Al-Qadisiya Journal For Engineering Sciences Vol. 4 No.

2 Year 2011

THE EFFECT OF SAND FILTER CHARACTERISTICS ON

REMOVAL EFFICIENCY OF ORGANIC MATTER FROM GREY
WATER
Zaidun Naji Abudi
Assist Lecturer
Al-Mustansiryiah University
College of Engineering

ABSTRACT
The specific goals of this study are to determine the optimum inflow to the filters (hydraulic
loading rates) and media characteristic such as the effective size, the particle size distributions
within the bed media. In addition to modify the traditional slow sand filter to gain better flow
control to treat grey water of high turbidity. Three set of experiments were carried out during nine
months from April to December in Mustansiryiah University, college of engineering, environmental
hydraulic laboratory.
The first set of experiments were achieved by using (1m height) sand of effective diameter
0.35 mm, uniformity coefficient, UC=2.2 and porosity (39%), the second set of experiments were
carried out by using sand of 0.75mm diameter, UC= 2.9 and porosity (43%) to study the effect of
grain size of sand on water head over sand surface and removal efficiency. While the other set of
experiments were done by using (0.7m height) sand of effective diameter 0.35 mm, UC=2.2 and
porosity (39%) to study the effect of filter height of sand on removal efficiency.
Measurements of chemical, physical and bacterial parameters were achieved during nine
months from April to December. These parameters include turbidity, pH, PO4-2, BOD5, COD, TDS,
TSS, and coliform removal for treated grey water.
Results show that filter loading rate has been determined to be not more than 680 L/hr/m2
which removal efficiency of BOD5 was (51%) and minimum filter loading rate has been tested to be
212 L/hr/m2 which removal efficiency of BOD5 was (83%).

Key words: Slow Sand Biofilter, Grey Water, BOD, COD, Effective Diameter.

:
( )

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Al-Qadisiya Journal For Engineering Sciences Vol. 4 No. 2 Year 2011

.( )
(UC=2.2) ( ٠.٣٥) ( ١)
( ٠.٧٥) .(%٣٩)
( ١) (%٤٣) (UC=2.9)
.(%٣٩) (UC=2.2) ( ٠.٣٥) ( ٠.٧)
:

.
BOD (٢ /‫ﺴﺎﻋﺔ‬/ ٦٨٠)
.(%٨٣) BOD (٢ /‫ﺴﺎﻋﺔ‬/ ٢١٢) (%٥١)

INTRODUCTION
The process Slow Sand Filtration (SSF) percolates untreated water slowly through a bed of
porous sand, with the influent water introduced over the surface of the filter, and then drained from
the bottom.
Biosand filter was designed and constructed for removing suspended organic, inorganic
matter and pathogenic organisms. The treatment technology must be economical to build, and
simple to operate and maintain given the adverse economic and environmental conditions. For these
reasons, slow sand filtration has been selected as a potential water treatment technology. The
resulting recommendations are planned around and sized appropriately for use by the many small
community for water treatment in Baghdad.
The major benefits of slow sand filtration are due to the microbiology of the filter. The
microbiological community must be kept alive for the filter to be effective. In a conventional slow
sand filter, oxygen is supplied to the organisms through dissolved oxygen in the water.
Consequently, they are designed to be operated continuously. In addition, because the water moves
through at a slow rate, the filter beds tend to be very large.
A bucket of water is poured into the top of the filter as necessary. The water simply flows
through the sand media and is collected in another bucket or gravel container at the base of the
spout. It normally takes a few minutes for the entire bucket to make its way through the filter. There
are no valves or moving parts and the design of the outlet system ensures that a minimum water
depth of five centimeters is maintained over the sand when the filter is not in use. When the water is
flowing through the filter, oxygen is supplied to the biologic layer at the top of the sand by the
dissolved oxygen in the water. When water sand levels is too shallow (less than 50mm) biolayer
dries out especially in hot days during periods from May to September causing major problem for
restarting after back washing and pause period. An other problems may be occurred when water
sand level is too deep (more than 500mm during continuous run and 100mm during pause period)
because of insufficient oxygen for biolayer.

