Eying rainwater harvesting as water crisis deepens

A girl collecting rainwater from an underground reservoir in MirpurPhoto: STAR

Harvesting rainwater on the rooftop can be a possible alternative solution to acute water crisis in Dhaka city, experts say.
Many agencies, including the Dhaka Water and Sewerage Authority (Wasa) are studying the feasibility of rainwater harvesting as alternative source of water and a way to recharge the depleted groundwater table.
Mizanur Rahman, senior hydrogeologist, Institute of Water Modelling (IWM), currently conducting a study said, “Rainwater harvesting has immense possibility in Dhaka city. Bangladesh is a country of heavy monsoon. The water that goes down the drain and causes waterlogging every year can serve a better purpose.”
The capital city averages nearly 1,700 to 2,200 mm rainfall per year while the nationwide average is around 2,200 to 2800. Monsoon usually lasts from May to October and there is occasional rainfall also in November.
Dhaka city is situated on around 370 sq km of land with a roof area of 75 sq km as there are around 6,75,000 concrete houses in the city. With the current amount of rainfall IWM estimates that around 149160 million litres of water can be harvested during the monsoon.
“This will reduce the dependency on groundwater at least for the six months of monsoon. About 15 percent of the total supply can be met by harvested rainwater,” Rahman said.
Rahman noted that apart from using it for public consumption, rainwater can also help conserve groundwater and artificially recharge the water table by digging wells and trenches on the ground.
Groundwater in Dhaka city is depleting alarmingly as the water supply is mostly dependent on groundwater. Current demand of water in Dhaka city is more than 200 crore litres per day while Wasa has the capacity to produce up to 180 crore litres.
Of the total water demand, 83 percent is being met by underground sources using 491 deep tube-wells. The rest 17 percent is supplied by three surface water treatment plants in and around Dhaka, using water from the adjacent rivers.
“This is a huge pressure on groundwater table. Over exploitation of groundwater is causing high depletion rate. The water table is continuously going down in Dhaka city posing environmental threat,” Rahman said.
According to Water Development Board, the groundwater table in upper aquifer goes down by 3.5 metres every year. In 1990 the rate was only 00.33 metre.
In Tejgaon, Khilgaon, parts of Banani and Mirpur, the groundwater table is more than 70 metres below the surface. The level varies in different parts of the city depending on the population density and water consumption rate.
There are also around 2,000 privately owned deep tube-wells of various capacities in the city running without any monitoring, making the situation even worse. These tube-wells are mostly owned by factories.
“These privately owned deep tube wells are one of the major reason for the depleted condition of underground water. They pump water indiscriminately without assessing the condition”, he pointed out.
“As the groundwater level is gradually going down in the city, many pumps can no longer lift water. Cost of water pumping is also increasing every year because the pumps needs to go further down,” Rahman said.
The three surface water treatment plants of Wasa cannot function properly as the pollution level in the rivers Balu, Shitalakhya, Turag and Buriganga is so high that the water is beyond treatment.
Considering the situation, Wasa is planning to implement a project to recharge groundwater level artificially with harvested rainwater and thus check the depletion. Wasa believes it is the easiest and cheapest solution.
The pilot project will be implemented on the roofs of the offices and staff quarters of Wasa, in parks and open spaces of the city, and the DU and Buet campuses.
Traditionally, rainwater harvesting has been practised in arid and semi-arid areas across the world for thousands of years. It is gaining further popularity in many countries.
Several states in neighbouring India, Thailand and the Maldives are already practising rainwater harvesting. The city of Kolkata will have similar projects. People of Rajsthan have been practising rainwater harvesting for long using indigenous technology.
Plan Bangladesh is currently working on 20 model harvesting plants in slum areas in Mipur and Jatrabari areas to assess the possibility of rainwater harvesting.
Md Zillur Rahman, water and environmental sanitation specialist of Plan Bangladesh, said, “Significant economic, social and environmental benefits can be achieved by using rainwater, especially at the household level.”
He said usually rainwater directly goes to the drainage or sewerage system from the roof. Rooftop harvesting of rainwater is a technique through which rainwater is collected on the roof catchments and then taken through pipes into the underground storage tank.
“Rainwater harvesting is not a new idea. It has been done for centuries in rural Bangladesh. Farmers used it for irrigation. It is an old practice that will be done in a new way,” Zillur said.
He mentioned that the technique is quite cheap and easy. It requires simple modification like construction of a reservoir and the linking pipes. The roof and the storage must be kept clean. This technique reduces the cost of water lifting and supply.
Zillur pointed out that rainwater is the purest water. It can be used for cooking, cleaning and toilet flushing at the household level. Large amount of water is used for these purposes. With a little chemical treatment or boiling it can become safe for drinking too.

Water for schoolchildren

Plan Bangladesh and Coca-Cola have joined hands to initiate a pilot project on rainwater harvesting in five schools in Dhaka and Barguna district. The project aims at supplying safe drinking water for schoolchildren from an alternative source.
The initiative also promotes rainwater harvesting as a sustainable and alternative source of water.
Four primary schools in Baunia embankment and Muktijoddha complex in Mirpur and one in Barguna district have been selected for harvesting rainwater on experimental basis.
Under the 'Rainwater Harvesting & Drinking Water Project' the two organisations will finance the installation of piping systems, construction of storage tank with 30,000 litres capacity in each school for rainwater collection and developing a safe distribution system directly to the classroom.
Classrooms will be specially equipped with filtration systems and water taps to ensure bacteriological safety of the water at the point of consumption.
“In most schools there is no source of safe water. Children usually bring water from home. This is why school has been chosen for the project. Barguna has been selected as water in this southern district has high salinity level,” said Md Zillur Rahman, water and environmental sanitation specialist of Plan Bangladesh.
At the initial stage, awareness campaign will start from this month involving schoolchildren, parents, school management committees and the community leaders. This will be followed by construction of necessary structures on the school premises.
Project managers hope that by next monsoon children in these schools will be able to consume safe drinking water.
The project will benefit several thousand children with access to safe drinking water, Zilliur said.