CHEMICAL CLEANING OF MF AND UF MEMBRANES IN

WATER TREATMENT

By: Javier Lopetegui, PhD in Science (*); Rakel Gutiérrez, Industrial Enginner (*); Elena Meabe, Chemical Engineer (**); Luis Sancho, PhD in Science (**).

(*) Likuid Nanotek S.L. PºManuel Lardizábal 15

20018 San Sebastián

Tel.: 943 223 841

Web: www.likuidnanotek.com

(**) Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT) PºManuel Lardizábal 15

20018 San Sebastián

Tel.: 943 212 800

Web: www.ceit.es

ABSTRACT: The successful operation and maintenance of a filtration system, as well as

membrane average life will depend on membrane fouling phenomena. To optimize that, a good

membrane system design is critical, considering both water and membrane features and

proposing good operational conditions. Therefore, the establishment of proper and versatile CIP

protocols, with the use of good and specific cleaning detergents is being more and more

important. Those protocols must be dynamics and must work all together with useful membrane

lab analysis and works.

A membrane is a semi permeable thin layer, able to separate different substances according to

their physical and chemical properties when driving force is applied through it.

Pressure-driven membrane operations (other operations known as GP and DIA) can be

classified in four groups mainly: IO, NF, UF and MF. OI and NF (also known as OI at low

pressure) are mainly used for salt removal (water desalination) and water softening respectively.

In this kind of operation, membrane fouling can generate production stops, high preventive

maintenance and shorten/reduce membranes’ average life.

The number of OI plants and their size has grown exponentially in recent years, being a very

attractive market for large water companies settled in international markets.

However, UF and MF market has grown steadily over the last 10 years, with annual growth

above 10 and 15%. Despite the fact that the type of installation is not as big as the latest

generations of OI plants, the problem of membrane fouling remains critical for the viability of the

system.

Global market for MF and UF tangential filtration systems, including modules and associated

equipment, was 3,8 billion dollars during 2006 [1], and it is expected to reach 5 billion dollars by

2010.

This kind of membrane can be applied on several sectors, such as agro-food sector (wine

clarification, filtration of milk-based products, juice clarification etc.), biopharma (crop

separation, metabolite concentration, etc.), chemical (oil separation and recycling etc.) and of

course, water market. Specifically in applications for water treatment, MF and UF membranes

are used for:

� Surface water and groundwater purification.

� Pre-treatment of NF and OI plants.

� Wastewater treatment (Direct filtration and MBR).

� Sludge treatment.

The appearance, development and growth of new technologies such as MBR, are essential for

the increase associated to water treatments, and the fact that the use of MF membranes in

purification is increasing.

The EU Water Framework Directive 2000/60/EC of the European Parliament favors MBR

technology, in cases like urban water with very strict requirements of effluent quality. In the

industrial sector IPPC directives and the Best Available Technology concept clearly favour the

application of this technology.

With reference to purification, MF provides water quality and reliability compared to

conventional technologies; it eliminates or minimizes the continuous addition of chemical

reagents and the formation of undesirable by-products.

The average life of a membrane used for water treatment range from 3 to 10 years. At this

point, it is important to take into account the properties of the fluid to be treated, to operate the

membranes under appropriate conditions and, to introduce a monitoring and a fouling detection

system. Moreover, it is important to develop, optimize and introduce cleaning protocols

(especially with industrial water) in order to extend the membrane’s life and to minimize

preventive and corrective maintenance (Table 1).

Purpose of the filtration Quality, performance, operating costs

Properties of the fluid to be treated DOM, COM, SS, SM, viscosity, hardness, FI

Operating conditions T, TMP, CFV, VCF

Cleaning and recovery protocol

Periodicity, type of reagent (acid, alkaline, enzymatic, degreaser, etc.), cleaning temperature, CT reagent combination, pH, etc.

Table 1. Points that should be taken into account for the design and operation of a tangential filtration

system of MF and UF MBR.

In order to optimize operating conditions and cleaning protocols, laboratory studies as well as

destructive tests of Membrane Autopsy are becoming extremely necessary (Figures 1 and 2).

