Sacha Journal of Environmental Studies, Volume 1 Number 2 ... · A further statistical analysis using least Square Deviation (LSD) and Dunnett statistic indicates that there is no

Sacha Journal of Environmental Studies, Volume 1 Number 2 (2011) pp. 103-129

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Comparative Study On The Filtration Properties Of Local Sand, Rice Hull And Rice Hull Ash

J.A IJADUNOLA1, I.O ADEWUMI

1, A.O ASHAYE

2, M.I OGUNTADE

2

and M.J OGUNLADE3

1Department of Agricultural Engineering, Federal College of Agriculture, P.M.B 5029, Moor

Plantation, Ibadan, Oyo-State, Nigeria 2Department of Crop Production Technology, Federal College of Agriculture, Moor Plantation,

Ibadan, Oyo-State, Nigeria 3Department of Animal & Dairy Science, Mississippi State University, United State of America

ABSTRACT

This paper investigates the potentials of some locally available materials namely local

sand, rice hull ash and rice hull, as a substitute for imported sand in raw water

filtration. The materials were subjected to various soil mechanics and hydraulics tests

including: particle size distribution, specific gravity, porosity, permeability and

filterability. The paper also investigated the physical characteristic of local sand, rice

hull and rice hull ash as filter media for water treatment, to investigate the filtration

performance or features of local sands, rice hull and rice hull ash, and; to compare the

filtration features of these three materials with existing standards.. The abilities of the

local sand, rice hull ash and rice hull to remove turbidity are found to be 21%, 58%

and 53%, while E. Coil removal is 41%, 94% and 100% respectively. Flocculation

pre-treatment only increased the turbidity removal using rice hull ask and rice hull,

while their E. coil was not affected. But for the local sand both the turbidity and E.

coli removal rates were 58% and 65% respectively. Significant difference was

established among these three materials using one way ANOVA at 95% confidence

level. A further statistical analysis using least Square Deviation (LSD) and Dunnett

statistic indicates that there is no significant difference in performance between rice

hull and rice hull ask, except between local sand and the other filter media considered.

Keywords: Rice Hull, Filtration, E. Coil

1. INTRODUCTION

The need for water treatment to have potable water for the rural dwellers is highly

important and need to be emphasised. The rural settings of developing countries like Nigeria,

are associated with a number of problems; namely: unlimited capital resources with nearly

unlimited demand for capital; lack of appropriate technology to suit the prevailing conditions;

the cost and availability of power, materials and labour is in reverse order; shortage of skilled

and trained personnel; inadequate facilities for repairs and maintenance etc.

The provision of safe and aesthetically acceptable drinking water to the community is of

vital importance for the maintenance of public health. The role of public water supplies that are

bacteriologically unsafe, as vectors of diseases, a vehicle for the spread of diseases and other

water – borne diseases have been established by many incidents and investigators. The genesis

and efficacy of water treatment to check water - borne diseases were convincingly

demonstrated, among other incidents, by the dramatic results of Altona, and Hamburg in


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Germany during the cholera epidemic of 1892. In these incidents, using water from the same

source viz. Teh Elbe River, Altona, which filtered its water supply, escaped entirely, while

Hamburg, which used the water unfiltered, suffered a severe outbreak of disease. Realising from

such incidents the importance of prevention being better than cure, treatment of water before its

consumption was initiated on a wider scale, especially in developed nations. Depending on the

prevailing transmission pathways, different intervention in water supply and sanitation are

required. More often, most of these diseases are transmitted in drinking water, thus making the

quality of drinking water of highest importance. The presence of a safe and reliable source of

water is thus an essential pre-requisite for the establishment of a stable community.

Man eventually devised various complex ways, which are either complementary or

supplementary to each other, in order to make raw water potable. The processes are collectively

referred to as the water treatment process or sections, which include: Sedimentation,

Coagulation, Filtration, Disinfection, Softening and Aeration among others. This has been

satisfactorily established and adopted in all the developed countries of the world and most urban

centres of developing countries. However, the adoption of this technology for the growing

population of the developing world (rural areas to be specific) is becoming a challenging task.

Because of the imposed constraints of paucity of financial resources coupled with the restricted

availability of skilled technical personnel and sophisticated equipment, two options are evident

viz: new water treatment methods have to be evolved or existing one modified. The main goal is

the development of water treatment unit, which is cheap, simple easy to construct, maintain and

operate. The units should not require either the incorporation of sophisticated equipment or

attendance by skilled technical labour.

Among the various unit operations of conventional water treatment plant, filtration

occupies a central and important place and perhaps the oldest and most widely used in the water

purification treatment (Schroeder, 1977). The occurrence of this method in nature had made its

early appreciation and utilisation possible.

The filtration process involves the removal of suspended particles, colloidal materials,

bacteria and other organisms by combination of hydraulic and mechanical straining process.

Basically the process of filtration consists of passing the water through a bed of sand, or other

suitable medium at low speed. In order to achieve the final degree of clarity, the influent water

from the settling basins must be of fairly low turbidity. This degree of settling may vary with

the type of filter adopted. Filtration is most effective when used as a final treatment process

after the use of sedimentation and/or coagulation, and can be loosely classified as pre-coat

filtration or depth filtration. The former involves the use of particulate coating on thin materials

support media such as cloth or finely woven wire while the latter utilizes a permanent, relatively

thick medium usually granular of varying porosity and density.

