Assistant Professor of Geology
Georgia Southwestern State University, Americus, GA 31709, USA. URL:
There is a common misconception among the villagers in Bangladesh that rainwater is pure. Since rain form by condensation of evaporated water (like a distillation process), it seems reasonable that rainwater would be of good quality. A myth about the purity of rainwater prevails even among many educated people. In fact, many quack doctors (I have seen one in our own village when I was a little boy) in Bangladesh use rainwater to substitute distilled water used in injections medicine in human body. Now that arsenic contamination of groundwater in Bangladesh poses serious threat to health, more and more people are turning to surface water sources. Rainwater harvesting offers a possible source of drinking water. Many agencies, including the Water and Sewerage Authority (WASA), are studying feasibility of rainwater harvesting in Bangladesh. The most important questions that need to be addressed before harvesting rainwater are: (1) is rainwater harvesting economically and technically feasible for rural people? and (2) does rainwater meet the quality for drinking water?
Amount of rainfall varies both spatially and temporally. While the maximum amount of average annual rainfall occurs in the northeastern districts (55 cm) of Sylhet and Moulivibazar, the minimum amount falls in the western/southwestern districts (15 cm) of Meherpur, Kushtia, Chuadanga, Chapai Nawabganj, Noagaon, and Rajshahi. Also, rainfall is mainly restricted during the months of April to September. Consequently, rainwater harvesting will be relatively easier during certain months of the year in the certain parts of Bangladesh. Groundwater contamination by arsenic is more severe in the western/southwestern districts, where rainwater harvesting would be more appropriate to solve the polluted drinking water problems. A 1982 study, by the United Nations Environment Program, showed that with an average rainfall of 72 inches and using 1,100-gallon storage tanks, enough water could be collected in 12 hours to serve a family of six for 45 days.
Rainwater harvesting -- in one form or another -- has been in practice for thousands of years. According to Paul Woods of Texas A & M University, extensive water harvesting systems in the Negev Deserts of Israel more than 2,000 years ago have been documented. Additionally, Roman villas and cities were planned in such a way to take advantage of rainwater for drinking and air-conditioning. Cisterns, containers to collect rainwater, were fairly common in the United States in rural areas until the 1920s. Rainwater harvesting requires an arrangement to collect, treat, and distribute the captured rainwater. The quality of rainwater harvesting technology varies widely depending on the availability of resources available. For example, a commercial company in Texas deploys sophisticated technology to collect, treat, and distribute captured rainwater. Villagers living on the rain-shadow areas of the Andes Mountains use a long and wide piece of thick cloth raised with long poles like sails of a boat to collect rainwater. Rooftops in buildings can be designed to collect rainwater, as is done in Kawranbazar office of the WASA. However, most buildings in Bangladesh are not designed for rainwater harvesting. Structural modification of buildings will be necessary to make them suitable for rainwater harvesting. Such an investment will be beyond the financial means of most families living in Bangladesh. In addition, millions of slum dwellers, who constitute the majority of the city population and who are in dire need of clean drinking water, will have difficulty to adopt such techniques to say the least. The situation in the villages is even worse as it pertains to rainwater harvesting. Although it will be financially burdensome, most tin-roofed houses can be modified to capture rainwater by adding a gutter system. However, most villagers living in houses made out of straws will not likely to have the means and resources to adopt such rainwater harvesting technique.
Next question is how to keep the rainwater for dry season? Saving rainwater in vessels for later use will be practically impossible, for it will require a huge number of such containers. Another possibility would be to dig ponds to gather rainwater for use during dry months. However, a series of problems will arise from such measure. First, there are not enough lands available in various parts of all villages in Bangladesh. Second, ponds will be subject to surface run-off that will carry different pollutants from non-point sources. Possible non-point sources of contamination include fertilizer, pesticides, chicken and cow manure, dissolved minerals, sediments, sewage, decaying plants, algae, bacteria, aerosol fallout, and detergents. The amount of surface run-off can be reduced by building elevated banks around ponds. Third, water will be lost to evaporation if the ponds are not completely covered. Fourth, ponds will be connected to groundwater flow and will be subject to contamination by dissolved chemicals. Most feasible option would be conjunctive and seasonal use of rainwater during wet months, and other sources of surface water and groundwater during dry months.
