The Impact of Design and Operating Parameters on Small-Scale Slow Sand Filtration Performance for...

7/29/2019 The Impact of Design and Operating Parameters on Small-Scale Slow Sand Filtration Performance for Household W

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The Impact of Design and Operating Parameters on Small-

Scale Slow Sand Filtration Performance for HouseholdWater Treatment in Developing Countries.

by

Sarah Clayton

Final Year Project

Department of Civil and Environmental Engineering

Imperial College London

Supervisor: Dr. Michael Templeton

Final Report

Submitted: 17th June 2010

A PRODECI & Engineers Without Borders Research Project


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PRODECI & EWB-UK Research: Slow Sand Filtration Sarah Clayton

29/09/2010 Department of Civil & Environmental Engineering ii

ABSTRACT

A Slow Sand Filter test-rig was designed and constructed to investigate the design

and operating parameters of small-scale (household scale) Slow Sand Filtration

(SSF). Tap water was mixed with light Kaolin clay to create turbid influent which

was run through the apparatus at three different filtration rates; 0.2, 0.3 and

0.4m/hr. The filter produced greater than 80% turbidity reduction in all

experiments, the resulting effluent meeting water quality guidelines.

Although both filtration rate and filter depth are shown in the literature to be

important parameters in turbidity reduction, the limitations of this research project

mean that the conclusions drawn from the experiments undertaken are incomplete.

The process of researching and designing the experimental set-up highlighted the

need for further research of the schmutzdecke and its properties.


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29/09/2010 Department of Civil & Environmental Engineering iii

ACKNOWLEDGEMENTS

I am grateful to all those who have helped me during this research project and in

particular would like to thank the following people for their assistance and

contributions:

Dr. Michael Templeton for his supervision, support in allowing me to choose an

external EWB-UK project and on his feedback on the report and poster.

Engineers Without Borders UK for the opportunity to take up the project and in

particular EWB Cambridge and Mott MacDonald for the Water Quality and Health

Training day.

Ian Baggs for writing the research proposal that lead to this project. Also for his

time and guidance towards the aims and benefits of his project.

Carol Edwards, Dr. Geoff Fowler and Dr. Thomas Bond for all their help in the

laboratory.

Kim and Beth Waterhouse for generously welcoming me to Clare Farm and

providing their time to show the author their working Slow Sand Filters and discuss

this project.

My family for all their support and proof-reading over the last four years.

Sarah x


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29/09/2010 Department of Civil & Environmental Engineering iv

TABLE OF CONTENTS

Abstract ...................................................................................................... ii

Acknowledgements .................................................................................... iii

List of figures .............................................................................................. v

List of tables .............................................................................................. vi

Acronyms ................................................................................................... vi

1. Introduction ............................................................................................ 1

2. Aims and objectives ................................................................................ 2

2.1. Objectives ........................................................................................... 2

2.2. Scope .................................................................................................. 2

3. Literature review ..................................................................................... 3

3.1. Types of slow sand filter ........................................................................ 3

3.2. What happens inside the filter? ............................................................... 5

3.3. Small-Scale Slow Sand Filtration ............................................................. 8

3.3.1. Continuous vs. Intermittent.............................................................. 8

3.3.2. Filter Depth .................................................................................... 8

3.3.3. Basic designs.................................................................................. 9

3.4. Discussion & Further Work ................................................................... 12

4. Materials & Methods .............................................................................. 12

4.1 Apparatus and Selection of Parameters .................................................. 12

4.2 Water Testing Method .......................................................................... 17

4.2.1 Turbidity Testing ............................................................................ 17

4.2.2 Microbiological Testing .................................................................... 18

4.3 Analysis .............................................................................................. 18

5. Results & Discussion ............................................................................. 19

5.1 Preliminary Results .............................................................................. 19

5.2 Comparison of Filtration Rates ............................................................... 19

5.3 Comparison of Filter Depths .................................................................. 22

5.4 Experimental Limitations ...................................................................... 24

6. Overall Project Conclusions ................................................................... 27

6.1 Conclusions ......................................................................................... 27

6.2 Future work ........................................................................................ 28


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29/09/2010 Department of Civil & Environmental Engineering v

7. References ............................................................................................ 29

8. Appendices ............................................................................................ 35

8.1 Appendix 1: Detail of Experimental Procedures ........................................ 35

8.2 Appendix 2: Results & Analysis .............................................................. 40

LIST OF FIGURES

Figure 1: Dominant particle transport mechanisms (Graham 2010) ...................... 5

Figure 2: Structure of a biofilm (Bruce and Hawkes 1983 p37) ............................. 7

Figure 3: Clay Pot Filter (CMS n.d.) .................................................................. 9

Figure 4: Galvanised Tin Filter (TILZ 2005) ...................................................... 10

Figure 5: Biosand Filter Components (CAWST 2009 p.2) ................................... 11

Figure 6: Experimental Set-Up ....................................................................... 12

Figure 7: the 100l water butt used (Homebase 2010) ....................................... 13

Figure 8: Filter Structure ............................................................................... 16

Figure 9: Turbidity standards of 10, 100, and 1000 NTU (Science Fair Project 2010)

.................................................................................................................. 17

Figure 10: Comparison of Initial Turbidities ..................................................... 21

Figure 11: Comparison of experiments ............................................................ 22

Figure 12: Filter Ripening Period (CAWST 2009 p.7) ......................................... 24


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29/09/2010 Department of Civil & Environmental Engineering vi

LIST OF TABLES

Table 1: Comparison of Traditional and Demand Operated Slow Sand Filtration

(Manz 2005 p.10) ........................................................................................... 4

Table 2: Advantages and Disadvantages of the CMS design ................................. 9

Table 3: Advantages and Disadvantages of the TILZ design ............................... 10

Table 4: Advantages and Disadvantages of the CAWST design ........................... 11

Table 5: Turbidity Standards (at tap) .............................................................. 18

Table 6: Results from varying Filtration Rates .................................................. 20

Table 7: Re-run experiments with filter depth 0.17m ........................................ 22

