Rainwater Harvesting Made Simple
Rainwater Harvesting Made Simple WHAT IS RAINWATER HARVESTING? Rainwater harvesting is essentially the capture of rainwater when it rains. This water can be stored and re-used, or can be recharged into the ground. Water is essential to all life forms on earth - human, animal and vegetation. It is therefore important that adequate supplies of water be developed to sustain such life. Development of water supplies should, however, be undertaken in such a way as to preserve the hydrological balance and the biological functions of our ecosystems. As land pressure rises, cities are growing vertical and in countryside more forest areas are encroached and being used for agriculture. In India the small farmers depend on Monsoon where rainfall is from June to October and much of the precious water is soon lost as surface runoff. Hence, it is important to conserve, reuse (in case of rainwater) and recycle (in case of wastewater) water to ensure that this important resource essential to life is not depleted. 'Rain Water Harvesting' (RWH), a technique that has been used in India for many centuries, is now gaining importance as a supplemental source of water. Water harvesting is essentially the activity of collection of rainwater, which can be stored for direct use or can be recharged into the groundwater.
NEED FOR RAINWATER HARVESTING:
As water is becoming scarce, it is the need of the day to attain selfsufficiency to fulfill the water needs. As urban water supply system is under tremendous pressure for supplying water to an ever increasing population. Groundwater is getting depleted and polluted due to excessive use. Soil erosion resulting from the unchecked runoff. Health hazards due to consumption of polluted water. More concrete and roads – lesser percolation of rainwater naturally unlike olden days. Surface water bodies like lakes being destroyed due to unscrupulous development.
Status in Bangalore:
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No perennial source of water near the city Lakes were the only source of water to the city Status of lakes: o 1960 – 262 lakes in the green belt area o 1986 – 127 left, only 81 alive 2009 – 17 good lakes 1 currently present o
http://en.wikipedia.org/wiki/Lakes_in_Bangalore accessed 2010
This is due to Encroachment and Eutrophication (excess nitrate in the lake causing growth of algae and plankton) Arkavathi River has minimal supply Currently, water supply is from the Cauvery river, which is located 510mts below Bangalore and 100 kms away The cost of pumping water alone is Rs 16 – Rs 18/Kl Taking into account the cost of distribution and transportation across the city, BWSSB spends about Rs. 40 /Kl However, domestic customers are charged only Rs 6/Kl Even after pumping Cauvery water, the shortfall of water in the city was 495MLD in 2001 Majority of the population depends on groundwater. Unsustainable use of ground water has resulted in a drop in the water table of 5.4 mts in 18 years
BENEFITS OF RAINWATER HARVESTING: 1. 2. 3. 4. 5. 6. 7. 8.
Environment friendly and easy approach for water requirements RWH is the ideal solution for all water requirements. Increase in ground water level. Mitigates the effects of drought. Reduces the runoff, which otherwise flood storm water drains. Reduces flooding of roads and low-lying areas. Reduced soil erosion. Improves the ground water quality, since rainwater is largely free from minerals, bacteria and other contaminants.. 9. Low cost and easy to maintain. 10. Reduces water and electricity bills.
WHO CAN HARVEST RAINWATER?
Rainwater harvesting is for any person planning to build a house or who is in the process of building a house on an independent plot. It makes ecological and financial sense not to waste a pure natural resource available in large quantity on one’s roof. In the case of a homebuilder, at an initial stage of construction, investments in time, design and money are minimal for adopting roof rainwater harvesting.2
HOW MUCH WATER CAN I HARVEST IN MY HOME? 2
http://www.rainwaterclub.org/index.htmaccessed August 2009
Bangalore receives about 970mm each year. This is spread across 6-8 months. Based on the plot size and rooftop we can calculate the harvestable rainfall endowment for each home. The table below illustrates the water that is available for rooftop harvesting: 3
Plot size (sqft) 1200 2400 4000 5000
Rooftop (sqm) ~ 75% of plot size 83 165 275 344
Water harvested annually for 970mm of rain (litres) 72,000 1,44,000 2,40,000 3,01,000
Value in Rs of water harvested annually (at current tanker water rates for Rs 50/- per 1000 litres) 3,607 7,214 12,024 15,030
In Ferns Paradise, the average monthly usage of water per household is 60kl (using monthly water meter data). This amounts to 720kl of water per household on an annual basis. Based upon the above calculation, this means that the water harvested will be sufficient for 150 days (even at 2000 litres per day). Alternatively, the rainwater can be used for re-charging the groundwater which will improve the ground water table (level as well as quality of water) over a period of time.
WHAT CAN I USE MY RAINWATER FOR? The rainwater thus collected can be used for most household purposes, like cleaning, gardening, flushing, laundary, car washing etc.
