Process optimization on production of lignin peroxidase of sewage treatment plant sludge in a stirred tank bioreactor and its biodegradation of synthetic industrial dyes

Abstract

The increasing volume of sewage sludge produced and the total managing cost every year has been one of the major environmental issues in Malaysia. Bioconversion of sewage treatment plant (STP) sludge by liquid state bioconversion (LSB) is proposed to solve these problems through the recovery of products such as lignin peroxidase (LiP) enzyme. A lab-scale study was carried out to produce LiP enzyme by white-rot fungus Phanerochaete chrysosporium using STP sludge as a major substrate. The experiments were conducted in two liter stirred tank reactor (STR). The optimization of aeration and agitation rates was done using full-factorial design (FFD). Using the statistical analysis, the optimum aeration and agitation rates determined were 2.0 vvm and 200 rpm respectively with maximum production of 225 UL-1 in 3 days of fermentation. The validation experiment showed that the maximum lignin peroxidase was 744 UL-1 in five day of fermentation. This enzyme activity was stable at pH 5.0 and temperature 55°C which maintained the activity more than 80% up to 12 hours of incubation. Optimization by one factor at-a time (OFAT) and statistical approach was carried out to evaluate the process conditions on decolorization of methylene blue dye using LiP enzyme in static mode. The OFAT technique indicated that the optimum conditions for decolorization of methylene blue (MB) dye was at temperature 55°C, pH 5.0 with hydrogen peroxide (H2O2) concentration 4.0 mM. The addition of veratryl alcohol to the reaction mixtures did not show any positive effect on decolorization. The initial concentration of MB and the activity of LiP enzyme were further optimized using response surface methodology (RSM). The contour and surface plots suggested that the optimum initial concentration of MB and LiP activity predicted were 15-20 mg/L and 0.687 U/ml respectively for removal of 63-65%. The final validation in static and agitate mode showed that agitation gave higher removal in decolorizing MB. The mixtures solution was colorless as it reached the equilibrium time within 60 minutes with 90% removal compared to only 70% color removal in static mode at the same conditions: initial dye concentration 15 mg/L, LiP activity 0.687 U/ml, H2O2 concentration 4.0 mM, at temperature 55°C in pH 5.0. In fact, this is a new biotechnological approach for the biodegradation and biosolids accumulation of sludge beside the production of industrial enzyme (LiP) which exhibits the benefit of low production cost as well as environmentally friendly.