Depth filtration is of greater significance in water and wastewater treatment. It utilises

such media as silica sand, anthracite coal, resin, and some local materials. However, silica sand

is mostly used in graded form though the recent practice involves the use of multi-media filters,

where two or more of the above materials are combined. A primary factor in choosing filter

materials is its resistance to abrasion. This is a major factor during back washing. Sand is used

as filtering medium in the following filters, namely: Rapid sand, Pressure, Slow sand, simple

domestic, Intermittent sand filter etc.

The efficiency of filtration depends large on the filter media. Filter media are officially

standardised when the traditional sand layer over gravel is used or finely crushed anthracite coal

or mixed media of the above or diatomaceous earth is used (De Zuane, 1990).

2. THEORETICAL BACKGROUND


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This section examines the theory guiding the experiments needed to be conducted and

their relevance to the study for a better understanding of principles and interpretation of data.

2.1 PARTICLE SIZE ANALYSIS

The range of particle sizes encountered in soils is very wide: this varies from 200mm

down to the colloidal size of some clays of less than 0.001mm. Although, natural soils are

mixtures of various-sized particles, it is common to find a predominance occurring within a

relatively narrow band of sizes. A number of engineering properties; e.g. permeability, frost

susceptibility, and compressibility are related directly or indirectly to particle size

characteristics.

The particle size analysis of a soil sample involves determining the percentage by

weight of particles within the different size ranges. The particle size distribution of a coarse-

grained soil can be determined by the method of sieving. A representative sample of the sample

of the sand is split systematically down to a convenient sub-sample and then oven-dried. The

sample is then passed through a series of standard test sieves arranged in descending order of

mesh size. Following agitation from the whole nest and then individual sieves, the weight of soil

retained on each sieve is determined and the cumulative percentage of the sub-sample weight

passing each sieve calculated. Where the soil sample consists of fine-grained particles, a

hydrometer analysis is used. The hydrometer a test is a sedimentation process based on stoke’s

law which governs the velocity at which spherical particles settle in suspension.

There are several classifications and descriptions given according to different

organisations depending on the particles size ranges. A typical classification referred to as

British standard is shown in Table 3.1. The particle size distribution of a soil is represented as a

curve in a semi-logarithmic plot, the ordinates being the percentage by weight of particles

smaller than the size given. The grading curve is a graphical representation of the particle size

distribution and is therefore useful in itself as a means of describing the soil. For this reason, it

is always a good idea to include copies of grading curves in laboratory and other similar reports.

It should also be noted that the primary objective is to provide a descriptive term for the type of

soil. This is easily done by estimating the range of sizes included in this most representative

fraction of the soil.

A further quantitative analysis of grading curves may be carried out using certain

geometric values known as grading characteristics. Initially, three points are located on the

grading curve to give the following characteristic sizes.

d10 = Maximum size of the smallest 10% of the sample.




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Source: Whitlow (1998)

From these characteristic sizes, the following grading characteristics are defined:

Effective size = d10 (mm)

Uniformity Coefficient, Cu = d60............................ 3.1

d10

Coefficient of graduation, Cg = (d30)2...................3.2

d60 x d10

One useful application is an approximation of the coefficient of permeability K, which

was suggested by Hazen. The coefficient of permeability K = Ck (d10)2 m/s; Weir Ck is a

coefficient of variation between 0.01 and 0.015. The parameter d10 and Cu are very important

in the choice of a suitable material as a filter. The percentages of a given material that is usable

can also be obtained from the parameter d10 and d60.

2.2 SPECIFIC GRAVITY

The specific gravity of the soil particles (Gs) is the ratio of the mass of a given value of a material

to the mass of the same volume of water at 40C.

Gs = Ms or ls ................. 3.3

Vslw lw

Where Ms is the mass of the one unit of solid volume

Lw is the density of water which may be taken as 1000kg/m3.

The determination of the specific gravity is imperative because it is used in calculating some other

parameters such as porosity, permeability, the equilibrium head loss etc.

2.3 SOLUBILITY

Sand cannot be affected to any appreciable (or noticeable) extent by acids because it is

mainly SiO2 compound. When soaked in an acid a change in the weight of the sand is usually

noticed in minutes. Any high or noticeable change in the weight of sand raise doubts about its


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purity as this suggests that the change in the weight is a representation of the impurities which

cannot be mechanically removed by washing but are now either dissolved or burnt by the acid.

Therefore, a sand sample that has a large solubility value is not good for filter medium as acids

are usually formed in water. If the initial weight of sand is Wi and the final weight is Wf then,

the solubility can be estimated from:

Solubility = 100 (wi – wf) .......................... 3.4

Wi

2.4 POROSITY AND PERMEABILITY

The porosity (n) of a material is a measure of voids (Vv) present in a given volume (V) of soil

materials that is the ratio of the volume of voids to the total volume of the soil.

n = Vv or V – Vs ........ 3.5

V V

Porosity of soil material is a major factor in determining the flow through such

materials. This flow through a porous medium is a common phenomenon occurring in

groundwater flow, seepage and infiltration; dewatering of slurries and sludge in industries;

clarification of industrial liquids, sewage treatment and water purification. In all the above cases

the flow rate is proportional to the pressure drop (head loss), given by Darcy’s law, where the

constant of proportionality is the permeability. In saturated conditions, one – dimensional flow

is governed by Darcy’s law, which states that the flow velocity (V) is proportional to the

hydraulic gradient (i).

