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\begin_body

\begin_layout Title
Stacked Rapid Sand Filtration
\end_layout

\begin_layout Author
Ziyao Xu, Alli Hill, William Maher, Min Pang, Michael Adelman
\end_layout

\begin_layout Abstract
Stacked filters have been demonstrated as a novel alternative to traditional
 rapid-sand filtration, and their efficiency makes them an appropriate process
 for sustainable municipal-scale drinking water facilities.
 In this study, a pilot-scale apparatus was set up as a model of the hydraulic
 controls for the stacked filter system.
 This pilot-scale system was used to develop and provide experimental justificat
ion for stacked filter design equations.
 In addition, a bench-scale apparatus was developed with a single-layer
 rapid-sand filter to study fundamental questions such as removal efficiency
 in upflow and downflow filtration.
 Results to date have successfully clarified aspects of the hydraulic design
 and provided some insight into flow within the layers of the filter.
\end_layout

\begin_layout Abstract
\begin_inset Newpage pagebreak
\end_inset


\end_layout

\begin_layout Section*
Background
\end_layout

\begin_layout Subsection*
Rationale
\end_layout

\begin_layout Standard
Traditional rapid-sand filtration is a familiar and widely-used water treatment
 process, but its technical complexity makes it impractical for small communitie
s especially in the developing world.
 Since the backwash cycle requires high velocity, expensive equipment such
 as elevated tanks or electric pumps must be used to provide backwash flow.
 It is possible to have fully hydraulic filters in traditional design, but
 the number of filter boxes would need to be the same as the backwash velocity
 over the filtration velocity, so that all of these parallel filters could
 backwash an additional one.
 The Stacked Rapid Sand Filter (SRSF) overcomes this drawback.
 It requires only one filter tank to accomplish filtration and backwash,
 and uses the same total flow rate for both cycles.
 Additionally, with only one valve, it can change from filtration mode to
 backwash mode and vice versa.
 It saves water during backwash, reduces cost of infrastructure, and eliminates
 requirement for pumps.
 
\end_layout

\begin_layout Standard
Our SRSF research team has conducted several experiment at both the pilot
 scale and the bench scale.
 We have studied the siphon system, flow distribution in the sand layers,
 control system configuration, upflow and down performance comparison, as
 well as a proposed sand removal systems.
 Our goals for this semester were to gain insight into important design
 and operating parameters for the SRSF, and investigate questions of fundamental
 importance to the viability of this technology.
\end_layout

\begin_layout Subsection*
Literature Review
\end_layout

\begin_layout Paragraph*
Mayer, V.
 (1983).
 
\begin_inset Quotes eld
\end_inset

Rapid Sand Filters for Advanced Wastewater Treatment: Upflow or Downflow?
\begin_inset Quotes erd
\end_inset

 GWF, Wasser-Abwasser, 124(5), 213-220.
\end_layout

\begin_layout Standard
This article comments on the upflow vs.
 downflow debate in rapid sand filtration design and operation, and compares
 the performance of rapid sand filters operating in upflow or downflow based
 on pilot-scale and full-scale data.
 The author concludes that downflow filtration is advantageous for the particula
r application of tertiary suspended solids removal at a wastewater plant.
 The effectiveness of filtration performance in the upflow and downflow
 directions is a question of great significance for the SRSF, which has
 both upflow and downflow layers during the filtration cycle.
\end_layout

\begin_layout Paragraph*
Han, S., Fitzpatrick, C.S.B., and Wetherill, A.
 (2008).
 “Mathematical Modelling of Particle Removal and Head Loss in Rapid Gravity
 Filtration.” Separ.
 Sci.
 Technol., 43(7), 1798-1812.
\end_layout

\begin_layout Standard
This paper presents a model that describes particle removal mechanisms and
 the development of head loss in the bed of rapid-sand filters.
 The model consists of three 
\begin_inset Quotes eld
\end_inset

stages
\begin_inset Quotes erd
\end_inset

 of particle removal that are manifest during the course of a filtration
 cycle.
 The results of the model were compared with full-scale data from a working
 water plant.
\end_layout

\begin_layout Paragraph*
O'Connor, J.T.
 and O'Connor, T.L.
 (2002).
 
