Sedimentation tank design for rural communities in the hilly regions of Nepal

Mathillo Semrang in rural Nepal relies on stream sources to provide drinking water. Erosion and deforestation of local terrain produces turbid water that requires treatment before distribution. The round gravity sedimentation technology currently employed is large in footprint and cannot handle large increases in in! uent ! ow rate and silt concentrations resulting from extreme weather events. In partnership with Nepal Water for Health (NEWAH) and Engineers Without Borders Australia (EWB), this project aimed to design a highly simpli" ed, small footprint inclined plate settler (IPS) to treat in! uent ! ow-rates ranging from 0.25 to 4 L/s. A 76 % decrease in footprint was achieved by the IPS design for source ! ow-rates up to 4 L/s. Laboratory analysis revealed that a large inlet area along with a highly sloped ! oor is essential to the design which may prove problematic in the reduction of footprint in the design. Further research is needed to con" rm this " nding along with further collaboration with rural Nepalese communities and NEWAH.


Drinking water in Thumi VDC -Nepal
The secure provision and distribution of drinking water is crucial in boosting socio-economical development and increasing standards of living in developing countries.Nepal ranks among the top nations in terms of fresh water potential but the majority of the population in rural areas do not have access to safe drinking water (Water Resources Management Committee, 2010).One such rural village is Mathillo Semrang in the Thumi Village Development Community (VDC) in the Western Region of Nepal.
Drinking water in Mathillo Semrang is sourced from hill and mountain streams that are subject to high sediment loads, particularly during monsoon and landslide events.Large concentrations of ne sediments in suspension result in highly turbid water sources.Deforestation and land degradation are key contributors to high sediment in ows (Julien & Shah, 2005).
Nepal Water for Health (NEWAH) is the national Nepalese non-government organisation established to address the water and sanitation (WASH) needs of the rural and remote communities of Nepal (Nepal Water for Health, 2011b).Many NEWAH WASH programs involve the upgrade or installation of water supply systems from source to tap.In the hilly western regions of Nepal, where sources are turbid, NEWAH and local workers install sedimentation facilities prior to the distribution of water to villages.These sedimentation facilities consist of round sedimentation tanks employing gravity sedimentation.However, it has been identi ed by NEWAH that in many cases there is insuf cient space for the existing technology.And where the technology is of an appropriate size, it is not robust enough to cater for extreme weather conditions that result in a rapid increase in volumetric ow, such as monsoonal rainfall or landslides.

Description of current design
The current system employed by NEWAH is a series of ve round "Ferrocement Water Filter" tanks.These tanks consist of a round clari cation device with a rectangular valve box attached at the rear (Figure 1 and Figures 2a/b).The tank is doughnut in shape and contains two settling basins.The in uent water enters the central basin where it is allowed to settle.The settled particles collect at the base of the vessel via a sloping oor.The clari ed ef uent in the central basin then enters the outer basin through a transfer pipe.This water travels in a clockwise direction to settle; this clockwise movement reduces the potential for short-circuiting in the system.The clari ed ef uent in this outer basin leaves the system through the outlet pipe and travels straight to distribution.The settled particles are removed from the system via a "washout cycle".This involves entering the valve box via the manhole.The valve servicing the outlet pipe is closed to allow the opening of the washout pipes.The normal operation of the system then allows the accumulated settled particles to be ushed from the system.The round tank design is a simple and cost effective technology consisting of basic building materials that are locally sourced.The two round chambers and the rectangular valve box are all constructed using Ferrocement.Simple pipe and valve ttings are included to channel the in uent and ef uent through the system.
The tank designs vary in cross-sectional area to suit the influent flow rate to the system (Figure 3). Figure 4 provides the operational range of the ve Ferrocement water tanks currently in use by NEWAH.The tanks cover ow-ranges from 0.25 L/s to 3 L/s.Flow-rates greater than 3 L/s require the design of a complex sedimentation tank.Mathillo Semrang has a maximum stream source ow-rate of 0.32 L/s and currently employs the use of the Type 2 tank with a maximum area of 7.55 m 2 or a tank diameter of 3.1 m.