ACRONYMS

PRODECI an Ecuadorean Non-Governmental Organisation

EWB-UK Engineers Without Borders UK

UN United Nations

SSF Slow Sand Filtration

CMS Church Mission Society

SERVE an Afghani charity

TILZ Tearfund International Learning Zone

CAWST Centre for Affordable Water and Sanitation Technology

NTU Nephelometric Turbidity Units


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29/09/2010 Department of Civil & Environmental Engineering 1

1. INTRODUCTION

The author has been involved with EWB-UK throughout the last four years and so

this research project was found via their website (EWB-UK 2010). Ian Baggs

experiences in Intag, Ecuador lead him to draw up a proposal for investigation into

SSF as an option for providing the local population with potable water. He describes

the problem: Due to farm-land distribution, many inhabitants of the Intag area live

in isolated areas, miles from communities with centralised water systems. Most of

these inhabitants drink untreated water from nearby springs and streams, which are

often contaminated due to nearby agricultural activity. As a result, there is an

extremely high rate of parasitic infection (80-90% from medical studies conducted

in 2007), leading to prolonged illness, low school attendance in children and

reduced productivity in agriculture. (Baggs 2008 p.1) Geographical barriers, such

as those described above, mean that centralised community scale treatment of

water is not feasible therefore a household scale method of water purification needs

to be implemented.

Within developing countries the need for potable water can be extremely hard

target to achieve. SSF is generally recognised to be a cheap, low maintenance and

easy to install system which is able to produce high quality results in terms of the

physical, chemical and biological quality of water treated. Hence it is seen as a very

useful tool in disaster relief and development work. The history of SFF has been well

documented. As Barrett et al. (1991) recount in detail the first slow sand filters

were used in industry within Scotland and their first use for piped public water

supply was in London in 1829.

Even though SSF has been used at a large scale since the 19th century but there has

been significantly less research into smaller scale use. So the purpose of this

research project is to rectify that by designing and operating a bench-scale filter in

a laboratory at Imperial College, London. This will enable investigations into the

design and operating parameters for small-scale (household scale) SSF.


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2. AIMS AND OBJECTIVES

2.1. Objectives

Research and present a thorough study of all SSF information available to be used

as a resource for future work

Quantitatively investigate the impact of filtration rate on turbidity removal for a

standard small-scale slow sand filter design

Investigate the maximum filtration rate for the test-rig filter designed and built

2.2. Scope

As discussed SSF has been used for the last few centuries to treat large volumes of

water in developed countries and more recently it has been used for disaster relief

by charities such as Oxfam. Even here the focus has been on community scale SSF.

This focus has come under fresh scrutiny as there is significant contamination of

water occurring between the source, i.e. the community treatment works, and

households. The problem is often due to a lack of education, for example the vessel

used for collecting water is dirty. To help combat the problem of source-to-mouth

contamination this project will concentrate on small-scale, specifically household

scale, SSF. This in turn leads to a new set of problems. The majority of the current

guidance is based on the larger scale models and is not always appropriate. The aim

of this research project is to draw upon others work and supplement it in order to

create practical guidelines on SSF. In particular the aim is to create some

recommendations of parameters which are able to help those in the field make

informed choices about how to implement SSF with the resources they have

available. This will mainly be achieved through a literature review which will try to

combine all current research on intermittent small-scale SSF in one document and

so be used as a source document for future reference.


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3. LITERATURE REVIEW

This literature review was compiled between October 2009 and January 2010. It

includes 44 sources which include theses, papers, books and electronic sources. A

thorough study of all SSF information available is presented.

3.1. Types of slow sand filter

There are two types of slow sand filter: 1) Traditional or Continuously Operated and

2) Intermittent or Demand Operated. The differences between traditional and

demand operated slow sand filters are driven by the number of people the Slow

Sand Filter is serving.

Traditional filters have generally been part of large, community based projects

which require very significant quantities of water to flow through them. The

necessity to cater for such quantities has generated the need for a large surface

area, to create a high flow rate. It has also been a requirement that the flow is

continuous. This often requires a large storage tank to allow for water provision

whenever it is needed. Contrary to this small scale SSF does not require the large

volumes of water to be treated and is also constrained by the amount of space

available for the filter and storage tank. It is due to these constraints that small

scale SSF is known as Demand Operated or Intermittent. With Demand Operated

SSF the user can turn the filter on and off at will. Due to the smaller demand for

water the filter can be scaled down to a size which is more appropriate for a

household.

Table 1 overleaf compares these two types of slow sand filter directly. In particular

it highlights that Demand Operator filters can achieve the same high performance

whilst operating under more severe Raw Water Quality and with higher Filter

Loading Rates.


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Table 1: Comparison of Traditional and Demand Operated Slow Sand Filtration(Manz 2005 p.10)


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3.2. What happens inside the filter?

Contrary to early understanding and the terminology used, SSF does not solely

surface filter the material out of the water. In reality although

SSF operates via all the mechanisms normally associated with

filtration, in addition there are biological removal mechanisms. The mechanicalremoval mechanisms may be classified into two general categories: 1) Transport

mechanisms which bring particles into contact with the sand grains and 2)

Catchment mechanisms which hold particles to the sand grain surfaces.

1) Transport Mechanisms

In all cases of filtration the flow through a filter is laminar even with a considerably

clogged filter media. Therefore in each of these mechanisms the particles have to

cross flow streamlines to come into contact with the sand grains. The different

mechanisms are:

a)Inertial Impaction: The particles own inertia enables it to cross flow streamlines

and collide with sand grains. This is of little importance in SSF due to the low

velocities.

b)Diffusion: Transport due to Brownian motion is important for very small particles,


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2) Capture Mechanisms

These mechanisms depend on the physical and chemical properties of the

contaminated water as well as the filter media and the rate of filtration.

Straining: The mechanical process by which the particles are simply too large to

fit through the voids left by the sand grains.

Interception: Particles approach sand grains within one particle radius, without

crossing the laminar flow streamlines. So the particles come into contact with and

attach to the sand grain.

Adhesion: The process by which a particle is attracted to another and so when

they come into contact they stick to each other.