\begin_inset Quotes eld
\end_inset

Rapid Sand Filtration.
\begin_inset Quotes erd
\end_inset

 In Control of Microorganisms in Drinking Water.
 Reston, VA: American Society of Civil Engineers, 127-148.
\end_layout

\begin_layout Standard
This article discusses the prevalence of rapid sand filtration as a means
 of removing microbial pathogens from drinking water.
 It discusses the role of rapid sand filtration, following flocculation
 and sedimentation, as the last barrier between suspended particles and
 the public drinking water distribution system.
 The article also discusses the importance of using coarse-grain filter
 media for meeting drinking water turbidity standards.
\end_layout

\begin_layout Paragraph*
Lin, P.-H.
 (2010).
 
\begin_inset Quotes eld
\end_inset

Filter media modification in rapid sand filtration.
\begin_inset Quotes erd
\end_inset

 Thesis (Ph.D.)--Cornell University, May 2010.
 
\end_layout

\begin_layout Standard
This thesis focuses on developing improved operating methods for rapid sand
 filtration.
 It discusses several different coagulants like alum, ferric chloride and
 poly-aluminum chloride which are applied to modify the sand filter medium,
 concerning their dosage and pretreatment.
 Moreover, it introduces a novel fluidized-bed pretreatment process which
 enhances the efficiency of turbidity removal.
 In addition, the thesis discusses and explains the mechanism related to
 the improvement of the filter performance by the presence of coagulants.
\end_layout

\begin_layout Section*
Materials and Methods
\end_layout

\begin_layout Subsection*
Pilot-scale Apparatus
\end_layout

\begin_layout Standard
First, the pilot-scale control system was set up to test the siphon.
 This pilot-scale apparatus includes a bucket representing the filtration
 inlet channel with four pipe stubs connected to hoses through the bottom
 of the bucket.
 Each of the four hoses runs from the bottom of the inlet bucket to one
 of the four filtration column inlet manifolds.
 The hose connected to the bottom inlet manifold is connected to the shortest
 pipe stub, and the hose connected to the top inlet manifold is connected
 to the second shortest pipe stub.
 The two hoses connected to the middle inlet manifolds are connected to
 the tallest pipe stubs.
 This configuration allows the water level in the inlet bucket to fall below
 the top of the higher pipe stubs during backwash initiation, thereby cutting
 off flow to the middle inlet manifolds.
 The siphon is configured to hold an air trap during filtration mode, and
 then backwash mode can be initiated by releasing the air trap and allowing
 the siphon to transport the total system flow to the backwash outlet.
 The control system was run in cycles beginning with filtration mode, followed
 by initiating backwashing by breaking the air trap, followed by stead-state
 backwashing, followed by initiating filtration by forming the air trap,
 and so on and so forth.
 Particular attention was paid to the water level in the inlet bucket and
 flow of air and water through the siphon during these cycles.
 A diagram of this apparatus is found in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Pilot-scale apparatus"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Pilot Scale Apparatus.png

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Pilot-scale apparatus"

\end_inset

Diagram of the pilot-scale apparatus for the control system study, showing
 the water levels during (a) the filtration cycle and (b) the backwash cycle
 along with important head losses.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
\align left
Second, the pilot-scale apparatus was modified for subsequent pilot-scale
 studies.
 The inlet bucket has been replaced by a clear 4
\begin_inset Quotes erd
\end_inset

 pipe with the same configuration of stubs and hoses.
 The goal of the new clear 4
\begin_inset Quotes erd
\end_inset

 pipe is twofold: first, it will allow for visual monitoring of water level
 in the inlet channel and second, it provides a reduced diameter of the
 wetted perimeter in the inlet channel and reduces the volume of water that
 sits in the inlet channel during filtration mode.
 