Current design methodology
Although it is not wholly certain where the speci c design methodology for the round technology originates, discussion with NEWAH suggests that the basic design principles, drawings and construction techniques have been adapted from a Nepalese NGO published text entitled "Rural Gravity Flow Water Systems" (Neku & Hillman, 1996).This text deals with rural water infrastructure design in a global context and makes no speci c mention of the design considerations necessary for rural Nepal, nor the effect of conditions such as extreme weather events may have on the design process.
Although the text does not detail speci cally, it is suggested that the cross-sectional area of the tank is calculated using the principal of Stokes' Law.NEWAH documentation states that the crosssectional area calculation of the NEWAH design

Fig ure 3:
Cross-sectional area of current tank design with respect to the influent flow-rate.

Fig ure 4:
Variation in the diameter of the current technology with respect to the influent flow-rate.

Limitations of current design
Observational evidence from NEWAH suggests that the current design is considered to be an appropriate low-cost method of water treatment, provided the influent flow-rate to the system is constant.The limitation inherent in such a design is that it provides little exibility for the design to handle shock increases in ow-rate greater than the design ow-rate.
Although there is no data to cite the difference between flow-rates during normal rainfall and extreme weather events, taking a reasonable gure such as 0.64 L/s (double the average maximum ow-rate experienced in Mathillo Semrang) would necessitate the construction of a 'Type 3' tank which requires an increase in diameter of 42 % (3.1 m to 4.4 m) and a corresponding increase in area of 101.5% (7.55 m 2 to 15.2 m 2 ).Therefore, a shock increase in ow-rate due to an extreme weather event is not catered for the in current design.Due to space limitations it is also sometimes dif cult to build overly large tanks to cater for ows that are larger than the design ow-rate.Therefore, it is necessary to consider an alternative technology that would provide similar water treatment capabilities within a smaller land footprint.

Criteria for new tank design
To remain a viable design alongside the already established technology, the proposed design needs to meet the following criteria: • The potential to produce water of equal or greater quality than that currently produced by the existing technology.• Low in cost with respect to both construction and operation.• Easy to construct and operate on steep gradients.
• Able to withstand extreme weather conditions, i.e. monsoonal rainfall and land slides.• Small enough to suit the Nepalese water supply system.• Requiring minimal or no machinery to construct and no electricity to operate.

Possible solution -inclined settling
Conventional gravity sedimentation tank design insists that the cross-sectional area of the design must allow for the adequate settling of the particles contained in the solution, given a known in uent ow-rate.This maximum speed is calculated using a simple principle called Stokes' Law (Appendix 1).Although this approach is adequate where space is unlimited and ow-rate conditions are constant, where space restrictions are necessary or the rate of particle settling is hindered, such as during increases influent water turbidity due to landslide events or monsoonal conditions, the traditional design approach is inadequate.
As the physical area of the tank cannot be adjusted due to space restriction, the other solution possible is to increase the settling rate of the particles.This approach both minimises the cross-sectional area necessary for design whilst still allowing for adequate particle settling.

Boycott Effect
The Boycott Effect describes the increase in particle settling rate due to the presence of an inclined surface.This phenomenon was rst described by Boycott in 1920 with the discovery that if "defribinated blood is put to stand in narrow tubes, the corpuscles sediment a good deal faster if the tube is inclined than when it is vertical" (Boycott, 1920).
The increase in settling rate can be described by imagining a settling particle within an infinite quiescent medium in a container with vertical walls.The particle must travel through the medium until it reaches the bottom surface of the container.But if the particle is inside a vessel containing an inclined surface, the particle has the opportunity to make contact with a surface and slide down to the bottom of the container without having traversed the height of the entire container.The increase in particle settling rate can therefore be seen as a decrease in settling distance (Demir, 1995) and an increase in the surface area available for settling (Davis & Acrivos, 1985).

