Design of Filtration Tank~FINAL FINAL FINAL

Demand Study and Design of Filtration Tank 2010

Contents

Chapter 1: Demand Analysis of Sophia Settlement

1.1 BACKGROUND.................................................................................................................................6

1.2 SCOPE OF WORKS............................................................................................................................7

1.3 LIMITATIONS....................................................................................................................................8

1.4 METHODOLOGY OF DEMAND ANALYSIS..........................................................................................8

1.5 CONSIDERATIONS............................................................................................................................9

1.6 LOCATION OF SOPHIA......................................................................................................................9

1.7 DATA COLLECTED.............................................................................................................................9

1.8 ANALYSIS OF DATA........................................................................................................................10

1.81 Demand Categories.......................................................................................................................10

1.82 Demand Growth over time............................................................................................................11

1.9 CONCLUSION.................................................................................................................................12

1.10 APPENDICES...................................................................................................................................13

2.1 ABSTRACT......................................................................................................................................17

2.2 AIM................................................................................................................................................17

2.3 EXECUTIVE SUMMARY...................................................................................................................17

2.4 INTRODUCTION.............................................................................................................................18

2.5 LITERATURE REVIEW......................................................................................................................19

2.6 LIMITATIONS..................................................................................................................................20

2.7 METHODOLOGY.............................................................................................................................20

2.71 Desk Study.............................................................................................................................20

2.72 Sophia Water Treatment Plant site visit................................................................................21

2.73 The design of the filtration tank.............................................................................................21

2.74 Building of a model of the proposed filtration tank...............................................................21

2.65 Influent and effluent testing..................................................................................................21

2.8 DESIGN...........................................................................................................................................22

2.81 Design Objective............................................................................................................................22

2.82 Design Constraints.........................................................................................................................22

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2.83 Design Functions............................................................................................................................23

2.84 Design Specifications.....................................................................................................................23

2.85 Design Solutions............................................................................................................................24

i. Ion Exchange..................................................................................................................................24

ii. Carbon Adsorption.........................................................................................................................25

iii. Micro-porous Basic Filtration.........................................................................................................27

iv. Ultrafiltration.................................................................................................................................28

v. Reverse Osmosis............................................................................................................................29

vi. Rapid Sand Filter............................................................................................................................32

vii. Slow Sand Filter.........................................................................................................................32

Comparison of the various filtration processes.........................................................................................34

2.86 Selection of Design Solution..........................................................................................................35

2.87 Description of Selected Solution....................................................................................................38

2.88 Actual Design.................................................................................................................................45

2.89 MODEL OF THE RAPID SAND FILTER SYSTEM.................................................................................58

2.90 TESTING OF WATER THROUGH THE SYSTEM.................................................................................58

i. Results...........................................................................................................................................59

ii. Discussion of Results......................................................................................................................59

2.91 APPENDICES...................................................................................................................................60

GLOSSARY..................................................................................................................................................67

REFERENCES..............................................................................................................................................68

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Table of Figures

Chapter One

Chart 1.1 Percentage water usage that is required by the categories of use

Chart 1.2 Graph showing Projected Demand increase for a period of 20 yrs

Map 1.1 Aerial Photograph of the Sophia Area

Map 1.2 Cadastral Plan of the Sophia Area

Table 1.1 Number of lots under the classified category

Table 1.2 Population under their category

Table 1.3 Consumption rate and demand

Table 1.4 Additional demand for the various factors that are considered

Chapter Two

Figure 2.1 Chemical Reaction in the Softening Method of Ion Exchange Process

Figure 2.2 Mechanism of the Carbon Absorption Process

Figure 2.3 Mechanism of the Micro-Porous Filtration Process

Figure 2.4 Mechanism of the Ultra Filtration Process

Figure 2.5 Mechanism of the Reverse Osmosis Process

Figure 2.6 Comparison of the Filtration Processes listed

Figure 2.7 Characteristics of Gravity Type Filters

Figure 2.8 Nozzle to be Used

Figure 2.9 Chosen Under-drain System

Figure 2.10 Illustration showing the arrangement of the Wash-water trough

Figure 2.11 Components of the Filtration Tank(Side View)

Figure 2.12 Arrangement of component parts of the Filtration Tank (Transverse View)

Figure 2.13 Cross-section of the Filtration Tank showing the components

Figure 2.14 Plan of Final Design

Figure 2.15 Elevation of Final Design

Figure 2.16 Section of Final Design

Figure 2.17 Improvised Apparatus Used for Testing

Table 2.1 Drinking Water Standards

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Table 2.2 Characteristics of Ion Exchange Process

Table 2.3 Characteristics of Carbon Absorption Process

Table 2.4 Characteristics of Micro-porous Filtration Process

Table 2.5 Characteristics of Ultra Filtration Process

Table 2.6 Characteristics of Reverse Osmosis ProcessTable 2.7 Characteristics of Slow Sand Filtration Process

Table 2.8 Properties of the sand for the Filter Medium from sieve analysis

Table 2.9 Results from Lab Tests

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Chapter 1

Demand AnalysisFor the Sophia Settlement

1.1 BACKGROUND

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Since May 30, 2002 the Guyana Water Incorporated (GWI) has been responsible for

providing a safe and dependable water supply to its customers throughout Guyana. Over

this period of time GWI has been gradually developing their potable water facilities to

meet the demands of the increasing population. In several regions across Guyana, they

have installed wells and treatment plants to enhance their water quality production.

Though GWI effort to provide the population with quality potable water, has been

growing, customers in the Sophia settlement are still to be provided with a dependable

water supply. There are residents in Sophia that does not have water connection,

therefore they are forced to break into the distribution lines, causing damages to the

pipe systems and wastage in the water supply.

Since the establishment of Sophia in the early 1990s, the population has increased,

which has resulted in high water demands. Sophia is divided into five sections that are

classified as A, B, C, D, E and F field, with E and F Fields being the most recent

addition. The settlement comprises predominantly of domestic dwellings and in order to

meet future demands, it is first necessary to predict what those demands will be over a

selected planning horizon.

To achieve a predicted demand of potable water for Sophia, a demand analysis will be

carried out. This evaluation is intended to cover all current and potential categories for

the use of potable water. The evaluation also review all available data for Sophia, which

will include the consensus, maps, rates of consumption and any other information that

may be beneficial to the analysis.

1.2 SCOPE OF WORKSThe scope of this analysis includes.

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1) Reviewing the population and housing consensus.

2) Categorizing the use of potable water under the following;

Domestic

Commercial

Industrial

Community type structure

3) Determining the projected water demand for a 20 year planning horizon,

considering various factors such as losses, emergencies, storage and development.

The scope of the analysis will cover the essential criteria’s needed to determine an

accurate population demand. In order to asses all impacts of the project, the planning

period should be at least as long as the economic life of the facilities. The U.S Internal

Revenue Service publishes estimates of the economic life of buildings, equipment etc.

Buildings have economic lives on the order of 20 years. Based on these estimates the

planning period of 20 years is established.

The population and housing consensus from the Bureau of Statistics will be used to

established the number of residents that are currently in need of potable water supply.

The predicted increase in demand from the established population, to a predicted

population increase over the 20 year period, will be determined by an exponential

growth rate. This relationship will help to better manage the water supply and increase

the plant capacity as the demand increases.

The population will then be divided into categories of usage, with each category having

a consumption rate that is used by GWI. It is these rates that are used in the demand

analysis.

