Flow Chart of Water Treatment Plant System

Raw water source

Water Intake Work

Screening

Aeration

Coagulation

Flocculation

Sedimentation

Filtration

Disinfection

Flouridation

Water Distribution

Addition of polyaluminium

chloride

ADDITION CLORINE LIME

FLOURIDE

Raw Water IntakeRaw water (untreated) is collected from a surface water source (such as an intake on a

lake or a river) or from a groundwater source (such as a water well drawing from an underground aquifer) within the watershed that provides the water resource. The raw water is transferred to the water purification facilities using uncovered aqueducts, covered tunnels or underground water pipes. Submerged and floating intakes are used for small water supply projects. Large projects utilize tower-like intakes that can be an integral part of the dam or a separate structure.

Function of water intake

To provide best quality water for treatment

To protect the pipes from foreign materials such as floating bodies or submerged marine

To protect the pipes from stuck and damaged

To ensure the system of water supply is smooth

Source of Water Intake

River Lake Reservoir Canal

The factor and design of water intake location

The sources of water intake must be near to treatment site to reduce the cost of design

The sources must be from a clean site to prevent the water from polluted

The intake must come from a high ground location to prevent water from mix with flood

water during rain/flood seasons

The design must take pipes and pump safety into account to prevent them from damaged

by debris or heavy currents

The intake of water must take place in a place with an average temperature to prevent

water loss due to high temperature

Water intake consist of :

An opening with a screening to filter the foreign material (woods, floating bodies, debris)

from joining the water flow to the treatment plants

A conduit must be installed to channel the flow of water to the treatment plants

An aeration system to filter gases and volatile organic which may cause an alteration in

odour and taste of water.

The water intake comes from the river. The structure of the intake usually designed one

the upstream of river. This will endure the water quality is at the best. With possibility,

the structure must be designed a quite below the surface to prevents the sediments at

lower level and debris and woods on surface level. As the intake from river, screening is

needed to block large quantities of debris from entering the pipes and water pump. This

will cause a blockage of water so schedule maintenance must be form. The river water

will pump to the treatment plant.

The conduit must be support with 1-2m above the bottom and kept 1m below the water

surface. This to prevent entry silt and to avoid floating particles. The velocity must be

kept lower than 0.20m/s to prevent fish from entering during intake process.

Intake structure

Intake pump

A pump is a mechanical device or machine and is used for lifting the water or any fluid to higher

elevations or at higher pressure. The operation of lifting water or any fluid is called “pumping”.

1. To increase the water pressure at certain points in the distribution system

2. To lift treated water to elevated storage tanks so that it may flow automatically under

gravity into distribution system.

3. To lift raw river or lake water to carry it to treatment plant

4. To lift well water to elevated storage tanks.

Screening

Initially, wood chips, leaves, aquatic plants and floating bodies are removed by screening process. After screening, the denser suspended matters are removed by allowing water to pass through chamber where it settles down to the bottom. Screening process help to protect pumps in water treatment plants and save cost to repair the pipes. These screen is a final steps in the pre-treatment process.

Advance water intake screen

Thru-flow water intake screen

Thru flow is a common type of water screen. They are placed into the intake at right angles to flow. The water passes through the rotating baskets. Only one side of the unit is utilized for screening thereby reducing the effectiveness of this design.

Dual-flow water intake screen

When water flow through the panels of the traveling water screen, debris is collected on the mesh panels. As the screen rotates, the debris is carried up to deck level where a pressure spray flushes it from the panels into the debris trough. The clean panels then rotate back into the water, resulting in continuous, uninterrupted screening.

Screening unit can be operated continuously or intermittently and can be provided with baskets in metallic or non-metallic construction with varying mesh sizes. The high pressure spraying system is provided with efficient nozzles, which ensures efficient and effective cleaning of the screens as it returns back to the water flow. The screening unit can also be provided either in carbon steel construction for normal surface water application or in alloy steel construction for abrasive/sea water application

Types of screen

1. Coarse screens or trash racks

2. Fine screens(wire-mesh screens)

Coarse screens

with bar spacings of the order of 75-100mm are designed to intercept only the largest materials and these are generally held back in the flow to be manually removed. Such materials will generally be rocks, branches and large pieces of timber with little organic contamination. Coarse screening of the order of 20 mm spacing have been found to have a high rag content. Such screenings will have a relatively high volatile solids content which can be up to 80% and will typically have a dry solids content in the order of 15-25%.

