final report_edited by jc and kp

RAINWATER CATCHMENT SYSTEM

PROJECT FINAL WRITTEN REPORT

TEAMJARED CORDES

PAIGE MOLZAHNKIET PHUNG

JUNE 11, 2014

CBEE 416 Cordes, Molzahn, and Phung Project Final Report

ABSTRACT

The project objective is to design a low cost and low maintenance system that can catch and clean water to be used for 50 in Haiti for hygienic purposes. The water must be caught, diverted, filtered and stored before used by the population. These steps required literature comparisons as well as experimental testing. The catchment surface area was determined to be 185 m2 and will be made of aluminum. The flow diverter consists of an initial diverter, a clarifier and a bleeder valve. This system will be constructed of 1” PVC pipes to handle the maximum amount of flow. There will be two operational sand filters to remove particulates and pathogens with diameters of 30 cm and heights of 1.1 m. These filters will have layers of fine sand, course sand, gravel, and small rocks. A bio layer will accumulate over time which will assist in the pathogen removal. This layer needs to be checked monthly to avoid overgrowth of biota. Finally, there will be two 5,000 gallon reservoirs made of polyethylene. They will be UV resistant and opaque to prevent bacteria growth within the tank. A bill of materials was created for the overall system design determining an expected cost range of $6,700 - $8,200.

DESIGN BACKGROUND

Obtaining clean water is a current struggle in undeveloped countries such as Haiti. The World Health Organization (WHO) requires 12.5 L/c/d to meet the basic hygiene needs for rural populations. An additional 7.5 L/c/d are required to meet water consumption needs. Many water sources used in developing countries come from streams containing pathogens. Rainwater harvesting has been one alternative water source that provides clean water for hygiene and consumption. Rainwater contains no pathogens or heavy metals, but it can be low in pH in deforested regions. Harvesting can only remain viable in areas of heavy rainfall with few drought seasons. Heavy droughts lead to an increase in required water holding time which increases the chance for contamination. Haiti received an average annual rainfall of 1600 mm between 1999 and 2009. May, September, and August had the most rainfall while December through March obtained less than 100 mm.1

A rainwater catchment system requires four main parts: a catchment area, a piping system, a purification filter and a storage reservoir. The inverted roof catchment area is a commonly used design for rainwater catchment systems. This design requires minimal maintenance and has the ability to easily divert flow. This system, shown in Figure 1,2 is the optimal catchment area design. The rainfall will be caught and then flow to a diverter system to remove the debris from the initial rainfall. Dust will accumulate on the catchment area during droughts. These particulates must be removed before entering the filter when it finally rains. The cleaner water will then move on to a sand filter to be purified.

The water enters the slow sand filter after the initial flow diverter to catch small debris that passed the initial flow diverter and to treat pathogens from the catchment surface. A sand filter is made up of three layers: fine sand to trap particles, coarse sand to keep the fine sand from leaving the filter, and gravel to keep the coarse sand from leaving the filter. Fine sand is made of particles that have around 100 μm of

1 "Climate Change Knowledge Portal 2.0." Climate Change Knowledge Portal 2.0. N.p., n.d. Web. 18 Mar. 2014.2 "1.1 Rainwater Harvesting from Rooftop Catchments." Rainwater Harvesting from Rooftop Catchments.Web. 08 June 2014.

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Figure 1 : This is a diagram of the catchment area inverted roof design. The rainfall will be caught and roll down the slope of the system and flow to the flow diverter.


equivalent diameter and the coarse sand has an equivalent length of 200 μm. Pathogen removal is carried out in three ways; they will get trapped in between sand particles and die from nutrient loss, they will be too large to go through the sand, or be removed through use of a bio layer that will destroy pathogens. Bacteria in this bio layer attack incoming contaminants in a process called predation. The bio layer, which grows on the top layer of fine sand, takes around one month to develop according to the EPA. This bio layer can be destroyed by agitation or removal of the top layer of sand, so it is important that the filter is not mishandled. The bio layer requires still water to be present or the bacteria will die. The bio layer will continually grow which can cause high pressure loss through the sand filter due to the extra layer. This growth is coined schmutzdecke, or dirty layer.

