MEMBRANE FILTRATION

Membrane Processes

• A membrane is a selective barrier that permits the separation of certain species in a fluid by combination of sieving and diffusion mechanisms

• The membrane is a synthetic material that is semipermeable. It’s highly permeable to some constituents and less permeable to others. Water is pumped against the surface of a membrane resulting in a separation of product.

Membrane Processes

Four common types of Pressure Driven membranes:

Reverse OsmosisNanofiltrationUltrafiltrationMicrofiltration

Membrane Processes are becoming popular because they are considered “Green” technology - no chemicals are used in the process.

Module Rack

• Membranes can be described by a variety of criteria:

- Membrane pore size- Molecular weight cutoff (MWCO) refers to the lowest molecular weight solute (in daltons) in which 90% of the solute is retained by the membrane.- Membrane material and geometry- Targeted materials to be removed- Type of water quality to be treated- Treated water quality

• RO & NF remove ions and used for water softening.• MF & UF separate suspended particles (colloidal matter,

viruses, microorganisms) from the water.

PROCESS PORE SIZE FLUX(L/m2 h)

PRESSURE (psi)

MF 0.1 to 2 mm 100 – 1000 15 - 60

UF 0.005 to 0.1 mm 30 – 300 10 – 100

NF 0.0005 to 0.005 mm

20 – 150 40 – 200 psig (90 typically)

RO < 0.5 nm 10 - 35 200 – 300

Flux or water flux is typically expressed as volume per area per unit of time. Flux is used to express the rate at which water permeates a reverse osmosis membrane

The materials most widely used in RO & NF are:

- Cellulosic Acetate (CA): it’s not tolerant to temperatures above 30˚C. It tends to hydrolyze when pH is less than 3 or greater than 8.

- Polyamide (PA) membranes: are resistant to biological degradation; stable over a pH range of 3 to 11;do not hydrolyze in water.

Pressure requirements are based on osmotic pressure for R.O., osmotic pressure and fluid mechanical frictional headloss (straining) for nanofiltration, and purely fluid mechanical frictional headloss (straining) for ultra- and microfiltration.

Applications of Micro- and Ultrafiltration:

•Conventional water treatment (replace all processes except disinfection).

•Pretreat water for R.O and nanofiltration.

•Iron/Manganese removal (after oxidation).

•Removal of DBP.Applications for R.O. and nanofiltration:

•R.O. application mostly desalination.

•Nanofiltration first developed to remove hardness.

•Nanofiltration can be used to remove DBP.

There are various ways to reduce this fouling such as:

• Periodic pulsing of feed• Periodic pulsing filtrate (backwashing)• Increasing shear at by rotating membrane•Vibrating membrane (VSEP technology )

MEMBRANE PERFORMANCE AND MAINTENANCE

The performance of a membrane depends on:

The characteristics of the membrane

The feed solution being treated

The operating conditions

The following are some parameters used to measuremembrane performance:

RecoveryFactor

100covRe Feed

permeate

QQ

ery

Where Qpermeate and QFeed are the permeate flow rate and the feedflow rate respectively.

Measures how much of the feed is recovered as permeate.

Rejection or Retention

100)(

Feed

PermeateFeed

CCCR

Where CFeed is the concentration of a particular species in the feedand Cpermeate is the concentration of the same specie in the purifiedstream.

Transmission Percentage of solute that is not retained by the membrane.

Measure of the fraction of solute that

is retained for the membrane.

100Feed

permeate

CC

T RT 100or

DecontaminationFactor

Permeate

Feed

CCDF

Useful to evaluate the performance of waste treatment

processes.

Fouling: Is the deposition of sub-micrometer particles (smaller than 1 μm) on the membrane surface and/or its pores. In general, there are four major types of fouling:

Comparison of Fouled and Clean Membrane

Generally, the different types of

fouling occur simultaneously.

Dissolved solids

Suspended solids

Non-biological organics

Biological organisms

Examples of applications and separation processes which compete with the respective membrane separation process.

Process Applications Alternative Processes

Microfiltration Separation of bacteria and cells from solutions

Sedimentation,Centrifugation

Ultrafiltration Separation of proteins and virus,

concentration of oil-in-water emulsions

Centrifugation

Nanofiltration Separation of dye and sugar,water softening

Distillation, Evaporation

Reverse Osmosis Desalination of sea and brackish

water,process water purification

Distillation, Evaporation,

Dialysis

REVERSE OSMOSIS MEMBRANE AND MODULES

HOLLOW FIBER

FLAT SHEET

TUBULAR

According to Geometric Shape,

membranes can be

classified in

Hollow Fibermodule

Spiral woundmodule

Plate and Framemodule

Tubularmodule

ADVANTAGES AND DISADVANTAGES OF MEMBRANE MODULES

SPIRAL-WOUND

HOLLOW FIBER

TUBULAR

PLATE AND FRAME

• Low manufacturing cost• Relatively easy to clean by both

chemical and hydraulic methods.• Has a very broad range of applications• High packing density

• Relatively low manufacturing cost.• Compact• High packing density• Modes energy requirement

• Can be operated on extremely turbidfeed water.

• Relatively easy to clean eithermechanically or hydraulically.

• Can process high suspended solidfeed with minimal pretreatment.

• Moderate membrane surface.• Well-developed equipment.

• It can not be used on highly turbid feed waters without extensive pretreatment.

• Susceptible to plugging by particulates

• Extremely susceptible to fouling due tovery small spacing between fibers.

• Difficult to clean.• Requires extensive pretreatment.• Limited range of applications.

• High capital cost.• Relative high volume required per unit

membrane area.

• Expensive to operate for large scale.• Susceptible to plugging by particulates

at flow stagnation points.• Potentially difficult to clean.

ADVANTAGES DISADVANTAGES

Determine the number of modules per rack and the number of racks for the following conditions:- Q = 10,000 m3/day- J = 0.1 m3/h.m2

- Membrane Area per module = 50 m2

- Backwash interval = 60 min- Backwash time = 8 min