Fine screenings

retained on screens with apertures of the order of 6mm will also have significant volatile solids contents and are likely to include 5-10% of influent suspended solids. Moisture contents are likely to be somewhat greater than for coarse screenings. They will also contain significant elements of grease and scum.

Aeration

Aeration also called aerification. It is a process which air is circulated through, mixed with or

dissolved in a liquid or subtances. In our context air is circulated in raw water material. Aeration

as a water treatment practice is used for the following operations:

Carbon dioxide reduction(decarbonation)

Oxidation of iron and manganese found in many water (oxidation tower)

Ammonia and hydrogen sulfide reduction (stripping)

Methods of Aeration

Three general methods may be used for the aeration of water. The most common in

industrial use is the water-fall aerator. Through the use of spray nozzles, the water is broken up

into small droplets or a thin film to enhance countercurrent air contact.

In the diffusion method, air is diffused into a receiving vessel containing counter current

flowing water, creating very small air bubbles. This ensure good air-water contacts for

“scrubbing” of undesirable gases from the water.

In gravity aerators, water is allowed to fall by gravity such that a large area of water is

exposed to atmosphere, sometimes aided by turbulence.

Addition of PAC (Polyaluminium Chloride)

To make sure coagulation and flocculation of water occur

PAC tank

Coagulation

In water treatment, the use of chemicals to make suspended solids gather or group into

small flocs

The small flocs also known as microflocs. They were not visible to naked eyes.

They water surrounding the microflocs should be clear, if not more coagulants is need to

be add.

A high energy, rapid mix to properly disperse the coagulant and promote particle

collisions is needed to achieve good coagulation. Proper contact time is 1-3 minutes.

Coagulation selection

The choice of coagulant chemical depends upon the nature of the suspended solid to be

removed, the raw water conditions, the facility design, and the cost of the amount of chemical

necessary to produce the desired result. Final selection of the coagulant (or coagulants) should be

made following thorough jar testing and plant scale evaluation.

Common coagulant chemicals used are alum, ferric sulfate, ferric chloride, ferrous sulfate, and

sodium aluminate. The first four will lower the alkalinity and pH of the solution while the

sodium aluminate will add alkalinity and raise the pH. The reactions of each follow:

ALUM

A12(SO4)3 + 3 Ca(HCO3)2 ------------> 2 Al(OH)3 + 3CaSO4 + 6 CO2

FERRIC SULFATE

Fe2(SO4)3 + 3 Ca(HCO3)2 ------------> 2 Fe(OH)3 + 3CaSO4 + 6 CO2

FERRIC CHLORIDE

2 Fe Cl3 + 3 Ca(HCO3)2 ------------> 2 Fe(OH)3 + 3CaCl2 + 6CO2

FERROUS SULFATE

FeS04 + Ca(HCO3)2 ------------> Fe(OH)2 + CaS04 + 2CO2

SODIUM ALUMINATE

2 Na2A12O4 + Ca(HCO3)2 ------------> 8 Al(OH)3 + 3 Na2CO3 + 6 H20

Na2Al2O4 + CO2 ------------> 2 Al(OH)3 + NaCO3

Na2Al2O4 + MgCo3 ------------> MgAl2O4 + Na2CO3

Rapid Mixing

In the rapid mixing, coagulant chemicals are added to the water and mixed quickly and violently.

This to ensure the chemicals is evenly distribute through the water. It usually last a minute or

less. If less than 30 seconds, the chemicals will not be properly mixed into the water. If more

than 1 minutes, the mixer blades will turn the flocs into small particles.

We use static mixer as it consists basically of a sequence of stationary guide plates which results

in the systematic, radial mixing of media flowing through the pipe. The formation of fine gas

bubbles in a water/gas mixture promotes intensive contact between the two phases. The results is

high mass transfer, for instance a high oxygen transfer rate or an excellent ozone utilization

factor.

In contrast to stirred tanks or empty pipe systems, the static mixers ensure that the complete fluid

stream is subjected to compulsory or enforced mixing or contacting.

