Fouling characterization is currently done primarily by measuring the silt density index (SDI) of feed water and monitoring the average permeate flux of full-scale RO processes

not capable to catch all possible foulants.

Fouling is the increase in membrane resistance caused by the formation of fouling layer on the membrane surface.

Studies indicated that scale inhibition and cleaning chemicals account for about 5–20% of the total cost for a typical seawater RO system and the pretreatment cost in RO systems in Middle East to range between 10 and 25% of the total cost.

Actions to mitigate fouling 1- pretreatment2- membrane cleaning.

The main fouling mechanism in RO processes is often associated with the formation of dense fouling layer on the membrane surface.

Factors Affecting Membrane Fouling

1. driving pressure.2. dimensions and physicochemical properties of membrane channel.3. feed water characteristics.

Foulants are usually brought onto the membrane surface by the downward drag of fluid (permeate) as shown in Fig:

(1) driving pressure.

As the permeate is primarily driven by the driving pressure, an increase in driving pressure will increase the accumulation of foulants on membrane surface. Driving pressure also affects the porosity of the fouling layer. A more compact fouling layer reduces the amount of water that is allowed to reach the membrane surface and increases the rate of fouling.

(2) dimensions and physicochemical properties of membrane channel.

One popular method, commonly found in spiral-wound and hollow-fiber membrane modules, is to pass the feed water through a narrow membrane channel, so that the cross flow velocity generates sufficient wall shear to remove the foulants from the membrane surface.

To enhance the shearing effect and promote mixing, the membrane channel is constructed in a way such that the feed water is forced to travel through a series of bends within the channel, generating turbulence that hinders foulants deposition.

This method to develop unstable feed water flow can be achieved with the use of feed spacers commonly found in spiral-wound membrane modules. However, it is important to note that the use of narrow channel and generation of turbulence can significantly reduce the downstream driving pressure. In addition, the presence of feed spacers may trap or capture the suspended foulants, leading to “clogging” problem.

Hydrophobicity of RO membranes is also recognized to affect membrane fouling in RO processes. It is believed that hydrophobic membranes attract organic foulants more easily than hydrophilic ones. This is why some hydrophobic RO membranes are pretreated to give them a hydrophilic property.

(3) Feed water characteristics

such as types and concentration of foulants, ionic strength, and pH – can have huge impact on membrane fouling in RO processes.

A higher foulants concentration naturally poses a greater fouling problem with the availability of larger poolof foulants for deposition. The foulants type and composition influence the porosity of the fouling layer, which in turn determines the hydraulic resistance of the fouling layer. For example, a fouling layer consisting of organic matters and colloidal particles can have higher resistance than that made up of same colloidal particles only.

Ionic strength and pH of feed water affect the membrane–foulants and foulants–foulants interactions by altering the surface properties of membrane and foulants. Generally, a high ionic strength and low pH enhance the accumulation of foulants on membrane surface.

Types of Fouling in RO Processes

1. colloidal fouling2. organic fouling3. biological fouling4. scaling.

(1) colloidal fouling

Colloidal particles are major foulants in all kinds of membrane processes. The size of colloidal particles ranges from a few nanometers to a few micrometers. They are ubiquitous in natural waters and examples of inorganic colloids include metal oxides, clay minerals, colloidal silica, and silicon. There are also plenty of colloidal particles of organic and biological origins. Most colloids carry negative surface charge in pH range of natural waters.

Under the drag force of permeate flux, these colloidal particles will accumulate on the RO membrane surface to form a cake layer. The formation of this cake layer of the deposited colloidal particles adds on an additional resistance to the membrane resistance.

(2) organic fouling

A common organic foulants is the natural organic matter (NOM), which is a complex heterogeneous mixture of different organic macromolecules from degradation and decomposition of living organisms. Another organic foulants is the residual organic matter in the effluent of wastewater treatment plants.

The ability of organic foulants to foul membranes includes their affinity for RO membrane, molecular weight, and functionality. Negative functional groups on organic polyelectrolytes may be repulsed by the negatively charged membrane surfaces of RO membranes. Greater charge density on the membrane surface is often associated with higher hydrophilicity.

Because most RO membranes are made of hydrophobic polymers, organic matters in the feed water usually tend to be preferentially adsorbed onto the membrane surfaces.

(3) Biological fouling

Biological fouling or biofouling occurs when living microorganisms in the feed water are transported onto the membrane surface where they absorb or adhere, forming a thin fouling layer. Once attached, microorganisms may grow and multiply at the expense of nutrients in the feed, forming a biological film or biofilm on membrane surface.

(4) scaling

Fouling by precipitation of the sparingly insoluble minerals to form an impermeable layeron the membrane surface is commonly known as scaling.

As the salt concentration in the feed increases downstream due to the loss of water through permeation, the dissolved inorganic minerals such as carbonates and sulfates of calcium, magnesium, and iron, and some silica minerals may reach their solubility limits and precipitate on the membrane surface.

Another possible reason that leads to precipitation is concentration polarization. Scaling can be reduced or avoided with the addition of acid to reduce anion species in feed water, pretreatment with lime softening to remove scale-forming metals, and addition of antiscaling agents such as hexametaphosphate.

Silt density index (SDI) is the most widely used fouling index to quantify fouling potential of colloidal particles in feed water.