.2 Inclined Plate Settlers
Inclined plate settlers (Figure 5) are high rate sedimentation devices consisting of a series of inclined parallel plates forming channels (plate stack) into which turbid waters can be fed for settling.The plate stack is installed between an inlet and outlet channel (Leung & Probstein, 1983).
Water enters through the inlet and ows through the channels created by the plate stack to the outlet area where the ef uent is collected in the outlet chamber (Foellmi & Bryant, n.d.;Leung & Probstein, 1983).As the water ows through the plate stack channels, the particles settle onto the downward facing walls of the plates and slide down to the bottom of the settler (Davis et al, 1989).
IPSs possess the ability to settle very ne suspended particles at a high rate.The settler capacity per unit volume can be made large without substantial increase in footprint.The ratio of oor area needed for conventional sedimentation basins to the oor area for IPS designs can range from 8:1 to 10:1 (Foellmi & Bryant, n.d.).
Industrial application of inclined plate settlers has been shown to be very effective in the optimisation of particle settling.Current designs allow for very compact design with very small footprints, as can be seen in Figure 6.The operation of the industrial IPS is analogous to the basic description presented above, although typically a variety of patented design features included in the Nordic Water unit, such as a ow control system and sediment thickener (Figure 6), allows for the unit to maintain very high reliability and ef ciency (Nordic Water Products, 2013a).
Although providing an elegant solution to the need for a small footprint treatment technology in Nepal, there are a variety of issues that do not allow the direct integration of an industrial IPS: • The design is complex and requires extensive expert knowledge and design expertise.
• The main materials used for the design (i.e.steel/ stainless steel) are costly and are unlikely to be available in rural Nepal.
• The construction of the system is unable to be performed using manual unskilled labour.
• The system is designed as a parallelepiped in order to minimise any area loss and minimise footprint.This necessitates the construction of rigorous reinforcement to stabilise the unit and makes it less conducive to construction and operation on sloped gradients.
• The plates included in the IPS are usually very thin and spaced very close together.They are also xed to the walls of the system.This limits the ability of the unit to be cleaned with methods other than high-pressure water or air systems and also poses a construction dif cultly.
• The design of the system often incorporates the use of electricity, particularly in the case of pumps, flow control units and stirrers for thickening sludge etc.

Project aim
Although an industrial IPS may be inadequate to adapt for the use of rural Nepal, the basic design principle present in this design is highly useful.The aim of this project was therefore to design a highly simpli ed IPS that harnessed the inclined settling principle whilst still adhering to the design conditions set forth by NEWAH (Section 1.4).As the IPS design would allow such large changes in footprint, it was decided that a single design, able to treat in uent ow-rates from 0.25 to 4 L/s, would be more practical than creating ve separate designs corresponding to different in uent ow-rate ranges.This would give the design added robustness to handle extreme events whilst still maintaining a relatively small footprint.

INCLINED PLATE SETTLER DESIGN
The approach settled upon in the construction of the IPS design was a basic rectangular cement tank that houses a simple inclined plate bundle (Figure 7).The tank structure comprises of an inlet zone, a plate bundle area, an outlet zone and a sloped oor leading to a washout pipe.
The tank will sit on a foundation of stone and may be dug into the ground if necessary.The outer walls and oors will be ferrocement with the use of adequate steel reinforcement where appropriate.A corrugated iron roof will protect the system from the weather.Any additional accessories necessary for the operation of the unit, such as valves and piping, will remain the same speci cation as is currently utilised.

Figur e 6:
Industry standard inclined plate settler (Nordic Water Products, 2013b).Vol 2 No 1 "Sedimentation tank design for rural communities in the hilly regions of Nepal" -Wisniewski In order to ensure the IPS design remains a viable construction in competition with the current round tank design, care has been taken to ensure that the building materials and construction methods align as closely with the current methods as possible.

Inclined plate bundle
The area most responsible for adequate particle settling is the inclined plate bundle area.This area consists of a series of closely spaced plates attached to the left and right walls of the structure.The crosssectional area of this structure is the most important variable for calculation and dictates the overall footprint of the design.The cross-sectional area was determined by examining the physics associated with inclined particle settling and the consideration of a geometrical principle developed by Huisman (1986).

