Arsenic contamination of aquifers is a major health problem in much of the world

Question

Arsenic contamination of aquifers is a major health problem in much of the world and is particularly severe in Bangladesh. One method of removing the arsenic is to pump water from an aquifer to the surface and through a bed packed with granular material containing iron oxide, which binds the arsenic. The purified water is then either used or allowed to seep back through the ground into the aquifer. In an installation of the type just described, a pump draws 69.1 gallons per minute of contaminated water from an aquifer through a 3-inch ID pipe and then discharges the water through a 2-inch ID pipe to an open overhead tank filled with granular material. The water leaves the end of the discharge line 80 feet above the water in the aquifer. The friction losses in the piping system are 10ft middot lb_f/lb_m. (a) If the pump is 70% efficient (i.e., 30% of the electrical energy delivered to the pump is not used in pumping the water), what is the required pump horsepower? (b) Even if we assume that the iron oxide binds 100% of the arsenic, what other factors limit the effectiveness of this operation?

Explanation / Answer

1. 10ft.lbf/lb= 10*2.99 j/Kg=29.9 J/kg

flow rate = 69.1 gallons/min= 69.1*3.78 l/min=261 L/min

density of water is assumed to be 1000g/L, mass flow rate of water= 261*1000 g/min*1Kg/1000gm = 261 Kg/min

power = mass flow rate* frictional losses =29.9* J/kg*261kg/60sec= 130 J/sec= 130 Watts

1Watt= 0.00134 Hp, 130 watts= 130*0.00134 HP=0.1742 HP

since pump is 70% , power =0.1742/0.7= 0.25HP

After Arsenic binds to iron oxide, disposal of bound material posses challenges. so the problem is law of conservation of waste -a waste from liquid is converted to a solid waste which can easily be handled.

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