Finally, the water will be stored in a reservoir. The rainwater reservoir is the most expensive component of a rainwater harvesting system. The cost depends on the type of construction materials used, water demand for daily consumption, and whether the tank is built above ground or underground. An above ground tank is preferred over an underground tank for water storage considering price, maintenance, and convenience. The above ground tank can be constructed easily from a wide variety of materials. Pump installation is not necessary because water extraction can be done by gravity in many cases. An above ground tank also allows easy inspection and maintenance for water leaks. The tank storage, however, must have opaque or dark color to block sunlight to stop the growth of algae. The underground cistern requires excavation, a pump system to extract water, and a higher cost of construction and maintenance; therefore, an above ground tank proves to be a better solution in Haiti.

METHODS AND MATERIALS

SYSTEM SCALING

With the 50 person scope of this project, a total of 625 L would be the daily minimum required clean water production. This water must be caught, diverted, filtered, and stored before accessed by the population. The average rainfall data3 shown in Figure 2 was used for determination of catchment and reservoir scaling. Adequate amounts of water must be caught and stored through the rainy seasons to continuously provide through the drier seasons.

The reservoir must fill to a maximum and then deplete in the course of one year to prevent the system from overfilling and to maintain the lowest cost possible. The average monthly rainfall data was divided among 50% of the days in the month, assuming that the rain does not fall in even amounts every day and some days have no rainfall. This strategy was applied to size both the catchment surface area and the reservoir giving a catchment area of 185 m2 and a reservoir size of 10,000 gallons.

The scaling of the sand filter requires the calculation of the pressure loss through the system. Pressure loss was calculated using the Ergun equation shown be Equations 1 and 2. ΔP represents the pressure loss, ε is the void space in the bed, ff is the friction factor, L is the length of the sand, ρ is the water

3 Climate Change Knowledge Portal. The World Bank Group, n.d. Web. 5 Apr. 2014.

2

0

50

100

150

200

250

300

Month

Rai

nfal

l Int

emsi

ty (m

m)

Figure 2 : The average rainfall data for Haiti, showing both the rainy and dry seasons


density, u is the superficial velocity, dpart is the equivalent diameter of sand, and v is the kinematic viscosity of water. These calculations were used with experimental data to optimize the diameter of the filter, the required height of sand and the number of filters to be in operation.

∆ P=1−εε3 f f Lρ

u02

d part

(1 ) f f =150 (1−ε)

( u0∗dpart

v )+1.75

(2)

MATERIALS AND COST

A bill of materials is attached in the Appendix that details all goods that would need to be purchased to complete the catchment system. Services such as construction and maintenance are variable in cost and are left out. A 10% decrease and increase was added to the price in the US for an estimate range of the cost for Haiti. The catchment is required to be a raised system due to the lack of available house roofs that could meet the area required. This drove the cost up to $1,289 for the US. Five beams would have to be anchored into concrete on the back of both aluminum sheets. Another five beams would anchor the middle of the aluminum sheets. The initial flow diverter was the cheapest at $78 US. The PVC should be shaded and wrapped with the drain pipe. This should decrease sun and heat exposure to the system. The filters cost $896 US. Obtaining and fabricating the plastic for the filter was costly. The material is reliable and should last up to 10 years though. The filters are clear so a basic tub should be put over them to avoid growth of photosynthetic bacteria and to reduce the heat on the filter. The reservoirs cost the most at $5,165 US. The tank sizes are massive so this cost was unavoidable. Haiti should expect an initial cost, for just the materials, of $6,685.95 up to $8,171.71.

EXPERIMENTAL TESTING

The catchment area and the reservoir system were not tested due to cost and time restrictions. These aspects would be constructed following designs obtained from the literature.

