Bioethanol and biogas production: an alternative valorisation pathway for green waste.

Biofuels are a research field of great interest given the environmental benefits they offer over conventional fossil fuels. Nowadays, it is urgent to find ways of utilizing waste in the direction of biofuels production. The aim of this paper was the utilization of green waste (branches, leaves and ligno-cellulosic residues from tree prunings, hedge cuttings and grass clippings) towards biofuels production and specifically towards bioethanol and biogas. The experimental plan that was followed included biogas production through anaerobic digestion and bioethanol production through alcoholic fermentation after the necessary chemical pretreatment (acid or alkaline hydrolysis) prior to enzymatic hydrolysis and fermentation. Based on the results obtained, three valorisation scenarios of green waste were designed and compared in terms of product mass intensity, product yield and energy content of biofuels produced. The optimal results for bioethanol production were 5.22 g/L ethanol, 70.61% saccharification yield and 33.67% ethanol yield with acid pretreatment using H2SO4 3% w/v, 475 µL/g cellulose CellicCtec2 and 10% solids loading. Regarding biogas, the highest biogas production observed was 267.1 mL biogas/g dry substrate resulting from anaerobic digestion of the alkaline stillage. Thus, the production of biofuels from green waste is technically feasible, although it provides moderate efficiencies. However, for a sustainable valorisation of green waste, other techno-economic factors such as the cost of enzymes, chemicals, energy, etc. must be taken into account.

Assessment of the effect of drying temperature and composition on the biochemical methane potential of in-house dried household food waste.

Household food waste management and treatment has been recognised as a significant issue worldwide and at a European Union level. Source-separation of household food waste following drying at source presents a viable solution to this problem. The present research aims at investigating the effect of drying of model household food waste at different temperatures (i.e. 63 ±3 °C and 83 ±3 °C) on its biochemical methane potential. The drying process was carried out using a prototype household waste dryer. The model sample consisted of 77%w/w vegetables and fruits (48%w/w and 29%w/w, respectively), 12%w/w pasta/rice, 6%w/w meat and fish, 3%w/w bread and bakery and 2%w/w dairy. Moreover, drying at the same temperatures was applied for two household food wastes samples with different composition, in order to assess the influence of the samples' composition on both the drying process and the methane generation. For all temperatures used, the higher %w/w mass reduction was observed for model waste (MD) (67.39%w/w and 75.79%w/w for 63 °C and 83 °C, respectively), then for rich-in-protein content (PRO) (66.18%w/w and 69.73%w/w for 63 °C and 83 °C, respectively) and finally for rich-in-fat content (FAT) samples (54.35%w/w and 66.31%w/w for 63 °C and 83 °C, respectively), which confirmed the effectiveness of the drying process. The biochemical methane potential experiments have confirmed that the substrate produced the highest methane yields was the FAT, producing 524.25 ±2.86 L CH4 kg-1 volatile solids.

Analysis of tar compounds and quantification of naphthalene from thermal treatment of household biowaste.

Household biowaste represent the organic fraction of municipal solid waste and are an underutilized resource. Although previous studies have performed pyrolysis of organic waste, the vast majority has been on specific presorted feedstock or conventional lignocellulosic streams. Therefore, there is a lack of pyrolysis applications on representative food waste as retrieved from households and this can be attributed primarily to their high water content and their degradability. But via the intermediate step of drying, long-term storage and thermal treatment have become possible. In the framework of this study, household biowaste were pyrolyzed for the production of carbonaceous materials with a main focus on the analysis of produced tar compounds. Tars can be corrosive or cause clogging and disrupt the operation of pyrolysis and gasification plants. Their analysis has faced several difficulties due to inconsistency in the methodologies that have been applied by various groups. The tar protocol has provided a solid framework for consistent analysis of tars but until now has been solely used for the case of gasification. This study aimed to apply the tar protocol for pyrolysis and to enhance the detectability of the method for a wider range of tars by means of elemental analysis, attenuated total reflectance (ATR) and gas chromatography-mass spectrometry (GC-MS). GC- MS was performed by means of a specific column for PAHs identification and calibration methods were developed for the proper quantification of naphthalene which is the dominant tar compound. The results of the analysis showed that naphthalene concentration increased from torrefaction to carbonization but then decreased significantly for high temperature pyrolysis at 860 °C.

