Journal of Global Resources

Volume 2
January 2016 Page 136-144
ISSN: 2395-3160 (Print), 2455-2445 (Online)

18
PERFORMANCE OF HORIZONTAL-FLOW ROUGHING FILTER UNITS (HRF) UNDERVARIED RAW WATER CONDITIONS IN KENYA
C.N. Mushila*b, G.M.M. Ochienga, F.A.O. Otienoc, S.M. Shitoteb and C.W. SittersbaFaculty of Engineering, Vaal University of Technology, South AfricabSchool of Engineering, Moi University, Eldoret, Kenyac
Masinde Muliro University of Science and Technology, Kakamega, Kenya*Email: collymush@gmail.com
Abstract: Horizontal flow Roughing filter units (HRF) have been found to be the most effective
and appropriate pre-treatment method used before slow sand filters (SSF) for application indeveloping countries. The main advantage of HRF being its simplicity in design, have low
capital cost, ability to separate fine solids particles over prolonged periods ( high solids
retention capacity) without addition of chemicals. Most research on HRF has concentrated on
turbidity (solids removal) whereas its role in removal of other parameters of concern such as
microbiological (Faecal and Total coliform), colour, dissolved iron and manganese hasreceived little attention. The general objective of this research was to evaluate theperformance of HRF units by way of their removal efficiencies for the various water quality
parameters under varied raw water conditions. In order to meet this objective a pilot plant unit
was designed and run. The pilot plant was monitored between the months of January to June
2013. The response of the respective HRF units in removal of selected parameters guiding
drinking water quality such as microbiological (Faecal and Total coliform), Turbidity,
Suspended Solids (SS), dissolved iron and Manganese was investigated. With respect tomicrobiological raw water quality improvement, HRF units reduced Faecal coliforms load by 36
percent and Total coliforms load by 38 percent respectively
Keywords: Horizontal-flow Roughing Filter, pre-treatment of drinking water, slow filtration
Introduction
In the year 2000 access to safe drinking water in Kenya was 58 percent (UNFPA report,
2003). In this report, access to improved water source for Kenya is reported as 46 percent for
the Rural and 89 percent for the urban populace. The 2008 population estimates for Kenya are
38,765,312 (WB 2010). It is clear that a significant number of people in Kenya, particularly inthe rural areas, do not have access to safe drinking water leading to numerous water-related
outbreaks such as typhoid fever and cholera. A simple low cost water treatment system isrequired in Kenya in order to alleviate the problem of in accessibility of the potable water to therural community. Slow Sand Filtration (SSF) has been recognized as an appropriatetechnology for drinking water treatment in rural areas, and is recognized as a suitable filtration
technology for removing waterborne pathogens (Galvis, G. 2006). It is capable of improving
the physical, chemical, and microbiological quality of water in a single treatment process
without the addition of chemicals, and can produce an effluent low in turbidity and free ofbacteria and viruses. In fact, Wegelin (1988) states, “no other single water treatment process
can improve the physical, chemical, and bacteriological water quality of surface water better
than Slow Sand Filtration”. In addition, the USEPA (1997) states, “when used with a source
water of appropriate quality, Slow Sand Filtration may be the most suitable filtration technology
for small systems”. These two statements elucidate the important role of Slow Sand Filtration
for treating surface water in small systems.