Flocculation: Using the processes described above particles adhere together and

so become too large to fit through the voids left by the sand grains.

In addition, as already mentioned, the filtration process is partly biological; it

harnesses the natural behaviour of microorganisms, namely converting any material

into biomass which can be straightforwardly separated from the water. The biofilm,

or schmutzdecke, which is inherent to this process, is a very complex environment

containing a multitude of different organisms including microorganisms. The

biological filter is essentially a food pyramid or web. These organisms do not need

to be added to wastewater as they occur naturally in the sewage. The rate of film

development depends primarily on the season i.e. due to the temperature. Hence

the biofilm will form faster in summer than in winter because microorganisms have

an optimum temperature for growth which summer temperatures are closer to. The

primary purification mechanism is biological oxidation. This is the process by which

the heterotrophic microorganisms, which require external organic compounds as

their source of carbon, oxidise the pollutants to be used for microbial growth. This

process is facilitated by the flow of wastewater over or through the biofilm.

Suspended solids and colloidal matter will be flocculated by extracellular polymers

and then adsorbed onto the surface of the film where some of this matter will be

directly ingested by metazoa and protozoa. Degradation will also occur by

extracellular enzymes, which create soluble organics and O2 that are able to diffuse

into the biofilm. Fungi hyphae play an important role in this process as they are able

to transport O2 to deeper layers of the biofilm more efficiently than diffusion.


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A mature biofilm has three main layers. The base layer contains bacteria, fungi and

algae. The middle layer, which is the largest, contains fungi and algae whereas the

outer layer contains only fungi. As Casey (1997) suggested, the biofilm will alter

depending on the nature and strength of the waste and also the rate and method at

which it is applied. The biofilm has a porous structure which enables effluent to flow

through the biofilm as well as over it, Figure 2 below, which enables mechanical

filtration producing very clear effluents. The biofilm increases in thickness during the

filters operation, which is due to two activities. Obviously as more wastewater flows

then the microorganism will thrive creating more biomass. Also more material will

flocculate and therefore become attached to the biofilm surface. This in turn will

increase the likelihood of physical entrapment of particles.

Figure 2: Structure of a biofilm (Bruce and Hawkes 1983 p37)


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3.3. Small-Scale Slow Sand Filtration

3.3.1. Continuous vs. Intermittent

As stated previously the main development in SSF has been the evolution of

intermittent small-scale slow sand filters. As Buzunis (1995) noted, this research

was initiated Dr. David Manz of the University of Calgary, Canada in the 1990s. It

was the realisation, followed by experimental verification, that the schmutzdecke

could be kept alive providing it was kept wet and aerated. Buzunis goes on to

explain that at the time there had been very little research undertaken focusing on

small-scale SFF. He is thorough in his explanation of the possible sources or error in

the research completed. In Section 5 of his thesis Buzunis describes how the depth

of standing water above the filter is dependent on a number of parameters. For

example oxygen is highlighted as a limiting factor as the schmutzdecke is aerobic.

Though how much oxygen is required is directly related to temperature. Once this

need is fulfilled the next requirement of the schmutzdecke is the amount of

substrate and this quickly becomes a limiting factor. Clearly the depth of standing

water is a delicate balance. Most importantly, it must not change significantly during

the pause time as this will affect the schmutzdecke greatly.

3.3.2. Filter Depth

There has only been one report detailing research into the depth of the sand filter

required. This was carried out by Way (2004) in an investigation into whether SSF

could be used in conjunction with rain water harvesting as an in-tank treatment.

Way acknowledges that traditional filters frequently have a sand layer of over 0.5m

which would be impractical for in-tank treatment. This allows the filter to be

cleaned, removal of ~5cm of top surface, when there is significant head loss,

several times before more filter media would need to be added. Through her

research Way proved that a much smaller layer of sand was equally effective. When

taken in conjunction with Manzs clean-in-place technology(2004 p.1), SSF may be

scaled down significantly and, combined with intermittent flow, to provide a very

realistic water treatment for household scale use.


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3.3.3. Basic designs

There is great variation in the design of small-scale slow sand filters. The author has

chosen the following three to demonstrate the range of modifications possible

depending on the local materials and circumstance.

(i) CMS

Figure 3: Clay Pot Filter (CMS n.d.)

Table 2: Advantages and Disadvantages of the CMS design

Advantages Disadvantages

Locally sourced materialsDifficult to transport due to weight

and fragility

Simple constituent parts therefore easy

to construct

Less quality control possibly leading

to variable results

Cheap Low Filtration Rate

This slow sand filter has been designed with the specific aim for individuals or

families to make their own. Hence the benefits become the designs weaknesses

when the design is considered for distribution and enterprise.


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(ii) SERVE/Tearfund house hold slow sand filter

Figure 4: Galvanised Tin Filter (TILZ 2005)

Table 3: Advantages and Disadvantages of the TILZ design


Made from existing materials/equipment

therefore easy to constructUnusual upward flow

CheapSmall surface area for the

schmutzdecke to form

Pre-filter

This filter design was thoroughly researched in order to make it feasible in terms of

cost and build-ability. The design is unusual for two reasons: 1) the pre-filter which

protects the main sand filter meaning that less maintenance is needed and 2) the

direction of flow which interestingly has not been adopted in any other designs.


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(iii) CAWST BioSand Filter

Figure 5: Biosand Filter Components (CAWST 2009 p.2)

Table 4: Advantages and Disadvantages of the CAWST design


CheapMade from concrete which involves

training and time to produce

Thoroughly researched and well

provenDifficult to transport due to weight

Availability of training materials Complicated instructions

This design is the most widely used small-scale sand filter which has been adopted

by a large assortment of charities. Nonetheless it is not necessarily the best design

for all situations since it requires specific materials and a trained labour force.


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3.4. Discussion & Further Work

Research into small-scale SFF has flourished over the last 20 years though there

remain areas which are underdeveloped in the literature. In particular, there is a

need for a rigorous assessment of the schmutzdecke and its properties. It would be

predominantly useful to investigate the relationship between ripening time and

parameters such as temperature or intermittency. Unfortunately this area of

research is beyond the scope of this project.

