Reverse Osmosis
The process of movement of solvent through a semipermeable membrane from the solution to the pure
solvent by applying excess pressure on the solution side is called reverse osmosis.
Reverse osmosis is a membrane treatment process primarily used to separate dissolved solutes from
water. Reverse osmosis is most commonly known for its use in drinking water purification, particularly
with regard to removing salt and other effluent materials from water molecules.
Table of Contents
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Introduction to Reverse Osmosis
What is Reverse Osmosis?
Reverse Osmosis Principle
Reverse Osmosis Process
Experiment of reverse osmosis
Advantages of Reverse Osmosis
Disadvantages of Reverse Osmosis
Frequently Asked Questions – FAQs
Introduction to Reverse Osmosis
Reverse osmosis is one of the oldest and most popular separation techniques used mainly for the
purification of water. The process was mainly adopted for the desalination of seawater in the year 1950
when the whole process was relatively slow and limited to certain laboratories. However, after a lot of
research and advancements in technology, there were significant developments, especially in the field
of polymers and the production of efficient membranes.
Today, this technique is extensively used by many around the world to purify water for industrial,
residential, commercial and scientific purposes. While reverse osmosis technology is one of humanity’s
important scientific innovations we will develop a basic understanding of the whole process here on this
page.
What is Reverse Osmosis?
Reverse osmosis which is also commonly referred to as RO is a type of filtration method used for the
removal of molecules and ions from a certain solution.
Reverse osmosis involves the application of pressure (usually greater than the osmotic pressure) on one
side of the solution where a semipermeable membrane is placed in between the solutions. This membrane
is used to filter out contaminants down to the smallest particles. The contaminants are often referred to as
RO concentrate.
Reverse Osmosis Principle
To break down the process further, due to the presence of a membrane, large molecules of the solute are
not able to cross through it and they remain on the pressurised side. The pure solvent, on the other hand,
is allowed to pass through the membrane. When this happens the molecules of the solute start becoming
concentrated on one side while the other side of the membrane becomes dilute. Furthermore, the levels of
solutions also change to some degree.
In essence, reverse osmosis takes place when the solvent passes through the membrane against the
concentration gradient. It basically moves from a higher concentration to a lower concentration.
Reverse Osmosis Process
Osmotic pressure is the minimum pressure required to stop solvent flow through the semipermeable
membrane. Therefore, when the solution side (the side where the solute concentration is high) is subjected
to a pressure greater than the osmotic pressure, the solvent particles on the solution side move through the
semipermeable membrane to the region where the solute concentration is low. Such inverse solvent
movement through the semipermeable membrane is called reverse osmosis.
It is important to note that the pressure applied to the solution side must be higher than the osmotic
pressure for the reverse osmosis process to proceed. Osmotic pressure is a colligative property, which
depends on the concentration of the solution. In water purification, the reverse osmosis process is very
important. Many water purifiers used today use reverse osmosis in the purification process as one of the
steps.
Experiment of Reverse Osmosis
The reverse osmosis process is explained below with the help of an experiment.
How does Reverse Osmosis work?
An easy experiment can be conducted by taking some freshwater and a concentrated aqueous solution.
The solutions should be kept on opposite sides with a semipermeable membrane placed in between to
separate the two solutions. Pressure should be applied on the side with the concentrated solution. Now
this will result in water molecules moving through the membrane to the freshwater side. This basically
sums up the process of reverse osmosis.
Benefits of Reverse Osmosis
Some of the benefits of reverse osmosis are discussed below –
1. This process can be used to effectively remove many types of dissolved and suspended chemical
particles as well as biological entities (like bacteria) from the water.
2. This technique has a wide application in treating liquid wastes or discharges.
3. It is used in purifying water to prevent diseases.
4. It helps in desalinating seawater.
5. It is beneficial in the medical field.
Advantages of Reverse Osmosis
Reverse Osmosis has several advantages, including the following:
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Bacteria, viruses and pyrogen materials are rejected by the intact
membrane. In this respect, RO water approaches distilled water in
quality.