136

The main drawback of Slow Sand Filtration is its inability to treat high turbidity surface water
without the rapid development of head-loss and frequent clogging of the filter. Ellis (1985). A
proven treatment method to cope with many of the limitations of Slow Sand Filtration is MultiStage Filtration (MSF). MSF is a robust, multi-barrier treatment method, which consists of pretreatment with roughing filtration followed by Slow Sand Filtration. The MSF technology is still
unknown in many countries including Kenya and hence the need to research on its adaptability
in such countries (Ochieng, 2001). Horizontal flow Roughing filter units (HRF) have beenfound to be the most effective and appropriate pre-treatment method used before slow sandfilters (SSF) for application in developing countries. This research endeavoured to investigate
the water quality improvement performance of HRF units beyond its normal role of protecting
the operational conditions of SSF against high turgidities and Suspended Solid (SS).
Implementation of this technology as a pre-treatment to SSF will mean increased accessibility
to potable water in Kenya.
Objectives
The general objective of this research was to evaluate the performance of HRF units by way of
their removal efficiencies for the various water quality parameters under varied raw waterconditions.
The specific objectives involved designing and constructing Laboratory model Horizontal-flow
Roughing Filters (HRF). The effect of varying HRF gravel pack ratios and Series connection of
two HRF units was also investigated. The parameters of interest were Coliform bacteria,
colour, dissolved iron and Manganese.
Study area
The pilot plant was set up at Moi University Water Treatment Works in Kesses Division, which
is about 36 kilometres South East of Eldoret town in Kenya. Eldoret falls within Uasin Gishu
County. Uasin Gishu County lies on coordinates: 0o310N 35o170E.The County has a cool andtemperate climate with mean annual rainfall 1100 to 1200mm. Temperature during wet season
drops to 180C and dry season records a temperature of 260C. There are four noticeable soil
types in the district: red loam, red clay, brown clay and brown loam.
Horizontal-flow Roughing Filter (HRF)
A Horizontal-flow Roughing Filter (HRF) consists of a horizontal filter box with 3 or 4compartments of decreasing length separated by baffles, in which water flows horizontally.
Each compartment is filled with gravel, with the coarsest media in the first compartment andthe finest media in the last compartment. The advantage of HRF is its extended bed lengths
and solids storage capacity, resulting in less cleaning frequency than up-flow Roughing Filters(Collins et al., 1994a). It is also more suitable for treating very high Suspended Solids
concentrations. The disadvantage of the HRF is its large space requirements. The horizontal
flow roughing filter (HRF) is commonly applied with SSF especially in the developing countries(e.g. Jayalath and Padmasiri, 1996; Mesfin, 1999; Torabian and Fazeli, 2004). To date, themost comprehensive model applied in HRF design is based on Wegelin design criteria
founded on the “1/3 – 2/3” filter theory (Wegelin, 1986; 1996).
Wegelin design criteria
Wegelin design criteria is founded on the “1/3-2/3” filter theory and is still to date the mostcomprehensive model applied in design of Roughing Filters (Wegelin, 1986; 1996). The filter
theory is based on Suspended Solids (SS) reduction. The “1/3-2/3” filter theory is classified asconceptual. The investigations of Wegelin et al (1996) describe this theory as follows; by logic
or experience, a particle in water can bypass a gravel grain either on the left or right or settle
on its surface. Hence the chance to fall on the grain is 1/3. However, the process continues, as
there is a second, third and many other gravel grains to settle on. This theory has been used137

to formulate models, which give a simple elucidation of the removal kinetics of the Roughing
Filters and hence further used to describe the filter efficiencies in the design of Horizontal-flow
Roughing Filters. According to the mathematical exercise description of this theory, it clearly
proves that solid matter separation by filtration can be described by an exponential equation.
Based on Fick’s law and other established filter theories, the filter efficiency can be expressedby the filter coefficient λ.

dc
= −λc
dx
……………………………………………………………................……… (1)where;
c = solids concentration.
x = filter depth.
λ = Coefficient of proportionality also known as filter coefficient.
Equation (1) states that the removal of suspended particles is proportional to the concentrationof the particles present in water. Assuming the total filter length as a multi-tore reactor
consisting of a series of smaller filter cells, the performance of an HRF can be calculated on
the basis of the filter cell test results. Neglecting straining mechanisms and further assuming
surface chemical conditions to be constant, the total suspended solids (SS) concentration after
an element ∆x can be estimated by the following expression;

C out = ∑ Ci ( in ) e − λi ∆x………………………………………………………….............…. (2)where;
ci = concentration of particles of size d pi
∆x = the length of the experimental filter cell.

λi = filter coefficient for each filter cell.
Equation (2) shows that in knowing the inlet SS concentration, the filter coefficient, and thefilter depth (length), the outlet SS concentration can be easily predicted and consequently thefilter performance efficiency. This aids in filter design. According to Wegelin (1986; 1996), theeffluent quality for an n number of compartments is given by the following expression;ce = co × E1 × E2 × ..... × En …………………………………………................…….…….. (3)where;
co = concentration in the HRF influent.
ce = concentration in the HRF effluent.
E i = are the filtration "efficiencies" for i =1,2......n compartments respectively.
The basic expression for the above relationship is;……………………………………………………………………..…...........….. (4)where;
L = is the length of filter.

λ = is the coefficient of filtration ( also known as filter coefficient).
Where;
L= is the length of filter.
λ= is the coefficient of filtration (also known as filter coefficient).
The filter efficiency is given by:138

