Membrane component of MBRs

The membrane component of MBRs is selected to retain the biomass within the system [8]. Details of the types of membranes typically used in MBRs and the parameters that affect their performance are discussed below.

2.2.1 Categories of membranes

Pressure-driven membranes are typically classified based on the size of the material that they can retain. Microfiltration and ultrafiltration membranes, also commonly referred to as low-pressure membranes, are predominantly used to remove particulate material. On the other hand, nanofiltration and RO membranes, also commonly referred to as high-pressure membranes, are used to remove soluble material. Considering that the primary role of the membrane component of MBRs is to separate the biomass particles from the treated wastewater, low-pressure membrane systems are typically used for MBR applications.

Low-pressure membranes, hereafter simply referred to as membranes, essentially function as sieves, retaining particulate material that is larger than the pore sizes of the membranes. Particles in MBRs typically range in size from 1 to 200 mm [2], suggesting that membranes with pore sizes slightly smaller than this would effectively separate the biomass. However, experience has shown that the performance of membranes with pore sizes of approximately 0.5—1 order of magnitude smaller than the size of the biomass is better than that of membranes with larger pore sizes. Membranes with larger pores tend to get clogged internally by biomass and other solids in the mixed liquor. As discussed below, the resistance to the permeate flow increases as membrane pore size decreases, and therefore membranes with pore sizes smaller than 0.04 mm are also seldom used in MBR applications. As a result, membranes with pore sizes ranging from 0.04 to 0.4 mm (i.e., microfiltration and ultrafiltration) are typically used in MBR applications. Table 1 lists some ofthe characteristics a few different types ofcommercially available membranes used in MBR applications.

As previously discussed, both external and submerged membrane configurations are used in MBR applications. External membrane configurations are stand-alone systems to which the solution to be filtered (i.e., mixed liquor) is pumped in a recirculating loop (Fig. 1b). A flow restriction on the return line provides the pressure necessary to drive the permeate through the membrane, while the relatively high cross-flow velocity scours the membrane surface, preventing excessive accumulation (i.e., fouling) of retained material. The solution being filtered is typically confined to the inside of the membrane and flows from the inside to the

Table 1 Characteristics of membranes commonly used in MBR applications

Ge-Zenon

Norit

Siemens/ Memcor

Kubota

Type

UF

UF

MF

MF

Pore size

0.04

0.03

0.02

0.4

Configuration

Hollow fiber

Tubular

Hollow fiber

Flat sheet

Material

PVDF

PVDF

PVDF

Polysulphone

Operation

Submerged air sparged

External air lift

Submerged air sparged

Submerged air sparged

Note: PVDF, polyvinylidene fluoride.

Note: PVDF, polyvinylidene fluoride.

outside of the membrane (i.e., inside-out flow). External membrane systems are usually operated with a constant pressure and variable permeate flux (i.e., permeate flux decreases as membrane fouls). Submerged membrane systems are integrated into the bioreactor component of MBRs (Fig. 1c). A permeate pump provides the vacuum necessary to drive the permeate through the membrane and air sparging is used to scour the membrane surface. The solution being filtered is not confined by the membrane and flows from the outside to the inside of the membrane (i.e., outside-in flow). Submerged membranes are typically operated with a constant flow and variable transmembrane pressure (i.e., transmembrane pressure increases as membrane fouls).

Tubular membranes are typically used in external membrane MBR configurations where the permeate flows from the inside to the outside of the membrane. These membranes have internal diameters typically greater than 5 mm [8] and are grouped into modules containing multiple tubes (Fig. 4a). Tubular membranes with smaller internal diameters are not typically used in MBR applications since they can be plugged by the particulate material present in the mixed liquor. Hollow fiber and flat sheet membranes are usually used in submerged MBRs configurations. In these systems, the solution to be filtered is not confined within the membrane (i.e., outside-in flow) and therefore plugging is typically not a concern. Hollow fiber membranes usually have external diameters ranging from 1 to 3 mm and are potted into modules consisting of a dozens to thousands of

Norit Membrane Technology
Figure 4 Membrane configurations. Source: 1, Norit Americas Inc.;2, GE Water and Process Technologies;3, Sanitherm Engineering.

fibers (Fig. 4b). Flat sheet membranes are typically mounted onto panels, which are stacked into assemblies (Fig. 4c).

