Submerged membrane bioreactor system for municipal wastewater by cometjunkie42


									Indian Journal of Chemical Technology
Vol. 15, November 2008, pp. 604-612

    Submerged membrane bioreactor system for municipal wastewater treatment
                           process: An overview
                                               Neha Gupta, N Jana & C B Majumder*
                     Department of Chemical Engineering, Indian Institute of Technology, Roorkee 247667, India
                                        Received 27 September 2007; revised 20 September 2008

            The submerged membrane bioreactor (SMBR) is a promising technology for wastewater treatment and water
      reclamation. This paper provides an overview of wastewater treatment in a submerged membrane bioreactor process with a
      special focus on municipal wastewater systems. SMBRs predict more than 95% organic removal with relative short
      hydraulic retention times (HRTs) of 1-8 h and NH3 removal of more than 90% in the municipal wastewater treatment. It
      achieves 30% more removal of organic matter than activated sludge process. The COD can be reduced by 95%. Nitrification
      was complete and up to 82% of the total nitrogen could be denitrified. Nitrification/ denitrification is sensitive to the feed
      quality, such as dissolved oxygen (DO) concentration, temperature, organic loads, inorganic/ organic compounds and pH.
      The feed water composition, membrane geometry/configuration, membrane materials, and hydrodynamic effects are
      responsible for membrane fouling. These drawbacks can be reduced by maintaining turbulent conditions, operating at
      critical flux, and by selection of a suitable fouling resistance membrane material. Membrane washing is performed when the
      permeability is less than 10% of initial permeability. The reactor performance and the stability of the process and the
      membrane capacity are also discussed. Details of the various methods for washing are also included.
      Keywords: Submerged membrane, Bioreactor, Wastewater treatment, Nirification, Hydrarulic retention time

Recently developed and one of the most promising                       bioreactor (Fig. 1). For municipal applications, it is
newer technologies is the membrane bioreactor                          expected      that     the    hollow-fiber  submerged
(MBR) consisting of both a biological stage and a                      configuration would be useful for medium-to-large
membrane module, a process involving membrane                          size plants. For small-to-medium size plants, plate-
filtration combined with biological treatment. In a                    and-frame technologies would have an advantage,
membrane bioreactor solid/ liquid membrane filtration                  whereas larger applications could be designed with
occurs either within the submerged configuration or                    secondary/tertiary treatment followed by membrane
externally through recirculation of bioreactor subject                 filtration and ultrafilteration.
to pressure drop across the membrane. Submerged                           For the submerged configuration, membranes act in
membrane bioreactor has been gaining lots of                           the aeration basin as filter and improve the biological
attentions for wastewater treatment for its better                     treatment of the effluent. In the external system1, the
effluent quality and lower sludge production                           amount of permeate flux varies between 50 and 120
compared to conventional activated sludge processes.                   L.h−1.m−2 and transmembrane pressure (TMP) is in the
This paper presents an overview of the submerged                       range of 1-4 bar. In the submerged configuration, the
membrane bioreactor technology suitable to treat the                   permeate flux is removed by suction which limits
municipal wastewater. A comparison of the                              transmembrane pressure at about 0.5 bar. The
membrane bioreactor to conventional wastewater                         submerged configuration appears to be more
treatment processes has been made and the                              economical based on energy consumption for two
characteristics of the submerged membrane bioreactor                   main reasons: no recycle pump is needed since
are discussed.                                                         aeration generates a tangential liquid flow in the
                                                                       vicinity of the membranes. Furthermore, the operating
Membrane module characteristics and comparison                         conditions are much more feasible in an external
with an activated sludge system                                        membrane bioreactor system because of the lower
  For a bioreactor, there are two types of                             values of the transmembrane pressure and tangential
configurations for the membrane module: the                            velocities. Lesjean et. al.2 established design
membrane can be placed either outside or inside the                    parameters of submerged membrane bioreactor
                   GUPTA et al.: SUBMERGED MEMBRANE BIOREACTOR SYSTEM: AN OVERVIEW                                      605