1.3 LIMITATIONSLimitations for this aspect of the study were minimal.

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1.4 METHODOLOGY OF DEMAND ANALYSIS

The method of projecting the demands will follow a defined sequence of steps. These

steps are outlined as follows.

1) Determining the number of lots and the number of residents per house hold.

2) Categorizing the lots under the various usages.

3) Establishing the consumption rates per each category.

4) Determining the present population demand.

5) Adjusting the present demand by accounting for losses, storage, emergencies,

agriculture and development.

6) Determining the predicted demand over the 20 year period based on an

exponential growth rate.

7) Establishing the demand that is required for the study area.

1.5 CONSIDERATIONSIn calculating the demand for the Sophia area, the following were taken into considerations:

Development of the community not to be instantaneous.

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Increase in demand proportional to development.

1.6 LOCATION OF SOPHIAMap of Sophia

The map of Sophia is attached to this demand study. It highlights the area covered by this

analysis.

Sophia is located approximately one and half miles east of central Georgetown with UTM

(Universal Traverse Mercator) coordinates 409336E, 726248N.

See map in Appendices.

1.7 DATA COLLECTEDThe following tables show the data that were collected from the various authorities. They

are represented in the following order.

Table 1.1 shows the number of lots under the classified category.

Table 1.2 shows the population under their category.

Table 1.3 shows the consumption rate and demand.

Table 1. 4 shows the additional demand for the various factors that were considered.

1.8 ANALYSIS OF DATA

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1.81 Demand Categories

95%

1%

2% 2%

DemandResidential Commercial Public Services Educational Institutions

Chart 1.1: showing the percentage water that is required by the categories of use.

1.82 Demand Growth over time

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0.24 1.2 6 301100000

1300000

1500000

1700000

1900000

2100000

2300000

Year

Pro

ject

ed

Dem

an

d (

Gal/

year)

Chart 1.2: Graph showing Projected Demand increase for a period of 20 yrs.

Q t = Qo ert

WhereQt = demand at time (t)Q0 = initial de-mandt = timer = constant (0.01733)

The graph above shows the exponential growth rate of the demand of the 20 year

planning horizon.

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1.9 CONCLUSIONBased on all the calculations and assumptions made, the estimated demand for sections A

to F of the Sophia Settlement, Greater Georgetown is 228488 gal/day and can be

approximated to 2.3 million gal/day.

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1.10 APPENDICES

Maps of Sophia

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Map 1: Aerial Photograph of the Sophia Area

(Compiled By: Vickram Manoo & Donald Britton)14 Group 3

F Field

C Field

B Field

A Field

D Field

E Field


Map 2: Cadastral Plan of the Sophia Area

(Provided By: GWI)

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Chapter 2

Design of Filtration Tank for a Water Treatment Facility

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2.1 ABSTRACTThe filtration process is deemed the second most important stage in the treatment of water.

Moreover, the major type of filter used in Guyana to treat water is the rapid sand filter due to

the economical nature. This research seeks to assess the efficiency of the present configuration

of the rapid sand filter used at the Sophia Water Treatment Plant and proposes a more efficient

configuration. In doing so the dimensions, inflow, outflow and the quality of water and the

composition of the present filter tank were assessed. A model of the proposed filter tank was

also built.

2.2 AIMThe basic aims of this report are:

To determine the filtration rate of the rapid sand filter at the Sophia treatment plant; and,

To design a filter with a sufficient rate of filtration water to achieve the projected demand for the Sophia community.

2.3 EXECUTIVE SUMMARYFiltration is a physical liquid-solid separation process used to removed colloidal particles (0.001

-1 µ) and if present, larger particles, by gravitational or pressure force through a porous

medium. A rapid sand filter was designed to meet the projected demand of 2.3 mgd (million

gallons per day) of the Sophia community, Greater Georgetown. Sand particles with effective

size 0.5 mm and coefficient of uniformity 1.6 were used in the model development and

construction of the filter. The dimensions of the filter (actual and model) were calculated based

on similar filters used to supply similar demands. A test procedure of this filter yielded a flow

rate for filtration which was computed to be 5.33mm/s. By laboratory testing, this filtration rate

is sufficient to supply water of the required quality and rate to the Sophia community.

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2.4 INTRODUCTIONWater filtration is a physical process for separating suspended and colloidal impurities from

water by passage through a porous medium, usually a bed of sand or other granular material.

Water fills the pores of the medium, and the impurities are left behind in the openings or upon

the medium itself. Filtration is an important and active process in the natural purification of the

underground waters, and it is an essential unit process utilized under controlled conditions in

water treatment plants throughout the world.

A number of mechanisms are involved in particle removal by filtration. Some of these

mechanisms are physical and others are chemical in nature. The effects of both the physical and

chemical actions occurring in a filter bed of granular substances must be combined to explain

fully the overall removal of impurities obtained.

Normally, there are two applicable types of filtration processes: slow sand filtration and rapid

sand filtration. However, for the purpose of this project only rapid sand filtration will be

discussed. The pre-treatment filtration removal mechanisms for rapid sand filtration include, in

the order of importance: aeration, coagulation, flocculation and sedimentation.

In this project, you will be exposed to the design of a filtration system to meet the current

demand that exist in the study area, followed by a model to demonstrate what will occur

should a prototype be built.

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2.5 LITERATURE REVIEWThe earliest recorded attempts to find or generate pure water date back to as early as 2000 B.C.

Early Sanskrit writings outlined methods for purifying water.

These methods identified that filtering water through crude sand or charcoal filters (Baker &

Taras, 1981) was the accepted technique to produce quality drinking water. These writings

suggest that the major motive in purifying water was to provide better tasting drinking water. It

was assumed that good tasting water was also clean.

The first record of experimentation in water filtration, after the blight of the Dark Ages, came

from Sir Francis Bacon in 1627 (Baker & Taras, 1981). Hearing rumours that the salty water of

the ocean could be purified and cleansed for drinking water purposes, he began experimenting

in the desalination of seawater using simple filtration techniques.

The first water treatment plant was erected in 1804 at Paisley, Scotland (Baker & Taras, 1981).

This plant provided filtered water to every household within the city limits. The Scottish water

treatment plant depended upon slow sand filters designed by Robert Thom, an important

scientist of the Scottish Enlightenment. However, due to increasing demands scientists in the

United States designed a rapid sand filter in the late 19th century (Baker & Taras, 1981). The

rapid sand filter was cleaned by powerful jet streams of water, greatly increasing the efficiency

and capacity of the water filter. It was therefore capable of supplying large demands based on

modifications of its dimensions (height, width, thickness of sand layer, etc.).

Therefore, filters can generally be classified hydraulically as rapid or slow filter depending upon

the rate of flow per unit surface area. Essentially slow filters operate at rates 1 to 10 mgd per

acre, and rapid filters at rates 1 to as much as 8 gpm per square foot. Filters may also be

classified based on the filter media used, such as sand, coal, multi-layered filter, etc.

It is evident that with increasing population, the need for larger quantities of potable water

supply will increase. The rapid sand filtration technique is therefore employed in most water

treatment plants in the developed and the developing countries largely due to its superior rate

of filtration and consequent discharge as compared to the slow sand filtration method.

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2.6 LIMITATIONS

The limitations encountered during the design of the filtration tank were:

i. Filter Medium: The water treatment plant at Sophia imports a special kind of black

sand with a larger effective size than that available in Guyana. Consequently, sand

was sampled from different locations and the minimum standard for the effective

size was chosen.

ii. Testing: In the testing of the flow rate or velocity of water passing through the sand

medium the following limited the results:

Nozzles for the under drain were unavailable for use in the testing,

A constant head could not be maintained during the exercise.

