Rainwater Data
In order to generate an accurate measurement for the capacity of the first-flush, it was necessary to investigate the anticipated volume of water to be collected by the rainwater catchment system.
While rainfall data was unavailable for the specific community of Miramar, rainfall data was collected for the relatively nearby capital of San Salvador during its rainy season. According to this information, the average rainfall during the rainy months (May-October) was determined to be approximately 270 millimeters per month, or roughly 9 mm per day [1].
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| Average Rainfall for El Salvador from 1990-2012 |
From measurements conducted from Google Earth for rooftops in Miramar, El Salvador, as well as estimations taken from EWB pictures and member testimony, the average roof size was judged to be 18 feet by 25 feet, totalling 450 square feet, or 41.8 square meters [2].
Assuming 9 mm of rainfall per day, a roof measuring 41.8 square meters would be calculated to collect approximately 99 gallons of water per day.
First-Flush Volume
Sources differ on the quantification of the first-flush volume. An analysis of roof contaminants revealed generally, that "for each mm of first-flush, the contaminant load will halve" [3].
Other research, including a study conducted at the Massachusetts Institute of Technology, suggested that "the first millimeter of runoff be diverted from the roof after three
consecutive days of precipitation less than one millimeter." This study, however, was conducted in a treeless Rwandan community, which fails to accurately take into account "the effect of tree-proximity and distance-to-roadway on runoff quality" as well as "the effects of roof- and tank-cleaning on
runoff quality" [4].
Given that the composition of Miramar roofs is less than ideal, consisting of a mixture of semi-rusted sheet metal and fungi-laden ceramic tiles, the first-flush would be expected to be higher than predicted for relatively cleaner American homes.
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| Miramar roofs are a mix of aluminum sheet metal and ceramic tiles. |
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| Ceramic tile roofs have the potential to host fungi and microorganisms. |
In fact, due to the contamination of the ceramic tiles, it was concluded that no duration of first flush would be capable of producing clean potable water. Therefore, when deciding capacities for the diverter materials, the size of the rainwater collection drum for the flush diverter was determined to be 35 gallons in order to account for circumstances of varying rainfall levels.
Taking into consideration the rule for “halving the contaminants,” the contaminant load for the collected water would decrease by a factor of 0.111, a sufficient decrease for purifying the water in one of the local bio-sand filters installed by the Drexel chapter of Engineers without Borders. Furthermore, assuming 9 millimeters of water fall per day in the rainy season, the first-flush system as designed was predicted to collect approximately 65% of rainfall per rain event per day.
Researching the flow rate of various sizes of PVC pipes, it was determined that the smallest size (2") would be sufficient to handle the rainfall of Miramar, even taking into consideration the torrential conditions of each rain event [5].
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| Water flow capacities for Schedule 40 PVC pipes |
Structural Support
Support for the flush diverter design will be restricted to pipe supports by nature of the design. The container meant to hold the first-flush water will sit on the ground; any overflow will be diverted to designated containers. For this reason, the design for support systems will retain the ability to be easily modified to fit individual homes. Bamboo is a common plant in the area, and was considered as a major component in the construction of a support structure. This building material is not only abundant, but also a strong and easily implemented substance.
The species of bamboo found in Miramar is most likely guadua aculeata [6]. The compressive strength for this genus of bamboo is extremely high. The measured results from a similar species, guadua angustifolia, is approximately 40 N/mm^2. However, this is limited by a lower shear strength, estimated to be about 4 N/mm^2.
Given an average diameter of 135 mm, the average area can be calculated to be 14313 mm2. Therefore using the relationship, Stress = Force/Surface Area, the maximum force supported by an average bamboo stalk was determined to be 57252 N.
This value, equivalent to 5842 kg, was shown to be more than enough to support the full 35-gallon drum totalling about 135 kg of water [7].
References
[1] "Average Monthly Rainfall for El Salvador from 1990-2012," The World Bank Group [Online]. Available: http://sdwebx.worldbank.org/climateportal/index.cfm?page=country_historical_climate&ThisRegion=Latin%20America&ThisCCode=SLV
[2] "Engineers Without Borders Drive" Engineers Without Borders - Drexel Chapter, Database (2016).
[3] D.B. Martinson and T.H. Thomas, “Quantifying the First-Flush Phenomenon: Effects of First-Flush on Water Yield and Quality,” in 14th International Rainwater Catchment Systems Conference, Kuala Lumpur, Indonesia, 2009.
[4] K.C. Doyle, “Sizing the First-Flush and its Effect on the Storage-Reliability-Yield Behavior of Rainwater Harvesting in Rwanda,” M.S. Thesis, Dept. Civil Environmental Eng. Massachusetts Inst. of Technology, Cambridge, 2008.
[5] "Engineering ToolBox." The Engineering ToolBox. The Engineering ToolBox, n.d. Web. 20 Apr. 2016.
[7] S. Schröder. (2015, April 12). Mechanical Properties of Bamboo [Online]. Available: http://www.guaduabamboo.com/facts/mechanical-properties-of-bamboo