These tests enable to locate, evaluate and correct fouling, breakages and malfunction in

membrane systems.

Figure 1. SEM Microscopy room – Membrane Laboratory at Likuid-CEIT

Figure 2. Jar-test to optimize cleaning protocols of organic membrane – Laboratory at Likuid-CEIT.

Types of fouling There are several types of fouling that are classified in three main categories: inorganic, organic

and biofouling. The latter is mainly organic and presents some peculiarities that will be

explained forward (that will be further developed).

Inorganic

Silica

Silica deposits are very hard, fragile and looks like/seem to porcelain. Precipitation of silicates is

always related to the presence of iron or aluminum, forming silico-aluminates of calcium and

iron. The cleaning of siliceous precipitates should be done in alkaline pH values and under

temperatures as high as possible with detergent products.

Aluminum

The aluminum found in membranes might have three origins: ionic, colloidal (aluminum

silicates) and an excess of coagulants (alumina sulfate and aluminum polychloride).

As these coagulants are very difficult to clean, frequently high pH and temperatures are used in

order to enhance the detergents’ action.

Iron

The iron from pipes’ corrosion, etc. is not problematic. However, there might be some problems

with the iron in solution, which is deposited in membranes as ferric oxide, as well as the iron

from coagulants, usually chloride and ferric sulfates as they originate a dark brown colored

deposit.

To eliminate this kind of deposits, they should be dissolved with acid solutions at high

temperatures if possible. Citric acid works very well, and if it is combined with surfactants agents

it works even better.

If the cleaning is done with pH around 4,1 the removal of iron is optimized, due to the fact that

the mentioned pH corresponds to maximum iron solubility.

Calcium Carbonate, CaCO3

Calcium Carbonate precipitates quickly, forming granulated and porous precipitates. Initially the

treatment is simple, since any kind of strong acid solution will redissolve it. Even small doses of

weak acids such as citric, working at pH 6,5-6,8 will eliminate this deposit

Calcium Sulfate, CaSO4, (gypsum)

It forms hard, dense and fragile precipitates. Its removal is more complicated than in the

previous case because it is highly insoluble in water, requiring a strong acid combined with

other elements as chelating agents.

Calcium Phosphate, Ca3(PO4)2

Phosphate deposits have a brown-gray colour and they are eliminated with a medium-soft acid

cleaning, although alkaline surfactants may also be used.

Barium Sulfate, BaSO4 and Strontium, SrSO4

Barium Sulfate is insoluble in water. It is eliminated with chelating agents.

Organic Material / Matter

The accumulation of particles on the surface of the membrane is controlled by two opposite

mass transfer processes: convection of particles towards the membrane, because of the

permeate flux, and diffusion of particles from the membrane due to the shear produced by the

tangential flow of the retentate [[[[2]]]]. Although the membranes are operated in cross flow mode,

to minimize the accumulation of soluble and colloidal material in the boundary layer near the

membrane’s surface (concentration – polarization), part of the feed water’s colloids are

transported to the membrane’s surface where they are adsorbed, forming a thing fouling layer

[[[[3]]]]. The size or degree of fouling depends on those summarized in Table 1, among others.

In order to minimize fouling problems, it is important to reduce as much as possible COM

especially in cases that the molecular size is similar to the pore size of the membrane. This is

not easy to control; therefore, a pre-coagulation system or coagulation/flocculation previous to

the membrane is commonly used, thus reducing the amount of COM in the circulation loop of

the membrane.

MBR systems present several advantages in this sense, as the biological system located just

before the membrane, eliminates efficiently the BOM. Thus, the design of the MBR should be

focussed on eliminating the BOM as much as possible, reducing the maximum COM in the

circuit of the membranes.

Temperature factor is also very interesting when minimizing organic biofouling. As MBRs

eliminate high volumetric loads (kgBODm3d) in exothermic processes, the temperature of

reactors is usually high. This favors the dissolution of colloidal species that might otherwise be

potential “blockers” of pores.