2.7 METHODOLOGYThe research done was carried out in the following format:

i. Desk top study

ii. Sophia Water Treatment Plant site visit

iii. The design of the filtration tank

iv. Building of a model of the proposed filtration tank

v. Testing

2.71 Desk Study

During the desk study of the research, literatures upon the design of filtration tanks were

reviewed. The primary source of the former was taken from the text Standard Handbook of

Environmental Engineering (Second Edition) by Robert A. Corbitt. Other pieces of literatures

and sources of information which were used to obtain the necessary information were the text

Mechanics of Fluids (Eight Edition) by Bernard Massey and handouts obtained from the Guyana

Water Authority (GWI) as well as the vast internet sources.

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2.72 Sophia Water Treatment Plant site visit

A visit was conducted at the Guyana Water Authority (GWI) in order to observe the operation

of the water treatment process particularly the filtration process. Information such as the

water demand of the community, the dimensions (length, width and height) of the filter tank

and filter bed used and the inflow and outflow velocities of water into and out of the tank

respectively were obtained during the visit. Water samples from test valves before and after

the filtration process were also collected and the turbidity measured to make a comparison

between the presently used filtration tank and the proposed filtration tank which was being

designed by the researchers.

2.73 The design of the filtration tank

In order to design the filtration tank, certain parameters had to be known. The effective size of

the sand, coefficient of uniformity and demand all had to be determined before the actual

design could have been done. Prior to determining the effective size of the sand, a laboratory

sieve analysis was done to determine the D10 which is the 10 % pass rate on a semi-log graph.

2.74 Building of a model of the proposed filtration tank

The model was established based on dynamic similarity between what was designed and what

was expected to happen should the structure be built. Materials used were perspex (for the

body of the tank), reef sand (for the filter bed) and polyvinyl chloride (PVC) pipes and fittings.

2.65 Influent and effluent testing

Raw water was collected from the Sophia treatment plant, harvested from its supply well. The

water was introduced into a specially made influent-effluent tester which was made of 1 ½”

diameter of 4’ 6”PVC pipe. In the pipe, there was 30” of reef sand and from the base of the

tester there were 8” of 1/4” diameter holes drilled to allow the effluents to pass through since

the bottom of the tester was blocked (see appendices for illustration). From this exercise, the

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actual effluent discharge was calculated. Samples were taken before and after the passage of

water through the tester. These samples were then taken to the Guyana Geology and Mines

Commission (GGMC) to determine the turbidity and pH.

2.8 DESIGN

2.81 Design Objective To design a filtration tank that will satisfy the demand estimated for the Sophia Area.

2.82 Design Constraints Several types of constraints were encountered during this design namely:

Cost- The design should be economical in terms of:

o Materials – all materials (components) must be easily sourced and filter

media should be locally sourced.

o Maintenance – the system must be easy and inexpensive to maintain.

Manufacturing – The system must allow ease of construction.

Safety – The system must be accident free.

Legal – There must be accommodations for disposal of waste.

Functional:

o Must be energy efficient compared to other systems.

o Must not occupy very large area.

o Materials used must have structural integrity.

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2.83 Design Functions

To provide a quality filtration process.

To remove matter such as silt, clay, colloids, micro-organisms like algae, bacteria and

viruses held in suspension.

To filter water at a rapid rate in order to meet demand.

2.84 Design Specifications

The following criterion must be satisfied in the design for the filtration tank:

Demand: From the demand study discussed in chapter one, the filtration tank

must have the capacity to handle 2.3 million gallons per day (MGPD)

Material: All materials used should be of the ASTM standards and in acceptance

with the World Health Organisation (WHO).

Water Quality: For the filtration process the following table explains the

parameters taken into consideration.

Parameters Standard

Physical

Characteristics

Turbidity 5 NTU

Colour Clear

Taste and odour None

pH 6-8

Table 2.1: Showing the Drinking Water Standards

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2.85 Design Solutions

The selection of type of filtration process to be used is generally a function of the raw water

quality. As filtration implies, water flows through a material that removes particles, organisms,

and/or contaminants. This flow is controlled by the force of gravity or the force of pressure.

Moreover, design solutions or options for the filtration process are examined as follows.

i. Ion Exchange

The ion exchange process percolates water through bead-like spherical resin materials (ion-

exchange resins). The principle behind this process is that the ions in the water are exchanged

for other ions fixed to the beads. The two most common ion-exchange methods are softening

and deionization.

Softening is used primarily as a pre-treatment method to reduce water hardness prior to

reverse osmosis processing. The softeners contain beads that exchange two sodium ions for

every calcium or magnesium ion removed from the "softened" water.

Figure 2.1: Chemical Reaction in the Softening Method of Ion Exchange Process(Source: www.allaboutwater.com/filtration)

Deionization beads exchange either hydrogen ions for cations or hydroxyl ions for anions. The

cation exchange resins, made of styrene and divinylbenzene containing sulfonic acid groups,

will exchange a hydrogen ion for any cations they encounter (e.g., Na+, Ca++, Al+++). Similarly,

the anion exchange resins, made of styrene and containing quaternary ammonium groups, will

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http://www.allaboutwater.com/filtration


exchange a hydroxyl ion for any anions (e.g., Cl-). The hydrogen ion from the cation exchanger

unites with the hydroxyl ion of the anion exchanger to form pure water.

Deionization can be an important component of a total water purification system when used in

combination with other methods discussed in this primer such as reverse osmosis, filtration and

carbon adsorption. Deionization systems effectively remove ions, but they do not effectively

remove most organics or microorganisms. Microorganisms can attach to the resins, providing a

culture media for rapid bacterial growth and subsequent pyrogen generation.

The advantages and disadvantages of this technology are summarized below.

Advantages

Removes dissolved inorganics effectively.

Regenerable (service deionization).

Relatively inexpensive initial capital investment.

Disadvantages

Does not effectively remove particles, pyrogens

or bacteria.

DI beds can generate resin particles and culture

bacteria.

High operating costs over long-term.

Table 2.2: Showing the Characteristics of Ion Exchange Process

ii. Carbon AdsorptionCarbon absorption is a widely used method of home water filter treatment because of its ability

to improve water by removing disagreeable tastes and odours, including objectionable chlorine.

Activated carbon effectively removes many chemicals and gases, and in some cases it can be

effective against microorganisms. However, generally it will not affect total dissolved solids,

hardness, or heavy metals. Only a few carbon filter systems have been certified for the removal

of lead, asbestos, cysts, and coliform. There are two types of carbon filter systems: granular

activated carbon, and solid block carbon. For more effective water purification, these two

methods can be employed along with a reverse osmosis system.

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Activated carbon is created from a variety of carbon-based materials in a high-temperature

process that creates a matrix of millions of microscopic pores and crevices. One pound of

activated carbon provides from 60 to 150 acres of surface area. The pores trap microscopic

particles and large organic molecules, while the activated surface areas cling to, or adsorb,

small organic molecules.

The ability of an activated carbon filter to remove certain microorganisms and certain organic

chemicals, especially pesticides, chlorine by-products and trichloroethylene, depends upon

several factors, such as the type of carbon and the amount used, the design of the filter and the

rate of water flow, how long the filter has been in use, and the types of impurities the filter has

previously removed.