BASIC ELEMENTS OF A RAINWATER HARVESTING SYSTEM Irrespective of complexity, rainwater-harvesting systems have three basic components. 1 Catchment : The surface from which rainwater is collected for storage. This could be a rooftop, a paved flooring surface, or a landscaped area. Catchment area is the area of that surface, usually calculated in m 2 (square metres).
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Calculation done by Biome Environmental Solutions Pvt. Ltd
2 Conveyance: They lead the water from the catchment surface to the storage tank. For rooftops rainwater gutters and rainwater down pipes are conveyance systems, which need to be designed appropriately so as to manage the severest intensity of rain as well as not to lose any water during the conveyance process. Storm water drains, French drains with pebbles are also conveyance systems. 3 Storage tanks: From the simplest ground level tank, to underground sumps, surface lined ponds and large lakes storage options are many depending on the context of the rainwater harvesting design. These can be above or below the ground. In some cases, the soil profile may also permit artificial recharge of rainwater to open wells and borewells where water can be stored to be retrieved later for productive use.
HOW DO I DESIGN A ROOFTOP RAINWATER HARVESTING SYSTEM FOR MY HOUSE? Step 1: Calculate the quantity of water Calculate the rooftop area – both the flat as well as the sloped areas. The area should be in m2 Calculate the total amount of water that can be harvested. Water harvested (l) = catchment area (m2) * runoff coefficient (0.8) * annual rainfall (mm) Runoff coefficient is the natural loss of water when flowing on a surface. For a concrete surface, the coefficient is 0.8. So, if the rooftop area = 1500 sq ft. = 140 sq.m. , the amount of water that can be harvested annually in Bangalore can be calculated as: Amt of water = 140*0.8*900 = 1,00,800 litres Step 2: Designing the roof area Some tips to design your roof top area to efficiently capture rainwater.
A flat roof can be gently sloped to drain water towards the downtake pipes. Ideally, it should be designed so that the slope is towards one side, so as to prevent the need for too many downtake pipes. A 'nahani trap' or 'floor trap' can be placed at the time of casting the roof just near the inlets of the downtake pipes
Sloping roofs should have a gutter of PVC or zinc sheet to collect water & channel it to the down water pipe system. Roofs should be uncluttered & should be easy to clean by sweeping & swopping if necessary.
Gutters to collect water from sloped roofs
It is important to keep the roof are clean so as to ensure that the rainwater collected is free from dust and bacterial contamination. Step 3: Downtake pipes 90mm dia. PVC pipes resistant to UV rays appear to be the best option for downwater pipes. Of course, this depends upon the roof area to be drained. The number of downtake pipes that will be required depends upon the area to be drained. About 3 to 4 downwater pipes seem sufficient for 1000 to 1200 sft area. These can be connected to each other either to form one or two outlets to lead to the storage system. Step 4: Filter It is important to filter the water before it enters the storage tank so as to remove the dirt and other particles. Filtering can be as basic as a floor trap placed before the water enters the downwater pipe or a piece of sponge placed at the inlet of the downwater pipe. However, a PVC bucket with gravel, sand & charcoal is a good filter before rainwater is stored. A small two chamber inspection/ filter tank can also be devised There are different types of filters in the market. Details of these filters are available in the Annexure.
Step 5: First flush diverter The first rain that falls after a long dry spell usually carries down with it a lot of dirt, dust, and debris that would have collected over the roof. It also dissolves many air pollutants on its way down. It is, therefore, a safe practice to divert the first run-off away from the storage tank for which a first-flush diverter is used. After the ‘first-flush’, water that is collected is cleaner. There are different types of first-flush diverters but most are not commercially available in India and Karnataka. The RAINY filter and the pop-up filter (see Annexure) also act as first-flush diverters, but if these filters are not used then one would have to fashion a first-flush diverter themselves. One that is relatively easy to build and maintain is the standpipe. The standpipe consists of a vertical PVC pipe perpendicular to the pipe that leads into the storage tank. It has a threaded plug at the bottom. Water from the downtake pipe fills up the standpipe and when full, water is allowed to go into the storage tank. The threaded portion at the bottom must be removed after each rainfall to drain out the water in the standpipe and keep it empty for the next rainfall. Step 6: Storage/Recharge Rainwater that is harvested can either be stored or can be used to recharge the groundwater. The ideal system would a combination of both – a storage tank, with an overflow pipe connected to a recharge well.
Storage Tank A storage tank could be: A roof-level storage tank Ground level drum or masonry tank Below the ground sump During the construction phase, it is most practical to opt for an underground storage tank, since it is easier to design and will work out to be less expensive. Some points to keep in mind are: Roof level storage tanks may need to be at the rear of the house or on the sides so that it is neither obtrusive nor visually offensive. Ground level drums or tanks occupy space and should not hinder movement or appear unsightly. Below the ground sump is a good option since most new constructions in Bangalore go in for sumps anyway. Sumps are hidden from view, less costly to build and do not obstruct movement.