Huisman's analysis
The geometric principle developed by Huisman shows the correlation between specific plate parameters and their effect on the increase in particle settling rate.As can be seen in Figure 8, the behaviour of a particle travelling in the path AC between two inclined plates can be subdivided into two separate behaviours: (a) the particular travelling linearly with velocity (V o ) in the direction of water ow or BC and (b) the particle settling due to gravity (S' o ) or AB.A geometrical principle relation the distances BC and AB can then be derived which includes the parameters of the plate height bundle (H), the plate thickness (t), the angle of inclination (#), the plate spacing (w).
The selection of the plate parameters depends on the design conditions.Once chosen, the 'enhanced' or 'improved' settling rate due to the inclined conditions can be described by Equation 1: where: S o = enhanced settling rate due to inclined conditions (m/s) S' o = particle settling rate due to gravity (calculated from Stokes' Law) (m/s) The over ow parameter (U) is the crucial parameter in design.It is expressed as a rate of ow per unit area (m 3 /m 2 s) (Demir, 1995) and is generally chosen to be half of the value of the Stokes' settling rate or as in this case, half the value of the enhanced settling rate.
Once the over ow rate is obtained, the required cross-sectional area (m 2 ) (Equation 2) can be obtained by dividing the design in uent ow-rate (Q) by the enhanced particle settling rate.(2)

Plate bundle dimensions
A summary of the design parameters chosen for the full-scale IPS plate scale design is listed in Table 1.
The height of the plate bundle (H) was considered to be the most the parameter in the design of the plate bundle, as its height would determine the resulting height of the remaining tank structure.It was necessary for this height to be less than 2 m as the remaining tank structure (i.e.walls and roof) needed to be at a height that intuitively considered safe for manual construction (i.e. 2 m).A plate bundle height of 1.5 m was chosen for the design.
A plate thickness of 0.01 m was chosen to allow structural integrity when inserting and removing the plates from between the tank walls.This differs to the industrial application of the IPS where extremely thin and xed plates are used in the design.This generally limits the ability of the units to be cleaned without the use of high-pressure water or air applications.
The decision to create thick plates allows for their removal from the tank structure, and the ability to clean the plates using ordinary cleaning utensils and low-pressure water application.
A plate spacing (w) of 0.05 m was chosen to allow the ease of plate removal and insertion into the system.Industry standards generally calls for plate spacing with the region of 25 to 50 mm in order to minimise the resulting cross-sectional area of the plate bundle (McKean et al, 2010).But as the plates are xed to the walls of the units, there is no need to remove the plates and the plate spacing can be minimal.A plate spacing of 0.05 m was chosen in order to allow for the removal of the plates for cleaning whilst still maintaining a reasonable distance to allow for adequate particle settling.
An inclination angle (# ) of 50° was chosen as a reasonable angle of inclination given the resulting cross-sectional area yielded by the Huisman principle, i.e. 5.1 m 2 .Sensitivity analysis conducted established that the angle of plate inclination was one the most sensitive parameters to increases in values, even with relatively small changes in angle between 10 to 30 % resulting in the increase in cross-sectional area from 11 to 43 % (Wisniewski, 2012).An angle of 50° is also said to allow for effective self plate cleaning and the minimisation of particle entrainment (Culp et al, 1968;Shamim & Wais, 1980).
Given the calculation of the plate bundle crosssectional area (5.1 m 2 ), a reasonable plate width was chosen that would be deemed suitable to construct easily using manual labour.A gure of 1.9 m was chosen as it resulted in a plate bundle length less than 3 m.Although both the plate bundle length and width are values that are ordinarily inconsequential, the maximum potential value of these parameters was considered in light of the construction practices that would be needed to construct the tank and the resulting volume of ferrocement necessary for construction.

Plate Bundle Construction Materials
The plates should ideally be constructed from sheet metal such as stainless steel, plastic (polyethylene, ABS or similar) or even a marine ply (Schulz & Okun, 1984) but these materials are currently unavailable in rural communities.An alternative suggestion would be the use of inclined plastic tubing or an ABS hexagonal matrix instead of a plate design.This would require the redesign of the system and may prove challenging in construction and maintenance; however, such systems are common in industry.
There is need for further research and liaison with the local community members as to the most appropriate material for use.

