By Larry Henke
Bangladesh is a nation approximately the size of Wisconsin and has a population that is estimated between 139 million and 155 million, or about half population of the United States. In the 1970s, 80s and early 90s, tube wells were drilled to provide irrigation during the dry seasons and for drinking water. It was thought then that groundwater was a safe alternative to the highly contaminated surface waters. It is ironic that a land immersed in water should find it hard to find acceptable drinking water, but that is the case.
By the mid-1990s, it became evident that ingestion of groundwater water was causing health consequences of a serious nature, and it was further determined that the primary cause was arsenic.
Bangladesh’s geology is characterized by the confluence of three rivers: the Ganges, the Brahmaputra and the Meghna. These rivers begin in India in the Himalayas and carry the combined waters of mountain snowmelt, monsoon and glacial melt. They also carry the effluent from thousands of miles of agricultural runoff, city and industrial wastes. They convey approximately 1 billion tons of sediment annually, a third of which is deposited in the river plains, a third in the submarine shelf, and a third carried out to sea. The nation is made from the deposits of these rivers since the last glacial period.
Bangladesh has a its annual monsoon from March to May and it, along with snowmelt from the Himalayas, often causes flooding. Upwards of one-third of the cropland is covered by water during those floods; in the southeast, the ground is so saturated with water that even at this time of the year, there is nowhere for additional moisture to go.
Despite the potential for contamination, 97 percent of the population receives drinking water from the ground, of which one-forth is tainted with arsenic, affecting from 30 to 70 million people. Add to that the affected population of East Bengal in India and the “at-risk” population is staggering.
Arsenic’s Toxic Effects
Arsenic has a well-deserved reputation as a poison. Although the different forms or species (inorganic As III, As V and organic arsenic) have different toxicity characteristics, in low but steadily accumulating doses any form can be harmful. Generally, As III is most toxic, but As V can be converted to As III in the stomach, so speciation is useful mainly for removal selection. The LD50 (Lethal Dose ─ where 50 percent of those exposed will die) for inorganic arsenic is 10-20 mg/kg body weight, the corresponding LD50 for organic arsenic varies with the specific organic chemical from 700 to 2600 mg/kg.
Arsenic can be ingested unknowingly for several years before the effects begin to show. In trace amounts, there is no taste or odor to the water. The first noticeable effects are darkened pigmentation, especially of the hands and feet. These patterns can have a “raindrop” appearance, followed by a thickening of the skin. After five years’ exposure there can be neurological damage and the internal organs can be harmed; lungs, liver, kidney, nerves and other organs can be seriously affected. Ultimately, cancers can form in the lungs, kidneys, bladder, liver or skin. Acute exposures where the lethal dose is approached can cause immediate problems: diarrhea, extreme cramps, discoloration and death.
The chronic onset is determined by age, weight, gender and genetic make-up, but the final result is fatal. Since the progression of the disease is long term, the total burden of arsenic poisoning will not be know for decades.
There is relatively little medical treatment for the condition now. Some therapies have been devised, including the use of chelating resins and of hemodialysis, with varying degrees of success, but the best medical procedure to date ─ especially for the poor ─ is to stop drinking the water.
The source of the arsenic is natural. Arsenic is the 20th most-common element in the earth’s crust; the 14th most-common in seawater and the 12th in the human body.
Arsenic is a metalloid, with an atomic weight of about 75 and has a reduction/oxidation (redox) relationship with iron that partly explains its release from geology into water and that can be exploited in its removal from water. Although either reducing or oxidizing conditions can be responsible, it is most commonly the reduction of arsenic from its combination with iron or other minerals that releases it and increases its solubility in water. Arsenic can also be locked into organic molecules, especially arseno-sugars, in plants and animals. The organic forms of arsenic can be accumulated in plants and animals and ingested as a part of the diet. Most organic forms are less of a problem than the straight inorganic forms.
Once in the water, how can you get it out? This question has received intense study in the past decade, with emphasis placed on the use of adsorptive media and membranes for application to small systems, including solutions for developing nations. The traditional methods of water treatment include coagulation and precipitation by aluminum or iron salts followed by settling or filtration, lime softening, anionic exchange with resins or natural iron exchangers, reverse osmosis or nano-filtration, co-filtration with natural iron and by adsorptive media, with special attention on iron-based media.