Studies of the chemical composition of rainfall have been carried on for many years starting in late 1880s in the United States and in Europe. Rainwater collected in various parts of the USA contains (in milligrams per liter): Fe (0.015) , Ca (0.075-1.41), Mg (0.027-1.2), Na (0.22-9.4), Ca (0.075-1.41), K (0.072-0.11), HCO3 (4-7), SO4 0.7-7.6), Cl (0.22-17), NO2 (0.02), NO3 (0.02-0.62), and Total Dissolved Solids 8.2-38), and pH of 4.9 to 6.4. Although most of these concentrations fall within the safe limit prescribed by the US Environmental Protection Agency, some exceed safe drinking water limit.
Rainwater in rural areas - away from atmospheric and industrial pollution - is fairly clean except for some dissolved gases it may pick up while traveling through the atmosphere. Some scientists consider rainwater as the "gold standard" of water. However, rainwater is not free of pollution. It contains most of the atmospheric gases in dissolved form in proportion to their abundance. In addition, rainwater contains sediments, dust, aerosols, particulates, and anthropogenic gases that result from industrial discharge, biomass and fossil fuel burning. Gases such as H2O, SO2, NH3, NO2, N2O, HCl, CO, and CO2 are produced in substantial amounts by burning of fuels, by metallurgical processes, and by other anthropogenic activities, and also by biochemical processes in soil and water. Carbonates, nitrates, and sulfates in the atmosphere can react with water vapor and form carbonic, nitric, and sulfuric acids, respectively. These acids washed down with rain and form acid rain, which is detrimental to ecosystem and water quality.
Since rainwater is not pure water, some precautions will have to be taken before the water is consumed. Sediments will have to be removed, and water further purified by using a reverse osmosis distillation system. This is a membrane permeation process that separates pure water from a less pure solution containing dissolved chemicals. Rainwater purifying techniques also involves passing through a pipe surrounded by an ultraviolet light, which kills most pathogens. Based on the Texas Water Development Board's "Texas Guide to Rainwater Harvesting", a scientist named Krishna developed a rainwater harvesting system in 1998 and received approval from the city of Portland, Oregon, to use his system for all household use (http://www.rdrop.com/users/krishna/rainwatr.htm). The rainwater harvesting system costs less than $1,500 and consists of the following components: a 1500 gallon plastic cistern, a 1/2 horsepower shallow-well pump, plastic (outdoor PVC and indoor CPVC) piping, two particulate filters in series, rated at 20 and 5 micron particle sizes, an ultraviolet light sterilizer, screen covering the cistern, a 20 gallon water butyl rubber diaphragm pressure storage tank, and a reduced pressure backflow prevention device. The cost to install a similar system in Bangladesh will be much less, because indigenous equipment will be cheaper than buying from the United States.
Rainwater harvesting is in use in many parts of the world. There is a long established tradition of rainwater collection in some parts of Alaska and Hawaii. City of Austin, Texas, offers rebate for using rainwater for some household uses. According to the "Sourcebook Harvested Rainwater", in some areas of the Caribbean, new houses are required to have rainwater capture systems. Hawaii apparently is currently developing (or has already developed) guidelines. The island of Gibraltar has one of the largest rainwater collection systems in existence. Rainwater offers advantages in water quality for both irrigation and domestic use. Rainwater is naturally soft (unlike well water), contains almost no dissolved minerals or salts, is free of chemical treatment, and is a relatively reliable source of water for households. Rainwater collected and used on site can supplement or replace other sources of household water. Rainwater can be used as drinking water if proper treatment is done before using. McElveen, a physian from Texas, also developed methods to treat rainwater for drinking purposes. For drinking water treatment (http://twri.tamu.edu/twripubs/WtrSavrs/v3n2/article-1.html), McElveen relies on 5-micron and 1-micron cartridge filters and an ultraviolet (UV) treatment. He runs an Environmental Protection Agency test every 8 months for the same contaminants as municipal utilities test for: heavy metals, volatile organic compounds, pH, and hardness. Bangladesh is faced with numerous water-related problems: flooding, droughts, arsenic contamination of groundwater, surface water pollution by point sources and non-point-sources of pollution. To solve these problems, Bangladesh will have to develop an integrated water resources development plan. Although rainwater harvesting will not be able to replace all other sources of driking water, it will certainly be able to ease the pressure on surface water and contaminated groundwater usage as the primary source of drinking water. The development of a rainwater harvesting plan that is economically and technically feasible for the majority of the people in Bangladesh should be given high consideration as a part of the integrated water resources management plan.