Experimental testing was completed on the flow diverter. This testing required the construction of a flow diverter system made out of PVC that consists of two parallel horizontal pipes with the top one linked to a biosand filter and two vertical pipes connecting to a clarifier and the other to a bleeder valve. The bleeder valve will be slightly open during the operation. A first flush of contaminated rainwater enters the flow diverter system where the debris from collected rainwater will travel through the pipes down into the clarifier and also deposit above the valve. The water level in the flow diverter system rises and the subsequent flow of cleaner rainwater is then automatically directed along the pipes to the biosand filters system. The bleeder valve is eventually opened after the catchment period to drain contaminated water at the bottom and allow for maintenance and cleaning purposes. These visible pollutants can be diverted and removed by initial flow of water into a flow diverter system as depicted in Figure 3. A lab-scale flow diverter system was constructed using ½‘’ PVC pipes, ½’’ T’s, ½’’ 90o elbow, and small plastic container acting as a clarifier in order to determine the feasibility of the system. A sample of 200 mL of imitation rainwater containing debris was introduced to the flow diverter, subsequently followed by adding 400 mL of DI water. The

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Figure 3 : : A process flow diagram of a flow diverter composed of PVC pipes, a clarifier, and a bleeder valve to remove larger contaminants from collected rainwater prior to further treatment from biosand filters


first experimental trial went until no water flowed through the bleeder valve. The second through fourth trial repeated adding imitation rainwater and then DI.

The experiment testing for the sand filter dimensions was conducted to determine the equivalent diameter of the particles within the filter, the flow of water, and the height of required sand. The equivalent diameter of the sand was the first parameter to be found by flowing water through the filter at 100 mL/min. The height of water remained constant at 15 cm above the top of the sand which fixed ΔP using Equation 1. The bed void space was estimated to be 0.45. The sand length was 15 cm. This gave an equivalent diameter of 115 μm for the fine sand. The pressure drop through the system is taken using this diameter for both sand layers which will slightly overestimate the actual pressure drop. This is a small safety factor to account for channeling, wall effects, and the effects of the schmutzdecke. The pressure drop through the gravel area is much smaller than that of the sand beds. Figure 4 shows the pressure drop through the filter as a function of diameter and length. The filter will be operating at a length of 60 cm of total sand with gravel. An ID of 30 cm is suggested for each filter. At these filter conditions, we expect a required pressure head of 1 m. The filter length is 70 cm, so a head of 30 cm is needed to overcome the pressure difference.

RESULTS

INITIAL FLOW DIVERTER

The experimental testing on the flow diverter system proved to be feasible as the rainwater condition improved significantly at the outlet connecting to biosand filter compared to the original rainwater entering the system. The cost of material and installation for constructing a flow diverter system is relatively inexpensive as PVC pipes are the primary material and easy to work with. The flow diverter must have covered PVC pipes or be made from UV resistant material to avoid PVC degradation from sunlight. The potential stress cracking of PVC due to constant thermal exposure is another major problem when using PVC. The flow diverter system should be built below the inverted roof system or in the shade to extend the in-service life of PVC pipes. Regular maintenance is recommended, for i.e. opening the bleeder valve to remove debris settled at the bottom to prevent clogging. Figure 5 below shows the cleanliness of each water sample after the experiment.

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0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.200.00.51.01.52.02.53.03.54.04.5

Diamater = 50cmDiameter = 40cmDiameter = 30cmDiameter = 20cm

Filter Length (m)

Pres

sure

Hea

d (m

)

Figure 4 : A graph of the pressure loss as head of water through the sand filter. The increase is linear with length, but varies based on diameter. The design should be carried out at 30 cm ID. The pressure loss more than doubles from 30 to 20 cm.

(a) (b) (c) (d)


BIOSAND FILTERS

Three filters were suggested for the design. This would allow two filters to be used at once while the other goes through maintenance or repairs. The filters will all be the same size for simplicity. The design of the filters are based on acceptable pressure loss through the system and adequate treatment of turbid water. The first design is to find the length of fine sand needed to treat turbid water. Turbidity represents the “cloudiness” of a water and is directly related to the amount of pathogens that could be present. Higher turbidity has an increased chance of carrying pathogenic species. The EPA suggests that an effective filter for removing pathogens can treat turbidity to under 5 Nephelometric Turbid Units (NTU). Rainwater was collected from Corvallis using a bucket in order to find the approximate turbidity of rain water. The rainwater sample had a turbidity of 22 NTU. Experiments were run using a solution of water and corn starch in order to create turbid solutions that ranged from 10 – 34 NTU, the expected turbidity of rainwater into the filters. Figure 6 shows an example of the difference in turbidity.

