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Bioplastics have entered everyday life as a potential sustainable substitute for commodity plastics. However, still further progress should be made to clarify their degradation behavior under controlled and uncontrolled conditions. The wide array of biopolymers and commercial blends available make predicting the biodegradation degree and kinetics quite a complex issue that requires specific knowledge of the multiple factors affecting the degradation process. This paper summarizes the main scientific literature on anaerobic digestion of biodegradable plastics through a general bibliographic analysis and a more detailed discussion of specific results from relevant experimental studies. The critical analysis of literature data initially included 275 scientific references, which were then screened for duplication/pertinence/relevance. The screened references were analyzed to derive some general features of the research profile, trends, and evolution in the field of anaerobic biodegradation of bioplastics. The second stage of the analysis involved extracting detailed results about bioplastic degradability under anaerobic conditions by screening analytical and performance data on biodegradation performance for different types of bioplastic products and different anaerobic biodegradation conditions, with a particular emphasis on the most recent data. A critical overview of existing biopolymers is presented, along with their properties and degradation mechanisms and the operating parameters influencing/enhancing the degradation process under anaerobic conditions.

The Circular and Green Economy principles is inspiring new approaches to municipal wastewater treatment plants (MWWTPs) design and operation. Recently, an ever-growing interest is devoted to exploring the alternatives for switching the WWTPs from being able to ‘simply’ removing contaminants from water to biorefinery-like plants where energy and material can be recovered. In this perspective, both wastewater and residues from process can be valorised for recovering nutrients (N and P), producing value added products (i.e. biopolymers), energy vectors and biofuels (i.e. bio-H2, bio-CH4 and bioethanol). As an additional benefit, changing the approach for WWTPs design and operation will decrease the overall amount of landfilled residues. In this context, the present research is aimed at evaluating the CH4 production potential of MWW screening units’ residues. While such a stream is typically landfilled, the expected progressive increase of biodegradable matter content due to the ban on single-use plastic along with the boost of bioplastics makes the investigation of different biochemical valorisation routes more and more interesting from an environmental and economical perspective. Thus, a full-scale data collection campaign was performed to gain information on screening residues amount and properties and to analyse the relationship with influent flowrate. The most relevant residue properties were measured, and lab-scale tests were carried out to evaluate the bio-CH4 potential.

The present work investigates a two-stage process scheme for cheese whey valorization through energy recovery in different forms by means of bio-electrochemical systems. The first stage consisted of an integrated bio-electrochemical process for H2 and electricity production. This combined dark fermentation with an electrochemical system with the aim of overcoming the typical thermodynamic/biochemical limitations of fermentation and enhancing H2 recovery. The second treatment stage involved a single-chamber microbial fuel cell, featuring an innovative configuration consisting of four air cathodes with fly ash as the oxygen reduction catalyst. The bio-electrochemical process performed in the first stage achieved promising results, displaying a three-times higher H2 production yield compared to conventional dark fermentation. In addition, the experiments using the MFC in the second stage were found to successfully exploit the effluent from the first stage, with COD removal yields of 86% ± 8% and energy recovery with a maximum current output of 1.6 mA and a maximum power density of 1.2 W/m3.

This study investigated the performance of a novel integrated bio-electrochemical system for synergistic hydrogen production from a process combining a dark fermentation reactor and a galvanic cell. The operating principle of the system is based on the electrochemical conversion of protons released upon dissociation of the acid metabolites of the biological process and is mediated by the electron flow from the galvanic cell, coupling biochemical and electrochemical hydrogen production. Accordingly, the galvanic compartment also generates electricity. Four different experimental setups were designed to provide a preliminary assessment of the integrated bio-electrochemical process and identify the optimal configuration for further tests. Subsequently, dark fermentation of cheese whey was implemented both in a stand-alone biochemical reactor and in the integrated bio-electrochemical process. The integrated system achieved a hydrogen yield in the range 75.5–78.8 N LH2/kg TOC, showing a 3 times improvement over the biochemical process. Graphical abstract

Volatile fatty acids (VFAs) are high-value chemicals that are increasingly demanded worldwide. Biological production via food waste (FW) dark fermentation (DF) is a promising option to achieve the sustainability and environmental benefits typical of biobased chemicals and concurrently manage large amounts of residues. DF has a great potential to play a central role in waste biorefineries due to its ability to hydrolyze and convert complex organic substrates into VFAs that can be used as building blocks for bioproducts, chemicals and fuels. Several challenges must be faced for full-scale implementation, including process optimization to achieve high and stable yields, the development of efficient techniques for selective recovery and the cost-effectiveness of the whole process. This review aims to critically discuss and statistically analyze the existing relationships between process performance and the main variables of concern. Moreover, opportunities, current challenges and perspectives of a FW-based and fermentation-centred biorefinery layout are discussed.

