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A Do it yourself Guide to Rainwater Harvesting…

RAINWATER HARVESTING:

            Rainwater Harvesting (RWH) is the act of capturing rainwater and either storing it for use or recharging it into the ground. In Apartments, water from clean catchments (roof) can be routed properly through suitable conduits into a filter. Filtered water can be stored in storage structures for subsequent use. Surface run off can be used to recharge the ground water. This is achieved by recharge wells and/or direct tube-well recharge
 

Rainfall pattern and facts about Rainfall in Bangalore

            The following are details about the rainfall pattern in Bangalore and form the base assumptions for all designs and calculations:
Parameter
Measure
Total Annual average rainfall
970 mm
Total no of rainy days
60 rainy days
Peak hour intensity of rain in Bangalore
60 mm/hr
 

The rainfall distribution pattern in Bangalore is as follows:

MONTH
DAYS
QUANTITY (mm)
JAN
0.2
2.70
FEB
0.5
7.20
MAR
0.4
4.40
APR
3.0
46.30
MAY
7.0
119.60
JUN
6.4
80.80
JUL
8.3
110.20
AUG
10.0
137.00
SEP
9.3
194.80
OCT
9.0
180.40
NOV
4.0
64.50
DEC
1.7
22.10
TOTAL
59.8
970.00

 

 
Monthly Average Rain Fall Data of Bangalore

            The design of storage or recharge structures is based on the above rainfall distribution. Normally for Apartments in Bangalore 30 mm rainfall intensity is used for all designs.


            Bangalore is blessed with a relatively well distributed rainfall and has a rainfall distribution which is Bi-modal (two peak rainfall seasons in a year).  In this context, the following strategy has been found most suitable for rainwater harvesting implementation in Bangalore:

a)      Storage of rainwater for direct use:  Priority is given to capture as much of the run-off rainwater in storages such as sumps, on-ground tanks or tanks on terraces at intermediate levels (eg: sit outs / balconies).  However for such a strategy, the run-off only from clean areas can be tapped.  It is important that these catchment areas be free from any form of chemical or other toxic contamination and dust content is as low as possible.  Typically roof areas qualify well for such a strategy.  The water from this run-off is first rain separated, filtered and then let into the storage.  The water can be used for all household purposes such as bathing, washing, cleaning, gardening etc directly and can even be used for potable purposes if subsequently it is passed through filters to deal with bacteriological contamination (Eg:  aquaguard filters, boiling etc).  However, this requires that roof areas be kept clean and there is no junking of material on the roof or movement of pets such as dogs and cats on the roof and there is no soap water washing of the roof area.  A water testing process prior to use for drinking and cooking purposes is recommended.  Subsequently regular potability tests are also recommended.
b)      Groundwater recharge: Excess run-off from above mentioned clean surfaces, run-off from other surfaces such as roads, garden area etc can then be redirected for groundwater recharge.  In the context of Bangalore, the most effective recharge structure has been found to be arecharge well whose depth is a minimum of around 20 feet.  These recharge wells recharge the shallow aquifer.  Water needs to be desilted adequately before allowing the water into recharge wells.  The location of the recharge wells need to be chosen strategically – both where significant run-off water passes through the recharge well location and which is close to existing bore wells.  Recharge wells, over time will help replenish groundwater.  If the ground water table rises above the bottom of the recharge well, the recharge well can be used as a withdrawal well.  The diagram below illustrates the principle of recharge.
 
 
 
Groundwater Geology of Bangalore
 

 

Components of Rainwater Harvesting:

1.      Conduit / Pipes
2.      Filtration
3.      Storage Structures
4.      Recharge Structures

Conduit / Pipes:

            Rainwater pipes/conduits carry water from the roof top to the ground. These pipes have to be connected and drawn to the required location by providing sufficient slope so that the water flows with gravity and no water is stored in the pipes after rain stops. The pipes can also be laid underground but sufficient care has to be taken in order to avoid damage to the pipe because of the vehicular movement.
The cost of piping depends on various factors like quality of pipe and length of piping works to be done and many other site conditions.

FILTRATION:

            Rainwater has to be filtered for silt and suspended impurities before allowing into any of the storage structures. 
Below are some of the filtration methods which can be implemented.

First Rain Separator:

            First rain separator is a mechanism where in first few minutes of rain which carries most of the impurities from the roof is not allowed to pass into the filter or storage structures. First rain separators play a major role in rainwater harvesting structures by reducing the amount of silt / debris flowing into the filter or storage structures. This in turn reduces maintenance of filter and cleaning of the storage structure.
First rain separators can also act as bypass arrangement if the rain water is not to be collected.
Design:
            Provide a capacity of 0.2 to 0.3 Litres per square meter of the roof area in case the roof is maintained clean. 
 

