Fouling in Membrane Process used for Water & Wastewater Treatment

Water scarcity is among the most serious issues and water crises faced by the world as a result of poor management of climate change. There are many places in India which have serious water crises. India’s sixth-largest city Chennai faces serious water crises problem. Maharashtra state’s Latur district experienced a great water shortage in 2019. Gujarat is already thinking to solve their water crises issues and they are going to install desalination systems in various parts of Gujarat.

To cope with this situation, investigators and engineers have been attempting to develop treatment methods of every sort, which aim to eliminate the pollutants in water bodies or to increase the water supply via the safe reuse of wastewater and efficient desalination of sea water as well as brackish water. Among all the process used for water and wastewater reuse, membranes technology is a best and very reliable process at present. The membranes-based process used for water and wastewater treatment include direct membrane filtration, such as microfiltration, ultrafiltration, Nanofiltration and reverse osmosis and hybrid membrane processors like MBR.

There are several issues that have not yet been fully understood and they still are a significant obstacle toward the broad application of membranes process. One of the major issues is the understanding and mitigating of the membranes fouling, which is inevitably associated with membrane process. Membrane fouling is also a big issue in wastewater treatment industry. Here, I oblige that throughout from last few years a strongly increasing number of investigations per year were performed.

What is membrane fouling?

Membrane fouling can be defined as the undesirable deposition and accumulation of particulate matter, micro-organism, colloids and solutes on membrane’s surface.

As shown in the figure, membranes fouling can be attributed to both membrane pore-clogging and cake deposition on membranes which is usually the predominant fouling component. Membrane fouling is a very complicated phenomenon and results from multiple causes. The particle size of the pollutant in wastewater may strongly affect fouling mechanism in a membranes filtration system. If the size of foulant is comparable with the diameter of the membrane’s pores (i.e., Colloids), or smaller than the pore size (i.e., solutes), adsorption at the internal pore surface and pore-blocking may occur. However, if foulant is much larger than the pores of the membrane, they tend to form a cake layer on the surface of the membrane. Membranes fouling result in a reduction in permeate flux and increase the transmembrane pressure (TMP), depending on the operation mode. When the cake layer formed on membranes surface then crossflow velocity affects the thickness of the boundary layer.

Generally, the intensity of membranes fouling results from the following mechanism:

• Adsorption of solutes or colloids within/on membranes, such as silica can precipitate at a concentration below saturation in the presence of aluminium or iron.

• Formation of cake layer on the surface of the membranes.

• Micro-organism or microbes.

• Organics, which provide the nutrients for microbes.

• Metal, such as iron and manganese that precipitate when oxidized; aluminium typically from alum, which is commonly overfed, particularly into municipal/surface sources; hydrogen sulphide, which releases elemental sulphur upon oxidation, a sticky material very difficult or not possible to remove from a membrane’s surface.

In fact, the occurrence of membrane fouling also strongly depends on the membranes used. From the viewpoint of fouling components, membrane fouling can be classified into three major categories; biofouling, organic fouling and inorganic fouling. A fundamental understanding of the formation of membrane foulant will help to develop more effective approaches for fouling control.

. Biofouling

Biofouling or biological-fouling is basically caused by deposition growth and metabolism of bacteria cells on membranes surface. Such accumulation is referred to as epibiosis when the host surface is another organism and the relationship is not parasitic. Biofouling may start with the deposition of individual bacteria on the membranes surface, after which the cells multiply and form a cake. Many researchers suggest that soluble microbial products (SMP) or extracellular polymeric substance (EPS) released by bacteria play important roles in the formation of biological foulant and cake layer on membranes surface.

The deposition of bacteria cells can be visualised by techniques such as scanning electron microscopy (SEM), we use this technique during membranes autopsy for analysing the bacterial growth on membranes surface.

The potential for biofouling of a membrane can be determined by considering the assimilable organic carbon (AOC). This test is a bioassay that measures the growth potential of micro-organism in a sample.

The degree of membrane fouling with microbes that has already occurred is determined by checking the number of colonies that slough off membrane into RO reject stream.

Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the micro-organism from the action of shear forces and biocides chemical used to attack them. These microbes can be destroyed using chlorine, ozone, UV radiation or some non-oxidizing biocides such as – DBNPA, SPCP, THPS, DDAC as well as ODDAC.

Organic Fouling

Organic fouling in membranes processes refer to the deposition of natural organic matter (NOM) or soluble microbial products (SMP), which is mainly composed of proteins, humic acids and polysaccharides on the membrane’s surfaces. NOM is the main substance causing fouling on membrane’s surface. For NOM, humic substances are most detrimental foulants, which can cause severe irreversible fouling through pore blocking. Another organic foulant 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.