This study investigated the performance of a novel integrated bio-electrochemical system for synergistic hydrogen production from a process combining a dark fermentation reactor and a galvanic cell. The operating principle of the system is based on the electrochemical conversion of protons released upon dissociation of the acid metabolites of the biological process and is mediated by the electron flow from the galvanic cell, coupling biochemical and electrochemical hydrogen production. Accordingly, the galvanic compartment also generates electricity. Four different experimental setups were designed to provide a preliminary assessment of the integrated bio-electrochemical process and identify the optimal configuration for further tests. Subsequently, dark fermentation of cheese whey was implemented both in a stand-alone biochemical reactor and in the integrated bio-electrochemical process. The integrated system achieved a hydrogen yield in the range 75.5 – 78.8 N LH2/kg TOC, showing a 3 times improvement over the biochemical process.

Factorial fermentation experiments on food waste (FW) inoculated with activated sludge (AS) were conducted to investigate the effects of pH and the inoculum-to-substrate ratio (ISR [g VSAS/g TOCFW]) on biohydrogen production. The two parameters affected the H2 yield, the fermentation rate and the biochemical pathways. The minimum and maximum yields were 41 L H2/kg TOCFW (pH = 7.5, ISR = 1.74) and 156–160 L H2/kg TOCFW (pH = 5.5, ISR = 0.58 and 1.74). The range of carbohydrates conversion into H2 was 0.37–1.45 mol H2/mol hexose, corresponding to 9.4–36.2% of the theoretical threshold. A second-order predictive model for H2 production identified an optimum region at low pHs and high ISRs, with a theoretical maximum of 168 L H2/kg TOCFW at pH = 5.5 and ISR = 1.74. The Spearman’s correlation method revealed several relationships between the variables, suggesting the potentially governing metabolic pathways, which turned out to involve both hydrogenogenic pathways and competing reactions.

Three different experimental sets of runs involving batch fermentation assays were performed to evaluate the influence of the experimental conditions on biological hydrogen production from the source-separated organic fraction of municipal solid waste collected through a door-to-door system. The fermentation process was operated with and without automatic pH control, at a pH of 5.5 and 6.5, food-to-microorganism ratios of 1/3 and 1/1 (wet weight basis) and with different working volumes (0.5 and 3 L). The experimental results showed that the pH control strategy and the reactor volume did not affect the final hydrogen production yield but played an important role in determining the time evolution of the process. Indeed, although the different experimental conditions tested yielded comparable hydrogen productions (with maximum average values ranging from 68.5 to 88.5 NLH2 (kgTVSOF)−1), the automatic pH control strategy improved the process from the kinetic viewpoint resulting in a t95 reduction from an average of 34.9 h without automatic pH control to an average of 19.5 h.

This paper evaluates the effects of ultrasonication (US) applied, individually or in combination with a mechanical treatment, to the effluent of anaerobic digestion (AD) of lignocellulosic waste, on methane (CH4) production. US of the substrate downstream of AD is a relatively novel concept aimed at improving the degradation of recalcitrant components in order to enhance the overall energy efficiency of the process. US tests were carried out on real digestate samples at different energies (500−50,000 kJ/kg total solids (TS), corresponding to sonication densities of 0.08−0.45 W/ml). AD tests were performed on mixtures of sonicated (Sus) and untreated (S) substrate at two different Sus: S ratios (25:75 and 75:25 w/w), simulating post-sonicated material recycling to the biological process. The US effect was estimated through the solubilization degree of organic matter, as well as the CH4 production yield and kinetics, which were all found to be enhanced by the treatment. At Sus: S = 75:25 and Es ≥ 20,000 kJ/kg TS (0.25 W/ml), CH4 production improved by 20% and the values of the kinetic parameters increased by 64–82%.

Batch factorial experiments were performed on cheese whey+ wastewater sludge mixtures to evaluate the influence of pH and the inoculum-to-substrate ratio (ISR) on fermentative H2 production and build a related predictive model. ISR and pH affected H2 potential and rate, and the fermentation pathways. The specific H2 yield varied from 61 (ISR =0, pH=7.0) to 371 L H2/kg TOCwhey (ISR =1.44 g VS/g TOC, pH =5.5). The process duration range was 5.3 (ISR =1.44 g VS/g TOC, pH =7.5) - 183 h (ISR =0, pH=5.5). The metabolic products included mainly acetate and butyrate followed by ethanol, while propionate was only observed once H2 production had significantly decreased. The multiple metabolic products suggested that the process was governed by several fermentation pathways, presumably overlapping and mutually competing, reducing the conversion yield into H2 compared to that expected with clostridial fermentation.