This work will focus on how a range of filtration rates affect the effectiveness of the

filter. The investigation will detail the range of filtration rates within which the filter

can work therefore maximising the potential of this technology.

4. MATERIALS & METHODS

4.1 Apparatus and Selection of Parameters

Figure 6: Experimental Set-Up

The limiting factor in terms of apparatus set-up is laboratory space. The slow sand

filter was made from rudimentary equipment based on the Centre for Affordable

Water and Sanitation Technologys Biosand Filter design (CAWST 2009) which is

used worldwide by many Non-Governmental Organisations and charities.


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(i) Representation of domestic household use and Slow Sand Filter:

A hypothetical familys situation may be represented by a theoretical demand of 150

litres a day through a Version 10 CAWST Biosand Filter with a 0.05m2 cross-

sectional surface area.

(ii) Equipment constraints:

Figure 7: the 100l water butt used (Homebase 2010)

The test-rig filter was constructed from a plastic water butt with across sectional

area of 0.1089m2. This larger cross-sectional surface area was chosen to minimise

edge effects. Although due to lack of storage space it was not be possible for 150

litres of water to be run through the test-rig every day. The fundamental differences

between the experiment set-up and the CAWST filter are the surface area of the

filter and depth of sand. These differences were not expected to affect the results

obtained, due to the fact that the surface area is larger and so the flow rate can be

easily matched. Secondly, Ways work (2004), aforementioned in the literature

review, has shown that a shallower filter depth is still successful at producing high

water quality.


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It was not possible to include a diffuser plate into the design, as shown in Figure 5.

Diffuser plates prevent disturbance of the sand surface and therefore the biofilm

when water is added to the top of the filter. Instead the water was added through a

funnel into some tubing which directed the influent away from the sand surface, see

Appendix 1.

(iii) Experimental calculations:

As the depth and composition of the test-rig was similar to the CAWST Biosand

Filter it was assumed that the optimum Filter Loading Rate or Filtration Rate would

also be comparable. Hence the range of Filtration Rates chosen to investigate the

maximum suggested by CAWST is:

Filtration Rate (m/hr) 0.2 0.3 0.4

Flow Rate (m3/hr) 0.0218 0.0327 0.0436

There are two parameters that could be changed in order to vary the flow rates

through the filter in the desired manner outlined above.

This is shown by Darcys Law:

Q = K*(A*h/L)

(Bioandfilter.org 2004)

Q = Flow Rate (m3/hr)

K = Hydraulic Conductivity (m/hr)

A = Surface Area (m2)

h = Head Loss (m)

L = Depth of media (m)

Accordingly the head loss and depth of media were the two parameters which could

be changed. In order to determine which parameter was easier to change the

Hydraulic Conductivity needed to be ascertained.


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This was achieved using:

K 100(D10)2

D10 = effective diameter (cm)

(Hazen 1930, as cited in Smith et al. 1992 p.1)

By assuming D10 = D = 0.07cm for standard builders sand it was found that with a

filter depth of 0.35m the levels of head loss needed to drive the above flow rates

were:


Head Loss (m)

(estimated)0.14 0.21 0.29

As these could be accommodated within the experimental set up it was decided that

it would be significantly easier to vary the head of water than the depth of sand. As

the maximum head space levels were within the water butt the volume of

contaminated water were:

Maximum Head (m) 0.14 0.21 0.29

Volume of water (l) 15.6 23.3 31.1

As Jenkins et al. (2009 p.1) reported changing the maximum head space levels

may achieve this as the flow rate will decrease as the water drains through the

filter.

(iv) Time Constraints:

Due to necessity and storage capacity it is highly unlikely that the filter would be in

use continuously. It is more likely that the filter would be used multiple timesthroughout the day to provide small quantities of water when needed. Unfortunately

due to time constraints it was not possible to model this use of the filter accurately

i.e. multiple runs per day. Instead the filter was run through at the chosen filtration

rate before each and every test.


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(v) Filter construction:

The filter was assembled using four layers of material as shown below in Figure 8.

The first layer of Drainage Gravel, 20mm diameter, was deep enough to ensure the

outflow pipe drained water from this layer. This meant that this layer was actually

100mm deep to cover the tap at the bottom of the water butt. The drainage gravel

supported two layers of Separating Gravel which were of a smaller diameter size.

Each layer was 50mm deep and the first contained 10mm diameter gravel whilst the

second was Pea Gravelof 6mm diameter. The change in particle size should be

sufficient to stop any gravel or sand entering the outflow pipe. The material used for

the filter layer was coarse builders sandwith a particle size of < 0.7mm and was

350mm deep. These sizings quoted are all approximate to the CAWST manual.

(CAWST 2009)

Figure 8: Filter Structure

Builders Sand~0.7mm diameter

Drainage Gravel~20mm diameter

Separating Gravel

~10mm diameter

Pea Gravel~6mm diameter

Clean Effluent

Turbid Influent


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Significant challenges were encountered upon the initiation of experimentation. No

data was collected before the Easter break, due to the use of unwashed gravel and

in the filter. The panning process was omitted due to time and space constraints. It

was deemed impractical for one person to wash the mass sand and gravel. The

experiment was carried out within the Roger Perry laboratory where space was

limited. It has been presumed that there would be adequate time to allow the filter

to wash through.

4.2 Water Testing Method

In order to test the effectiveness of the filter two tests were planned on the treated

water. The first of the two tests was a Turbidity Test and then, if time permitted, a

set of experiments measuring the Thermotolerant (faecal) Coliform Count were to

be completed.

4.2.1 Turbidity Testing

Turbidity is the measurement of transparency of a liquid, see Figure 9 below. It is

caused by suspended solids so small that they do not settle out and is measured in

Nephelometric Turbidity Units (NTU).

Figure 9: Turbidity standards of 10, 100, and 1000 NTU (Science Fair Project 2010)

Turbidity reduction has been used as a measure of filtration efficiency for the last

one hundred years (OConnor 2009). The assumption behind using turbidity removal

as a surrogate indicator for microbiology is that the majority of the microorganisms

within the water are actually attached to the surfaces of larger particles and

therefore removal of these equals the removal of microorganisms (OConnor 2001).