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 micro-meters. 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. This type of fouling is known as colloidal fouling.

Silt Density Index (SDI)

Silt density index (SDI) is the most widely used fouling index to quantify fouling potential of colloidal particles in feed water. Most notably, the SDI is only measuring the fouling rate associated with particles larger than 0.45 mm, since it uses a 0.45-mm filter membrane. In many RO processes where feed water is prefiltered before reaching RO systems, membrane fouling is usually caused by particles much smaller than 0.45 mm. In addition, the SDI adopts dead-end flow mode, which is different from the crossflow mode commonly employed in conventional full-scale RO processes. As the operating flow mode affects the fouling behaviour of the particles on the membrane surface, fouling potentials measured when operating in these flow modes may be different.

The accuracy of SDI also became questionable when it was reported that the index given by SDI is not proportional to the colloidal concentration. Other indices, such as modified fouling index (MFI), have been proposed in view of the deficiencies in SDI. Unlike the SDI that is empirically derived, MFI is based on the theory of cake filtration and is observed to be proportional to the colloidal concentration in the reported tests. Unfortunately, this method to measure colloidal fouling potential adopts the same type of filter membrane and operating flow mode as the SDI. Hence, the index may not be suitable for use in RO processes.

. Prevention from Membranes Fouling

Pre-treatment

Membrane cleaning reduces production time and increases operating costs. In addition, excessive cleaning may damage the membranes, resulting in more frequent membrane replacement. To reduce the cleaning frequency and duration, it is critical to slow down the rate of membrane fouling by keeping the fouling potential of the feed water to a minimum. This can be achieved by pre-treating the feed water before it enters into the RO membrane modules. Some examples of pre-treatments are as follows:

1. Removal of large particles using coarse strainer. For smaller particles, microfiltration is used

2. Ultrafiltration membranes can also be used for preventions from SDI, Turbidity, small unit micro-organism

3. Water disinfections with chlorine or using the UV light

4. Clarification and hardness removal using lime

5. Addition of anti-foulant chemicals

To ensure optimized performance of the full-scale RO system, the pre-treated feed water must satisfy the requirements set by the plant operators. For example, to reduce the potential of organic fouling, designers can limit the total organic carbon (TOC) at 3 mg/L, biochemical oxygen demand (BOD) at 6 mg/L, and 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.

Membranes Cleaning of CIP (Cleaning in Process)

Membrane cleaning is a direct method to alleviate membrane fouling by restoring the permeability of RO membranes. Membrane cleaning is an important part of full-scale RO operation, without which an early plant failure may occur or frequent replacement of membranes may be required. The membrane cleaning efficiency is affected by numerous factors, including the types of cleaning agents, types of foulant, chemical dosages, frequency of cleaning and contact time. The efficiency of membrane cleaning and its effects on the performance of fullscale RO process are usually evaluated from past experiences or through pilot studies.

As a guide, cleaning is usually done when there is a 10% decrease in water production at constant operating conditions, or a 10% increase in the driving pressure to maintain the same production at constant temperature, or 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, as shown in the chart given below.

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 organism

Challenges for RO Membranes

Since RO membranes were first used in water and wastewater treatment, membrane fouling has emerged as one of the most serious obstacles to the technology. Scientists and engineers in this field have worked for decades to mitigate membrane fouling.

Although many pre-treatment processes and membrane cleaning techniques are invented, membrane fouling has not been totally eliminated and still remained as the major threat of RO processes. One of the reasons for ineffective efforts on membrane mitigation and control is the lack of effective fouling characterization methods. There are two obvious shortcomings in current fouling characterization the commonly used SDI and related indices are inadequate to include all possible foulants and fouling development in a full-scale RO process is not quantitatively related to the fouling indices. Therefore, SDI and related indices cannot be used as a rigorous parameter in process design. Pilot tests usually have to be conducted for observation of fouling development in the full-scale RO processes to generate the needed design parameters in fouling mitigation and control. The duration of pilot test may take months or over a year to obtain the meaningful information. However, because pilot tests are usually costly and time consuming, only limited scenarios can be tested and evaluated.

Hence, the information obtained for fouling characterization is quite limited and incomplete. A quick, cost-effective, and reliable method for fouling characterization is therefore the key to a successful fouling control.

