|
|
Environment
Raindrops on the Roof Top – 2
by V.K. Joshi
 Recharge
through hand pumps and dug wells
Working/abandoned hand pumps and abandoned dug wells (see figures) near
the houses in alluvial areas where the aquifer is comparatively at a
shallow depth are ideally suited for recharge.
Hand pumps
are useful for roofs up to 150 sq m, whereas the dug wells can recharge
the aquifer from roof tops larger than 1000 sq m.
 Recharge
Trench
For high rise
residential complexes with sufficient open space a recharge trench (see
figure) is useful. This involves digging a trench 0.50 to 1m wide and
1-2 meter deep trench around the perimeter of the complex. The trench is
filled with pieces of bricks and water from the roof is diverted to the
trench. This technique is useful for roofs up to 200 to 300 sq m area.
Precautions
Water from first rain should not be harvested as it brings along with
the atmospheric pollutants too. Many of them are injurious to life. Use
of proper filters between the water outlet from the roof and the
recharge structure is strongly advocated by experts. In case of
rainwater storage periodic chlorination is necessary to avoid bacterial
contamination.
Advantages
Rooftop rainwater harvesting augments the availability of water, checks
the declining water table, it is eco-friendly, dilutes the fluorides,
nitrates and salinity of groundwater and prevents soil erosion and
water-logging particularly in urban areas.
Problems
Easier said than done, the most common complaint of people is lack of
information regarding the availability of technical guidance, masons and
technicians to do erect the structures. Applying common sense some
people are lucky enough to install a structure that really harvests
water. On the contrary some have lost money along with water in the
process. Actually the issue needs a wider publicity and awareness.
Metros move ahead
Delhi
Countrywide though rainwater harvesting is just a mere dot in the map,
the scene in localized pockets is gradually changing for better. The
Residents Welfare Association (RWA) of a colony in south Delhi has been
able to harvest approximately 48,000 cu m of water annually continuously
for past two years. The RWA was assisted by the Centre for Science and
Environment (CSE). A well laid out water collection system from the
roofs and the streets takes water to recharge structures strategically
located in the colony. The costs of such structures vary from Rs 8000
for a private recharge tank to Rs 30,000 for a recharge tank for the
entire block.
The depleting and overstretched water table of Delhi needs hundreds of
such structures. Collectively they can change the shape of the things.
But the government is perhaps more interested in transporting water from
the Ganga River!
Mumbai
Compared to Delhi , Mumbai is colossal. It is one of the ten mega cities
of the world and most densely populated amongst the Indian metros with a
population density of 36600 persons per square kilometer. Demand of
water for the metro is 3900 million liters per day (MLD) against a
supply of 3000 MLD. The original water supply scheme of Mumbai was
commissioned in 1810 and subsequently many more schemes were added.
Today the water supply meets 95% quota from sources as far as 110 to 120
km away. Obviously with this scenario and anticipated rise of population
there is not going to be much change in the demand and supply ratio.
During monsoon billions of liters of water flows down to the sea in
Mumbai. Rainwater harvesting is the only answer to save Mumbai from
remaining thirsty on the sea shore. Taking into account the area of
Mumbai and average annual rainfall, an estimate says that if roof top
rainwater is religiously practiced nearly 590 MLD water can be easily
harvested. This would considerably ease the problem of water logging too
during the monsoon.
Mumbaikars have to take cue from the RWA of Delhi and self implement
rain water harvesting in their societies. Waiting for the government to
implement such schemes may not be of much use. Like anywhere it seems
such 'small measures' are of not much interest to the government.
Kolkata
Unlike Mumbai, Kolkata is located on the terminal part of
Ganga-Brahmputra Alluvium. Precipitation is also sufficiently high yet
Kolkata faces water shortage. Indiscriminate use of tube wells is one of
the root causes. In Kolkata an individual can sink a deep tube-well by
paying a pittance of Rs 4000 per annum to the Kolkata Municipal
Corporation.
While water levels are going down in central and south Kolkata, West
Bengal Pollution Control Board has installed as an example Rainwater
Harvesting structures in Loreto Convent, Sealdah and Rajbhawan. These
structures will replenish 6, 00,000 liters of water and also store
10,000 liters water. The state government has to lay and implement
stringent rules and clear maps of new housing projects only if rain
water harvesting structures are included by the builders.
Chennai
Geohydrological condition of Chennai is worst compared to other metros.
Despite a big 'water trade' through tube wells in the farms near Chennai
the government has laid strict rules for water harvesting and recycling.
A year ago this author was surprised to observe a 'leak-proof' network
of water supply pipes in Chennai. Investigations revealed that these
pipes can not leak because there is no water in them. Majority of the
population gets water supplied through water tankers. A water tanker
carrying 12,000 liters of water costs between Rs 7000 to Rs 10000. A
family of five needs about 2,46,375 liters water per annum at an average
cost of about Rs 20,530. A house plot of 2400 sq. feet can yield up to
2, 55, 500 liters water in a year. It is economical because only one
time installation cost involved. Law abiding, disciplined residents of
Chennai did attempt rain water harvesting seriously and succeeded to
quite an extent. A little encouragement from the government like
reducing the house tax of an owner who implements rainwater harvesting
will go a long way to improve the situation.
Metros are the places where the richest of the rich and also the poorest
of the poor live. The media also keeps a regular eye on the activities
of the metros. Even the efforts made by individuals, society, NGOs and
Institutions towards rain water harvesting get well publicized.
We have read something about the rain water harvesting in the metros. In
the next issue we will read about some success stories from some remote
places.
July 23, 2006
Images by
courtesy of CGWB
Previous
Page
Previous : Alluvium:
Nature's Store of Water
Next: On the Footsteps of Kautilya!
|
|
