In responses to coping up with the ever-increasing water crisis, particularly freshwater crisis due to various anthropogenic causes and also groundwater depletion problem resulting in the over-extraction of it, harvesting rainwater is now considered as one of the sustainable solutions for different countries or regions of the world that receive a good amount of rain annually. This system is not a new practice as it was a common practice in ancient India and Egypt used for households and agricultural purposes and in Thailand; this practice has prevailed for centuries.
But, in recent times, the system, being clean and healthy sources of water, has gained much more popularity in different parts of the world where rainfalls are available in sufficient quantities, taking into consideration of the portable and freshwater inadequacy. Countries like Japan, Singapore, the USA, Germany, India, Brazil, Australia, China, and South Korea are the best examples of harvesting it. And most surprisingly, recently, Indian Railways has installed rainwater harvesting systems including rooftop rainwater harvesting systems at more than 2,400 different locations, including station buildings across the country in order to replenish the groundwater table and meet its growing demand for water.
Concept of Rainwater harvesting in city areas
However, Rainwater harvesting is a technique used for collecting, storing, and using rainwater for potable and various non-potable uses, and the rainwater is collected from various hard surfaces such as rooftops and/ or other types of man-made above ground hard surfaces (Rainwater Harvesting, Dublin city council). There are generally three components in any rainwater harvesting system include catchment, conveyance, filters, and storage.
In urban or city areas, rainwater harvesting can be done mainly in two ways: one is surface runoff harvesting, and another is rooftop rainwater harvesting.
In the case of surface runoff harvesting: landscapes, open fields, parks, gardens, roads and pavements, driveways, and other available open areas of the environment are used to harvest the rainwater.
On the other hand, in the rooftop rainwater harvesting system, rainwater is captured from roof catchments, and stored in reservoirs, and it is can be harvested in the city’s household buildings, offices, factories; particularly, where adequate land spaces are not available. And in this regard, according to the United Nations Environment Program (UNEP), some cities around the world have already the best examples of rainwater harvesting and its utilization including Singapore, Tokyo, Berlin.
Rainwater harvesting in Dhaka city: Need of the hour
Dhaka is the 11th largest megacity in the world which is the home to more than 18 million people, and its population size is estimated to have reached more than 27 million in 2030 (The World’s cities in 2016, UN). Since the independence of the country, the population of Dhaka city has increased more than 10 folds due to the rapid urbanization, growth of informal settlements, and industrialization in and around the city. Like many other utilities and services crisis, and problems (i.e. electricity, gas, traffic congestion, municipal solid waste collection, and management, etc.) in the city, water scarcity is the ones that the dwellers in several parts of the city are severely facing this problem almost round the year over the decades. And each year, the situation is going from worse to worse.
Though Dhaka Water and Sewerage Authority (DWASA) continues to make several efforts and initiatives to bring the situation under control, it is not being much able to deal with the ever-rising water demand resulting in rapid growth of population in the city.
And most shockingly, the city exclusively depends on the groundwater for meeting the water demand need for its dwellers as the most of the existing surface water sources (i.e. ponds, rivers, lakes, etc.) in the city have lost its natural state in terms of physical, chemical, and microbiological composition because of indiscriminate discharge of industrial effluents, household wastages into them, and finally, they have turned in to large drains. In the city, groundwater extraction is solely responsible for meeting over 80% of the water demand whereas, the remaining demand is trying to fill up by treating the surface water. But, this extensive dependency and over-pumping of groundwater for meeting the water need of city dwellers, factories, and commercial establishments have caused severe groundwater level depletion, and which in turn causes the water severe water scarcity in the city particularly, in the dry seasons.
As per the DWASA’s annual report 2014-15, the groundwater level is declining by 2-3 meters per year because of the continuous over-extraction of water. Against this backdrop, rainwater harvesting can be a sustainable solution to mitigate both severe groundwater depletion problem and water demand crisis in the city.
Like many areas in Bangladesh, Dhaka city is also blessed with rain, receiving a plenty amount of rainwater annually which is from May month through until the end of September month, while particularly the months of April, June, July, and August bring the highest amount of rainfall, having about 20-25 rainy days with average monthly rainwater of 300mm- 340mm (World weather & climate, Bangladesh information, 2017). In these months, rainwater can be harvested in the city, and harvested rainwater can be stored properly (for 4-5 months) for later use. It is also stunning to note that this year in the month of April; Dhaka city has experienced a record amount of rainfall (i.e. around 9,000 mm in the first three weeks of the month) in 35 years.
Therefore, in Dhaka city, considering the natural blessing of rainfall, and the inadequate availability of land spaces, rooftop rainwater harvesting can be the best and sustainable water supply option for its households, offices, commercial and industrial establishments, particularly in the time of an emergency situation (i.e.in the event of the breakdown of supply water) or drought times. And, from this perspective, it is now the need of the hour.
Potential benefits of rooftop rainwater harvesting in Dhaka city
Rooftop rainwater harvesting in Dhaka city can result in three different types of benefits: economic, environmental, and social. These are as follows:
- Allowing the city dwellers and other users (i.e. owners of the offices, factories, commercial establishments, and also government buildings and educational institutions, etc.) to have the access to safe and clean water both for potable(in this case, collected rainwater should be treated properly prior to use), and other possible non-potable usages including cooking and drinking, bathing and laundry, flushing toilet, washing car, landscape irrigation, and other cleaning purposes at minimal costs.
- Cutting down the annual water bill significantly for individual users.
- Reducing the pressure felt by the DWASA steadily for supplying water to the growing population and also helping it to minimize the cost burden for supplying it.
- Reducing excessive pumping of groundwater and thereby minimizing the current accelerated rate of groundwater depletion.
- Reducing ever-increasing and prolonged waterlogging on the roads, and lanes, and also lessening the problem of soil erosion in the city.
- Saving the city from the potential land subsidence that can result from excessive groundwater extraction.
- Promoting social consciousness about water conservation, and also an efficient use of water.
- Helping the people of the city to a great extent particularly the children, to save from various types of water-borne illnesses such as diarrhea, typhoid, and jaundice, dysentery, and so on.
- Making people conscious of the water resources and thereby keeping adherence to the country’s water conservation efforts.
- Creating new job opportunities for the number of builders and plumbers who will be engaged in installing, repairing to run this system.
- In order to mitigate Dhaka city’s ever-increasing water crisis and the continual depletion of groundwater level, to solve the prolonged and ever-increasing water logging crisis, and also to derive the above mentioned potential benefits of the harvested rainwater, a specific policy framework should be developed for rooftop rainwater harvesting in the city immediately by the government in which the following aspects can be covered:
- An institutional arrangement should be made, and this should include: relevant ministries, development authorities, real estate companies, NGOs, and other relevant institutions, academicians, experts having experience on rainwater harvesting practices for the promotion, implementation, and monitoring, and management of the system in the city.
- Accelerating the implementation of RAJUK’s proposed plan to install the rainwater harvesting systems at over 3, 00, 000 existing buildings of them, nearly 275,000 are residential and 40,000 are commercial establishments in the two Dhaka city.
- Incorporating the rainwater harvesting system into the Bangladesh National Building Code (BNBC) for the design and construction of new residential, commercial, and industrial buildings.
- Initiatives for sensitization and mass awareness building via several mass-media for the promotion of rooftop rainwater harvesting system, and also arranging workshops and implementing training programs on it for its proper storage, maintenance, and usage.