In my last post I talked about issues regarding access to potable water in Grand Ravine, Haiti. This time, I will talk about the first part of our solution and design decisions we made.
The housing for Grand Ravine is typically single family homes with 1-2 rooms. The rainwater harvesting process can be divided up into three parts: Catchment, Conveyance, and Storage.
The first portion of the design is responsible for catchment of the rainwater. The roofs act as the first contact point between the rain and the harvesting system. Said catchment facilities would be sprayed with hydrophobic spray for increased efficiency and collection. The supply potential is directly dependent upon the amount of rainfall and roof collection area. The formula is S = R x A x Cr, where S is the supply potential, R is the mean annual rainfall in mm/year, A is the surface area of catchment in m2, and Cr is the runoff coefficient. Cr accounts for losses due to splashing, evaporation, leakage, and overflow.
The next portion of the process is conveyance. The gutters, pipes, and various filtration systems collect water and move it towards storage. The gutters being used should be V-shaped gutters placed slightly lower than the roof so that water can fall off of the sides for collection. The sheets should be connected to the roof every 3 feet or so to prevent the gutters from falling under the weight of the water. Also placed periodically throughout the gutter should be leaf screens that act as filters for large organic debris, rodents, and the helicopter toilets, which are popular methods of waste disposal in the Grand Ravine area. Lastly, at the end of the gutter before the rainwater reaches the downspout it should pass through a screen filter to prevent smaller organic matter that passed through the gutters from getting inside and clogging up the pipes. The various filters ensure quality deliverance of water through the system. Poor conveyance can lead to high levels of water loss, poor system efficiency, and contamination.
Once through the gutters, the water should pass through downspouts made of 4 inch PVC pipes. The smaller size pipes are being used because of the petite house sizes and lack of sturdy infrastructure. The pipes should lead to another filtration portion called the First Flush. This filter practices a roof washing technique that gets rid of the water from the first portion of the rainfall. The earliest water released is generally the most polluted, absorbing contaminants in the air and debris that has yet to be washed off of the roofs and out of the gutters. The first flush filter’s purpose is to prevent buildup and major contamination. Though the water collected is not safe to drink, it can be used for other purposes such as irrigation.
The last piece of the conveyance section is another filter. Its purpose is to get rid of smaller particulate matter. The ideal design is 10 cm of gravel, 10 cm of activated charcoal, 25 cm of sand, and 25 more cm of gravel layered on top of each other, separated by mesh wire. The gravel and sand acts to remove silt and other organic matter, while the charcoal helps with water odor and chemical adsorption. This filter is important in improving the overall quality of rainwater being harvested, and allows for the water to sit safely in storage.
Next time I will talk about the storage portion of our design. Stay tuned!
Photo: Water Charity
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