Determination of hydrodynamic cavitation assisted naoh pretreatment and enzymatic hydrolysis process's impacts on biogas production potential of greenhouse residues

Lignocellulosic greenhouse residues are vastly produced in Turkey due to agricultural activities and these residues have an important energy potential. In this thesis, impacts of hydrodynamic cavitation assisted NaOH pretreatment and enzymatic hydrolysis processes on biogas production from lignocellulosic greenhouse residues were investigated and process optimization was done. In this context, primarily, characterization of the mixture sample of lignocellulosic greenhouse residues was determined via total solids, volatile solids, total Kjeldahl nitrogen, protein, soluble chemical oxygen demand, soluble sugar, extractive matter including lipids, Van Soest fraction (cellulose, hemicellulose, lignin, soluble fraction) and elemental analyses. Elemental C, H, N and S composition of greenhouse residues mixture sample was determined as 35.20%, 5.73%, 2.25% and 0%, respectively. Van Soest fractions of soluble matter, cellulose, hemicellulose, and lignin of greenhouse residues mixture sample were defined as 37.83%, 30.51%, 10.54% and 21,12%, respectively. The results of elemental analysis and Van Soest analysis show that greenhouse residues are an appropriate organic material source for biogas production. Hydrodynamic cavitation assisted NaOH pretreatment was investigated based on the impacts of selected independent variables on response variables. Independent variables with a potential impact on hydrodynamic cavitation assisted NaOH pretreatment were selected as NaOH concentration (0-1%), cavitation number (0.6-0.7) and reaction time (60-300 min.). Investigated response variables were determined as sCOD concentration, sSugar concentration and biochemical methane potential (BMP). For the investigation of independent variables impacts on response variables, central composite design (CCD) within the statistical response surface methodology (RSM) was used, CCD pretreatment trials were performed, models were developed for the changes of sCOD, sSugar and BMP and model validations were investigated with ANOVA test. Regression coefficients of sCOD, sSugar and BMP models were calculated as 0.7302, 0.8750 and 0.2636, respectively. Eventhough BMP model found significant with ANOVA test, the test resulted with a low regression coefficient for BMP model. As a result of CCD trials of hydrodynamic cavitation assisted NaOH pretreatment, it was determined that the increase of reaction time and cavitation number causes negative impacts on sCOD and sSugar concentrations. While NaOH concentration increase had a negative impact on sSugar change, the increase on NaOH concentration in low cavitation number and shorter reaction time conditions caused positive impacts on sCOD change. It was determined that hydrodynamic cavitation assisted NaOH pretreatment has a negative impact on BMP in general. While maximum increase on BMP values were observed by increasing the NaOH vi concentration and cavitation number, maximum decrease on BMP values were obtained from the sample pretreated at 180 min. reaction time and 0.65 cavitation number conditions. Due to the different behaviors of sCOD, sSugar and BMP within the aim of improving the BMP production, different approaches were evaluated for the optimization of hydrodynamic cavitation assisted NaOH pretreatment. Process optimization of hydrodynamic cavitation assisted NaOH pretreatment was done according to process cost and maximum methane production criteria by using the models developed for sCOD, sSugar and BMP changes. 0% NaOH concentration, 0.7 cavitation number and 90 min. reaction time conditions were determined for the process cost optimization and 0.44% NaOH concentration, 0.7 cavitation number and 180 min. reaction time conditions were found for the maximum BMP production optimization. For the process cost optimization of hydrodynamic cavitation pretreatment, sCOD, sSugar changes and BMP changes were estimated as 149,919 mgCOD/gVS, -0.11% and 8.43% by the model, respectively. At the end of validation experiment, sCOD, sSugar changes and BMP changes were found as 155.52 mgCOD/gVS, -2.84% and -6.90%, respectively. For the maximum methane production optimization of hydrodynamic cavitation assisted NaOH pretreatment, sCOD, sSugar changes and BMP changes were estimated as 145,09 mgCOD/gVS, -15.13% and 25.94% by the model, respectively. At the end of validation experiment, sCOD, sSugar and BMP changes were found as 148.20 mgCOD/gVS, -8.58% and 0.97%, respectively. Estimated and experimentally determined sCOD values for both optimizations were found to be close. However, estimated values of sSugar and BMP changes were not close to the experimentally obtained values at the end of validation experiments. It is thought that low estimation result for BMP change originated from the low regression coefficient of it's model. Enzymatic hydrolysis (10-60 FPU/gTS cellulase and 10-80 IU/gTS β- glucosidase) was applied to the samples after hydrodynamic cavitation pretreatment in order to enhance the biogas production. To determine the impacts of enzymatic hydrolysis, enzymatic hydrolysis was applied to raw greenhouse residues and pretreated sample at cost optimization conditions (0% NaOH, 0.7 cavitation number and 90 min. reaction time) by hydrodynamic