\end_layout

\begin_layout Standard
It is worth mentioning that the bottom of the backwash siphon-capture pipe
 has been replaced with clear PVC to allow for the visual monitoring of
 flow through the siphon.
 The goal is to visually identify whether air is completely trapped in the
 siphon or if some bubbles flow out through the bottom of the siphon as
 the water level in the filtration column rises.
\end_layout

\begin_layout Subsection*
Pilot-scale Experiments
\end_layout

\begin_layout Standard
Specific experiments required certain specific modifications to the pilot-scale
 apparatus, as described below.
\end_layout

\begin_layout Standard
First, a test was conducted to demonstrate the sand removal system.
 The test apparatus was modified to model the full-scale sand removal system
 that has been constructed in Tamara, which is intended to allow filter
 media to be readily removed for maintenance as needed.
 The modification required the drilling of a hole in the filtration column
 at the arbitrarily selected place of about 10 cm higher than elevation
 of the backwash exit weir.
 A bulkhead fitting was installed in hole in the filtration column, and
 a 1/4
\begin_inset Quotes erd
\end_inset

 ID tube was connected to the bulkhead fitting on the interior of the filtration
 column.
 The tube ran lengthwise all the way down the filtration column so that
 the end of the tube was embedded in the sand at an elevation about 5 cm
 higher than the lowest filter column inlet.
 The tube was installed during backwash because the tube can only be inserted
 into a fluidized bed.
 A tube of the same diameter was connected to the exterior of the filtration
 column by the bulkhead fitting, and that tube was run to the sink, with
 the tip having an elevation of 76 cm below the backwash exit weir to provide
 the driving head for this system.
 The sand slurry was discharged into a sieve in the sink to collect the
 data for this study.
 
\end_layout

\begin_layout Standard
The following diagram (
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Sand removal system"

\end_inset

) shows the schematic for this test on the pilot-scale.
 The design for the selection of ID for the sand removal tube is dominated
 by the need to achieve flow velocity in the tube within the range between
 the minimum scour velocity of sand and the maximum allowable velocity,
 which is the highest velocity that will not affect the normal performance
 of the system in backwash mode.
 This maximum allowable removal velocity can be calculated by solving for
 the removal flow rate that would bring the backwash velocity in the filter
 column to its threshold of 5 mm/s.
 This algorithm helped us select a tube of OD 3/8
\begin_inset Quotes erd
\end_inset

, which provides a sand removal flow rate of 1.277 L/min and a removal velocity
 of 0.672 m/s, while maintaining an upflow velocity of 7 mm/s in the filter
 to keep the sand fluidized.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Sand Removal Setup.png
	scale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Sand removal system"

\end_inset

Diagram of sand removal study apparatus and key dimensions.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
Another modification was made to test the possibility of an alternate design
 for the backwash pipe, basin, and weir that has been constructed in Tamara.
 The backwash basin was completely removed from the test apparatus, and
 the lower end of the backwash siphon pipe was connected to a series of
 two additional elbows and two additional pipes to create a U-shaped plumbing
 trap, or water seal.
 This modification is motivated by the need to investigate affordable alternativ
es to the initial design used in Tamara.
 The large diameter of the backwash basin incorporated by the initial design
 comes at a higher cost than a U-shaped water seal configuration made of
 pipe with the same diameter as the backwash siphon.
 The new configuration for the U-shaped water seal maintained the same final
 backwash weir elevation as had been implemented by the prior apparatus
 configuration.
 A diagram of this modification to the pilot-scale equipment is found in
 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Water seal apparatus"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Alternative siphon setup.png
	scale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Water seal apparatus"

\end_inset

Diagram of water seal apparatus in the pilot-scale experimental system,
 with key dimensions.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
A third modification was made to the pilot-scale apparatus to measure and
 compare the flows through the six individual sand layers of the filtration
 column.
 The filtration column pipe was tapped, with two holes and fittings installed
 on each of the six layers.
 The holes were oriented so that they lined up with the vertical axis of
 the filtration column, and the holes were spaced regularly such that they
 each had 10 cm elevation spacing between them, centered on each sand layer.
 The spacing was selected to include an area with only vertical flow in
 the sand, and exclude the area of influence of the inlet and outlet slotted
 pipes in the sand bed.
 Pressure sensors were installed across each of the six pairs of tapped
 holes, and the sensors were calibrated so that when calculations were applied,
 they reported positive flow in each layer during filtration mode.
 The motivation for this test is to analyze the uniformity of flow distribution
 between the six layers during filtration mode.
 The setup of these pressure sensors is shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Pressure sensor apparatus"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Flow Sensor Diagram.png
	scale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Pressure sensor apparatus"

