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Fly ash based low cost method for COD removal from domestic waste water
Experimental conditions Study for contact time: These studies were conducted by agitating 100ml sample with initial COD concentration 1080 ppm. And known amount of fly ash as an adsorbent agitated it for different time period, 30 - 300 minutes. After the predetermined time intervals, the sample were withdrawn, filtered and determined the residual COD concentration. Study for adsorbent dose: the studies were conducted by varying the amount of adsorbent. A known volume of sample was treated with different doses of fly ash, 2.0- 7.0g/100ml. The samples were agitated for specific time, filtered and then analyzed for the residual COD. Study for pH: pH effect was performed taking a specific concentration, adsorbent dose, & contact time and varying the pH values from 1-12 using dilute NaOH/ HCl solutions. The samples were agitated for specific time, filtered and then analysed for the residual COD concentration. Study for initial COD concentration: These studies were performed by keeping all the conditions constant except changing the initial COD conc. by using simulated COD bearing solutions prepared by dissolving known amount of glucose in distilled water. Studies for agitation time were: performed by varying agitation speed from 100 rpm to 700 rpm and keeping all conditions constant. Finally analysed the sample for residual COD concentration. Similarly, studies for adsorbent particle size were performed by keeping all conditions constant and varying the particle size. The residual COD concentration was determined after each run. The removal of COD was quoted (as %) relative to the values measured for the untreated effluent. Similar kinds of experiments were performed for standard commercial activated carbon as an adsorbent for the shake of comparison. Results and Discussion The results observed after the physico-chemical analysis of the wastewater as depicted in table: 1 showed that the domestic waste water is highly polluted with the organic load and suspended matter. Organic load is depicted in terms of COD and BOD values. The COD concentration is much higher than the permissible limit. The composition of typical Indian fly ash in table: 2 depicted that the fly ash is predominantly silicious followed by the insoluble oxides of aluminium, iron, calcium, magnesium, titanium, alkali oxides and a negligible amount of phosphorus pentoxide and sulphur oxides. In the case of fly ash as an adsorbent the metal salts hydrolyses in the presence of natural alkalinity to form metal hydroxides. The multivalent cations present in fly ash can reduce the zeta potential while the metal hydroxides are good adsorbents. They form monomolecular layer on the surface of suspended organic matter and removes it by enmeshing them and settling. 
Fig. 1 represents the percent removal of COD for different contact time both by the fly ash and commercial activated carbon. It seems that COD removal has been achieved to the extent of more than 67% by fly ash at a maximum time period of 250 minute and the trend of percent COD reduction with fly ash was comparable to that of commercial activated carbon. From the removal curve (fig. 1), it has been seen that equilibrium attained in 250 minutes. The smooth and independent nature of curve indicates formation of monolayer cover of the adsorbate on the outer surface of the adsorbent. The adsorption process for the studied adsorbent follows first order kinetics and Freundlich adsorption pattern. 
Fig. 2 indicates the effect of adsorbent dose on the percent COD reduction by fly ash and also compared its trend with that of commercial activated carbon. It was observed that maximum removal occur at the dose of 60 g/l for fly ash and 50 g/l for commercial activated carbon and that is 79.31 % and 98.59 % respectively. After that the equilibrium was set up by further addition of adsorbent dose. Fly ash shows fairly the same trend to that of commercial activated carbon. The trend of dose effect on percent COD reduction both by fly ash and activated carbon are also represented. The results showed the tremendous increase in percent COD removal with the increment of adsorbent dose, owing to the increase in the number of sites (Mancy et al., 1964). At lower doses, the significant small adsorption is possibly due to the saturation of surface active sites with the adsorbate molecules. 
Fig. 3 depicts the effect of pH on percent COD reduction by fly ash and commercial activated carbon. The runs were taken at the constant initial COD concentration, adsorbent dose and the contact time. The results indicate that at all pH levels below 5.0, the fly ash has consistently higher adsorption capacity for COD. There is more than 80 % drop in percent COD by fly ash. |