معالجة مياه الصرف الصناعى لمصانع الورق والكرتون بطريقة الترشيح بالاغشية كاحد مراحل المعالجة المتقدمة

ALTERNATIVE PROCESSES

COLONEL.DR
BAHAA BADR
WATER TREATMENT CONSULTANT
membrane filtration system

Using membrane filtration system, pulp and paper mills can now effectively treat the effluent streams. Using latest membrane filtration system, effluent streams can be treated without the fouling problems. The treatment with membrane system effectively reduce BOD,COD, TDS, TSS and color bodies from effluent streams that is discharged pulp and paper mills. The liquid fraction is filtered freely without the building up of solids over membrane, thereby reducing the fouling possibilities.

Membrane that is popularly used are ultra filtration or nano filtration membrane to treat the effluent. These membrane separate fibers, fines, fillers and organic materials or dissolved solids and hence generates stream which meets water discharge or reuse criteria. If there is a possibility of TDS buildup, reverse osmosis filtration can be used. The treated wastewater with membrane distillation can be discharged or recycled back into the mill for reuse.


Waster Water Treatment In Pulp Industry

As per diagram, the membrane in used in secondary and tertiary stage. The effluent from the primary clarifiers is treated by using membrane in secondary treatment step whereas the effluent from the secondary clarifiers is treated again in a tertiary treatment step using membrane. The water so treated can be either reused or discharged. In every stage of water treatment biological oxygen demand (BOD) and (COD) is reduced considerably. Then there is the process of membrane separation that is used to clean the paper and pulp wastewater.

Membrane Filtration Process in Pulp and Paper Wastewater Treatment
What is Membrane Filtration?
Membrane filtration is the process of separating solid particulates from liquid in order to purify it. The process of membrane filtration is useful to separate both the large as well as small particles.

Membrane Filtration
Types of Membrane Filtration
There are basically four types of membrane filtration. These are:
• Micro Filtration
• Ultra Filtration
• Nano Filtration
• Reverse Osmosis
Micro and ultra membrane filtration is used when the particle size is big whereas when the slat is to be removed from the liquid then nano filtration and reverse osmosis is applied. In nano as well as reverse osmosis, separation occurs because of the difference in pressure. But in micro and ultra the removal takes place on the principle of pores.
Benefits of Membrane Filtration
This technique of separation is used in pulp and paper wastewater treatment or in other water treatment as it has many benefits over the other techniques of purifying water.

Membrane filtration can take place even at low temperature. So if the matter is heat sensitive then you can easily use this process for purification.

Total consumption of energy in the overall process is very low as compare to other processes like evaporation. The energy is just required to transfer the water and particulates across the membrane.

It is really very easy to expand the entire process. In the entire system of filtration no chemicals or food additives are required. With this the operating cost can be reduced to a great extent.
More About Membrane System
The filtration membrane that you are going to pick will depend upon the size, cost, cleaning and maintaining prospects and risk or plugging. Mostly these membranes are never used as a single large sheet because it involved high cost of installing and maintaining. Instead of this small units consisting of dense system is made in a smalled possible volume.

The permeability of the membrane depends upon the pore size hence the particulate having size larger than the pores cannot pass through it. Most of the membrane filtration systems are designed for the industrial use. In industries the successive membrane filtration is very popular that consists of series of membranes through which the solvent passes. The pore size keeps reducing with the membrane. With this there are less chances of clogging.

The technique of membrane filtration is extensively used in different kinds of industries where the wastewater treatment is done. Industrial food processing, laboratory and medicines are other important industries where this technique of filtering and cleaning water is applied.

Ultra filtration is process by which suspended materials and macromolecules are separated from wastewater by using membrane and pressure differential. The pressure differential in this method is lower than that of reverse osmosis. Unlike reverse osmosis it does not rely on overcoming osmotic effects. For dilute solutions of large polymerized macromolecules, this process is fruitful.

In microfiltration, membrane filter separates particles according to pore size. The membrane used in this ultrafiltration system acts as a molecular sieve. The ultrafilter used in the process is selectively permeable membrane which does not allow macromolecules above a certain size to pass through. It also retains colloids, microorganisms and pyrogens. However smaller molecules like solvents and ionized contaminants pass into the filtrate.



The function that Ultrafiltration processes perform are feed clarification, concentration of rejected solutes and fractionation of solutes. Ultrafiltration (UF) however is not so effective against organic streams.


Pores of the surface layer of the membrane is relatively smaller than the pores in the support layer of the membrane. Material that passes through the fine pores can readily be transported through the sponge-like structure of the support layer.


Whenever the mixture of solvent passes through the membrane some of the materials are retained that when get concentrated resist the flow. Thus when the solution is processed this localized concentration of solute usually lead to the precipitation of a solute gel over the membrane. Due to this, by controlling the rate of transport through the polarization layer the permeate rate can be effectively controlled.