LITERATURES SURVEY
Originating in Europe, slow sand filtration is classified as the first, modern water treatment
technology (Ellis, 1985). This filtration process removes particles and microorganisms by the slow
percolation of water through a porous sand media. Unlike other water treatment technology (i.e.
rapid sand filtration), conventional slow sand technology does not involve chemical or physical pre-
treatment applications (Collins etal, 1992).
Problems associated with highly turbid waters made conventional slow sand treatment
impractical for communities plagued with such source water. Conventional slow sand filters
clogged under such conditions, and the technology of choice became rapid sand filtration, due to its
ability to produce large quantities of acceptable finished water from highly turbid source water

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(Ellis, 1985). An additional factor influencing the move to rapid sand filtration was public support
for the newest technology available, regardless of community size (Logsdon, 1991).
Recently, however, slow sand filtration technology has received a resurgence of interest in
the United States (Logsdon, 1991). Increased concerns regarding the persistence of Giardia cysts in
many municipal water systems has led to a greater interest in slow sand technology (Lange,
Bellamy, Hendricks and Logsdon, 1986; Fogel, Isaac-Renton, Guasparini, Moorehead and Ongerth,
1993). With the 1989 passage of the Surface Water Treatment Rule (SWTR) in the United States,
many previously unfiltered surface water sources now require filtration (Logsdon, 1991; Brink and
Parks 1996). The United States Environmental Protection Agency (EPA) has set a turbidity standard
< 1 nephelometric turbidity unit (NTU) 95 percent of the time, never to exceed 5 NTU's.
Furthermore, the removal or inactivation of Giardia cysts is to be > 3-logarithmic (log) and virus
removals are to be > 4-log removal. Removals of microorganisms in slow sand filters have proven
to be 2 – log to 4 – log in effluent of slow sand filters (Hendricks and Bellamy, 1991). The
effectiveness of slow sand filtration in removing Giardia cysts is well documented (Fogel et al.,
1993; Bellamy, Hendricks and Logsdon, 1985; Ellis, 1985). Research in the United States and Great
Britain has shown the effectiveness of slow sand filtration in removing viruses and bacteria
(Wheeler and Lloyd, 1988; Poynter and Slade 1977 as cited by Hendricks and Bellamy, 1991).
The effectiveness, affordability and ease of operation available with slow sand filtration
systems is appealing to small communities (those under 10,000 people) that lack significant capital
for constructing, operating and maintaining rapid sand filtration facilities (Riesenberg, Walters,
Steele, and Ryder, 1995; Li, Ma and Du, 1996). As of 1984, a survey by Simms and Slezak
identified 71 slow sand filtration facilities in operation in the United States. Brink and Parks (1996)
stated that a preliminary report compiled for the American Slow Sand Association indicated that
225 such facilities were in use in the United States. It is anticipated that additional facilities will be
built by small communities needing affordable, effective water treatment technology to comply
with the surface water requirements established in 1989 (Logsdon, 1991; Brink and Parks, 1996).
Based on slow sand filter research, the biosand filter may also remove some heavy metals
(Muhammad, 1997; Collins, 1998). There is also a design modification known as the KanchanTM
Arsenic Filter that is effective in removing both pathogens and 85-90% of arsenic from source
waters (Ngai, 2007).
Preliminary health impact studies estimate a 30-40% reduction in diarrhea among all age
groups, including children under the age of five, an especially vulnerable population (Liang, 2007;
Sobsey, 2007).

EXPERIMENTAL WORK
Grey water is pumped to the slow sand biofilter made from PVR of 1800mm height and
300mm diameter (1) to remove suspended organic matter, algae etc. Overflow outlet (4) back to
holding tank (2) maintains constant depth to water layer as shown in Figure (1). The slow sand
biofilter consists of: a water storage layer (5), a bed of fine sand or media filter bed (6) (Plates (1
and 2)), diffuser plate (7) and gravel layers (9) (Plate (3))to support filter bed. The outflow (10)
may be fitted with a flow regulator valve to control the filtration rate, flow meter (8) is used for
measured treated water flow, and a piezometers (open tube) (11) to measure filter head loss. A
small collection tank (12) collects filtered water for distribution by pump (13). The thermometer (3)
is put in the middle of water storage zone to measure water temperature.
The first set of experiments were achieved by using sand of effective diameter 0.35 mm,
UC=2.2 and porosity (39%) (Plate (2)), while the other set of experiments were carried out by
using sand of 0.75mm UC= 2.9 and porosity (43%) (Plate (3)) to study the effect of grain size of
sand on water head over sand surface and removal efficiency.
The effective size ES or d10 is defined as the sieve size in mm that permits passage of 10%
by weight of the sand. The uniformity coefficient (UC) of a sand is defined as d60/d10, uniform
(the UC is 2.6), and be washed free of loam, clay, and organic matter. Fine particles will quickly