Figure 2.2: Mechanism of the Carbon Absorption Process(Source: www.allaboutwater.com/filtration )

The carbon adsorption process is controlled by the diameter of the pores in the carbon filter

and by the diffusion rate of organic molecules through the pores. The rate of adsorption is a

function of the molecular weight and the molecular size of the organics. Certain granular

carbons effectively remove chloramines. Carbon also removes free chlorine and protects other

purification media in the system that may be sensitive to an oxidant such as chlorine.

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Carbon is usually used in combination with other treatment processes. The placement of

carbon in relation to other components is an important consideration in the design of a water

purification system.

The advantages and disadvantages of the system is show below:

Advantages

Removes dissolved organics and chlorine

effectively.

Long life (high capacity).

Disadvantages

Can generate carbon fines.

Table 2.3: Showing the Characteristics of Carbon Absorption Process

iii. Micro-porous Basic Filtration

There are three types of micro-porous filtration: depth, screen and surface. Depth filters are

matted fibres or materials compressed to form a matrix that retains particles by random

adsorption or entrapment. On the other hand, screen filters are inherently uniform structures

which, like a sieve, retain all particles larger than the precisely controlled pore size on their

surface. While surface filters are made from multiple layers of media. When fluid passes

through the filter, particles larger than the spaces within the filter matrix are retained,

accumulating primarily on the surface of the filter.

Figure 2.3: Mechanism of the Micro-Porous Filtration Process

(Source: www.allaboutwater.com/filtration)

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The distinction between filters is important because the three methods serve very different

functions. Depth filters are usually used as prefilters because they are an economical way to

remove 98% of suspended solids and protect elements downstream from fouling or clogging.

Surface filters are used to remove 99.99% of suspended solids and may be used as either

prefilters or clarifying filters. Micro-porous membrane (screen) filters are placed at the last

possible point in a system to remove the last remaining traces of resin fragments, carbon fines,

colloidal particles and microorganisms.

The advantages and disadvantages of the system is show below:

Advantages

Absolute filters remove all particles and

microorganisms greater than the pore size.

Requires minimal maintenance.

Disadvantages

Will not remove dissolved inorganics,

chemicals, pyrogens or all colloidals.

Potentially high expendable costs.

Not regenerable.

Table 2.4: Showing the Characteristics of Micro-porous Filtration Process

iv. Ultrafiltration

While a microporous membrane filter removes particles according to pore size; an

ultrafiltration membrane functions as a molecular sieve. It separates dissolved molecules on the

basis of size by passing a solution through an infinitesimally fine filter.

The ultra filter is a tough, thin, selectively permeable membrane that retains most

macromolecules above a certain size including colloids, microorganisms and pyrogens. Smaller

molecules, such as solvents and ionized contaminants, are allowed to pass into the filtrate.

Thus, ultra filter provides a retained fraction (retentate) that is rich in large molecules and a

filtrate that contains few, if any, of these molecules.

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Figure 2.4: Mechanism of the Ultra Filtration Process

(Source: www.allaboutwater.com/filtration)

Ultrafilters have several advantages and disadvantages which are listed below:

Advantages

Effectively removes most particles, pyrogens,

microorganisms, and colloids above their rated

size.

Produces highest quality water for least

amount of energy.

Regenerable.

Disadvantages

Will not remove dissolved inorganics.

Table 2.5: Showing the Characteristics of Ultra Filtration Process

v. Reverse Osmosis

Reverse osmosis is the most economical method of removing 90% to 99% of all contaminants.

The pore structure of reverse osmosis membranes is much tighter than that of the

ultrafiltration membranes. Reverse osmosis membranes are capable of rejecting practically all

particles, bacteria and organics >300 daltons molecular weight (including pyrogens). In fact,

reverse osmosis technology is used by most leading water bottling plants.

Natural osmosis occurs when solutions with two different concentrations are separated by a

semi-permeable membrane. Osmotic pressure drives water through the membrane; the water

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dilutes the more concentrated solution; and the end result is equilibrium. However, water

purification systems utilise a hydraulic pressure which is applied to the concentrated solution to

counteract the osmotic pressure. Therefore, pure water is driven from the concentrated

solution and collected downstream of the membrane.

Since reverse osmosis membranes are very restrictive, they yield slow flow rates; storage tanks

are required to produce an adequate volume in a reasonable amount of time.

Reverse osmosis also involves an ionic exclusion process. Only solvent is allowed to pass

through the semi-permeable reverse osmosis membrane, while virtually all ions and dissolved

molecules are retained (including salts and sugars). The semi-permeable membrane rejects

salts (ions) by a charge phenomena action: the greater the charge, the greater the rejection.

Therefore, the membrane rejects nearly all (>99%) strongly ionized polyvalent ions but only

95% of the weakly ionized monovalent ions like sodium.

Reverse osmosis is highly effective in removing several impurities from water such as total

dissolved solids (TDS), turbidity, asbestos, lead and other toxic heavy metals, radium, and many

dissolved organics. The process will also remove chlorinated pesticides and most heavier-

weight VOCs. Reverse osmosis and activated carbon filtration are complementary processes.

Reverse osmosis is the most economical and efficient method for purifying tap water once the

system is properly designed for the feed water conditions and the intended use of the product

water. Reverse osmosis is also the optimum pre-treatment for reagent-grade water polishing

systems.

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Figure 2.5: Mechanism of the Reverse Osmosis Process (Source: www.allaboutwater.com/filtration)

The following are the pros and cons of the reverse osmosis process:

Advantages

Effectively removes all types of contaminants to

some extent (particles, pyrogens,

microorganisms, colloids and dissolved

inorganics).

Requires minimal maintenance.

Disadvantages

Flow rates are usually limited to a certain

gallons/day rating.

Table 2.6: Showing the Characteristics of Reverse Osmosis Process

vi. Rapid Sand FilterRapid sand filters use relatively coarse sand and other granular media to remove particles and

impurities that have been trapped in a floc (flocculated particles formed by chemicals typically

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http://en.wikipedia.org/wiki/Flocculation

http://en.wikipedia.org/wiki/Floc



salts of aluminium or iron). Water and flocs flow through the filter medium under the force of

gravity or under a pumped pressure where the floc 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. A disinfection system (typically using chlorine or ozone) is commonly used

following filtration. Rapid sand filtration has very little effect on taste and smell and dissolved

impurities of drinking water, unless activated carbon is included in the filter medium.

vii. Slow Sand FilterSlow sand filters 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.

Slow sand filters work through the formation of a gelatinous layer (or biofilm) called the

hypogeal layer in the top few millimetres of the fine sand layer. The hypogeal layer is formed in

the first 10-20 days of operation and consists of bacteria, fungi, protozoa, rotifera and a range

of aquatic insect larvae. As the hypogeal layer ages, more algae tend to develop and larger

aquatic organisms may be present including some bryozoa, snails and Annelid worms.

The hypogeal is the layer that provides the effective purification in potable water treatment,

the underlying sand providing the support medium for this biological treatment layer. As water

passes through the hypogeal layer, particles of foreign matter are trapped in the mucilaginous

matrix and dissolved organic material is adsorbed and metabolised by the bacteria, fungi and

protozoa. The water produced from a well-managed slow sand filter can be of exceptionally

good quality with 90-99% bacterial reduction.