Two filters should remain operational at all times. This allows for one filter to work while the other has an overflow at the top section in case of heavy rainfall. The highest designed rainfall was for 0.83 mm/hr. An outlet at the top of one of the filters will flow straight into a reservoir instead of being filtered. This overflow water will accumulate in another reservoir that will need to be treated, ran through a filter, before it should be used. The filters will need to 1.1 meters tall to allow space for the water to accumulate. The accumulation of water can build up to 20 cm above the top layer of sand before the rain water flows into the overflow. The design of the filter is shown by Figure 7.

The last part of the filter is the flow diverter that sits inside of the filter. Its purpose is to slightly divert the incoming water so that it does not damage the top layer of sand. The plate can also be used to remove the

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Figure 6 : A picture of two water samples. The sample on the right that flowed through the filter had a turbidity of 34 NTU, and the treated water on the left had a turbidity of 1.3 NTU.

Figure 7 : The overall design of the three filters. Each level is detailed out with dimensions. The gravel layer can be split into 5 cm of gravel and 5 cm of larger rocks if needed. The outlet to overflow will wither connect two filters together lead to a reservoir. That will depend on which filter is hooked to the overflow reservoir. A union is suggested to be built on the swan neck, which will allow the PVC to easily be removed from the filter. The swan neck should use the same materials as the PVC flow diverter


top layer of schmutzdecke if it accumulates more than 5 cm on top of the fine sand. The design is shown in Figure 8. The filter will be built from polyethylene or any other plastic material. Another cylindrical plastic cylinder, with dimensions around 35 cm ID with 1.12 m tall, should be put over the filter to reduce UV light damaging the filter. The sides of this cylinder need to have holes in them in order to allow air to enter the filters. The bio layer should be run under aerobic conditions where oxygen is present. The filter could also be put under constant shade. This doesn’t decrease the heat significantly so the filters would need more maintenance.

Two filters were used for this experiment. One filter had an internal diameter (ID) of 13 cm and the other 18 cm. Both filters contained the same length of sand. Flow rates were set at 1 m 3/hr making a superficial velocity of 2.1 cm/s for the 13 cm ID filter and 1.1 cm/s for the 18 cm ID filter. A Vernier turbid meter was used to find the turbidity of the water samples. The outflow turbidity of water from the filters ranged from 10 to 30 NTU with only the coarse sand. The length of the coarse sand was reduced, and 15 cm of fine sand was added to the system. The new filters treated the 10 – 34 NTU solutions to less than 5 NTU for all runs that were conducted. This was based on 3 trial runs for 3 different turbidities. Figure 9 shows the data with a 90% confidence interval. The suggested design is to use 50 cm of fine sand for the scaled up version. This would allow the schmutzdecke to form on the top layer of sand, which was not accounted for in the turbidity tests. The extra amount will also account for increased flows which can occur during large rain events.

MAINTENANCE

The required maintenance on the filters is minimal under ideal conditions. The initial sand used for the filter can be bought from a local store for simplicity. Sand from beaches and other areas can be used, but they would require cleaning with safe water before they can be used. The top layer of the bio layer will have to be looked at monthly. This is to avoid overgrowth of the bio layer. The filters flow diverter can be used to take out some of the top layer of schmutzdecke without disturbing the top layer of sand. Every four months the schmutzdecke should be scraped to a low level if the flow diverter can’t remove it. The layer needs to be minimally disturbed in order to have the best treatment. The bio layer will take about 1 month to accumulate, so the filters should collect rainwater for one month before the water can be used for hygiene. This procedure applies to any filter that hasn’t been used in over a month. The sand is suggested to be replaced yearly. This should be done at different times of the year for each filter. The sand can be replaced by simply removing the contents of the filter, cleaning the sand off, and replacing it back in the filter. The layer of fine sand may fall into the coarse sand, so it is important to check the filters every year. The plastic closed cylinder around the filter should be repaired, or replaced, yearly. The sun quickly damages the material so it is best used in a shaded area. The filters should be inspected every month, by eye, to make sure that there are no cracks or leaks.

