TR 15897:2018) has been prepared by Technical Committee CEN/TC 165
“Wastewater engineering”, the secretariat of which is held by DIN.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document is based on the CWA 15897:2008, Submerged Membrane Bioreactor (MBR) Technology
which was prepared by the CEN Workshop 34 – 'Submerged' Membrane Bioreactor (MBR) technology.
This document supersedes CWA 15897:2008.
Introduction
This document deals with custom designed MBR systems for more than 500 PT. It became clear that it is
not possible to have interchangeable membrane modules without considering a complete system. So
this led to the conclusion that this document deals with the entire membrane system rather than the
membrane modules alone.
It was realized that today’s market is a growing one with fast developments in membrane technology.
Standards might be too early and may hamper the technological development. So it was decided at this
stage to create a basic document for submerged MBR technology by means of a Technical Report.
Regarding interchangeability of MBR systems, this document especially focuses on separate membrane
tanks as there is a tendency that large MBR systems (more than 10 000 m3/d) are designed with
separated membrane tanks.
Although there are differences between hollow fibre and flat sheet membrane manufacturers’ designs,
it is believed that there is no need for separate guidelines because these are focused on membrane
tanks. Furthermore, it is clear that interchangeability between hollow fibre membrane systems is not so
easy and the same is true for flat sheet membrane systems. Thus, producing two sets of guidelines
would be of no real benefit to interchangeability.
1 Scope
This Technical Report defines terms commonly used in the field of membrane bioreactor technology.
This document aims at submerged MBR systems for the treatment of municipal wastewater with MBR
Separate Systems and MBR Integrated Systems.
This document establishes general principles for MBR filtration systems interchangeability between
different MBR filtration systems from different manufacturers.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 12255-11, Wastewater treatment plants - Part 11: General data required
EN 16323, Glossary of wastewater engineering terms
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 16323 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
NOTE Some manufacturers may use different terms for their products, but nevertheless the following terms
and definitions are used in this document.
3.1
backwashing
backflush
backpulse
short-term reversal of the flow direction through the membrane in intervals to remove the particles
accumulated during the filtration process (covering layer), usually with permeate
3.2
biofouling
development of a biofilm on the membrane surface or in the membrane due to the growth of micro-
organisms
Note 1 to entry: See Figure 1.
Note 2 to entry: Biofouling causes a reduction of the performance or the permeability (see also fouling and
scaling).
Key
a
W irreversible cake layer adsorption, compaction precipitation, inclusion
of colloids, etc.
b
X pore blocking particle diameter approximately Pore diameter
c
Y inner pore adsorption permeable substances with affinity to membrane
material
d
Z biofouling microorganisms in film consisting of EPS
Figure 1 — Principle of biofouling
3.3
cleaning interval
interval of time between successive cleanings
Note 1 to entry: Depending on the manufacturer there might be different types of cleanings (see 3.26 and 3.34).
3.4
clogging
accumulation of solids within the membrane system
3.5
concentrate
partial flow of the material mixture in which the activated sludge retained by the membrane is
concentrated
Note 1 to entry: It is usually recycled as return sludge into the activated sludge tank.
3.6
covering layer
accumulation of substances retained by the membrane surface
3.7
cross flow
transverse flow which develops at the membrane surface and serves to control fouling
Note 1 to entry: The term cross flow comes from the configuration of the dry-arranged membrane systems
operated in a pressure vessel. During this process, the membranes are subjected to liquid flows parallel to the
surface that limit the development of a covering layer on the membrane surface.
3.8
cross flow aeration
aeration required to induce cross flow
Note 1 to entry: As a result of the two-phase flow, the effective mechanisms clearly differ from the principle of
classic crossflow operation of pressure tube systems with inside flow.
3 2
Note 2 to entry: The cross flow aeration flow rate per unit membrane surface area is expressed in Nm /m /h.
