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.