OVERALL SYSTEM DESIGN

The overall design of the system is shown in Figure 10 (Page 8). This design includes the overall surface area of the catchment system, the initial flow diverter design, the sand filter dimensions and the sizing of the reservoirs. The specifics of the design are provided in previous sections of the report.

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10 15 20 25 30 35 400.0

0.5

1.0

1.5

2.0

2.5

3.018 cm ID Filter13 cm ID Filter

Inlet Turbidity (NTU)

Outl

et

Turb

idit

y

(NTU

)

Figure 9: A graph of the turbidity of the water that went into the filter on the x axis, and the turbidity of the treated water out of the filter on the y axis. All runs contained treated water under the EPA standards of 5 NTU.


ACKNOWLEDGEMENTS

The rainwater catchment team would like to thank the project sponsor, Dr. Yokochi, for providing this opportunity, Dr. Harding for weekly advising to ensure our work stays on task, Andy Brickman for assistance in obtaining materials, Manfred Dittrich for assistance with construction tools, OSU Engineers Without Borders for cost analysis insight and Lauren Kelly for providing access to the turbid meter.

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Figure 10: Overall design of a rainwater catchment system to be used in Haiti, which consists of a catchment area, an initial flow diverter, two biosand filters, and two main reservoirs.


APPENDIX

Bill Of Materials

BOM Item Number Item Name Quantity Size US price Procurment Haiti Low Haiti High Notes

1 10-001 Aluminum Sheet 2 92 m2

$644.00 Metals Depot $579.60 $708.40

2 10-002 High Wood Posts 10 1.7 m $89.70 Home Depot $80.73 $98.67 4 in by 4 in beams

3 10-003 Low Wood Posts 10 1.3 m $89.70 Home Depot $80.73 $98.67 Same beams that will need to be cut

4 10-004 Concrete 20 60 lbs $247.00 Home Depot $222.30 $271.70 Rapid set concrete to hold posts

5 10-005 Post Anchors 20 4 by 4" $159.40 Home Depot $143.46 $175.34

6 10-006 Joist Hanger 20 18 gauge $7.00 Home Depot $6.30 $7.70 Simpson Strong Tie A21Z Z-MAX

7 10-007 Deck Screws 3 inch $30.00 Home Depot $27.00 $33.00

8 10-008 Hinged Gutter Screen 10 6 by 36" $22.30 Home Depot $20.07 $24.53

9 10 Catchment Total $1,289.10 $1,160.19 $1,418.01

10 20-001 PVC Pipe 3 1" ID, 10' $10.92 Home Depot $9.83 $12.01

11 20-002 Drain Pipe 3 4" ID, 10' $19.65 Home Depot $17.69 $21.62

12 20-003 90o PVC Elbow 3 1" $5.91 Home Depot $5.32 $6.50

13 20-004 PVC Tee 6 1" $4.44 Home Depot $4.00 $4.88

14 20-005 Ball Valve 1 1" $5.15 Home Depot $4.64 $5.67

15 20-006 PVC Glue 3 8 oz. $9.74 Home Depot $8.77 $10.71

16 20-007 Container 1 1 L $0.00 $0.00

17 20-008 Bulkhead Fitting 1 1" $16.24 Home Depot $14.62 $17.86

18 20-009 Gutter 1 10' $5.23 Home Depot $4.71 $5.75

19 20 Flow Diverter Total $77.28 $69.55 $85.01

20 30-001 Acrylic Clear Sheet 3 1.1 by 0.94 m $531.36 Plas-Tech $478.22 $584.50 Rough estimate based on professional plastic

21 30-002 Acrylic base 3 0.6 by 0.6 m $90.00 Plas-Tech $81.00 $99.00

22 30-003 Plastic Circle 3 30 cm D $60.00 Plas-Tech $54.00 $66.00 Used for filter flow diverter