In general, this kind of biofouling can be treated with a combination of sodium hydroxide and

hypochlorite. In the case that organic fouling is nor effectively removed with these reagents, the

use of alkaline detergent with enhancers and other complementary reagents is recommended to

obtain higher removal efficiency. In some cases, the use of enzymatic detergents with protease

and lipases, which act specifically over certain compounds, is recommended, as well as

additives to eliminate fat, etc.

Biofouling

As in the previous case, the microorganisms are transported towards the surface of the

membrane where they are adsorbed; they grow and multiply at the expense of water’s nutrients,

forming a biological layer or bio-layer that can condition the performance of the system.

The bio-layers formed may or may not cover the membrane uniformly. However they usually

consist of multiple layers of alive and dead microorganisms together with their extracellular

products (EPS, glycoprotein, lipids, etc.)

This type of fouling is critical to IO systems and MF/UF systems that filtrate directly the water,

especially when this water has a high organic load. This effect is diminished in MBR systems

due to the low BOM concentration in the membrane circuit, despite the presence of a large

amount of biomass, in form of.

One of the critical factors in order to reduce biofouling is to work with higher CFV which drag

and prevent the consolidation of bacterial layers on the membrane’s surface.

The chemical cleaning suitable for this kind of biofouling is very similar to that described before,

but sometimes the cleaning action might be combined with biocide agents, chelating agents as

EDTA and some enzymatic detergents that denature certain proteins of the bacterial cell

membrane.

Types of membranes and cleaning protocols

There are many kinds of membranes depending on their composition, geometry, etc. In order to

simplify and in relation to MF and UF for water treatment, the most commonly used nowadays

are organic (HF tubular and flat) and inorganic (ceramic tube).

In all cases the principle of performance is cross-flow filtration, and due to the high sheart

imposed, a cake layer is not quickly formed on the surface of the membrane, enabling a

continuous performance during a long period of time. On the other hand, in submerged systems

the CFV necessary for this continuous “self-cleaning” is given by the air flow injected into the

bottom of the membrane module. In external membranes, CFV is created by recirculation of

water along the membrane.

External membranes’ cleanings protocols are more developed compared to those for

submerged membranes. There are several reasons for this:

� Costs: submerged membrane plants predominate in urban sector, with very high flow

rates and volumes. Working with these volumes and with a large number of

membranes, the purpose is to save as much as time and reagents as possible.

However, this might not be the best option considering the global system.

� Type of fouling: the operation of submerged system with urban wastewater generates a

repetitive fouling nature, generally less severe than in the industrial systems. Moreover,

fouling is generated at lower TMP values. All this favors the efficiency of the chemical

cleaning, despite the comments of the preceding paragraph.

The gradual introduction of submerged membrane systems in MBRs for industrial water

treatment is generating more complicated fouling problems and they are becoming more difficult

to treat. On the other hand, the use of submerged membranes in industrial MBRs is usually

associated to medium-low size plants. Therefore, the use of detergents with combined and

potentiated formulas instead of hypochlorite or a combination with it, is becoming widespread

and optimized from a technical-economic point of view.

Tubular Membrane – Ceramic or Organic

Tubular membranes are mainly used in external configuration of medium-small size plants, and

cleaning protocols have been adapted to industrial sector’s necessities. Therefore, there are

already a large number of chemical agents used for different cleaning protocols adapted to each

case (Table 2).

Chemicals commonly used in membrane cleaning Caustics Hydroxide, carbonates and phosphate

Oxidants/disinfectants Hydrogen peroxide, peroxyacetic acid, sodium hypochlorite and metabisulphite

Acids Citric, nitric, phosphoric

Chelating agents EDTA, citric acid

Surfactants Anionic, nonionic

Enzymes Proteases y lipases Table 2. Chemical agents mainly used for cleaning membranes.