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clog the filters and frequent cleaning will be required. A sand that is not uniform will also settle in
volume, reducing the porosity and slowing the passage of water.
Measurements of chemical, physical and bacterial parameters were achieved during six
months from April to December. These parameters include turbidity, pH, PO 4-2, BOD5, COD, TDS,
TSS, and coliform removal for treated grey water.

RESULTS
The efficiency of SSF depends on the particle size distribution of the sand, the ratio of
surface area of the filter to depth and the flow rate of water through the filter.

FLOW RATES
The biosand filter has been designed to allow for a filter loading rate (flow rate per square
meter of filter area) which has proven to be effective in laboratory and field tests. This filter loading
rate has been determined to be not more than 680 liters/hour/square meter (Q=48L/h) which
removal efficiency of BOD5 was (51%) and the minimum filter loading rate has been tested to be
212 liters/hour/square meter (Q=15L/h) which removal efficiency of BOD5 was ( 83%)
(Figure(2)).
The percentage removal of contaminants is inversely proportional to the flow rate through
the filter because the biologic reduction of contaminants takes time. The amount of water that flows
through the biosand filter is controlled by the size of sand media contained within the filter. If the
rate is too fast, the efficiency of bacterial removal may be reduced. If the flow rate is too slow,
there will be an insufficient amount of treated water, the users will become impatient and may use
contaminated sources of water.
The micro-organisms are more closely confined near the surface of the sand bed in a
continuously operated slow sand filter because the oxygen supply is limited by diffusion from the
surface. Because of the thin biologic zone, there is a shorter contact time between the bio film and
water during filter runs. Slower filtration rates are therefore required in a biosand filter to produce
water of similar bacteriological quality as a continuously operated filter.

WATER DEPTHS
Changes in the water depth will change the depth of the bio zone and removal efficiency of
biofilteration. A greater water depth (500 mm) results in lower oxygen diffusion and consequently a
thinner bio zone and reduce the removal efficiency. With increasing water depth (more than 500
mm), the bio-layer moves upwards in the sand bed and thus oxidation and metabolism decrease.
Eventually, the filter becomes a non living system. Changes in the water depth above the sand
surface will cause a change in the biological zone disrupting the removal efficiency of the filter.
Results of analysis measured such as turbidity and BOD5 show that the removal efficiency of
turbidity and BOD5 in the first biosand filter (sand of effective diameter 0.35 mm and porosity
(39%) and 500mm water depth was 89% and 43% respectively, while the removal efficiency of
turbidity and BOD5 in the second biosand filter (sand of effective diameter 0.75 mm and porosity
(43%) and 500mm supernatant water depth) was 81% and 64% respectively. The removal
efficiencies of different depth on BOD5 and turbidity values is shown in Figures (3 to 5).
A water depth of greater than 500 mm results in lower oxygen diffusion and consequently a
thinner biological zone. A high water level can be caused by a blocked outlet spout or by an
insufficient amount of sand media. As the water depth increases, the oxidation and metabolism of
the microorganisms within the biological zone decrease. Eventually the layer dies off and the filter
becomes ineffective.
The water passes through the sand from top to bottom. Any larger suspended particles are
left behind in the top layers of sand. Smaller particles of organic sediment left in the sand filter are
eaten by microscopic organisms including bacteria and protozoans which 'stick' in the layers of
slime that form around the sand particles and the clean water which passes through the filter is safe

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to drink. Provided that the grain size is around 0.1mm in diameter, a sand filter can remove all fecal
coliforms (bacteria that originate from feces) and virtually all viruses.