23 30-004 Silicone Sealant 2 300 mL $25.98 Ace Hardware $23.38 $28.58 Red Devil heat resistant industrial grade

24 30-005 Fine Sand 10 50 lbs $32.50 Home Depot $29.25 $35.75 Can also use local sand that is treated

25 30-006 Coarse Sand 8 1 ft3

$59.92 Agri Supply $53.93 $65.91 " "

26 30-007 Gravel 15 0.5 ft3

$45.00 Home Depot $40.50 $49.50

27 30-008 Union 3 1" $14.58 Home Depot $13.12 $16.04

28 30-009 90o PVC Elbow 6 1" $11.82 Home Depot $10.64 $13.00

29 30-010 PVC Male Adapter 6 1" $3.54 Home Depot $3.19 $3.89 Connect slip PVC with threaded hole on filter

30 30-011 PVC Cutter 1 $6.48 Home Depot $5.83 $7.13

31 30-012 Cylindrical Tub 3 $0.00 $0.00 Put over filters to reduce sun exposure

32 30 Filter Total $881.18 $793.06 $969.30

33 40-001 Storage Tank 2 5000 L $4,711.26 Tanksforless.com $4,240.13 $5,182.39

34 40-002 Overflow Tank 1 500 L $418.52 Tanksforless.com $376.67 $460.37

35 40-003 Brass Gate Valve 3 1" $33.72 Home Depot $30.35 $37.09

36 40-004 PVC Male Adapter 3 1" $1.77 Home Depot $1.59 $1.95 Connects PVC

37 40 Reservoir Total $5,165.27 $4,648.74 $5,681.80

38 50 Total Cost $7,412.83 $6,671.55 $8,154.11

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Required Volume of Reservoir:*The value calculated will be based on the rainfal intensity per month. Future calculations may be done to find daily rainfal intensitys.**Assume that the catchment system will be built in April

Month Total Days RainfallRain

capturedRain

ConsumedReservoir Volume

[mm] [m3] [m3] [m3]April 30 136 19.26 18.8 0.51May 31 243 34.43 19.4 15.57June 30 123 17.46 18.8 14.28July 31 114 16.17 19.4 11.08

August 31 124 17.60 19.4 9.31September 30 184 26.13 18.8 16.69

October 31 280 39.68 19.4 36.99November 30 149 21.12 18.8 39.37December 31 68 9.67 19.4 29.67

January 31 60 8.55 19.4 18.84February 28 54 7.70 17.5 9.05

March 31 73 10.33 19.4 0.00

*Excess was calculated by: rain captured - rain used + previous month excess

The largest excess was at 39.37 m3. This value can be used for a one year cycle

Suggested volume of reservoir for a one year cycle [m3]: 43

% filterdesign.m% a file in order to find a suitable filter design to minimize %pressure loss and calculate the required head %to offset the pressure loss% Team Rainwater Catchment, CBEE 416, 6/11/14clear all, format compactA = 141.9; % minimum required catchment area (m^2)dpart = 0.0001148; % equivalent diameter (m) eps = 0.45; % void space of bedro = 1000; %density of water (kg/m^3)kvisc = 0.995*10^(-6); % kinematic viscosity of water 20 C (m^2/s)r = input('Estimation of daily rain intensity (18.65) : '); % (mm/d) d = input('Diameter of filter estimate (0.4-0.7) : '); % (m)L = input('Desired length of filter (0.3-0.6) : '); % (m) V = r/1000/86400 * A % volumetric flow of rain (m^3/s)u = V /((d/2)^2*pi) % velocity of water throught the filter (m/s)Re = u * dpart / kvisc % Reynolds number f = 150*(1-eps) / Re + 1.75 % friction factor

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F = (1-eps)/eps^3 * f*L*u^2/dpart %friction losses (m^2/s^2)Ploss = F * ro % Pressure lost across filter (pa)% a mechanical energy balance can be done for the required catchment% height by setting g*z = F. All other losses are negligible. z = F / 9.8 % Catchment height to maintain atm pressure (m)

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