Rain shouldn't go in vain; Our houses newly built RWH System

For those of you who don’t know, I’ve been deeply involved with water conservation, and advocating the efficient use of rainwater harvesting for more than 3 years now. My journey began with my participation in a competition called First Lego League, where my team came up with an intra-community rainwater harvesting model and showed how it saved more water. Since then, as I became more and more involved, I’ve been featured in the YourStory website link, interviewed by Godrej in their event Design Dekko, and got the Best Paper Award in the World Water Summit, New Delhi twice, and even got nominated for the Namma Bengaluru Award for my efforts.

Saving water should not be an ordeal - it should be a part of our lifestyle.So the natural question that arose was how to conserve water in our own homes. Besides practices of conserving water, is there a possibility of using rainwater that falls on the area of our house? Most of the rainwater was ending up in the storm water drain, which eventually ended up in sewer lakes. So this was an opportunity to make more constructive use of the rainwater. As part of my research during this hydrophilic journey, I got statistics about the availability of rainwater in Bangalore, which is the city of my residence. Bangalore gets 986 mm of rain every year(as per the Indian Meteorological Dep.). Our home has a roof area of about 1500 square feet. If we were to harvest all this rainwater, we would get 137,403L liters of water every year!

With the average monthly requirement for our home being about 30,000 liters, which includes water used for gardening, all the rainwater collected throughout the year should last for about 4.5 months.

As I discussed these with my parents, interests among us rose as we realized that rainwater can make a sizable contribution to our water needs. We started a plan to increase our use of rainwater for daily consumption. Since our home was already built, however, there were limitations on what changes we could make.

At the beginning, we had one underground sump that stored water supplied by the municipal(government) bodies, which came in twice a week, and one overhead tank in the terrace into which this soft water(~300 tds) would get pumped for our daily household use(drinking, cooking, bathing, etc).

Occasionally, when we begin to run out of Municipal water, we get access to water from a number of borewells dug in the neighbourhood. This hard water(~700 tds) would be delivered to the same underground sump.

Taking stock of the ground realities, we decided to make the modifications required to harvest the maximum amount of rainwater possible. During the design process, we took advice from experts in this domain, did further research on the types of filter systems that would be needed, and converged on a design that would be reliable, required minimal maintenance and had the least disruptions to our daily activities.

The following diagrams illustrate how our water supply system was before rainwater harvesting and after rainwater harvesting:

Before rainwater harvesting, referring to Figure 1, we had a single sump (underground tank) S1, which would receive municipal water a couple of times a week. This had a storage capacity of about 6000 litres. Water from S1 would be pumped up to the overhead tank T1 (500 liters) as and when we needed it (about once daily; done automatically using a level controller device), and this water from Tank T1 would serve all the water needs of the house.

When we planned for rainwater harvesting, we were unsure about the quality of the harvested water. So we decided to separate the water supply to the house into two sections. The first ‘cleaner’ section supplied water to the kitchen and for bathing. The second section supplied water to toilet flushing, garden, and general washing. The intention was to continue using municipal water for the first section and the harvested rainwater for the second section.

Referring to Figure 2, these sections are served by tanks T1 and T2 respectively. Here is how the rainwater harvesting system works:

1. All the rainwater that falls on the terrace is collected by a network of pipes that empties the water into a filter tank F. In addition, rain water collected in the basement sump S4(rain falling on the driveway) is also pumped up to the filter, thus increasing the collection area. The first 5 minutes of rain is discarded through a simple mechanism called ‘first flush’ for the sole purpose of getting rid of water contaminated with surface dust, small insects, etc that are always present in the terrace. (I’ll probably explain this mechanism in a later blog)

2. The filter tank, consisting of stones, gravel and charcoal, filters the collected rainwater to a considerable level and delivers this clean water to the sump S3 (up to this point, no pumps were needed to pump water to any location as gravity does this job for us).

3. Sumps S2 and S3 are linked internally at the top such that when S3 gets nearly full, water automatically overflows into sump S2. In effect these two sumps function as one large sump during heavy rains. We decided to have two separate sumps because we can isolate both the sumps for different purposes. For example, during summers, one sump can be isolated for rainwater, and the other can be filled with recycled sewage water(provided by our community) that we use to water our garden.

4. We also dug a 20-foot, 3-foot diameter recharge well ‘W’, which was connected to sump S3 in such a way that when S2 and S3 would get filled(in the case of really heavy rains), the excess water would flow into the well. We also had a mechanism that allows excess water collecting on the ground to slowly seep into the recharge well via a section of small porous stones that surround the well. This well serves to recharge the groundwater table. Thus, little to no rain water is left to flow out down the drain

5. For ‘clean water activities’ such as cooking, drinking, and bathing, we use the municipal water which is pumped up to tank T1. Water from the S2-S3 combo is pumped up to tank T2 for the second section(flushing, gardening, general washing).

Building the Underground Sumps

There are two systematic approaces to the way we can utilize the rainwater:

Important numbers:

Monthly usage of water: 30,000L

Yearly catchment of rainfall: 140,000L

• We can use all the rainwater that we receive during the rainy season over 6 months, thereby reducing our dependence on municipal water drastically during that interval. If we roughly assume that 130,000L of the 140,000 come in the 6 rainy months, we get 23,000L of rainwater every month. As our monthly consumption is 30,000L, we can fully utilize the rainwater as and when it comes, and then turn to the municipal water for the extra 7000L as and when we need it.

  • The advantage of this system is that it allows for minimum storage of rainwater, thus saving us from the trouble of building huge underground sumps(which is especially helpful when building a system in an already built house, such as in our case).

• The other method, which is a more sustainable one in the long term, is to use both municipal and rainwater in a complementary way all throughout the year. The sole purpose of this is to reduce our dependence on the Municipal(river) water during the dry seasons, when rivers lose large amounts of water due to sparse rains, by storing rainwater until the dry season.

  • Let’s assume the dry months to be 3.5 - 4 continuous months of the year. Say we reduce our dependence on Municipal water by half, which means half the water needs would come from rainwater. Half of 30,000L/month for 3.5 months would be about 50,000L of rainwater. Thus, we would need 50,000L of water stored at the beginning of the dry season to last us until the next rains come.

  • One obvious drawback of this system is the necessity of building large tanks to store a large volume of rainwater for the whole dry season.

Conclusion:

At the end of this project, we have now reduced our dependence on municipal water by over 70% during the rainy season, which is about 5 - 6 months in a year. I am content that my interest in water conservation played a part in motivating my family to take up this project to help conserve water.

During this process, I quickly realized that the water during the rainy season is abundant, and we need a way to store water for a longer duration to help in the dry season. After repeated observations of the sumps overflowing at certain times during the year, we realized that an ideal sump would have to be much larger than the 16,000L sump that we built. This limitation - the mere 16,000L sump - reduces our dependence on rainwater from 4-5 months down to 2-3 months throughout the year.

If I were to rebuild our house , I think the enhanced storage capacity of 50,000L would be incorporated into the design of the house itself to maximize the conservation water. One place that comes to mind is the space below the house. Our house has a footprint of about 1500 square feet. If we were to build a storage tank below the house of just 21 by 21 feet to a depth of just 4 feet, that would hold our desired 50,000 liters of rainwater.

[Figure 3 shows how this might look like if I were to build it again. S0 is the first sump to be built along with the house, and it will be connected to the external sumps. We might also consider installing additional filters to ensure purity so that we can use filtered rainwater for all needs - not just gardening, flushing, and general washing]