                       Fig. 1 — Bioreactor system (a) submerged membrane, and (b) external membrane

system (plate-and-frame and hollow fiber) which                     Table 1 — Design parameters of submerged membrane
indicated that the hollow fiber membranes are                                          bioreactors35
economical to use in submerged membrane bioreactor             Membrane/module type          Unit      Plate-and- Hollow
(Tables 1 and 2). Overall, the membrane bioreactor                                                       Frame       fiber
offers following advantages over the traditional                                                      (Flat sheet) (Bundles)
activated sludge system:                                       Net flux                     L/h.m2      15-25       20-30
                                                               Recommended MLSS           G MLSS/L      10-15       10-15
(i)   Energy consumption and capital costs for                 Fraction of aerobic volume     %         30-10       10-40
      fabrication and maintenance are lower for                occupied by membrane
      submerged system.                                        Energy consumption          Kwh/m3       0.3-0.6    0.3-0.6
                                                               (membrane system only)
(ii) The volume of the aeration tank can be reduced            Cost                          m−2         High      Medium
      since a higher concentration of biomass can be           pH range                        -         1-12       2-11
      stored in the bioreactor.                                T0 residence                   °C         <60        <40
(iii) The traditional secondary clarifier is replaced by
      a membrane module. This module is more                   microbial process using porous carrier has been
      compact and the quality of rejected water is             applied in view of high concentration culture of
      independent of the variations of sludge settling         microbes5 or multifunctional microbial reaction such
      velocity.                                                as simultaneous removal of carbonaceous and
(iv) The membrane bioreactor allows the biomass                nitrogenous substances from municipal wastewater6, 7.
      concentration to be higher than for traditional
      treatment plants. Jefferson et. al.3 utilized a          Carbonaceous removal
      20 g.L−1 biomass concentration while Yamamoto              The membrane bioreactor is characterized by a
      et. al.4 utilized 30 g.L−1 in membrane bioreactor,       complete retention of the biomass inside the
      whereas conventional processes utilize biomass           bioreactor because of the use of membrane separation,
      concentration of less than 5 g.L−1 in order to           which controls and increase the sludge retention time
      avoid problems inherent to settling of                   (SRT), and is a significant operational factor for the
      concentrated flocs.                                      biological process being independent from the
(v) The process can be run at high solids retention            hydraulic retention time (HRT). High SRT enables
      time, which favours the development of slow              one to increase the sludge concentration and the
      growing micro-organism and ultimately lead to            applied organic load, thereby increasing the pollutant
      better removal of refractory organic matter and          degradation. The organic decomposition rate
      making the system robust to load variations and          decreases while the sludge retention time increases. A
      toxic shocks.                                            small reduction in COD consumption is observed in
                                                               the bioreactor with short sludge retention times. In the
Biological activity of submerged membrane                      membrane process, COD removal remains constant at
module                                                         95% whatever may be the sludge retention time.
  Membrane processes have been extensively studied             Dufresne et. al.8 were the first to undertake a
for biological wastewater treatment. Retained                  comparative study of the traditional activated sludge
606                                INDIAN J. CHEM. TECHNOL., NOVEMBER 2008

process (ASP) and the membrane bioreactor. This            bioreactor system and aerated biological filters for
study confirmed that the performance of membrane           gray water recycling. Once again, the membrane
bioreactor is better than that of conventional activated   bioreactor system was found to be more efficient in
sludge processes, especially for COD removal and           the removal of biological oxygen demand, turbidity
solid suspension separation. Jefferson et. al.9 carried    and coli forms.
out study of the performance of a membrane
          GUPTA et al.: SUBMERGED MEMBRANE BIOREACTOR SYSTEM: AN OVERVIEW                                        607

         Table 2 — Examples of membrane types and characteristics used in submerged MBR system

                   Membrane      Membrane           Pore size       Wastewater       Reference
                                 geometry         characteristics   (µm)
                   Hollow        MF-                    -           Municipal           36
                   fiber         polypropylene                      and
                   Hollow        MF                     0.1         Municipal           20
                   Hollow        Zenon                  0.1         Municipal           16
                                 MF-                 0.2-0.4 17
                   Flat          MF-                    0.4         Domestic            16
                   Hollow        MF-                    0.1         Municipal           27
                   fiber         polyethylene
                   Plate         MF-                    0.4         Municipal           27
                   Hollow        Hydrophilic            0.1         Municipal           11
                   fiber         polyethylene
                   Hollow        Polypropylene          0.1         Synthetic           12
                   fiber                                            municipal
                   Hollow        Polyethylene           0.1         Municipal           15
                   Plate and     Polysulphone           0.4         Domestic            6
                                 Polypropylene         0.5-5        Domestic            6
                   Flat          MF-                    0.4         Municipal           37
                   hollow        polyolefin
                   Hollow        Polyethylene           0.1         Domestic            37
Table 3 — Organic loading rates of different biological treatment processes of submerged membrane bioreactor32