. Challenges for Ultrafiltration Membranes

UF membranes are different membranes from the RO membranes. Numerous polymers, including poly (ether sulfone) (PES), poly-sulfone (PSF), poly (vinylidene difluoride) (PVDF) and polyacrylonitrile (PAN) are commonly used for UF. PVDF Properties such as a good mechanical strength and physicochemical stability, excellent film-forming properties, stability over a wide range of pH, and thermal stability (high glasstransition temperatures) make these polymers good membrane materials. UF membranes are the separating process of extremely small suspended particles and dissolved macromolecules (surface pore size range = 50 – 1 nm) that passes them through the membranes.

Fouling in UF membranes causes a build-up material on the surface of the membrane. UF membranes foul external and internal from both sides. Fouling may cause irreversible loss of the permeability of a membrane. The different types of organic matter or organic matrix can cause different degrees and forms of UF membranes fouling. The foulants does not only physically interact with the membrane surface, but also chemically degrade the membrane material.

Despite the enormous applications of UF membranes in various fields, their permeability and selectivity deteriorate over time because of an accumulation of solids, suspended particles, colloids and bacteria on the membrane surface and within the membrane pores; this is known as membrane fouling. Fouling is the deposition of retained particles, colloids, macro-molecules, salts, biomolecules and so on.

Fouling reduces the membrane flux either temporarily or permanently. The main mechanisms of fouling are:

• The adsorption of partially rejected matter within the membrane pores (pore constriction.

• The plugging of individual pores by particles similar in size to the pores (pore blocking.

• The accumulation of completely rejected particulate matter on top of the membrane surface also (cake formation).

• Fouling is due to the overall effects of concentration polarization, adsorption and cake layer deposition.

The fouling phenomenon is caused by the interaction between the membrane surface and the foulants, which include inorganic, organic and biological substances in many different forms. Colloidal particle like NOM physically interact with the membrane surface, but also chemically degrade the membrane material.

Ultrafiltration membranes have wide uses of applications for water and wastewater treatment. Different applications have different types of foulants.

Recently we faced a serious fouling issue in UF membranes. The application is the reuse of industrial wastewater and industry is basically the “silicate processing” industry.

The treated wastewater has SiO2, Na2SiO3 from 140 ppm to 200 ppm in rage. The temperature of the treated water is approx. 45 o C. They are doing aeriation to maintain the dissolve oxygen in the water and treatment scheme is – Treated Effluent >> Cartridge Filter >> UF Membranes >> High recovery RO.

After a certain time, UF train flow has decreased by up to 90% and pressure increase up to 10 times. We have tried to clean the UF membranes with the different chemicals. After analysis of foulant, we found that the fouling is because of Na2SiO3.

We have changed the CIP chemical and do the shock treatment of 0.1 % of HF acid. UF membranes have recovered with their original flow at the designed pressure.

. Challenges for Membrane Bio-Reactor (MBR)

MBR is basically combination of Ultrafiltration and microfiltration process along with biological wastewater treatment process. This is the best technology for treatment of domestic wastewater. Now-a-days, it is widely used for municipal and industrial wastewater treatment.

Biological treatment process is very important for using the MBR membranes. Biological treatment (or biotreatment) processes are those which remove dissolved and suspended organic chemical constituents through biodegradation, as well as suspended matter through physical separation. Biotreatment demands that the appropriate reactor conditions prevail so as to maintain sufficient levels of viable or living micro-organisms (or, collectively, biomass) to achieve removal of organics. The latter are normally measured as biochemical or chemical oxygen demand (BOD or COD, respectively); these are indirect measurements of organic matter levels since both refer to the amount of oxygen utilized for oxidation of the organics. The micro-organisms that grow on the organic substrate on which they feed generate cellular material from this organic matter, and can be aerobic (oxygen-dependent) or anaerobic (oxygen-independent). They are subsequently separated from the water to leave a clarified effluent that has a reduced level of organic matter.

. The major issue in MBR is clogging of physical particles. Clogging is the agglomeration of solids within or at the entrance to the membrane channels. Whilst this is to be clearly distinguished from membrane surface fouling regarding both its mechanism and amelioration, the impact of both fouling and clogging is identical in that both are manifested as a decrease in the membrane permeability. However, whereas fouling can generally be substantially removed through the application of an in-situ chemical clean, i.e. cleaning in place (CIP), this course of action is not necessarily effective against clogging since in this case the materials are physically lodged between the membrane surfaces rather than coated onto them. Severe clogging is generally only countered by removal of the membrane from the tank and cleaning the membrane modules individually with a low-pressure hose. Such a level of manual intervention risks compromising the integrity of the fibres and MBR sheet.