Also turbidity is easily, quickly, and cheaply detected and quantified(Heller 2007

p.337) which is why it has been adopted worldwide as a surrogate indicator. The

limitations of this assumption are discussed in Section It is hypothesised that the

reason for this result is related to the removal of the top half of the previous filter


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media. This meant that any fine particulate matter that was within this media was

removed and as the experiment was carried out within the filter ripening period

subsequent to the other two the average effluent turbidity was reduced. Therefore

the limitations of the experiment discussed below, see Section Error! Not a valid

bookmark self-reference., again become relevant.


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5.4 Experimental Limitations.

The test-rig is designed to compare different flow rates through a single

intermittently used Slow Sand Filter. The outflow pipe is controlled by a tap so the

flow can be stopped leaving a standing water zone. Fresh contaminatedwater is

added to the system before each cycle by mixing tap water with light kaolin clay.

Although there is no health-based guideline it is seen as an important parameter

especially for the effectiveness of disinfection. Hence it is included in all water

quality guidelines and regulations such as those shown below in Table 5.

Table 5: Turbidity Standards (at tap)

World Health Organization

(WHO 2008)

UK

(The Water Supply (Water Quality)

Regulations 2000)


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using a t-Test. The 95% Confidence Interval was calculated so that error bars could

be shown. All analysis was carried out in Microsoft Excel.

5. RESULTS & DISCUSSION

5.1 Preliminary Results

The filter was not run for a period of 8 weeks, between the end of March until the

end of May 2010, consequently for the first couple of days the effluent being

produced was more turbulent than the influent tap water. The filter had to be run

continuously over 2.5 working days before clean water was produced. From

preliminary tests it was found that the baseline turbidity in the tap water was 2NTU.

5.2 Comparison of Filtration Rates

The results below, Table 6, show that the filter built for this project was successful

in consistently reducing the turbidity levels to below the guidelines figures for

turbidity at tap. The percentage reduction was not as high as expected but this is

probably due to the short length of time the filter was run for discussed below in

Section It is hypothesised that the reason for this result is related to the removal of

the top half of the previous filter media. This meant that any fine particulate matter

that was within this media was removed and as the experiment was carried out

within the filter ripening period subsequent to the other two the average effluent

turbidity was reduced. Therefore the limitations of the experiment discussed below,

see Section Error! Not a valid bookmark self-reference., again become

relevant.


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Table 6: Results from varying Filtration Rates20NTU

Filtration Rate(m/hr)

TestInitial Turbidity

(NTU)Final Turbidity

(NTU)PercentageReduction

Average InitialTurbidity (NTU)

Average FinalTurbidity (NTU)

Average PercentageReduction

0.4

1 14 1.7 87.9%

17 2.4 85.9%

2 17 2.8 83.5%

3 23 3 87.0%

4 17 2.4 85.9%

5 15 2.2 85.3%

0.3

1 13 2.2 83.1%

16 2.5 84.4%

2 18 2 88.9%

3 20 3 85.0%

4 16 2.6 83.8%

5 13 2.7 79.2%

0.2

1 15 2.1 86.0%

15 2.1 86.1%

2 15 2 86.7%

3 19 1.9 90.0%

4 14 2.5 82.1%

5 13 2.1 83.8%

40NTU

Filtration Rate(m/hr)

TestInitial Turbidity


(NTU)PercentageReduction




0.4

1 41 3.2 92.2%

36 3.0 91.7%

2 31 2.3 92.6%

3 37 2.8 92.4%

4 38 3.4 91.1%

5 32 3.1 90.3%

0.3

1 38 2.5 93.4%

35 2.6 92.4%

2 33 3.4 89.7%

3 38 2 94.7%

4 34 2 94.1%

5 30 3.2 89.3%

0.2

1 33 2.2 93.3%

30 2.2 92.7%

2 28 3.5 87.5%

3 30 1.1 96.3%

4 32 2.7 91.6%

5 27 1.4 94.8%


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Figure 10: Comparison of Initial Turbidities

The means from the two initial turbidity conditions produced statistically significant

results with greater than 95% confidence interval based on a t-Test comparison. So

therefore it can be said that there is a low probability that the difference in results

between the two experimental conditions happened by chance. (StatPac Inc. 2009)

It was expected that with a higher initial turbidity the filter would become less

efficient. The graph above, Figure 10, seems to suggest that the higher the initial

turbidity the more effective the filter is at removal which is counterintuitive. Again

this is probably due to the limitations of the experiment discussed below in Section

It is hypothesised that the reason for this result is related to the removal of the top

half of the previous filter media. This meant that any fine particulate matter that

was within this media was removed and as the experiment was carried out within

the filter ripening period subsequent to the other two the average effluent turbidity

was reduced. Therefore the limitations of the experiment discussed below, see

Section Error! Not a valid bookmark self-reference., again become relevant.


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It was also expected that the percentage reduction would increase with decreasing

filtration rate. This was not seen in the results above. This suggests that the range

of filtration rates chosen were not at the limit of the capability of the filter. This

confirms that it is very inefficient to translate and recommend large-scale SSF

parameters on to small-scale projects. Also it implies that the maximum filtration

rate is very specific to the design of the filter. Even though the filter built for this

project is similar to the CAWST design it seems to have a higher maximum filtration

rate than suggested by CAWST (p.7). This means that it could be dangerous to build

filters in the field from modified designs without the necessary equipment to test

the effluent water quality.


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5.3 Comparison of Filter Depths

Table 7: Re-run experiments with filter depth 0.17mFiltration Rate

(m/hr)Test

Initial Turbidity(NTU)

Final Turbidity(NTU)

PercentageReduction




0.4

1 78 3 96.2%

174 2.5 98.5%

2 240 1.6 99.3%

3 195 2.4 98.8%

4 180 1.9 98.9%

5 175 3.8 97.8%

0.3

1 80 2.4 97.0%

123 2.0 98.4%

2 145 1.7 98.8%

3 120 1.5 98.8%

4 120 2.5 97.9%

5 150 1.7 98.9%

0.2

1 79 1.5 98.1%

99 1.9 98.1%

2 120 2.4 98.0%

3 95 2 97.9%

4 93 1.5 98.4%

5 110 1.9 98.3%

Figure 11: Comparison of experiments


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In the previous experiments the maximum filtration rate was not found. This can be

seen as the breakthrough point was not reached. The breakthrough point is the

point at which the filter fails and turbid effluent is produced. In order to expand the

research further it was decided that the results would be repeated but with only half

the filter media depth.