 Available units are relatively compact and require little space. They are
well suited to home dialysis.
 In average use, the membrane has a life of a little more than one to two
years before replacement is necessary.
 Periodic complete sterilization of the RO system with formalin or other
sterilant is practical.
Disadvantages of Reverse Osmosis
The disadvantages of RO systems include the following;
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Cellulose acetate membranes have limited pH tolerance. They degrade at
temperatures greater than 35oC. They are vulnerable to bacteria. They
eventually hydrolyze.
Polyamide membranes are intolerant of temperatures greater than 35oC.
They have poor tolerance for free chlorine.
Thin-film composites are intolerant of chlorine. High flux polysulfones
require softening or deionization of feed water to function properly.
Table of Contents
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1. Types of Grit Chambers
o a. Horizontal flow grit chambers
o b. Aerated Grit chamber
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2. Uses
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3. Composition of Grit
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4. Working Principle of Grit Chamber
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5. Advantages of Grit Chamber
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6. Disdvantages of Grit Chamber
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7. FAQ
A grit chamber is the second step procedure that is used in the primary treatment of wastewater.
The grit chamber removes sand and other weighty matters which are inorganic such as metal fragments,
rags, etc. If not removed in primary treatments, grit in the primary settling tank can induce abnormal
abrasive wear and tear on mechanical equipment and sludge pumps, choke by deposition, and collection
in sludge holding tanks and digesters. Therefore grit removal is essential to save the moving mechanical
equipment and pump parts from abrasion.
Grit removal relies upon the differences in specific gravity between organic and inorganic solids to affect
their separation. The specific gravity of the grit is usually 2.4 to 2.65.
1. Types of Grit Chambers
These chambers are divided into 2 types that are as follows:
1. Horizontal flow grit chambers
2. Aerated grit chamber
a. Horizontal flow grit chambers
These chambers are a lean open channel, which is about 10-18 meters long and 1 to 1.3 m in depth.
Velocity in the grit chamber should be held in such a way that the velocity will carry most of the organic
particles through the chamber and will re-suspend any that are designed.
A control Aerated settle but will allow the settling of weightier grit materials particles. It is to keep a
velocity of 0.3 m/s.
A control department is utilized, in order to keep a fairly consistent velocity of flow.
b. Aerated Grit chamber
An aerated grit chamber is a unique form of grit chamber having a standard spiral flow aeration tank
equipped with air diffusion tubes set at one end of the tank at about 0.6 to 1m from the bottom.
The weightier grit particles with their higher settling velocities settle down to the floor, whereas more
lightweight organic particles will stay in suspension and carry with the roll of spiral motion because of the
diffused air and finally move out of the tank.
It is further classified into two types, based on the cleaning mechanism.
a. Mechanically cleaned
b. Manually cleaned
a. Mechanically cleaned
Mechanically cleaned grit chambers are equipped with mechanical equipment for the accumulation and
washing of grit chambers, which are served either on a continuous or intermittent basis.
b. Manually Cleaned
Manually run grit chambers should have adequate capacity for hold of grits between the time of cleaning.
The easiest method of cleaning is with the help of a shovel.
2. Uses
The uses of the grit chamber are as follows:
✔ It is used to prevent equipment from clogging.
✔ It is used to slow down the flow to settle heavy solids.
✔ It is used to save the waste treatment cost.
✔ It is used to control grit collection in sludge digesters.
3. Composition of Grit
The composition of grit varied depending on the following conditions✔ Types of street surfaces encountered
✔ Relative areas served
✔ Climatic conditions
✔ Types of inlets and catch basins
✔ Amount of stormwater diverted from combined sewers at overflow points Sewer grades
✔ Construction and condition of the sewer system
✔ Ground and groundwater characteristics
✔ Industrial wastes and
✔ Social habits.