First Rain separator by using Y Joints in the Conduit:

            This can be designed by providing a Y joint for the rainwater pipes at about 8 – 10 feet height and closing the end of the down flow pipe using and threaded end cap. The Other end of the Y joint can be connected to the Filter or storage Structure.
 
First few minutes of rain collects in the pipe till the water level rises to the Y joint level .Once the water level rises above the Y Joint level the water starts flowing to the filter or storage structure. The silt and dirt in the water collects at the bottom of the pipe which has to be cleaned by opening the end cap at least once in a week. A drain hole has to be provided on the cap which ensures that the pipe gets emptied after every rain. It is recommended that the cap is opened after every rain. 
 
The first rain separator can be designed by using the conduits as shown in the below diagram.
 
 
First Rain Separator



Filter:

            Rainwater from the roof carries dust and suspended particles from the roof top. These impurities have to be filtered before allowing the water for storage. This can be achieved by passing the rain water through a chamber having layers of graded aggregates and a layer of charcoal. The suspended particles get trapped in the aggregate bed and charcoal adsorbs gases and odour if any. Filter also reduces the velocity of water and hence helps in sedimentation of fine particles in the filter.
Design:
            The size of the filter depends on the roof area and the cleanliness of the roof. The filter can be designed to a capacity of 0.5 to 0.8 Liters per square meter of the roof area.
Below is the schematic diagram of a filter;

 

Rain Water Filter
 

 

Typical filter constructed at site:

Rainwater Filter Installed at Site
 

 

 
Approximate Cost of Filters:
Length
(Feet)
Breadth
(Feet)
Depth
(Feet)
Volume
(Litres)
Approximate Cost (Rs.)
1.5
1.5
3
190
4,350
1.5
2
3
250
5,700
1.5
2.5
3
310
7,000
2
2
3
335
7,650
2
2.5
3
420
9,500
2.5
2.5
3
525
11,950
2.5
3
3
630
14,350
3
3
3
750
17,000
3.5
3.5
3
1030
23,000
 

Underground / Subsurface Filters:

            In case it is not possible to provide filters over the ground, underground filters can be constructed. The principle of filtration remains the same but the filter will have 2 chambers with the filter media and the water flows through the filter media and moves to the second chamber through the underground pipe and
Below is the schematic diagram of subsurface filter:
 
Underground Rainwater Filter

Typical Subsurface filter installed at site:

Three chamber underground filter
Three chamber underground filter
 
Underground Rainwater FIlter Constructed at Site

Approximate cost for subsurface filters

Length
(Feet)
Breadth
(Feet)
Depth
(Feet)
Volume 3 Chambers
(
Liters)
Approximate Cost
(Rs.)
1.5
1.5
3
560
11,500
1.5
2
3
750
15,200
1.5
2.5
3
940
19,000
2
2
3
1000
20,200
2
2.5
3
1250
25,200
2.5
2.5
3
1575
31,500
2.5
3
3
1890
37,800
3
3
3
2250
45,360
3.5
3.5
3
3080
61,740
 
 

Different Filters:

 
Rainwater Filters Installed at  site
 
 

Storage Structures:

            Clean water from the roof can be stored in Underground sumps or Rain barrels for further use after the suggested filtration methods.
How to calculate the Required Storage or Recharge structure capacity?
Calculation of Storage volume as per BBMP Regulation:
            The required storage can also be calculated as per the BBMP regulations. As per the regulation the storage structure should be designed at 20 Litres per square Meter of the roof.
Eg: If the roof area is 100 SqM then the storage or recharge structure capacity will be
            100 x 20 = 2000 Ltrs
Calculation of storage / recharge volume based on the average rain fall:
            The storage or Recharge structure capacity can be calculated by the amount of runoff from your roof taking into consideration the losses due to evaporation, leakages etc…
The optimum storage = Roof Area in SqM x Runoff Co-Efficient (0.9) x Average Rainfall in mm
 