\end_inset

Diagram of showing the setup of pressure sensors to measure flow in each
 layer of the SRSF.
 The white lines are conceptual flow lines in the sand, and the pressure
 sensors were placed to keep the pressure sensor outside of the area of
 influence of the inlets and outlets.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Subsection*
Bench-scale Apparatus
\end_layout

\begin_layout Standard
A filter has been set up at the bench scale to test turbidity removal performanc
e of upflow versus downflow.
 This has involved rigging a series of tubes and valves to pump water, dose
 alum, dose clay, and monitor influent and effluent turbidity.
 Each dose and sampling system has a pump which requires calibration, and
 solenoid valves are controlled by the process controller software to control
 the system in upflow and downflow.
 The bench-scale apparatus to compare upflow and downflow performance is
 shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Upflow/downflow PFD"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Bench Scale Apparatus.jpg
	scale 75

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Upflow/downflow PFD"

\end_inset

Process-flow diagram of the bench-scale apparatus for (a) upflow and (b)
 downflow filtration.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
This apparatus was modified to study the self-healing capabilities of nonuniform
 flow among layers in a stacked filter.
 Two parallel filter columns were set up, each with a flow sensor and turbidimet
er.
 Standard rapid-sand filtration sand (effective size 0.5 mm, uniformity coefficie
nt 1.65) was passed through a #30 soil sieve to create coarse and fine fractions,
 and one column was loaded with each fraction.
 Water with 10 NTU influent turbidity and 1.5 mg/L of coagulant as alum was
 passed through these filters, and the distribution of flow between the
 columns was logged throughout the test.
 Flow sensors and turbidimeters were placed in this system downstream of
 a drop tube, so that the head loss in the sensors would not affect flow
 distribution between the columns.
 It is hypothesized that a scenario like this might be seen in an SRSF with
 insufficiently uniform filter media, but we further expect that the problem
 of uneven flow will be self-correcting as increased solids loading to one
 column will increase head loss and re-allocate flow to the other column.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Self-healing PFD Revised.png
	scale 75

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Self-healing PFD"

\end_inset

Process flow diagram of the bench-scale apparatus to study flow distribution
 and the self-healing nature of uneven flow in filters.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Section*
Results and Discussion
\end_layout

\begin_layout Subsection*
Siphon Failure Limits
\end_layout

\begin_layout Standard
The first pilot-scale experiments involved putting the siphon through control
 system tests, which has shown siphon failure in the system.
 When the system is in filtration mode, the water rises in the filter column
 and increases pressure on the air trap that is in the siphon.
 What we have found, through simple visual observation, is that this pressure
 becomes too much for the air trap and the water begins to flow into the
 siphon pipe.
 Our experimentation consisted of putting the system through its normal
 filtration and backwash cycle, while carefully taking measurements of the
 heights of the water in the inlet bucket as well as in the column when
 the siphon just began to leak.
 Water elevations at the failure point are shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Siphon failure diagram"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Siphon Failure.png

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Siphon failure diagram"

\end_inset

Diagram showing water levels observed at siphon failure, when water began
 leaking into the siphon.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
All of our data in this part of the experiment was pretty consistent, and
 also coincides with the predictions from the design equations that tell
 us the water is supposed to go over the siphon only about 56 cm.
 The air trap at equilibrium is described by the following system of equations:
 
\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{equation}
P_{Air}=\rho_{Water}g\left(H_{Rise}-H_{1}\right)+1atm
\end{equation}

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{equation}
P_{Air}=\rho_{Water}gH_{2}+1atm
\end{equation}

