Nikhil Water Harvesy

7/31/2019 Nikhil Water Harvesy

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Presentation on rainwater

harvesting system

Nikhil Kumar Sharma,

2nd sem, section-b


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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.


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NEED OF RAIN WATER

HARVESTING SYSTEM

This is perhaps one of the most frequently asked question, as to why one should harvestrainwater.There are many reasons but following are some of theimportant ones.To arrest ground water decline and augment groundTo beneficiate water quality in aquifers.To conserve surface water runoff during monsoon.To reduce soil erosionTo inculcate a culture of water conservation


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Design

Selected surface Area

Collection System

Storage system


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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 greaterthe storage capacity required to achieve the same efficiency of fewer people underthe same roof area.

Per capita water requirement: This varies from household to household based onhabits 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 willdetermine the storage requirement. The more distributed the pattern, the lesser thesize.

Type and size of the catchment:Type of roofing material determines the selection ofthe runoff coefficient for designs. Size could be assessed by measuring the areacovered by the catchment i.e., the length and horizontal width. Larger thecatchment, larger the size of the required cistern (tank).


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Dry season demand versus supply

approachFor 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 itsusers

Choosing a tank size that is appropriate in terms of costs, resources andconstruction methods.

In practice the costs, resources and the construction methods tend to limitthe tanks to smaller capacities than would otherwise be justified by roofareas or likely needs of consumers. For this reason elaborate calculationsaimed 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.


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Illustration

Suppose the system has to be designed for

meeting drinking water requirement of a five-

member family living in a building with arooftop area of 100 sq. m. The average annual

rainfall in the region is 600 mm (average

annual rainfall in Delhi is 611 mm). Dailydrinking water requirement per person

(drinking and cooking) is 10 litres.


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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.

1. Calculate the maximum amount of rainfall that can beharvested from the rooftop:Annual water harvesting potential = 100 x 0.6 x 0.85

= 51 cu. m. (51,000 litres)

2. Determine the tank capacity:

This is based on the dry period, i.e., the period between thetwo consecutive rainy seasons.

For example, with a monsoon extending over four months, thedry season is of 245 days.


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3. Calculate drinking water requirement for the familyfor the dry season

= 245 x 5 x 10= 12,250 litres

As a safety factor, the tank should be built 20 per centlarger than required, i.e., 14,700 litres. This tank canmeet the basic drinking water requirement of a 5-member family for the dry period. A typical size of arectangular tank constructed in the basement will beabout 4.0 m x 4.0 m x 1.0 m


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Design of groundwater recharge

structures

In places where the withdrawal of water is more than the rate of rechargean imbalance in the groundwater reserves is created. Recharging of aquifersare 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 importantfactor. Once the required capacity is calculated, length can be calculated by

considering a fixed depth and width.


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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 (alreadyexisting 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 percolation3) Define depths to water table and groundwater flow directions4) Establish the maximum rate of recharge that could be achieved at

the site.

i f h d


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Design of recharge structures and

settlement tank

For designing the optimum capacity of the tank,the following parameters need to beconsidered:

1.) Size of the catchment 2.) Intensity of rainfall

3.) Rate of recharge, which depends on the

geology of the site


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The capacity of the tank

The capacity of the tank should be enough to retain the runoff occurringfrom conditions of peak rainfall intensity. The rate of recharge incomparison to runoff is a critical factor. However, since accurate rechargerates are not available without detailed geo-hydrological studies, the rateshave to be assumed. The capacity of recharge tank is designed to retainrunoff 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 peakrainfall 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)


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Design of a recharge trench

The methodology of design of a recharge trench is similar to that for asettlement tank. The difference is that the water-holding capacity of arecharge trench is less than its gross volume because it is filled with porousmaterial. A factor of loose density of the media (void ratio) has to beapplied 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


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ways of harvesting rainwater


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Ways of harvesting rainwater

Roof top rain water harvesting system

Surface runoff rain water harvesting system
http://rooftop.ppt/http://surface%20runoff.ppt/http://surface%20runoff.ppt/http://rooftop.ppt/


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Rainwater harvesting in Singapore

The average annual rainfall of Singapore is2400 milimeters. Inspite of 50% of theland area being used as a water

catchment, almost 40-50 percent of itswater requirement are imported.


