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O. Khadijah et al.: Statistical Experimental Designs: Optimization of Medium CompositionThe optical densities (OD) measured were then convertedto the dye concentrations using the respective standardcurves. The efficiency of colour removal was expressed asthe percentage ratio of the decolorized dye concentration tothat of initial one based on the following Equation 6 (Chenet al. 2003),Dye (i) – Dye (r) 100%Colour removal (%) =Dye (i)where Dye (i) = initial dye concentration (mg/l), Dye (r) =residual dye concentration (mg/l).RESULTS AND DISCUSSIONScreening of Variables Required for Decolourisationof PR-MX8B by Plackett-Burman DesignThe results of the Plackett-Burman design experimentsare shown in Table 4. The table shows the magnitudeof effect, its standard error (S.E.), the t-test (t (5) ) and itsassociated P-value and percentage confidence level. Thet-test estimates the probability of finding an observedeffect on decolourisation (the measured output) if P < 0.01i.e. confidence level >99%. A positive effect value, E (X i )indicates that the growth factor i exert a beneficial effect ondecolourisation when present at a high amount, and viceversa.Of the various organic and inorganic nitrogen sourcestested, yeast extract had the most significant positiveeffect on the decolourisation of all dyes. This result was inaccordance to findings in other studies (Chen et al. 2003;Khehra et al. 2005; Moosvi et al. 2007). Yeast extracthas been widely reported as the most effective carbonnitrogensource (Nigam et al. 1996; Kapdan et al. 2000),where complete decolourisation of several dyes was(6)obtained in 24 h. In addition to yeast extract, peptone andcorn steep solids also showed significant positive effectson decolourisation. Contrasting results however, wereobtained by several researchers with regards to the peptoneeffect on decolourisation. A positive effect of peptone wasreported in stimulating biodecolourisation of ReactiveBrilliant Blue KN-R (Nam & Renganathan 2000), whilefor Remazol Violet 5R, poor decolourisation was observedin its presence (Moosvi et al. 2007). The positive effect ofcorn steep solids on decolourisation was an advantage interms of cost since corn steep solids were much cheapercompared to yeast extract and peptone. The additionof organic nitrogen sources were considered essentialmedium supplements for the regeneration of nicotinamideadenine dinucleotide (NADH) that act as electron donorfor the reduction of azo dyes by microorganisms (Nam &Renganathan 2000).The decolourisation of dyes by bacteria has beenreported to have been attained in synthetic mediacontaining various extra carbon sources (Nigam et al.1996a; 1996b). Of all the carbon sources tested, starchand glucose gave the most significant positive effects ondye decolourisation. A similar observation was obtainedbased on decolourisation of Reactive Turquoise Blue(RTB) and it was suggested that glucose is a readilybiodegradable carbon and energy source and sufficientglucose promotes microbial proliferation, thus producingmore bacterial cells capable of removing RTB (Fu et al.2002). However, contrasting results where the additionof glucose reduced the decolourisation efficiency ofvarious dyes had also been observed and in addition,lactose and sucrose had caused nearly completedecolourisation of the dyes (Kapdan et al. 2000). Theseobservations suggest that the suitability of the carbonsources varied according to the consortium since differenttypes of micro-organisms required different nutrients tosupport growth.Table 4. Statistical analysis of the effect of medium components on decolourisation of PR-MX8B based on Plackett-Burman designresults.Variables Medium component Effect S.E. t (5) P-value Confidence level (%)X 1 Yeast extract 11.55 0.9168 12.60 6 × 10 –5 99.99X 2 Peptone 6.45 0.9168 7.035 0.0009 99.91X 3 Corn steep solids 7.25 0.9168 7.906 0.0001 99.94X 4 Ammonium sulfate 1.35 0.9168 1.473 0.2009 79.91X 5 Urea –1.55 0.9168 1.691 0.1517 84.83X 6 Sodium nitrate –3.45 0.9168 3.763 0.00232 46.29X 7 Nutrient broth –0.65 0.9168 0.709 0.5100 49.00X 8 Glucose –0.95 0.9168 1.036 0.3476 65.24X 9 Maltose 1.05 0.9168 1.145 0.3039 69.61X 10 Mannitol –0.85 0.9168 0.9272 0.3964 60.36X 11 Starch 6.65 0.9168 7.254 0.0008 99.92X 12 Sodium acetate 0.65 0.9168 0.709 0.5100 48.99X 13 pH 0.65 0.9168 0.709 0.5100 49.00X 14 Inoculum size 8.75 0.9168 9.544 0.0002 99.98107paper 1.indd 1071/20/2011 11:14:13 AM