             Reactor        Membrane         Organic loading        HRT   Percentage        Reference
                            geometry               rate
                                            kgBOD5 kgCOD            (h)    removal
                                              m−3        m−3
                                             day−1      day−1
             Activated      Sequencing      0.08-          -        12-     85-95              38
             sludge         batch            0.24                   50
             Process        Conventional     0.32-         -        4-8     85-95              38
                            Complete-       0.08-          -        3-5     85-95              38
                            mix              1.92
                            High-rate       1.6-16         -        2-4     75-90              38
             Submerged      Plate     and   0.39-0.7       -        7.6         99             39
                            Hollow fiber    0.03-          -         1      98-99              40
                            Plate    and    0.005-         -         8          98             14
                            frame            0.11
                            Hollow fiber       -         1.5        0.5     87-95              13
                            Plate    and       -         0.45        8      88-95              41
608                                  INDIAN J. CHEM. TECHNOL., NOVEMBER 2008

   It has been reported that the membrane in a MBR                 to feed water quality determinants and operational
contributes approximately 30% to the removal of                    parameters such as dissolved oxygen (DO),
organic matter, this roughly equates to the insoluble              temperature, organic loads, inorganic and organic
fraction10, with the soluble fraction being removed via            compounds, and pH. Nitrification can be maintained
the active biomass. From Table 3, it can be seen that              at higher rates with lower DO concentrations
organic removal is often greater than 95% even with                (<5 mgL−1). Denitrification, the reduction of nitrate to
relatively short HRTs of 1-8 h (ref. 11-13). Organic               various gaseous end-products such as molecular
loading rates of MBRs are typically higher than                    nitrogen and nitrogen oxide, can proceed alongside
conventional ASP, owing to the shorter HRTs                        nitrification if:
(Table 3).
   Increasing organic loads to an ASP produces                     (i) Aeration is performed intermittently,
increased heterotrophic activity, as organic matter                (ii) Hydrodynamics are such that an anoxic area
removal follows first-order kinetics, and this can be                    results,
assumed to be the case in MBRs. Organic matters                    (iii) High organic loads are added allowing anoxic
removal in MBRs appears not to be significantly                          micro-sites to develop within the flocs.
affected by low temperatures, such as those between                   In aerobic MBRs denitrification can be achieved by
5 to 20°C, which may be due to the number of                       the addition of an anaerobic tank prior to the aeration
heterotrophic bacteria in the biomass remaining                    tank, with conventional recycle17.
constant, albeit with a decreased activity at lower
temperatures14.                                                    Membrane modules fouling
                                                                      Membrane fouling is influenced by the chemical
Nitrogen removal                                                   nature of the membrane and by the membrane
   Biological nitrogen removal has been done by                    operational parameters such as suction and bubbling.
using membrane systems to concentrate the nitrifier in             Hollow-fiber microfiltration membrane induces
a reactor15,16. Small increase in the oxygen supply rate           transmembrane pressure gradients, which have an
and the improved nitrification rate17,18 facilitates the           impact on flux rates. The magnitude of the flux
growth of microbial flocs. In spite of such improved               depends on the design of the hollow-fiber (length,
nitrification capability in membrane processes, an                 internal diameter, permeability) and on the properties
anaerobic denitrification reactor is still needed for              of the cake. The structure of the membrane pores play
achieving complete nitrogen removal. Nitrification                 a significant role on the fouling and the rougher the
has been shown to be greater in an MBR than with a                 surface of the membrane faster the fouling by
conventional ASP owing to the longer retention times               attachment of colloids and the particulate matters on
of the nitrifying bacteria (high sludge age, low food/             the membrane surface.
microorganism ratio) and the smaller floc sizes,                      The limiting factor for further process development
allowing slightly greater mass transport of nutrients              is membrane fouling resulting from the following:
and oxygen into the floc.
   MBRs achieve almost complete nitrification owing                (i)  Formation of a layer or cake on the membrane
to the retention of the slow-growing autotrophic                        and/or the intrusion of molecules, colloids and
bacteria. At nitrogen loads of 0.1 and 3.3 kg NH3                       particles in the porous structure;
m−3 day−1, ammonia removal is greater than 90%. As                 (ii) Preferential adsorption on the membrane
with conventional processes, nitrification is sensitive                 surface. Fouling induces transmembrane flux
                           Table 4 — Flux decline rate of selected submerged membrane bioreactors