With the scope of the problem however, and with the introduction of the Grainger Prize (a $1 million prize for the development of low cost, sustainable, effective arsenic remediation technology) the number of prospective media choices has soared. Selections and reports on media ranging from highly sophisticated, such as titanium dioxide or zirconium doped resin, to those less processed such as coal plant fly ash, hyacinth root powder, newspaper pulp. Many schemes employ variations on iron based media, whether zero-valent iron, processed iron hydroxides, dried iron oxides or other minerals coated with iron. Each of these systems has its role, but there is no single mechanism for application in all cases. Most groundwater treatment cases are highly site specific.
Many removal systems aimed toward the rural populations include multiple “buckets” with sand filtration as part of the process. In one case, a “tea bag” of adsorbent is stirred in a container, with the tea bag later disposed.
One novel system, Solar Oxidation and Removal of Arsenic (SORAS) uses readily available PET bottles, lime or lemon juice (citric acid) and sunlight to oxidize iron and arsenic. After overnight settling, the supernatant water can be decanted and used.
The effectiveness of a removal plan will depend on the conditions of the water, in particular the redox potential (Eh), the pH (inorganic arsenic species are pH dependent), other minerals in the water that might affect oxidation or compete for adsorption sites on media such as silica, phosphorous, nitrates and sulfates.
In all cases however, treatment plans for developing nations must consider costs and simplicity of operation. Like people worldwide, water is an extremely important commodity for Bangladeshi women (many of whom walk up to 6 km each way to carry a 20 liter ─ 45 pound ─ container) and ease of access and availability is of utmost importance. Initial and continuing costs, and the problem of disposal of concentrated waste (whether water or spent media) must be factored into any removal program.
In some cases, especially in developing countries, but also in areas of the US where funds are limited, alternative water sources may be the best solution.
Other water sources
In Bangladesh, harvesting rainwater may be a good solution in some rural areas. The average annual rainfall is 80 inches, and where it can conveniently be captured and stored, rainwater may provide a good source. Of course, surface storage invites other concerns, microbial contamination foremost among them, but disinfecting powdered bleach is available for reasonably low cost. Boiling the water is not usually an option: fuel is limited and costly.
With the volume of surface water available, some of the cities may choose a central treatment plant and distribution as the best long-term solution. Dhaka City, the nation’s largest with over 10 million people, receives most of its water from some 400 wells that draw from the Dila Tipa Aquifer. While this aquifer is arsenic free, heavy withdrawals have resulted in a lowering of the ground water to dangerously low levels. Despite frequent flooding, aquifers do not recharge as rapidly. Increased use of surface water, however difficult to treat because of upstream contamination, may be the best overall source.
In rural areas, central treatment with distribution would be difficult. Many villages are strung together by raised walking paths around ponds and rice paddies. Even central elevated storage would, aside from its costs, be a complex engineering problem.
Another solution is deeper wells. Wells at about 1,000 feet generally have low arsenic levels. These wells can be drilled in many places by a hand-turned, rotary water/mud drill in five days. The cost for such a well is about $1.00/foot, half of which is for materials.
The attention Bangladesh is getting is well founded: solutions and insights offered here can be applied elsewhere. But the urgency of the crisis can not be underestimated. Millions of people are drinking tainted water, and so both immediate and long-term solutions are necessary. Although we tend here to focus on the technical problems, there are profound political problems as well. These must be solved before the Bangladeshis can all be provided safe drinking water. Perhaps as Bangladesh matures as a nation (it was only formed as such less than 35 years ago) the conditions for large civil engineering projects will be met and the nation can enjoy a network of roads, communications and water treatment distribution. For now however, small, local, village and family level water treatment solutions are imperative.
About the author
Larry Henke is an independent water treatment consultant in the Minneapolis area serving industrial filter and disinfection applications and small non-community and community public drinking water systems. A graduate of the University of Minnesota, he has more than 20 years of experience in the water treatment industry. He’s a member of the American Water Works Association and the National Ground Water Association, as well as WC&P’s Technical Review Committee. Henke can be reached at (612) 599-7477 or [email protected].