DPHE-Danida Arsenic Mitigation Pilot Project

Two-Bucket Arsenic Mitigation Method

 

The arsenic mitigation method developed by the Department of Public Health Engineering (DPHE) and Danida is based on the oxidation of all aqueous arsenic to As(V), or arsenate, and subsequent co-precipitation with aluminum sulfate (alum).

Materials

The materials required are:

    1. 2 20-liter plastic buckets (one red and one green)
    2. 2 plastic taps
    3. 1 plastic funnel with nipple and elbow
    4. 10" length of ˝ " PVC pipe
    5. 5 kg coarse sand
    6. flat metallic cover for lower bucket
    7. stirring rod
    8. measuring scoop
    9. chemical powder (4g alum and 0.03g potassium permanganate per 20L)

The above are all locally available, and the total cost is approximately Tk300 to Tk350 (US$6.00 to $7.00) per system. A wooden stirring rod and a measuring scoop designed especially for the 20L buckets are made at a school in Dhaka, but any rod or measuring scoop can be used. The chemical powder will be prepared and packed by local businesses based on manuals now being prepared. The chemicals are both familiar to potential users of the system, and their cost per 20L of water is about Tk0.12 plus packing cost of about Tk0.05. A family that treats two buckets of water a day—enough for about 5 people—will face a monthly running cost of about Tk10, or slightly less than 1% of their income (research suggests that poor families in Bangladesh are willing to spend at most 3% of their income for water and sanitation).

Method

The buckets are colored in analogy to the nationwide practice of painting arsenic-affected tubewells red and safe ones green. The red bucket is placed on top of the green one, and they are connected via a plastic tap about 10cm from the bottom of the top bucket, which empties into plastic tubing that channels the water into a filtering device inside the bottom bucket bucket. The filter is a ten-inch length of PVC pipe filled with sand. The water enters the pipe at the top, passes through ten inches of sand, and exits through a screen at the bottom. Water is drawn from the bottom (green) bucket via another plastic tap about two centimeters from the bottom of the bucket.

Untreated tubewell water is poured into the top bucket, the alum and potash powder is added, and the solution is stirred vigorously for 10-15 seconds. After roughly an hour of coagulation, flocculation, and sedimentation, the tap can be opened for the water to flow into the green bucket. The sand filter provides additional protection to keep the flocculate out of the drinking water, which is drawn directly from the green bucket.

Weekly cleaning of the sludge from the top bucket seems to be sufficient.

Testing

The method has been tested in the DPHE-Danida laboratory in Noakhali with water from four different handpumps, ranging in arsenic concentration from 0.16 mg/L to 1.1 mg/L. After treatment, arsenic concentrations in all samples were below 0.02 mg/L. However, the method is sensitive to pH and will only achieve high arsenic removal efficiency at pH levels between 6.5 and 8.

19 bucket units were installed on 1 December 1998 in households in four slum areas of Noakhali, where arsenic concentrations range from 0.12 to 0.46 mg/L. The units were monitored for two and a half months. In all households, arsenic removal efficiency was consistently high enough to bring arsenic concentrations below the Bangladesh standard of 0.05 mg/L. All stayed in operation throughout the monitoring period, with each family treating an average of two buckets of water per day. All users took drinking water from the system, and two-thirds took water for cooking from the green buckets. Users were satisfied with the operating procedure as well as the appearance and taste of treated water, and the bucket system is compact enough for use in slum households. Surrounding communities have expressed great interest in the system.

One problem with the production of the chemical powder is that it must be very fine in order to dissolve in an acceptable amount of time and to work with adequate efficiency. However, the production of such fine powder is not impossible for local organizations and entrepreneurs and should not pose a long-term obstacle to making the system affordable and accessible to people throughout the arsenic-affected areas of Bangladesh.