WATER QUALITY
Biosand filters have been shown to remove most pathogens found in raw water. If the grey
water is highly contaminated, the outlet water may still have some contaminants. The same source
of water should be used consistently because the biolayer cannot quickly adapt to different water
quality. Over time, the micro-organisms in the biologic layer become adapted to conditions where a
certain amount of food is available.
The turbidity of the source water is also a key factor in the operation of the filter. If the
turbidity is greater than 50 NTU, the raw water should be settled or strained before it goes though
the biosand filter.
In a slow sand filter, the filter bed is constructed of a medium with high surface area which
can be colonized by suppressive micro-organisms. This fine media also presents a physical barrier
to the passage of spores of plant pathogens.
In a SSF, plant pathogens recirculation in the irrigation water are captured in the filter
media, and at slow rates of water filtration (100-200 l/hr/m2 surface area of the first filter), are acted
upon by the antagonistic micro organisms that colonized the filter bed. The maximum water level
may be automated by using a float and a control valve or by periodically adjusting the valve
manually to maintain the water lever near the overflow line. The depth of the filter bed has a strong
influence on the effectiveness of the filtration and should be at least 0.75 - 1.0 meters.
High turbidity levels in the raw water will prematurely block the slow sand biofilter, leading
to a much shortened time span between cleanings and an overall deterioration of the water quality.
High turbidity in the raw water may shorten the filter life from several months to a matter of days.
Other means of turbidity reduction include holding ponds and sedimentation tanks.
The processes that occur in the schmutzdecke are enormously complex and varied, but the
principal one is the mechanical straining of most of the suspended matter in a thin dense layer in
which the pores may be very much less than a micron. The thickness of this layer increases with
time from the initial installation to the point where the flow rates become unacceptably small, when
it is usually about 25 mm. The greatest benefit of the SSF lies in its ability to trap bacteria and
viruses in this schmutzdecke. Bacterial and biological activity maximizes in there but will continue
at a decreasing level down into the sand of the filter bed. A certain minimum level of dissolved
oxygen should be present to support the aerobic actions that occur in the bed. After the initial
installation of the SSF, the formation of the schmutzdecke and bacterial/biological activity in the
bed may take days or weeks depending strongly on the ambient temperature. The comparsion
between biosand filters (1 &2) and previous works is shown in Table (1).
Slow sand filters consistently demonstrate their effectiveness in removing suspended
particles with effluent turbidities below 1.0 nephelometric turbidity units (NTU), achieving 90 to 99
+ percent reductions in bacteria and viruses, and providing virtually complete Giardia lamblia cyst
and Cryptosporidium oocyst removal. The parameters limitations are shown in Table (2).
Chlorination processes are most widely used for water disinfestations. The effectiveness of
such chemical treatments in controlling plant pathogen depends on correct dosages and treatment
times, control of suspended particulate matter in the recycled water, and foolproof monitoring
systems. Chemical treatments have proven effective when used properly, however they are
relatively expensive and present safety issues to the handlers and the environment.
Sand filters cannot cope with heavy metals or other excessive pollutants. Their prime
purpose is to remove bacteria and particles. It is not appropriate to use the technology to clean up
water contaminated by chemicals. If water source does have a high level of contamination, ideally
you should locate a new one. If this isn't possible other methods of filtration may be used,
depending on the level of contamination. If the water contains sediment, it should be passed through
an initial settling tank before it gets to the sand filter.

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TIME AND TEMPERATURE
It normally takes a period of three weeks for the biologic layer to develop to maturity in a
new filter. During that time, both the removal efficiency and the oxygen demand of the filter
increase as the biologic layer grows and increase the pressure head drop Figure (6). The filters are
cleaned by stirring up the thin layer of sand at the surface and scooping out the resulting dirty water.
After cleaning, the removal efficiency declines somewhat, but increases very quickly to its previous
level as the bio-layer is re-established. Over time, the pore opening between the sand grains will
become clogged with sediment. As a result, the water flow rate through the filter will slow down.
To clean the filter, the surface of the sand must be agitated to re-suspend the sediment in the
standing water. The dirty water can be removed using a small container. The process can be
repeated as many times as necessary to regain the desired flow rate. After cleaning, it will take the
biolayer up to a week to re-establish itself and return the removal efficiency to its previous level.
The diffuser plastic plate of 10 mm mesh diameter was designed and laid on the free sand
surface to damp the strong shock of turbid grey water on sand bed and to prevent the disturbance of
the sand layer when water is poured into the filter.
Different experiments were conducted to study the water temperature affects. Water
temperatures ranged from 16 oC to 32 oC were experimented for biosand filter of (0.35 mm dia.)
and 1 m depth as shown in Figure (7).
The removal efficiency for biological treatment also depends upon empty bed contact time
(EBCT), results show that efficiency of COD and BOD5 increases when EBCT increases
(Figure(8)).