                    Membrane        Pore       Flux        Pressure      Time      Flux decline   Reference
                                     size                                              rate
                    configuration   (µm)    (m3m−2day−1)    (bar)        (days)   (mday−2bar−1)
                    Plate     and   0.4         0.5         0.06         0.04         208            41
                    Plate     and   0.4         0.1          0.7          0.2         0.79           14
                    Plate and       0.4         0.9         0.06          39         0.38            13
                    Hollow fiber    0.1         0.2         0.7           70         0.004           10
                   GUPTA et al.: SUBMERGED MEMBRANE BIOREACTOR SYSTEM: AN OVERVIEW                             609

     reduction; when the flux reaches a threshold           declines exponentially with time to a semi-stable low
     value, membrane washing becomes necessary.             value after a few hours. This decline is caused by
                                                            concentration of retained material.
   The complexity of fouling is increased by                   Fouling also contributes to the increased pressure
biological activity, and the progression in this field is   drop and thus the permeate flux increases with it. The
relatively slow18.                                          presence of dissolved substances in the solution
                                                            causes an accumulation of solutes on the retaining
Permeate flux decline                                       side of the membrane. This layer is less permeable to
   Permeate flux decline means decrease in flux and it      water, resulting in higher osmotic pressure and the
is influenced by a number of factors relating to the        permeate pressure decline but with the increase in
feed water (composition), the membrane (element             fouling it rises due to the rise in trans membrane
geometry/ configuration, pore size area and material        pressure difference and permeate flux vary according
composition), and operation (hydrodynamics)19. Flux         to Eq. (3),
decline rate is dependent on the fouling mechanisms
of various membrane systems. For ultra filtration of        J α Lv( P – σ Π )                               … (3)
colloidal species, there are three main mechanisms.
                                                            where σ is the reflection coefficient, Π is the
(i) Standard pore blocking model                            osmotic pressure difference and Lv is the solvent
(ii) Pore blocking model and                                permeability coefficient.
(iii) Cake build up model                                      It can be concluded by Eq. (1) and Eq. (3) that the
                                                            reduction of trans membrane pressure is due to the
  The permeate flux generally decreases with time           effect of osmotic pressure20,
but increases with increasing operating flux or
pressure (Table 4). The flux of clean water across a        J = P− σ Π /µR                                  … (4)
membrane with no materials deposited on the surface
can be described by,                                           Hirose et al.21 showed the existence of a linear
                                                            relation between the surface roughness of membrane
J = P/µR                                          … (1)     and flux, as it has been mentioned earlier that the
                                                            permeate flux increases with the fouling. Surface
where J is the permeate flux (m3/m2s), P is the             roughness increases fouling due to the increase in the
pressure drop across the membrane (N/m2), µ is the          attachment of colloids and the particulate matters on
absolute viscosity of the water (Ns/m2), R is equal to      the membrane surface. They concluded that greater is
total resistance of the membrane against the flux           the membrane roughness, greater is the local
(1/m).                                                      turbulence, wall shear stress and the permeate flux.
                                                               In submerged membrane bioreactor the permeate
R= Rm + Ri + Rc                               …(2)          flux is made stable at particular value. There are also
Rm= hydraulic resistance of the membrane (m−1) in           two modes of operation - constant transmembrane
pure water,                                                 pressure (TMP) or constant flux. In a constant TMP
Ri= irreversible fouling resistance of the membrane         operation, deposition and fouling cause a decline in
(m−1) and                                                   flux that is initially rapid, but later becomes more
Rc = resistance due to particle deposit at the              gradual. For constant flux the effect of deposition and
membrane surface and it increases with roughness of         fouling is to increase TMP, which is initially gradual,
membrane (m−1).                                             but accelerates prior to cleaning. Constant flux is the
                                                            preferred mode of operation for membrane
   A comparison of the resistance caused by fouling         bioreactors because it ensures a steady throughput21.
with the performance of the membrane was quantified            Depending upon biomass concentration and the
in terms of specific flux i.e. the permeate flux            permeate flux variation, Vyas et. al.22 showed two
produced per unit pressure applied (l/m2h bar). It was      distinct zone, increasing the biomass concentration
observed that initially fouling of the membrane             from 0.65 to 2.5 g.L−1 a flux reduction, beyond
surface causes a high rate of flux decline. Thus,           2.5 g.L-−1 the biomass concentration had no
permeate flux decreases with the increase in                significant effect on the membrane fouling and
resistance of membrane due to fouling. The flux
610                                 INDIAN J. CHEM. TECHNOL., NOVEMBER 2008