3.9
cycle
temporal sum of filtration phase and following backwashing phase and/or relaxation phase
3.10
Dalton
Da
molecular mass relative to that of a hydrogen atom
3.11
feed flow
flow to the membrane bioreactor system at the inlet of the aeration tank
3.12
filament
single hollow fibre or capillary tube
3.13
flux
specific filtrate volume per unit surface area per time unit
Note 1 to entry: The flux is expressed in litres per square metres of membrane surface area, per hour, [l/(m h)].
In some cases the abbreviation LMH is used.
3.14
flux, instantaneous
flux, gross
actual flux during filtration
3.15
flux, net
overall flux achieved during the filtration cycle including periods of filtration and relaxation and/or
backwashing
3.16
flux, corrected
flux viscosity-corrected for temperature
Note 1 to entry: The water temperature has a major impact on the maximum allowable flux, due to the fact that
the transmembrane pressure (TMP) is theoretically proportional to the water viscosity.
Note 2 to entry: The following equation gives a good approximate value of the viscosity vs. temperature:
ν
t
=0,3804+−0,1696 * EXP(0,040*(20 t)
ν
Where
ν is the water viscosity at t °C
t
ν is the water viscosity at 20 °C
3.17
flux, critical
flux below which permeability decline is considered negligible
3.18
flux, sustainable
flux for which the transmembrane pressure increases gradually at an acceptable rate, such that
chemical cleaning is not necessary
3.19
fouling
deposition of existing solid material in the feed stream on the element of the membrane at or in the
pores
Note 1 to entry: Fouling can either be reversible or irreversible.
Note 2 to entry: Depending on the material involved, a distinction can be made between organic fouling, inorganic
fouling and biofouling. Fouling always results in a reduction of the performance or the permeability of the
membrane (see also biofouling and scaling).
3.20
lumen
interior of a hollow fibre membrane
3.21
maintenance cleaning
cleaning with low concentrations of chemicals to maintain membrane permeability
Note 1 to entry: Maintenance cleaning is usually carried out in situ.
Note 2 to entry: Maintenance cleaning uses less aggressive procedures and/or chemicals than recovery cleaning.
3.22
MBR integrated system
system where the membranes are placed in the aeration tank
3.23
MBR Separate System
system where the membranes are placed in the separate membrane tank
Note 1 to entry: See 3.35.
3.24
membrane element
smallest unit of operation typically combined in assemblies known as modules, units, racks or cassettes
Note 1 to entry: A membrane element could also be called a panel or cartridge.
3.25
membrane area
feed side area of the membranes
Note 1 to entry: The membrane area is expressed in m .
3.26
membrane packing density
membrane area per unit volume of a membrane assembly
2 3
Note 1 to entry: The membrane packing density is expressed in m /m .
3.27
panel
flat sheet membrane element
3.28
permeate
filtrate
portion of the feed stream that passes through the membrane
3.29
permeability
property of a material characterising its ability to selectively permit substances to pass through it
Note 1 to entry: The permeability is expressed in l/(m ·h·bar).
Note 2 to entry: The permeability can be corrected to a reference temperature in order to allow a more accurate
comparison of values.
3.30
permeability, corrected
permeability corrected for the effect of temperature on viscosity
Note 1 to entry: The corrected permeability is expressed in l/(m ·h·bar) considering a reference temperature.
3.31
pore diameter
pore size
size of the pores in the membrane
Note 1 to entry: As a rule, the pores are not uniform, i.e. they show a relatively wide pore size distribution.
Note 2 to entry: The pore diameter with a maximum in pore size distribution is called the nominal pore diameter.
Note 3 to entry: The pore diameter is expressed in μm.
Note 4 to entry: The maximum pore diameter can be determined with the help of the bubble point method
according to DIN 58355-2, which is used to determine the pressure required to extrude the first air bubbles
through the membrane. The maximum pore diameter is then calculated by means of a formula.
3.32
recovery cleaning
intensive cleaning
cleaning with high concentrations of chemicals to recover membrane permeability
Note 1 to entry: Recovery cleaning is either carried out in situ or in a separate cleaning tank.