This experiment was set-up and run over a period of two days. Due to time

constraints it was only possible to run once. Instead of removing all of the filter

media and adding new sand the top half of the previous bed was removed. This had

the advantage of need a shorter start up time before the effluent was running clear.

Again the filter was very effective at removing high turbidities, see Table 7 above.

Interestingly this set of results has a higher average percentage reduction, Figure

11, than the previous experiments even though the initial turbidities are extremely

high. It is hypothesised that the reason for this result is related to the removal of

the top half of the previous filter media. This meant that any fine particulate matter

that was within this media was removed and as the experiment was carried out

within the filter ripening period subsequent to the other two the average effluent

turbidity was reduced. Therefore the limitations of the experiment discussed below,

see Section Error! Not a valid bookmark self-reference., again become

relevant.


New Head Loss (m)

(estimated)0.07 0.01 0.14

New Volume of water (l) 7.6 11.3 15.1


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5.4 Experimental Limitations(i) Timescale:

Figure 12: Filter Ripening Period (CAWST 2009 p.7)

Filter ripening is the time period over which the biofilm develops on the filter media.

Even though it is unlikely that a significant biofilm developed on the filter used,

given that the influent water passing through the filter contained very little

microbiological contamination, the filter would still need adequate time for the filter

media to settle and optimum performance to be reached. Then with time, length of

which is dependent on the influent water contamination, the treatment efficiency

would start to decline or the filtration rate becomes impractically slow because of

the large head loss due to the filter media clogging. This indicates that the filter has

broken through which means that maintenance is needed after which there is,

again, another filter ripening period.

The main limitation to this project was the condensed timescale. The results indicate

that the filter ripening period was not completed over the short, 3 weeks, timescale

that the experiments were conducted over. This is the reason that the experiments

completed later yet with a higher initial turbidity show better percentage removal

simply because they were completed after the filter had been running for a longer

period of time.


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(ii) Intermittency:

During the preliminary testing it was noted that the filter was particularly sensitive

to stoppages even to the extent that after the tap had been turned off to empty the

effluent container the effluent turbidity spiked. This may be as a result of the tap at

the base of the water butt not being located at the very bottom, but instead

approximately 5cm up from the base. Therefore sediment could build up instead of

being washed through resulting in a turbidity spike when flow resumed. To

counteract this after every pause period the filter needed to be filled to beyond the

0.4m/hr head level and the excess, ~20l for the 35cm filter depth experiments and

~7l for the 17cm filter depth experiment, run through the filter before the effluent

became clear. The amount of water that needed to run through the filter reduced as

the experiments continued indicating that the filter was improving in effectiveness.

Therefore if the experiments had continued on this filter it is likely that true

intermittency would have been achieved.

(iii) Influent Turbidity

It was decided to carry out the lower turbidity experiments first so as to minimize

the risk of filter clogging due to the high turbidity levels. A difficulty was

encountered in trying to control the influent turbidity. This arose because the filter

had to be left wet, i.e. with approximately 5cm of standing water above the sand

surface, and that flow had to be stopped during lunch and then overnight which

meant that the clay had time to settle. As the turbid water could only be added in 3l

quantities, see Appendix 1, it was assumed that within the filter there was sufficient

mixing and the turbidity of the influent water was therefore constant. Both of these

factors contributed to the measured turbidity of influent water being variable, both

between and within runs, and lower than expected.

In the third experiment due to the decreased depth of filter media the head loss for

the same filtration rate decreased. This meant that when the influent was added via

the funnel the filter media surface was disturbed much more than in previous

experiments. This is what contributed to the very high influent turbidities and also

added colour to the influent water.


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(iv)Turbidity as a Microbiological Surrogate

Although turbidity has been used as a surrogate indicator for microbiology, and

therefore as a parameter for filter treatment efficiency, for a long time, and still is in

use, there has been recent research to suggest that the relationship between

turbidity and microbiological removal is not as strong as previously thought. This is

due to the limitations of the underlying assumption that the majority of the

microorganisms are attached to or embedded the larger particles within the

suspension (OConnor 2001). OConnor continues to explain that the problem lies in

the diversity of microorganisms found within the raw water and that the assumption

that they are have the same removal efficiencies is weak. This is particularly

significant in terms of viruses and protozoa. It is also interesting to note that the

water temperatures had a negative effect on the microbiology removal although this

was for Rapid Sand Filtration. Heller et al. (2007) discuss that there is significant

disagreement in the literature as to whether or not there is an association between

turbidity and microbiology. In their results no such correlation could be found and

so they advise against solely using turbidity as a surrogate for SSF.

With increasingly stringent water quality guidelines the discrepancies between

turbidity removal and microorganism removal become more significant. This is the

reason why turbidity levels are much more severe,


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6. OVERALL PROJECT CONCLUSIONS

6.1 Conclusions

The filter built for this project was extremely successful at removing turbidity from

the influent water at a variety of filtration rates. It did this with relatively little

variation in terms of percentage removal, even when the filter media was reduced

to half the original depth. This would suggest that although both filtration rate and

filter depth are shown in the literature to be important parameters in turbidity

reduction, the limitations of this research project, discussed above, mean that the

conclusions drawn from the experiments undertaken are incomplete.

In particular this research project has highlighted the following key issues when

investigating small-scale SSF design:

Individuality of filter: The maximum filtration rate is very specific to the design of

the filter and therefore care must be taken when modifying existing designs to

allow for specific materials available in the field.

Filter Ripening Period: This research corroborates that completed by Heller et al.

(2007) which found filter maturity as one of the most important factors for

microbiological removal efficiency. The filter must be given time to mature and

then reliable and consistent results and conclusions can be drawn.