permeate flux remained stable. Defrance et. al23            (usually associated with the biomass growth) and by
reported similar results, the first zone with               biomass decay. PMS must be regarded as one of
concentration range of 0.8-1.5 g.L−1 and second zone,       significant factor in the process of biological
that is stabilization zone, of the fouling layer with       treatment. PMS is classified as:
concentration range of 1.5-5 g.L−1. When the
                                                            (i) Utilization-associated product (UAP).
concentration increases between 5 to 10 g.L−1, the
                                                            (ii) Biomass-associated product (BAP).
third zone corresponds to a flux decrease. This could
be due to the suspension of rheological properties
                                                               The UAP represents the PMS associated to the
(increase of non-Newtonian viscosity).
                                                            substrate metabolism and the biomass growth; the rate
                                                            of production of biomass is proportional to the
Critical flux                                               substrate consumption rate. The BAP are PMS
   Field et al.24 were the first to introduce the concept   associated with biomass death and biomass
of the critical flux. The membrane fouling can be           production is proportional to the concentration; the
neglected below critical flux, and thus membrane            BAP is a by-product of the endogenous respiration of
cleaning is not required. It is important, therefore, to    the cell mass. Because of the concentration of
choose an adequate initial permeate flux or                 activated sludge in the reactor and the long sludge
transmembrane pressure.                                     retention times, it is impossible to ignore the
   Liu et. al.25 observed that the hydrodynamic             formulation of the microbial products in the
characteristics have a strong influence on liquid mass      membrane bioreactor system.
transfer of submerged membranes bioreactor. Critical           The disposal of undesirable organic and mineral
flux depends on hydrodynamics, particle size (it            matter occurs with the help of flocculating bacteria,
reaches very quickly for small particles), interaction      which are the base of sludge. The biological flocs are
between colloids and membrane and suspension                disintegrated by circulation inside the membrane
properties (pH, salinity and conductivity).                 bioreactor. Changes in the particle size and there
   The system hydrodynamics is also important in            distribution modify the fouling properties of the
determining the critical fouling point, as has been         suspension; the presence of smaller particles, resulting
demonstrated by Muller et al.26. The airflow rate           due to the breakdown of the flocs increases the
affects the liquid tangential circulation, which in turn    membrane fouling. For an external type of membrane
affects the transmembrane pressure; furthermore the         bioreactor, the membrane fouling depends on the
dimensions of the bioreactor also act on the liquid         intensity of shear stress imposed on the bacterial flocs
circulation.                                                by the recycling pump. The pump breaks down the
   The various biological conditions within the reactor     flocs, generating more of the colloidal particles and
are supposed to act differently on membrane fouling.        releasing the extracellular polymeric substances (EPS,
It is, therefore, clear that a purely hydrodynamic          principal compounds of the soluble organic matter)
analysis is not sufficient to explain the fouling           from the interior of the flock to the bioreactor. The
problem. Ogoshi and Suzuki et al. 27 showed that in a       bacterial suspension is composed of three fractions29:
bioreactor membrane fouling can be neglected below          (i) The soluble fraction
critical flux, as low transmembrane pressure prevents       (ii) The colloidal fraction
an irreversible membrane fouling and a reduction in         (iii) The particulate fraction
the membrane cleaning frequency. Generally, there
appears to be much corroboratory evidence from              Washing and regeneration of membrane modules
MBRs to support the critical flux hypothesis; many             Effective washing requires an understanding of the
submerged (plate-and-frame) plants run at <0.4 bar          interaction between the fouling products and the
TMP with no noticeable steady-state flux decline27.         membranes as well as the effect of the washing
                                                            procedures on elimination of the deposit. Membrane
Concentration and sizes of the particles and flocs          washing is performed when the permeability is less
  Barker et al.28 showed that the main part of the          than 10% of initial permeability. Details of the
soluble organic matter present in the effluents of          different methods in chronological order of their
biological treatment process is the produced microbial      applications are as follows.
soluble (PMS). PMS is composed of the organic
compounds released by the metabolism of substrate           Intermittent filtration
                     GUPTA et al.: SUBMERGED MEMBRANE BIOREACTOR SYSTEM: AN OVERVIEW                                 611