Note: always provide for an overflow pipe for excess collection of water from the storage system The size of the storage tank would depend primarily on the space available. If we assume an average daily rainfall of 30 mm, then the amount of rainwater that can be collected from a catchment area of 140 sq m will be 3360 litres. Typically, the size of the storage tank can vary from 3000 to 6000 litres.
Recharge Well A recharge well is used to recharge the groundwater. While it does not directly reach the water table, a recharge well is a good way to moisturize the soil and let water seep its way to the water table. Infiltration through the soil also helps in filtering out the impurities in the water. A recharge well is typically 25 – 30 ft deep, and can be of a 3’ to 8’ diameter, depending on the quantity of water as well as space and budget. It is lined with concrete rings, but the bottom of the well is unlined. Rainwater fills the well and slowly infiltrates into the ground. It typically takes about 3- 4 hours for the water to completely seep into the ground.
WHAT IS RECOMMENDED IN FERNS PARADISE ? Since Ferns Paradise is entirely dependent upon borewells for its water requirement, it makes both environmental and economic sense to recharge our groundwater. While proposals are being evaluated to implement a community rainwater harvesting system, we would like to recommend that every house implements a rainwater harvesting system with groundwater recharge. Constructing a storage system would help in re-using the water for various uses such as landscape. To work towards the common goal of enhancing our water sources for the future generation, we recommend all new constructions to have a storage tank (for immediate re-use), with an overflow pipe leading to a recharge well.
CASE STUDIES Case Study 1 – Construction phase with storage option
Plot size: 50ft by 100 ft Roof area: Approximately 1500 sqft. Since the plan for rainwater harvesting was conceived during the construction stage, the owners decided to build a rainwater harvesting (RWH) tank which was separate from the drinking water sump. The capacity of the RWH tank is 6000 litres. It is an underground tank and is located at the back of the house. The tank is lined with 5’ diameter cement rings. The tank is fitted with a 3-layer filter of gravel, sand and activated carbon, at the top. There are 5 downtake pipes – 3 coming down from the left side of the house and 2 from the right side. In each side, the downtake pipes are joined underground so that there are 2 pipes finally entering the tank. The water entering the RWH tank passes through the filter before being stored in the tank. There is a self-priming pump that is used to pump water from the tank. A gate valve is used to divert the water either for gardening or to the overhead tank. There are 2 overhead Syntex tanks, each of 1000 litre capacity. One tank is connected to all the bathrooms, and the other is connected only to the kitchen. The connecting pipe between the tanks has a gate valve. The water in the RWH tank can be used both for gardening as well as in the bathrooms. Since there is a basic level of treatment, the water is perfect to use in bathrooms. If it has to be used for drinking, then additional treatment for bacterial contamination is necessary. However, the owners did not want to use the water for drinking purposes. The total cost of the tank and the filter was Rs. 10,000/By using rainwater, the consumption of supplied water has dropped by 10-20 kl during the rainy season. Case Study 2 – Construction phase with storage plus recharge option Plot size: 10000 sq ft Roof area: approx 3000 sq ft The rainwater harvesting system was implemented during construction. Hence the roof was sloped so as to collect the rainwater in five downtake pipes. Each downtake pipe has a filter to remove the grit and leaves, and they are all connected
to an underground sump (separate from the drinking water sump). The capacity of the sump is 8000 Litres. The overflow pipe from this sump is connected to a recharge well, which is 5’ diameter and 25’ deep. It is lined by concrete rings, and the bottom is unlined. A first flush diverter is present before the water enters the sump. 2 mm of rainfall is collected in the pipe and this is diverted directly to the recharge well, hence the storage tank is kept clean. A valve has been installed so as to allow by-pass of the storage tank, and enable all the rainwater to go into the recharge well. Rainwater harvested = approx. 240,000 litres annually Case Study 3 - Retrofit4 ROOF: The roof area is approximately 650 square feet. The roof was given a slope to bring all the rainwater to one point. The rainwater is brought down through one 90 mm diameter down-pipe to a first rain separator. When the terrace is being cleaned, the cap of the first rain separator can be opened and the dirty water allowed to flow in to the front garden. After cleaning the cap is closed and the first rain separator collects the first 1.50 mm of rain falling on the terrace every time it rains. The maximum amount of silt and dust on the roof is picked up in this vertical pipe first rain separator leaving cleaner rainwater to be filtered and stored for use. FILTER: Then the rainwater is filtered using a 90 litre blue colored HDPE drum. Rainwater enters from below the drum and passes through two layers of sponge kept at intervals. The water is then picked up at the top and led into the sump tank of capacity 6000 litres. During heavy rains if the sump tank fills up, the overflow of rainwater is led into an open well on site. The house is a ‘zero runoff’ house in so far as rainwater is concerned. All rainwater falling on the plot or the roof is either collected or allowed to recharge into the open well. In fact the side setback area is left unpaved to allow for infiltration of rainwater too. Rainwater available = 82,080 litres Rainwater harvested = 65,664 litres