Improvement of home composting process of food waste using different minerals.

This article presents the experimental study of the process of composting in a prototype home-scale system with a special focus on process improvement by using different additives (i.e. woodchips, perlite, vermiculite and zeolite). The interventions with different bulking agents were realized through composting cycles using substrates with 10% additives in specific mixtures of kitchen waste materials. The pre-selected proportion of the mixtures examined was 3:1:1 in cellulosic:proteins:carbohydrates, in order to achieve an initial C/N ratio equal to 30. The control of the initial properties of the examined substrates aimed at the consequent improvement of the properties of the final product (compost). The results indicated that composting process was enhanced with the use of additives and especially the case of zeolite and perlite provided the best results, in terms of efficient temperature evolution (>55 °C for 4 consecutive days). Carbon to nitrogen ratios decreased by 40% from the initial values for the reactors were minerals were added, while for the bioreactor tested with woodchips the reduction was slight, showing slowest degradation rate. Moisture content of produced compost varied within the range of 55-64% d.m., while nutrient content (K, Na, Ca, Mg) was in accordance with the limit values reported in literature. Finally, the composts obtained, exhibited a satisfactory degree of maturity, fulfilling the criterion related to the absence of phytotoxic compounds.

Thermodynamic modelling of an onsite methanation reactor for upgrading producer gas from commercial small scale biomass gasifiers.

Small scale biomass gasifiers have the advantage of having higher electrical efficiency in comparison to other conventional small scale energy systems. Nonetheless, a major drawback of small scale biomass gasifiers is the relatively poor quality of the producer gas. In addition, several EU Member States are seeking ways to store the excess energy that is produced from renewables like wind power and hydropower. A recent development is the storage of energy by electrolysis of water and the production of hydrogen in a process that is commonly known as "power-to-gas". The present manuscript proposes an onsite secondary reactor for upgrading producer gas by mixing it with hydrogen in order to initiate methanation reactions. A thermodynamic model has been developed for assessing the potential of the proposed methanation process. The model utilized input parameters from a representative small scale biomass gasifier and molar ratios of hydrogen from 1:0 to 1:4.1. The Villar-Cruise-Smith algorithm was used for minimizing the Gibbs free energy. The model returned the molar fractions of the permanent gases, the heating values and the Wobbe Index. For mixtures of hydrogen and producer gas on a 1:0.9 ratio the increase of the heating value is maximized with an increase of 78%. For ratios higher than 1:3, the Wobbe index increases significantly and surpasses the value of 30 MJ/Nm3.

Study and assessment of segregated biowaste composting: The case study of Attica municipalities.