Taken from “Universidad de Granada-Tesis Doctoral 2006-José Edgar Zapata Montoya”

In general, the tubular membranes are less sensible than HF, for example, to changes in viscosity, SS, T and BOM as they can be adapted to to a wide range of TMP and CFV, in a more intensive operation that requires less membrane surface to be installed and thus reduce the inversion. Tubular membranes do not need a small mesh size pre-filtration of the influent and they are

easy cleaned. In the case of ceramic membranes (Figure 3), the recovery that can be achieved

is the best of all, as it is possible to use a wide range of pH and temperature. This also affects

the average life of the membrane, which can approximately duplicate lifetime of an organic

membrane.

Figure 3. Likud’s multitubular inorganic membrane samples

The cleaning protocol must be adapted to the operating conditions of the modules which can vary significantly in the case of external membranes. TMP values between 0.5 and 6 bar can be applied, together a wide range of CFV values.

In general, it would be advisable:

� Maintenance cleaning, low foam medium alkaline chlorinated detergent at 0,6-1%

during 40-60 min.

� Clean shock, with a medium-high alkaline detergent (depending on the tolerance of the

membrane to pH) followed by a mild acid detergent at 0,5-1% during 30-60 min.

In the case of difficult fouling associated to proteins, grease or other substances, it is necessary

to use specific enzymatic products, degreasers, etc.

Hollow Fiber (HF)

FH membrane is usually used with outside-inside filtration mode. The permeate is collected

inside the fiber, which can be as thin as 0.5 mm. Hollow fiber is used both in external and

submerged configuration. In the external one, HF is applied to treat superficial and underground

waters with low turbidity and low OM concentrations. In MBR systems the submerged

configuration is mainly used, with reinforced fibers of 2 mm diameter.

HF usually gets more clogged in the end of the fibers (Figure 4), because the degree of

agitation of the fiber in these areas is lower.

Figure 4. Solids accumulation and organic material on the top of HF module.

However, before initiating discontinuous cleaning protocol it is recommendable, if possible, to

flush the membranes with tap water at high pressure

In the case of HF membranes, apart from optimizing the chemical cleaning protocol, it is highly

important to operate the system in the optimum conditions in order not to vary the

characteristics of the solution to filter. This fact is due to the low pressure applied for the

filtration, which makes the membrane performance to be highly influenced by any change in the

liquid to filter. For example, in hollow fiber membranes applied to MBR processes, the control of

SS and BOM concentration, as well as the temperature, is critical for the good performance of

the system. An increase in SS, which generates an increase in the viscosity, is the cause of two

important deficiencies:

1. Filterability of mixed liquor decreases.

2. Coalescence of air bubbles occurs, reducing the scouring on the membrane surface

and consequently, the “cross-flow” efficiency.

Chemical cleanings have been shown to be very efficient. In lab scale, filtration tests are done

in specific devices (filtration cells Figure 5), operating in dead-end filtration mode). By using

these techniques and trying different cleaning solutions, it is possible to obtain interesting

recovery graphics (Figure 6) in order to optimize the cleaning protocol.

As an example of a standard cleaning protocol:

� Backwashes: 2-5 ppm of hypochlorite in opposite direction, with a flux 2.5 times higher

than nominal flow.

� Maintenance cleaning:

o 200-800 ppm of hypochlorite.

o 200-800 ppm of hypochlorite combined with medium alkaline detergent with low

foaming potential.

� Recovery cleaning:

o 600-1.000 ppm of hypochlorite combined with medium alkaline detergent (low

foaming potential), followed by citric acid at 1% concentration.

As in the previous case, in cases of severe fouling of the membranes, adaptation of cleaning

protocols is needed: more specific reagents are used and cleaning conditions (contact time,

temperature, etc) are modified.

Figure 5. Dead-end filtration cell AMICON 8200.

y = 95.994x

y = 509.13x

y = 215.27x

y = 250.55x

0

5

10

15

20

25

30

35

40

45

0 0.05 0.1 0.15 0.2 0.25

TMP (bar)

J(2

0ºC

) (l

/hm

2)

New Fouled Alk. Cl. Acid Cl.

Figure 6. Clean water permeability evolution (slope of each line) after different chemical cleanings.