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{equation}
P_{Air}=\frac{n_{Air}RT}{V_{Air}}
\end{equation}

\end_inset


\end_layout

\begin_layout Standard
where 
\begin_inset Formula $P_{Air}$
\end_inset

 is the gauge pressure of the air; 
\begin_inset Formula $H_{Rise}$
\end_inset

 is the height that water has risen in the filter column over the inlet
 of the siphon; and 
\begin_inset Formula $H_{1}$
\end_inset

 and 
\begin_inset Formula $H_{2}$
\end_inset

 are the heights of water in the siphon, within the filter column and in
 the backwash box, respectively.
 For a water rise as shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Siphon failure diagram"

\end_inset

, these equations predict that the water in the siphon pipe will rise sufficient
ly to begin spilling over into the horizontal part of the siphon.
 The experimental observations show that these equations are valid for the
 purposes of siphon design.
 They also allowed us to select a higher position to place the siphon pipe
 so that it would not leak during the filtration cycle.
\end_layout

\begin_layout Subsection*
Riser Pipe Configuration Test
\end_layout

\begin_layout Standard
Three possible configurations were proposed for the riser pipes in the filter
 inlet box.
 Configurations tested with the pilot-scale apparatus are illustrated in
 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Proposed pipe stub configuration"

\end_inset

, and include:
\end_layout

\begin_layout Itemize
Configuration (a): three riser pipes of the same height, to automatically
 turn the top three filter inlets on and off during the mode switch
\end_layout

\begin_layout Itemize
Configuration (b): the reduced size of the riser pipe for the topmost inlet
 allows most water to drain from the inlet box and exit the filter without
 passing through the sand and possibly causing excess fluidization
\end_layout

\begin_layout Itemize
Configuration (c): staggered inlet pipes to fluidize the layers of the filter
 two at a time instead of all at once, possibly reducing the initial head
 requirement during backwash
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Pipe Stub Configurations.png
	scale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Proposed pipe stub configuration"

\end_inset

Proposed configurations of inlet box riser pipes and rationale for each.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
After testing the three different pipe stub configurations in the pilot
 scale model, we have come to the conclusion that Configuration (a) is the
 most effective configuration.
 We have concluded that the necessary height for the three variable pipe
 stubs lies between the steady water height during filtration mode and the
 steady water height during backwash mode.
 As to what the heights of the individual pipes should be has proven to
 be irrelevant.
 With Configuration (b), there was little excess fluidization observed with
 the pilot-scale system at the beginning of backwash.
 This leaves us with little reason to put much time into the testing of
 this configuration.
 Likewise, Configuration (c) shows a water level behavior similar to that
 of (a), but with more activity between the filtration and backwash water
 levels, which gives no further effect to the system.
 Therefore, the simplest and most effective configuration would be the one
 which puts three equal-height pipe stubs at a level happily between the
 water levels during filtration and during backwash.
\end_layout

\begin_layout Standard
Water level logs are shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Water level logs"

\end_inset

, which plot the height of water in the inlet box and the height of water
 over the top of the sand in the filter box.
 These results show that both cycles of operation work effectively with
 the control system configured as in (a) or (b).
 The equilibrium heights of water and head losses, both for backwash and
 clean-bed filtration, are the same with either configuration.
 The major difference can be seen in the data from the inlet box water level
 during the transition to filtration mode, where the flat lines on the curve
 reflect the heights of the pipe stubs in the inlet.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Water level logs a.png
	scale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Graphics
	filename Water level logs b.png
	scale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Water level logs"

\end_inset

Water level data for the filtration and backwash cycles, for (a) pipe stubs
 of equal size and (b) pipe stubs of staggered size in the inlet box.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Subsection*
Water Seal Siphon Study
\end_layout

\begin_layout Standard
It was conceived to implement a U-shaped water seal instead of the backwash
 basin for holding the siphon's air trap.
 The expectation is that in full scale implementation, the U-shaped water
 seal, consisting of a length of pipe with two elbows, will cost less to
 construct than a concrete basin.
 Furthermore, draining a pipe will prove to be simpler than draining a basin
 in the field, especially if the backwash effluent contains sedimentary
 particles.
\end_layout

