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PROCESS WATER RECYCLING TECHNOLOGIES

MEMBRANE FILTRATION PROCESSES

In membrane filtration processes, the range of particle size which can be

removed from a solvent is increased from the normal range of particulate and

colloidal matter as in normal filtration to include dissolved constituents (typically

0.0001 to 1.0 microns in size). The role of the membrane is to serve as a

selective barrier that will allow the passage of certain constituents and will retain

other constituents found in the solvent.

Membrane filtration processes include microfiltration (MF), ultrafiltration (UF),

nanofiltration (NF), reverse osmosis (RO), dialysis and electrodialysis (ED). Each

of these different membrane filtration processes operate in different manners

and have different characteristics which are shown in the table below.

Membrane

processes

Membrane

driving

force

Typical

separation

mechanism

Operating

structure

(pore size)

Typical

operating

range, μm

Permeate

description

Typical

constituents

removed

Microfiltration Hydrostatic

pressure

difference or

vacuum in

open vessels

Sieve Macropores

(>50 nm)

0.08 – 2 Water +

dissolved

solutes

TSS, turbidity,

protozoan

oocysts and

cysts, some

bacteria and

viruses

Ultrafiltration Hydrostatic

pressure

difference

Sieve Mesopores

(2-50 nm)

0.005 – 0.2 Water +

small

molecules

Macromolecules,

colloids, most

bacteria, some

viruses, proteins

Nanofiltration Hydrostatic

pressure

difference

Sieve +

solution/

diffusion +

exclusion

Micropores

(<2 nm)

0.001 – 0.01 Water + very

small

molecules,

ionic solutes

Small molecules,

some hardness,

viruses

Reverse

osmosis

Hydrostatic

pressure

difference

Solution/

diffusion +

exclusion

Dense

(<2nm)

0.0001 –

0.001

Water, very

small

molecules,

ionic solutes

Very small

molecules,

colour,

hardness,

sulphates,

nitrate, sodium,

other ions

Dialysis Concentration

difference

Diffusion Mesopores

(2-50 nm)

- Water +

small

molecules

Macromolecules,

colloids, most

bacteria, some

viruses, proteins

Electrodialysis Electromotive

force

Ion exchange

with selective

membranes

Micropores

(<2 nm)

- Water +

ionic solutes

Ionised salt ions
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Membranes that are used in water or waste water treatment typically consist of a

thin skin having a thickness on about 0.2 to 0.25 μm supported by a more

porous structure of about 100μm in thickness. Most commercial membranes are

produced in flat sheets fine hollow fibres or in tubular form. Membranes can be

made from a number of organic and inorganic materials but the membranes

typically used in waste water treatment are organic. These include

polypropylene, cellulose acetate, aromatic polyamides and thin film composite

(TFC). The choice of membrane and system configuration is based on minimising

membrane clogging and deterioration, typically using pilot plant studies.

Microfiltration methods separate particles primarily by straining as does

ultrafiltration. In Nanofiltration and reverse osmosis small particles are rejected

by the water layer adsorbed on to the surface of the membrane which is known

as a dense membrane. Ionic species are transported across the membrane by

diffusion through the pores of the macromolecule comprising the membrane.

Typically nanofiltration can be used to reject constituents as small as 0.001μm

where as reverse osmosis can reject particles as small as 0.0001μm. Straining is

also an important aspect of nanofiltμration especially at the larger pore size

openings.

It can be seen from the previous table that there is a considerable overlap in the

size of particles that can be removed by each of the membrane filtration

processes.

The principle type of membrane modules used in the treatment of waste water

are tubular modules, hollow fibre modules and spiral wound modules. Plate and

frame modules are also available but are used more commonly in industrial

applications.

In tubular modules the membrane is cast on the inside of a support tube and

these tubes, either singly or in bundles are then placed in an appropriate

pressure vessel. The feed water is pumped through the feed tube and the

product water is collected on the outside of the tubes while the concentrate

continues to flow through the feed tube. These units are generally used for water

with high suspended solids or plugging potential since these tubular units are the

easiest to clean which is accomplished by circulating chemicals and pumping a

“foamball” or “spongeball” through to mechanically wipe the membrane.

Hollow-fibre modules consist of a bundle of hundreds to thousands of hollow

fibres. The entire assembly is then inserted in to a pressure vessel. The feed can

be applied to either the inside or the outside of the hollow fibre.

In spiral wound membrane units a flexible permeate spacer is placed between

two flat membrane sheets and rolled up in to a tight circular configuration. The

membranes are sealed on three sides and the open side is attached to a

perforated pipe the feed is passed in between the two membrane sheets and the

permeate flows through the membranes to the perforated pipe at the centre of
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the roll and the concentrate continues through the roll in between the membrane

sheets.

