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KSRM COLLEGE OF ENGINEERING
The paper outlines rainwater harvesting is being frequently used these days, however, the concept of water harvesting is not new for India. Water harvesting techniques had been evolved and developed centuries ago.
Living creatures of the universe are made of five basic elements, viz., Earth, Water, Fire, Air and Sky, Obviously, water is one of the most important elements and no creature can survive without it. Despite having a great regard for water, we seem to have failed to address this sector seriously. Human being could not save and conserve water and it sources, probably because of its availability in abundance. But this irresponsible attitude resulted in deterioration of water bodies with respect to quantity and quality both. Now, situation has arrived when even a single drop of water matters. However. “Better late than never", we have not realized the seriousness of this issue and initiated efforts to overcome those problems.
Ground water resource gets naturally recharged through percolation. But due to indiscriminate development and rapid urbanization, exposed surface for soil has been reduced drastically with resultant reduction in percolation of rainwater, thereby depleting ground water resource. Rainwater harvesting is the process of augmenting the natural filtration of rainwater in to the underground formation by some artificial methods. "Conscious collection and storage of rainwater to cater to demands of water, for drinking, domestic purpose & irrigation is termed as Rainwater Harvesting”.
Why rainwater harvesting?
In many regions of the world, clean drinking water is not always available and this is only possible with tremendous investment costs and expenditure. Rainwater is a free source and relatively clean and with proper treatment it can be even used as a potable water source. Rainwater harvesting saves high-quality drinking water sources and relieves the pressure on sewers and the environment by mitigating floods, soil erosions and replenishing groundwater levels. In addition, rainwater harvesting reduces the potable water consumption and consequently, the volume of generated wastewater.
Rainwater harvesting systems can be installed in both new and existing buildings and harvested rainwater used for different applications that do not require drinking water quality such as toilet flushing, garden watering, irrigation, cleaning and laundry washing. Harvested rainwater is also used in many parts of the world as a drinking water source. As rainwater is very soft there is also less consumption of washing and cleaning powder. With rainwater harvesting, the savings in potable water could amount up to 50% of the total household consumption.
Criteria for selection of rainwater harvesting technologies
Several factors should be considered when selecting rainwater harvesting systems for domestic use:
• type and size of catchment area
• local rainfall data and weather patterns
• family size
• length of the drought period
• alternative water sources
• cost of the rainwater harvesting system.
When rainwater harvesting is mainly considered for irrigation, several factors should be taken into consideration. These include:
• rainfall amounts, intensities, and evapo-transpiration rates
• soil infiltration rate, water holding capacity, fertility and depth of soil
• crop characteristics such as water requirement and length of growing period
. Components of a rooftop rainwater harvesting system
Although rainwater can be harvested from many surfaces, rooftop harvesting systems are most commonly used as the quality of harvested rainwater is usually clean following proper installation and maintenance. The effective roof area and the material used in constructing the roof largely influence the efficiency of collection and the water quality.
Rainwater harvesting systems generally consist of four basic elements:
(1) a collection (catchment) area
(2) a conveyance system consisting of pipes and gutters
(3) a storage facility, and
(4) a delivery system consisting of a tap or pump.
Fig. 2: A schematic diagram of a rooftop rainwater harvesting system.
(1) A collection or catchment system is generally a simple structure such as roofs and/or gutters that direct rainwater into the storage facility. Roofs are ideal as catchment areas as they easily collect large volumes of rainwater.
The amount and quality of rainwater collected from a catchment area depends upon the rain intensity, roof surface area, type of roofing material and the surrounding environment. Roofs should be constructed of chemically inert materials such as wood, plastic, aluminium, or fibreglass. Roofing materials that are well suited include slates, clay tiles and concrete tiles. Galvanised corrugated iron and thatched roofs made from palm leaves are also suitable. Generally, unpainted and uncoated surface areas are most suitable. If paint is used, it should be non-toxic (no lead-based paints).
(2) A conveyance system is required to transfer the rainwater from the roof catchment area to the storage system by connecting roof drains (drain pipes) and piping from the roof top to one or more downspouts that transport the rainwater
through a filter system to the storage tanks. Materials suitable for the pipework include polyethylene (PE), polypropylene (PP) or stainless steel.
Before water is stored in a storage tank or cistern, and prior to use, it should be filtered to remove particles and debris. The choice of the filtering system depends on the construction conditions. Low-maintenance filters with a good filter output and high water flow should be preferred. “First flush” systems which filter out the first rain and diverts it away from the storage tank should be also installed. This will remove the contaminants in rainwater which are highest in the first rain shower.
(3) Storage tank or cistern to store harvested rainwater for use when needed. Depending on the space available these tanks can be constructed above grade, partly underground, or below grade. They may be constructed as part of the building, or may be built as a separate unit located some distance away from the building. .
(4) Delivery system which delivers rainwater and it usually includes a small pump, a pressure tank and a tap, if delivery by means of simple gravity on site is not feasible.
Disinfection of the harvested rainwater, which includes filtration and/or ozone or UV disinfection, is necessary if rainwater is to be used as a potable water source.
Storage tanks or reservoirs
The storage reservoir is usually the most expensive part of the rainwater harvesting system such that a careful design and construction is needed. The reservoir must be constructed in such a way that it is durable and watertight and the collected water does not become contaminated.
Materials and design for the walls of sub-surface tanks or cisterns must be able to resist the soil and soil water pressures from outside when the tank is empty. Tree roots can also damage the structure below ground.
The size of the storage tank needed for a particular application is mainly determined by the amount of water available for storage (a function of roof size and local average rainfall), the amount of water likely to be used (a function of occupancy and use purpose) and the projected length of time without rain (drought period).