This empirical test, shown in Fig, involves filtering the feed water through a 0.45-μm filter membrane at constant pressure in dead-end flow.

As shown in Eq, the index is calculated based on the time taken to filter a fixed volume of feed water of a clean membrane and the time required to filter the same volume of feed water after the membrane has been used for a defined length of time.

To ensure optimized performance of the full-scale RO system, the pretreated feed watermust satisfy the requirements set by the plant operators.

For example, to reduce the potential of organic fouling, designers can limit:1. the total organic carbon (TOC) at 3 mg/L2. biochemical oxygen demand (BOD) at 6 mg/L3. chemical oxygen demand (COD) at 8 mg/L.

SDI is widely used to determine the potential of colloidal/suspended fouling. Generally, the maximum allowable SDI of the feed water is about 5 or less.

Membrane Cleaning

The membrane cleaning efficiency is affected by numerous factors, including:

1. types of cleaning agents2. types of foulants3. chemical dosages4. frequency of cleaning5. contact time.

Membrane cleaning is usually done when there is :

1- 10% decrease in water production at constant operating conditions2- 10% increase in the driving pressure to maintain the same production at constantTemperature.3- an increase of 15–20% in the pressure differential between feed and reject flows.

In full-scale RO processes where spiral-wound membrane modules are the predominant membrane configuration, membrane cleaning is commonly done with chemicals.

Chemical cleaning is generally divided into low and high pH cleanings, which are used to remove inorganic and organic foulants, respectively. Cleaning agents are categorized into strong/weak acids and bases.

In practice, both inorganic and organic fouling occur together and membrane cleaning usually starts off with acid cleaning to remove inorganic scale or soluble colloidal materials, before the membranes are subjected to high pH cleaning to remove any remaining insoluble inorganic colloidal material, organic material, and/or biological organisms.

Concentration Polarization

This phenomenon describes the increase in retained solids or solute concentration at the membrane surface.According to a simple stagnant film theory, there will be a concentration gradient over the thickness of the stagnant film. Thus, the concentration will be the highest on the membrane surface and will decrease to bulk concentration in the well-mixed liquid.

This will cause the diffusion of solids in the direction opposite to the main flux and, therefore, areduction in the overall flux.

Other negative results include an increase in solute flux resulting from the increased concentration gradient across the membrane, and scaling or particle scaling induced by the precipitation of the solute.Consequently, the separation properties of membrane are reduced.

The extent of concentration polarization can be reduced by promoting good mixing of the bulk feed solution with the solution near the membrane wall.

Mixing can be enhanced through membrane module optimization of turbulence promoters, spacer placement, hollow fiber diameter, and so on, or by simply increasing axial velocity to promote turbulent flow

concentration polarization modulus β = cio / cib = E / E ͦ

E = cip /cib

Eo = cip / cio the modulus can be written as

This Equation shows that concentration polarization increases exponentially as the total volume flow Jv

through the membrane increases. This is one of the reasons why modern spiral-wound reverse osmosis membrane modules are operated at low pressures.

The effect of changes in:

1. boundary layer thickness δ2. membrane enrichment Eo3. membrane flux Jv4. solute diffusion Di

on concentration gradients in the stagnant boundary layer

Gas sparging , i.e., injecting gas (e.g. air, nitrogen) in the form of bubbles into the feed stream has proved to be an effective, simple and cost-effective technique for controlling concentration polarization. Gas sparging works by formation of bubble induced secondary flow, physical displacement of the mass transfer boundary layer, scouring of the membrane surface and pressure pulsing caused by gas slugs. The contributions of these mechanisms in controlling concentration polarization depend of the type of membrane module and their mode of operation.

Slug (or plug) flow: In this type of flow some of the gas bubbles have nearly the same cross section as that of the channel and move along in the characteristic bullet-shaped Taylor bubbles. The Taylor bubbles are separated by liquid slugs which may or may not contain smaller spherical gas bubbles.

Two main mechanisms for flux enhancement have been identified:

1- Bubble induced secondary flow: The secondary flow is generated by the buoyancy driven bubble motion. The wake following bubble promotes local mixing in and near the mass transfer boundary layer, thereby decreasing concentration polarization.

2-Physical displacement of the concentration polarization layer: Taylor bubbles in tubular membranes disrupt the concentration polarization layer by physically eroding them, leading tohigh mass transfer rates. With hollow fiber membranes, the thickness of the liquid film betweenthe membrane wall and a gas slug is usually lower than the calculated boundary layer thickness.

In addition to these two mechanisms, other factors such as pressure pulsing caused by passing gas slugs, increase in superficial cross-flow velocity and vibration (in the case of hollow fiber membranes) are also responsible for flux enhancement.

Compaction

Compaction is another phenomenon in membrane mass transport. When a polymericmembrane is put under pressure, the polymers are slightly reorganized and the structure ischanged.

This results in:1. lowered volume porosity2. increased membrane resistance 3. dense structure with smaller pores4. lowered flux. For example, after long-term compaction of the asymmetric CA membranes, the selectivity was found to be unchanged but the flux decreased. The flux loss could be attributed to a compaction of the porous sub layer and the unchanged skin. It was concluded that under all circumstances, compaction results in a notable membrane permeability loss. Thus the correct application of pressure is very important.