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Research and development

workResearch 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 educationalinstitutions.

Singapore has a rising demand for water and is on the lookoutfor alternative sources and innovative methods of harvestingwater.


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Rooftop water collection systems:

1. Changi Airport:

Changi Airport system collects and treats rainwater, whichaccounts for 28 to 33% of its total water used, resulting insavings of approximately S$ 390,000 per annum. The potentialfor using these rooftops as catchments are high. The systemdeveloped have been result of intensive research. A simplecomputer programmed was developed and monogramprepared relating the roof area, tank size and roof wateravailable.


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1 .Changi Airport:


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2. High Rise Buildings:

In this system implemented in a 15 storey building, thecollected roof water was diverted to two rainwater tanks andthe water was used only for flushing. The water quality wasacceptable in terms of color, turbidity and bacteriologicalcontent though the total solids and chloride levels were

marginally higher. A simple dual mode system wasincorporated in the collection tanks which were placed on theroof of the building. An economic appraisal established thatthere was an effective saving of 13.7% of the water. The costof 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).


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3. Urban Residential Area:

In an urban residential area of about 742 hectareshaving a total of 49,000 flats, rainwater harvestingwas very effective. Using further modified computerprogrammed, it was possible to compute the volumeof potable water to be pumped when there was nostored rainwater and also determine the frequency ofsuch pumping.


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4. Capturing Urban Runoff:

By 1986, the growing need for water led to the establishmentof the lower Seletar-Bedok water scheme where almost nineper cent of the total land area was used. The most importantfeature of this scheme is that almost one-quarter of thiscatchment is in urban area having high rise buildings and

industries and surface run-offs were subject to a wide veritiesof contaminants. Hence the control of the water pollution andrelevant technologies were the main priorities of the scheme.The lower involved the damming of the Sungei Seletarestuary, which has a catchment area of 3200 ha, and forming

of lower Seletar reservoir. The reservoirs are interconnectedand raw water from Bedok reservoir is treated to potable levelsbefore distribution. The rest of the catchment of 2,625 ha wasprimarily urban and both the runoffs are directed to Bedokreservoir.


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Advantages

Rainwater harvesting technologies are simple to install andoperate.

Local people can be easily trained to implement suchtechnologies, and construction materials are also readilyavailable.

It is convenient in the sense that it provides water at thepoint of consumption, and family members have fullcontrol of their own systems, which greatly reducesoperation and maintenance problems.

Running costs, also, are almost negligible.

Water collected from roof catchments usually is ofacceptable quality for domestic purposes.


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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 continuoussource of water supply for both the rural and poor.

Depending upon household capacity and needs, both thewater collection and storage capacity may be increased asneeded within the available catchment area.


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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* approachrather than the more usual *top down* approach employed inother water resources development projects.

This may make rainwater harvesting less attractive to somegovernmental agencies tasked with providing water supplies indeveloping countries, but the mobilization of local governmentand NGO resources can serve the same basic role in thedevelopment of rainwater-based schemes as water resources

development agencies in the larger, more traditional publicwater supply schemes.


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Tips to ensure quality of harvested rain

For high quality performance. Following aspects should be taken careof:

Just before the arrival of monsoon, the rooftop/catchmet area has to becleaned properly.

The roof outlet on the terrace should be covered with a mesh to prevententry of leafs or other solid waste into the system.

The filter materials have to be either replaced or washed properly beforethe monsoon.

The diversion valve has to be opened for the first 5 to 10 minutes of rain todispose 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 onthe actual site condition


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Do's and Don'ts

Harvested rainwater is used for direct usage or for recharging aquifers sofollowing precautionary measures should be taken while harvestingrainwater:-

Roof or terraces uses for harvesting should be clean, free from dust, algalplants etc.

Roof should not be painted since most paints contain toxic substances andmay 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.


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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 andpreferably cleaned after every dry period exceeding a month.

At the end of the dry season and just before the first shower of rain isanticipated, the storage tank should be scrubbed and flushed off allsediments and debris.

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