Effect of Operational Conditions on Performance of Deep Sand Filter in Turbidity Removal
Hani Mahanna*, Kamal Radwan, Moharram Fouad and Hoda Elgamal
Department of Public Works Engineering, Mansoura University, Egypt
Submission: July 02, 2018; Published: August 02, 2018
*Corresponding author: Hani Mahanna, Department of Public Works Engineering, Mansoura University, Egypt, Email: email@example.com
How to cite this article: Hani M, Kamal R, Moharram F, Hoda E. Effect of Operational Conditions on Performance of Deep Sand Filter in Turbidity Removal. Trends Tech Sci Res. 2018; 2(5): 555597. DOI: 10.19080/TTSR.2018.02.555597
In this study, laboratory pilot plant was constructed to study the performance of deep sand filter in water treatment. Sand was used as a filtration media under different filtration rates ranged from 4 m/hr to 8 m/hr. Down flow was applied to the filter through sand media with size of 0.7-1.0 mm, while sand depth was changed from 80 cm to 140 cm. The used synthetic turbid water was prepared in different turbidity levels varying from 10 to 30 NTU. Aluminum sulfate (alum) was used as coagulant in different doses varied from 20 to 40 mg/l. Turbidity removal and head loss were investigated as functions of sand depth, filtration rate, alum dose and influent turbidity. From analysis of the experimental results, it was found that the changing of operational conditions (changing media depth, filtration rate, alum dose and influent turbidity) have significant impact on the turbidity removal efficiency, head loss and ripening period. Also, the effective depth of media is influenced by operational conditions and its value was obtained in this paper.
Keywords: Deep sand filter; Effective depth; Filtration rate; Head loss; Turbidity removal
With an exponentially increasing of population, better and more economical water treatment methods and strategies have become the need of the hour. One such sustainable water treatment method is using deep bed filters which has great potential for treating of contaminants in drinking water [1-3].
Deep bed filtration is an effective process in removing particles from water. This is achieved by passing the suspension through a deep layer (usually between 0.5 m and 2.0 m depth) of granular material, typically sand. The suspension particles are retained within the depth of the filter media. This removal may be achieved by different mechanisms [4-6].
In filtration process, there are different stages, ripening, working, and breakthrough stage . The removal efficiency during ripening stage increases, while this efficiency is nearly constant during working stage. Then the removal starts to deteriorate in breakthrough stage . During ripening period, the effluent turbidity is relatively high. It takes some minutes to be allowable . The effluent water during this period is mostly wasted thus this period has significant effect on the quantity of water. This period has been studied in many researches [10-13], but the effect of different operational conditions on this period was not clear. So, this paper will study that effect.
Numerous empirical equations, which are used to predict initial filter head loss, have been developed. Some commonly used equations are Carmen-Kozeny, Ergun, Fair-Hatch, Hazen, Rose, and Gregory equations [14,15]. These equations may be unacceptable for specific types of media such as crumb rubber. it was found that Both the Kozeny and Ergun equations were unacceptable for clean-bed head loss prediction in crumb rubber filters . A neuro-fuzzy model was conducted to estimate head loss in dirty sand filters. Hydraulic loading rate, influent iron concentration, bed porosity, and operating time were selected as input variables .
Many researchers studied the deep bed filter and its performance. One of these studies showed that the removal efficiency during the ripening stage of filtration can be improved significantly by increasing the ionic strength of the water to be treated . In addition, the structure of porous media (i.e., pore size distribution) has a clear effect on deep bed filter performance . Moreover, impact of backwash procedures on deep bed filtration productivity in drinking water was presented . From modeling point of view, many models were introduced to explain the efficiency and performance of deep bed filters, [21-25].
Through deep bed filters, not all used media depth is effective in turbidity removal but there is an effective depth which give the required or permissible limit. The effective depth for media from burned oil palm shell granular has been obtained . Moreover, knowing the effective depth of turbidity removal will assist in identifying depth of the sand during filter design.
Using granular sand filters has a limitation of influent quality. When it used with drastic influent quality with small depth it don’t give the required quality. Therefore,
another technique is required such as using deeper filter. The
main objectives of this study is to investigate the effect of
operational conditions on the performance of deep sand filter
and to determine the effective depth for turbidity removal.
For this study, a Laboratory pilot plant was designed and
constructed in the laboratory of sanitary engineering in the
faculty of engineering – El- Mansoura University, Egypt. Figure
1 shows the schematic diagram of the laboratory pilot plant.
a. Feeding tanks: The synthetic turbid (raw) water
was prepared in four plastic feeding tanks. The four tanks
divided into two groups each group consist of two tanks.
