The efficiency of decolorisation and COD removal from pulp and paper mill effluent using ozone, activated carbon and microorganism were compared. The results showed that the highest color removal efficiency of 83.42% was obtained when ozone method was used with optimal pH 9 and retention time 60 min. However, the COD was decreased only by 5.50% at pH 9 and retention time 40 min. Similar COD removal efficiency of 68.99% was observed when activated carbon with pH 2 and retention time 180 min was employed and for microbial method, the COD removal efficiency of 77.67% was obtained when pH 7 and retention time 6 days was applied. While the efficiency in color removal using the microbial method was lower (36.2%) which was at pH 4.5 and retention time 2 days than that removed using activated carbon which was 79.79% at pH 2 and retention time 180 min.

When an operation cost for each method was compared, it was found that the microbial treatment consumed the cost of 587.82 US dollar/1,000 L for color removal and 29,760 US dollar/1,000 L for COD removal whereas the ozone treatment consumed the lowest cost for both color and COD removal of 24.0 US dollar/1,000 L and 9.3 US dollar/1,000 L, respectively. Similarly, the activated carbon adsorption method consumed low cost for both color and COD removal of 40.62 US dollar/1,000 L.

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