4. Working Principle of Grit Chamber
The working principle of the grit chamber is as follows:
It operates as a sedimentation tank which is prepared to distinguish the planned weightier inorganic
materials (specific gravity of about 2.65) and to move ahead of the more lightweight organic materials.
Differential sedimentation and differential scouring velocity are held in the chamber so that the flow
velocity should neither be less as to cause the settling of more lightweight organic matter nor should it be
more as not to cause the settlement of the silt and grit attending in the sewage.
The critical velocity of flow vc beyond which particles of a particular size and density once settled should
always be smaller than the scouring velocity of grit particles. The critical velocity of scouring is provided
by Schields formulaV = 3 to 4.5 (g(Ss - 1)d)1/2
The horizontal velocity of flow of 15 to 30 cm/sec is utilized at peak flows. This identical velocity is to
be held at all change of flow to make sure that only organic solids and not the grit is scoured from the
bottom.
5. Advantages of Grit Chamber
The advantages of this chamber are as follows:
✔ To save running mechanical equipment from abrasion and abnormal wear.
✔ To decrease maintenance cost in the frequency of digester cleaning caused by an extreme collection of
grit.
✔ To control weighty deposits in pipelines and channels.
✔ It also saves the cost of waste treatment by stopping solid materials in it.
6. Disdvantages of Grit Chamber
The disadvantages of the grit chamber are as follows:
✔ They are more probable to release toxic odors and toxic organic matter.
✔ Aeration system control and maintenance will affect different human resources.
✔ Compared to other girt removal technologies, they need more energy resources.
✔ Initial construction cost is high.
✔ Regular maintenance is needed.
UV DISINFECTION SYSTEM FOR WATER TREATMENT
What is UV Disinfection System and How Does it Work?
UV Disinfection System is an extremely effective way to combat microbial contamination in water.
However, microbes have to be exposed to UV-C light in the proper amount in order to effectively
disinfect the water. UV Disinfection Systems are used in many different applications ranging from the
purification of drinking water in individual homes to disinfecting the water supply of entire townships
to industrial wastewater treatment. UV treatment for water is recognized as a safer and more costeffective way to disinfect water for industrial applications
Industrial Water Treatment
UV sanitization is useful in almost any application where microbial-free, safe and pure water is required;
and where there is a chance of the water being contaminated before it reaches the final point of use.
1] What Is UV Disinfection System?
In UV water disinfection technology, Ultraviolet light of wavelength 253.7 nanometers is used for the
disinfection of bacteria, viruses, molds, algae, and other microorganisms, which multiply and grow. UV
disinfection technology destroys the DNA of microorganisms which leaves them dead and unable to grow
further. UV disinfection technology can be used for drinking water disinfection, process water
disinfection, wastewater disinfection, and surface disinfection. Other than disinfection applications, this
technology can also be used for TOC removal and Ozone destruction. Plus, there is a UV sterilizer for
hospitals, factories, and offices.
2] How Does UV Disinfection System Work?
In the UV water disinfection technology, the UV light disinfects by penetrating microorganisms and
destroying their DNA. DNA plays an important role in organisms’ functions and reproduction hence
destroying the DNA prevents the organism from being active and multiplying. This UV energy
(wavelength of 240-280 nm) is also naturally found in sunlight in very small quantities. The same energy
is produced in stronger intensities with the help of high mercury discharge lamps, commonly known
as UV lamps.
No bacteria, viruses, molds, or spores can survive when exposed to the correct dose of UV light.
Therefore UV is considered as the best solution for water sterilization, and for room sterilization, there is
the UV mobile sterilizer machine that can be used.
