Rainwater Harvesting

What is Rainwater Harvesting

Myths about the cost and complexity of RHW systems have meant that many people do not take it seriously enough.

Rainwater Harvesting is getting a lot of publicity these days but there is still a lot of confusion in the average person’s mind. I’ve often heard people giving their opinions without actually understanding what it’s all about. One will say that it’s impossible because most buildings in Bombay (Mumbai) have no place to build a large tank. Another wonders how they can store enough water for 12 months and still others are put off by the fact that the water will “be quite filthy by that time anyway”.

There are numerous resources on the internet as well as books published by NGOs that will give you detailed information. However, on this page, I will only give you a broad understanding from which you can proceed if you so wish.

To begin with, let’s clear the confusion by first making a distinction between types of urban rainwater harvesting systems:

  1. Water collected from roofs and terraces is stored for year-round use.
  2. Water that is harvested is sent back into the ground to raise the water-table.
  3. A hybrid system where some amount is collected and the remainder goes into the ground.

Collecting Water for Later Use

The water that you store is useful to you especially if you live in a place where you have to buy water from tankers because the municipal supply is weak. The drawback of this system is that the initial investment is high. Especially in multistorey buildings, the storage system has to be designed at the outset in, say, a basement. Also, in such cases, you have to store an enormous amount of water which leads to its own set of engineering problems.

However, the picture is not entirely bleak and there are a few misconceptions that need to be clarified. Firstly, you don’t need to store water for 12 months but, at most, 8 months worth because for the remainder, the rain is refilling the tanks. In fact, even if you can store only 3 months worth, it is still an option to keep for the summer or when the municipal supply falters.

Another misconception is about the quality of water. Provided that no sunlight enters the tank, there is no chance of algae growing and “spoiling” it. The tanka system has been in practice for centuries in parts of Gujarat and the water stored there is used for everything from drinking to ablution throughout the year.

However, there are many situations where it not feasible to store such large amounts of water so let us look at the second option – recharging the ground.

Recharging Ground Water

In their obsession to be urban, our planners pave every square inch of land they find. We, the general public, are no better I can assure you. Almost all private urban properties are concreted – “otherwise where would we park our (expensive) cars”? The net result is that rain water which, under natural circumstances would have seeped into the ground and raised the water-table is now channeled into storm water drains and we desperately try to throw it away! The drains are usually unable to keep up with the discharge resulting in our roads getting flooded – leaving us floating in those very same expensive cars. This is a poor joke on us because just a few months later, we’re sure to be complaining that we have too little water!

In places like Bombay (Mumbai) the “waste” rainwater is pushed into the sea reducing the salinity and causing problems for marine life. In addition, as the level of fresh water falls, it allows the sea water, in turn, to push its way inland and make our remaining groundwater brackish. In places where the natural drainage is into rivers, it leads to sudden flooding as the rivers break their banks – a situation that is usually referred to as a “natural disasters”. Let’s not delude ourselves; there is nothing natural about it.

Recharging ground water is a lot less complicated – and costs a fraction compared to the first system. Simplistically, you only need to have a soak pit that can be filled with gravel. Of course the size and design would depend on the amount of water discharged but that, in essence, is what it is. Alternatively, if you have a borewell, it is the ideal way to send a large amount of water into the aquifier below. As I said earlier, there are numerous sources for detailed information; a brief list is given at the bottom of this page.

But there is little tangible benefit from just recharging ground water. Yes, you do know you’re doing the “right thing” but and you’re still receiving your tap water from a dam far, far away – a dam that has been built on someone else’s land. Which might lead you to wonder, couldn’t there be a way in which we can store water and recharge the ground?

I thought you’d never ask…

The Hybrid System of Water Harvesting

As you may have guessed, this is the best of both worlds. You store some; as much as you can afford – space-wise and cost-wise – and you feed the rest into the ground; as much as the ground will take anyway. The proportion of water stored can be decided after speaking with a consultant but, as usual, I will arm you with the basic knowledge so you can take an active part in the discussions.

Let’s start with the amount of water you use. This is measured in Lpd (litres per person per day). The BMC considers that the average person consumes 180 Lpd. Of this, 120 litres are for flushing and the remaining 60 are for bathing, washing, cooking and drinking. Rules for new buildings state that dual flush cisterns with low-flow must be installed. This brings the the daily total down to 110 L. If you’ve read my advice on saving water, you’ll know that this is plenty!

Let us, for the sake of calculation, assume that you have 5 members in your family and you live in a four storey apartment building that has two flats of 150sqm (~1500 sft) each per floor.

That means your own household needs 550 litres and, in total, your building requires 4,400 litres every day.

Every month you, together, need 1,32,000 litres. So, if you have a 3m (10′) deep tank, it’ll have to be about 6m x 6m (20′ x 20′) in size to hold a month’s water.

For 3 months that becomes almost 4,00,000 litres. Keeping the same depth, you now need 11.5m square (35′ x 35′) on the surface. This could be a basement (if it is designed for the purpose) or in your compound – provided the space exists.

For 6 months, the required amount increases to 8,00,000 litres and we suddenly hit a wall. No, I’m no longer talking about the lack of storage space here; I’m talking about the ability to even catch such a huge quantity of water on your roof.

Keeping the example of Bombay which I am familiar with since I’ve lived here most of my life; the rainfall here is in the region of 2.20 m each year. Now, since you have 2 flats of 150sqm per floor, if you add the staircase area, you get in the region of 325 sqm as catchment on your terrace/roof. Efficiency is rarely more than 90% (losses are due to evaporation and draining of dirty water) so, in effect, you are able to catch:

325 x 2.2 x 90% = 643.5 cubic meters, or 6,43,500 litres in a normal year.

But you remember, your requirement for 6 months is 8,00,000 litres so let’s accept that it is impossible to collect your annual requirement this way. Instead, how about you collect enough water to last you 3 months and put the rest into the ground? If you like, you can use municipal water during the monsoon and keep your stock for the summer when the supply is scarce. That takes a lot of pressure off the system and if enough people do it, we won’t even need dams like the middle Vaitarna project. In addition, this keeps storm-water off the streets and helps to prevent flooding.

There are a lot of assumptions in the calculations above so the likelihood of it matching your needs is quite remote. Give it a shot based on your own permutations; I’ve made a small spreadsheet which will, if you fill in parameters like number of persons, annual rainfall and your catchment area, give you an idea of how much water you can collect.

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