V = -Ki.........................3.6

V = -K dh....................... 3.7

Dl

Where dh is the difference in total head over a flow path of length dl.

The permeability concept is a characteristic of both fluid and the porous media. A number of

appropriate empherical relationships have been suggested between permeability K and other soil

properties

e.g. K α d10; K α e2 (e = void ratio)#

1 + e

It is clear from comparative studies, however, that none are particularly reliable and that

it is far more realistic to obtain estimates for K using field pumping tests or a laboratory method

(Whitlow, 1998). The most frequently used approximation is one suggested by Hazen for filter

sand.


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K = Ck d210m/s................................. 3.8

Experimental evidence suggests that acceptable approximate values for K can be

obtained when Hazen formular is applied over a wide range of soils. Table 3.2 gives the range

of suggested values for the coefficient Ck

2.5 FILTERABILITY

Deep beds of porous granular media are in widespread use in municipal and industrial

practice to filter liquids to improve their clarity. Prominent among these uses is the filtration of

drinking water and industrial water, although the filtration of sewage as a tertiary stage of

treatment is increasing. Other liquids are also being filtered through granular media in the

processing of beverages and food products. Filterability is not a property of just suspension, but

is an interactive property between a suspension and some filter media. If the properties of one of

these say a standard medium is kept constant then changes in the filterable if it can pass rapidly

through a porous medium, giving a clear filtrate with little clogging of filter medium clogging

is reflected in the loss of permeability, as seen in the increase in pressure drop.

A simple measure of whether the liquid is filterable is useful, to enable assessment of

whether filtration is an appropriate process, and if so what type of pre-treatments and filter

medium required. Although the normal methods of chemical and physical analysis may with

experience indicate whether a suspension may be filtered, they give no direct measures of this

property. The early researchers as reported by Sangodoyin (1981), have proposed a number of

measures of filterability. Such include; usage of woven Micromesh, or lint pads, or membrane

filters, which have application where these materials represent the filter media to be used. The

weakness observed in using these membrane or similar thin filters to determine a filterability

index for granular filter is that the membrane results do not indicate the depth removal that may

or may not be achieved with the granular filter and they are not applicable to deep bed filtration.

Other techniques or approaches as propagated by researchers are the use mathematical equation.

(i) Filter performance index (FPI) given as:

FPI = Filtration rate x length of filter run

Headloss at the end of filter run [3.9a]


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(ii) Filterability index FI = ᵞ x ᵟ

Headloss mg/L-cm2. [3.9b]

Where ᵞ is impediment modules, and ᵟ is specific deposit.

(iii) The filterability Number F given as;

F = H C

V C0 F t [3.9c]

Where H is the head loss (liquid gauge)

C is the average filtrate quality

C0 is the inlet suspension quality

V is the approach velocity (Volumetric flow rate per unit face area).

T is the time of the filter run.

This dimensionless parameter F takes into account the quality of the influent and

effluent, loss in permeability of clogged filter medium and resulting increases in head loss. C/C0

is a ratio. The unit’s frequently used for C and C0 include turbidity, coliform number and total

suspended solids, provided that they are maintained for both inlet and outlet.

The actual numerical value of F has no significance attached to it but relative values of

F indicate relative filterability. For example a pre-treatment of suspension, which decreases F

gives rise to better filterability, even though nothing could be inferred from numerical value of

the minimum F. However, a low F-number expresses a good filterability. For a good

filterability, the numerator should be low, with a low head loss (clogging) and loss filtrate

concentration. Also the denominator should be high flow rate (approach velocity), accepting

high inlet concentration during a long time of operation.

One other important factor would be the limit F could have in order to produce

satisfactory values of which something could be inferred. The F-number has not been derived

vigorously from filter theory therefore; it cannot be used at extreme value, or with boundary

conditions; for example, the extreme value or with boundary conditions. For example, the

extremely high value of V. E.g. 2m/min though might give low F values; they would probably

lead to C/ C0 approaching 1. This means that there is no filtration and no clogging, which is

unrealistic operation. Also with very coarse media a negligible value of H would result,

implying C/ C0 approaching 1 that is no filtration.

2.6 METHODOLOGY

2.6.1 SAMPLE COLLECTION AND PREPARATION

The three materials used for this work include the local sand (sample I), Rice hull ash

(sample II) and rice hull (sample III). Sample I was collected from the bed of the river Ara,

which is the major sand deposit rive in Offa, Kwara State. The collection was done manually

with hand trowel into a porous sack, so as to allow water to drain easily. Thereafter, the sand


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was thoroughly washed, sun dried properly packed and made ready for experiments. This

washing is necessary to remove all organic materials, dirts and rubbish that may be present in

the sample.

Sample II was collected at a rice-milling farm in Offa Kwara State. No thorough

cleaning was carried out except the handpicking of any visible dirt from it. The sample was then

air-dried in a girded manner to prevent being blown away by air and was later bagged for use.

Sample III was prepared from part of sample II. After being air-dried, it was packed

inside a drum have its upper ends opened and burnt to ashes. The incineration of rice hull took

up two days. Later, it was allowed to cool for the next 24hrs. Thereafter, the ashes were

collected in a bag and made ready for experiments.

