design of filtration tank~final final final
TRANSCRIPT
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
Demand Study and Design of Filtration Tank 2010
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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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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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
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Demand Study and Design of Filtration Tank 2010
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
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Figure 2.6: Showing a comparison of the Filtration Processes listed(Source: www.allaboutwater.com/filtration)
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Demand Study and Design of Filtration Tank 2010
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.
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Demand Study and Design of Filtration Tank 2010
Table 2.7: Showing the Characteristics of Gravity Type Filters
(Source: www.watertreatments.com/water-filters/rapid-sand-filters)
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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.
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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
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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
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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.
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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
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Demand Study and Design of Filtration Tank 2010
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.
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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.
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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
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Demand Study and Design of Filtration Tank 2010
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’
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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
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Demand Study and Design of Filtration Tank 2010
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
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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:
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Demand Study and Design of Filtration Tank 2010
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
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Demand Study and Design of Filtration Tank 2010
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.
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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
Demand Study and Design of Filtration Tank 2010
Figure 2.12: Illustration showing the arrangement of component parts of the Filtration Tank
(Stimulation Done By: Yonnick Pratt)
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Figure 2.13: Cross-section of the Filtration Tank showing the components
(Stimulation Done By: Yonnick Pratt)
54 Group 3
Filter Medium (Reef Sand)
Under drain Nozzles Under drain
Demand Study and Design of Filtration Tank 2010
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Demand Study and Design of Filtration Tank 2010
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
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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.
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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
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Demand Study and Design of Filtration Tank 2010
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
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Sample 2 (Reef Sand)
GRAIN SIZE ANALYSIS
Project: CIV322 (FILTRAION TANK DESIGN) Job No.: 1
Sample No.: 2 Location: UOG Lab
Description of Sample: Brown sand (Reef sand)Depth of Sample: Surface
Tested By: Group 3 Date: 04/19/2010
Soil Sample SizeWt. of dry sample + container (g) 2264.50
Wt. of container (g) 500.00
Wt. of dry sample, W1 (g) 1764.50
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 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
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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
Sample 2 (Reef Sand)
From graph,
Effective Size, D10 = 0.29mm
Average Size, D50 = 0.45mm
To determine the coefficient of uniformity (Cu)
Cu=D60
D10
Where D60 (From Graph) = 0.48mm
Therefore,
Cu=0.48mm0.29mm
=1.66
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Sample 3 (Reef Sand)
GRAIN SIZE ANALYSIS
Project: CIV322 (FILTRAION TANK DESIGN) Job No.: 1
Sample No.: 3 Location: UOG Lab
Description of Sample: Brown sand (Reef sand)Depth of Sample: Surface
Tested By: Group 3 Date: 04/19/2010
Soil Sample SizeWt. of dry sample + container (g) 3359.80Wt. of container (g) 500.00Wt. of dry sample, W1 (g) 2859.80
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 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
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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
Sample 3 (Reef Sand)
From graph,
Effective Size, D10 = 0.50mm
Average Size, D50 = 0.75mm
To determine the coefficient of uniformity (Cu)
Cu=D60
D10
Where D60 (From Graph) = 0.80mm
Therefore,
Cu=0.80mm0.50mm
=1.6
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Demand Study and Design of Filtration Tank 2010
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
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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.
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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
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Demand Study and Design of Filtration Tank 2010
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
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