3.33
relaxation
ceasing permeation whilst continuing to scour the membrane with air bubbles
3.34
scaling
precipitation of inorganic solids in or on the membranes
3.35
separate membrane tank
membrane tank
filtration tank
separate basin containing submerged membranes where the primary function is filtration
Note 1 to entry: The volume of the separate membrane tank filled with mixed liquor can be considered as
biological treatment volume.
3.36
surface porosity
percentage of the membrane surface occupied by the pores
3.37
transmembrane pressure
TMP
pressure loss across the membrane
Note 1 to entry: The transmembrane pressure is expressed in kPa or bar. For practical measurement see 7.4. In
practice this measurement includes losses attributable to the hydrodynamics of the system.
3.38
viscosity
property of a fluid to resist internal movements (turbulences) or global movements (flowing)
Note 1 to entry: Viscosity contributes to pressure loss in water flowing in pipes or through membranes.
There are two types of viscosities:
— dynamic (or absolute) viscosity μ (Pa·s);
µ
— kinematic viscosity ν= (m /s).
ρ
Where
Ρ is the specific gravity of the fluid (kg/m ).
3.39
clean water permeability
corrected permeability of membrane in clean water
4 General system — requirements
4.1 Basic considerations
Possible negative effects on MBR system performance can arise from:
— fibres and/or hair;
— organic solvents;
— fats, oils and greases;
— synthetic polymers;
— limited biodegradability;
— temperature;
— abrasive substances (e.g. sand);
— silicon;
— calcium;
— alkalinity;
— flow (sewer infiltration);
— type of sewer system;
— unwanted short-circuiting of raw, untreated wastewater directly to the membrane.
In the case of severe negative effects predictable from the wastewater characteristics - especially in the
case of unfavourable COD/BOD ratios and of industrial wastewater fractions - a feasibility membrane
test should be carried out to assess the suitability of the membrane bioreactor process. The aims of such
a test are to evaluate the principal filterability and to estimate the filtration performance over time.
Whereas for reverse osmosis a simple parameter such as SDI (silt density index) is used to evaluate the
feasibility of the process application, such a parameter does not exist for membrane bioreactors. The
main difference is that for membrane bioreactors the feed is activated sludge and not the raw
wastewater. Thus the feasibility test has to be conducted with activated sludge as the raw wastewater
characteristics are of limited use. As membrane fouling interactions are always dependent on the
properties of a fouled membrane, a prediction of membrane filterability based on the characteristics of
a clean/new membrane is unreliable.
Comparative pilot-scale tests are the most reliable means of predicting the suitability of membrane
bioreactors for a specific wastewater. Under the given wastewater conditions the comparative studies
should be directed towards resolving the following issues (adopted from [13]):
— functionality and performance of the membrane (peak flux, critical or sustainable flux);
— biological treatment (COD removal, nitrogen removal, phosphorus removal, sludge characteristics);
— membrane fouling (TMP evolution, fouling rate);
— achievable effluent quality;
— system operability;
— cleaning procedures.
Even if bench-scale or pilot-scale tests were performed the relative contribution of biomass
supernatant to the overall membrane fouling varies in a wide range from 17 % to 81 % [7] and
emphasizes the need of a comparable protocol to minimize the impact of differing test conditions.
Because of different module designs general specifications of applicable pilot plants are impractical.
Therefore because of a well-established membrane bioreactor market with several suppliers [6] the
know-how of the suppliers should be used for the design and operation of the pilot plants.
The following criteria have been recommended [13] for the pre-selection of membrane suppliers for a
comparative pilot-scale test series:
— world-wide experience with full-scale applications of membrane bioreactor technology;
— expected technical suitability for application of the membranes for the given wastewater and the
given circumstances;
— future membrane production capacity and pricing;
— liability of the companies involved.
The full self-supporting and independent pilot plants should be equipped with all features necessary for
automatic operation including data collection and processing. The size of the pilot plants should be
according to the prerequisite of a representative scale [13]. The use of a standard full-scale membrane
module is necessary for reliable results.
Prior to the start-up of an installed membrane bioreactor the clean water permeability (PWP) and the
membrane integrity should be determined. These two parameters are reliable tools for the quality
control and the monitoring of the membrane status during the life-time by the end-users.