Flocculation

Flocculation is a process which clarifies the water. Clarifying means removing any turbidity or colour so that the water is clear and colourless. Clarification is done by causing a precipitate to form in the water which can be removed using simple physical methods. Initially the precipitate forms as very small particles but as the water is gently stirred, these particles stick together to form bigger particles - this process is sometimes called flocculation. Many of the small particles that were originally present in the raw water adsorb onto the surface of these small precipitate particles and so get incorporated into the larger particles that coagulation produces. In this way the coagulated precipitate takes most of the suspended matter out of the water and is then filtered off, generally by passing the mixture through a coarse sand filter or sometimes through a mixture of sand and granulated anthracite (high carbon and low volatiles coal). Coagulants / flocculating agents that may be used include:

Iron (III) hydroxide.

This is formed by adding a solution of an iron (III) compound such as iron(III) chloride to pre-treated water with a pH of 7 or greater. Iron (III) hydroxide is extremely insoluble and forms even at a pH as low as 7. Commercial formulations of iron salts were traditionally marketed in the UK under the name Cuprus.

Aluminium hydroxide

widely used as the flocculating precipitate although there have been concerns about possible health impacts and mis-handling led to a severe poisoning incident in 1988 at Camelford in south-west UK when the coagulant was introduced directly into the holding reservoir of final treated water.

PolyDADMAC

an artificially produced polymer and is one of a class of synthetic polymers that are now widely used. These polymers have a high molecular weight and form very stable and readily removed flocs, but tend to be more expensive in use compared to inorganic materials. The materials can also be biodegradable.

Sedimentation

Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity.   The process is also known as settling or clarification.  Most water treatment plants include sedimentation in their treatment processes.  However, sedimentation may not be necessary in low turbidity water of less than 10 NTU.  In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters. 

 The most common form of sedimentation follows coagulation and flocculation and precedes filtration.  This type of sedimentation requires chemical addition (in the coagulation/flocculation step) and removes the resulting floc from the water.  Sedimentation at this stage in the treatment process should remove 90% of the suspended particles from the water, including bacteria.  The purpose of sedimentation here is to decrease the concentration of suspended particles in the water, reducing the load on the filters.  

Sedimentation can also occur as part of the pretreatment process, where it is known as presedimentation.  Presedimentation can also be called plain sedimentation because the process depends merely on gravity and includes no coagulation and flocculation.  Without coagulation/flocculation, plain sedimentation can remove only coarse suspended matter (such as grit) which will settle rapidly out of the water without the addition of chemicals.  This type of sedimentation typically takes place in a reservoir, grit basin, debris dam, or sand trap at the beginning of the water treatment process.

While sedimentation following coagulation/flocculation is meant to remove most of the suspended particles in the water before the water reaches the filters, presedimentation removes most of the sediment in the water during the pretreatment stage.  So presedimentation will reduce the load on the coagulation/flocculation basin and on the sedimentation chamber, as well as reducing the volume of coagulant chemicals required to treat the water.  In addition, presedimentation basins are useful because raw water entering the plant from a reservoir is usually more uniform in quality than water entering the plant without such a holding basin.  

The rest of this lesson will be concerned with sedimentation following coagulation and flocculation.  We will consider types of sedimentation basins and parts of a typical sedimentation basin, as well as the disposal of sludge.  Then, in the next lesson, we will learn to design a sedimentation basin and will consider some problems which may affect sedimentation basins.

Types of Basins

Three common types of sedimentation basins are shown below:

Rectangular basins are the simplest design, allowing water to flow horizontally through a long tank.  This type of basin is usually found in large-scale water treatment plants.  Rectangular basins have a variety of advantages - predictability, cost-effectiveness, and low maintenance.  In addition, rectangular basins are the least likely to short-circuit, especially if the length is at least twice the width.  A disadvantage of rectangular basins is the large amount of land area required.Double-deck rectangular basins are essentially two rectangular sedimentation basins stacked one atop the other.  This type of basin conserves land area, but has higher operation and maintenance costs than a one-level rectangular basin.

 

Square or circular sedimentation basins with horizontal flow are often known as clarifiers.  This type of basin is likely to have short-circuiting problems.

FiltrationFiltration is now required for most water treatment systems.  Filters must reduce turbidity

to less than 0.5 NTU in 95% of each month's measurements and the finished water turbidity must never exceed 5 NTU in any sample.  

