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Integrated solid organic waste treatment and valorization in the Mediterranean area using anaerobic digestion

Pellera Frantseska-Maria

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Year 2017
Type of Item Doctoral Dissertation
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The purpose of the present doctoral thesis is to study the treatment and valorization of some of the most produced types of solid organic waste of the Mediterranean area and especially of Greece, using the anaerobic digestion technology. Specifically, four solid organic waste originating from agroindustrial activities, namely winery waste (WW), cotton gin waste (CGW), olive mill waste (olive pomace) (OP) and juice industry waste (JW) (orange waste), were studied.The first step of this study was the determination of the methane potential of the investigated substrates in their raw form, by evaluating the influence of different substrate to inoculum ratios (SIR) and inoculum types. To this purpose, Biochemical Methane Potential (BMP) assays were conducted, in which four SIR, i.e. 0.25, 0.5, 1 and 2 (on a volatile solids (VS) basis), were tested and three different inocula, namely anaerobic sludge, landfill leachate and gravitationally thickened anaerobic sludge, were compared. Ultimately, anaerobic sludge was found to be the most adequate inoculum among tested samples, while landfill leachate and thickened anaerobic sludge showed lower efficiencies. The optimum SIR for determining the methane potential of the investigated substrates were of 0.5 for WW and JW, and of 0.25 for CGW and OP, yielding 446.2, 446.0, 268.0 and 258.7 mLCH4,STP/gVSsubstrate, respectively. The complexity of the anaerobic digestion of the investigated substrates was manifested by the association of different SIR with 2- and 3-parameter kinetic models, while a multiple-stages modeling approach, appeared to be suitable for describing the experimental data.The next part of the study focused on the application of two pretreatment methods prior to the anaerobic digestion of the investigated substrates, namely microwave and chemical pretreatment. In both cases, the objective was to evaluate the effect of such pretreatments on the solubilization and the degradability of the substrates. The effect on substrate solubilization was evaluated by analyzing the liquid fractions obtained after pretreatment for soluble chemical oxygen demand (sCOD) and total phenols (TPH) concentrations, while the effect on substrate degradability was assessed through BMP assays performed on the respective solid fractions. The conditions adopted in these BMP assays were based on the results of the first part of the study. Microwave pretreatment was carried out using a laboratory scale microwave reaction system, and by investigating the variation of four operational parameters, i.e. solid to liquid ratio (50, 75 and 100 g/L), heating rate (2.5, 5 and 10 °C/min), holding time (5, 10, 15 and 30 min) and temperature (75, 125, 150, 175 and 200 °C). On the other hand, for chemical pretreatment the use of eight different chemical reagents i.e. sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3), sodium chloride (NaCl), citric acid (H3Cit), acetic acid (AcOH), hydrogen peroxide (H2O2), acetone (Me2CO) and ethanol (EtOH), was investigated, under three condition sets resulting in treatments of varying intensity, depending on process duration (16, 8 and 4 h), reagent dosage (0.25, 0.5 and 1 mmol/gVS) and temperature (25, 60 and 90 °C). Different reagents were used in order to also determine the impact of different reagent natures (alkaline, acidic, saline, oxidative, organic) on the final results.The results obtained from microwave pretreatment showed that temperature had the most important effect among the four investigated operational parameters, while optimum solid to liquid ratio, heating rate and holding time were determined and correspond to 50 g/L, 10 °C/min, and 5 min, respectively. Microwave pretreatment appeared to have exerted different effects on each investigated substrate. More specifically, WW and JW were mainly affected regarding their solubilization, while in the cases of CGW and OP, pretreatment most likely induced structural changes on these materials. Ultimately, the obtained results indicated that microwave pretreatment at temperatures between 125 and 150 °C could eventually result in the generation of samples that are more suitable for methane production. On the other hand, the results of chemical pretreatment indicated that the application of more severe conditions for this kind of process, is more effective on the solubilization of substrates such as those investigated in this study, with H3Cit, H2O2, and EtOH appearing as the most effective reagents for this scope. However, in terms of methane production, moderate to high severity conditions were found to generally be the most satisfactory. More specifically, maximum specific methane yield values were obtained for samples generated after moderate severity treatments using EtOH, H3Cit and H2O2 for WW, OP and JW, while a high severity treatment using EtOH had an analogous result for CGW. Solid fractions obtained with both pretreatments had lower methane yields compared with untreated substrates. Nevertheless, chemical pretreatment was proved better than microwave pretreatment in three out of four cases, i.e. for WW, CGW and JW. On the contrary, in the fourth case, that of OP, microwave pretreatment showed a better methane efficiency than chemical pretreatment.In the third part of the study the anaerobic digestion of the four agroindustrial substrates in semi-continuous mode was investigated. Each substrate was digested separately in mono-digestion assays, as well as in combination with a synthetic organic fraction (SOF) sample, which was used as a co-substrate, in co-digestion assays. Further division of the assays in two Groups aimed at studying the application of different operational conditions, in both mono- and co-digestion systems. More specifically, in the assays of Group I, the variation in two operational parameters, namely organic loading rate (OLR) and hydraulic retention time (HRT), was investigated, whereas in the assays of Group II, different feeding materials were fed to the reactors in a sequential order, based on their seasonality. It was ultimately observed that co-digestion of the four agroindustrial waste with SOF, resulted in higher methane yields compared with mono-digestion. Maximum methane yields in the first group of assays were obtained after halving the HRT and setting the OLR to 1.0 gVS/L/d, while further reduction of the HRT coupled to an increase of the OLR, led to a significant decrease of methane yields, due to system overloading and possibly, washout phenomena. The latter was true for the majority of assays, except those being fed with OP-substrates. Severe system overloading, which eventually resulted in system failure, was observed only for the assays being fed with a JW-substrate in mono-digestion mode. Feeding the assays of the second group, with different substrates in a sequential order, led to a more equilibrated operation, especially for co-digestion systems. Moreover, higher methane yields were observed during the periods in which WW- and JW-substrates were being fed to the reactors. Characterization analyses on the digestates obtained from all semi-continuous assays, suggested a potential suitability of these materials for land application.

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