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Introduction

A water catchment system is a system used to capture clean water from rainfall and direct it into a storage container. It is an easy method of obtaining potable water and storing it for future use.

One of the requirements of a water catchment system is that they require a large exposed surface to collect water. Undoubtedly this large surface will be prone to collecting pollutants such as fecal coliforms, heavy metals (Zn, Pb, Cu, Cd, Cr, Ni), ammonia (NH4+), particulates, and other organic debris due to its direct exposure to the climate, heat, moisture, animals, etc. As the whole purpose of these RRCS’s is to deliver potable water to developing communities, it is crucial to reduce the level of these pollutants in the water before it is put into storage.

Polluted water can be very dangerous in communities where health care and sanity are generally neglected. Contaminated water can cause serious illness; acute affects range from diarrhea, nausea, vomiting and lung irritation, and chronic affects include cancer, birth defects and organ damage. Pollutants accumulate on the runoff surfaces over periods of no or little rain. This is why it is especially important to get rid of the pollutant build-up before the start of heavy rainfall or rainy season. In order to accomplish this effectively and inexpensively, the “first-flush” method is widely used.

This method uses rainwater to wash away materials accumulated on the collection surface before the run-off water is put into storage for potable use. The first water collected can be used to fertilize the land, but should not be consumed. There are a variety of factors which may affect the amount of pollutants that build up on the collection surface, and thus affect how much run-off water needs to be disposed of before being collected. Climate and geographic location play a large role in this, as this affects temperature, humidity, weather patterns, flora and fauna, etc. Generally there is greater ability for pollution built up in warm, tropical climates, as they are better breeding grounds for bacteria and organic debris. Geographic location, proximity to large pollution cities, can also affect the pH level of rain-water, and acid rain may react with the roof material and displace heavy metals into the water.

Rainfall intensity and frequency are also very important factors to examine when trying to determine first flush requirements. The harder it rains, the greater the force and thus it is easier to displace and flush away pollutants stuck to the collection surface. In addition, if it rains frequently, there is less chance for pollutants to build up on the surface between rainfalls. The material used for the collection surface is also very important. Metal roofs are more prone to heavy metal leakage when exposed to acid rain – however they are generally rather smooth and as such accumulate fewer particulate pollutants. Tile roofs have a coarser surface and tend to gather more pollutants. Due to these different possibilities, roof rainwater catchment systems need to be evaluated on a case by case basis.


Water Catchment

Every water catchment system has three basic parts:

1) The collection surface

2) The collection basin

3) The gutter system

The design of this optimized water catchment system focuses on the design of a first-flush system, which primarily encompasses the gutter system, and somewhat of the collection basin. The collection surface is briefly looked at, but the focus is on the requirements for a first-flush system.

It should also be noted that although a first-flush system is a great method of diverting the large contaminants from the main collection basin, there will still be some smaller contaminants that need to be filtered out before the water can be considered potable. To do this, a filtration device is required.


Collection Surfaces

Collection surfaces must have minimal opportunities for contamination, inert or essentially inert materials, a large surface area, and a design which ensures maximum water flow. The table below gives a basic outline of various roof materials that can be used for collection. Essentially, any roof that is not made of organic material can be used in a rainwater catchment system.

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Collection Basins

The only difference for a collection basin in a first flush system is that a second, smaller basin is needed for the diverted, non-potable water. The selection of the main storage basin is not important to making a first flush system work. However, there are many other considerations for choosing/designing a collection basin. The only purpose of a collection basin is to provide a container in which to store water for a significant amount of time. The additional factors to consider are whether the basin is above or below ground, and the shape. The following table provides a list of pros and cons for above vs. below ground collection basins.


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Gutter Systems

The gutter system is the most important aspect of a water catchment system that uses a first-flush approach to decontamination. The gutter system must direct the water from the collection surface to the collection basin. In a first-flush approach, the gutter system is also responsible for directing the non-potable water from the first rainfall to a separate collection basin.


The Proposed Design

The design of this water catchment system using the first-flush method of primary decontamination is relatively simple.

Overview of Designed Water Catchment System
Overview of Designed Water Catchment System

An overview of the system can be seen above. The materials used in this depiction are for the sole purpose of demonstrating the layout of the system.

In this design, the water being collected on the roof of the building flows into the gutter, then proceeds down the pipe into the first, smaller collection basin. The size of this basin is based on the surface area of the roof, and the amount of water required to wash the majority of the buildup on the collection surface (and is further described in the sections below).

The key to this design is to seal the smaller collection basin after it has filled with the contaminated water from the roof. In order to do this, a flotation device within the vertical pipe may be used. This flotation device can be anything that floats in water, but should be as spherical as possible to ensure the best seal is made. At the base of the vertical pipe, and directly above the small collection basin, a mesh cross section must be in place to house the flotation device as the basin is filling. Once the water reaches the level of the flotation device, it will lift the device with it as it travels up the height of the pipe. At the top of this vertical section of pipe is a smaller section of pipe jammed inside the first pipe, creating a smaller inner diameter that must be smaller than the flotation device. When the device reaches this smaller diameter pipe, a seal is created, and all the water that is collected from this point onwards is directed to the large, second collection basin, which houses the potable water.

The design of this system depends on the location (and the amount of precipitation per year) and the surface area of the collection system. These factors will affect the volumes of both collection basins, and the required diameter of the pipes.

Regional Considerations

The location in which this design is implemented will be a determining factor in how to size the system, and with what materials the system can be built. To narrow the scope of this project, only regions in Africa and South America were considered.

The size of the system depends on the amount of annual rainfall in a region. Following are links to precipitation maps of both Africa and South America.

[Precipitation Map of Africa]

[Precipitation Map of South America]

The following section outlines different tables to help size a water catchment system based on different levels of annual precipitation.

Please refer to Table 1 in the following section of your location corresponds to either:

• The dark blue areas in the precipitation map of Africa; or

• The green or blue areas in the precipitation map of South America

Please refer to Table 2 in the following section if your location corresponds to either:

• The medium blue areas in the precipitation map of Africa; or

• The yellow areas in the precipitation map of South America

Please refer to Table 3 in the following section if your location corresponds to either:

• The light blue areas in the precipitation map of Africa; or

• The orange or red areas in the precipitation map of South America

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