RAIN WATER HARVESTING SYSTEM

Project based Learning RAIN WATER HARVESTING SYSTEM by Rajashri Dinkar Dahibhat & Priyanka Sampat Datir Guide Prof. Rajashri Bam

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RAIN WATER HARVESTING SYSTEM

INTRODUCTION • Rainwater harvesting is a technology used for collecting and storing rainwater from rooftops, the land surface or rock catchments using simple techniques such as jars and pots as well as more complex techniques such as underground check dams.

NEED OF RAIN WATER HARVESTING SYSTEM • This is perhaps one of the most frequently asked question, as to why one should harvest rainwater. • There are many reasons but following are some of the • important ones. • To arrest ground water decline and augment ground • To beneficiate water quality in aquifers. • To conserve surface water runoff during monsoon. • To reduce soil erosion • To inculcate a culture of water conservation

Design • Selected surface Area • Collection System • Storage system

Design of storage tanks The volume of the storage tank can be determined by the following factors: • Number of persons in the household: The greater the number of persons, the greater the storage capacity required to achieve the same efficiency of fewer people under the same roof area. • Per capita water requirement: This varies from household to household based on habits and also from season to season. Consumption rate has an impact on the storage systems design as well as the duration to which stored rainwater can last. • Average annual rainfall • Period of water scarcity: Apart from the total rainfall, the pattern of rainfall -whether evenly distributed through the year or concentrated in certain periods will determine the storage requirement. The more distributed the pattern, the lesser the size. • Type and size of the catchment:Type of roofing material determines the selection of the runoff coefficient for designs. Size could be assessed by measuring the area covered by the catchment i.e., the length and horizontal width. Larger the catchment, larger the size of the required cistern (tank).

Dry season demand versus supply approach For determining the volume of storage: • Matching the capacity of the tank to the area of the roof • Matching the capacity of the tank to the quantity of water required by its users • Choosing a tank size that is appropriate in terms of costs, resources and construction methods. • In practice the costs, resources and the construction methods tend to limit the tanks to smaller capacities than would otherwise be justified by roof areas or likely needs of consumers. For this reason elaborate calculations aimed at matching tank capacity to roof area is usually unnecessary. However a simplified calculation based on the following factors can give a rough idea of the potential for rainwater collection.

Illustration Suppose the system has to be designed for meeting drinking water requirement of a five-member family living in a building with a rooftop area of 100 sq. m. The average annual rainfall in the region is 600 mm (average annual rainfall in Delhi is 611 mm). Daily drinking water requirement per person (drinking and cooking) is 10 litres.

Design procedure: Following details are available:Area of the catchment (A) = 100 sq. m.Average annual rainfall (R) = 611 mm (0.61 m)Runoff coefficient (C) = 0.85 1. • Calculate the maximum amount of rainfall that can be harvested from the rooftop:Annual water harvesting potential = 100 x 0.6 x 0.85 = 51 cu. m. (51,000 litres) • Determine the tank capacity: • This is based on the dry period, i.e., the period between the two consecutive rainy seasons. • For example, with a monsoon extending over four months, the dry season is of 245 days.

3. Calculate drinking water requirement for the family for the dry season = 245 x 5 x 10 = 12,250 litres As a safety factor, the tank should be built 20 per cent larger than required, i.e., 14,700 litres. This tank can meet the basic drinking water requirement of a 5-member family for the dry period. A typical size of a rectangular tank constructed in the basement will be about 4.0 m x 4.0 m x 1.0 m

Design of groundwater recharge structures • In places where the withdrawal of water is more than the rate of recharge an imbalance in the groundwater reserves is created. Recharging of aquifers are undertaken with the following objectives: • To maintain or augment natural groundwater as an economic resource • To conserve excess surface water underground • To combat progressive depletion of groundwater levels • To combat unfavorable salt balance and saline water intrusion • = 4.25 cu. m. (4,250 litres) • In designing a recharge trench, the length of the trench is an important factor. Once the required capacity is calculated, length can be calculated by considering a fixed depth and width.

Design of an aquifer recharge system The proper design will include the following considerations:Selection of site: Recharge structures should be planned out after conducting proper hydro-geological investigations. Based on the analysis of this data (already existing or those collected during investigation) it should be possible to: 1) Define the sub-surface geology. 2)Determine the presence or absence of impermeable layers or lenses that can impede percolation 3) Define depths to water table and groundwater flow directions 4) Establish the maximum rate of recharge that could be achieved at the site.

Design of recharge structures and settlement tank For designing the optimum capacity of the tank, the following parameters need to be considered: • 1.) Size of the catchment • 2.) Intensity of rainfall • 3.) Rate of recharge, which depends on the geology of the site

The capacity of the tank • The capacity of the tank should be enough to retain the runoff occurring from conditions of peak rainfall intensity. The rate of recharge in comparison to runoff is a critical factor. However, since accurate recharge rates are not available without detailed geo-hydrological studies, the rates have to be assumed. The capacity of recharge tank is designed to retain runoff from at least 15 minutes rainfall of peak intensity. (For Delhi, peak hourly rainfall is 90 mm (based on 25 year frequency) and 15 minutes peak rainfall is 22.5 mm/hr, say, 25 mm, according to CGWB norms). • IllustrationFor an area of 100 sq. m.,volume of desilting tank required in Delhi = 100 x 0.025 x 0.85 = 2.125 cu. m. (2,125 litres)

Design of a recharge trench The methodology of design of a recharge trench is similar to that for a settlement tank. The difference is that the water-holding capacity of a recharge trench is less than its gross volume because it is filled with porous material. A factor of loose density of the media (void ratio) has to be applied to the equation. The void ratio of the filler material varies with the kind of material used, but for commonly used materials like brickbats, pebbles and gravel, a void ratio of 0.5 may be assumed. Using the same method as used for designing a settlement tank: Assuming a void ratio of 0.5, the required capacity of a recharge tank = (100 x 0.025 x 0.85)/0.5

Ways of harvesting rainwater

Ways of harvesting rainwater • Roof top rain water harvesting system • Surface runoff rain water harvesting system

Rainwater harvesting in Singapore The average annual rainfall of Singapore is 2400 milimeters. Inspite of 50% of the land area being used as a water catchment, almost 40-50 percent of its water requirement are imported.