CONCLUSIONS:
Three sets of experiments were achieved to evaluate the optimum hydraulic loading rate for two
different grain sizes of sand media and two different depths of sands filter, all these experiments
were achieved on grey water.
1. The removal efficiency of BOD5 increases from 51% at hydraulic loading 680 L/hr/m2 to
83% when filter loading rate be tested at 212 L/hr/m2.
2. The removal efficiency of turbidity for the first biosand filter (sand effective diameter
d10=0.35 mm, porosity = 39%) was 89%, while the removal efficiency of the second
biosand filter (d10 = 0.75mm and porosity = 43%) was 81%.
3. The removal efficiency of COD increases when empty bed contact time (EBCT) increase.
The COD removal efficiency mesursed from 41% at EBCT = 48 min to 76% at EBCT = 9
min.
4. The increasing in water temperature increases the removal efficiency of BOD 5 and COD.
The BOD5 removal efficiency measured from 64% at T = 18 oC to 78% T = 32oC.

REFERENCES
1. Bellamy, W.D., Hendricks, D.W., Logsdon, G.S. (1985). Slow Sand Filtration:Influences of
Selected Process Variables. Journal of the American Water Works Association,77 (12), 62-
66.

2. Brink, D.R. and Parks, S. (1996). Update on Slow Sand/Advanced Biological Filtration
Research. In N. Graham and R. Collins (Eds.), Advances in Slow Sand and Biological
Filtration (pp. 11-18). England: John Wiley & Sons Ltd.

3. Collins, M.R., Eighmy, T.T., Fenstermacher J. M., Spanos, S. (1992). Removing

NaturalOrganic Matter by Conventional Slow Sand Filtration. Journal of the America Water
Works Association, May 1992, 80-90.

4. Ellis, K.V. (1985). Slow Sand Filtration. CRC Critical Reviews in Environmental control,
14 (4) pp315-354.

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Al-Qadisiya Journal For Engineering Sciences Vol. 4 No. 2 Year 2011

5. Fogel, D., Isaac-Renton, J., Guarsparini, R., Moorehead, W., Ongerth, J., (1993)Removing
Giardia and Cryptosporidium by Slow Sand Filtration. Journalof the American Water Works
Association, November 1993, 77-84.

6. Haarhoff, J., and Cleasby, J. L., (1991). “ Biological and Physical Mechanisms in Slow
Sand Filtration”, In G.S. Logsdon (Ed), Slow Sand Filtration: A report prepared by the Task
Committee on Slow Sand Filtration. New York: American Society of Civil Engineers.

7. Hendricks, D. (Ed.). (1991). Manual of Design for Slow Sand Filtration. Denver: American
Water Works Association (AWWA).

8. Hendricks, D. W. and Bellamy, W.D. (1991). “ Microorganism Removals by Slow Sand

Filtration”, In G.S. Logsdon (Ed), Slow Sand Filtration: A report prepared by theTask
Committee on Slow Sand Filtration. New York: American Society of Civil Engineers.

9. Li, G.B., Ma, J. and Du, K.Y. (1996). “Multi-Stage Slow Sand Filtration for the Treatment
of High Turbid Water”, In N. Graham and R. Collins (Eds.), Advances in Slow Sand and
Biological Filtration. England: John Wiley & Sons Ltd.

10. McMeen, C.R. and Benjamin, M. (1997). NOM Removal by slow sand filtration through
iron oxide-coated olivine, Journal of the American Waterworks Association, 89(2), 57-71.

11. Poynter, S. F. B. and Slade, J.S. (1977). The removal of viruses by slow sand filtration.
Progress in Water Technology. 9, 75-88.

12. Riesenberg, F., Walters, B.B., Steele, A., and Ryder, A.R. (1995). Slow sand filters for
small Water system. Journal of the American Waterworks Association, 87 (11), 48-56.