3] Industrial Applications of UV Disinfection System
An ultraviolet disinfection system is not simply a lamp inside a pipe. The UV Reactor must be designed
to ensure that all microbes receive sufficient exposure to the UV light (dose). Based on the hydraulic
properties of water; the reactor needs to be optimized to guide the flow in a manner so as to maximize
residence time and boost turbulence. Well designed Industrial UV water disinfection systems are
producing consistently exceptional results in the industrial applications
Few Examples :
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Food and Beverage – A UV disinfection system can help to achieve High-quality
water as per specifications laid down by the FDA ( Food and Drug
Administration)
Bio-Pharmaceutical – Water used in Pharmaceutical and healthcare products and
for CIP (Cleaning in Place) must be free of chemicals like chlorine, ozone, and
pathogens. Most pharmaceutical companies depend on UV systems for water
disinfection.
Cosmetics – Water that is free of microorganisms and toxins ensures quality and
enhances the shelf life of cosmetics. UV Sterilization is the preferred choice for
the cosmetic industry across the globe.
Centralized Drinking Water – A UV drinking water disinfection system is an
easy, affordable solution to ensure pure water in each and every tap of your home
or office
Waste Water Disinfection and Reuse – To combat the problems of water scarcity
and the rising cost of fresh water, UV Disinfection can help by treating the
wastewater in the tertiary stage. UV systems that are specially designed for
wastewater can thus disinfect wastewater so that the water can be reused for
secondary purposes such as flushing and gardening.
Swimming Pools – Traditionally, chlorine has been in use to ensure clean water in
swimming pools. However, it is increasingly being known that with chemical
disinfection, chemical reacts with many other organic matters to form hundreds of
new chemicals which are harmful. While UV is recognized as a safer and more
cost-effective way to disinfect swimming pools.
4] Benefits of UV Disinfection System
Natural – UV is nature’s way of purification.
Environmentally Friendly – No Toxic by-products are formed during the UV disinfection process
Effective – All known microorganisms are susceptible to UV light
Economical – Lowest operating cost among disinfection systems
Safe and Chemical-Free – No addition of chemicals hence no danger of overdosing
Fast – It is In-contact purification therefore Instant
Easy to Manage – Well-designed systems like the Alfaa UV systems come with many advanced features
like CFD (Computational Fluid Dynamics), high-efficiency electronic ballasts, and extremely precise UV
intensity monitors which make them highly effective and hence easy to manage.
5] Does a UV Disinfection System need periodic maintenance?
There can be some cases where the water is not adequately pre-treated and turbidity levels are low. In
such cases, routine inspection and cleaning can be carried out every 6 months. In the case of high
turbidity and hardness, the cleaning frequency might need to be increased. Finally, the UV lamp has a
limited life and must be replaced once it is exhausted. In the unlikely event of premature failure of the
lamp, the monitoring circuit will provide the signal to advise replacement.
Comparison of UV Disinfection System, Chlorination and Ozonation
Ultra Violet
Chlorination Ozonation
Low
Lowest
High
Capital Cost
Lowest
Low
High
Operating Cost
Excellent
Good
Complex
Ease of Installation
Excellent
Good
Poor
Ease of Maintenance
<10 seconds
20-30 minutes 10-20 minutes
Contact Time Required
Analysis of Water Softening
Related information:
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Industrial water softener information
Industrial water calculators
Industrial reverse osmosis information
Ion Exchange Water Softening vs. Lime Softening
Ion-exchange water softening
The Various Types of Ion Exchange Materials
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Cation
Anion exchangers
Cation exchange materials react only with
positively charged ions such as Ca++ and Mh++.
Anion exchanger materials react only with the
negatively charged ions such as carbonate (CO3-)
and sulphate (SO4-)
Lime-soda softening
The Various Methods of Lime-Soda Softening
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The older method of intermittent softening
consists of mixing the chemicals with the water in
a tank, allowing time for reaction and settling of
the sludge, and drawing off the clear water.