2.6.2 DETERMINATION OF PARTICLE SIZE DISTRIBUTION

Apparatus * Complete set of sieves

* Shaking machine

* Weighing balance

The sieve analysis was performed with sieve numbers 7, 25, 36, 52, 60, 72, 100 and 200

that is in accordance with the U.S standard. Sample I was weighed and poured into the sieve.

The set of sieves was then vigorously shaken using a shaking machine for ten minutes (see plate

I). The sieves were then dismantled and the quantity of the sample retained in each of the sieve

was weighed on weighing balance and recorded. The same procedure was repeated for sample II

and III. The results of the sieve analysis are presented in Table 5.1.

2.6.3 SPECIFIC GRAVITY

Apparatus * Measuring cylinder

* Weighing machine

Some quantity of water was poured into the measuring cylinder and the height inside the

cylinder was noted and recorded. 50 grams of the sample material was then poured into the

measuring cylinder containing water, stirred properly and left for five minutes to settle before

reading the new height of water in the cylinder. The difference in these readings gave the

volume of quantity of the same material that was added. The experiment was repeated three

times for each of the sample material and the average volumes used to determine their

respective relative densities (special gravities).

2.6.4 SOLUBILITY

Apparatus used include; * Crucible dishes

* Weighing balance,

* Dilute H2So4 and Dilute HCI.


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The samples were prepared using the materials finer than 2.36mm but courser than

0.25mm. A known weight of each sample was put in a crucible dish, some quantity of dilute

tetraoxosulphate VI acid (0.4NH2So4) was added and stirred very well to ensure proper mixing.

Samples were washed clean of the acid, allowed to dry and then re-weighed to determine the

loss in weight. This procedure was repeated three times and the average loss in weight obtained.

The same experiment was performed on the three samples using the dilute hydrochloric acid

(0.6 N HCI) and the average loss in weight.

2.6.5 POROSITY AND PERMEABILITY

Apparatus * Permeability apparatus

* Glass beaker and Measuring Cylinder#

* Stop watch

* Sieves (No. 20 and 30)

* Weighing balance

The falling Head permeameter is used for measuring the permeability of clays, silts and

fine sands. It consists of non-ferrous sample cylinder with a cutting edge. The most common

size is 101.6mm internal diameter and 101.6mm effective height. Other sizes used are 76mm

diameter and 101.6mm height, 76mm diameter and 76mm height and 60.3mm diameter sample

tubes. Number 200 B.S mesh brass gauze fits over the top and bottom of the cylinders and

supports the sample. The cylinder is mounted in a cage, consisting of a perforated circulated

brass base supporting three vertical tie rods, threaded at their upper ends. Butterfly nuts hold

down a non-ferrous cap, which is threaded to take central nipples of various sizes.

The apparatus also has a vessel containing de-aired water in which the sample cylinder

stands during the test and it is fitted with a weir overflow. A number of calibrated and graduated

standpipe tubes, graduated with a minimum of three marks were also connected to the

apparatus. Each tube of uniform internal diameter along its length is of a different internal

diameter, decreasing from about 2 ¼ cm to ¼ cm. Any tube can be connected through a series of

valves to the nipple on the sample cylinder cap, and to a reservoir, which is supplied air free

distilled water.

The following is the procedure for conducting this experiment as highlighted in the

apparatus manual. The average grain size of 0.715m of the dry sample was poured into a beaker

and little water is added to eliminate air trapped. These sample materials were treated as

cohensionless materials. First, the sample is compacted to the requested density after which the

sample is treated as undisturbed sample and trimmed to fit the sample container/cylinder.

Thereafter the sample was trimmed in the cylinder, then weighed and recorded. The

cylinder was mounted into the caps, with gauze on top and bottom. Above the gauze at the top,

the cylinder was packed with steel wool and the cap screwed down. The cylinder was then

placed in a soaking tank, which was slowly filled with air-free treated water. At the same time a

rubber tube (hose) connected the cap, to a vacuum line and a small vacuum of two or three

inches of mercury was applied. The rubber tube has a section of glass tube in it that acted as a

sight glass. The treated air-free water was carefully placed in the tank so as to avoid aeration by

agitation as the tank is filled. The vacuum in the sample draws air from the sample and pulls air-

free water up into the sample, thus completely saturating it and operation was carried out slowly

to avoid air being trapped in the sample. The vacuum was maintained until no more air is seen

to rise from the sample.


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The air-free sample completely saturated was then connected up through tubing filled

with air-free treated water in the selected standpipe tube, which has also been filled with air-free

treated water, to slightly above the starting head, which is indicated by the upper graduation on

the tube. For the start of the test, water was allowed to fall in the stand-pipe tube and hence

through the sample and the times required for it to pass the successive graduations on the tube

was recorded. The tube was then re-filled with water and the test repeated four more times, the

times being recorded during each test.

The whole procedure was repeated for the other sample materials being tested. The

initial head of water in the standpipe tube H1, head of water indicated at the end of a particular

period of time H2, starting time t1 and time t2 corresponding to H2 were recorded during the

test for each of the samples. The dry (bulk) densities of each sample were determined from the

dry weight of the sample, at the end of the test.