Flat-sheet organic membrane

Organic flat-sheet membranes can be used in external and submerged configuration. In both cases, fouling nature and chemical cleaning protocols are similar to those mentioned previously for tubular and HF membranes, respectively. These membranes do not have any particular characteristic which can affect the fouling mechanisms and the chemical cleaning conditions. A fragment of PAN organic membrane is shown in Figure 7, before and after being cleaned with several acid solutions. Clean water permeability was measured after each cleaning test. In this specific case, as alkaline cleaning was shown to be inefficient, it was concluded that the membrane presented inorganic fouling. Afterwards, different acid cleanings were made in order to compare their efficiency. It must be highlighted that the low efficiency of nitric and oxalic acids are due to the pH and temperature limitations typical of organic membranes, which make not possible to apply high concentrations of these acids and require the use of other commercial detergents with moderate pH values.

REAGENT

DIVOS 35 Citric acid Nitric acid Oxalic acid

CWP (l/hm2bar)

475 400 190 60

Figure 7. Fragments of PAN flat-sheet organic membrane before and after being cleaned with acid DIVOS35 reagent. Clean water permeability values after the cleaning with different solutions are included.

Summary

For a correct design of a MF/UF system, it is necessary to consider the characteristics of both

the wastewater and the membrane, as well as select the most appropriate operating conditions

in order to find the optimum combination between them.

In addition, a correct chemical recovery of the membrane, no matter what kind it is, is essential

so as to fulfill the technical-economic requirements of a filtration system.

When a chemical cleaning protocol is settled, it is necessary to adapt it to the nature of fouling

that will be generated, especially when working with industrial wastewater and variable

composition water.

Consequently, the maintenance operations are minimized and membrane’s lifetime is extended.

Adaptation of cleaning protocol can be based on CIP procedures or on lab tests. These

observations can be complemented with a membrane autopsy when membrane performance

and cleaning efficiency is not good.

The basis for a good cleaning protocol is almost the same independently of the configuration

(tubular, HF, flat-sheet) and operation mode (external or submerged). Nevertheless, it is

necessary to adapt the chemical cleanings to the specific operation conditions under normal

operation of the membrane (TMP and temperature).

In the case of fouling by organic material and/or biofilm, temperature and pH are very important.

Thus; the higher they are, the better the efficiency of the cleaning.

Finally, the effectiveness of the cleaning will depend directly on the type of membrane, as

Depending on it, stronger products and higher pH, temperature and contact time can be

selected.

The same happens with acid cleanings, where higher cleaning efficiency is obtained with lower

pH values. Temperature is also very important.

If the fouling is inorganic, the efficiency of the cleaning will depend on the type of active principle

used, and the adjustment of the pH in relation to the solubility index of the compound intended

to be removed from the membrane.

As a general rule, it is important to ask the membrane manufacturer for advice if there are any

doubts concerning the pH, temperature or active principle that will be used during the cleaning

protocols.

Acronyms BOM Biodegradable Organic Matter

CIP Cleaning In Place

CFV Cross- flow velocity

COM Colloidal Organic Matter

CT Contact Time

CWP Clean Water Permeability

DIA Diafiltration

FI Fouling index

GP Gaseous Permeation

HF Hollow Fiber

IO Inverse Osmosis

MBR Membrane Bioreactor

MF MicroFiltration

MOD Dissolved Organic Matter

NF NanoFiltration

OM Organic Matter

PAN Polyacrylonitrile

SS Suspended Solids

TMP Transmembrane pressure

UF UltraFiltration

VCF Volumetric concentration factor

References

[1] News. (September 2005). Membrane Technology, pg.4.

[2] Wang, L and Song, L. (1999), Flux decline in microfiltration and ultrafiltration:

experimental verification of fouling dynamics. Journal of Membrane Science 160, pp.41-

50.

[3] Lepore, J.V. and Ahlert, R.C., 1988, “Fouling in Membrane Process”, in B.S. Parekh

(ed.), Reverse osmosis Technology:Application for High Purity Water Production,

Marcel Dekker, New York, pp. 141-184.