Case5-February-2013
Rainwater harvesting is an ancient technique enjoying a revival in popularity due to the inherent quality of rainwater and interest in reducing consumption of treated water. Archeological evidence attests to the capture of rainwater as far back as 4,000 years ago, and the concept of rainwater harvesting in China may date back 6,000 years. Rainwater is valued for its purity and softness. It has a nearly neutral pH, and is free from disinfection by-products, salts, minerals, and other natural and manmade contaminants. Rainwater provides a water source when groundwater is unacceptable or unavailable, or it can augment limited groundwater supplies. The zero hardness of rainwater helps prevent scale on appliances, extending their use. Rainwater harvesting at households apart from fulfilling a families water needs, reduces their utility bills.
Rain water harvesting do away with the need for water distribution infrastructures, thereby avoiding the health risks associated with storage and distribution of drinking water. But along with the independence of rainwater harvesting systems comes the inherent responsibility of operation and maintenance like flush removal system, regularly cleaning roof surface and tanks, maintaining pumps, and filtering water. For potable systems, responsibilities include all of the above, and the owner must replace cartridge filters and maintain disinfection equipment on schedule, arrange to have water tested, and monitor tank levels. Rainwater used for drinking should be tested, at a minimum, for pathogens. Rain is the first form of water that we know in the hydrological cycle, hence is a primary source of water for us
Rivers, lakes and ground water are all secondary sources of water. In present times, we depend entirely on such secondary sources of water. In the process, we have forgotten that rain is the ultimate source that needs all these secondary sources and remain ignorant of its value. Though the average rainfall in our country is 800 mm, it occurs in short spells of high intensity which gets wasted rapidly through surface runoff. Water harvesting means, to understand the value of rain, and to make optimum use of rainwater at the place where it falls
One of the Water Harvesting Technology Option is rooftop harvesting system. Rooftop catchment tanks are storage containers installed to receive runoff water from the roof of a house, a shed or a public building via a gutter and a down pipe.
The most suitable roofs for the purpose are those covered with iron sheet, tiles and fiberglass-sheet or open terrace floors. Existing asbestos sheets can also be used (because the health hazards from inhaling and ingesting asbestos fibre is only related to the production and construction process) Thatched roofs too have been used particularly for traditional systems. But they give a much lower yield and often produce coloured water.
The catchment of a water harvesting system is the surface which receives rainfall directly and contributes the water to the system. It can be a paved area like a terrace or courtyard of a building, or an unpaved area like a lawn or open ground. Temporary structures like sloping roofs can also act as catchments.
Conduits are the pipelines or drains that carry rainwater from the catchment or rooftop to the harvesting system. Conduits may be of any material like polyvinylchloride (PVC), asbestos or galvanized iron (GI),
Rooftop harvesting has been practiced since ages, and even today it is practiced in many places throughout the world. In some cases, the rooftop harvesting system is little more a split pipe or bamboo directing runoff from the roof into an old oil drum placed near the roof
Generally, runoff from only paved surfaces is used for storing, since it is relatively free of bacteriological contamination. Drain pipes that collect water from the catchment (rooftop) are diverted to the storage container.
To prevent leaves and debris from entering the system, mesh filters should be provided at the mouth of the drain pipe.Further, a first-flush device should be provided in the conduit before it connects to the storage container. If the stored water is to be used for drinking purposes, a sand filter should also be provided.
An underground RCC/masonry tank can be used for storage of the rainwater. The tank can be installed inside the basemen of a building (see figure 9) or outside the building. Pre-fabricated tanks such as PVC can be installed above the ground. Each tank must have an overflow system for situations when excess water enters the tank. The overflow can be connected to the drainage system.
Rooftop water harvesting systems can provide good quality potable water if the design features outlined below are taken into account.
Rooftop catchment system has three main components, viz. a roof, a guttering and first flush device and a storage tank.
Guttering is intended to protect the building by collecting the water running off the roof and direct it, via a down pipe, to the storage tank. Gutters should have a uniform slope of 0.5 per cent large enough to collect the heavy runoff from high-intensity rain.