Although turbidity is not harmful on its own, turbid water is difficult to disinfect for a variety of reasons.  Microorganisms growing on the suspended particles may be hard to kill using disinfection while the particles themselves may chemically react with chlorine, making it difficult to maintain a chlorine residual in the distribution system.  Turbidity can also cause deposits in the distribution system that create tastes, odors, and bacterial growths.  

Location in the Treatment Process

In the typical treatment process, filtration follows sedimentation (if present) and precedes disinfection.  Depending on the presence of flocculation and sedimentation, treatment processes are divided into three groups - conventional filtration, direct filtration, and in-line filtration.

The most common method of filtration is conventional filtration, where filtration follows coagulation/flocculation and sedimentation.  This type of filtration results in flexible and reliable performance, especially when treating variable or very turbid source water.

Some treatment plants operate without some or all of the sediment removal processes which precede filtration.  If filtration follows coagulation and flocculation, without sedimentation, it is known as direct filtration.  This method can be used when raw water has low turbidity.

Another type of filtration, known as in-line filtration, involves operating the filters without flocculation or sedimentation.  A coagulant chemical is added to the water just before filtration and coagulation occurs in the filter.  In-line filtration is often used with pressure filters, but is not as efficient with variable turbidity and bacteria levels as conventional filtration is. 

Mechanisms of Filtration

Straining

Passing the water through a filter in which the pores are smaller than the particles to be removed.  This is the most intuitive mechanism of filtration, and one which you probably use in your daily life.  Straining occurs when you remove spaghetti from water by pouring the water and spaghetti into a strainer.  

Adsorption

The second, and in many cases the most important mechanism of filtration, is adsorption.  Adsorption is the gathering of gas, liquid, or dissolved solids onto the surface of another material. Coagulation takes advantage of the mechanism of adsorption when small floc particles are pulled together by van der Waal's forces.  In filtration, adsorption involves particles becoming attracted to and "sticking" to the sand particles.  Adsorption can remove even very small particles from water.

Biological Action

The third mechanism of filtration is biological action, which involves any sort of breakdown of the particles in water by biological processes. This may involve decomposition of organic particles by algae, plankton, diatoms, and bacteria or it may involve microorganisms eating each other. Although biological action is an important part of filtration in slow sand filters, in most other filters the water passes through the filter too quickly for much biological action to occur.

Absorption

The final mechanism of filtration is absorption, the soaking up of one substance into the body of another substance. Absorption should be a very familiar concept - sponges absorb water, as do towels. After the initial wetting of the sand, absorption is not very important in the filtration process

Type of filtration

Slow sand filter

Slow sand filter are used in water purification for treating raw water to produce a potable product. They are typically 1 to 2 metres deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water. The length and breadth of the tanks are determined by the flow rate desired by the filters, which typically have a loading rate of 0.1 to 0.2 metres per hour (or cubic metres per square metre per hour).

Slow sand filters differ from all other filters used to treat drinking water in that they work by using a complex biological film that grows naturally on the surface of the sand. The sand itself does not perform any filtration function but simply acts as a substrate, unlike its counterparts for UV and pressurized treatments.

Rapid Sand Filter

The rapid sand filter or rapid gravity filter is a type of filter used in water purification and is commonly used in municipal drinking water facilities as part of a multiple-stage treatment system. The first modern rapid sand filtration plant was designed and built by George W. Fuller in Little Falls, New Jersey. Fuller's filtration plant went into operation in 1920 and its success was responsible for the change to this technology in the U.S. Rapid sand filters were widely used in large municipal water systems by the 1920s, because they required smaller land areas compared to slow sand filters.

Rapid sand filters use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals—typically alum. The unfiltered water flows through the filter medium under gravity or under pumped pressure and the floc material is trapped in the sand matrix.

Mixing, flocculation and sedimentation processes are typical treatment stages that precede filtration. Chemical additives, such as coagulants, are often used in conjunction with the filtration system.

DisinfectionDisinfection is usually the final stage in the water treatment process in order to limit the

effects of organic material, suspended solids and other contaminants. Like the disinfection of wastewater, the primary methods used for the disinfection of water in very small (25-500 people) and small (501-3,300 people) treatment systems are ozone, ultraviolet irradiation (UV) and chlorine. There are numerous alternative disinfection processes that have been less widely used in small and very small water treatment systems, including chlorine dioxide, potassium permanganate, chloramines and peroxone (ozone/hydrogen peroxide).