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http://www.rainwaterclub.org/domestic_prithvi.htm accessed August 2009
CONTACTS Below is a list of organizations that design and implement rainwater harvesting systems in Bangalore: 1. Biome Environmental Solutions Pvt. Ltd Contact persons: Shubha or Avinash 1022, 6th Block, 1st Floor, HMT Layout Vidyaranyapura Main Road Vidyaranyapura, Bangalore – 560 097 INDIA Phone: 91-80-41672790 Email:
[email protected] Website: http://www.biome-solutions.com/ 2. Farmland Rainwater Harvesting: Contact persons: Vijayraj or Michael 648, 11th Cross, 7th Block, Jayanagar Bangalore - 560 082 Ph : 94481 30524, 94480 76595 Telefax: 080 - 2676 6252 e-mail :
[email protected] website : www.rainwaterharvestingindia.com www.rainyfilters.com 3. Karnataka State Council for Science and Technology Contact person: A. R. Shivakumar Indian Institute of Science, Bangalore - 560 012 Phone : 080 23341652 TeleFax : 23344880 E-mail :
[email protected] /
[email protected] Website: http://kscst.org.in/rwh.html 4. Rainwater Club Contact person: S. Vishwanath, 1022, 6th Block, 1st Floor, HMT Layout, Vidyaranyapura Main Road, Vidyaranyapura, Bangalore - 560 097 India Phone: 91-80-41672790 Email:
[email protected] Website: http://www.rainwaterclub.org/
Annexure - Details on Filters There are several filters available in the market and can also be made at home. The list provided is not meant to be exhaustive – it is only an indication of some of the filters that are available.
Option 1 - Varun Varun is a slowsand filter constructed in a 90-litre (HDPE) drum. The lid is turned over and holes are punched in it. This is the first sieve which keeps out large leaves and twigs. Rainwater then passes through three layers of sponge and a 150-mm thick layer of coarse sand. The filter removes suspended particles from the harvested rainwater. Varun has been developed by S Vishwanath, a Bangalore-based water-harvesting expert. According to him, ‘Varun’ can handle a 50mm-per-hour intensity rainfall from a 50-m2 (square metre) roof area on a decently clean roof. Based on these calculations, when a rainwater-harvesting system is being designed for a new house, the optimal number of filters can be recommended. Maintenance The sponge needs to be cleaned periodically, and the top layer of sand to a depth of about 3 cm needs to be cleaned at the end of a rainy season. Sponge can be removed and soaked/cleaned in a bucket of water and replaced. Sponges can have fungal growth if prescribed maintenance is not followed. If charcoal is used, it must be changed every year. It is also advised to clean the meshes and the top layer of gravel. Cost A filter that can service a rooftop area of 100 m2 costs about Rs 4500 (Price in Bangalore in 2006). Option 2 – RAINY filter
A product made by Farmland Rainwater Harvesting Systems Ltd, this wall-mounted filter is designed to be inserted directly into the vertical section of rainwater downtake pipes. It consists of a very fine (0.20 mm) SS-304 steel mesh which is housed in a HDPE casing. It works on the cohesive force principle. As the water passes through the filter, silt, insects, and debris continue to move down the pipe into a drain,
while clean water flows outwards through the mesh into a pipe that is perpendicular to the inlet pipe. The filter is stated to have an efficiency of 85%. Two models of RAINY, suitable for different roof areas are available. Model FL-150 is suitable for roof areas up to 110 m2 (square metre) and Model FL-250 for roof areas up to 230 m2. The patent for this filter has been applied for. Maintenance: This is a relatively maintenance-free filter, since there is automatic flushing during heavy rains. If required the mesh can be removed and washed to eliminate the suspended particles blocked inside. This can be done after the rainy season. Cost (Prices in Bangalore in 2006): Model FL-150 costs Rs 4750 and Model FL-250 costs Rs 8750
Option 3 – Ground filter Water collected from the rain water pipe. It passes through this filter at Ground level and then flows to the sump tank. It has a valve to divert the first rain collected to ground or the drain. The filter is multilayer fibre mesh. They are available in 3” and 4” sizes. If the catchment area is more than 700 sq ft, a 4” size would be preferable. Maintenance: The taken out rains.
mesh will need to be periodically after 4 – 5
Contact person: Mr. 9980522434
Venkatesh -