Membrane filtrate unit operation

The operation of a membrane unit is quite simple. A pump is used to pressurize

the feed solution and to circulate it through the membrane module. A valve is

used to maintain the pressure of the retentate and the permeate is withdrawn

usually at atmospheric pressure. As constituents from the feed water accumulate

on the membranes (called membrane fouling) the pressure builds up on the feed

side of the membrane. The membrane flux (flow through the membrane) starts

to decrease and the percent rejection also starts to decrease. When the

performance of the membrane has reached a certain level the membrane

modules have to be taken out of service and backwashed and/or cleaned

chemically

Microfiltration & Ultrafiltration

Three different process configurations are used with microfiltration and

ultrafiltration. The first is known as cross flow where the feed water is pumped

with cross flow tangential to the membrane. Water that does not pass through

the membrane is recirculated through the membrane after blending with

additional feed water. The second configuration is also known as cross flow and

is very similar to the first with the exception that water that does not pass

through the membrane but flows to a storage chest where it is blended with

additional feed water before again being fed to the membrane. The third

configuration is known as direct feed in that there is no cross flow. All of the

water applied to the membrane passes through the membrane. Raw feed water

is used periodically to flush the accumulated material from the membrane

surface.

Reverse osmosis

When two solutions are separated by a semi-permeable membrane, a difference

in chemical potential will exist across the membrane. Water will tend to diffuse

through the membrane from the lower concentration side to the higher

concentration side. In a finite volume system this flow will continue until the

pressure difference balances the chemical potential difference. The balancing

pressure difference is known as the osmotic pressure and is a function of the

solute characteristics, concentration and temperature. If a pressure gradient

opposite in direction and greater than the osmotic pressure is imposed across the

membrane, flow from the more concentrated side to the less concentrated side

will occur and this is called reverse osmosis.
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Membrane fouling

The term fouling is used to describe the potential deposition and accumulation of

constituents in the feed stream on the membrane. Membrane fouling is an

important consideration in the design and operation of a membrane system as it

affects pre-treatment requirements, cleaning needs, operating conditions, cost

and performance. Fouling can occur in three general forms these are pore

narrowing, pore plugging and gel/cake formation.

Pore narrowing occurs when particles that are smaller than the molecular weight

cut off for the particular membrane attach themselves to the interior surface of

the pores which results in the narrowing of the pores.

Pore plugging occurs when particles the same size as the pores become stuck in

the pores of the membrane. Gel/cake formation, caused by concentration

polarization, occurs when the majority of the solid matter in the feed is larger

than the pore sizes or the molecular weight cut off of the membrane.

Concentration polarization can be described as the build up of matter close to or

on the membrane surface that causes an increase in resistance to solvent

transport across the membrane. Some degree of concentration polarisation will

occur in the operation of any membrane but the formation of a gel/cake layer is

an extreme case of concentration polarisation where a large amount of matter

has actually accumulated on the membrane surface forming a gel or cake layer.

Typically there are three approaches to control membrane fouling. These are pre

treatment of the feed water, membrane back-flushing and chemical cleaning of

the membrane.

Pre-treatment of the feed water is used to reduce the TSS and bacterial content

of the water and often the feed water will be treated chemically to limit chemical

precipitation within the membrane unit.

The most commonly used method of eliminating the accumulated material from

the membrane surface is back-flushing with water and/or air. Chemical

treatment is used to remove constituents that are not removed during

conventional backwashing.

Pre-treatment for nanofitration and reverse osmosis is usually required since a

very high quality feed is required for efficient operation of these membrane units.

To assess the need for pre-treatment for nanofiltration and reverse osmosis

membrane units fouling factors can be calculated from simple membrane tests.

The water sample must be passed through a 45μm Millipore filter with a 47 mm

internal diameter at 30 psi gauge to determine the SDI (silt density index)

Each of the different membrane processes removes different constituents from

waste water. It can be seen from the table below that a membrane system would
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need to be tailor made for the specific waste water regarding which constituents

were in the waste water and which constituents it is necessary to remove from

the waste water.

Comments

Constituent MF UF NF RO

Biodegradable organics X X X

Hardness X X

Heavy metals X X

Nitrate X X

Priority organic pollutants X X X

Synthetic organic compounds X X

TDS X X

TSS X X TSS removed during pretreatment for NF and RO

Bacteria X X X X Used for membrane disinfection.

Removed as pretreatment for NF

and RO with MF and UF

Protozoan cysts and oocysts and

helminth ova

X X X X

Viruses X X Used for membrane disinfection

Application of membranes

Microfiltration has been used most commonly as a replacement for depth

filtration to reduce turbidity, remove residual suspended solids and reduce

bacteria to condition the water for effective disinfection and as a pre-treatment

step for reverse osmosis. A relatively recent development is the use of

membranes for biological treatment of waste water. Membranes have been used

for both aerobic and anaerobic treatment of wastewaters. In typical membrane

bioreactors (MBRs) the membrane separation unit is internal and immersed in

the bioreactor with the treated effluent withdrawn from the bioreactor via the

application of a vacuum. Typical performance data for am membrane bioreactor

used to treat waste water is shown below.