A number of polymeric materials are commonly used in membrane applications (Table 1). Membranes made from hydrophobic polymers, such as polyvinylidene fluoride (PVDF), can withstand repeated exposure to the relatively harsh cleaning agents commonly used for membrane cleaning (e.g., hypochlorous and citric acids) and therefore are commonly used in commercial membrane applications. However, membranes made from hydrophobic polymers tend to foul more readily than those made from hydrophilic polymers. For this reason, most membranes used in MBRs consist of proprietary blends of hydrophobic and hydrophilic polymers to maximize stability and minimize fouling. Inorganic membranes, made from materials such as ceramics, are increasingly being considered for wastewater treatment applications because they are much more durable than polymeric membranes. Although extensively used in many industrial applications, the current cost of these membranes makes them prohibitively expensive for municipal wastewater treatment applications. However, this may change in the near future as innovative inorganic membrane designs and manufacturing approaches are developed.

2.2.2 Flux and fouling in membrane systems

Flow through clean, low-pressure membranes can be modeled as Poiseuille flow through a network of capillary tubes (i.e., pores) using Eqs. (6) and (7) for constant-pressure, variable-flux systems, and for constant-flux, variable-pressure systems, respectively:

J mRm

fr 2

where J is the permeate flux [L3/L2T]; f the fraction of open pore area on the membrane surface [—]; r the radius of the membrane pores [L]; DP the pressure drop across the membrane, also commonly referred to as the transmembrane pressure [F/L2]; m the viscosity [FT/L2]; t the pore tortuosity factor [—]; d the effective thickness of the membrane [L]; and Rm the membrane resistance to the permeate flow [1/L].

As presented in Eqs. (6) and (7), the resistance to the permeate flow is proportional to the thickness of the membrane and inversely proportional to the square of the size of the membrane pores. The permeate flux, or transmembrane pressure, that is achievable is also a function of the viscosity of the liquid being filtered [19]. As illustrated in Fig. 5, the biomass viscosity

MLSS Concentration (g/L)

Figure 5 Relationship between MLSS concentration and bulk mixed liquor viscosity in MBRs treating wastewater. Adapted from Takemura et al. [62], Nagaoka et al. [58], and Sato and Ishii [61]. Solid line: Exponential relationship fitted to reported data as per Krauth and Staab [59].

MLSS Concentration (g/L)

Figure 5 Relationship between MLSS concentration and bulk mixed liquor viscosity in MBRs treating wastewater. Adapted from Takemura et al. [62], Nagaoka et al. [58], and Sato and Ishii [61]. Solid line: Exponential relationship fitted to reported data as per Krauth and Staab [59].

tends to increase very rapidly at concentrations exceeding approximately 10,000mg/L (measured as MLSS). For this reason, the biomass concentration in an MBR is typically less than approximately 12g/L. The viscosity also affects the back-transport of foulants away from a membrane surface, as discussed below.

During operation, the material that is retained by the membrane can accumulate, plugging membrane pores, reducing the diameter of membrane pores, and/or forming a cake layer on the membrane surface (Fig. 6), resulting in an increase in the overall resistance that the permeating flow must overcome to pass through the membrane.

