Working Principle of DM06-880

In the in-situ treatment method, the aerated tube well water is stored in feed water tanks and released back into the aquifers through the tube well by opening a valve in a pipe connecting the water tank to the tube well pipe under the pump head (Figure 1). Because there is little or no oxidant in the deep soil pores and the permeability of the soil layers are usually low, there is a strong driving force for the oxidant to diffuse into these locations and oxidize the contamination. The dissolved oxygen in aerated water oxidizes arsenite to less-mobile arsenate, the ferrous iron to ferric iron and Manganese(II) to Manganese(III), followed by adsorption of arsenate on Fe(III) and manganese(III) and subsequent precipitation resulting in a reduction of the arsenic content in tube well water. Oxidation is further enhanced biologically by bacteria living in the subsurface and is termed bioremediation process. Insitu oxidation process can work in tandem with bioremediation by chemically oxidizing recalcitrant compounds and creating products that are readily biodegradable.

The process of in-situ oxidation of groundwater virtually transfers the oxidation and filtration process of the conventional above ground water treatment plants into the aquifer. The underground aquifer is used as a natural biochemical reactor. A part of the delivered groundwater is recirculated back into the aquifer carrying oxidising agent generally atmospheric oxygen. Oxygen can be introduced into water by aerating it and then recharging the water into the aquifer.

The basic configuration for subterranean processing consists of an oxidation station, either a spray nozzle or water jet air pump, a storage tank and pipelines for delivery from the aquifer and for enrichment (recharge) into the aquifer . The schematic arrangement is shown in Figure 1. The water is pumped from the groundwater by means of a submersible pump fitted within the well, aerated in the aeration chamber by means of spray nozzles fitted inside the tank and the water is stored in a feed water tank. The oxygen rich water is then re-infiltrated into the aquifer using the filter pipes of the delivery system. The ratio of the delivered volume to the recharged water volume is termed as 'efficiency coefficient' and is varied between 2 and 12 as per the requirement depending on raw water quality and the aquifer characteristics.

Because of the input of oxygen, the redox potential of the water is increased. A number of different physical, chemical and biological processes are intensified in the surrounding area of the well screen section , the so called oxidation zone (Figure 2). The alternate operation of the wells for delivering groundwater in the tank top and infiltration of the oxygen rich water into the aquifer induces alternating oxidation and adsorption periods on the surface of the solid material in the aquifer.

During the groundwater delivery period (discharge) Fe(II), Mn(II) and As(III) are adsorbed to the surface of the soil grain, which are partially coated by previously deposited oxidation products of the previous cycle of the operation and bacteria. In the following recharge period, the bivalent ions are oxidised to insoluble ferric hydroxides and manganese oxides( Mn (III)) by the oxygen transported with the infiltration water into the pores of the aquifer and get precipitated and separated from water. As(III) first oxidises to As(V) and then gets adsorbed on iron hydroxide and manganese hydroxide. Figure 3 illustrates the adsorption and oxidation process in the aquifer. The oxidation processes are accelerated by autocatalytic effects of the oxidation products and by autotropic micro-organisms utilizing energy from the oxidation process. Additionally, the dissolved iron and manganese are adsorbed on the bacteria sheaths by the bio-film.

The in situ method is a very cost effective and eco-friendly process for arsenic removal. The greatest advantage of this process is there is no need for sludge handling. The arsenic which is trapped into the sand along with the iron flocs constitute a infinitesimal volume of the total volume being handled and hence pose very little environmental threat in its precipitated form. The whole mass remains down below unlike other processes where there is extra cost of sludge handling and messy disposal problem. The process is chemical free, simple and easy to handle. There is no restriction to the volume it can handle as long as proper time is allowed for the oxygen rich impregnated water to create the adequate oxidizing zone in the deep aquifer. It is also quite flexible with respect to the raw water quality as the efficient coefficient could be varied depending on the quality of the raw water. It involves low capital cost and minimum operating cost . The results obtained in the test site is quite promising as the process is able to reduce the arsenic content from 100-250 ?g/l to permissible limit. It is ideal for a rural set up where people really cannot afford to pay a substantial amount for water supply. The only disadvantage is that it takes some time for the whole system to stabilise because of the slow kinetics of the oxidation process. However, once stabilised, it remains steady for years to come.