  The suction pump is switched off for approximately             membrane clean with a 0.1% sodium hypochlorite
10 min, and then the filtration operation is reinitiated.        solution. Cicek et al.33 showed that a ceramic
During the intermittent filtration, the recirculation            membrane required weekly cleaning for about 2 h
pump continues to make the retentate circulate29.                using 5.25% NaOCl heated to 60-80°C along with the
                                                                 concentrated nitric acid.
Forward flush
  The filtration is stopped when the reactor is filled           Membrane bioreactors design and operational
up with pure water. Then, the water is recirculated for          options
10 min without filtering.                                           Membranes can be incorporated into the design of
                                                                 wastewater bioreactors as the solids / liquid separation
Back flush
                                                                 stages into submerged membranes bioreactor (Fig. 1).
   Back flush consists of reversing the filtration               In this case surface shear and/or backflushing are
direction for 5 to 30s in every 30-60 min or in every            applied to control cake formation and fouling. In the
hour, possibly accompanied by the air sparing. Ognier            submerged system the membranes are located in the
et al.30 showed that the pure water was filtered in the          mixed liquor tank, where bubbling is applied to
opposite direction of the normal filtration operation            produce surface shear and permeate flux is kept stable
with a TMP of 0.5 bar. Marcucci et al.29 cleaned UF              which ensure the TMP is < 1 atmosphere. There are
membranes by backwashing for 90s at every 20 min                 also two modes of operation - constant TMP or
under a TMP of 0.4 bar. When the hydraulic                       constant flux (Fig. 2). At constant TMP operation,
performance markedly decreases, the membrane must                deposition and fouling cause a decline in flux that is
undergo chemical washing by acid and alkaline                    initially rapid, but becomes more gradual. For
compounds.                                                       constant flux the effect of deposition and fouling is to
Back pulsing                                                     increase TMP, which is initially gradual, but
   High-frequency back pulsing is different from                 accelerates prior to cleaning. Constant flux is the
backwashing. High-frequency impulses (typically                  preferred mode of operation for membrane
0.1-2 Hz) can also be applied over a very short period           bioreactors (MBRs) because it ensures a steady
(nearly a second for microfiltration and ultrafil-               throughput34.
tration). High-frequency back pulsing can be                        Configuration of various submerged membrane
performed every couple of seconds after a few                    bioreactor is as follows:
minutes of filtration. The problem of this technique is
                                                                 Hollow- Fiber-Capillary
that it requires high-pressure-resistant membranes30.
                                                                   • Very small diameter membranes (< 1mm).
Chemical cleaning                                                  • Consist large number of membranes in a
   Generally in first step of chemical cleaning,                       module and self supporting.
membrane is chemically cleaned with a slightly                     • Density is above 600 to 1200 m2/m3 (for
diluted solution of nitric acid at room temperature.                   capillary membrane), up to 30000 m2/m3 (for
Bouhabila et al.31 showed that in case of less                         hollow fiber membrane).
permeability, membrane are soaked in chloride water                • Size is smaller than other module for given
(2000 ppm Cl2/L) for 24 h.                                             performance capacity.
   Marakami et al.32 cleaned a submerged plate and                 • Process “inside-out”, permeat is collected
frame configuration by employing a twice-yearly                        outside of the membrane.
                                                                   • Process “outside-in”, permeat passes into
                                                                       membrane bore.
                                                                 Plate and Frame
                                                                   • Structure is simple and membrane replacement
                                                                        is easy.
                                                                   • Similar to filter press.
                                                                   • Density is about 100 to 400 m2/m3.
                                                                   • Membrane is placed in a sandwich style with
Fig. 2 — Constant TMP operation and constant flux operation [a          feed sides facing each other.
                     & b respectively]
612                                INDIAN J. CHEM. TECHNOL., NOVEMBER 2008

