How Much Water Can We Collect?

Rainwater harvesting systems generally have the following main components. These components are listed here:

•  Roof/catchment area
•  Gutters and connecting pipes
•  Leaf screens and first flush diverters and graded filter filled with coarse sand, gravel and pebbles
•  Storage Tank
•  Water Treatment system
•  Water distribution system

Though in some domestic rainwater harvesting systems you may not find the graded filter. In some large-scale rainwater harvesting systems, it may be advantageous to have a sedimentation tank before graded filter to settle the dust particles. Main components of a domestic rainwater harvesting system shown in Figure

i. The Roof / Catchment Area

Rainwater can be collected from any inert roofing, that is, it won’t leach out any chemicals into rainwater. It is important that no lead is used as roof flashing or gutter solder, as the slightly acidic quality of rain can dissolve the lead and contaminate the water supply. Be aware that some composite asphalt, asbestos, concrete tiles and lead based paints can leach pollutants and affect the quality, colour and taste.Therefore we need to pay special attention to roofing, if we are going for rainwater harvesting. We may paint the roof with non-toxic paints to make the roof less porous and increase the collection efficiency.

Rainwater Harvesting System

ii. Gutters and Pipes

These are the conduits from the collection surface to storage tank. Size and slope of these will affect collection efficiency of the rainwater harvesting system. We need bigger size to avoid spillage and overflow during most intensive rainfall anticipated. But increasing size will increase the cost of installation, which is will give added advantage only during heavy rains. So we need to optimize the size.During off-season, rodents might make these pipes as their homes. So check gutters and pipes periodically for rodent infestation.

iii. Leaf Screens, First Flush Diverters And Graded Filters

The function of leaf screens is to prevent leaves and other debris from entering the rainwater collection system. Primary leaf screening devices may have a 6mm wire mesh in a metal or plastic frame installed on the top of gutter. The rainwater entering gutter will pass through the screen and leaves and rags etc. will be retained by the screens. If there are trees nearby and leaves pose a problem, a leaf screen may be installed along the entire length of the gutter.The first part of the rainfall picks up most of the dirt, bird droppings and debris.This initial rainwater needs to be diverted. First flush diverters are the devices used for removal of this dirt and debris (shown in the Figure 20.2). The first flush diversion devices are nothing but a short pipe/ small chamber near the entrance of gutter where rainwater from roof is collected. As the rain fall starts, the initial water carrying dust and debris will fill this pipe or chamber. After that the rest of rain water find its way to graded filter and storage tank. The small chamber/pipe has a valve in the base that allows it to empty before the next rainfall. Typically 40 liters of water for every 100 square meters of roof area is diverted.Graded filters are structures used for removal fine suspended particles. These are filled with coarse sand, gravel and pebbles sequentially to make a filter bed. The size of particles in each layer and depth of layer will influence the in efficiency of particle removal and head loss or pressure difference across the bed. Higher efficiencies can be achieved by use of small size of particles with increased depth. But this will lead to more head loss in the bed. These filters are prone to clogging. So you need to clean it or have provision for backwashing. In backwashing, pressurized water enters from bottom and leaves through top layer. This operation will remove particle adhered to pebble, gravel and sand.

iv. Storage Tanks

The characteristics of a good storage tank include durability, leak proof, opaque exterior and a clean, smooth interior. The tank should be located near a cool place where less light falls. This will inhibit the growth of algae. If tanks are not opaque and if lot of sunlight is there, the algae growth will be prevalent. We can construct the tank from many materials such as plastic, steel, concrete and fiberglass. Tank should be covered properly to prevent evaporation, mosquito breeding and to keep insects, rodents, birds, and children out of reach. Provide a manhole for tank cleaning and suitable overflow outlet for excess water removal. The elevation of tank can be above ground, below ground, or at the ground level. It could be wall mounted just under the gutter. It is best to locate the tank in a cool place, out of sunlight to inhibit the algae growth.We need sufficient storage capacity of the tank to store the water. It depends on many factors. The quantity of water needed, the amount and pattern of rainfall and the surface area of catchment area are the criteria used in deciding the size of storage tank.

v. Water Treatment
The treatment operation depends upon the intended uses of rain water. Treatments for non-drinking purposes include simple filters, addition of chemical coagulants for flocculation and sedimentation. Fine filters and microbiological disinfection are only necessary for potable use, i.e. for drinking water. The disinfection can be achieved either by addition of chlorine or exposure of water to ultraviolet (UV) light.Chlorination is more commonly used. The fine filters need regular cleaning and maintenance.

In spite of installation of leaf screens and first flush diverters, dirt, rust, scale, bird and rodent droppings and airborne bacteria may still reach the storage tank.Therefore, even for non drinking purposes, it may be a good idea to have gravity settling of solids and filtration of dispersed solids. Optionally, fine filters may be installed prior to the end use.Consult with experts before adopting different water treatment methods. Have water tested by an approved laboratory periodically to monitor the water quality.

vi. The Distribution System for the Treated Rainwater

The treated water needs to be conveyed to the point of use. Proper distribution system is needed for this. If the tank is installed at higher elevation than the taps, the water will flow by gravity. Otherwise, we need to install a pump in distribution system. Effective plumbing is important for efficient rainwater collection and distribution.