…………………………………………………………..........………....…..... (5)............…………………………………………………………..............…..... (6)The values of Ei (i = 1, 2… n) are obtained either from tables or graphical nomograms as
developed by Wegelin (see mentioned reference for further details).
The HRF pilot unit
To investigate the performance of the HRF units under varied raw water conditions, a pilot
plant was built at Moi university water treatment works in the year 2013. This site wasconvenient because of the readily available raw water. The topography of the site allowed theflow of water by gravity thus avoiding pumping equipment and associated costs. The pilot unit
comprised of a Constant head feeder tank supplying raw water to three Horizontal roughing
filters. There was no addition of chemicals to the raw water.
Constant Head Feeder Tank
The constant head feeder tank or the Mixing Tank (MT) maintained a constant water head forthe units and acted as the first sampling point of raw water. It also served as a unit where thechosen raw water parameters were varied to different levels and the response of the HRF unit
investigated. The MT was positioned adjacent to the intake of the raw water at a lower head so
as to allow flow of raw water by gravity from the point of abstraction to the SSF unit. The MTunit measured 1.2 m Length, 0.8 m width and 1 m height. The unit was fabricated using Mild
Steel (MS) plates (3mm thick).
Horizontal-flow Roughing Filters (HRFs)
The pilot unit consisted of three lines of Horizontal-flow Roughing Filters (HRFs) (refer figure1) processing raw water from the MT.The design and sizing of the pilot-plant HRFs wereguided by the Wegelin design criteria (Wegelin, 1986) based on the preliminary raw water
quality data obtained prior to the commencement of the full pilot-plant study.The units had thesame design specifications and were fabricated using Mild Steel (MS) plates (3mm thick).
Each measured 5.4m length, 0.5m width and 1m height. The designed HRF unit had threecompartments with different sizes of gravel separated with perforated steel plates. Sampling
points were identified along the length of each of the HRF unit and also at both inlet and outlet
points to monitor the raw water quality improvement. HRF consisted of three parts: the inlet
structure, the filter bed and the outlet structure. Inlet and outlet structures had flow control
installations (valves). This was meant to maintain the designed flow velocity and water level
along the filter bed. An “Equal distribution chamber” was provided at both the inlet and outlet toestablish an even flow distribution along and across the filter. The filter bed was composed of
3 gravel packs of different sizes. The filter material was arranged from coarse to fine in the
direction of water flow. The coarsest material diameter was in the range 15-24mm, medium
material was in the range of 8-15mm and the finest from 4-8mm. These filter media packs
were separated with perforated Mild Steel (MS) plates to avoid mixing. The filter bed wasprovided with under drainage system to enable hydraulic sludge extraction to be carried out
after a certain running period. The HRF unit was operated at a constant filtration velocity of
0.5m/h.
RESULTS AND DISCUSSIONS
Bacteriological quality improvement
The HRF unit played a significant role in both Faecal and Total coliform reduction. On
average, the HRF unit reduced Faecal coliform load of SSF influent by 36 percent and Total
coliforms load by 38 percent respectively. Figure 1 shows Total Coliform removal trend in HRF
units. It is important to note that efficiencies may be somewhat site specific as there is somevariation in the findings from several authors.

139

Figure 1 Total Coliform removal trend in HRF units.

Total coliform removal trend
800
Coliform units -CFU/100ml

700
600
Raw water
HRF out

500
400
300
200
100
0
5

26

40

54

62

76

84

91

98

Run time -days

Removal of Iron and Manganese
It was noted that HRF units also played a critical role in both Iron and Manganese reduction.
On average, HRF units reduced dissolved Iron load of SSF influent by 42 percent andManganese by 57 percent. Figure 2 and 3 show the Manganese and dissolved Iron removal
trends.
Figure 2 Manganese removal trend

Manganese removal trend

Manganese level (mg/l)

0.06
0.05

Raw water
HRF out

0.04
0.03
0.02
0.01
0
1

5

7

9 12 14 26 48 54 58 61 65 69 72 79 82 86 91

Run time ( days)

140Figure 3 Dissolved Iron removal trend

Dissolved Iron removal trend
Dissolved Iron level (mg/l)

1.2
Raw water
HRF out

1
0.8
0.6
0.4
0.2
0
1

5

7

9 12 14 26 48 54 58 61 65 69 72 79 82 86 91

Run time (days)Y
Removal of Turbidity
From the field data, it was observed that on average, the HRF unit registered 86 percent
Turbidity and 85 percent SS removal. Figure 4shows the Turbidity removal trends by HRF
units. The average Turbidity of water flowing from HRF unit was 8.28 NTU. According to mostresearch findings in the field of Slow Sand Filtration, Turbidity levels to the Slow Sand Filters
should be less than 10 NTU, hence the average Turbidity values from HRF units was within
acceptable standards.
Figure 4 Turbidity removal trend by HRF units

Turbidity removal trend

400
350

Turbidity units-NTU

300
Raw Water
HRF out

250
200
150
100
50
0
1

5

9

14 28 33 37 42 45 49 54 58 63 69 72 79 86 91 Run time (days)

Colour removal trend by HRF
The colour removal trend in the three HRF units proved that they are not as efficient in colour
removal relative to values gotten for turbidity and SS respectively. Figure 5 shows colour
removal trend in HRF units. The HRF units achieved an average of 50 percent colour removal.

141

Figure 5 Colour removal trend by HRF units

Colour removal trend.

160
140
120

Colour units (mg/l pt.)

