Fly ash based low cost method for COD removal from domestic waste water


At high pH the capacity of the adsorbent get recessed. The reason for the better adsorption capacity observed at low pH levels may be attributed to the larger number of H+ ions present, which in turn neutralise the negatively charged adsorbent surface, thereby reducing hindrance to the diffusion of organics at higher pH. The reduction in adsorption may be possible due to the abundance of OH-ions, causing increased hindrance to diffusion of organics (contributing to COD) ions. Oxides of aluminium, calcium, silicon, iron etc. are abundant in fly ash. The hydroxylate surfaces of these oxides in aqueous medium is reported to undergo the following dissociation:

Similar observations have also been reported by the workers (Mott et al., 1992; Liskowitz et al., 1980; Mall et al., 1997)



Fig. 4 represents the effect of initial COD conc. on % COD reduction by fly ash and commercial activated carbon at the optimum pH, adsorbent dose and the contact time as predicted from table 3,4, and 5 respectively. The fly ash seems to be fairly active adsorbent even at higher initial concentrations. At lower initial concentrations, the ratio of the initial number of moles available to the adsorbent surface area is low and subsequently the fractional adsorption becomes independent of initial concentration. At higher concentrations, the available sites of adsorption become fewer and hence the % removal of COD depends upon the initial concentration. The COD removal of over 65% -81% obtained with fly ash within the concentration range investigated. The comparison in trend of % COD reduction by fly ash with respect to commercial activated carbon under this condition is depicted in the figure.



Fig. 5 indicates the effect of agitation speed on % COD removal by fly ash and commercial activated carbon. The agitation speed varied from 100 rpm to 800 rpm, keeping the initial conc., pH, contact time and adsorbent dose constant. The results indicate that there is a definite improvement in the extent of adsorption with increase in the speed of agitation and then equilibrium was set up after 700 rpm. And after that, decreased which the film covering the adsorbent surface due to the rate of adsorption owing to the mass transfer resistance contributes.



Fig. 6 represents the effect of particle size on % COD reduction. It was observed that the extent of adsorption decreased with increasing particle size. In general, the intra-particle mass transfer effect will increase with the increasing particle size. However, the surface area per unit mass of adsorbent as well as diffusional transport might be larger in case of smaller particles, which increases the adsorption rate. Rao et al., 2000, observed similar characteristics for the adsorption of COD on activated carbon. In case of 0.053 mm particles, 87.89% COD reduction was observed within 250 minutes, but it was only 75.31% for 2.8 mm particles.

Conclusions
It is revealed from the studies that the treatment of domestic wastewater can be done by fly ash generated from thermal power plant to reduce the organic load. It is observed that the COD can be reduced upto the extent of 87.89 % by use of fly ash. Trend of % COD removal by fly ash are fairly comparable to that of commercial activated carbon. It is physically viable and economically viable approach.

References
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