Slow sand filters slowly lose their performance as the hypogeal layer grows and thereby

reduces the rate of flow through the filter. Eventually it is necessary to refurbish the filter. Two

methods are commonly used to do this. In the first, the top few millimetres of fine sand is

scraped off to expose a new layer of clean sand. Water is then decanted back into the filter and

32 Group 3

http://en.wikipedia.org/wiki/Adsorbed

http://en.wikipedia.org/wiki/Annelid

http://en.wikipedia.org/wiki/Mollusca

http://en.wikipedia.org/wiki/Bryozoa

http://en.wikipedia.org/wiki/Rotifera

http://en.wikipedia.org/wiki/Protozoa

http://en.wikipedia.org/wiki/Fungi

http://en.wikipedia.org/wiki/Bacterium

http://en.wikipedia.org/wiki/Hypogeal

http://en.wikipedia.org/wiki/Biofilm

http://en.wikipedia.org/wiki/Raw_water

http://en.wikipedia.org/wiki/Water_purification

http://en.wikipedia.org/wiki/Activated_carbon

http://en.wikipedia.org/wiki/Ozone

http://en.wikipedia.org/wiki/Chlorine

http://en.wikipedia.org/wiki/Disinfection

http://en.wikipedia.org/wiki/Sedimentation

http://en.wikipedia.org/wiki/Media_filter

http://en.wikipedia.org/wiki/Iron

http://en.wikipedia.org/wiki/Aluminium


re-circulated for a few hours to allow a new hypogeal layer to develop. The filter is then filled to

full depth and brought back into service. The second method, sometimes called wet harrowing,

involves lowering the water level to just above the hypogeal layer, stirring the sand and thereby

suspending any solids held in that layer and then running the water to waste. The filter is then

filled to full depth and brought back into service. Wet harrowing can allow the filter to be

brought back into service more quickly.

Advantages

As they require little or no mechanical

power, chemicals or replaceable parts, and

they require minimal operator training and

only periodic maintenance, they are often

an appropriate technology for poor and

isolated areas.

Slow sand filtration may be not only the

cheapest and simplest but also the most

efficient method of water treatment.

Disadvantages

Due to the low filtration rate, slow sand

filters require extensive land area for a large

municipal system.

Table 2.7: Showing the Characteristics of Slow Sand Filtration Process

Comparison of the various filtration processes

33 Group 3

http://en.wikipedia.org/wiki/Appropriate_technology


Figure 2.6: Showing a comparison of the Filtration Processes listed(Source: www.allaboutwater.com/filtration)

34 Group 3



2.86 Selection of Design Solution

Based on the design constraints mentioned, the design would be limited to the gravity type

sand filter systems. The reasons for eliminating the pressure and generic type systems are as

follows:

i. The availability of raw material as for the pressure type systems is limited,

ii. The lack of available information to establish the design criterion,

iii. Other systems, like reverse osmosis are very expensive to set up and require large

amounts of energy to function,

iv. High operating costs and expertise is needed for effective operation.

The justification for the choice of the gravity types are:

i. Filter media (sand) are readily available in Guyana,

ii. Construction is simple and relatively cheap,

iii. Easy and rapid maintenance.

Gravity Type Sand Filters Comparisons

Sand filtration can be either rapid or slow. The difference between the two is not a simple

matter of the speed of filtration, but in the underlying concept of the treatment process. Slow

sand filtration is essentially a biological process whereas rapid sand filtration is a physical

treatment process. The table that follows gives a general comparison of the slow and rapid

sand filters.

35 Group 3


Table 2.7: Showing the Characteristics of Gravity Type Filters

(Source: www.watertreatments.com/water-filters/rapid-sand-filters)

36 Group 3


Choice of Filter

Slow sand filters have an advantage over rapid sand filters in that they produce

microbiologically "clean" water which should not require disinfection to inactivate any bacteria,

although the addition of a disinfectant to provide a residual for the distribution system is still

advisable. However, because of their slow flow rate, slow sand filters require large tracts of

land if they are to supply large populations and can be relatively labour intensive to operate

and maintain.

The rapid sand filter differs from the slow sand filter in a variety of ways, the most important of

which are the much greater filtration rate and the ability to clean automatically using

backwashing. Rapid sand filtration is now commonly used worldwide and is far more popular

than slow sand filtration. The principal factor affecting the decision is the smaller land

requirement for rapid sand filters and lower labour costs. Conversely, rapid sand filters do not

produce water of the same quality as slow sand filters and a far greater reliance is placed on

disinfection to inactivate bacteria. However, once the proper pre-treatment processes are

implemented prior to the filtration, this filter system will be just as effective.

Therefore, rapid sand filter system is chosen on the basis that the filtration tank must be able to

supply the estimated demand of the Sophia.

37 Group 3


2.87 Description of Selected Solution

Rapid Sand Filter

Filtration by rapid sand filters, as the name suggest, is the separation of colloidal and other

particles from water by passage through a porous medium at rapid rates of approximately 2 to

8 gpm/ft2. Rapid sand filters do not use biological filtration but depend primarily on mechanical

straining, sedimentation, impaction, interception, adhesion and physical adsorption.

Filters that must be taken off-line periodically to back wash are classified operationally as semi-

continuous. Filters in which filtration and backwash operations occur simultaneously are

classified as continuous.

Types of Rapid Sand Filter

There are a number of different types of rapid sand filters depending upon bed depth (e.g.,

shallow, conventional and deep bed) and the type of filtering medium used (mono-, dual-, and

multi-medium).

A further classification can be made based on the driving force as gravity or pressure filters.

Typically sand is used as the filtering material in single medium filters. Dual- medium filters

usually consist of a layer of anthracite over a layer of sand. Multi-medium filters typically

consist of a layer of anthracite over a layer of sand overlying a layer of garnet.

The principal filtration methods now used with reference to the rate of flow through gravity

filters may be classified as:

Constant-rate of filtration with fixed head

Constant -rate filtration with variable head

Variable- declining-rate filtration

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Constant-rate Filtration with fixed head

In constant-rate filtration with fixed head, the flow through the filter is maintained at a

constant rate. They are either influent controlled or effluent controlled. Pumps or weirs are

used for influent control whereas an effluent modulating valve that can be operated manually

or mechanically is used for effluent control.

Constant-rate Filtration with variable head

In constant-rate variable filtration head, the flow through the filter is maintained at a constant

rate. Pumps or weirs are used for influent control. When the head or effluent turbidity reaches

a preset value, the filter is backwashed.

Declining-rate filtration with fixed or variable head

In declining-rate filtration, the rate of flow through the filter is allowed to decline as the rate of

head loss builds up with time. Declining-rate filtration systems are either influent controlled or

effluent controlled.

In the effluent controlled type of filters, the filter effluent lines are connected to a common

header. A fixed orifice is built into the effluent piping for each filter so that no filter after

washing will take an undue share of the flow. The filtered water header pressure may be

regulated by a throttle valve which discharges to filtered water reservoir. Costly rate controllers

are replaced with fixed orifices and therefore, would make the units economical particularly in

large water works involving batteries of filters. For equal duration of filter runs the total output

per day from a declining rate filter is higher than that in the conventional filters. In group of

filters operating at an average rate of 10 m3/m2/hr, the fixed orifice will be so designed that a

recently cleaned filter will begin operation at 15 m3/ m2/hr while the filter next in line for

39 Group 3


cleaning will have slowed down to about 5 m3/m2/hr. Usually the depths of filter boxes for

declining rate filters are more than those for the conventional ones. These would permit longer

filter runs and consequent reduced wash water requirements.