The more modern method of continuous lime-soda
softening involves the use of specially
compartmented tanks with provisions for
1. Proportioning chemicals continuously to the
incoming water
2. Retention time for chemical reactions and settling
of sludge, and
3. Continuous draw-off of softened water. Lime-soda
softening may also be classified as hot or cold,
depending on the temperature of the water. Hot
process softeners increase the rate of chemical
reactions and give better quality water.
Advantages
Ion – exchange water softening
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Lower capital costs
The main advantage of zeolite softening is
ease of control.
Running this system does not require any
training nor does it require a staff person to
take care of it.
When paired with bulk brine tank, system
can run 100% automatically.
Ordinary variations of hardness in the raw
water or in flow rate do not affect
completeness of softening.
The use of acid exchangers has advantages
when a low alkalinity soft water is required.
Better quality water than can be obtained
by any other method.
It has compact size and small footprint.
The chemicals used (salt) are safer for the
operator to handle and operation is much
easier.
Disadvantages
Lime-soda softening
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The main advantage is that in reducing
hardness, alkalinity and silica can also be
reduced.
In addition, prior clarification of the water is
not usually necessary since suspended matter
and turbidity are also removed in the process.
Another advantage is that with continuous hot
process softening some removal of oxygen and
carbon dioxide can be achieved.
Operational costs may be lower than ion
softening. However, careful analysis needs to
be done to consider the costs of full time staff
to run equipment.
Ion – exchange softening
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Lime-soda softening
Could have higher operating costs.
The total solids, alkalinity and silica
contents of the raw water are not reduced.
A problem encountered with cation
exchange on the hydrogen cycle is
corrosion from acidity of the effluent. This
does not apply for standard sodium
exchange softeners.
In some cases, fouling of the ion exchange
material with suspended or colloidal matter
in the raw water can produce difficulties
and some water impurities cause
degradation of the material.
Softeners have to be backwashed in a
manner similar to a filter, and the recharge
water, known as brine, could cause disposal
problems.
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Initial capital cost is higher.
Generally requires full-time, trained
personnel to run equipment
Hardness is reduced it is not completely
removed.
Wide variations in raw water composition and
flow rate also make control of this method
difficult since this involves adjusting the
amounts of lime and soda ash being fed.
Handling and monitoring of chemicals that
are less safe than salt for an ion exchanger
Process
Ion-exchange water softening
What is Ion Exchange?
When minerals dissolve in water they form electrically charged particles called ions. Calcium carbonate,
for example, forms a calcium ion with plus charges (a cation) and a carbonate ion with negative charges
(an anion). Certain natural and synthetic materials have the ability to remove mineral ions from water in
exchange for others. For example, in passing water through a simple cation exchange softener all of
calcium and magnesium ions are removed and replaced with sodium ions. Ion exchange materials usually
are provided in the form of small beads or crystals which compose a bed several feet deep through which
the water is passed.
Types of Ion Exchange Materials
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Ion exchange materials are basically of two types: cation and anion exchangers.
Cation exchange materials react only with positively charged ions such as Ca++ and Mh++.
Anion exchanger materials react only with the negatively charged ions such as carbonate (CO3-)
and sulphate (SO4-).
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Zeolite materials are cation exchangers composed chiefly of sodium, aluminum and silica.
There are several other types of cation exchange materials of an organic or resinous nature.
The anion materials are usually organic in nature and are of two basic types: weak base and
strong base types.
Weak base exchangers don’t take out carbon dioxide or silica (actually carbonic acid and silica
acid) but remove strong acid anions by a process that is more like adsorption than ion exchange.
Strong base anion exchangers on the other hand can reduce silica and carbon dioxide to very low
values.
Cation exchangers usually poles which settle out readily. In these cases clarification equipment
merely involves the use of settling basins use either a sodium or hydrogen cycle. That is, they
may be designed to replace all cations in the water with either sodium or hydrogen.
Strong base anion exchangers are generally operated on a hydroxide weak base on a carbonate
cycle. Chloride anion exchange is also used in some processes.