\begin_layout Standard
The pilot-scale apparatus was adjusted to implement the water seal by completely
 removing the backwash basin and extending the siphon pipe down to the floor,
 adding two elbows, and running the pipe back up to the elevation required
 for the backwash weir.
 The new water seal configuration successfully held the air trap and allowed
 for cycling between modes.
 Since the new water seal pipes in the pilot-scale apparatus are clear,
 it allowed for direct observation of water levels in the siphon pipe during
 filtration mode, while the air trap was in place.
\end_layout

\begin_layout Standard
Measuring from the datum at the top of the sand during filtration, it was
 observed that at the beginning of filtration, the top of the water in the
 siphon pipe within the filter column reached 130 cm above the datum.
 Simultaneously, the top of the water in the siphon pipe leading to the
 backwash weir reaches 30 cm below the datum.
 These results are shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Water seal siphon data"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Water seal data.png
	scale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Water seal siphon data"

\end_inset

Steady-state equilibrium in filtration mode for the water-seal siphon study.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
These observations concur with the expected water levels based on the prediction
 from theoretical hydrostatics.
 The pressure head from the water level in the filtration column over the
 water level in the adjacent siphon pipe equals the pressure head from the
 backwash weir over the water level in the water seal pipe.
 This hydrostatic equilibrium holds the air trap in place because air has
 negligible density.
 Equations (1)-(3) can be used to show that the observed steady-state equilibriu
m for filtration was consistent with our hydrostatics model.
\end_layout

\begin_layout Subsection*
Sand Removal System Test
\end_layout

\begin_layout Standard
During the sand removal system experiment, we have successfully removed
 the sand down to the end of the pipe, which is halfway between the inlet
 and outlet pipes of the lowest layer.
 We notice that the bed remains fluidized during the process and sand is
 removed as a slurry.
 However, we also encounter problem related to the air entering the siphon.
 As sand being removed, water level in the inlet falls, it goes below the
 bottom of the inlet box and entrains significant amounts of air.
 The air goes into the siphon system and increases its head loss dramatically
 and creates a risk of breaking the siphon.
\end_layout

\begin_layout Standard
According to the calculation, the sand removal flow rate at the beginning
 should be 1.27 L/min.
 In our experiment, we measured the flow rate one minute after the removal
 system started, and we observed a 1.22 L/min flow.
 The initial value we measured was almost the same as the design prediction.
 As we observe in the test that the flow rate decreases during the process,
 there is a great possibility that the little difference between the predicted
 and observed value was due to time difference.
 The data taken through the course of a test of this sand removal system
 is shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Sand removal data"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Sand Removal Plots.emf

\end_inset


\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Sand removal data"

\end_inset

Water levels and sand flow rate over time during the sand removal system
 test.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
When sand is being removed from the filter, the aggregate density of the
 filter bed volume is decreasing while the density in the sand removal tube
 is roughly constant.
 This density difference reduces the net available head driving the sand
 removal system, and therefore causes the sand flow rate to decrease during
 the test at the beginning.
 However, as shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Sand removal data"

\end_inset

, sand flow rate stops decreasing near 600 s into the run, at the same time
 the water level in filter column starts to rise.
 That is because the air trapped in the siphon causes the water level in
 the filter column to rise, thus increasing the driving head on the sand
 removal system.
 As a result, the sand flow increases.
 
\end_layout

\begin_layout Standard
As more head loss in the siphon system increases the sand flow rate, we
 suggest that adding a screen over the inlets in order to add more head
 loss.
 This may be required to effectively use the sand removal system in the
 field.
 