The filter beds are operated by scheduled cleaning in such a way that each of beds will be in

different stage of filter cycle producing the required average flow. When the rate of flow is

reduced to the minimum design rate, the filter is removed from service and backwashed. In an

inlet-controlled filter, the rate of flow is controlled proportional to the rate of filtration with

float control arrangement to the inlet valve. Inlet control reduces the amount of work which

has to be done on the filter to just clean it.

Components of Rapid Sand

The major parts of a gravity rapid sand filter are:

Filter tank or filter box,

Filter media,

Gravel support,

Under drain system, and

Wash water troughs

Filter Tank

The filter tank is generally constructed of concrete and is most often rectangular. Filters in large

plants are usually constructed next to each other in a row, allowing the piping from the clarifier

basins to feed the filters from a central pipe gallery or from the inlet channel. The sizes of the

filters vary according to the quantity to be treated. The number of filters is selected to minimize

the effect of removing the filter from service for washing on remaining filters. Ideally it should

be possible to take three filters out of service simultaneously (one draining down, one washing

40 Group 3


and one for maintenance). A minimum of four filters is desirable, although two to three filters

may be used for small plants.

Filter bed sizes vary from 25 to 100 m2 with lengths in the range of 4 to 12 m, widths in the

range of 2.5 to 8 m and length to breadth ratio of 1.25 to 1.33. The wash water collection

channel is located on one side along the length of the filter. A minimum overall depth of 2.6 m

including a free board of 0.5 m is adopted.

Filter media

The filter media is the important component of the filter which actually removes the particles

from the water being treated. The filter media must have the following properties: coarse

enough to retain large quantities of floc, sufficient fine particles to prevent passage of

suspended solids, deep enough to allow relatively long filter runs, and graded to permit

backwash cleaning.

Filter media is most commonly sand, though other types of media can be used, usually in

combination with sand. The sand used in rapid sand filters is coarser than the sand used in slow

sand filters. This larger sand has larger pores which do not fill as quickly with particles removed

from the water. Coarse sand also costs less and is more readily available than the finer sand

used in slow sand filtration. The filter sand used in rapid sand filters is prepared from stock sand

specifically for the purpose. Most rapid sand filters contain 60 to 75 cm thickness of sand, but

some newer filters are deeper. The sand used as filter media in rapid sand filtration is generally

of effective size of 0.4 to 0.7 mm and uniformity coefficient of 1.3 to 1.7. The standing water

depth over filter varies between 1.0 and 2.0 m.

41 Group 3


Graded Gravel

The filter gravel at the bottom of the filter bed is not part of the filter media and it is merely

providing a support for media above the under drains and allowing an even distribution of flow

of water across the filter bed during filtering and backwashing. The gravel also prevents the

filter sand from being lost during the operation. The filter gravel is usually graded of size from

2.5 to 50 mm (largest size being at the bottom) in four to five layers to total thickness of 45 to

50 cm, depending on the type of under drain system used. In case the under drainage system

with porous bottom or false floor no gravel base is required. The filter gravel shall be classified

by sieves into four or more size grades, sieves being placed with the coarsest on top and the

finest at the bottom.

Under-drainage System for Rapid Sand Filters

The under-drainage system of the filter is intended to collect the filtered water and to

distribute the wash water during backwashing in such a fashion that all portions of the bed may

perform nearly the same amount of work and when washed receive nearly the same amount of

cleaning. Since the rate of wash water flow is several times higher than the rate of filtration, the

former is the governing factor in the hydraulic design of filters and under drainage system,

which are cleaned by backwashing.

The under-drainage system can be one of the following types, connected to main drain:

Pipe laterals

False floor

Porous plates or strainer nozzles

The most common type of under-drain is a central manifold with laterals either perforated on

the bottom or having umbrella type strainers on top. Other types such as wheeler bottom, a

42 Group 3


false bottom with strainers spaced at regular intervals or a porous plate floor supported on

concrete pillars are all satisfactory when properly designed and constructed.

Wash-water Troughs

Wash-water troughs placed above the filter media collect the backwash water and carry it to

the drain system. Proper placement of these troughs is very important to ensure that the filter

media is not carried into the troughs during the backwash operation and removed from the

filter. The upper edge of the wash-water trough should be placed sufficiently nearer to the

surface of sand so that a large quantity of dirty water is not left above the filter sand after

completion of washing. At the same time, the top of the wash-water trough should be placed

sufficiently high above the surface of the sand so that the sand will not be washed into the

gutter.

Width of the filter bed must be equally divided by the troughs so that each trough covers an

equal area of the filter. Maximum clear spacing between the troughs may be 180 cm. The

horizontal travel of wash-water to trough should not be more than 90 cm. All the wash water

troughs must be installed at the same elevation so that they remove the backwashed water

evenly from the filter so that an even head is maintained across the entire filter. The troughs

may be made with the same cross-section throughout its length or it might be constructed with

varying cross-section increasing in size towards the outlet end. The bottom of the troughs

should clear the top of the expanded sand by 50 mm or more. These wash water troughs are

constructed in concrete, plastic, fiberglass, or other corrosion-resistant materials. The troughs

are designed as free falling weirs.

43 Group 3


Backwashing

Proper backwashing for cleaning the filter is a very important step in the operation of a filter. If

the filter is not backwashed periodically, it will eventually develop additional operational

problems. If a filter is to operate efficiently during a filter run it must be cleaned regularly at

every 24 to 48 hours. Treated water from storage is used for the backwashing. This treated

water is generally taken from elevated storage tanks or pumped in directly from the clear water

drain by passing in the reverse direction from under drains to the media.

During filtration, the grains of filter media become coated with the floes, which plug the voids

between the filter grains, making the filter difficult to clean. Backwash should, therefore, be

arranged at such a pressure that sand bed should expand to about 130 to 150% of its

undisturbed volume so as to dislodge the deposited floes from the filter media during the

backwash. Washing causes the sand grains to impinge on one another and thus dislodging

adhering floc and, the rising wash water carries the material and discharge into the gutters. The

backwash flow rate has to be great enough to expand and agitate the filter media and suspend

the floes in the water for removal. On the other hand an unduly high rate of flow will cause

more expansion than needed, so that the sand grains will be separated further and scrubbing

action will be decreased and the media will be washed from the filter into the troughs and out

of the filter. A normal backwash rate is 600 Lpm/ m2 of filter surface area without any other

agitation. The pressure of the wash water to be applied is about 5 m head of water as

measured in under drains. Backwashing normally takes about 10 minutes, though the time

varies depending on the length of the filter run and the quantity of material to be removed.

Filters should be backwashed until the backwash water is clean. For high rate back wash, the

pressure in the under drainage system should be 6 to 8 m with wash water requirement being

650 to 850 Lpm/ m2 of filter (40 – 50 m/hr) for a duration of 6 to 10 minutes.

44 Group 3


2.88 Actual Design

i. Variables affecting the Filtration Process

For the rapid sand filter, there are several limiting factors which should be

considered in the design. This involves:

o The rapid sand filters utilise flow rates of 1 – 2.5 gpm/ft2,

o Head loss will increase the run length of the process; however, coarse

medium is used to maintain a balance,

o Inadequate pre-treatment will result in a reduction of the flow rate

(< 2gpm/ft2),

o Weak flocculation will cause break through in the filter medium leading to

degradation of water quality at the end of the filtration process, and

o Any rate of change during filtration will alter the effects of the process.

ii. Filter Calculations

Each component part of the filtration system requires separate calculations. Therefore, each

aspect is clearly described below.

