Roof top Rain Water Harvesting for Rosemary & G. Viswanath

Address: N48, 9th B main, Jeevan Bhima Nagar, Bangalore 560075

House type: Row house

Roof area: 1000 sq feet/ 100 sq mts in two levels

Number of residents: 3 family members and 2 dogs

Nature of Water supply: 

Connected to BBMP mains with 1 sump and a 1000 lts overhead tank. House is located at an elevation within the society. Mains subjected to illegal extraction with pressure pumps meaning often even on supply days sufficient water would not reach the sump. Regular need for tankers.

Date of Execution of RWH: 

May 2008

Design Parameters:

1. Roof was being used to exercise the dogs, set them free for some time. Clients were not sure whether they could stop that altogether.

2. Clients were not confident enough in the first phase to direct water directly into the sump. Partly because of the dogs using the roof but also because the quality of rainwater collected would have to be tested first.

3. Strip of land available around the house for placing a tank was limited

4. The terrace was in two levels, with a smaller terrace being at the lower level

5. The whole terrace had 2 rain water down take pipes both on the same side of the house. Half the larger higher terrace flowed into the lower terrace, and then from there to the ground. The other half of the higher larger terrace had another downtake pipe at the back.

6. The costs seemed high and had to be controlled to make the venture economically viable.

Design: 

The design could be divided into four parts,

1. A 500 litre tank on the smaller lower roof (to be used for a garden) into which the water from half the higher terrace flowed in. The overflow from this tank and the water from the lower terrace then flowed through the downtake pipe into the 200 litre filtration tank.

2. The other half of the higher terrace had its water flowing straight down to a 200 litre filtration tank.

3. A 2000 litre tank located on a slightly raised platform at the back that received water from the two downtakes via the 200 litre filtration tank.

4. A small recharge chamber, lined with bricks and about 3ft x 3ft, with a 5 ft depth into which overflow pipes from the 2000 litre tank were laid.

A first rain separator detail was part of both down take pipes

The platform for the tank was designed in a way that it enhanced the little space around it

Budget: 

The precise budget for the Rain Water harvesting itself is difficult to determine because along with the RWH design some other minor civil and plumbing works were undertaken. However the cost could be put somewhere around Rs 45 - 50,000, including, material, labour, design and supervision.

The plumber had already executed Rain Water Harvesting on a earlier site. His previous knowledge allowed for a speedier execution.

Actual Design & Usage:

1. Considering that the height difference between the two terrace levels was not much we needed to have a rectangular 500 litre tank. Since it was going to be exposed to the sun it had to be black PVC equivalent of the commonly found rectangular white PVC loft tank. This had to be specifically ordered and took time. It has now been installed.

2. In the first year the water was primarily used to wash cars (the residents have two cars) and water their small garden. Also to wipe the floors and other washing.

3. Care was taken to keep the roof clean and stop the dogs from using the roof. However intermittent use of the roof by the dogs continued.

4. It was found virtually impossible to open the cap of the rain separator by the residents.

Water Quality:

In the month of November, at the end of the rain period water samples were taken for testing to State Ground Water Cell, Bangalore, Department of Mines & Geology, Government of Karnataka. The Chemical Analysis Report of Water Sample said “All the parameters are within the permissible limit”. Dr Shashirekha added verbally that the water sample was excellent.

This was followed by the Bacteriological analysis which said

Total Coliforms: 3.6 mpn/100ml

Faecal Coliforms: 3.6 mpn/100ml

Source contaminated, Not potable

This was obviously due to the usage of the roof by dogs. However the scientist in charge, Dr Shashirekha added verbally that the water was potable after boiling.

Present Scenario:

After the results of the test the residents are enthused to drain the water collected in the 2000 litre tank into the sump for daily use.

Rain Water Harvesting Potential of the Andaman & Nicobar Islands

Some Basic Calculations on Roof top Rainwater Harvesting Systems for Port Blair

By Peeyush Sekhsaria

Rainfall data was kindly provided by John Swamy, a member of this e group.

Month	Average Rainfall for Port Blair in mm (based on 5 to 27 years average from 1930 - 1960)	Rain water harvesting potential for a 100 sq m/1000 sq feet roof	Number of days water rainwater can provide (based of 400 litres/day for a family of 4)
January	27	2700	6.75
February	26	2600	6.5
March	20	2000	5
April	82	8200	20.5
May	355	35500	88.75
June	564	56400	141
July	417	41700	104
August	432	43200	108
September	514	51400	128.5
October	309	30900	77.25
November	193	19300	42.25
December	140	14000	35
		307900	769.75

Rain Water Harvesting can alone Provide for the water needs of a family of four for 9 months of the year

Notes

1. Rainfall data for Port Blair is based on 5 to 27 years average from 1930 - 1960

2. Sample calculations done for a 100 square metre / 1000 square feet roof area

3. Average consumption assumed at 100 lpppd (100 litres per person per day) - This data is based on national standards and may not represent ground reality where in actual consumption may be found lower.

4. Family size assumed of 4 members, hence average daily consumption comes to 400 litres /day

5. Depending on the roof type there may be a maximum correction of 0.2 to the effective rainwater harvested to take care of cleaning, loss to absorption, etc

6. The cost of setting up a Rain Water Harvesting System vary a great deal depending on the individual site conditions, roof type, capacity of storage to be created.

7. Rain Water Harvesting systems can be mostly easily retrofitted on existing buildings

8. The calculations here are keeping in mind a harvest - store and a use as you store system. Meaning to say water is not stored only to be used during lean months

9. With the rainfall pattern of Port Blair it can be safely said that rain water alone can completely suffice for 8 months of the year, play a major part in the month of April. For the months of January it is possible to take advantage of the rainfall in December. Also depending on the storage capacity of the system rain water may suffice for the lean months of February & March

10. Though roofs will have to be kept clean, rainwater will have to be filtered before storage and water collected tested, it can be safely said that properly collected rainwater is fit for drinking after boiling.

11. More work will be needed if one is looking at actually designing and implementing a system, but I hope this gives an idea of the fantastic potential of Rain Water harvesting in the islands

12. Just to give an idea of the likely costs, a RWH system for a 100 sq m roof in Bangalore with a 2500 litres storage capacity, inclusive of design and supervision charges, material, labour cost roughly 40-50000 rupees. This is life long investment and depending on cost of water money is recovered in a few years. After which money is made!!