4.2 Pre-treatment and internal sieving
In order to keep a safe operation of MBR, advanced mechanical pre-treatment (AMP) of the wastewater
is necessary that exceeds the standard methods of screening and grit removal. AMP of the raw
wastewater is needed to avoid braid formation, sludge accumulation and, as a consequence hereof,
deterioration of hydraulic efficiency of the membranes.
As membranes form a physical barrier for solids, including hairs and fibres, it is evident that the content
of hairs and fibres in a MBR will increase if the amount of incoming hairs and fibres exceeds the
removal of hair and fibres – as such or as braid – via the excess sludge. Thus, withholding hairs and
fibres from entering the MBR part of the WWTP by means of screens and sieves is of prime importance.
Although gap width is the most important property, gap geometry, type of construction (structural
shape), and flow direction through the sieve have to be regarded. Slit geometries offer less efficiency
than mesh or hole geometries [16]. Also, operational issues like amount of residues, type of cleaning,
usage of water or air, usage of auxiliary materials and attendance required are to be considered.
Recommendations on maximum gap width can be found in [6], [16], [17], [21] and in manufacturers’
specifications.
No leakage or bypass of untreated wastewater is acceptable. Standby equipment shall be present.
Primary sedimentation can improve the measures for AMP but not replace them. The impact of primary
sedimentation for the overall plant design shall be considered.
Research on braid formation [16] revealed that the phenomenon of braid formation in general is not
dependent upon membrane module types (HF or FS membranes). It could be shown that the most
important factors are turbulence, length of hairs and fibres, SS concentration and fat content. Even in a
basin without any installation (such as membrane modules, for example), braids can be formed if
energy input, amount of long hairs and fibres and MLSS and grease/fat are sufficient.
Thus, as an alternative, internal sieving according to Figure 2 can be applied when using a separate
MBR Separate System.
As in this case braid formation will occur in the aeration tank, it is sufficient to use a standard pre-
treatment, allow braid to be formed in the aeration tank and remove it via the internal sieve.

Key
1 advanced pre-treatment 3 conv. pre-treatment
2 aeration tank 4 sieve
Figure 2 — Possible design of mechanical treatment [16]
4.3 Characteristics of biological systems used in MBR plants
4.3.1 General
Carbon removal is required and complete nitrification is recommended in MBR systems. Short circuits,
high concentrations of extracellular polymeric substances (EPS) or incomplete biodegradation of
wastewater may affect filterability.
4.3.2 Mixed liquor suspended solids (MLSS)
Membrane systems are usually operated with MLSS concentrations considerably higher (e.g. 6 g/l to
18 g/l) than conventional activated sludge systems.
Higher concentrations of suspended solids may have an influence on
— aeration (lower alpha-factor);
— volume of the aeration tank (lower volume of the AT).
4.3.3 Hydraulic retention time (HRT (or detention time))
Due to high suspended solids concentrations, the volume of the aeration tank may be decreased (see
4.3.2). This lowers the HRT.
Lower HRT may have an influence on effluent quality and consequently on membrane performance due
to:
— shorter biological contact time;
— lower equalization effects in times of high influent loads.
4.3.4 Sludge age (or sludge retention time (SRT))
The SRT should be designed to allow an advanced carbon degradation and full nitrification (if required),
at any time and in particular during peak loads and at low temperature. The volume of excess sludge
will be lower as SRT increases.
NOTE A typical range for a minimum aerobic SRT is 12 d to 15 d (at 12 °C).
4.3.5 Chemical phosphorus removal
Metal salts can be used to precipitate soluble phosphorus from MBR systems. However precipitant(s)
dosage should be carefully controlled in order to protect the membrane from inorganic fouling. This
may require more frequent low-acid cleaning.
4.3.6 Aeration
It should be recognized that the high MLSS content in the aeration tank will adversely affect the oxygen
transfer coefficient.
Due to the low HRT the dissolved oxygen control loop should have a short reaction time in order to be
more responsive.
4.4 Membrane filtration system
The membranes can be installed directly in aeration tanks or in separate filtration tanks. Examples are
given in Figure 3.