The capacity of each tank was about 250 liters.
b. Feeding pump: The synthetic turbid water was
transported from feeding tanks to a constant head tank
by feeding pump. The horsepower of pump were 0.45 HP
(Discharge 0.45 m3/hr).
c. Constant head tank: The raw water has been fed
from the feeding tanks to the sand filter via a constant head
tank. The capacity of constant head tank was 45 liters and
has dimensions of 30 * 30 * 50 cm. Constant head tank has
confirmed a constant discharge to the plant whatever the
difference in water levels before and after it.
d. Filtration column: The major part of the laboratory
pilot plant was the filtration column. The filtration column was made from galvanized steel with height 2.5 meters.
The column consists of two parts each part 1.25 m height
and it has square cross section (20 *20 cm). The filtration
column has one glass face to allow monitoring what is inside
the column. Along the filtration column several connection
points were fixed each 20 cm as samples ports.
e. Backwash Pumps: Two pumps were used for
backwashing. The horsepower of each pump were 0.45 HP
(Discharge of each pump 0.45 m3/hr).
f. Piezometers: Manometer tubes installed behind
filtration column to determine the head loss through the
filtration media depth at different times.
g. Fittings and Plastic connecting pipes: There were
different service connections such as elbow, tee, reducer
and connecting pipes with different diameters due to its
location and use.
In this study, the filtration media was uniform sand media
rested on the gravel layer. The sand depth in the filtration
column was changed from 80cm to 140cm with size of 0.7-
1.0mm and its porosity was 0.38. The dry density of the sand
was 1.65 gm/cm3 and its specific gravity was 2.55. The gravel
depth under the sand was about 20cm with different sizes
ranged from 2.0mm to 9.0mm. The gravel layer is located
immediately below the filter sand media to separate the filter
media from the underdrain system, to prevent media particles
from clogging the underdrain orifice, and to dissipate the
backwash water jets from the orifice of the underdrain system
By using fine clay soil and tap water it was able to achieve
the synthetic raw water which can be used in the experimental
work. The raw water was prepared by dispersing fine clay,
having a size less than 0.074mm, in tap water. In this research,
the synthetic raw water turbidity was ranged up to 30 NTU
which is a suitable range in the River Nile and its branches in
This study has been extended to cover the various operation
conditions for the sand bed filter. In present research, different
positions for Aluminum sulfate (alum) feeding were tested as
a coagulation process. it was found that feeding alum dose in
feeding tanks gives high filtration efficiency with doses varied
from 20 to 40mg/l. The operation of the laboratory pilot plant
was controlled by 12 valves (shown in Figure 1). These valves
facilitate two modes of filter operation (down flow filtration
mode and filter backwash mode).
a. Filtration mode: The experimental runs were
conducted under the conditions of a direct constant filtration rate. The operated filtration rate was changed
from 4m/hr to 8m/hr (suitable for rapid sand filter). The
effluent turbidity was measured at different depths each
1 minute for the first 30 minutes (to assess the ripening
period) then it was measured each one hour throughout
the remaining part of the run. The turbidity of influent
and effluent water was measured by using turbidimeter in
Nephelometric Turbidity Unit (NTU). Its model is Orbeco
TB300-IR Lab Turbidimeter. In addition, the head loss
through sand media was measured by Piezometers each
one hour. The effluent turbidity measurements at different
depths will help in determining of effective sand depth and
developing of simplified model for filter efficiency.
b. Backwash mode: The end of filtration run was
considered when effluent turbidity started to increase
(turbidity breakthrough) or head loss reaches the
maximum value or the run length reaches defined period
(in this research, one day). Once either of this condition
was reached, the filter run was terminated and the filter
needs to backwash mode to remove the accumulated solids
inside its bed [29,15]. Water only with high rate (50m/hr)
was used as a backwash method . The backwash period
was extended to 18minutes and during this period, the
wasted backwash water turbidity was measured each 1
minute to appraise the required backwash duration.
Various runs were conducted under different operational
conditions (varying the media depth, filtration rate, influent
turbidity and alum dose) to study the performance and get the
effective depth for deep bed sand filter.
In this study, the effluent turbidity within the first 30
minutes was measured at each of tested filtration rates (4, 5,
6, and 8m/hr) and through the different media depths (80, 100,
120, and 140cm). The results were obtained with 3 different
influent turbidity (10, 20, and 30 NTU). The coagulation
process was done by using alum with different doses (20,
30, and 40mg/l). the effluent turbidity was observed until it
reaches the acceptable limit in the Egyptian drinking water
code (1 NTU) to obtain the ripening period. Figure 2 shows the
effect of increasing media depth on the ripening period for 10
NTU influent turbidity and alum dose of 40mg/l. As shown in
that figure, the effluent turbidity in the beginning of filtration
run was nearly 1 NTU and this turbidity was due to the
remaining backwash water in the underdrainage system. The
turbidity increases over few period of time nearly 4minutes.
