Subsurface aeration of ground water for the development of low cost iron removal device

Abstract

The relative purity and abundance of ground water increase it's usage. In some places the iron content in ground water is so high that this metal mU:5tbe :removed before water can be used for drinking or industrial purposes. Iron occurs in two states of oxidation in nature- ~e ferrous (divalent)and ferric (trivalent)forms. The subsurface aeration technology developed oxidizes the ferrous ion, which is soluble in water, to the ferric ion, which is insoluble before it comes to the treatment unit for further action and separation. This method achieves a high degree of oxidation in the ground water during it's lifting. An airlift pump is employed in the tube well instead of turbine or submersible pumps. Air is forced down into the well under water and an air-water mixture is build up which has a lower density and thus come out of the well. The mixture obviously becomes enriched with oxygen. The air compressor used in an air lift pump selVes at least four purposes- lifting of water, aeration of water; regeneration of encrusted tube well and frequent cleaning of iron sludge from the interstices of the course media flocculator in the flocculation chamber. The encrustation of tube well strainer develops higher draw down, causes failure of shallow tube well and hand pump tube well. The regeneration is necessary for the encrusted tube wells for ensuring required discharge resulting energy saving- thus the operating cost. On the other hand regeneration of flocculators increases the efficiency of the coarse media bed. The unit cost of production of treated water in the subsurface aeration technology has been found to be Tk.1.20/ m3 while those of DPHE urban water supply is Tk. 1.81/ m3, calculated assuming lSOJ'6 discount factor, in areas where ground water has higher Iron content (Fe>5ppm). The dose of sodium Hexa-metaphosphare for the regeneration iron encrusted tube well has been calculated and has been found to be suitable for the use in regeneration work supported by air surging from the air compressor. The process of precipitation of iron in the aquifer has only a slight affect on aquifer permeability. The clogging of aquifer surrounding the well will not occur for a period many times longer than the life span of a typical well.