Waste Management & Research: The Journal for a Sustainable Circular Economy

The olive oil industry in Australia has been growing at a rapid rate over the past decade. It is forecast to continue growing due to the steady increase in demand for olive oil and olive products in the local and regional market. However, the olive oil extraction process generates large amounts of solid waste called olive husk which is currently underutilized. This paper uses life-cycle methodology to analyse the carbon emission reduction potential of utilizing olive husk as a feedstock in a mobile pyrolysis unit. Four scenarios, based on different combinations of pyrolysis technologies (slow versus fast) and end-use of products (land application versus energy utilization), are constructed. The performance of each scenario under conditions of uncertainty was also investigated. The results show that all scenarios result in significant carbon emission abatement. Processing olive husk in mobile fast pyrolysis units and the utilization of bio-oil and biochar as substitutes for heavy fuel oil and coal is likely to realize a carbon offset greater than 32.3 Gg CO₂-eq annually in 90% of the time. Likewise, more than 3.2 Gg-C (11.8 Gg CO₂-eq) per year could be sequestered in the soil in the form of fixed carbon if slow mobile pyrolysis units were used to produce biochar.

The production of renewable energy, a reduction of waste and prevention of environmental pollution promote the industrial application of anaerobic co-digestion for the treatment of agro-industrial organic waste. In this paper production of biogas/methane was studied by performing a series of laboratory batch experiments using whey and cow manure as substrates. The influence of substrate concentration, temperature and pH on biogas production was analysed. A mathematical model has been developed that describes the co-digestion process. The hydrolysis of proteins, lipids and cellulose has been modelled using first-order kinetics. Fermentation of sugars and amino acids, anaerobic oxidation of long chain fatty acids (LCFA), acetogenesis and methanogenesis have been described using an unstructured model based on Monod kinetic equations taking into account different inhibitory effects. Model applicability was demonstrated by comparing experimental results with the model simulation results.

Fast pyrolysis is an effective technology for conversion of biomass into energy and value-added chemicals instead of burning them directly. In this study, fast pyrolysis of waste furniture sawdust (pine sawdust) was investigated under various reaction conditions (reaction time, pyrolysis temperature, heating rate, residence time and particle size) in a tubular reactor. The optimum reaction conditions for bio-oil production was found as reaction time of 5 min, pyrolysis temperature of 500 °C, heating rate of 300 °C min⁻¹ under nitrogen flow rate of 400 cm³ min⁻¹. At these conditions, maximum bio-oil yield was obtained as 42.09%. Pyrolysis oils were characterized by using various elemental analyses, fourier – transformation infrared (FT-IR) spectrometry and gas chromatography–mass spectrometry (GC–MS). The results of the GC–MS showed that cracking of large molecular phenolics was followed by partial conversion into phenol and alkylated phenols (45%) during the pyrolysis. According to the experimental and characterization results; the liquid product could be used as feedstock for the chemical industry or petroleum crude for refinery.

For five agricultural biogas plants with a high share of energy crops in the input material, a detailed balance of greenhouse gas emissions (GHG) and cumulated energy demand (CED) was calculated for the years 2007 and 2010. The results vary considerably between plants and over time. In 2010 compared with 2007, all of the five biogas plants reduced their impact on climate change and four of them also reduced their consumption of fossil energy. The strongest influence was from the enhanced utilization of surplus heat energy, whereas variations of environmental impact due to direct emissions from the biogas plants were less distinctive. Compared with a reference system based on fossil resources, electricity production in the biogas plants avoided GHG emissions of 603 g to 940 g carbon dioxide equivalents (CO₂-eq)•kilowatt hours electrical energy (kWh_el^-1) and saved 2.48 to 3.23 kilowatt hours primary energy from fossil energy carriers (kWh_fossil)•kWh_el^-1 CED (results for 2010).

The present study investigated the utilization of an industrial by-product, lignite fly ash, in oil pollution treatment, with the further potential profit of energy production. The properties of lignite fly ash, such as fine particle size, porosity, hydrophobic character, combined with the properties, such as high porosity and low specific gravity, of an agricultural by-product, namely sawdust, resulted in an effective oil-sorbent material. The materials were mixed either in the dry state or in aqueous solution. The oil sorption behaviour of the fly ash–sawdust mixtures was investigated in both marine and dry environments. Mixtures containing fly ash and 15–25% w/w sawdust performed better than each material alone when added to oil spills in a marine environment, as they formed a cohesive semi-solid phase, adsorbing almost no water, floating on the water surface and allowing total oil removal. For the clean-up of an oil spill 0.5 mm thick with surface area 1000 m², 225–255 kg of lignite fly ash can be utilized with the addition of 15–25% w/w sawdust. Fly ash–sawdust mixtures have also proved efficient for oil spill clean-up on land, since their oil sorption capacity in dry conditions was at least 0.6–1.4 g oil g⁻¹ mixture. The higher calorific value of the resultant oil–fly ash–sawdust mixtures increased up to that of bituminous coal and oil and exceeded that of lignite, thereby encouraging their utilization as alternative fuels especially in the cement industry, suggesting that the remaining ash can contribute in clinker production.