Turbidity: Although turbidity is used worldwide as a surrogate indicator for

microbiology care should be taken as there are serious limitations to this

approach. If possible a second parameter should be measured, for example

coliform count, to verify filter efficiency. The turbidity limits imposed in design

recommendations are purely to limit the likelihood of clogging and therefore the

amount of maintenance the filter requires. This research project has shown that

SSF can produce high drinking quality water even with high initial turbidities.


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6.2 Future work

This research would benefit from supplementary study into the several areas,

presented briefly below.

Extended testing

This experimental set-up would benefit from a longer period of testing for three

reasons:

1)To enable the maturation of the filter leading to more reliable results.

2)The order of filtration rates could be randomised to counter any time-

dependency.

3)To establish intermittency.

Apparatus Modifications

This experimental set-up would benefit from two changes:

1)An adaptation to allow higher filtration rates.

2)A diffuser plate.

Microbiological testing

It would be interesting to see if these results reflect how the filter behaves when

tested with microbiologically contaminated water. It could also be incorporated to

test different influent water temperatures to see if that affects microbiological

removal in small-scale SSF.

SchmutzdeckeThe process of researching and designing the experimental set-up highlighted the

need for further research of the schmutzdecke and its properties. It would be

especially useful to investigate the relationship of ripening time with parameters

such as temperature and intermittency.


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http://h2oc.com/pdfs/Rapid_Sand_Filtration.pdfhttp://h2oc.com/pdfs/Rapid_Sand_Filtration.pdfhttp://publications.oxfam.org.uk/oxfam/display.asp?K=002H0009&sf1=series&st1=Oxfam+Humanitarian+Guidelines+and+Manuals&sort=sort_date/d&m=4&dc=13http://publications.oxfam.org.uk/oxfam/display.asp?K=002H0009&sf1=series&st1=Oxfam+Humanitarian+Guidelines+and+Manuals&sort=sort_date/d&m=4&dc=13http://publications.oxfam.org.uk/oxfam/display.asp?K=002H0009&sf1=series&st1=Oxfam+Humanitarian+Guidelines+and+Manuals&sort=sort_date/d&m=4&dc=13http://www.cruzroja.org/salud/redcamp/docs/aguasan-e/delagu.pdfhttp://www.cruzroja.org/salud/redcamp/docs/aguasan-e/delagu.pdfhttp://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Turbidityhttp://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Turbidityhttp://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Turbidityhttp://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Turbidityhttp://www.isope.org/publications/journals/ijope-02-4/abst-2-4-p318-KK-7-Smith.pdfhttp://www.isope.org/publications/journals/ijope-02-4/abst-2-4-p318-KK-7-Smith.pdfhttp://www.isope.org/publications/journals/ijope-02-4/abst-2-4-p318-KK-7-Smith.pdfhttp://www.statpac.com/surveys/statistical-significance.htmhttp://www.statpac.com/surveys/statistical-significance.htmhttps://icex.imperial.ac.uk/owa/redir.aspx?C=cc4e64ebbadf4ea08b38be0b986bc4ac&URL=http%3a%2f%2fwww.slowsandfilter.org%2findex.htmlhttps://icex.imperial.ac.uk/owa/redir.aspx?C=cc4e64ebbadf4ea08b38be0b986bc4ac&URL=http%3a%2f%2fwww.slowsandfilter.org%2findex.htmlhttp://tilz.tearfund.org/Publications/Footsteps+31-40/Footsteps+35/The+household+slow+sand+filter.htmhttp://tilz.tearfund.org/Publications/Footsteps+31-40/Footsteps+35/The+household+slow+sand+filter.htmhttp://tilz.tearfund.org/Publications/Footsteps+31-40/Footsteps+35/The+household+slow+sand+filter.htmhttp://tilz.tearfund.org/Publications/Footsteps+31-40/Footsteps+35/The+household+slow+sand+filter.htmhttp://tilz.tearfund.org/Publications/Footsteps+31-40/Footsteps+35/The+household+slow+sand+filter.htmhttp://tilz.tearfund.org/Publications/Footsteps+31-40/Footsteps+35/The+household+slow+sand+filter.htmhttps://icex.imperial.ac.uk/owa/redir.aspx?C=cc4e64ebbadf4ea08b38be0b986bc4ac&URL=http%3a%2f%2fwww.slowsandfilter.org%2findex.htmlhttp://www.statpac.com/surveys/statistical-significance.htmhttp://www.isope.org/publications/journals/ijope-02-4/abst-2-4-p318-KK-7-Smith.pdfhttp://www.isope.org/publications/journals/ijope-02-4/abst-2-4-p318-KK-7-Smith.pdfhttp://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Turbidityhttp://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Turbidityhttp://www.cruzroja.org/salud/redcamp/docs/aguasan-e/delagu.pdfhttp://publications.oxfam.org.uk/oxfam/display.asp?K=002H0009&sf1=series&st1=Oxfam+Humanitarian+Guidelines+and+Manuals&sort=sort_date/d&m=4&dc=13http://publications.oxfam.org.uk/oxfam/display.asp?K=002H0009&sf1=series&st1=Oxfam+Humanitarian+Guidelines+and+Manuals&sort=sort_date/d&m=4&dc=13http://h2oc.com/pdfs/Rapid_Sand_Filtration.pdf


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The Water Supply (Water Quality) Regulations 2000 (2000) SCHEDULE 1

PRESCRIBED CONCENTRATIONS AND VALUES Part II: National Requirements

[Online] Available from:http://www.opsi.gov.uk/si/si2000/20003184.htm#sch2

[Accessed May 2010]

Way, C. (2004) In-Tank Slow Sand Filtration, BEng Final Project Report, University

of Warwick
http://www.opsi.gov.uk/si/si2000/20003184.htm#sch2http://www.opsi.gov.uk/si/si2000/20003184.htm#sch2http://www.opsi.gov.uk/si/si2000/20003184.htm#sch2http://www.opsi.gov.uk/si/si2000/20003184.htm#sch2