  •   Feed flows from its sides and permeate comes          membranes tend to have higher TMPs than
      out from the top and the bottom of the frame.         membranes for a given flux, particularly at the
  •   Membranes are held apart by a corrugated              beginning of an operational cycle.
      spacer.                                                 Nanofiltration (NF) membranes have pores that are
                                                            approximately 2 nm in size and retain most species,
Following design equation can be used to design a           except for certain monovalent ions and low molecular
single SMBR:                                                weight organics. They are rarely applied to MBRs
   Sludge retention time (SRT) can be defined as,           because of their high hydraulic resistance, but they
SRT = (Total mass of organism in the reactor)/(Total        may be of interest in niche applications.
mass of the organism leaving the system per day).             Membrane materials are either organic polymers or
   SRT can be calculated by the following equation:         inorganic, i.e. ceramics. The relatively high cost of
SRT = V.X/Q.X = V/Q                               …(5)      inorganic membranes puts them at a disadvantage in
                                                            wastewater MBRs, where low cost components are
where V is the reactor volume; X is the biomass             required. When inorganic membranes have been
concentration in the reactor; Q is the wastage flow         evaluated they have usually involved high flux
rate per day.                                               operation (promoting fouling) and high energy input.
                                                            Certain physical and chemical properties of
S = Ks µm−( kd + 1/SRT)]/(kd + 1/SRT)              ...(6)   membranes favour their use in MBRs. These are:

where So and S are the initial and substrate                (i)   Hydrophilicity — it is well known that
concentration at time t respectively, and µm, kd, Ks are          hydrophilic polymers are less prone to fouling
biokinetic coefficients.                                          by biosolids and biosolutes. This favours
  Death rate constant kd, and yield (y) can be                    cellulosic materials, but does not preclude more
determined by using following equation,                           hydrophobic materials such as polyolefins and
Q/X.V (So− S ) = [(1/SRT). (1/Y)] + [kd / Y]       ...(7)   (ii) Robustness — the material should be resistant to
                                                                  chemical cleaning agents and able to handle
Membrane characteristics                                          cyclic stresses, particularly if blow-back or
   The primary role of the membrane in a SMBR is to               bubbling are applied.
provide a barrier against suspended solids, but mixed       (iii) Modest cost — as mentioned previously many
liquor from the bioreactor is often a complex mixture.            applications require low cost components.
The removal or partial removal of other species is          (iv) Ease of fabrication — some materials are more
possible, but it is dependent on the choice of                    readily processed into microporous barriers or
membrane34.                                                       extruded into hollow fibers than others.
   Microfiltration (MF) membranes have pore sizes
down to 0.1-0.2 microns. They provide high removals            The choice of polymer is a compromise, but those
of suspended solids including most bacteria, as well        generally favour include polyolefins, polysulphone
as partial removal of viruses and macro solutes. The        and polyvinylidene35.
principle macro solute in the liquor is extra cellular      Module characteristics
polymeric substance (EPS) produced by bacteria, and            The module design defines how the membranes are
this is removed by adsorption and gradual retention as      arranged or housed and also provides the fluid
the membranes foul. Thus, MF membranes provide              management, i.e. distribution of feed across the
partial biochemical oxygen demand (BOD) removal             membrane surface. The design also determines other
(i.e. primarily suspended solids), which is retained in     characteristics such as the relative energy demand, the
the bioreactor. Membranes have low TMPs for a               ability to handle suspended solids, ease of cleaning
given flux when unfouled, but this can be gradually         and replacement and the packing density. Table 2
lost because of fouling.                                    summarizes these characteristics for a range of
   Ultrafiltration (UF) membranes have pores ranging        module concepts - the contained modules are used in
from 0.1 microns down to less than 5 nm. These              external loop arrangements. The standard MBR
membranes provide high removals of virus and                (direct membrane filtration of mixed liquor) requires a
reasonable removals of EPS. As a result they help to        module that can handle suspended solids, has a
retain BOD, which is maintained in the bioreactor. UF       relatively low energy demand and can accommodate
                     GUPTA et al.: SUBMERGED MEMBRANE BIOREACTOR SYSTEM: AN OVERVIEW                                     613

