Plumbing Africa PA October 2018 | Page 55

HEALTH AND SANITATION 53 also taking into account losses due to spillage, leakage, catchment surface wetting, and evaporation (Singh, 1992). b. Rainwater quality The quality of rainwater run-off from a catchment surface can be affected in two ways. Dirt and debris can collect on the roof surface from direct atmospheric deposition, from overhanging foliage or bird and rodent droppings. Alternatively, the roof material itself can contribute both particulate matter and dissolved chemicals to run-off water. CONVEYANCE SYSTEM Rainfall collected by the catchment is transferred to the rainwater storage tank through the conveyance network (Figure 4.4). Figure 4.4: A typical conveyance network for an above ground rainwater storage tank. Rooftop rainwater harvesting conveyance systems consist of two basic elements: 1. gutter(s) that collect water from the roof; and 2. downspout(s) that channel the water collected by the gutters into the storage tank. DESIGN AND INSTALLATION OF CATCHMENT AREA AND CONVEYANCE NETWORK Selection of the catchment area • Only roof surfaces are recommended; • Ensure that your rooftop is suitable for RWH. Corrugated iron or metal rooftops are best for harvesting rainwater, followed by tiled and concrete rooftops. Thatched roofs, green roofs or any roof painted with paint-releasing chemicals are not suitable for practical rainwater harvesting; and • Sections of the roof with overhanging foliage or trim where possible should be avoided as much as possible. To maximise the volume of rainwater collected by the RWH system • The catchment surface should be as large as possible; • When a roof catchment material can be selected and installed in conjunction with the RWH system, material with run-off coefficient close to 1, such as steel, should be selected to convey rainwater using appropriately sized and sloped components, including gutters, downspouts, and/or conveyance drainage piping; and • where possible, combine the surface area of several multiple roof catchments and connect them to a central rainwater storage tank. www.plumbingafrica.co.za October 2018 Volume 24 I Number 8