The manufacturer should give recommendations for maximum allowable MLSS.
a)
b)
Key
1 inlet 3 excess sludge
2 permeate 4 return sludge
Figure 3 — Schematic of MBR Integrated System a) and MBR Separate Systems b)
Both systems have advantages and disadvantages, depending on application as well as the size of the
plant. When choosing the membrane filtration system design, the following aspects should be
considered:
— the cost of civil works for separate membrane tanks can be higher;
— separate membrane tanks allow for draining membranes with minimized loss of biological volume
as a separate membrane tank can be drained while the biological system and remaining membrane
tanks still operate;
— depending on the specific layout of the membrane system, the maintainability of integrated and
separate systems may be different and/or may require more expensive equipment, e.g. lifting
equipment;
— in order to control the level of biomass in the system, with separate membrane tank systems sludge
can be drawn from the return sludge line, and in integrated systems, directly from the aeration
tank;
— in a MBR Separate System, it is possible to adjust the MLSS concentration in the biological tank(s)
and in the membrane tank(s) independently in a wide range, which may be of importance when
upgrading plants utilizing existing tank volumes, seasonal varying loading, expected increase of
biological load etc. Higher MLSS has a positive impact on reducing the bioreactor volume and a
negative effect on the aeration system due to a decreasing alpha factor. In some systems it can also
affect the necessary membrane area. The design should optimize all of these factors;
— choosing a MBR Separate System makes it possible to introduce internal sieving (see 4.2);
— when utilizing a MBR Separate System, a high flexibility concerning tank volumes in operation can
be applied. The number of tanks being in operation for denitrification and nitrification and the
number of membrane tanks can be chosen independently. For example, for seasonal operation the
biological volume can be reduced while the full hydraulic capacity is maintained;
— when utilizing separate membrane tanks, it is necessary to incorporate pumps to either feed or
return. Integrated tanks will eliminate this need if sufficient mixing is provided to eliminate MLSS
gradients;
— in designs with simultaneous or intermittent nitrification / denitrification, having membranes
located directly in the aeration tanks, scouring air could impact denitrification. Therefore filtration
off time due to scouring air off time should be considered in the hydraulic process system design;
— plant hydraulic control is largely defined by the upstream buffering capacity, as such regardless of
whether the membranes are located in separate membrane tanks or within the aeration tanks, a
similar control strategy can be utilized.
4.5 Mixed liquor recirculation
Mixed liquor recirculation should be designed to prevent excessively high MLSS concentrations across
the membranes. The second purpose of the recirculation is to bring the bacteria back to the head of the
biological treatment process.
The oxygen concentration of the return flow from the filtration system can be significant and shall be
considered in the design of the biological system. For nitrification this oxygen contribution can be
beneficial but is detrimental where denitrification is required. In applications where total nitrogen in
the effluent is critical, the return sludge shall be directed to a point in the system so as not to disrupt the
anoxic process, such as return directly to the aerobic tank.
4.6 Permeate extraction system
Depending on the hydraulic profile and the membrane characteristics, permeate extraction can be
performed by means of suction pumps or by gravity flow.
System design shall ensure proper air removal from the permeate extraction system.
4.7 Desired effluent system
The treatment of wastewater with a MBR system shall ensure that limits, e.g. for hygienic parameters or
suspended solids, meet the requirements of national or local regulations or the relevant authority.
5 Material characteristics
5.1 General
This clause deals with membrane materials.
Common organic materials for membranes are polyvinylidene difluoride (PVDF), polyethersulfone
(PES), polysulfone (PS), polyacrylonitrile (PAN), polypropylene (PP), polyethylene (PE), chlorinated
polyethylene. These materials can have hydrophilic or hydrophobic characteristics. This may have an
effect on the storage and the start-up conditions of the membrane.
Ceramic materials can be used for membranes.
Membranes may consist of a support providing mechanical stability and the membrane layer itself.
The limits for the operating conditions (e.g. temperature, chemicals, pH, pressure) are given by the
membrane manufacturer and may not only be related to the membrane material itself.