Filter Tank/ Filter Box

Demand – 2.3 mgd

GWI uses 12” = 0.305 m pipes for inlets; therefore this diameter was used since it is readily

available.

Number of Filters required ¿ 2.7√Q , where Q is in mgd

¿2.7√2.3

¿1.36 Which we round up to 2

45 Group 3


A basin of depth of 10’ is used, which is a standard for rapids and filters and adequate for our

design

Velocity of Inlet - Q=AV

V=Q /A

Since 2 filters are being used, the demand is divided by 2; therefore each filter must satisfy a

demand of 2.3mgd

2=1.15mgd

But 1 m3 = 264.17 gal

Therefore supply (Q) = 4353.26 m3 per day = 3.02 m3 per min = 0.05 m3 per sec

A = πR 2 = 0.073m2

V= 0.050.073

=0.69m/ s

Filtration velocity for rapid sand filter is between 1-5 mm/s

Slower velocity gives a better filtration, therefore use 2 mm/s = 0.002m/s

Demand (Q) = 0.05 m3 per sec

A=Q /V

A= 0.050.002

=25m2

With a square tank, use a 5.0m x 5.0m tank ≈ 15’ x 15’

46 Group 3


Flow Rate through Filter Media

Testing was also done to find the Velocity at which water flows through 30” of reef sand, the

only limitation to this experiment was that a constant head was impossible to maintain. Since

there was a lack of equipment in the Laboratory to conduct the test, so the group members

improvised and used a 1.5” diameter pipe, drilled holes in the bottom, placed 6” of gravel inside

to prevent the sand from escaping through the holes and then filled it with 30” of sand. Then,

let water flow through (steady head could not be maintained) and timed it taking the volume

for a specific time.

Volume Collected =1 Gallon = 0.0038m3

Time Elapsed =10 min.

Diameter = 1.5” = 0.038m

Area = 0.0012m2

Discharge = 0.1gal/min = 0.00038m3/min

Velocity = 0.00038/0.0012

= 0.32m/min

= 5.33 mm/sec

This design was done considering a velocity of 2mm/sec, to achieve this velocity, so the inflow

will have to be monitored and a constant head is create throughout the system.

Under-drain System

47 Group 3


The under-drain system we chose was the false floor with strainer nozzles, which prevent the

medium from passing with the filtered water and eliminate the need for a course medium,

therefore only one medium would be required.

The amount of nozzles to be used varies from 50-90 per square metre.

A 70 per square metre was chosen, each having a diameter of 1.25”

Therefore, number of nozzles required = 25 x 70 = 1750

Figure 2.8 Showing Nozzle to be Used

( Source : http://www.oasen.nl/oasen/Documents/Oasen%20in%20Indonesi%C3%AB/Filtratie%20ontwerp%20en%20inrichting_eng.pdf)

Figure 2.9 Showing Chosen Under-drain System

( Source : http://www.oasen.nl/oasen/Documents/Oasen%20in%20Indonesi%C3%AB/Filtratie%20ontwerp%20en%20inrichting_eng.pdf)

Backwashing

48 Group 3

http://www.oasen.nl/oasen/Documents/Oasen%20in%20Indonesi%C3%AB/Filtratie%20ontwerp%20en%20inrichting_eng.pdf

http://www.oasen.nl/oasen/Documents/Oasen%20in%20Indonesi%C3%AB/Filtratie%20ontwerp%20en%20inrichting_eng.pdf


The pressure in the under drainage system should be 6 to 8 m with wash water requirement

being 650 to 850Lpm/ m2 of filter (40 – 50 m/hr) which would cause a bed expansion between

130% - 150% for a duration of 6 to 10 minutes.

The design for wash water of velocity 40m/hr for duration of 10mins was considered.

Area of nozzle = 0.0085m2

Total Area of Nozzles = 0.0085x 1750

= 14.875m2

Total Backwash Discharge = 14.875 x 40

= 595m3/hr

Storage Volume Required for Backwash = Backwash Discharge x Backwash Duration

= (669.375/60) x 10

= 99.17 m3 = 26196.86 Gal

Wash Water Trough

The horizontal travel of wash-water to trough should not be more than 90cm ≈ 6’

Therefore two (2) wash water troughs would be require; the arrangement of which is shown

below:

49 Group 3


Figure 2.10: Illustration showing the arrangement of the Wash-water trough

(Diagram By: Sudarshan Sukha)

Since there are two wash troughs the wash-water will be divided evenly between. Therefore

each takes off a discharge of – 595/2 =275.5m3/hr.

Q=2.49b h3/2

Q - Rate of discharge in m3/sec = 275.5m3/hr = 0.077m3/sec

b - Width of trough = we use 12” = 0.31m

h - Maximum water depth in trough.

0.077=2.49 x0.31 x h3/2

h = 0.215m = 8.36” ≈ 9”

Since the bed expansion would be between 130% and 150%, the trough was placed at the

maximum bed expansion which would be a bit over the actual bed expansion since the design

50 Group 3


utilises the minimum backflow velocity. This will prevent the washing away of the filter

medium.

Height of trough = 150/100 (30”)

= 45” above the filter media = 1.143m

iii. Filter Media Selection

Guyana has sand readily available. For choosing the filter media; sieve analysis was done on

two types of sand found in Guyana, Silica Sand and Reef Sand.

Parameters Recommended Sample #

Silica Sand Reef Sand

Sample 1 Sample 2 Sample 3

Effective size (mm) 0.45 – 0.7 0.18 0.29 0.5

Coefficient of Uniformity 1.2 – 1.7 2.78 1.66 1.6

Table 2.8: Showing the Properties of the sand for the Filter Medium from Sieve Analysis

Based on these results, the reef sand from sample three was selected as the filter medium. The

standard thickness of the media for the rapid sand filter is 30”; thus, this thickness is used in the

design.

iv. Final Design Specifications

The final design specifications are illustrated in the following diagrams. These diagrams

annotate the filtration system arrangement as well as the dimensions of the component

parts.

51 Group 3


Figure 2.11: Illustration showing the components of the Filtration Tank

(Stimulation Done By: Yonnick Pratt)

52 Group 3

Inlet

SupplyRetention Tank

Wash-water trough

Tank

Back wash pipe

Outlet

Wash-water Outlet


Figure 2.12: Illustration showing the arrangement of component parts of the Filtration Tank


53 Group 3


Figure 2.13: Cross-section of the Filtration Tank showing the components


54 Group 3

Filter Medium (Reef Sand)

Under drain Nozzles Under drain


55 Group 3


56 Group 3


57 Group 3


2.89 MODEL OF THE RAPID SAND FILTER SYSTEMA model of the rapid sand filtration tank was built for demonstration purposes. The scale for

the model to prototype was established as 1” = 1ft. The materials used were ¼ inch Perspex for

the walls and floors, and ½ inch and ¾ inch male and female adaptors and pipes.

Also a model of the filter bed was made to test raw water samples. This was done by using a

4.5’ length of 1.5” diameter pipe, drilling holes in the bottom placing gravel at the bottom to

prevent the sand from escaping and then filling it with 30” of reef sand. This apparatus was

used to filter water for testing and also to find the velocity of the water.