Source
http://www.hamadaboiler.com/en/water/qa.htm#46
http://www.thewatertreatments.com/water-softener/ion-exchange-softening
Lime-soda softening
What are the Various Methods of Lime-Soda Softening?
The two general types are intermittent (batch type) and continuous. The older method of intermittent
softening consists of mixing the chemicals with the water in a tank, allowing time for reaction and settling
of the sludge, and drawing off the clear water. The more modern method of continuous lime-soda
softening involves the use of specially compartmented tanks with provisions for
1. Proportioning chemicals continuously to the incoming water
2. Retention time for chemical reactions and settling of sludge, and
3. Continuous draw-off of softened water. Lime-soda softening may also be classified as hot or cold,
depending on the temperature of the water. Hot process softeners increase the rate of chemical
reactions and give better quality water.
Why are Coagulants Used in the Lime-Soda Process?
Just as coagulants are used for removing suspended matter in clarification processes, they serve to clump
together precipitates in the softening process. Coagulants can speed up settling of sludge as much as 25 –
50 per cent. Sodium aluminate has a special advantage as a coagulant in lime-soda softening since unlike
most other coagulants it is alkaline and also contributes to the softening reactions’, particularly in
reducing magnesium. Effective use of coagulants helps remove silica in the softening process. Silica tends
to be absorbed in the floc produced by coagulation of sludge.
Source: http://www.hamadaboiler.com/en/water/qa.htm#46
Initial cost & operating costs
Ion-exchange water softening
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Compared with lime-soda ash softening, ion-exchange has certain advantages.
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It is compact and has a low capital cost.
The chemicals used are safer for the operator to handle and operation is much easier.
The ion exchange process is more cost-effective when treating ground waters, as they are
typically a non-carbonate form of water hardness.
Approximate Costs of Lime Softening
Design Flow (mgd)
0.01
0.1
1.0
10
Average Flow (mgd)
0.005
0.03
0.35
4.4
Capital cost ($/gal)
$10.00
$5.00
$3.00
$1.50
Annual O&M Cost ($/kgal)2
$35.00
$6.00
$1.00
$0.50
Water treatment is the process of making water ready for human use. While there are several critical
aspects, sedimentation water treatment is of particular importance. It is essential to understand the whole
water treatment process in order to ensure the process is completed safely and efficiently for the general
public.
What Is Sedimentation?
Sedimentation is the process of separating small particles and sediments in water. This process happens
naturally when water is still because gravity will pull the heavier sediments down to form a sludge layer.
However, this action can be artificially stimulated in the water treatment process. This mechanical
assistance is called thickening.
Why Is Sedimentation Used?
The sedimentation process is used to reduce particle concentration in the water. The advantage of
sedimentation is that it minimizes the need for coagulation and flocculation. Typically, chemicals are
needed for coagulation and flocculation, but improved sedimentation controls the need for additional
chemicals. Additionally, sedimentation can be used after coagulation to increase the effectiveness of
ongoing filtration in the process.
What Are Technical Aspects of Sedimentation?
Although sedimentation is an accepted process within the water treatment industry, it is still theoretical.
The process can be varied depending on the concentration of particles. For example, small concentrations
often settle unhindered or without mechanical assistance. As concentrations increase, there are more
hindrances to settling and additional support will be needed to aid the process.
Types of Sedimentation Tanks
Sedimentation water treatment requires the use of specialized tanks. A sedimentation tank provides the
necessary support to make sure that the particles settle. Sedimentation will happen naturally over time,
but water treatment requires a tank to streamline the process.
Horizontal Flow Tank
Horizontal flow tanks are the simplest option. These rectangular tanks allow water to flow horizontally,
ensuring that particles are separated from the water during the movement through the tank. This way, the
sediment has been collected before the water leaves the far end of the tank. The tank is equipped to clean
the sediment out periodically in order to allow the process to continue.