13. Sims, R.C. and Slezak, L.A. (1991). “Slow Sand Filtration: Present Practice in the United
States”, In G.S. Logsdon (Ed), Slow Sand Filtration: A report prepared by the Task
Committee on Slow Sand Filtration. New York: American Society of Civil Engineers.

14. Weber-Shirk, Monroe L. and Dick, R. (1997a). Physical-Chemical Mechanisms in Slow

Sand Filters. Journal of the American Water Works Association, 89(1) 87.

15. Weber-Shirk, Monroe L. (1997b) Biological Mechanisms in Slow Sand Filters.Journal ofthe
American Water Works Association, 89(2)72.

16. Wheeler, D., Bartram, J., and Lloyd, B. (1988). The removal of viruses by filtration through
sand. (As presented in Slow Sand Filtration: Recent Developments in Water Treatment
Technology 1988 edited by N.J.D.GrahamHalsted Press: a Division of John Wiley & Sons.

17. Collins, M. R. 1998. “Assessing Slow Sand Filtration and Proven Modifications.’’ In Small
Systems Water Treatment Technologies: State of the Art Workshop. NEWWA Joint
Regional Operations Conference and Exhibition. Marlborough, Massachusetts.

18. Collins, M. R. Assessing Slow Sand Filtration and Proven Modifications. In Small Systems
Water Treatment Technologies: State of the Art Workshop. NEWWA Joint Regional
Operations Conference and Exhibition. Marlborough, Massachusetts. 1998.

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19. Muhammad, N., Parr, J., Smith, M.D., and A.D. Wheatley. Removal of heavy metals by
slow sand filtration. Proceedings of the 23rd WEDC International Conference on Water
Supply and Sanitation, Durban, South Africa. (1997) 167-170.

20. Stauber, C.E.,Elliott, M.A., Koksal, F., Ortiz, G.M., DiGiano, F.A., and M.D. Sobsey.
Characterization of the biosand filter for E. Coli reductions from household drinking water
under controlled laboratory and field use conditions. Water Science & Technology. 54:3; 1–
7. IWA Publishing (2006).

Table (1) Shows The Comparison Between

Biosand Filters (1 & 2) and Previous Works

Biosand Filter Biosand Filter

Parameter (R1) Removal (R2) Removal Previous Works
Efficiency Efficiency
E. Coli - An Indicator of Fecal > 97% (Duke, 2006;
> 91% > 81%
Contamination Stauber, 2006)
Protozoa and Helminthes > 93% 90% > 99% (Palmateer, 1999)
Could not be Could not be
Viruses 80-90% (Stauber, 2005)
tested tested
Organic and Inorganic
56-81% 42-86% 50-90% (Palmateer, 1999)
Toxicants
Iron 90-95% (Ngai, 2007)

Most Suspended Sediments 98% 95% --------

Table (2) Shows The Parameters Limitations

Parameter Limitations
Turbidity <1.0 NTU
Coliforms 1-3 log units
Enteric Viruses 2-4 log units
Giardia Cysts 2-4+log units
Cryptosporidium Oocysts >4 log units
Dissolved Organic Carbon <15-25%
Biodegradable Dissolved Organic Carbon <50%
Trihalomethane Precursors <20-30%
Heavy Metals
Zn, Cu, Cd, Pb >95-99%
Fe, Mn >67%
As <47%

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Figure (1) Filtering Systems

Plate (1) Sand Of Plate (2) Sand Of Plate (3) Gravel

Effective Diameter 0.35mm Effective Diameter 0.75mm Layer Material

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Figure (2) Effect Of Flow Rate On Removal Of BOD5 Efficiency

Figure (3) Relationship Between Turbidity Of Influent

Water (NTU) And Removal Efficiency Of Trubidity

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Figure (4) Relationship Between COD (Mg/L) Of Influent

Water And COD Removal Efficiency For Filters (R1 And R3)

Figure (5) Relationship Between COD (Mg/L) Of Influent

Water And COD Removal Efficiency For Filters (R1 And R2)

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Figure (6)Effect Of Grey Water Temperature On BOD5

Removal Efficieny

Figure (7) Relationship Empty Bed Contact Time (EBCT) And

Water Head Per Unit Depth Of Sand (M/M Sand ∆H/∆L)

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Figure (8) Effect Of Grey Water Temperature On Relationship Empty

Bed Contact Time (EBCT) And COD Removal Efficiency

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