2.90 TESTING OF WATER THROUGH THE SYSTEM

Figure 2.17 Showing Base of Improvised Testing Apparatus with Holes

58 Group 3


Figure 2.18 Showing Entire Testing Apparatus

i. Results

The results for the testing are summarised in the table below:

Sample Turbidity (NTU) pH

Unfiltered 14 6.04

Filtered 4 6.42

Table 2.9: Showing the results for the blab tests

ii. Discussion of Results

From the results the change in turbidity from 14 to 4 NTU, makes the water physically fit for human consumption since the EPA regulation for drinking water has a limit of 5 NTU. Also the filtration altered the pH of the water sample, it slightly reduced the acidity of the water.

59 Group 3


2.91 APPENDICES2.81 Filter Media Selection

Sample 1 (Silica)

GRAIN SIZE ANALYSIS

Project: CIV322 (FILTRATION TANK DESIGN) Job No.: 1

Sample No.: 1 Location: UOG Lab

Description of Sample: White sand (Silica)Depth of Sample: Surface

Tested By: Group 3 Date: 04/19/2010

Soil Sample SizeWt. of dry sample + container (g) 3424.20Wt. of container (g) 500.00Wt. of dry sample, W1 (g) 2924.20

Sieve Analysis and Grain Distribution

Sieve No.Diameter of opening (mm)

Weight Retained (g)

Percentage of Sample Retained (%)

Percentage of Sample Passing (%)

7 2.000 6.50 0.22 99.7810 1.680 14.10 0.48 99.3014 1.200 58.70 2.01 97.2925 0.600 672.10 22.98 74.3035 0.420 537.10 18.37 55.9450 0.300 980.80 33.54 22.4070 0.210 324.20 11.09 11.31100 0.150 164.30 5.62 5.69200 0.075 108.00 3.69 2.00Pan - 58.40 2.00 0.00

60 Group 3


Sample 1 (Silica)

From graph,

Effective Size, D10 = 0.18mm

Average Size, D50 = 0.42mm

To determine the coefficient of uniformity (Cu)

Cu=D60

D10

Where D60 (From Graph) = 0.50mm

Therefore,

Cu=0.50mm0.18mm

=2.78

61 Group 3


Sample 2 (Reef Sand)

GRAIN SIZE ANALYSIS

Project: CIV322 (FILTRAION TANK DESIGN) Job No.: 1


Description of Sample: Brown sand (Reef sand)Depth of Sample: Surface


Soil Sample SizeWt. of dry sample + container (g) 2264.50

Wt. of container (g) 500.00

Wt. of dry sample, W1 (g) 1764.50



Weight Retained (g)



7 2.000 0.00 0.00 100.00

10 1.680 17.70 1.00 99.00

14 1.200 39.70 2.25 96.75

25 0.600 289.10 16.38 80.36

35 0.420 421.10 23.87 56.50

50 0.300 738.80 41.87 14.63

70 0.210 172.90 9.80 4.83

62 Group 3


100 0.150 50.00 2.83 1.99

200 0.075 29.10 1.65 0.35

Pan - 6.10 0.35 0.00


From graph,




Cu=D60

D10


Therefore,

Cu=0.48mm0.29mm

=1.66

63 Group 3



GRAIN SIZE ANALYSIS

Project: CIV322 (FILTRAION TANK DESIGN) Job No.: 1


Description of Sample: Brown sand (Reef sand)Depth of Sample: Surface


Soil Sample SizeWt. of dry sample + container (g) 3359.80Wt. of container (g) 500.00Wt. of dry sample, W1 (g) 2859.80



Weight Retained (g)



7 2.000 0.00 0.00 100.0010 1.680 0.00 0.00 100.0014 1.200 0.00 0.00 100.0025 0.600 2260.00 79.03 20.9735 0.420 545.00 19.06 1.92

64 Group 3


50 0.300 35.00 1.22 0.6970 0.210 5.60 0.20 0.50100 0.150 5.30 0.19 0.31200 0.075 6.30 0.22 0.09Pan - 2.60 0.09 0.00


From graph,




Cu=D60

D10


Therefore,

Cu=0.80mm0.50mm

=1.6

65 Group 3


2.82 Conversion Factors

Classification To convert Into Multiply by Conversely multiply by

Length Inches Centimetre 2.540 0.3937Inches Feet 12 0.0830

Area Sq Metre Sq Feet 10.764 0.0929Volume Litres Cubic metre 0.001 1000

Litres Gallons 0.222 4.500

66 Group 3


GLOSSARYBack washing: The purpose of filter back washing is to remove from the bed all of foreign material collected in the bed during the preceding filter run. It is the reverse flow of water through the filter tank; which is required to flush out loose particles from the pore spaces, and agitate the grains of the media to remove accumulated coatings.

Break through: The penetration of part of the coagulated material into the bed.

Demand: In the context of water demand; the daily amount of water consumed by the population for all types of usage.

Exponential Growth: This is exponential representation of the increase in demand over time.

Floc: An alternative word for floccule. The large particles formed when small suspended

particles aggregate in the flocculation process.

67 Group 3


REFERENCES

National Bureau of Statics. (2010). Population Estimation (Research Department: no report no.).

National Exhibition Site Sophia: Authur not stated.

Guyana Lands and Survey. (2010). Cadastral Plans (Plans Department: no report no.). Durban

Backlands, Georgetown: Authur not stated.

Ministry of Housing. (2010). Number of Lots (Engineering Department: no report no.). Brickdam,

Georgetown: Authur not stated.

Ministry of Education. (2010). School Population (Population Department: no report no.).

Brickdam, Georgetown: Authur not stated.

Wikipedia, “Rapid Sand Filter” retrieved on April 15th , 2010 from http://en.wikipedia.org/wiki/Rapid_sand_filter

The Water Treatments, “Rapid Sand Filters”, retrieved on April 15th , 2010 from http://www.thewatertreatments.com/water-filters/rapid-sand-filters

Harvey A. Gullicks, “Optimisation of Rapid Sand Gravity Filters”, retrieved on April 15th , 2010 from, http://www.mnawwa.org/about/councils/training/research/workshop404/physicaloptimization.pdf

68 Group 3

http://www.mnawwa.org/about/councils/training/research/workshop404/physicaloptimization.pdf

http://www.mnawwa.org/about/councils/training/research/workshop404/physicaloptimization.pdf

http://www.thewatertreatments.com/water-filters/rapid-sand-filters

http://en.wikipedia.org/wiki/Rapid_sand_filter


All About Water, “Filtration”, retrieved on April 16th , 2010 from: http://www.allaboutwater.org/filtration.html

Water Supply, “The Rapid Sand Filter”, retrieved on April 16th, 2010 from: http://www.allaboutwater.org/filtration.html

Oasen, “Filtration and Design Installation”, retrieved on April 16th, 2010 from:

www.oasen,nl-Documents-Oasen%20in%20indonesi%C3AB-Filtratie%20ontwerp%20en%20inrichting_eng.url

Wikipedia, “Water Purification” retrieved on April 15th , 2010 from http://en.wikipedia.org/wiki/Water_Purification

Filtration, “Filtration Maths” retrieved on 16th April, 2010 from:

http://water.me.vccs.edu/courses/env110/lesson6_5.htm

Water and Wastewater Engineering, “Typical Rapid Gravity Filter Flow Operation”, retrieved on

April 18th , 2010, from:

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/wasteWater/Lecture

%2011.htm

69 Group 3

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/wasteWater/Lecture%2011.htm

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/wasteWater/Lecture%2011.htm

http://water.me.vccs.edu/courses/env110/lesson6_5.htm

http://en.wikipedia.org/wiki/Water_Purification

http://www.allaboutwater.org/filtration.html

http://www.allaboutwater.org/filtration.html

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