Inlet design
The typical design for the inlet section of an IPS would necessitate the construction of an area separate to the plate bundle with the addition of baffles in order to increase momentum dispersion in the in uent and to increase effective particle settling.
In order to simplify the design as much as possible and to minimise the footprint of the technology, the addition construction of a separate inlet area was discouraged, instead the inlet area was designed to make use of the wasted volume associated with the inclination of the plate bundle.Ordinarily, this volume would be eliminated by the construction of the tank in the shape of a parallelepiped.But in the case of rural Nepal, where ease of construction and simplicity in design is an imperative, the use of this space as an inlet area provide an elegant solution to the problem of increased footprint.
In order to provide adequate momentum dispersion of the in uent ow, in place of additional vertical baf es, the rst plate of the plate bundle was extended

Parameter Value
Design "Sedimentation tank design for rural communities in the hilly regions of Nepal" -Wisniewski to create a preliminary baf e.The inclination angle of this baf e also serves to provide a medium for some inclined settling of the in uent ow prior to reaching the plate bundle.

Outlet design
The outlet design consists of an overflow weir leading to an ef uent collection basin containing a submerged outlet to prevent the carry over of scum into distribution.The depth of the collection basin is merely indicative and can be adjusted to increase or decrease the ow-rate of the ef uent as necessary or to minimise the volume of water contained.

Sludge collection design
The oor of the tank has been constructed with a slope of 2° to channel the settle particles to the washout area where they can be ushed through the washout pipe during the washout cycle.This slope angle was chosen as it minimises the vertical height of the associated tank walls to allow for adequate clearance at the bottom of the plate bundle.The sloping oor ends at a wall corresponding to the last plate in the plate bundle.It is designed to sit ush against this wall to dissuade the entrance of particle sludge into the space between the plate bundle at the outlet area.
Given the maximum particle concentration of the in uent entering the tank, additional mechanical features such as the thickening devices employed in industrial design are unnecessary.Where increased particle loading is experienced, the washout pipe can simply be left open slightly to discourage the build up of sludge at the bottom of the vessel.

Tank walls and roof
Figure 7 indicates the dimensions of the four walls to be built to house the plate bundle.In order to ensure safe construction, the height of the walls was limited to a height of 2.4 m.The length of the left and right tank walls allows some added room to allow for the in uent ow to navigate the preliminary baf e and also concession has been made for the inclusion of an outlet zone.The length of the front and back walls was determined by the width of the plate bundle.Although mechanical design was not the main scope of this paper, the thickness of the walls was decided to be a minimum of 100 mm, this allows for the creation of grooves in the left and right walls to accommodate the plate bundle whilst also allowing for the addition of steel rebar for structural support.
The internal structure of the tank is to be protected from the elements by a simple roof created from corrugated iron or a similar material.The roof may be attached via hinges or secured in some manner that would allow it to be easily removed to reveal the contents of the tank.

LABORATORY TESTING
A Perspex model was built to test the integrity of the full-scale design; the model could not be built to scale as scaling of the tank walls resulted in nonrealistic dimensions for the plate bundle.For ease of construction, the model did not include the over ow weir or submerged outlet or a sloping oor included leading to a washout system.
A maximum ow-rate of 421 mL/min was calculated "Sedimentation tank design for rural communities in the hilly regions of Nepal" -Wisniewski using Huisman analysis for the scaled design.
A laboratory experiment was conducted to test the ability of the scaled model to clarify solutions containing calcium carbonate particles.The in uent solution was pre-prepared by settling a small amount of Omycarb 40 (calcium carbonate) in a beaker to collect the 15 micron particles needed for experimentation.The sediment was diluted with an appropriate volume of water to provide a solution with a turbidity of 20-25 NTU.
A peristaltic pump allowed the calibration of the in uent to an initial 55 mL/min, or approximately 13% of the maximum ow-rate.The model tank was lled with water and a particle solution added.When the inlet chamber solution was recorded as having a turbidity of approximately 25 NTU, the ef uent turbidity was measured approximately every 10 minutes to gauge operation of the device.When the ef uent turbidity reached the same value as the in uent, the process was stopped and the tank was emptied and cleaned.
The procedure was repeated at ow rates of 150 and 253 mL/min (36 and 60% of the maximum ow-rate respectively) and at 421 mL/min (maximum owrate).