A large number of things can lead to a diminution in flux or permeability. Generally, it is change in the feedwater flow and/or quality, or that of the sludge directly, which causes changes in permeability. This includes temperature, hydrophobicity (possibly from FOG in the feed) and shock loads of salinity or toxic chemicals which may promote EPS generation. Other factors impacting on permeability through EPS concentration include a high F/M ratio and low DO concentration. The extent of potentially onerous colloidal fouling can be assessed through a comparison of permeate and supernatant COD, from standard centrifugation, which gives a measure of the levels of fine flocculant materials and colloidal particles in the sludge, which are retained by the membranes, leading to membrane pore plugging. Such fouling may be ameliorated by chemical cleaning, but in such instances, it is better to identify the root cause.

In general, surface membrane fouling is a greater operational impediment in industrial effluent treatment than in municipal and, also in most cases, is ameliorated primarily by chemical cleaning. The plethora of research into membrane foulant (and specifically extracellular polymeric substances or EPS) characterization in municipal wastewater treatment has arguably done little to inform actual operation and maintenance of full-scale plant. Generally, for these applications the use of a combination of cleans based on hypochlorite, sometimes adjusted to an alkaline pH, and citric acid, or occasionally oxalic acid and in either case often supplemented with mineral acid. Any departure from this practice can generally be attributed to clogging, when greater intervention is required, constraints on waste discharge, when hydrogen peroxide may be used instead of sodium hypochlorite, or changes in wastewater quality.

For industrial applications the range of candidate cleaning chemicals is more extensive, and may include detergent and chelating or anti-scaling chemicals at a pre-defined temperature and duration of application. Thus, whilst the use of hypochlorite is almost ubiquitous in MBR membrane cleaning, it is not necessarily the most effective reagent for some industrial applications where more foulant appraisal may be required, particularly for more challenging effluents and/or unusual membranes.

The protocol of a clean place can, in the case of HF membranes, involve repeated short backflush intervals (or pulsing) hence, resemble a chemically enhanced backflush. The sequence of cleaning agents is usually (alkaline) hypochlorite followed by organic acid, and is particularly prevalent for municipal wastewater treatment. This arises because it is generally considered that finishing with an alkaline cleaner can promote precipitation of metal hydroxides and carbonate salts, and as such the acid clean should always follow an alkaline clean for waters containing significant concentrations of scaling compounds. However, reversing the sequence has been shown to be effective at some sites or for some membrane products. Maintenance cleaning is applied regularly, often twice weekly for HF systems, and it is nearly always more effective to employ both reagents consecutively on every clean, rather than alternating between cleans. For FS systems, with no maintenance cleaning, recovery cleaning is usually applied based on a set threshold pressure, but also time limited if extended operation without reaching the threshold pressure is encountered. The cleaning frequency is then generally between quarterly and annually.

Other components of the system may also require cleaning; aerator flushing with sludge is normally conducted according to the manufacturer’s recommendations, the standard frequency being daily and for each start-up of the blower for flushable centipedal or ring aerators typically used for FS systems. This is essential to remove any sludge which might otherwise collect inside the aerators and dry out in the air flow to form a tenacious solid residue. For HF systems using cyclic aeration, aerator flushing is not considered necessary. Another important issue is the management of the chemical waste stream generated from chemical cleaning, and recovery cleaning in particular. For maintenance cleaning, provided the total load of sodium hypochlorite exerted is not too large relative to the bioreactor, it can be flushed into the mixed liquor (through displacement with permeate) and consumed by it without sacrificing significant biomass activity.

This can generate EPS as a consequence of stresses imposed on the biomass, but this is generally not significant. In the case of recovery cleaning, where membranes are soaked in tanks filled with more concentrated cleaning reagent, the quantities of reagent involved are much larger. In such cases the chlorine residual can be quenched by dosing it with some of the sludge which has been displaced by the cleaning reagent.

The spent waste reagent must then be disposed of appropriately, normally to the head of works. If quenching with sludge is not appropriate then chemically de-chlorination with alkaline bisulphite solution may be necessary before returning the spent solution to the head of works. Notwithstanding the general guidelines provided, the control system should provide sufficient flexibility to allow different cleaning reagent concentrations and cleaning sequences to be applied. It can also be advantageous to study the impact of the head of sludge or water in the tank, since this imposes a back pressure which can influence the cleaning efficacy.

Fouling in Membrane Process used for Water & Wastewater Treatment

Anil Baghel