2.6.6 DETERMINATION OF FILTERABILITY

Apparatus * Filterability apparatus

* Glass beakers

* Thermometer

* Stop clock

* Funnel (100mm)

The filterability apparatus consists of Perspex column (A), 38mm bore, 65mm long with

inlet and outlet connections so that liquid flows downward through the column. At the base of

the column is a 0.5mm brass gauze mesh (B.S. 30 sieve mesh) to retain granular media. The

top-capping piece to the column can be quickly removed by unscrewing the knurled screw. It

has a small air release screw at the top.

Liquid suspension is introduced in this column from a 1.5 litre capacity glass conical

funnel (B), and flows from the base unit of the Perspex column through a flow control valve (c)

and a G.A. Platon Gap meter, flow meter (D) to drain. The flow control valve is needle-type, the

flow rate is indicated by the top level of the float in the flow meter (range 0.0012-0.018m3/s).

The capping piece above the column and the base unit below it are made of rigid PVC,

and are both connected to glass tube liquid manometers (E). The difference in level gives the

head loss (liquid gauge) directly, although the manometer tubes are connected at the top, to keep

the air pressure above atmospheric. The value and tubing connectors are made of chrome plated

brass. All tubing is 9mm bore translucent plastic except the manometer connections that are

7mm bore. Details of the filterability apparatus are illustrated in fig. 4.1.

The filter media are pre-sieved to a uniform size fraction, that is materials passing

0.60mm ᵠ (B.S. 25 mesh) and retained on 0.50mm ᵠ. A quantity of media is weighed in the dry

state, to provide a bed 40mm deep in the Perspex column (A) and a reproducible porosity of 0.4

for sand. The apparatus was filled with clean water to remove air bubbles. This was achieved by

reverse flow filling through the drain outlet tube with the help of a small funnel. During this

preliminary operation, the manometer air release plug was opened to allow liquid to rise in the

manometer tubes, appropriately to the 250mm mark, then the air-released plug closed.

A litre of test suspension was poured carefully into the inlet funnel. The control valve

(c) was then adjusted to give the required flow rate (93cm3/min) on the flow meter (D). About

40 seconds was allowed for the displacement of the clean water above the inlet surface of the


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bed of porous media. The stop clock was then stared to obtain the filterability test run time t. A

further time of 75 seconds was allowed for displacement of the cleaning liquid in the filter

pores, base unit of the column and the outlet tubing before the filtrate was collected in the 1L

beaker for quality analysis (See plate 2).

While the suspension water was filtering through the porous media, a sample was taken

from the inlet funnel for analysis Co. A thermometer was inserted in the filtrate-receiving

beaker, to determine the temperature of the filtrate. During the filtration, the control value was

continuously adjusted to ensure a constant flow rate as clogging of the filter media and falling

inlet level tends to reduce the flow rate. The difference in the manometer levels is the head loss

across the porous media and the support gauze mesh, which was read when the inlet suspension

level has fallen to the base of the inlet funnel, and the timing clock stopped giving time t.

The whole process was then repeated but with new test water prepared by adding single

5mg/1 alum (rapid mix for 15 seconds) to the original test suspension water. The two

experiments were performed on each of the sample for three times and their mean values were

performed on each of the sample for three times and their mean values were used for

calculation.

3. RESULTS

3.1 PARTICLE SIZE DISTRIBUTION

The result of the sieve analysis carried out in order to determine particle size

distribution is as shown in Table 5.1. Using the data, the percentages passing through the sieve

were plotted against sieve sizes to obtain a particle size distribution curve as shown on Fig. 5.1.

From the curve, effective size d10 (10th percentile), 60

th percentile (d60) were read and

uniformity coefficient, coefficient of graduation and other parameters were evaluated. Similarly,

the followings were determined and corresponding values were read on the particle size

distribution curve.

(a) The percentage usable Pu, from du = 2 (d60 – d10)

(b) The percentage fine Pf; from df = d10 – 0.1du

Or df = d10 – 0.2 (d60 – d10)

(c) The percentage Coarse Pc, from

dc = du + df

Or dc = d10 + 1.8 (d60 – d10)

The summary of the parameters calculated is presented in Table 5.2. From these results,

it could be seen that the three sample materials considered have their effective sizes and

uniformity coefficient outside the standard range of 0.35<d10<0.70mm and 1.2<Cu<1.7. These

values are commonly quoted for filter media in water treatment as specified in Standard

Handbook of Environmental Engineering (Robert, 1999). The discrepancy in effective sizes and

uniformity coefficient does not nullify the use of these materials as filter media. All it indicates


114

is that the materials are too fine and uniformly graded. Therefore, there will be a need for

careful screening and adequate grading of the materials before they can be used effectively as

filter media. However, the percentage usable obtained from the three samples vis.: sand (69%),

rice hull (95%) and rice hull ash (66%) are satisfactory. This is an indication of maximization in

the usage of the materials.

3.2 SPECIFIC GRAVITY

The specific gravity (Gs) of any substance as earlier defined is the ratio of its weight in

air to the weight of an equal volume of water at 40C. This was calculated using the relation;

Gs = M

Vsβw

The average specific gravity for each of the samples is presented in Table 5.3. The

densities of all samples are higher than that of water with sand averaging 2.57g/cm3. Rice hull

ash and rice hull samples are much lighter when compared with the local sand sample.