Properties of Membrane
Membrane characteristics include porosity, morphology, surface properties, mechanical strength and chemical resistance. Following have been used successfully as a Polymeric materials like polysulfone, polypropylene, nylon 6, PVC, acrylic copolymer etc. Some of the inorganic materials like ceramics, carbon based membranes, zirconia etc. are also sometimes used as ultrafiltration membrane. These membrane come in sheet, capillary and tubular forms. The liquid is filtered in two streams - dilute permeate passes perpendicularly through the membrane whereas, concentrate passes out the end of the media.

Advantages
Following are the advantages of Ultrafiltration:
• Removes particles, pyrogens, microorganisms, and colloids above their rated size effectively.
• Highest quality of water is produced using least amount of energy.
• They are easy to install.
• Maintenance is cheaper and easier.
Applications of Ultrafiltration
Ultrafiltration is used for following applications:
• For biological molecule concentration.
• For recovering electropaint.
• Waste treatment of oil emulsion.
• Whey treatment in dairy industries.
• Waste treatment of pulp mill.
• producing pure water for electronics industry.
• For concentrating textile sizing.
• Heat sensitive proteins concentration for food additives.
• Gelatin concentration.
• Preparing Enzyme and pharmaceutical products.

Nanofiltration like other membrane separation processes uses membranes to separate different fluids or ions.

Organic semipermeable membrane, forms a separation layer where the filtration process takes place.

Once the pressure difference between the feed (retentate) and the filtrate (permeate) is created, the separation process starts.

Membrane, because of its selectivity, retain one or many components of a dissolved mixture whereas water and substances having a molecular weight < 200 D are able to permeate the semipermeable separation layer.

The most important aspect of nanofiltration membrane is that, monovalent ions pass the membrane whereas divalent and multivalent ions are rejected.

Nanofiltration uses cellulosic acetate and aromatic polyamide type Membranes which can reject salts from 95% for divalent salts to 40% for monovalent salts.

The membrane used by Nanofiltration have pores that are typically much larger than the membrane pores that are used in reverse osmosis.

The rejection rate of divalent and multivalent cations especially calcium and magnesium is higher in nanofiltration due to this, it is used extensively in industrial water softening applications as well as pre-treatment for reverse osmosis.

Nanofiltration can operate at lower pressure and is capable of passing some of the inorganic salts hence they can be used where high organic removal and moderate inorganic removals are desired.


Nanofiltration Versus Reverse Osmosis
The membrane pore structure of nanofiltration is larger than reverse osmosis membrane.

That is why it is also sometimes referred to as "loose" RO. More salt is allowed to pass in nanofiltration as compared to reverse osmosis.

NF can operate at higher recoveries as compared to reverse osmosis, which enables the conservation of water usage because of lower concentrate stream flow rate.

The other main difference between reverse osmosis and the NF lies in the removal of monovalent ions such as chlorides.

The monovalent ion level removed by the RO is 98-99% level at 200 psi. But in case of NF membrane's removal of monovalent ions ranges between 50% to 90%.

The removal of monovalent ion by NF depends on the material and manufacture of the membrane due to this, there is a variety of Nanofiltration membranes available.

Nanofiltration uses less fine membrane hence the feed pressure required in the system is lower as compared to RO systems. The fouling rate in NF system as compared to RO system is also lower.


Application of Nanofiltration

Industry Applications


Food
• Whey demineralization

• Sugar solution-- demineralization

• Nutrient recycling in fermentation processes


• Separates sunflower oil from solvent


• Treating Effluent

• Recovers regenerated liquid from decolourized resins in sugar industry

• Organic acid Purification

Textile


• Separation of amino acid

• Dye removal from wastewater

Chemical
• Bleaching solution Recovery

• Bromide Preparation

• Recovers caustic solutions in cellulose and viscose production

• Precipitation of CaSO4

Water production
• Recovering LiOH while treating battery waste

• Removes degreasing agents from water

• Removes hardness in water

• Natural organic matter is removed

• Pesticide removal • Heavy metals like (As, Pb), Fe, Cu, Zn and silica is removed


• Treats brackish water

Landfills

• Phosphate, sulphate, nitrate and fluoride removal

Agriculture

• Toxins like algal is removed


• Landfill leachate Purification


• Selenium removal from drainage water


Clothing and leather
• Salts and water recovery from wastewater

Paper and graphical

• Chromium(III) and Chromium(II) recovery and reuse


• Water recovery from wastewater or wastewater treatment effluent

Others
• Desalination of slightly brackish water

• Food and pharmaceutical applications

• Demineralization
• Can concentrate sugars, divalent salts, particles, bacteria, proteins, dyes, and constituents that have molecular weight above 1000 daltons.

Properties of Nanofiltration
Following are the properties of nanofiltration:
• The nanofiltration membranes pore size is equivalent to a molecular weight cut off value of approximately 300-500 g/mole.