This work aims to assess the operation of the first large scale segregated biowaste composting scheme in Greece to divert Household Food Waste (HFW) from landfill and produce a material which can be recovered and used as compost. The source separation and collection of HFW was deployed in selected areas in Attica Region serving about 3700 households. Sorted HFW is collected & transported to the Mechanical and Biological Treatment (MBT) plant in Attica Region that has been designed to produce Compost Like Output (CLO) from mixed MSW. The MBT facility has been adjusted in order to receive and treat aerobically HFW mixed with shredded green waste in a dedicated composting tunnel. The composting process was monitored against temperature, moisture and oxygen content indicating that the biological conditions are sufficiently developed. The product quality was examined and assessed against the quality specifications of EU End of Waste Criteria for biowaste subjected to composting aiming to specify whether the HFW that has undergone recovery ceases to be waste and can be classified as compost. More specifically, the heavy metals concentrations (Cr, Cu, Ni, Cd, Pb, Zn and Hg) are within the set limits and much lower compared to the CLO material that currently is being produced at the MBT plant. In regard to the hygienic requirements of the product it has been found that the process conditions result in a pathogen free material (i.e. E. Coli and Salmonella) which does not favor the growth of viable weeds and plant propagules, while it acquires sufficient organic matter content for soil fertilization. Noticeable physical impurities (mainly fractions of glass) have been detected exceeding the quality control threshold limit of 0.5% w/w (plastics, metals and glass). The latter is related to the missorted materials and to the limited pre-treatment configurations prior to composting. The above findings indicate that effective source separation of biowaste is prerequisite for good quality production and marketing of compost and special consideration should be made to minimize glass impurities prior composting (i.e. awareness raising and pretreatment stage). Therefore, it is feasible to gradually replace the production of questionable quality CLO in MBTs with biowaste compost which is in line with the required quality control standards especially when heavy metals concentrations is concerned.

Thermochemical valorization and characterization of household biowaste.

Valorization of municipal solid waste (MSW), by means of energy and material recovery, is considered to be a crucial step for sustainable waste management. A significant fraction of MSW is comprised from food waste, the treatment of which is still a challenge. Therefore, the conventional disposal of food waste in landfills is being gradually replaced by recycling aerobic treatment, anaerobic digestion and waste-to-energy. In principle, thermal processes like combustion and gasification are preferred for the recovery of energy due to the higher electrical efficiency and the significantly less time required for the process to be completed when compared to biological process, i.e. composting, anaerobic digestion and transesterification. Nonetheless, the high water content and the molecular structure of biowaste are constraining factors in regard to the application of thermal conversion pathways. Investigating alternative solutions for the pre-treatment and more energy efficient handling of this waste fraction may provide pathways for the optimization of the whole process. In this study, by means of utilizing drying/milling as an intermediate step, thermal treatment of household biowaste has become possible. Household biowaste has been thermally processed in a bench scale reactor by means of torrefaction, carbonization and high temperature pyrolysis. According to the operational conditions, fluctuating fractions of biochar, bio-oil (tar) and syngas were recovered. The thermochemical properties of the feedstock and products were analyzed by means of Simultaneous Thermal Analysis (STA), Ultimate and Proximate analysis and Attenuated Total Reflectance (ATR). The analysis of the products shows that torrefaction of dried household biowaste produces an energy dense fuel and high temperature pyrolysis produces a graphite-like material with relatively high yield.

Utilisation of biomass gasification by-products for onsite energy production.

Small scale biomass gasification is a sector with growth and increasing applications owing to the environmental goals of the European Union and the incentivised policies of most European countries. This study addresses two aspects, which are at the centre of attention concerning the operation and development of small scale gasifiers; reuse of waste and increase of energy efficiency. Several authors have denoted that the low electrical efficiency of these systems is the main barrier for further commercial development. In addition, gasification has several by-products that have no further use and are discarded as waste. In the framework of this manuscript, a secondary reactor is introduced and modelled. The main operating principle is the utilisation of char and flue gases for further energy production. These by-products are reformed into secondary producer gas by means of a secondary reactor. In addition, a set of heat exchangers capture the waste heat and optimise the process. This case study is modelled in a MATLAB-Cantera environment. The model is non-stoichiometric and applies the Gibbs minimisation principle. The simulations show that some of the thermal energy is depleted during the process owing to the preheating of flue gases. Nonetheless, the addition of a secondary reactor results in an increase of the electrical power production efficiency and the combined heat and power (CHP) efficiency.

Combination of decentralized waste drying and SSF techniques for household biowaste minimization and ethanol production.