Chlorination

Chlorine Demand

When chlorine enters water, it immediately begins to react with compounds found in the water. The chlorine will react with organic compounds and form trihalomethanes. It will also react with reducing agents such as hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions.

Let's consider one example, in which chlorine reacts with hydrogen sulfide in water. Two different reactions can occur:

Hydrogen Sulfide + Chlorine + Oxygen Ion Elemental Sulfur + Water + Chloride Ions

H2S + Cl2 + O2- S + H2O + 2Cl-

Hydrogen Sulfide + Chlorine + Water Sulfuric Acid + Hydrochloric Acid

H2S + 4Cl2 + 4 H2O H2SO4 + 8 HCl

I have written each reaction using both the chemical formula and the English name of each compound. In the first reaction, hydrogen sulfide reacts with chlorine and oxygen to create elemental sulfur, water, and chloride ions. The elemental sulfur precipitates out of the water and can cause odor problems. In the second reaction, hydrogen sulfide reactions with chlorine and water to create sulfuric acid and hydrochloric acid.

Each of these reactions uses up the chlorine in the water, producing chloride ions or hydrochloric acid which have no disinfecting properties. The total amount of chlorine which is used up in reactions with compounds in the water is known as the chlorine demand. A sufficient quantity of chlorine must be added to the water so that, after the chlorine demand is met, there is still some chlorine left to kill microorganisms in the water.

FlouridationFluoridation is the process of adjusting the concentration of fluoride in public water

supplies for the prevention of dental decay. Fluoride has been added to drinking water in the United States since about 1945 and it has been estimated that every dollar spent on fluoridation has saved $50 in dentists' bills.

Fluoride in water has been proven to prevent tooth decay among children and to prevent root tip rot. The chemical acts by strengthening the tooth enamel and by making the enamel more resistant to decay. This is a long-term process, with results usually being noticeable only after about 4 to 6 years.

Concentration of Floridation

In fluoridation, we also set an optimal fluoride concentration, which is about 1 ppm in drinking water. However, fluoridation has a different goal from chlorination and from other instances of chemical addition in water treatment. In chlorination, the chlorine must react with substances in the water, so the optimal chlorine concentration depends primarily on water characteristics. Fluoride, in contrast, is not meant to react with substances in water. Instead, the goal when adding fluoride to water is to control the amount of fluoride which each customer will ingest per day. You can think of fluoride as being similar to a vitamin or mineral for which each person has a recommended daily allowance.

Dosage

The amount of fluoride to be fed into water is influenced by several factors. The climate of the region will determine the optimal concentration in the water, as discussed above. But dosage will also be influenced by the amount of fluoride already existing in the raw water. For example, if raw water contains 0.3 ppm fluoride and the recommended concentration is 0.9 ppm, then it will only be necessary to add 0.6 ppm of fluoride to the water being treated.

Dosage also depends on the type of chemical used to fluoridate the water. Several chemicals can be used to supply fluoride to water, and each chemical has a different fluoride concentration

Chemicals

Hydrofluosilicic acid Sodium silicofluoride Sodium fluoride

Water distribution Water distribution objective is to supply potable water, at sufficient quality, pressure and quantity, to the consumers.

Requirement for a good water distribution system:

Sufficient capacity of water – for domestic, industrial and other uses.

Sufficient pressure

Low cost – provide pipe network (about 55 – 70% of total cost of water supply scheme)

Easier to maintain and economical

Stable condition

Maintain the degree of water purity – complete water-tight

Emergency period – able to supply sufficient amount of water

Method of Distribution System

Gravitional

Reliable only when source water level > service area

Force of Gravity

Most Reliable

Limit use

Pressure at customer end is low

Direct Pumped System

-Level of source similar or lower than service area

-Pumped direct to consumers

-Dependent on Mechanical & Electrical Power

-Expensive & less effective

Combination of Gravity & Pumped System

Gravity & pumping utilized simultaneously

Water Flow by Gravity to service area

System is fairly reliable

More economical

EAT 237 WATER SUPPLY ENGINEERING

ASSIGNMENT

WATER TREATMENT PROCESS AND FUNCTION

NAME:MUHAMMAD ZAEEM BIN ZAHARIN

MATRIC NO:131201493

PROGRAM:CIVIL ENGINEERING

DATE OF SUBMISSION:29TH MAY 2015