Plumbing Africa March 2018 | Page 27

HEALTH AND SANITATION 25 << Continued from page 23 RECOMMENDED MINIMUM WATER GUIDELINES FOR RAINWATER HARVESTING In order to maintain the quality of harvested rainwater, the minimum water guidelines developed should address the six basic components that make up a rainwater harvesting system: • Catchment surface, environment, atmospheric conditions. • Gutters and downspouts through which water moves from the roof into the tank. • First flush diverters, leaf screens and roof washers — remove large pieces of debris and dust before the harvested water goes into the tank. • Storage tank(s). • Controls and pumps — determine the level of water in the tank, minimise air gaps, and prevent backflow. • Filtration, treatment, and disinfection — depends on the end use of the water. For potable use, filtration, treatment, and disinfection need to be used. While for non-potable use, filtration and treatment might be sufficient. GUIDELINES FOR CATCHMENT SURFACES, GUTTERS, DOWNSPOUTS, STORAGE TANKS, AND FIRST FLUSH DIVERTERS Improving the roof run-off quality of rainwater minimises the contamination levels that could be subsequently observed in stored rainwater. Removal of vegetation from around the catchment area and the installation of leaf guards prevent large debris from entering the storage tank. Birds, rodents, and small animals should also be monitored so that they do not wander around the catchment area, thereby introducing microorganisms into the run-off rainwater, primarily through their faecal droppings. The material the roof or catchment area is made of, has an influence on the quality of the water harvested. A metal roof made out of powder-coated steel is a smooth surface that allows efficient water run-off from the roof and furthermore, it is highly resistant to corrosion. Slate is also ideal as a catchment surface; however, compared to metal, it is costly. Irrespective of the type of roofing material used, testing of the run-off must be conducted to determine the microbial and chemical quality of water obtained from the various roofing materials. Guttering systems are often designed to maximise the amount of water that can be harvested. Guttering is usually made of polyvinylchloride (PVC), vinyl, and seamless aluminium. These systems can also be critical www.plumbingafrica.co.za control points when it comes to the introduction of contamination into stored rainwater. Some of the designs of guttering systems that minimise contamination include the use of a primary filter such as coarse screen or fine screen. A leaf screen is a type of coarse screen that is able to prevent larger particulate matter from entering the tank. For rainwater harvesting systems installed in areas with dense vegetation, it should be a requirement to have one of these types of screens. The fine screen, such as a strainer bucket, sits between the delivery pipe and the tank, preventing small insects and possibly rodents from having access to the storage tank. The sizes of these filters can be regulated depending on what type of particulate matter needs to be removed. The first litres of rainwater that are harvested should be drained off through the use of first flush diverters. This reduces the contamination that can arise from a dirty roof or catchment surface area. The first flush of water need not be wasted and can be diverted to an area with vegetation and plant life that would require watering, with the exclusion of small gardens and anything grown for ingestion. There are various types of first flush diverters that can be used, with the simplest ones being made out of a PVC standpipe. Various factors need to be taken into consideration when determining how much water must be diverted initially. These factors include the duration between rainfall events, the slope and smoothness of the catchment area, as well as the intensity of the rainfall event. In general, first flush diverters should be able to divert at least 38ℓ of water per 92m 2 of roof area. The storage tank is another critical control point in the design of a rainwater harvesting system. Although there are a variety of options when it comes to choosing a storage tank, there are certain characteristics that all tanks must have in order to optimise the quality of rainwater. All tanks must have lids and be covered at all times, and preferably with vents, which hamper mosquito breeding. To block sunlight, thereby preventing algal growth, tanks must be opaque. If the tank is not opaque at the time of purchase, it should be painted. With continual use of the tank, sediment build up and biofilm formation might occur in the tank. It is necessary to have easy access to the tank in order to clean and perform the necessary maintenance, thereby maintaining the quality of stored rainwater. The location of the tank can have negative impacts on the quality of stored water, especially if the tank is stored Continued on page 27 >> March 2018 Volume 24 I Number 1