This increase was due to not removed particles that were
dislodged from the media during backwash . At nearly 4 to
5 minutes the effluent turbidity reaches its maximum values
and these values vary according to the filter media depth. The
reason for increasing turbidity to this degree is that influent water enters the filter and mixes with the backwash remnant
water in the upper region of the filter. This is consistent with
the results in paper for Colton et al.  which showed that
the collapse pulsing duration was 4minutes. Then the effluent
turbidity decreases until it nearly becomes constant where the
media is able to capture newly influent particles .
From Figure 2, it was obvious that the effluent turbidity
reaches 1 NTU after 12minutes when the sand media depth
was 80cm. If the sand media increased to 140cm, the ripening
period will be virtually 7 minutes. This means that increasing
sand depth from 80cm to 140cm reduced the ripening period
by 42%. So, it help in increasing of water treatment plant
production. The effluent turbidity was influenced by increasing
filtration rate as shown in Figure 3. When the filtration rate
increased to 5m/hr the required ripening period increased.
At sand depth of 140cm the required ripening period was
10minutes, while depth of 80cm gives the required turbidity
after 32minutes. In addition, the ripening period in case of
100cm depth was 18minutes but in case of 120cm depth the
ripening period becomes 13minutes. Increasing filtration
rates may result in an inability to use a small depths as shown
in F igure 3 . A t f iltration r ate o f 6m/hr t he e ffluent t urbidity
reaches the acceptable limit after nearly 18, 25 and 40minutes
with sand depths of 140, 120 and 100cm respectively. But
depth of 80cm can’t be used due to its ripening period. This is
due to increasing filtration rates resulted in greater shearing
actions within the media pores, which enhanced the transport
of deposited turbidity matters through the filter bed. With
increasing filtration rate to 8m/hr, media depth of 100cm
can’t give acceptable effluent turbidity. Figure 3 summarizes
the required ripening period for each sand depth at different
filtration rates. Also, this figure shows that the influent
turbidity has significant effect on the ripening period. Figure
3 show great effect of operational conditions on ripening
periods, so it could be guided in determining either ripening
periods thus the productivity of treatment plant or required
media depth at different filtration rates and influent turbidity.
Head loss is a loss of pressure (also known as head) by water
flowing through the filter media. In the current study, the head
loss through filter media depth was measured for different
filtration rates and various influent turbidity with alum doses
20, 30, and 40mg/l. Figure 4 shows the variation of head loss
along run time at filtration rate of 5m/hr and alum dose of
40mg/l for different depths of sand media. It was noticed that
the observed head loss values were close to the calculated head
loss from Hazen equation and Kozeny equation for sand depth
from 80cm to 140cm. It was noted from the experimental
observations that the alum dose variation has not vital effect
on the initial head loss at the same depth and filtration rate.
but it has significant effect on the increasing rate of head loss
with run time. Further, increasing filtration rate increases the
increasing rate of head loss. From Figure 4, the head loss value
after 24hours of run was nearly 110cm at sand depth of 140cm
and filtration rate of 5m/hr. This means that using deep sand
filter up to 140cm have not a critical impact on head loss value.
Figure 5 shows the observed head loss at specific time (24
hr) for sand depth of 100cm at different filtration rates. It also
shows the effect of changing alum dose and influent turbidity on head loss values. It was noticed that the head loss increased
from 58.7 cm to 72.9cm by increasing alum dose from 20mg/l
to 40mg/l at filtration rate and influent turbidity of 4m/hr
and 10 NTU respectively after 24hours of run. In addition, at
filtration rate 8m/hr, increasing alum dose from 20mg/l to
40mg/l increased the head loss from 112.4cm to 129.5cm.
Thus, the effect of increasing alum dose on head loss at high
rates is more obvious than low rates. Figure 5 shows also the
impact of raw water characteristics which represented in
influent turbidity on head loss. It was noted that increasing
influent turbidity from 10 NTU to 30 NTU increased head loss
from 81cm to 102.5cm at filtration rate of 5m/hr and alum
dose of 40mg/l. This is due that raw water with high turbidity
which contains larger number of suspended particles leads to
increasing of shear action when flowing through sand grains.
This shear action increases with increasing filtration rate so
the effect of influent turbidity on head loss at high rates is more
significant than at low rates as shown in Figure 5.