Research and development work Research and development work had been done for • 1) Utilization of the roofs of high-rise buildings, • 2) Use of run-off from airports for non-potable uses, • 3) Integrated systems using the combined run-off from industrial complexes, aquaculture farms and educational institutions. Singapore has a rising demand for water and is on the lookout for alternative sources and innovative methods of harvesting water.

Rooftop water collection systems: 1. Changi Airport: • Changi Airport system collects and treats rainwater, which accounts for 28 to 33% of its total water used, resulting in savings of approximately S$ 390,000 per annum. The potential for using these rooftops as catchments are high. The system developed have been result of intensive research. A simple computer programmed was developed and monogram prepared relating the roof area, tank size and roof water available.

1 .Changi Airport:

2. High Rise Buildings: • In this system implemented in a 15 storey building, the collected roof water was diverted to two rainwater tanks and the water was used only for flushing. The water quality was acceptable in terms of color, turbidity and bacteriological content though the total solids and chloride levels were marginally higher. A simple dual mode system was incorporated in the collection tanks which were placed on the roof of the building. An economic appraisal established that there was an effective saving of 13.7% of the water. The cost of the rainwater was s$0.395 (us$0.25) per cubic meter (cum) as against the cost of potable water which was s$0.535 (us$ 0.33).

3. Urban Residential Area: • In an urban residential area of about 742 hectares having a total of 49,000 flats, rainwater harvesting was very effective. Using further modified computer programmed, it was possible to compute the volume of potable water to be pumped when there was no stored rainwater and also determine the frequency of such pumping.

4. Capturing Urban Runoff: • By 1986, the growing need for water led to the establishment of the lower Seletar-Bedok water scheme where almost nine per cent of the total land area was used. The most important feature of this scheme is that almost one-quarter of this catchment is in urban area having high rise buildings and industries and surface run-offs were subject to a wide verities of contaminants. Hence the control of the water pollution and relevant technologies were the main priorities of the scheme. The lower involved the damming of the Sungei Seletar estuary, which has a catchment area of 3200 ha, and forming of lower Seletar reservoir. The reservoirs are interconnected and raw water from Bedok reservoir is treated to potable levels before distribution. The rest of the catchment of 2,625 ha was primarily urban and both the runoffs are directed to Bedok reservoir.

Advantages • Rainwater harvesting technologies are simple to install and operate. • Local people can be easily trained to implement such technologies, and construction materials are also readily available. • It is convenient in the sense that it provides water at the point of consumption, and family members have full control of their own systems, which greatly reduces operation and maintenance problems. • Running costs, also, are almost negligible. • Water collected from roof catchments usually is of acceptable quality for domestic purposes.

It is collected using existing structures not specially constructed for the purpose. It has few negative environmental impacts compared to other water supply project technologies. Although regional or other local factors can modify the local climatic conditions, rainwater can be a continuous source of water supply for both the rural and poor. Depending upon household capacity and needs, both the water collection and storage capacity may be increased as needed within the available catchment area.

Disadvantages • Disadvantages of rainwater harvesting technologies are mainly due to the limited supply and uncertainty of rainfall. • Adoption of this technology requires a *bottom up* approach rather than the more usual *top down* approach employed in other water resources development projects. • This may make rainwater harvesting less attractive to some governmental agencies tasked with providing water supplies in developing countries, but the mobilization of local government and NGO resources can serve the same basic role in the development of rainwater-based schemes as water resources development agencies in the larger, more traditional public water supply schemes.

Tips to ensure quality of harvested rain For high quality performance. Following aspects should be taken care of: • Just before the arrival of monsoon, the rooftop/catchmet area has to be cleaned properly. • The roof outlet on the terrace should be covered with a mesh to prevent entry of leafs or other solid waste into the system. • The filter materials have to be either replaced or washed properly before the monsoon. • The diversion valve has to be opened for the first 5 to 10 minutes of rain to dispose off the polluted first flush. • All polluted water should be taken away from the recharge structures. • The depth of bores (of recharge structures) shall be finalised depending on the actual site condition

Do's and Don'ts • Harvested rainwater is used for direct usage or for recharging aquifers so following precautionary measures should be taken while harvesting rainwater:- • Roof or terraces uses for harvesting should be clean, free from dust, algal plants etc. • Roof should not be painted since most paints contain toxic substances and may peel off. • Do not store chemicals, rusting iron, manure or detergent on the roof. • Nesting of birds on the roof should be prevented. • Terraces should not be used for toilets either by human beings or by pets. • Provide gratings at mouth of each drainpipe on terraces to trap leaves debris and floating materials.

Do's and Don'ts • Provision of first rain separator should be made to flush off first rains. • Do not use polluted water to recharge ground water. • Ground water should only be recharged by rainwater. • Before recharging, suitable arrangements of filtering should be provided. • Filter media should be cleaned before every monsoon season. • During rainy season, the whole system (roof catchment's, pipes, screens, first flush, filters, tanks) should be checked before and after each rain 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 off all sediments and debris.

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RAIN WATER HARVESTING SYSTEM