Multi-Layer Tank
A variation of the horizontal flow tank is the multi-layer tank. The process is still the same in a multilayer tank. However, multiple decks have been built in the tank. Water is passed from one layer to the
next until the sediment is properly separated.
Radial Flow Tank
Radial flow tanks approach this process differently. These tanks are circular, and sediment is moved
centrally to be collected and discharged. Radial tanks can be enhanced for flocculation and recirculation
in some cases.
Settling Tank
Another tool used for sedimentation is a settling tank. A settling tank is inclined to assist with the
collection of sediment. Inclined settling tanks can be unhindered, which means they may work without
additional mechanical stimulation. Instead, the process is facilitated by the size of the tank, the depth of
the water and the placement of the inclined plates at the bottom. The flow of the water can move in
multiple directions depending on the sedimentation needs.
Ballasted Sedimentation
Ballasted sedimentation is another option. This is preferred when additional flocculation is needed to help
with coagulation. Ballasted sedimentation relies on the application of high molecular weight polymers.
These polyelectrolytes are used to increase particle density, which promotes separation. In particular,
ballasting agents are used. In most cases, this is a fine sand or Bentonite.
Floc Blanket Sedimentation
Another option is floc blanket sedimentation. These tanks look like inverted pyramids and feature a short
vertical section. Floc is circulated in the tank, attracting particles. Eventually, the floc and sediment turn
into sludge on the floor. Because of the shape of these tanks, the suspension is moved downward into the
pyramid and eventually discharged.
Sirofloc®
Finally, a Sirofloc® process can also be used. This process is used selectively for waters with little
mineral turbidity. In a Sirofloc® process, fine magnetite is prepared with high acidity. This attracts
certain particles in the water. As water is passed through a magnetic field, the magnetite particles start to
clump together. Then, the water is passed through a radial flow tank to allow the magnetite to be
collected. One great thing about a Sirofloc® process is that the collected magnetite can then be recycled
for a fresh batch.
The coagulation process in water treatment
This destroys the process whereby tiny particles repel each other and promotes their consolidation to bigger
ones that are able to stick together. The bigger the particle, the easier it is to separate from the liquid. The
use of coagulants for treating water goes all the way back to around 2000 BC when the Egyptians used
almonds, smeared around vessels, to treat river water.
These larger ‘clumps’ of particles are called micro-flocs and still cannot always be seen by the naked eye.
The water surrounding these newly formed particles should be clear – and this will signal that the particles’
charges have been neutralised. If it isn’t, more coagulant may be needed. Too much coagulant and the
particles will revert to repelling each other – but predominantly by the reverse charge.
Rapid mixing ensures the coagulant is properly dispersed to promote particle collisions. The metal
coagulant hydrolysis products formed within 0.01 to 1.0 seconds tend to be the most effective for
destabilisation – this is why adjustment of pH and post-dosing of more coagulant is rarely effective after
the initial coagulant addition.
One common type of rapid mixer is called a back-mix reactor, which normally consists of square tanks with
vertical impellers. In many instances, they produce poor results, and WCS tends to design in-line mixers
with velocity gradient control to provide the best conditions for rapid mixing.
Types of coagulants
Today, there are two types of coagulants that are most commonly used in water and wastewater treatment.
Organic and inorganic.