5.2 Porous membranes
Microfiltration (MF) and ultrafiltration (UF) membranes applied in MBR systems contain pores. In
contrast to the free volume within a polymer, a pore is defined as a position- and time-invariant
discontinuity in the dense membrane material [4].
The commercially available porous membranes are characterized by means of:
i) the nominal pore diameter or;
ii) the molecular weight cut-off;
iii) pore diameter distribution.
NOTE 1 Membranes with narrow pore size distributions in the MF range, so-called microsieves, exist as well.
For pore diameter measurement, see the example given in Annex E. For MWCO point determination, solutions
containing macromolecules of different molecular weights (e.g. dextran solutions [11]) are commonly used as a
model substance for membrane characterization. With this test method, the molecular weight of the key
component which is rejected to a degree of 90 or 95 %, is determined [9].
NOTE 2 For the characterization of both MF and UF membranes, it is essential to operate the test cell in cross-
flow mode with high cross-flow velocities and low TMPs. Otherwise, cake layer formation is more significant,
which in turn leads to an overestimation of the rejection of feed components by the membrane. Additional
information can be found in standard test method for molecular weight cut-off evaluation of flat sheet
ultrafiltration membranes [15].
For pore diameter distribution measurements see [22]. For the impact of pore size distribution on
membrane fouling see Annex F.
6 Configuration
6.1 Flat Sheet
A flat sheet membrane configuration is characterized by:
— an active separation layer on a support layer, which may be of different materials;
— single sheets of membrane materials featuring an even surface;
— typically two membrane sheets are attached (glued, welded, squeezed) to a support plate
(membrane panel), which is also the permeate carrier;
— also two membranes can be attached to the interface material or drainage layer (so called
membrane bags);
— to enhance the permeate transport some interface material (spacer) might be used between the
membrane sheet and the support plate.
A flat sheet membrane element, also called cartridge (typical thickness between 2 mm and 8 mm) or
bag (typical thickness between 1 mm and 2 mm) is made of the components mentioned above.
Several flat sheet membrane elements are assembled to modules.
Important for the mixed liquor flow between the membrane elements is the channel width (typical
width between 5 mm and 10 mm).
Two or three modules might be stacked on top of each other. These are called 2-stack modules or 3-
stack modules (“double decker” or “triple decker”) respectively.
The membranes can be arranged in rotational systems as well.
6.2 Hollow fibre
A hollow fibre membrane configuration is characterized by:
— single fibres typically made from a woven backing material (so called “braid”, made from polyester
for example) which is covered by the membrane material (for example PVDF or PES). Those fibres
are called “reinforced” fibres. Non reinforced fibres are also on the market.
Important are the inner and outer diameter of the fibres.
— several fibres are assembled into bundles or sheets and potted into headers where the filtrate is
withdrawn. Both equal spacing of the fibres in the header and the distance between the fibres are of
some importance regarding the solids tolerance.
In wastewater applications the typical fibre orientation is vertical. Dependent on the manufacturer
there are bottom and top headers or bottom headers or top headers only.
— several bundles / sheets are assembled into membrane elements. Again the distance between the
fibre bundles or sheets is of importance.
— several membrane elements are combined into membrane modules and cassettes, characterized by
a central port for filtrate withdrawal.
7 Design and operating parameters
7.1 General
For basic design data requirements related to wastewater treatment see EN 12255-11.
7.2 Influent characteristics
For the design of the filtration system the end user should at least provide the following information:
— minimum flow;
— daily and long term (seasonal) average flows;
— peak flows (duration and frequency);
— minimum and maximum temperature;
— flow balancing capacity.
Additionally the worst case situation should be taken into consideration.
7.3 Fouling
Membrane fouling has 2 possible origins:
— organic solids deposited on the membrane surface and/or inside the pores;
— mineral solids deposits due to chemical reactions such as metal oxides and scaling.
Furthermore, activated sludge deposits shall be controlled by removal at periodic intervals. This is
performed in 3 different ways:
— scour aeration;
— water backwashing and / or relaxation (scour aeration without permeation). These
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