Gutters are installed to capture rainwater running off the eaves of a building. Some gutter installers can provide continuous or seamless gutters. For potable water systems, lea cannot be used as gutter solder, as is sometimes the case in older metal gutters. The slightly acidic quality of rain could dissolve lead and thus contaminate the water supply. The most common materials for gutters and downspouts are half-round PVC, vinyl, pipe, seamless aluminum, and galvanized steel. Seamless aluminum gutters are usually installed by professionals, and, therefore are more expensive than other options. Regardless of material, other necessary components in addition to the horizontal gutters are the drop outlet, which routes water from the gutters downward and at least two 45-degree elbows which allow the downspout pipe to snug to the side of the house. Additional components include the hardware, brackets, and straps to fasten the gutters and downspout to the fascia and the wall.
The quantity of water stored in a water harvesting system depends on the size of the catchment area and the size of the storage tank. The storage tank has to be designed according to the water requirements, rainfall and catchment water availability.
Various kinds of recharge structures are possible which can ensure that rainwater percolates in the ground instead of draining away from the surface. While some structures promote the percolation of water through soil strata at shallower depth (e.g., recharge trenches, permeable pavements), others conduct water to greater depths from where it joins the groundwater (e.g., recharge wells). At many locations, existing features like wells, pits and tanks can be modified to be used as recharge structures, eliminating the need to construct any structures afresh.
Average annual rainfall, Size of the catchment and Drinking water requirement are the factors to be considered. Suppose the system has to be designed for meeting drinking water requirement of a five member family living in a building with a rooftop are of 100 sq m. Average annual rainfall in the region is 900 mm (average annual rainfall in Madurai is 2200 mm). Daily drinking water requirement per person (drinking and cooking) is 10 litres.
We shall first calculate the maximum amount of rainfall that can be harvested from the rooftop.
Following details are available Area of the catchment (A) = 100 sq.m
Average annual rainfall (R) = 900 mm (0.9 m)
Runoff coefficient (C) = 0.85
Annual water harvesting potential from 100 sq.m roof
= A x R x C = 100 x 0.9 x 0.85 = 76.5 cu.m (76,500 litres)
The tank capacity has to be designed for the dry period, i.e., the period of dry spell. With the two monsoons extending over seven months, dry season is of 152 days. Drinking water requirement for the family during the dry season.
= 152 x 5 x 10 = 76000 litres
As a safety factor, the tank should be built 20 per cent larger than required, i.e., 14700 litres. This tank can meet the basic drinking water requirement of a 5-member family for the dry period.
Rooftop catchment tanks, like all water supply systems, demand periodic management and maintenance to ensure a reliable and high quality water supply. If the various components of the system are not regularly cleaned water use is not properly managed, possible problems are not identified or necessary repairs not performed, the roof catchment system will cease to provide reliable good quality supplies.
Just before the rainy season, the whole system (roof catchment, gutters, pipes, screens, first-flush and overflow) should be checked and preferably cleaned after every dry period exceeding a month. At the end of the dry season and just before the first shower of rain is anticipated, the storage tank should be scrubbed and flushed of all sediment and debris (the tank should be re-filled afterwards with a few centimeters of clean water to prevent cracking). Ensure timely service (before the first rains are due) of all tank features, including replacement of all worn .screens and servicing of the outlet tap or hand pump.
Control over the quantity of water abstracted from the tank is important to optimize water use. Water use should be managed so that the supply is sufficient to last through the dry season. Failure to do so will mean exhausting all the stored water. In effect it will mean going back to where the user began, i.e. trekking long distances for poor quality water. On the other hand, under-utilization of the water source due to severe rationing may leave the user dissatisfied with the level of the service provided. Quality of stored water
Rainwater collected from rooftops is free of mineral pollutants like fluoride and calcium salts which are generally found in groundwater. But, it is likely to be contaminated with air pollutants and surface contaminants (silt, dust). All these types of contaminations can be prevented to a large extent by ensuring that the runoff from the first 10-20 minutes of rainfall is flushed off. Most of the debris carried by the water from the rooftop like leaves, plastic bags and paper pieces is arrested by the grill at the terrace outlet for rainwater.
Remaining contaminants like silt and blow dirt can be removed by sedimentation (settlement) and filtration. Additionally, biological contamination can be removed by disinfecting the water through boiling, chemical disinfection (chlorination) and filtration (activated charcoal and sand filters) and ceramic filters.