The results of the demonstration of an innovative household biowaste management and treatment scheme established in two Greek Municipalities for the production of lignocellulosic ethanol using dehydrated household biowaste as a substrate, are presented within this research. This is the first time that biowaste drying was tested at a decentralized level for the production of ethanol using the Simultaneous Saccharification and Fermentation (SSF) process, at a pilot scale in Greece. The decentralized biowaste drying method proved that the household biowaste mass and volume reduction may reach 80% through the dehydration process used. The chemical characteristics related to lignocellulosic ethanol production have proved to differ substantially between seasons thus; special attention should be given to the process applied for ethanol production mainly regarding the enzyme quality and quantity used during the pretreatment stage. The maximum ethanol production achieved was 29.12g/L, approximately 60% of the maximum theoretical yield based on the substrate's sugar content. The use of the decentralized waste drying as an alternative approach for household biowaste minimization and the production of second generation ethanol is considered to be a promising approach for efficient biowaste management and treatment in the future.

Research on the drying kinetics of household food waste for the development and optimization of domestic waste drying technique.

Domestic food waste drying foresees the significant reduction of household food waste mass through the hygienic removal of its moisture content at source. In this manuscript, a new approach for the development and optimization of an innovative household waste dryer for the effective dehydration of food waste at source is presented. Food waste samples were dehydrated with the use of the heated air-drying technique under different air-drying conditions, namely air temperature and air velocity, in order to investigate their drying kinetics. Different thin-layer drying models have been applied, in which the drying constant is a function of the process variables. The Midilli model demonstrated the best performance in fitting the experimental data in all tested samples, whereas it was found that food waste drying is greatly affected by temperature and to a smaller scale by air velocity. Due to the increased moisture content of food waste, an appropriate configuration of the drying process variables can lead to a total reduction of its mass by 87% w/w, thus achieving a sustainable residence time and energy consumption level. Thus, the development of a domestic waste dryer can be proved to be economically and environmentally viable in the future.

Design of an innovative, ecological portable waste compressor for in-house recycling of paper, plastic and metal packaging waste.

Waste management in Greece relies heavily on unsustainable waste practices (mainly landfills and in certain cases uncontrolled dumping of untreated waste). Even though major improvements have been achieved in the recycling of municipal solid waste during recent years, there are some barriers that hinder the achievement of high recycling rates. Source separation of municipal solid waste has been recognised as a promising solution to produce high-quality recycled materials that can be easily directed to secondary materials markets. This article presents an innovative miniature waste separator/compressor that has been designed and developed for the source separation of municipal solid waste at a household level. The design of the system is in line with the Waste Framework Directive (2008/98/EC), since it allows for the separate collection (and compression) of municipal solid waste, namely: plastic (polyethylene terephthalate and high-density polyethylene), paper (cardboard and Tetrapak) and metal (aluminium and tin cans). It has been designed through the use of suitable software tools (LS-DYNA, INVENTROR and COMSOL). The results from the simulations, as well as the whole design process and philosophy, are discussed in this article.

Photocatalytical degradation of 1,3-dichloro-2-propanol aqueous solutions by using an immobilized TiO2 photoreactor.

The photocatalytic oxidation of 1,3-dichloro-2-propanol (1,3-DCP) was studied by following the target compound degradation, the total organic carbon removal rate and by identifying the oxidation products. The reaction was performed in a batch recycle reactor, at room temperature, using UV radiation, H2O2 as oxidant, and immobilized TiO2 as catalyst. 1,3-Dichloro-2-propanone, chloroacetyl-chloride, chloroacetic acid, formic and acetic acid were detected as reaction intermediates and a possible pathway for the oxidation of 1,3-dichloro-2-propanol is proposed. The effect of the oxidative agent's initial concentration was investigated and it was established that higher concentrations of H2O2 slow down the reaction rate. The investigation of the effect of the 1,3-DCP initial concentration showed no influence on the degradation process. The carbon and chloride ion mass balance calculations confirmed the fact that chlorinated intermediates are formed and that they degrade with a lower rate than 1,3-DCP.