RESULTS
Figure 9 indicates that an increase in ow-rate from 13 to 60 % of the maximum ow-rate (421 mL/ min) resulted in an increase in ef uent turbidity of 40%.With the exception of the maximum ow-rate, the system produces reasonable ef uent less than 5 NTU within the first 60 minutes of operation.After this period, a dense build up of particles was observed in the sludge collection area.This build up of particles resulted in short-circuiting of the system either through the rst or last channel for the low and higher ow-rates respectively.This result was noted by an increase in ef uent turbidity after the 60-minute period.This time frame may signal the point at which a washout period is necessary; there is little indication as to whether this is a viable timeframe for operation without further testing at pilot scale.Although to stem the possible frequency of the washout cycle, the washout may be designed in such a way that enables a continuous low ow discharge to ensure adequate clari cation.
The experimental results also revealed the importance of the design of the inlet chamber and sloping oor.
An increase in area of the inlet chamber and a steepening of the sloping oor leading to a sump would allow an increase in momentum dissipation and an increase in system performance.These changes are a challenge for the design, as the increase in the dimension of these areas results in an increase in footprint and vertical height of the system, proving dif cult for construction.
If the experimental results are somewhat representative of the potential operation of the system at full-scale, it may be suggested that the simplistic nature of the design does not allow for adequate clari cation, but as the laboratory model was not a perfect representative of the system, this conclusion requires further validation with the construction of a pilot scale model.Experimental testing with a wide range of feed ow-rates, particle concentrations and turbidities would be necessary to fully understand system performance.

CONCLUSIONS AND RECOMMENDATIONS
A simplified IPS was designed to combat the turbidity issues arising from stream sources in the rural hilly community of Mathillo Semrang, Nepal.
The footprint of the design is 10.5 m 2 with the theoretical capacity to treat a maximum in uent ow-rate of approximately 4 L/s.This is a reduction in footprint of up to 76% from the current round sedimentation design (42 m 2 ) required to treat an in uent ow of 4 L/s.The design allows for the construction of a single device to replace the ve separate devices currently implemented.The design is gravity driven and requires no electricity to operate.Its construction is similar to that of the current design and is therefore assumed to be economically viable.
Although the theoretical basis for the design is promising, laboratory experimentation identi ed necessary changes in the design structure to allow for adequate sedimentation, such as increase in the slope of the oor and an increase in the area of the inlet zone to allow for adequate momentum dispersion.
The overall success of the design in handling a design in uent ow-rate of 4 L/s and the ability of the system to handle extreme ow-rate is dif cult to determine without the construction of a pilot system but further bench experimentation such as particle size distribution testing of the ef uent and ow modelling of the system may be useful in revealing any shortcomings of the design.
Collaboration with NEWAH and the Mathillo Semrang community is necessary to supplement the design particularly in the elds of material selection and construction.Vol 2 No 1 "Sedimentation tank design for rural communities in the hilly regions of Nepal" -Wisniewski used for the calculation of settling rates can be seen in Table A1.
The tank dimensions of the current technology (Figure 1) provided by NEWAH are listed in Table A2.Both the apparent footprint of the technology (D) and the Stokes' settling rate of a 10 micron particle (E) were calculated using Stokes' Law (Equation A1).
The average of the settling rate was calculated to be approximately 1.08 × 10 -4 m/s.A comparison to the settling rates obtained by Equation A1 by varying the particle diameter (Table A3) con rms that an average particle size of approximately 1.08 microns was used as a basis for the calculation of the tank dimensions.This validates the use of a 10 micron particle as a basis for this project.

F igure 1 :
Fig ures 2a/b: a: View of the inside of the round sedimentation tank showing the two separate settling chambers and transfer piping.b: View of the round sedimentation technology as currently constructed in Nepal showing the stone foundation and manhole covering.a b

Figure 7 :
Figure 7:Isometric cross section of full-scale IPS design.
Figure 8:Schematic of settling particle between two inclined plates.

Figure 9 :
Figure 9:Effluent turbidity as a function of time for varying influent flow-rates for the lab scale model.

Table 1 :
Parameters chosen for the fullscale IPS design using the Huisman analysis.Vol 2 No 1

Table A1 :
Parameters used in the calculation of the settling rates seen in TableA2.

Table A3 :
Variation in Stokes' settling rate with respect to particle size.

Table A2 :
Dimensions and settling properties of the current round ferrocement technology established in Nepal.