100
80

RAW WATER
HRF out

60
40
20
0
1

2

5

7

9 12 14 26 28 30 33 35 37 40 42 44 45 47 50

Run time (days)
Varying Horizontal Roughing .Filter (HRF) gravel pack ratio.
Gravel was used as the filter media in the HRF unit, the unit had three chambers holding
different sized Gravel material (Gravel packs) as discussed earlier. In order to achieve theobjective of this research, the HRF was designed to allow variation of the ratios of the gravel
packs. This was meant to examine its impact on raw water quality improvement. Sampling
points were identified at the inlet, along the filter media and outlet of each HRF unit to examine
raw water quality improvemen
improvement. The following gravel pack ratios were analysed in thisresearch;
1) 2:2:1 {i.e.
i.e. 2 metres (dg 15 -24mm), 2 metres (dg 8-15mm),15mm), 1 metre (dg4-8mm).}2) 1:2:2{i.e.
i.e. 1 metre (dg 15 -24mm), 2 metres (dg 8-15mm),15mm), 2 metre (dg4-8mm).}(dg43) 1.5:1.5:2 {i.e.
i.e. 1.5 metres (dg 15 -24mm), 1.5 metres (dg 8-15mm),15mm), 2metre (dg4-8mm).}(dg4
During the first month of this study, the HRF units operated at the same ratio of 2:2:1.
Water tests done on HRF effluent showed similar trends with respect to Turbidity andsuspended solid removal. A constant minimal filtration rate of 0.5 m/h was maintained in thethree HRF units to give optimal results. After the first month, each HRF
HRF units were set at adifferent ratios and the impact on overall raw water quality improvement noted.
HRF UNITS
HRF 1
HRF2
HRF 3

Table 1: HRF units Percentage removal (different ratios)“Gravel pack” ratio
Percentage removal.
Percentage removal.
Turbidity
SS
2:2:1
88
87
1:2:2
89
88
1.5:1.5:2
88
88

Source: Field data

Table 1 shows the average percentage removal of Turbidity and SS by the three HRF
units. It was noted that there wasn’t great variation with respect to percentage Turbidity andSuspended Solids removal by the units. HRF 2 gave the best performance with 89 percent
removal of turbidity and 88 percent removal of Suspended solids. This was attributed to thefact that this unit provided a large length or surface area of fine filter media material, hence it
was possible for this unit to intercept fine suspended matter that escaped through the first142chamber thus resulting into better water effluent. However, it was noted that HRF2
experienced frequent clogging relative to the other two HRF units- especially during heavy rain
events, hence a short filter run period. This necessitated frequent hydraulic cleaning in this unit
whenever the raw water had high turbidity and SS levels. It was on these grounds that it was
concluded that although HRF 2 gave good results, it is not advisable to operate such gravel
pack ratio combination in high turbid waters since it would result in short filter runs and tediouscleaning and maintenance activities. This would compromise the sustainability of such asystem. On the other hand, HRF1 was considered as the best alternative in high turbid water.
This was because it gave the longest filter run period of about 2 to 3 weeks depending on the
raw water quality.
Connecting two HRF units in series
The HRF was connected in series when the raw water turbidity levels were higher than 120
NTU. This was because, it was realized from field analysis that whenever Turbidity levels werehigher than 120 NTU, HRF effluents recorded high values of more than 10 NTU. By
connecting the HRF in series it was possible to minimize the Turbidity level to less than 10
NTU and achieve high efficiencies in removal of SS. Two HRF units connected in series
achieved very high efficiency levels, recording an average of 95 percent in terms of turbidity
removal and an average of 98.4 percent with respect to Suspended solids removal. The highefficiency with respect to turbidity removal was attributed to a high surface area of filter media
provided by connecting the two units in series
CONCLUSIONS AND RECCOMMENDATIONS• Performance of HRF
HRF unit registered 86 percent turbidity and 85 percent Suspended Solids removal. HRF
units apart from being used as a pre-treatment of turbidity and Suspended Solids it also
played a significant role in both Faecal and Total coliform reduction. On average, HRF
units reduced Faecal coliforms load by 36 percent and Total coliforms load by 38 percent
respectively. It was also noted that HRF units also played a crucial role in both Iron andmanganese reduction. On average HRF, units reduced dissolved iron load of SSF influent
by 42 percent and manganese by 57 percent.
The HRF operated in the range 1 to 3 weeks depending on the raw water quality. The main
pointer to this observation is because Horizontal Roughing Filter (HRF) this unit had verylarge surface area which allowed storage of high volume of Suspended Solid or Sludge
before hydraulic cleaning.
Connecting the HRF units in series yielded very high efficiencies in terms of Turbidity andSS solids removal recording levels as high as 98 percent with respect to SS and turbidity.
Recommendations
Research to examine the impact of media size and hydraulic loading rate on the removal of
colloidal matter in HRF filters should be carried out.
Further research targeting the optimization of HRF units should be conducted.
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