Management of biodegradable waste is one of the most important environmental problems in the grain-processing industry since this waste cannot be dumped anymore due to legal requirements. Biodegradable waste is generated in each stage of grain processing, including the waste-water and air emissions treatment processes. Their management causes some environmental and financial problems. The majority of Lithuanian grain-processing enterprises own and operate composting sites, but in Lithuania the demand for compost is not given. This study focused on the analysis of the possibility of using biodegradable waste for the production of solid recovered fuel, as a local renewable fuel with the purpose of increasing environmental performance and decreasing the direct costs of grain processing. Experimental research with regard to a pilot grain-processing plant has proven that alternative fuel production will lead to minimizing of the volume of biodegradable waste by 75% and the volume of natural gas for heat energy production by 62%. Environmental indicators of grain processing, laboratory analysis of the chemical and physical characteristics of biodegradable waste, mass and energy balances of the solid recovered fuel production, environmental and economical benefits of the project are presented and discussed herein.

Digested sewage sludge gasification in supercritical water was studied. Influences of main reaction parameters, including temperature (623–698 K), pressure (25–35 Mpa), residence time (10–15 min) and dry matter content (5–25 wt%), were investigated to optimize the gasification process. The main gas products were methane, carbon monoxide, carbon dioxide and traces of ethene, etc. Results showed that 10 wt% dry matter content digested sewage sludge at a temperature of 698 K and residence time of 50 min, with a pressure of 25 MPa, were the most favorable conditions for the sewage sludge gasification and carbon gasification efficiencies. In addition, potassium carbonate (K₂CO₃) was also employed as the catalyst to make a comparison between gasification with and without catalyst. When 2.6 g K₂CO₃ was added, a gasification efficiency of 25.26% and a carbon gasification efficiency of 20.02% were achieved, which were almost four times as much as the efficiencies without catalyst. K₂CO₃ has been proved to be effective in sewage sludge gasification.

The problem of the high carbon dioxide emissions linked to power generation makes necessary active research on the use of biofuels in gas turbine systems as a promising alternative to fossil fuels. Gasification of biomass waste is particularly of interest in obtaining a fuel to be run in gas turbines, as it is an efficient biomass-to-biofuel conversion process, and an integration into a combined cycle power plant leads to a high performance with regard to energetic efficiency. The goal of this study was to carry out an energetic, exergetic and environmental analysis of the behaviour of an integrated gasification combined cycle (IGCC) plant fuelled with different kinds of biomass waste by means of simulations. A preliminary economic study is also included. Although a technological development in gasification technology is necessary, the results of simulations indicate a high technical and environmental interest in the use of biomass integrated gasification combined cycle (BioIGCC) systems for large-scale power generation from biomass waste.

An assessment of recent technical advances on pretreatment processes and its effects on enzymatic hydrolysis as the main steps of a proposed citrus processing waste (CPW) biorefinery is presented. Engineering challenges and relevant gaps in scientific and technical information for reliable design, modeling and scale up of a CPW biorefinery are also discussed. Some integrated physico-chemical pretreatments are proposed for testing for CPW, including high speed knife-grinding and simultaneous caustic addition. These new proposed processes and the effect of parameters such as particle size, surface area and morphology, pore volume and chemical composition of the diverse fractions resulting from pretreatment and enzymatic hydrolysis need to be evaluated and compared for pretreated and untreated samples of grapefruit processing waste. This assessment suggests the potential for filling the data gaps, and preliminary results demonstrate that the reduction of particle size and the increased surface area for the CPW will result in higher reaction rates and monosaccharide yields for the pretreated waste material.

An economic evaluation of ethanol (EtOH) production from a thermo-chemical process derived from biomass/waste feedstocks was conducted. The influence of feed amounts, catalytic conversions, and EtOH selling prices was examined as these are the major variables for the economic evaluation of biomass/wastes conversion to EtOH. Among the three feedstock systems of biomass, high-moisture municipal solid waste (MSW), and plastic waste, the plastic waste has far better economic feasibility, with a payback period of 2–5 years at maximum CO conversion (40%) from syngas to ethanol, due to its higher heating value in comparison with biomass and high-moisture MSW. The heating value of the feedstock is a key factor in determining the overall economic efficiency in a thermo-chemical EtOH production system. Furthermore, enhancement of the CO conversion (related to catalytic activity) from syngas to EtOH using a low cost catalyst is necessary to retain economic efficiency because the CO conversion and cost consideration of catalyst are crucial factors to reduce the payback period.

In the present research study, the weight composition, physical–chemical composition and net calorific values of unsorted municipal solid waste (MSW) generated by a population of around 1,080,000 inhabitants in a region of northern Spain were determined. The unsorted MSW was composed of 38.1% organic fraction and 42.3% combustible fraction, with paper/cardboard constituting the most important part of the latter fraction (20.6%). The high content of textiles (10.9%) is worth noting, being practically equal to the content in plastics. The unsorted fractions present an average moisture content of 28.5% and an ash content of 29.4% (dry basis). The average lower heating value (LHV) is 10,744 kJ kg^-1. Likewise, variations in regard to the season of the year and the source of the waste were taken into consideration. A new correlation is proposed for estimating the LHV as a function of the physical composition of the waste.