reasonably high membrane packing densities. These               12 Churchouse S, Personal Communication, 1998.
requirements tend to exclude tubular modules,                   13 Chiemchaisri C & Yamamoto K, Membrane Sci, 87 (1994)
contained hollow fibers and the spiral wound module,            14 Kishino H, Ishida H, Iwabu H & Nakano I, Desalination, 106
suggesting the use of contained flat sheet systems or              (1996) 115.
submerged membranes35.                                          15 Ueda T & Hata K, Water Res, 33(12) (1999) 2888.
                                                                16 Bouhabila E H, Ben Aim R & Buisson H, Desalination, 118
                                                                   (1998) 315.
Conclusion                                                      17 Mignani M, Nosenzo G & Gualdi A, Desalination, 124
   The nitrogen removal in the membrane bioreactor                 (1999) 287.
system is about 30% higher than conventional                    18 Sakairi M A C, Yasuda K & Matsumura M, Water Sci
treatment process. The nitrogen removal is not solely              Technol, 34 (1996) 267.
due to microbial nitrification/ denitrification but also        19 Shimizu Y, Okuno Y I, Uryu K, Ohtsubo S & Watanabe A,
                                                                   Water Res, 30(109) (1996) 2385.
due to assimilation derived from microbial growth.              20 Mulder M, The Netherlands, 2nd edn (Kluwer Academic
The alkalinity of the suspension can cause the                     Publishers), 1996.
precipitation as a cause of the system instability, even        21 Hirose M, Ito H & Kamiyama Y, Membrane Sci, 121 (1996)
if the system works in sub critical conditions. COD                209.
                                                                22 Vyas H, Bennett R J & Marshall A D, Int Dairy, 10 (2000)
removal efficiency in the submerged membrane                       855.
bioreactor could be maintained at over 95% regardless           23 Defrance L, Jaffrin M Y, Gupta B, Poullier P & Geaugey V,
of STR. Submerged membrane systems, run at low                     Bioresource Technol, 73 (2000) 105.
TMP, achieve a steady-state flux that does not decline          24 Field R W, Wu D, Howell J A & Gupta B B, Membrane Sci,
over the time after the initial fouling period; this is            100 (1995) 259.
                                                                25 Liu R, Huang X, Wang C, Chen L & Qian Y, Process
hypothesized to be due to sub-critical flux. Critical              Biochem, 36 (2000) 249.
flux decreases with increase of STR, indicating that            26 Muller E B, Stouthamer A H, Van Verseveld H W &
membrane fouling has started to occur even at low                  Eikelboom D H, Water Res, 29 (1995) 1179.
flux condition. Membrane fouling in the presence of             27 Ogoshi M & Suzuki Y, Water Sci Technol, 41(10-11) (2000)
microorganism is linked to microbial products,                  28 Barker D J, Mannucchi G A, Salvi S M & Stuckey D C,
concentration, and sizes of the particle. The                      Water Res, 33 (11) (1999) 2499.
membrane fouling rate increases with STR, due to                29 Marcucci M, Nosenzo G, Capannelli G, Ciabatti I, Corrieri D
large amount of foulant. These strategies of                       & Ciardelli G, Desalination, 138 (2001) 75.
membrane washing or backwashing are proposed in                 30 Ognier S, Wisniewski C & Grasmick A, Desalination, 146
                                                                   (2002) 141.
order to maintain a stable permeate flux in the                 31 Bouhabila E H, Aim R B & Buisson H, Sep Purif Technol,
membrane bioreactor systems.                                       22-23 (2001) 123.
                                                                32 Marakami T, Usui J, Takamura K & Yoshikawa T, Water Sci
                                                                   Technol, 41(10-11) (2000) 295.
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