Evaluation of effects of chemical pretreatment methods on anaerobic digestibility of agricultural residues

In this study agricultural residues composed of lignocellulosic structure were pretreated thermo-chemicaly and effects of thermo-chemical pretreatment on anaerobic digestibility were evaluated. Agricultural residues obtained from Antalya province had been used in experimental studies. Effective parameters on pretreatment process (acid concentration, reaction temperature, reaction time, solid concentration and agitation speed) were determined and its effects on responses (sCOD increase, soluble sugar concentration and VSS concentration) were evaluated by statistical Plackett-Burman design. Acid concentration and reaction temperature, as the most effective parameters, have been optimized using the response surface method, RSM. Two constraints werepplied in optimization tests; first, soluble COD and soluble sugar concentrations were maximized secondly, independent variable acid concentration was maximized while reaction temperature was minimized. Batch BMP tests were conducted in the soluble phase and the mixture phase after pretreatment to evaluate methane production efficiency.As a result of the runs, acid concentration and reaction temperature were found to be the most effective parameters on the pre-treatment process. After the optimization, 2.59% acid concentration and 100 °C reaction temperature were found as optimum values for thermo-chemical pretreatment. After validation experiments, COD increase and soluble sugar concentration have been found 75.80% and 28.63 mg Glucose/g VS, respectively as the highest yield of the desired optimization. The highest methane value of soluble phase preatreated with 5% acid concentration at 60 °C was measured as 196.93 ml CH4/g COD in BMP tests. The highest amount of methane obtained from mixture phase was found 350.39 ml CH4/g VS at 80 °C reaction temperature under acidfree pretretment conditions. However, BMP results of pretreated mixture samples were rather low after the application of optimum conditions (%2.59 acid concentration and 100 °C reaction temperature). At the end of the pretreatment, methane production was expected to be higher with the increase of soluble COD and soluble sugar, however BMP results have shown low values. Therefore, as a better option, for the maximization of methane production in acidic-thermal pretreatment of lignocellulosic greenhouse residues; the optimization was decided to be done in accord with BMP results. BMP tests have been conducted on pretreated samples with RSM runs and a process model has been proposed to simulate the results. Optimum values for maximum methane production were found as 0% acid concentration and 78.63 °C reaction temperature.