| Parameter | Influent | Effluent |
|---|---|---|
| Arsenic (mg/L) Oxidation-reduction potential (millivolts) pH Conductivity (microsemans) Temperature (°C) Total iron (mg/L) Sulfate (mg/L) Sulfide (mg/L) Chloride (mg/L) Phosphate (mg/L) |
0.16 -38 7.06 514 27.2 NA b NA NA NA NA |
< 0.002a 19 6.47 344 28.2 1.4 < 1 < 0.03 16 1.3 |
The ideal water treatment system for the economic and demographic situation in Bangladesh will effectively remove arsenic and other toxic elements, be inexpensive to build and operate, and be simple to use. Such a system might use atmospheric oxygen as the oxidant and ambient iron as the coagulant; therefore, the long-term expense of purchasing water treatment chemicals would be avoided. Atmospheric oxygen delivered by waterfall or bubble aeration has been routinely used to oxidize As(III) to As(V) in water treatment systems, and should be evaluated in future studies. Ambient iron without the addition of another coagulant might adequately separate precipitated arsenic and other toxic metals from water in a large settling tank or an inclined-plate clarifier. Excellent guidance for the construction of low-input water treatment systems for the developing world is provided by the International Reference Centre for Community Water Supply and Sanitation.
The apparent reduction in arsenic concentration from 0.16 to < 0.002 mg/L shown in Table 5 supports the hypothesis that aeration followed by settling without the addition of coagulant can remove arsenic in tubewell water to below the more stringent WHO drinking standard of 0.01 mg/L. The apparent increase in oxidation-reduction potential suggests the water was aerated when pumped into the storage tank. The apparent decrease in pH, conductivity, and arsenic concentration suggests that ambient iron was oxidized, hydrolyzed water, and precipitated as a ferric hydroxide coagulant of arsenic. The water pump supplying this large tank does not operate at night due to the diversion of electricity to the capital; therefore, coagulated arsenic has many hours of relatively turbulent-free water to settle each evening.