A number of mechanisms and relatively complex models have been developed to mechanistically describe fouling processes [2,8,20]. Although these models can provide insight into the different fouling mechanisms, no comprehensive model currently exists that can effectively describe fouling in MBRs. However, the overall effect of fouling on the permeate flux through a membrane can be estimated using a simple empirical resistance-in-series relationship as presented in Eqs. (8) and (9) for constant-pressure, variable-flux systems, and for constant-flux, variable-pressure systems, respectively. Over time, as fouling progresses, the resistances due to pore plugging, pore constriction, and cake formation increase, resulting in a decrease in the permeate flux (for constant pressure systems) or an increase in the transmembrane pressure (for constant permeate flux systems) [21], as presented in Fig. 7. A scanning electron microscope image of a clean and

Membrane

Figure 6 Pore plugging, pore constriction, and cake formation.

Membrane

Figure 6 Pore plugging, pore constriction, and cake formation.

o Low sparging intensity □ Intermediate sparging intensity/ a High sparging intensity o Low sparging intensity □ Intermediate sparging intensity/ a High sparging intensity

0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Volume filtered (L) Vo|ume Fi|tered (L]

a) Constant pressure variable flux system b) Constant flux variable pressure system

Figure 7 Effect of fouling on membrane performance.

0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Volume filtered (L) Vo|ume Fi|tered (L]

a) Constant pressure variable flux system b) Constant flux variable pressure system

Figure 7 Effect of fouling on membrane performance.

a) Clean membrane b) Partially fouled membrane c) Heavily fouled membrane

Figure 8 Fouled membrane surface (hollow fiber polymeric membrane system) [60].

a) Clean membrane b) Partially fouled membrane c) Heavily fouled membrane

Figure 8 Fouled membrane surface (hollow fiber polymeric membrane system) [60].

fouled membrane is presented in Fig. 8.

m(Rm + Rp + Rr + Rc) m(Rt) DP - Jm(Rm + Rp + Rr + Rc) - Jm(Rt)

where Rm is the resistance offered by the membrane [1/L], Rp the increased resistance due to pore plugging [1/L], Rr the increased resistance due to a reduction in the membrane pore size [1/L], R the increased resistance due to the formation of a layer (i.e., cake) of retained material on the membrane surface [1/L], and RT the total resistance to the permeate flow [1/L].

It should be noted that although fouling negatively affects membrane performance by increasing the overall resistance to the permeate flow, fouling can also positively affect the performance of membranes. This is because the foulant layer can act as a more selective secondary membrane capable of removing material that is smaller than the membrane pores.

Fouling can be minimized by enhancing the rate of back-transport of retained material away from the membrane surface. When dealing with MBRs, the principal mechanisms responsible for the back-transport of retained material are expected to be inertial lift, shear-induced diffusion, and surface transport [20]. Although significantly different, each of these mechanisms can be promoted by providing high shear forces at a membrane surface. In external tubular systems, this is usually achieved by maintaining relatively high cross-flow velocities within the membranes. Typical cross-flow velocities inside tubular membranes range from 2 to 4m/s. Higher cross-flow velocities can negatively affect biomass activity while lower velocities are not sufficient to prevent fouling. In submerged membrane systems, high surface shear forces are usually achieved by sparging air at the base of the membranes. Although the resulting bulk cross-flow velocity induced by the rising air bubbles is relatively low (0.2-0.5 m/s), surface shear forces comparable to those maintained in external tubular systems can be generated by the turbulent conditions that surround rising air bubbles [22]. The relatively high shear forces present in MBRs tend to break up the flocs in the mixed liquor, reducing the overall size of the particles in solution [2]. The diffusion resistance associated with smaller flocs can be less than that for larger ones, and as a result, the overall contaminant removal rate can be higher when the size of particles in the mixed liquor is smaller [23].

Membrane relaxation or back-flushing cycles can be used to further reduce the extent of fouling. During a relaxation cycle, the permeate flow is interrupted while the high surface shear forces are maintained. This provides an opportunity for accumulated foulants to be transported away from the membrane surface while no new foulants accumulate. During back-flushing cycles, the permeate flow is reversed while the high surface shear forces are maintained. The reversed flow can further enhance foulant back-transport by lifting away accumulated foulants from a membrane surface. The extent of fouling control can be enhanced by using a solution containing a chemical cleaning agent (e.g., chlorine or citric acid) for back-flushing. Relaxation/back-flush cycles typically last 10-60 s and typically occur every 5-60 min. The effect of back-flushing on the permeate flux in a submerged hollow fiber MBR is presented in Fig. 9.