Inorganic coagulants include:
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Aluminium coagulants - e.g. aluminium sulphate, aluminium chloride and sodium aluminate
Iron coagulants - e.g. ferric sulphate, ferrous sulphate, ferric chloride and ferric chloride sulphate
Organic coagulants include:
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Polyamines
Polydiallyldimethylammonium chloride (Poly DADMACs)
Polytannate
Inorganic coagulants
Both aluminium and iron coagulants have been proven to be very effective at removing most suspended
solids. They offer a number of advantages:
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Enable highly charged ions to give a high charge density to neutralise suspended particles, which
allows hydrated inorganic hydroxides to form and produce short polymer chains that enhance
microfloc formation and heavy floc
Capable of removing a portion of the organic precursors which may combine with chlorine to form
disinfection by-products
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Low unit cost and widespread availability
They have some disadvantages:
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They create large volumes of floc, rich in metal, which must be disposed of in an environmentally
appropriate manner, which can add significant cost to disposal
They can significantly alter the pH of the water, where pH is critical for effective coagulation,
necessitating pH control. They also require corrosion-resistant storage and feed equipment.
Aluminium sulphate and chloride, ferric sulphate and chloride and ferrous sulphate are highly
acidic, destroy alkalinity and lower pH. Sodium aluminate, on the other hand, will add alkalinity
and raise pH.
Organic coagulants
Both polyamine and poly-DADMAC coagulants have been proven to be very effective at removing most
suspended solids. Tannates are particularly good at oils and fats. They offer a number of advantages:
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Enable relatively low charge density to neutralise lower charged suspended particles, more
effectively. Produce longer polymer chains that enhance microfloc formation without metals or
hydroxides
Capable of removing a portion of the organic precursors which may combine with chlorine to form
disinfection by-products
Produce small floc volume
Liquid forms, non-corrosive, ready for direct use.
Do not impact and are rarely or marginally affected by pH
They have some disadvantages:
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Higher unit cost
High dosages are needed if charge demand is high
Low-density floc – does not always settle well.
The most commonly used inorganic chemical coagulants in water treatment
Aluminium sulfate is the most commonly used chemical for coagulation in wastewater treatment.
Additional commonly used coagulants include sodium aluminate, ferric sulfate, and ferric chloride.
Aluminium Sulfate
Aluminium sulphate is the most widely used aluminium coagulant. It is available in a number of solid forms
such as block, kibbled, or ground and is also available as a solution. When added to water, the acidic
coagulant and the natural alkalinity of the water reacts to form an aluminium hydroxide floc, which usually
consists of calcium bicarbonate. pH control is important in coagulation, for both the removal of turbidity
and colour and also to maintain satisfactory minimum levels of dissolved residual aluminium in the clarified
water.
Sodium Aluminate
Sodium aluminate is formed by combining sodium oxide and aluminium oxide. Solid forms of this chemical
usually contain 70-80% sodium aluminate, while liquid forms contain around 30% sodium aluminate. Due
to the low molecular weight of AI, sodium aluminate solutions reduce chemical sludge production
compared to iron. Moreover, aluminates raise the alkalinity of the water, eliminating the need for lime or
hydroxides.
Ferric Sulfate
Ferric sulfate is a type of iron coagulant that is often used in conjunction with chlorine and can provide a
denser floc than aluminium sulfate. Compared to alum, ferric sulfate has some advantages; for example,
the flock particles of ferric hydroxides have a higher density than alum flocks and are more easily removed
by sedimentation. However, there are also disadvantages as it produces a significantly heavier hydroxide
sludge and it is difficult to dissolve.
Ferric Chloride
Ferric Chloride works as a flocculant and coagulant. It is versatile in the water treatment industry and is an
alternative to ferric sulfate. It generally promotes faster sedimentation, especially in cold water. However,
it is the less popular choice, as chloride can increase water’s corrosivity.
Which coagulant should you choose for water treatment?
In water treatment, metal coagulants such as the ones listed above are commonly used. Availability and
affordability are the key considerations that generally influence the coagulant used. Aluminium sulfate is
commonly available and affordable as well as being very effective.
However, other types of coagulants are also available:
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Synthetic coagulants may have high charge densities on relatively large molecules. Depending on
how they are made, some of the synthetic derivatives may behave as a flocculant.
Biopolymer coagulants from natural sources (such as fungi and plant sources). Generally, these
produce less sludge, are less toxic, and are considered to be safer.