Deep bed sand filter is usually characterized by filtration
efficiency, which determines the level of removal of particles
from influent water. The turbidity removal efficiency was
measured in the present study under various operational
conditions. Figure 6 shows the turbidity removal efficiency
with depth at different rates. The removal efficiency was
measured as the following equation:
Figure 6 shows the removal efficiency with alum dose
20 mg/l through different sand depths at 4, 5, 6 and 8m/
hr filtration rates. It was clear that increasing media depth
increases the removal efficiency where at 20cm depth the
removal ratio was 78.4% while at 140 cm depth the ratio of
removal increased to 94.7%.
Increasing filtration rate decreases the turbidity removal
efficiency, where at 20cm depth, the removal percents for 4, 5,
6 and 8m/hr were 78.4, 75.35, 71.43 and 66.3% respectively. If
the depth of media equals 140cm, the removal percents will be
94.7, 92.36, 89.72 and 87.41% at the previous rates respectively.
It was also noticed that sand depths less than 100 cm have
drastic variation in turbidity removal efficiency, while depths
more than 100cm have slightly variation in removal efficiency
at low filtration rates.
Increasing alum dose from 20mg/l to 40mg/l has significant
impact on the percent of removal. Figure 7 show the removal
efficiency at alum dose 40mg/l with different sand depths at
4, 5, 6 and 8m/hr filtration rates. This figure also confirms
that the efficiency of removal increased by increasing sand
depth and decreasing filtration rate. The removal efficiency at
20cm depth was 78.4% with alum dose of 20mg/l but 82.7%
with alum dose 40mg/l. In case of depth 140cm, the efficiency
was 94.7% with alum dose 20mg/l but 97.9% with alum dose
of 40mg/l. This means that increasing alum dose enhanced
the coagulation process so it increases the percent of removal,
but it occurs at a certain limit of alum. After this limit, any
increasing of dose has no effect.
After knowing turbidity values at different depths with
various rates and various influent turbidity, effective depth of sand could be determined. The effective depth is the smallest
sand depth which give the allowable limit of turbidity (>1 NTU). The effective depth is determined as shown in Figure 8
1. Plotting the average turbidity at different depths for
each filtration rate.
2. Identify the permissible or required limit of effluent
turbidity (1 NTU).
3. The intersection points at each rate could give the
Figure 9 summarizes the effective depth for each rate with
influent turbidity 10, 20, and 30 NTU. The results in the figure
were obtained when the alum dose was 40mg/l, which gives
high removal efficiency.
From Figure 9, it was observed that sand depth of 50cm was
more suitable for filtration rate 4m/hr and influent turbidity
10 NTU. To increase the filtration rate to 8m/hr, it was needed
to increase the sand depth to 110 cm. For influent turbidity
30 NTU, 73 cm depth was suitable for 4m/hr filtration rate
whereas 130 cm depth was suitable for 8m/hr filtration rate.
Moreover, at filtration rate 8m/hr and influent turbidity less
than 30 NTU, 130 cm sand depth is sufficient to give acceptable limit of effluent turbidity. From these results, in order to
double the filtration rate nearly 60 cm of sand should be added
to the existing depth. Since the filtration rate commensurate
inversely with filter area so the filter area could be reduced by
increasing sand media depth.
In the current study, backwash rate of 50m/hr was
used, which expands the media 20 percent. This rate was in
recommended range by AWWA . The backwash period
extended to 16minutes until the effluent backwash water
turbidity became acceptable. The effluent turbidity during
backwash was measured and plotted as shown in Figure 10.
The wasted effluent turbidity was very high during the first
two minutes. It exceeds 700 NTU then it begins to come down
to reach 10 NTU in average after 6 minutes. From 6 minutes to
the end of backwash period, there is slight decrease in effluent
backwash turbidity. Changing sand media depth from 80 cm to
140 cm has small effect on the backwash period and duration
of 15 min is sufficient for backwashing whatever the depth of
media, as shown in Figure 10.
The value of required backwash duration was close to
reported value in previous studies [33,34]. During backwash
mode, Turbidity (4-5 NTU) was left in the backwash stream
at cutoff to preserve the bed ripening and allow the filter to
produce quality effluent more quickly for the followed run as it
was recommended by Pizze .
From experimental results, it was noted that, Changing
operational conditions (increasing media depth, filtration
rate, influent turbidity, and alum dose) have significant impact
on ripening period of filter, head loss, and turbidity removal
efficiency. This study also shows the effective depth for
different rates and it was proportional with influent turbidity
and filtration rate directly. The effective depth could be changed
by changing the nature of influent water and sand media size.
From the head loss results, it was noted that increasing rate
of head loss is influenced by alum dose, filtration rate and
influent turbidity. Moreover, using deeper sand filter helps in treatment of high turbidity water under high filtration rate and
it can reduce the area of filters.