Therefore, precaution must be taken when using rice hull ash and rice hull as filter media, so

that these materials do not get fluidised. The density parameter is an indication that during


115

backwashing of the filter media, those with low densities will require less critical fluidization

velocity or force for bed expansion. It becomes more difficult to separate the particulate, since it

is collected over water during this process. However, the low densities of rice hull ash and rice

hull does not suggest that it cannot be used as filter material. Their densities are still very close

to that of anthracite Coal (1.4 – 1.6g/cm3) that is considered to be a good filter material and

commonly used (Robert, 1999).

Statistically, the differences in the densities of the samples were verified using one-way

Anova. The test shows that at 95% confidence interval, the densities of the three samples were

significantly different. A further test was carried out to identify interaction between their means

(mean separation) using Least Square Deviation (LSD) and Dunnet t – test, taken sand as a

control. These indicated a significant difference between sand and rice hull and between sand

and rice hull ash. Between rice hull ash and rice hull, there is no significant difference at this

confidence level (Table 5.4a and 5.4b).

Table 5: Relative Densities of Sample Materials

Sample Mass (g)

Average

Volume

Change in

specific density

∆V(cm3) (g/cm

3)

I 50 19.5 2.57

II 50 34 1.47

III 50 38.33 1.3

Table 6: Oneway Anova for Specific Gravity

ANOVA

Sum of squares df Mean square F Sig

Specific Between

Gravity Groups

Within

Groups

Total

2.801

4.7E-02

2.849

2

6

8

1.401

7.9E-03

177.545 .000


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Table 7: Post Hoc Tests

• The mean difference is significant at the .05 level.

a. Dunnett t-test treat one group as a control, and compare all other groups against it.

3.3 SOLUBILITY

The percentage of the samples that are soluble in H2 SO4 and HCI were calculated using the

relationship below:

Solubility = 100(wi – wf)

Wi

The mean solubility of the various sample materials were determined and tabulated

(Table 5.5). The solubility of the sand sample was very good as the loss in weight in H2SO4

and HCI is insignificant. The rice hull sample solubility was very high (see table 5.5), and was

still within the allowable 10% maximum. The solubility of the rice hull ash was a little bit

higher than the allowable standards in each of the solvent viz.: 10.7% and 10.2% respectively in

H2SO4 and HCI. This may suggest the presence of large impurities in the rice hull and rice hull

ash samples. Moreover, the materials are bio-degradable. They cannot be used for a longer

period unlike sand before they are replaced.

Statistically, there is a significant difference in the solubility of the samples in H2 SO4

(see Table 5.6a). Further test on mean separation between the group by LSD and Dunnett show

no significant difference in between rice hull and rice hull ash (Table 5.6b). Within the samples,

Anova show a significant difference at 95% confidence interval for the solubility in HCI, while

between each of the samples there is a significant difference (Table 5.7a and 5.7b).


117

Table 9: Oneway Anova for H2SO4

ANOVA

Sum of

squares

df Mean

square

F Sig

Solubility Between

In Groups

Sulphur Within

Acid Groups

Total

175.241

7.862

183.103

2

6

8

87.620

1.310

66.865 .000


Multiple Comparisons


a.Dunnett t-test treat one group as a control, and compare all other groups against it.


118

Table 11: Oneway Anova for HCI

ANOVA

Sum of

squares

df Mean

square

F Sig

Solubility in Between

hydrochloric Groups

Acid Within

Groups

Total

154.906

13.260

168.166

2

6

8

77.453

2.210

35.047 .000





3.4 POROSITY AND PERMEABILITY

The porosity of the samples were determined using the relation

n = Vv

V

The procedures together with other parameters are detailed in Appendix I. The summary

of the porosity results is given in Table 5.9. The data from the falling heading permeability test


119

is presented in Table 5.8a-5.8c. The values of Log. H (where H is the head of water) were

plotted against time for each sample as shown in Fig. 5.2-5.4.

A linear relationship is always an ideal case, but there is a kink in these graphs thus

indicating a discrepancy in the testing procedure. A comparison was made with this rate of

decline in the head of water, using mean values of time as reflected in Fig. 5.5. Results indicate

that it takes more time for the same head of water to fall in local sand, followed by rice hull ash

and rice hull. This is an indication that rice hull is more permeable to water than others.

The coefficient of permeability values were also determined using the relation;

K = 2.3026 a.l log10 (H1/H2)

A t2 – t1

S = log10(H1/H2), i.e. the slope of graph

T2 – t1

Therefore,

K = 2.3026 a.l x s

A

The mean values of porosity and coefficient of permeability are presented in Table 5.9

and these follow a particular trend. They are in agreement with theoretical expectation that,

porosity is directly proportional to permeability. These values fall within the ranges of values

expected i.e. porosity ranges from 0.38 for spherical shape materials to 0.48 for crushed

materials while permeability ranges from 1.0cm/s for coarse sand to 0.001cm/s for fine sand

(Berida, 1994). Statistically, one-way Anova show that, there is a significant difference between

the three samples at 95% level of confidence. A post-hoc tests (i.e. LSD and Dunnett) confirmed

this significant difference when samples are grouped together i.e compared with one another

(See Fig 5.10a and 5.10b).

The porosity and permeability parameters are very important in the choice of a suitable

filtering material. This is because if permeability is too high, no meaningful filtration can take

place and if too low, the bed gets easily clogged and it becomes uneconomical to operate since it

will require frequent backwashing.