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8. APPENDICES

8.1 Appendix 1: Detail of Experimental Procedures

Filter Depth: 35cm1. Ensure the water butt tap is turned off

2. Mix the influent turbid water

i. Calculate the amount of Light Kaolin Clay needed to add to 3 litres of water.

ii. Measure the correct amount of clay into a 50ml beaker.

iii. Mix well in beaker then add to 3l of tap water

3. Add 3l at a time through the funnel until 12l has been added to the water butt


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4. Then turn the water butt tap on

5. Add the final 6l of turbid water to the water butt

6. Take a test turbidity reading of the influent and effluent water not recorded

7. Allow to drain until the 0.4m/hr head level

8. Collect first sample

i. Standardise the turbidimeter to 100NTU

ii. Start timer

iii. Collect the sample from water on top of filter using a syringe

iv. Fill up a test tube and place in turbidimeter

v. Record reading


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9. Allow continuous flow for 5 minutes

10.Collect second sample

i. Standardise the turbidimeter to 1NTU

ii. Collect the sample from water on top of filter using a syringe

iii. Fill up a test tube and place in turbidimeter

iv. Record reading

11.Ensure that the test tubes are clean, rinse with Reverse Osmosis water, and dry

before taking readings

12.Allow drain until the 0.3m/hr head level

13.Repeat from Step 8.

14.Allow drain until the 0.2/hr head level

15.Empty the effluent water bucket when necessary

16.Once final reading has been taken turn water butt tap off and repeat from

Step 2.


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Filter Depth: 17cm

1. Drain water butt

2. Remove filter media until at the appropriate depth

3. Ensure the water butt tap is turned off

4. Mix the influent turbid water

i. Calculate the amount of Light Kaolin Clay needed to add to 3 litres of water.

ii. Measure the correct amount of clay into a 50ml beaker.

iii. Mix well in beaker then add to 3l of tap water

5. Add 3l at a time through the funnel until 6l has been added to the water butt

6. Then turn the water butt tap on

7. Add the final 6l of turbid water to the water butt

8. Take a test turbidity reading of the influent and effluent water not recorded

9. Allow to drain until the 0.4m/hr head level

10.Collect first sample

vi. Standardise the turbidimeter to 100NTU

vii. Start timer

viii. Collect the sample from water on top of filter using a syringe

ix. Fill up a test tube and place in turbidimeter

x. Record reading


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11.Allow continuous flow for 5 minutes

12.Collect second sample

v. Standardise the turbidimeter to 1NTU

vi. Collect the sample from water on top of filter using a syringe

vii. Fill up a test tube and place in turbidimeter

viii. Record reading

13.Ensure that the test tubes are clean, rinse with Reverse Osmosis water, and dry

before taking readings

14.Allow drain until the 0.3m/hr head level

15.Repeat from Step 8.

16.Allow drain until the 0.2/hr head level

17.Empty the effluent water bucket when necessary

18.Once final reading has been taken turn water butt tap off and repeat from

Step 2.


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8.2 Appendix 2: Results & Analysis

Preliminary Results

Date TimeFiltration Rate

(m/hr)Initial Turbidity


(NTU)

25-May15:47 0.4 4 25

16:04 0.3 2.75 18

16:20 0.2 5.5 13

16:32 0.2 3.3 13

16:50 0.3 3.7 15

17:05 0.4 4.5 18

26-Jun

11:30 >0.4 5

11:47 >0.4 20

12:12 >0.4 25>0.4 7.2

>0.4 5.5

12:20 >0.4 5.8

12:38 >0.4 2.8

12:41 >0.4 2

12:50 0.4 1.8 17

13:07 0.3 1.4 4

13:23 0.2 1.4 2.7

15:29 0.4 3.3 4.5

15:46 0.3 1.5 316:04 0.2 1.4 4.5

16:14 0.2 1.7 1.7

16:29 0.3 1.5 3

16:50 0.4 3.5 2.7


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First Experiment Set ~ 20NTU Initial Turbidity

Date Time TestFiltration Rate



(NTU)

27-May

12:08

1

0.4 14 1.7

12:24 0.3 13 2.2

12:46 0.2 15 2.1

15:10

2

0.4 17 2.8

15:35 0.3 18 2

15:50 0.2 15 2

16:30

3

0.4 23 3

16:50 0.3 20 3

17:05 0.2 19 1.9

28-May

11:12

4

0.4 17 2.4

11:32 0.3 16 2.6

11:46 0.2 14 2.5

13:44

5

0.4 15 2.2

14:05 0.3 13 2.7

12:27 0.2 13 2.1


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Second Experiment Set ~ 40NTU Initial Turbidity




(NTU)

01-Jun

10:06

1

0.4 41 3.2

10:22 0.3 38 2.5

10:41 0.2 33 2.2

15:10

2

0.4 31 2.3

15:32 0.3 33 3.4

15:50 0.2 28 3.5

16:28

3

0.4 37 2.8

16:51 0.3 38 217:10 0.2 30 1.1

02-Jun

10:25

4

0.4 38 3.4

10:46 0.3 34 2

11:03 0.2 32 2.7

14:23

5

0.4 32 3.1

14:42 0.3 30 3.2

15:01 0.2 27 1.4


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t-Test: Two-Sample Assuming Equal Variances

Variable 1 Variable 2

Mean 0.85211 0.92228

Variance 0.00076 0.00056

Observations 15 15

Pooled Variance 0.00066

Hypothesized Mean Difference 0

df 28

t Stat -7.47311

P(T


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Third Experiment Set 17cm Filter Depth




(NTU)

09-Jun

15:15

1

0.4 78 3

15:34 0.3 80 2.4

15:53 0.2 79 1.5

16:40

2

0.4 240 1.6

17:00 0.3 145 1.7

17:15 0.2 120 2.4

10:12

3

0.4 195 2.4

10:33 0.3 120 1.510:51 0.2 95 2

09-Jun

11:38

4

0.4 180 1.9

11:59 0.3 120 2.5

12:15 0.2 93 1.5

14:36

5

0.4 175 3.8

14:57 0.3 150 1.7

15:16 0.2 110 1.9


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