Parameter Unit Typical

Effluent BOD mg/l <5

Effluent COD mg/l <30

Effluent NH3 mg/l <1

Effluent TN mg/l <10

Effluent turbidity NTU <1

It can be seen from the above table that the effluent is ideal for a number of

reuse applications or for further processing by nanofiltration or reverse osmosis.
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Ultrafiltraton membranes are used for many of the same applications for

microfiltration. Some ultrafiltration units with small pore sizes have also been

used to remove dissolved solid compounds with high molecular weight, such as

colloids, proteins and carbohydrates. The membranes do not remove sugars or

salt. Ultrafiltration is used typically in industrial applications for the production of

high purity process rinse water.

Nanofiltration can reject particles as small as 0.001μm. Nanofiltration is used for

the removal of selected dissolved constituents from waste water such as the

multivaelent metallic ions responsible for hardness. The advantages of

nanofiltration over lime softening include the production of a product water that

meets the most stringent reuse water quality requirements. Because both

inorganic and organic constituents and bacteria and viruses are removed,

disinfection requirements are minimised. Although most NF facilities use

polyamide TFC membranes in a spiral wound configuration, more than ten

different types of membranes are available.

Reverse osmosis is used primarily for desalination. In waste water treatment

reverse osmosis is used for the removal of dissolved constituents from waste

water remaining after advanced treatment with depth filtration or microfiltration.

The membrane excludes ions, but require high pressures to produce de-ionised

water.

ADVANCED OXIDATION PROCESSES

Advanced oxidation processes are used to oxidise complex organic constituents

found in waste water that are difficult to degrade biologically in to simpler end

products. When chemical oxidation is used it may be necessary to oxidise

completely a given compound or group of compounds. In many cases partial

oxidation is sufficient to render specific compounds more amenable to

subsequent biological treatment or to reduce their toxicity. The oxidation of

specific compounds may be characterized by the extent of degradation of the

final oxidation product as follows.

1. Primary degradation: A structural change in the parent compound

2. Acceptable degradation (defusing): A structural change in the parent

compound to the extent that toxicity is reduced.

3. Ultimate degradation (mineralisation): Conversion of organic carbon to

inorganic CO2.

4. Unacceptable degradation (fusing): A structural change in the parent

compound resulting in increased toxicity.
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Advanced oxidation processes typically involve the generation and use of the

hydroxyl free radical (HO•) as a strong oxidant to destroy compounds that

cannot be oxidised by conventional oxidants such as oxygen, ozone and chlorine.

The hydroxyl radical is one of the most active oxidants known. The hydroxyl

radical reacts with the dissolved constituents initiating a series of oxidation

reactions until the constituents are completely mineralized. Non-selective in their

mode of attack and able to operate at normal temperatures and pressures,

hydroxyl radicals are capable of oxidizing almost all reduced materials present

without restriction to specific classes or groups of compounds, as compared to

other oxidants.

Advanced oxidation processes differ from other treatment processes because

waste water compounds are degraded rather than concentrated or transferred in

to a different phase. Because secondary waste materials are not generated then

there is no need to dispose of or regenerate materials.

At present there is a variety of technologies available to produce the hydroxyl

radical but there are only 4 of them being used on a commercial scale. These are

Ozone / UV, Ozone / Hydrogen Peroxide, Ozone / UV / Hydrogen Peroxide and

Hydrogen Peroxide / UV.

Ozone/UV

The photolysis of ozone with UV light in wet air results in the formation of

hydroxyl radicals, in water the photolysis of ozone with uv radiation results in the

formation of hydrogen peroxide, which is subsequently photolyzed to form

hydroxyl radicals. Therefore the use of ozone in this application is not cost

effective.

Ozone/Hydrogen peroxide

For the destruction of compounds that do not adsorb UV, advanced oxidisation

processes involving Ozone/H2O2 may be more effective. Compounds in water

such as trichloroethylene and perchloroethylene have been significantly reduced

with advanced oxidisation processes using hydrogen peroxide and ozone to

generate the hydroxyl radicals.

Hydrogen peroxide/UV

Hydroxyl radicals are also formed when water containing hydrogen peroxide is

exposed to UV radiation. In some cases the use of the hydrogen peroxide/UV

process has not been feasible because H2O2 has a small molar extinction

coefficient requiring high concentrations of H2O2 and not using the UV energy

efficiently.
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Most recently the hydrogen peroxide/UV process has been applied to the

oxidation of trace constituents in treated water.

Based on numerous studies it has been found that combined advanced

oxidisation processes are more effective than any of the individual agents (e.g.,

Ozone, UV, Hydrogen peroxide). Advanced oxidisation processes are usually

applied to low COD waste waters because of the cost of ozone and/or hydrogen

peroxide required to generate the hydroxyl radicals. Material that was previously

resistant to degradation may be transformed in to compounds that will require

further biological treatment.

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