First flush and filter screens
The first rain drains the dust, bird droppings, leaves, etc. which are found on the roof surface. To prevent these pollutants from entering the storage tank, the first rainwater containing the debris should be diverted or flushed. Automatic devices that prevent the first 20-25 litres of runoff from being collected in the storage tank are recommended.
Screens to retain larger debris such as leaves can be installed in the down-pipe or at the tank inlet. The same applies to the collection of rain runoff from a hard ground surface. In this case, simple gravel-sand filters can be installed at the entrance of the storage tank to filter the first rain.
Rainwater harvesting efficiency: The efficiency of rainwater harvesting depends on the materials used, design and construction, maintenance and the total amount of rainfall. A commonly used efficiency figure, runoff coefficient, which is the percentage of precipitation that appears as runoff, is 0.8.
For comparison, if cement tiles are used as a roofing material, the year-round roof runoff coefficient is about 75%, whereas clay tiles collect usually less than 50% depending on the harvesting technology. Plastic and metal sheets are best with an efficiency of 80-90%.
For effective operation of a rainwater harvesting system, a well designed and carefully constructed gutter system is also crucial. 90% or more of the rainwater collected on the roof will be drained to the storage tank if the gutter and down-pipe system is properly fitted and maintained. Common materials for gutters and down-pipes are metal and plastic, but also cement-based products, bamboo and wood can be used.
Benefits of rainwater harvesting
Rainwater harvesting in urban and rural areas offers several benefits including provision of supplemental water, increasing soil moisture levels for urban greenery, increasing the groundwater table via artificial recharge, mitigating urban flooding and improving the quality of groundwater. In homes and buildings, collected rainwater can be used for irrigation, toilet flushing and laundry. With proper filtration and treatment, harvested rainwater can also be used for showering, bathing, or drinking. The major benefits of rainwater harvesting are summarised below:
• rainwater is a relatively clean and free source of water
• rainwater harvesting provides a source of water at the point where it is needed
• it is owner-operated and managed
• it is socially acceptable and environmentally responsible
• it promotes self-sufficiency and conserves water resources
• rainwater is friendly to landscape plants and gardens
• it reduces stormwater runoff and non-point source pollution
. High Initial investment Costs - The main cost of a rainwater collection system generally occurs during the initial construction phase and no benefit is derived until the system is completed.
2. Regular Maintenance - Regular maintenance, cleaning and repair will be required for the operation of a successful rainwater collection system.
3. Vulnerable Water Quality - The quality of rainwater can be affected by air pollution, insects, and dirt or organic matter. The type and kind of construction materials used can also adversely affect water quality.
4. Water Supply is Climate Dependent - Droughts or long periods of time with little or no rain can cause serious problems with your supply of water.
5. Storage Capacity Limits Supply - The supply of water from a rainwater collection system is not only limited by the amount of rainfall but also by the size of the collection area and your storage facilities.
Effectiveness of technology
The feasibility of rainwater harvesting in a particular locality is highly dependent on the amount and intensity of rainfall. As rainfall is usually unevenly distributed throughout the year, rainwater harvesting can usually only serve as a supplementary source of household water. The viability of rainwater harvesting systems is also a function of the quantity and quality of water available from other sources, household size, per capita water requirements and available budget.
Accounts of serious illness linked to rainwater supplies are few, suggesting that rainwater harvesting technologies are effective sources of water supply. It would appear that the potential for slight contamination of roof runoff from occasional bird droppings does not represent a major health risk. Nevertheless, placing taps at about 10 cm above the base of the rainwater storage tanks allows any debris entering the tank to settle on the bottom, where it will not affect the quality of the stored water, provided it remains undisturbed.
Finally, effective water harvesting schemes require community participation which is enhanced by:
• sensitivity to people’s needs
• indigenous knowledge and local expertise
• full participation and consideration of gender issues, and
• taking consideration of prevailing farming systems as well as national policies and community by-laws.
Valid data on the economic efficiency of rainwater harvesting systems is not possible. Dependent on the regional conditions (water and wastewater prices, available subsidies), the amortization period may vary between 10 and 20 years. However, it should be taken into consideration that for the major investment (storage and pipe work) a period of use of several decades is expected.
The associated costs of a rainwater harvesting system are for installation, operation and maintenance. Of the costs for installation, the storage tank represents the largest investment which can vary between 30 and 45% of the total cost of the system dependent on system size. A pump, a pressure controller and fittings in addition to plumber’s labor represent other major costs of the investment.
Rainwater quality standards
The quality of rainwater used for domestic supply is of vital importance because, in most cases, it is used for drinking. Rainwater does not always meet drinking water standards especially with respect to bacteriological water quality. However, just because water quality does not meet some arbitrary national or international standards, it does not automatically mean that the water is harmful to drink.
Compared with most unprotected traditional water resources, drinking rainwater from well-maintained roof catchments is usually safe, even if it is untreated. The official policy of the Australian Government towards the question “Is rainwater safe to drink?” is as follows: “Providing the rainwater is clear, has little taste or smell and is from a well-maintained system, it is probably safe and unlikely to cause any illness for most users”. For immuno-compromised persons, however, it is recommended that rainwater is disinfected through boiling prior to consumption.
Drinking water from rainwater
In many countries of the world where water resources are not available at a sufficient quality fit for human consumption, rainwater acts as a substitute for drinking water and other domestic uses. In some remote islands around the globe, rainwater may even act as the major potable water source for their population.
The most important issue in collecting rainwater is keeping it free of dirt such as leaves, bird droppings and dead animals, and avoiding contamination with pollutants like heavy metals and dust.
Rainwater can be also treated for use as a potable water source. The use of slow sand filtration has proved to be a simple and effective treatment technology for the elimination of most of the organic and inorganic pollutants that may be present in rainwater, as well as producing a virtually pathogen-free water for drinking.