120


121

Table 17: Oneway Anova for Permeability

ANOVA

Sum of

squares

df Mean

square

F Sig

permeability Between

Groups

Within

Groups

Total

44.777

1.026

45.803

2

12

14

22.388

8.6E-02

261.771 .000


122




4. DISCUSSION

4.1 FILTERABILITY TEST

After the sets of experiments, the filterability numbers of filter index of the various

media samples were determined. The data collected and procedure taken in calculation are

detailed in Appendix II. The results are tabulated in Table 5.11a and 5.11b.

The ability to filter using the rice hull ash and rice hull in terms of turbidity removal

appears encouraging as turbidity values of <5NTU were recorded, even without any pre-

treatment. Sand does not appear to be good as compared with the other two samples, since it

recorded turbidity of about 7NTU. This observation is in agreement with other investigators on

local sand (Adeyemi, 1984; Berida, 1994). But in the flocculation process, the turbidity removal

is greatly enhanced for the three samples. Sample II (Rice hull ash) has the best turbidity

removal in the two processes with 58% without pre-treatment and 63% with flocculation

process.


123

The specific E.coli removal ability of the samples was also determined. The results

(Table 5.11a and 5.11b) show that rice hull and rice hull ash filter media removes E.coli bacteria

far better than sand. This is also in agreement with the finding of Barnes and Mampitiyarachchi

(1983) as reported by chaudhuri and Sattar (1990) that filter made of rice hull ash removed

turbidity better than sand filter. Between 90-99% E.coli was removed by rice hull ash as

compared to 60 – 96% removed by sand filter. However, considering the headloss parameter,

rice hull ash recorded a high headloss, which shows that the filter is easily clogged. This

indicates that only a small portion of the top layer is actually involved in the filtration process,

which becomes easily clogged. Whereas, if the clogged layer is either disturbed or removed, the

headloss will be low. Generally, the headloss and the time of filtering 1litre of raw water

through the three samples were considered satisfactory compared with ranges given in the W4

apparatus manual as standard.

As earlier stated, no particular significance can be attached to the numerical value of the

filterability number (F), but relative values indicate relative filterability. Therefore, the

filterability number of all the sample materials can only be compared with one another. This

was successfully carried out statistically by one-way Anova. ANOVA test shows that there is

significant difference within the filterability numbers of the samples. Mean separation analysis

of LSD and Dunnett t-test show that though there is a significant difference at this level of

confidence within the samples but between sample II and III there is no significant difference in

the index (see Table 5.12a and 5.12b).

Moreover, for flocculated effluent, statistically there is no significant difference at 95%

level of confidence neither within nor between the samples as shown in Table 5.13a and 5.13b.

The filtrate from the filter media were closely observed and it was noticed that the filtrates from

the rice hull ash media had some objectionable odour. Except this questionable odour in rice

hull ash, rice hull ashes are still considered to be better off than local sand in terms of

filterability.


124

Table 21: One-way Anova for Filterability without Pre-treatment

ANOVA

Sum of

squares

df Mean

square

F Sig

Filterability Between

A Groups

Within

Groups

Total

20808.7

945.333

21754.0

2

6

8

10404.3

157.556

66.036 .000






125

Table 23: Oneway Anova for Filterability without Pre-treatment

ANOVA

Sum of

squares

Df Mean square F Sig

Filterability Between

A Groups

Within

Groups

Total

380.667

435.333

816.000

2

6

8

190.333

72.556

2.623 .152





5. CONCLUSION

The following conclusion can be drawn from the experimental study:

• In this study, rice hull, rice hull ash and local sand have their effective sizes and uniformity

coefficient outsides the standard range of 0.35<d10<0.70mm and 1.2<u<1.7 respectively.


126

Thus for suitability and effective use, careful screening and re-grading of these materials

are required.

• Local sand has solubility in the range 0.28 – 0.68% in acid and it can be considered very

clean and suitable for treatment of water with low pH. Rice hull and rice hull ash solubility

range from 7.15 – 9.2% and 10.2% - 10.7% respectively. These are considered to contain

impurities and not suitable for treatment of water with low pH value.

• The porosity, permeability and filterability of all the materials followed the same trend and

found to have direct relationship with media sizes (particle size) and density, as established

by other investigators.

• The results of the turbidity and E.coli removal show that rice hull has 53 and 100%

respectively and rice hull ash with 58 and 94% removal respectively, are better than local

sand media with 21% turbidity and 41% E.coli removal.

• A flocculation pre-treatment process has partial effect on the rice hull and rice hull ash

effectiveness as filter. The effect was pronounced on local sand medium. However, the

overall effect of flocculation is noticed in reduction of headloss, thereby reducing clogging.

• The overall view of the study shows that rice hull and rice hull ash have greater potential

for water filter media than local sand, except for the objectionable odour noticed in the rice

hull ash filtrates. ANOVA and post-hoc tests have shown no significant difference between

rice hull and rice hull ash but between local sand and others.

6. RECOMMENDATIONS

In view of the findings and observations in this study and for further research, the following

suggestions and recommendations are made:

• W4 filterability apparatus used in carrying out filterability tests on the materials as earlier

stated is not intended for filter design purposes but to serve as preliminary assessment of

filter media. Therefore, a further study of these filter media using a pilot plant of standard

column is required. This is to determine their durability and filter media design bases.

