CONSTUCTING A RAINWATER HARVESTING SYSTEM A step-by-step guide for local governments in Lagos State

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1 COMMUNITY CONSERVATION AND DEVELOPMENT INITIATIVES CONSTUCTING A RAINWATER HARVESTING SYSTEM A step-by-step guide for local governments in Lagos State yinka jones (c)2012

2 STEPS TO CONSTRUCTING A RAINWATER HARVESTING SYSTEM Parts of a rainwater harvesting system 1. The gutter and downspouts s are channels constructed all around the edge of a roof to collect from the roof. s can be semi-circular, rectangular or hexagonal and could be made using: Locally available material such as plain galvanized iron sheet (20 to 22 gauge), folded to required shapes. Semi-circular gutters of PVC material can be readily prepared by cutting the pipes into two equal semi-circular channels. Bamboo or betel trunks cut vertically in half. 2. Downspouts - these are pipes conveying rainwater away from the gutters to the storage tank. They can be made with the same material as the gutters. 3. Leaf screens: Unwanted materials like leaves, animal droppings, etc, are often on the roof-top which need to be prevented from getting into the storage. These help to remove debris that gathers at the roof-top. Leaf guards are usually ¼-inch mesh screens in wire frames that fit along the length of the gutter. 4. First flush diverter: The first flush is a simple technology or device that ensures that runoff from the first spell of rain is flushed out and does not enter the system. This needs to be done since the first spell of rain carries a relatively larger amount of pollutants from the air and catchment surface. This device is important especially for impervious roof surfaces. 5. Storage tank: this could be cylindrical, rectangular, square or jar-shaped. The material for constructing a tank could be reinforced cement concrete, (RCC), ferrocement, plastic (polyethylene) or metal (galvanized iron) sheets. Steps to constructing a rainwater harvesting system The gutters Construct gutters the gutters can be constructed using thin metal sheets folded into a rectangular (A), semi-circular (B) or hexagonal cross-section (C) as shown in the diagram below... 2

3 i. Fix or attach gutters to the eaves of the roof. Roof Facing board ii. You could cover the whole length of the gutter with a 0.2 inch mesh to prevent unwanted materials from getting into it and clogging up the water-way. Facing board Fastener Mesh The gutter is fixed to the facing board of the roof just under the eaves. The downspouts iii. At predetermined spots along the length of the gutter, downspouts are fitted to convey the harvested water from the roof catchment to the storage tank. The number of downspout needed depends on factors like The length of the gutter, The rainfall history of the area and The area of catchment surface 3

4 Note: the gutter should be fitted in a way that allows free flow of water towards the downspouts. Facing board Downspout Facing board Downspout a. b. iv. The downspout could be fitted with a first-flush diverter for hygiene and maintenance purposes. First flush device v. Direct downspout(s) to the inlet of a storage tank. Roof Downspout Tank 4

5 The tank vi. vii. This could be made of a variety of materials as stated earlier, could be placed at any level in relation to the ground level and could be stand-alone or connected in series. Whichever the level, the water should be able to flow freely under gravity. Appropriate tank size should be chosen depending on volume. Overflow device Inlet from downspout First flush device Water outlet In constructing or choosing a tank, however, the following must be put in mind; i. Tank must be light-tight or opaque to discourage algae growth. ii. If water is for domestic use, the tank should not have been used to store toxic materials. iii. Tank must be covered and vent screened to prevent mosquito breeding. iv. Tank must be accessible for cleaning. v. Tanks should be located as close as possible to supply and demand to reduce distance of conveyance. vi. If possible, tank should be shielded from direct sunlight. vii. Locating tank very well over grade reduce the need for a pump as water flows under gravity. However, the tank inlet must be lower than the outlet of the lowest downspout to allow for a free flow of rainwater into the tank. viii. Tank must be placed on a stable level pad. In some areas, sand or pea gravel over wellcompacted soil may be sufficient for a small tank. Otherwise, a concrete pad should be constructed which must be protected from being undermined either from normal erosion or from the tank overflow. ix. The tank should be fitted with an overflow device which consists of a 4-inches pipe fixed at about 2 inches to the top of the tank. The overflow outlet should be fitted with a mesh of not greater than 0.25 inches opening. Maintenance Generally, there should be a regular check-up on the system. Worn-out or bad parts should be changed; rusting parts should be protected by coating or replaced if necessary; and parts that need to be cleaned should be adequately cleaned. 5

6 s: s should be periodically flushed to clear them of organic matter and help eliminate any clogs. Downspouts: Downspouts should be checked occasionally, especially where they connect to the gutter. Any debris should be removed to ensure the water flows through freely. Filters: in the case of older filters, this will need to be replaced occasionally. However, the newer filters only need minimal maintenance and do not require replacement. Tanks: If a first flush filter is not used, tanks will require yearly cleaning to remove organic debris buildup. If a first flush filter is used, tanks will not require cleaning as the biofilm on the bottom of the tank improves water quality by adding oxygen. In areas where acid rain is a problem, water ph should be tested periodically. A neutralizing agent can be added to the tank to combat ph problems. Pieces of limestone rock can be placed in the tank to assist in neutralizing the water. Other common neutralizing agents are quicklime, hydrated lime, soda ash and caustic soda. For protection of the rainwater harvesting system, the system must be used for what it is intended for the system should be protected from the introduction of foreign materials of any kind by people Water Management the story of three local governments in Lagos Community Conservation and Development Initiatives CCDI has been working for several years now raising awareness about a wide range of environmental issues. Recently CCDI has been working with three local governments (Lekki, Eti-Osa and Yaba Local Governments) in Lagos to address climate change issues and assist in the development of strategies for climate change adaptation. One major issue is that of water management and one of the key strategies for adaptation is rainwater harvesting. In view of this, the three local governments have identified an interest in rainwater harvesting systems. Although they expressed their concerns about the health safety and quality of the harvested water, it was, however, pointed out that the water so harvested is mainly meant for washing clothes, dishes, flushing toilets, bathing pets, and some other domestic needs. Any other use beyond these may require the treatment of the harvested water. 6

7 CCDI ROOFTOP RAINWATER HARVESTING PROJECTS 7 CCDI Rainwater Harvesting System in Igbo-Ora, Oyo State

8 CCDI Rainwater Harvesting System with underground water storage For further information on Rainwater Harvesting Systems, please contact Community Conservation and Development Initiatives, Plot 2B, Road A4, Zone A, Carlton Gate Estate, Off Chevron Drive, Lekki Epe Expressway, Lagos. P.O.Box 56267, Falomo, Ikoyi, Lagos. Phone: , Fax: Website: 8