cavitation. sCOD, sSugar and BMP values of raw greenhouse residues as substrate control sample were determined as 751.91 mgsCOD/gVS, 77.76 mgsSugar/gVS and 300.92 mLCH4/gVS, respectively. After the enzymatic hydrolysis of raw greenhouse residues, highest sCOD and sSugar values were obtained as 1363.29 mgsCOD/gVS and 361.65 mgsSugar/gVS under the 60 FPU/gTS cellulase and 45 IU/gTS β-glucosidase enzyme application conditions and compared to substrate control sample, 81.31% and 365.08% increase were obtained, respectively. Highest BMP value was obtained under 10 FPU/gTS cellulase and 45 IU/gTS β- glucosidase enzyme conditions and measured as 273.21 mLCH4/gVS. However, compared to substrate control, 9.21% decrease was determined at enzymatic hydrolysis process. sCOD, sSugar and BMP values of the substrate control sample which was prepared from the hydrodynamic cavitation pretreatment applied sample for enzymatic hydrolysis, were determined as 839.64 mgsCOD/gVS, 40.14 mgsSugar/gVS and 315.37 mLCH4/gVS, respectively. After the enzymatic hydrolysis of hydrodynamic cavitation vii pretreatment applied sample, highest sCOD and BMP values were obtained as 1084.41 mgsCOD/gVS and 411.44 mLCH4/gVS under the 60 FPU/gTS cellulase and 45 IU/gTS β-glucosidase enzyme application conditions and compared to substrate control sample, 29.15% and 30.46% increase were obtained, respectively. Highest sSugar value was obtained as 327.03 mgsSugar/gVS under 60 FPU/gTS cellulase and 10 IU/gTS β- glucosidase enzyme application conditions with an increase of 714.72% compared to substrate control sample. Enzymatic hydrolysis process (10 FPU/gTS cellulase and 45 IU/gTS β-glucosidase) decreased BMP value (273.21 mLCH4/gVS, 9.21% compared to substrate control), however, BMP value of sample (411.44 mLCH4/gVS) pretreated by hydrodynamic cavitation and enzymatic hydrolysis (60 FPU/gTS cellulase and 45 IU/gTS β-glucosidase enzyme concentrations) was increased 30.46% compared to substrate control sample. In order to determine the impacts of combined hydrodynamic cavitation and enzymatic hydrolysis pretreatment, sCOD, sSugar and BMP values of sample pretreated by hydrodynamic cavitation combined with enzymatic hydrolysis were compared to the substrate control sample which was prepared for the enzymatic hydrolysis of raw greenhouse residues. Total highest impact of combined hydrodynamic cavitation and enzymatic hydrolysis pretreatment on sCOD and BMP was observed under 60 FPU/gTS cellulase and 45 IU/gTS β-glucosidase enzyme application conditions and 60 FPU/gTS cellulase and 10 IU/gTS β-glucosidase enzyme application conditions for sSugar and caused 44.22%, 36.73% and 320.56% increase on sCOD, sSugar and BMP values, respectively compared to raw greenhouse residues as substrate control sample. As a result, synergistic impact of combined hydrodynamic cavitation and enzymatic hydrolysis pretreatment on BMP increase was observed. Changes on the molecular bond characterization and surface properties of raw greenhouse residue sample, hydrodynamic cavitation assisted NaOH pretreatment applied samples, enzymatically hydrolised samples and combined hydrodynamic cavitation and enzymatic hydrolysis pretreatment applied samples were investigated via Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM), respectively. Spectral profiles and relative densities of bands of raw greenhouse residue sample, hydrodynamic cavitation assisted NaOH pretreatment applied samples, enzymatically hydrolised samples and combined hydrodynamic cavitation and enzymatic hydrolysis pretreatment applied samples were determined to be generally similar, however, on observed spectral profiles densities of certain structures, slight density changes were determined. SEM images revealed that the raw greenhouse residues sample has a compact, continuous, uniform and rigid surface. After hydrodynamic cavitation assisted NaOH pretreatment, sample surface observed as more loose, nonuniform, curly and seperated and became structurally disordered. It was determined that, depending on the increase on reaction time and NaOH concentration, smaller pieces became more deformated, fibrils got seperated and surface suffer from serious erosion. When raw greenhouse residues sample and sample pretreated at cost optimization condition by hydrodynamic cavitation were loaded with maximum enzyme concentrations, larger pieces surfaces became disturbed, lost their uniform structures and fibrils got released. On the other hand when the same samples were loaded with minimum enzyme concentrations, larger pieces showed no changes and surface kept it's intact structure. viii However, smaller pieces revealed flakey structures and micropores observed on their surfaces. There was no study found during the literature survey investigating the effects of combined hydrodynamic cavitation assisted NaOH pretreatment and enzymatic hydrolysis on greenhouse residues with optimization of both processes. This study is the first research in literature which investigates the optimization of hydrodynamic cavitation assisted NaOH pretreatment and enzymatic hydrolysis with the aim of biogas production enhancement from greenhouse residues and impacts of pretreatment processes on surface structure and molecular bond characterization and determines the BMP potential. Therefore, obtained results from this study will be a reference for future literature.