\end_layout

\begin_layout Subsection*
Flow Distribution Study
\end_layout

\begin_layout Standard
The flow distribution among layers of a stacked filter plays an important
 role in the performance of the filtration cycle.
 We have successfully measured the flow distribution among each of the six
 layers of the pilot-scale SRSF and logged data by Process Controller.
 The interpretation of our flow data will be based on Darcy’s Law.
 Darcy's law is a simple proportional relationship between the instantaneous
 discharge rate through a porous medium, the hydraulic conductivity of the
 medium, and the pressure drop over a given distance.
 By simplifying the origin equation, we derive the equation below:
\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{equation}
v=K\nabla P=K\frac{\Delta H}{\Delta L}
\end{equation}

\end_inset


\end_layout

\begin_layout Standard
Where 
\begin_inset Formula $v$
\end_inset

 is flow velocity to hydraulic gradient, 
\begin_inset Formula $\nabla P$
\end_inset

 is hydraulic gradient and 
\begin_inset Formula $K$
\end_inset

 is hydraulic conductivity.
 Note that the hydraulic conductivity
\begin_inset Formula $K$
\end_inset

 is a material property of the porous media that describes the ease with
 which water can move through pore spaces or fractures.
 The hydraulic conductivity of the sand we are using in the pilot-scale
 SRSF is estimated to be about 5.3 mm/s.
\end_layout

\begin_layout Standard
Before each run of the filtration and backwash cycles, we zeroed each of
 the six layers.
 This must be based on the condition that there is no flow (zero flow) in
 the SRSF column.
 Assuming flow is uniformly distributed across all layers, the sensors should
 show a near consistent pressure drop during filtration mode, and a pressure
 drop of around 3.4 cm is expected to be observed.
 Due to the upflow through all the six layers during backwash, the sensors
 installed on Layers 1, 3 and 5, which normally experience downflow, will
 record negative head losses whereas the sensors installed on Layers 2,
 4 and 6 still show positive head losses.
 We expected that the measured head loss should spike to just under 10 cm
 in absolute value.
 By switching between the filtration mode and backwash mode, we obtain a
 beautiful diagram as shown below (
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Clean bed flow distribution"

\end_inset

).
 Note that we usually run the filtration longer than backwash, which is
 also consistent with the SRSFs in reality.
 In addition, we periodically shut off flow during the test to check whether
 the sensors maintain zero as expected.
 It appears from this data that flow is relatively well distributed among
 layers during filtration, at least initially.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Clean-bed Flow Distribution.jpg

\end_inset


\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Clean bed flow distribution"

\end_inset

Plot of flow sensor readouts for the clean-bed tests.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
Additional tests were carried out where 5-10 NTU influent water with 1.5
 mg/L coagulant as alum was loaded into the SRSF, and the distribution of
 flow among the layers was tracked during the course of a 7-hour filtration
 cycle.
 Data is shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Loaded bed flow distribution"

\end_inset

 below.
 The results of this test are consistent with our hypotheses.
 First of all, head loss in all layers increased steadily over the course
 of the experiment as contaminants filled the pores in the filter and lowered
 the effective Darcy K.
 Secondly, even though one layer started out receiving more flow than the
 others, the flow became more uniform over the course of the cycle as solids
 loading pushed the system closer to a uniformly-distributed equilibrium.
 Note also that performance was good, with effluent turbidity generally
 below 0.3 NTU.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Loaded-bed flow distribution.png

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Loaded bed flow distribution"

\end_inset

Plot of flow sensor readouts from the tests with 5-10 NTU influent over
 a 7-hour filtration cycle.
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Subsection*
Upflow-Downflow Performance Comparison
\end_layout

\begin_layout Standard
In our bench-scale study of the effectiveness of upflow and downflow through
 the layers of stacked sand, we measured the turbidity of the influent/effluent
 water in both modes.
 This experiment is meant to show us whether or not the assumption of equality
 of the upflow and downflow in the pilot-scale was correct.
 Each run of the experiment was preceded by backwash and vibration-assisted
 settling to a 20 cm bed depth.
 We collected 6 hours of data for downflow, followed by an automatic switch
 to a 6 hour data collection for upflow.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Upflow-downflow Results Spikes.png

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Upflow/downflow bad"

\end_inset

Upflow/downflow comparison results with spikes of turbidity in the effluent
 during the upflow cycle.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard

\end_layout

\begin_layout Standard
As you can see in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Upflow/downflow bad"

\end_inset

, there is a clear and systematic difference between the downflow and upflow
 cycles, with downflow performance significantly better.
 The major difference is the “spikes” of turbidity in the upflow effluent,
 as the baseline of the effluent and pC* graphs appear to be similar in
 upflow and downflow.
\end_layout

\begin_layout Standard
We hypothesized that the spikes in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Upflow/downflow bad"

\end_inset

 are due to contaminants that settled out on top of the sand during the
 downflow cycle, then were carried into the effluent from time to time during
 the upflow cycle.
 Therefore, an additional upflow/downflow comparison test was run with a
 backwash cycle to clean the filter bed and remove any settled contaminants
 between cycles.
 The results of this test are shown in 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Upflow/downflow good 1"

\end_inset

 and 
\begin_inset CommandInset ref
LatexCommand ref
reference "fig:Upflow/downflow good 2"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Upflow-downflow Performance Good.png

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Upflow/downflow good 1"

\end_inset

Upflow/downflow turbidity removal data for a 14-hour test with a backwash
 between cycles
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename Upflow-downflow pC Good.png

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
\begin_inset CommandInset label
LatexCommand label
name "fig:Upflow/downflow good 2"

\end_inset

Upflow/downflow performance as pC* during a 14-hour test with a backwash
 between cycles
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
These figures show that, under comparable operating conditions and using
 an entire filtration cycle of data, there does not appear to be a significant
 difference in particle removal performance between upflow and downflow
 filtration.
 This is an important result for the viability of the SRSF concent.
\end_layout

\begin_layout Section*
Conclusions and Recommendations
\end_layout

\begin_layout Itemize
The placement of the siphon pipe through the walls of the filter is an important
 design constraint to make sure that the air trap will work correctly during
 the filtration cycle.
 The design equations used for the field-scale filter appear to be valid
 for the failure modes of concern.
\end_layout

\begin_layout Itemize
The riser pipes in the inlet box should be placed between the steady-state
 backwash water level and the clean-bed water level during the filtration
 cycle.
 Within this window, either pipe stubs of a constant size or staggered size
 appear to be viable.
\end_layout

\begin_layout Itemize
The sand removal system, which has already been implemented in the field,
 is functioning properly and effectively removes all sand from the column.
 The only problem with the system is that the flow rate of the sand decreases
 significantly as sand is removed from the filter.
 A head loss element may be required for the siphon in the field to increase
 the flow of sand.
\end_layout

\begin_layout Itemize
At the bench scale, there does not appear to be a significant difference
 between the upflow and downflow filtration performances.
 
\end_layout

\begin_layout Itemize
Our tests of the flow rate using the sensors in the pilot scale show that
 the flow during clean bed conditions is relatively uniform, and the flow
 in each layer converges during the course of a filtration cycle.
 
\end_layout

\begin_layout Section*
Future Work
\end_layout

\begin_layout Standard
In the bench scale, we are in the process of adding a second column to the
 system.
 This second filter will be filled with sand of a different grain size than
 is in the current bench-scale filter.
 The purpose of this test is to find whether the flow distribution evens
 out in the two parallel filters over time.
 We expect that the flow within these two parallel filters will experience
 a somewhat 
\begin_inset Quotes eld
\end_inset

self healing
\begin_inset Quotes erd
\end_inset

 effect, distributing the flow proportionally through each filter.
 We expect that reducing the sand grain size will lower the range of filtration
 and backwash velocity, and work better for filtration at low flow rates.
 Other future priorities for the SRSF research will include:
\end_layout

\begin_layout Itemize
Experimental studies to specify the optimal filter medium, in terms of sand
 grain size and uniformity.
\end_layout

\begin_layout Itemize
Tests of a 4-layer instead of 6-layer SRSF configuration, to investigate
 the possibility of using higher velocities with deeper sand layers.
\end_layout

\begin_layout Itemize
Test of an air elimination system for the filter inlet channel, to address
 a problem that has been observed in the field.
\end_layout

\end_body
\end_document
