Investigation of effects of combined alkaline and thermal pretreatment methods on anaerobic digestibility of lignocellulosic wastes

In this thesis; thermal-NaOH pretreatment method was evaulated in detail to enhance anaerobic digestion for increasing the biogas amount and decreasing the digestion time by increasing the effective substrate amount which would be used in anaerobic digestion proses. In this context the process has been optimized, the pretreated greenhouse residues biochemical methane potential have been measured and the changes on lignocellulosic structure which are caused by thermal-pretreatment have been evaluated with the aim of determining the optimum conditions for thermal-NaOH pretreatment. In these experimental studies greenhouse residues obtained from Antalya province had been used and mixture sample of these greenhouse residues were characterized. Greenhouse residues mixture sample has been evaluated suitable as a lignocellulosic biomass source for biogas production with the content of 40.85% total cellulose and hemicellulose and 34.16% carbon.Combined thermal-NaOH pretreatment has been evaluated by the effect of selected independent variables on dependent variables. The independent variables which may have an effect on combined thermal-NaOH Pretreatment process have been determined as reaction temperature, reaction time, NaOH concentration and mixing speed. Determined dependent variables were sCOD concentration, sReducing sugar concentration and BMP. For the determination of the effect of independent variables on dependent variables, central composite design (CCD) method which takes places in statistical response surface method (RSM) has been used. CCD pretreatment experiments have been planned, suggested CCD pretreatment trials have been done, sCOD, sReducing sugar and BMP models have been set and the validity of these models have been tested by ANOVA in Design Expert software. The regression coefficients of sCOD, sReducing sugar and BMP models are 0.9497, 0.9967 and 0.9083, respectively. As a result to combined pretreatment experiments, it is found out that reaction time, mixing speed and reaction temperature have positive effect and NaOH concentration has a negative effect on BMP response variable.In determined factor levels, the optimization conditions of pretreatment process have been suggested by taking process economy and maximum BMP into account. The limitations of models have been determined by validation experiments. While sCOD and sReducing sugar were optimized together in optimization suggestions, three approaches (BMP, sCOD+sReducing sugar+BMP and sReducing sugar+BMP) were used in BMP optimization. In sCOD and sReducing sugar optimization, independent variables (reaction temperature, NaOH concentration and reaction time) were minimized and the other independent variable, mixing speed, was chosen in range for the benefit of process economy. In this optimization, sCOD and sReducing sugar production has been maximized. As a result of the optimization for the concentration of sCOD and sReducing sugar, the pretreatment process conditions (0.67% NaOH - 65.3°C ? 1 hour ? 500rpm) were selected and by validation experiments sCOD and sReducing sugar were mesuared under these conditions, respectively as 711.95 mg COD/gVS and 235.76 mg sugar/gVS. The pretreated samples? BMP were 34.97% higher than raw samples? BMP. In three approaches of BMP optimization (BMP, sCOD+sReducing sugar+BMP and sReducing sugar+BMP) NaOH concentration and reaction time were minimized and reaction temperature and mixing speed both minimized and left in range by taking proces economy and BMP amount increase into consideration. As the results of optimization suggestions, the pretreatment process conditions were determined. According to the results of validation experiments, the increases in BMP compared to raw sample BMP were determined as 29.12% (60°C - 0% NaOH - 1 hour ? 0 rpm), 3.42% (100°C - 0% NaOH - 1 hour ? 0 rpm), 38.03% (70.98°C ? 0.23%NaOH - 1 hour ? 0 rpm) and 19.41% (84.85°C - 0% NaOH - 1 hour ? 0 rpm).