Time (minutes)

Figure 9 Transmembrane pressure in a submerged membrane MBR during successive back-flush cycles (constant-flux, variable-pressure system;unpublished operating data for University of British Columbia pilot-scale MBR;arrows indicate when a back-flush cycle was performed).

Time (minutes)

Figure 9 Transmembrane pressure in a submerged membrane MBR during successive back-flush cycles (constant-flux, variable-pressure system;unpublished operating data for University of British Columbia pilot-scale MBR;arrows indicate when a back-flush cycle was performed).

Regardless of the fouling reduction measures used, some irreversible fouling will eventually occur. When the extent of irreversible fouling becomes too large, the membrane must be chemically cleaned. Fouling in MBRs is typically due predominantly to the accumulation of organic and/or biological material on the membrane surface and/or within membrane pores. Caustic chlorine solutions are commonly used to effectively remove organic and/or biological foulants. However, chlorinated by-products of concern, such as trihalomethanes (THMs) and haloacetic acids (HAAs), can be generated during cleaning with chlorine compounds. As an alternative to chlorinated cleaning agents, citric acid is also commonly used to clean membranes in MBRs. Whichever cleaning agent is used, care must be taken to ensure that the cleaning agent is chemically compatible with the membrane material.

A number of other parameters can also affect membrane fouling and a detailed analysis of these is beyond the scope of this discussion. A summary of these parameters is presented in Table 2 and additional information on how they affect membrane fouling can be found in Chang et al. [24] and Berube et al. [25].

Table 2 Parameters that affect permeate flux and fouling in MBRs

Membrane material

Membrane material. Hydrophilic membranes tend to foul less extensively than hydrophobic membranes.

Polarity and charge. Negatively charged membranes tend to foul less extensively than positively charged membranes.

Pore size. Membranes with pore sizes greater than 0.2 mm tend to foul more readily than those with smaller pore sizes.

Membrane configuration. Fouling control mechanisms are different for submerged and external membrane systems.

Membrane operation

Cross-flow velocity. Relatively high cross-flow velocities (2—4 m/s) are typically required to prevent fouling in external membrane systems.

Gas sparging. Air sparging is typically used in submerged membrane systems to prevent excessive fouling.

Transmembrane pressure (TMP). At a low TMP, the permeate flux increases linearly with TMP. At a high TMP, the permeate flux is mass-transfer-limited and does not increase linearly with TMP.

Operating flux. Surface fouling can be more extensive at a high operating flux.

Membrane cleaning. Relaxation and back flushing cycles can minimize fouling. Caustic bleach solutions and acidic solutions are typically effective at removing organic/biological foulants and inorganic foulants, respectively.

Operating temperature. Higher temperatures lower the viscosity of the mixed liquor, and therefore, increase the permeate flux. Greater biodegradation of soluble organic foulants also occurs at high temperatures.

Mixed liquor characteristics

Suspended solids. Higher solids concentrations typically increase the extent of fouling.

Colloidal solids. Colloidal solids have a greater tendency to accumulate at the surface of membranes and can clog pores or form dense foulant layers.

Soluble products. Soluble microbial products are likely responsible for most of the long-term irreversible fouling in MBRs.

Inorganic precipitates. Depending on the wastewater being treated and the operating conditions, inorganic material can precipitate onto the membrane surface.

Bioreactor operation

Loading rate/hydraulic retention time.a Higher loading rates or lower hydraulic retention times typically lead to greater biological growth and therefore higher biomass concentrations.

Solids retention time.a Linked to the production of excess biomass and therefore MLSS concentration.

a The loading rate/hydraulic retention time and the solids retention time also affect the concentration of soluble microbial products in the MBR.

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