Runoff co-efficient is the percentage of water from the roof that comes down in the rainwater pipes which is taken as 90% for RCC roofs , as about 10% will be lost by evapotranspiration, leakages etc…
Average design Rain fall of Bangalore can be taken as 30mm
Eg: If the roof area of a building is 100 SqM then the storage or recharge structure capacity will be
            100 x 0.9 x 30 = 2700 Litres 
Design of Underground Sump:
            If a new underground sump has to be constructed for the above capacity the size of the sump would be (1.5 M x 1.5 M x 1.2 M) or 4 feet x 5 feet x 5 feet (Every Cubic feet holds approximately 28 Ltrs of water)
            Underground sumps up to 20,000 Litres can be constructed of brick or solid concrete masonry. RCC sumps are recommended for higher capacities. 
NOTE: In most cases additional infrastructure like Sumps or recharge wells may not be required. Existing tanks can easily be used to store rainwater.
Cost of Construction of a New Sump:
            A new underground sump constructed out of brick masonry will cost approximately Rs. 11 per liter and RCC sump costs approximately Rs. 14 per liter
For the above tank capacity, the cost of construction in Brick Masonry will be 2700 x 11 = Rs. 29,700
The cost of Construction in RCC would be 2700 x 14 = Rs. 37,800
Storage in Rain Barrels:
            Rainwater can also be stored in HDPE tanks for secondary uses. This procedure involves construction of a small pedestal and placing the HDPE Tank on the Pedestal and connecting the rainwater pipe to the Tank after First rain Separation unit. The water stored in the rain barrel can only be used for non potable purposes like gardening, car washing etc… Proper overflow pipe should be provided in order to avoid water logging near the rain barrel.
Typical Rain barrel installed at site:
 
Rain-barrel Installed at Site
 
 
Approximate Cost of Rain barrel Installation:
Rain barrel Capacity (Ltr)
Cost of Tank (Rs.)
Pedestal Cost (Rs.)
Total Cost (Rs.)
300
1950
2000
3,950
500
3250
3000
6,250
750
4875
3500
8,375
1000
6500
6000
12,500
1500
9750
6000
15,750
2000
13000
6000
19,000
5000
32500
9000
41,500
 

RECHARGE STRUCTURES:

            Ground water can be recharged using recharge wells. Ground water recharge helps in reviving the underground water level and can act as an additional source of water if recharged consistently.
Recharge well:
            Recharge wells are constructed by digging the earth, placing concrete rings and packing the outer space of the ring using boulders.
 

Schematic Diagram of Recharge Well:

 
Schematic Diagram of Recharge Well
 
 
 

            Recharge Wells constructed at site:

 

 
Below table can guide you to decide on the size and depth of the recharge well for the required capacity.
Diameter
Height
Approximate Volume in Litres
Approximate Cost (Rs.) *
2 Feet
10 Feet
880
9,000
3 Feet
10 Feet
1978
15,000
3 Feet
15 Feet
2967
18,600
3 Feet
20 Feet
3956
24,000
4 Feet
10 Feet
3517
22,500
4 Feet
15 Feet
5275
33,500
4 Feet
20 Feet
7034
44,500
4 Feet
25 Feet
8792
56,500
5 Feet
10 Feet
5495
38,400
5 Feet
15 Feet
8243
55,800
5 Feet
20 Feet
10990
69,600
5 Feet
25 Feet
13738
84,000
5 Feet
30 Feet
16485
94,000

Note:  The Cost mentioned is approximate and varies on many factors like the soil condition, ground water level, presence of rocks etc.


Recharge Structures :

Recharge Structures

 

Precautions to be taken while constructing a recharge well:

1.     Make sure that the recharge well is not close to any of the walls or foundations or column footings. The distance of the centre of recharge well from the footing should be at least twice the depth of the footing.
Eg: If the depth of footing from the ground level is 1.5 Mtr the recharge well should be at least 3 M away from the footing.
2.  Make sure that the recharge well is not close to the basement wall or any other underground structure. This will cause water seeping into the basement / cellar through the walls during rainy season.  The centre recharge well should be at least 3 times the distance of the depth of basement.
3.      Make sure that the soil is not too loose while digging the recharge well. Extreme care is required while digging recharge well in loose soil. Also sufficient safety measures like shoring, safety helmets are taken care.
4.      Make sure to cover the recharge well properly while construction.
5.      Make sure to provide overflow pipe for the recharge well to avoid water logging around the recharge well in case of overflow.
6.    Recharge well depth can be restricted if the water table is high in the area. Do not create a recharge well if the water table is within 5 feet from the ground level.
7.  The Boulder packing should be done by skilled people in order to avoid caving or collapse of recharge well.
8.      Provide sufficient concrete / brick lining and make sure that water is not entering into the recharge well from the sides as this may lead to collapse.
 
 

Major Advantages of Rainwater Harvesting:

  • A well planned RWH system can reduce up to 30% of the water requirement from other conventional sources like bore well or municipal supply.
  • Rainwater Harvesting provides reliable supply of water in rainy season.
  • Can be a major source of drinking water if the other available sources of water are not fit for drinking.
  • Reduces the electrical power consumption as pumping water from deeper bore wells is reduced.
  • Reduces the consumption of municipal water hence reduces the stress on centralized water supply board.
  • Reduces chocking or flooding of the storm water drains as most of the water is captured at the household level.
  • Replenishes the groundwater and secures the availability of water for dry seasons.
Solar Project at Chandpur Village
 

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