Hence the components possessing this molecular weight can be easily separated

from the components possessing higher molecular weight.

• The surface of the NF membranes is slightly charged.

Charge interaction plays major role because the dimensions of the pores are less than one order of magnitude larger than the size of ions.

This property also helps in separating ions of different valencies.
Transfer of Mass in Nanofiltration
Mass transfer can be graphically represented as follows:


Where:
P = External pressure
(J) = Solvent flux
(R) = Rejection
solvent flux is given by:


P = Effective transmembrane pressure [N/m2]
n = The permeate viscosity [Pa.s]
Rtot = The total resistance towards solvent flow [m-1].

For solute behavior characterization, the rejection is given by:

Cb = Concentration level l
= Final concentration of solute in the permeate

the Reverse Osmosis System
The process through which solvent is pushed from region of high concentration solute to region of low concentration solute using semipermeable membrane is called as Reverse osmosis(RO) process.

When solution is pushed through semipermeable membrane, the pure solvent passes to the other side whereas solute is trap on one side.

The membranes that are normally used in this process is made of dense polymer barrier layer.

In this process high pressure is exerted from the region of high concentration side of the membrane.

The pressure exerted is more than the normal osmotic pressure.

Reverse Osmosis (RO) is considered as one of the most suitable process for desalination.

To achieve desalination, the feed water is forced with high pressure through the semi permeable membrane that results in the separation of salts from the water.

The concentration of salts in the feed water, pressure applied and the salt permeation constant of the membranes are some of the key factors that decides the quality of the water.

To further enhance the quality of the water, the water obtained during the first phase can be made to pass through the membrane second time.



Technical Process of Desalination

In the RO process, using water-permeable me
mbrane, dissolved salt and water is sep
arated from a pressurized saline solution.

The pressure difference created between the pressurized feedwater and the product water enables the permeate (the liquid flowing through the membrane) to flow through the membrane.

In this process no change of heat or phase takes place. It is only during the initial pressurization of the feed water the maximum energy is required.

To pressurized the feed water it is pumped into a closed container against the membrane.


The passing of the product water through the membrane results in the concentration of brine solution and the feed water.

The concentrated feedwater-brine solution is withdrawn from the container in order to reduce the concentration of remaining dissolved salts.


This is essential because this enables to prevent the the continuous concentration of dissolved salts in the feed water.

Desalination through Reverse Osmosis

Major Components of Reverse Osmosis System
Following processes are followed in reverse osmosis system:

Pretreatment
The feed water before introduction to membrane is pretreated for adjusting the pH, removing suspended solids, and to control scaling which is the caused by constituents such as calcium sulphate.

Pressurization

The pressure of the pretreated feed water is raised by the pump to make it suitable for the membrane and the salinity of the feedwater.

Separation
The dissolved salt is obstructed by the permeable membranes whereas desalinated water is allowed to pass through.



Thus the membrane separates the product feed water into freshwater product stream and a concentrated brine reject stream. However, small amount of salt passes through the membrane and remains in the product water no membrane is as perfect as to filtrate all the salts present in the water. The popular membrane that are used for reverse osmosis process (RO) are spiral wound and hollow fine fiber membranes. These membranes are mainly used for brackish water and seawater desalination.

Spiral Bound Semipermeable Membrane for Reverse Osmosis

Click here for large view

Stabilization

The pH adjustment and degasification of the product water is necessary before it is made fit for consumption. The pH value of the water is raised from the level of 5 to 7 by passing the water through an aeration column.

Applications of Reverse Osmosis Process


Reverse Osmosis technology is applied for
various purposes apart from desalination of sea and brackish water.

Some other applications are:


• It is used for desalination of ground water.
• In industries where the concentration of a wet process stream is needed.

The popular industries in which they are used are:
o Electronic parts manufacturing
o Specialty foods
o Pharmaceuticals
• For purifying water to be used in green house irrigation

Maintenance of the Reverse Osmosis System
If the reverse osmosis system is carefully planned and designed they can work for the long time provided that day to day monitoring of the system is done and a systematic program of preventive maintenance is followed. For preventive maintenance following steps need to be taken
• Chemical feed inspection and adjustment
• Pump adjustment
• Instrument calibration
• Leak detection and repair
• Structural repair of the system on a planned schedule.
Fouling of the membrane is one of the most common problem of the reverse osmosis system which is caused by the clogging of salts over membrane.

This fouling can be corrected cleaning, backwashing and replacing cartridge filter elements thrice in a year.


Advantages
The Reverse Osmosis (RO) system and process has following advantages:
• The system is simple which is easy to operate and install.
• Cost of installation is low.
• Space to production capacity ratio of the plant is quite high which range between 25 000 to 60 000 l/day/m2.
• The use of chemicals in whole process is quite low.
• The impact on environment is low.
• It is suitable both for organic and inorganic contaminants
• For processing brackish water energy use range from 1 to 3 kWh per 10001 of product water