• The local sand from different location may show significant difference in performance as

filter media. A comprehensive study of such should be undertaken on rice hull and rice ash

from different location and species. This is to be very sure of any variation in these

potentials and to guide against any error that might likely come-up when results are

applied.

• Government should formulate and implement policy on the use of appropriate technology

in water treatment. This will reduce some over – burden or extra cost in the importation of

materials or technology and create market for local industries, as well as encourage the

researchers.


127

REFERENCES

Adeloye,O. (1986)- The Geochemistry of Groundwater from Boreholes in Ibadan. Nigeria

Journal of Engineering. 5(4) pp 16-25

Adeyemi, S.O. (1980) – “Local Materials as Filter Media” 6th WEDC Conference, Water and

Wastewater Engineering in Africa, pp. 153 – 156

Adeyemi, S. O (1984) – “Development of Design and Operating Specification for Local Sand as

Filtration Media” Nigeria Journal of Engineering Vol.2, N0 5 pp. 16 – 28

Armfiled Technical Education Company Ltd., Manual on W4 Filterability Apparatus

Baker, M.N (1948) – The Quest for Pure Water” The American Water Works Association Inc.

U.S.A.

Berida O.T. (1994) – “Filtration Properties of Local Sands” Unpublished B.Sc. Civil Engr.

Project University of Ibadan.

Chaudhuri M. and Sattar, S.A. (1990) – Domestic Water Treatment for Developing Countries.

Drinking Water Microbiology Reports. Springer – New York Inc. 168 – 184.

CWC Internet Website www.cwc.org. (1997) Evaluation of Crushed Recycled Glass As a

Filtration Medium in Slow Rate Sand Filtration (No. GL-95-4)

De Zuane, P.E. (1990) Handbook of Drinking Water Quality Standards and Controls. Van

Nostrand Reinhold, New York. Okun, D.A (1968) – “Element of Water Supply

and Wastewater Disposal” Willey and Sons New York

Huisman, L., Van Gorkum, N., and Kompahan K. (1982) – Water Supplies in Rural Areas of

Developing Countries. Pp 147 – 185 in Reid, G.W (editor), Appropriate Methods

for Treating Water and Wastewater in Developing Countries; Ann Arbor Science

Ann Arbor.

Josh,N.S.; Kelkar, P.S.; Dhage. S.S., Paramasivam, R. And Gadkari, S.K. (1982) Water Quality

Changes During Slow Sand Filtration. Indian Journal of Environmental Health.24:

261 – 276

Logsdon, G.S. and Lippy E.C. (1982) – The role of Filtration in Preventing Waterborne Diseases.

Journal of the American Water Works Association 74:649-655

Logsdon, G.S (1990) – Microbiology and Drinking Water Filtration. Drinking Water

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WEDC

conference-Reaching the Unreached: Challenges for the 21st century, New Delhi,

India pp. 275 – 278

Micheal, M.I. (1986) – “Local Materials as Filter Media” Unpublished B. Eng (Civil Engr)

Project University of Ilorin, Ilorin.

Muhammad, N.; Parr, J., Smith, M.D.; and Wheatley, A.D. (1997) Removal of Heavy Metals by

Slow Sand Filtration 23rd

WEDC Conference Durban, South African. Pp. 167 –

170

Oni – Orisan, I.Y. (1984) – Fluidization Properties of Local Sands. Unpublished E. Engr. (Civil

Engr) Project University of Ilorin

Prasad, V.S. (1986) – Development of Filtration / Absorption Media for removal of Bacteria

and Turbidity from Water. Indian Institute of Technology, Kanpur, India.

Robert, A.C. (1999) – Standard Handbook of Environmental Engineering 2nd

Edition Mc Graw

– Hill. Pp. 5.89 – 5.96


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Sangodoyin A.Y. (1981) “Assessment of the Filterability Number”, M.Sc Project Report in

Water Resources Technology, Department of Civil Engineering University of

Birminghan.

Schroeder; E.D. (1971):- Water and Wastewater Treatment. Mc. Graw Hill, U.S.A.

Smith N.A.F (1975) – Man and water, A History of Hydro-technology Charles Scribner’s and

Sons, U.S.A.

Starley, N, and Roger, J.M.D. (1966) – Hydrogeology – Willey and Sons Inc New York

Tebbutt, T.H.Y. (1992) – Principles of Water Quality Control 4th Edition, Pergamon Press.

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Edition – Longman


129

APPENDICES

A. Raw water from the dam

Co. = 9.5 NTU C = 7.5 NTU Q = 93cc/min

Have = 28.5mm = 2.85cm Tave = 10.9 min

V = Q/A = 93/11.34 = 8.2cm/min

F = HC

V Cot = 2.85 x 7

8.2 x 19 x 9.5 = 0.0289

B. Using Flocculation Treatment

Co. = 9.5 NTU C = 4.0 NTU V = 8.2cm/min

Have = 2.28cm t = 10.9 min

F = 2.28 x 4 = 0.0123

8.2 x 9.5 x 10.9

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Sacha Journal of Environmental Studies, Volume 1 Number 2 ... · A further statistical analysis using least Square Deviation (LSD) and Dunnett statistic indicates that there is no