One- and two-stage anaerobic digestion of food waste aimed at recovering methane (CH4) and hydrogen and methane (H2 + CH4), respectively, were compared in order to assess the potential benefits from the two-stage process in terms of overall energy recovery. Results suggest that a two-stage process where the first reactor is properly operated in order to achieve a significant net hydrogen production, may display a 20% comparatively higher energy recovery yield as a result, mainly, of enhanced methane production as well as of the associated hydrogen production. The highest methane production of the two-stage process was due to improved hydrolysis and fermentation of food waste, with increased amounts of volatile fatty acids being readily available to methanogenesis.

The paper reports an investigation on the compositional characteristics and environmental behaviour of eight fractions deriving from an advanced separation process applied to waste incineration bottom ash to recover ferrous and non-ferrous metals. Specifically, five mineral fractions and three weakly magnetic mixed metal/mineral fractions were analyzed for their physical properties, chemical composition and leaching behaviour in order to assess their recycling potential and estimate the expected environmental impacts. While the mineral fractions were found to contain only small amounts of undesired waste glass fragments, the weakly magnetic materials turned out to be commingled with a consistent portion of mineral components, resulting in a heterogeneous mixture with limited utilization potential. Their bulk and chemical composition as well as leaching properties were strongly affected by the presence of mineral particles associated with molten magnetic metals. The most critical elements with a view to leaching turned out to be, for both the mineral and the weakly magnetic fractions, Sb and Cr, which exceeded the quality standards for inert waste disposal in landfills.

The concept of biorefinery expands the possibilities to extract value from organic matter in form of either bespoke crops or organic waste. The viability of biorefinery schemes depends on the recovery of higher-value chemicals with potential for a wide distribution and an untapped marketability. The feasibility of biorefining organic waste is enhanced by the fact that the biorefinery will typically receive a waste management fee for accepting organic waste. The development and implementation of waste biorefinery concepts can open up a wide array of possibilities to shift waste management towards higher sustainability. However, barriers encompassing environmental, technical, economic, logistic, social and legislative aspects need to be overcome. For instance, waste biorefineries are likely to be complex systems due to the variability, heterogeneity and low purity of waste materials as opposed to dedicated biomasses. This article discusses the drivers that can make the biorefinery concept applicable to waste management and the possibilities for its development to full scale. Technological, strategic and market constraints affect the successful implementations of these systems. Fluctuations in waste characteristics, the level of contamination in the organic waste fraction, the proximity of the organic waste resource, the markets for the biorefinery products, the potential for integration with other industrial processes and disposal of final residues are all critical aspects requiring detailed analysis. Furthermore, interventions from policy makers are necessary to foster sustainable bio-based solutions for waste management. peer-reviewed 2022-07-16

An assessment of the energetic requirements of slurry-phase accelerated carbonation for CO2 sequestration using steelmaking slag was conducted. The results of dedicated lab-scale carbonation experiments in which the CO2 sequestration yield of basic oxygen furnace slag was investigated at different operating conditions were used for the energy requirement calculations. The operating variables of the process and the associated values adopted were total gas pressure, temperature, and CO2 concentration in the gas phase. The energy duties of the slurry-phase carbonation layout were calculated for the different unit operations, which included slag milling, mixing, slurry pumping, heating, CO2 compression, solid/liquid separation, and CO2 capture (when required). The estimated energy requirements were found to lie in the range 980−6300 MJ t−1 CO2 sequestered, where the lower end of the range was associated with the use of diluted CO2. In all cases, the use of a concentrated CO2 flow proved largely energetically unfavorable over the use of diluted gas streams, since the energy duty of the required CO2 capture stage by far overcame the benefits associated with improved sequestration yields at increased CO2 concentrations in the gas phase. The calculated energy requirements were also processed to derive a second-order model accounting for the main effects and interactions between the operating variables of the carbonation process. The model developed is meant to predict the expected energy demand of the carbonation process under different operating conditions and to define the optimal combination of these in terms of the energetic profile of the process itself. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd.

This paper evaluates the effect of ultrasonication on anaerobic biodegradability of lignocellulosic residues. While ultrasonication has been commonly applied as a pre-treatment of the feed substrate, in the present study a non-conventional process configuration based on recirculation of sonicated digestate to the biological reactor was evaluated at the lab-scale. Sonication tests were carried out at different applied energies ranging between 500 and 50,000kJ/kg TS. Batch anaerobic digestion tests were performed on samples prepared by mixing sonicated and untreated substrate at two different ratios (25:75 and 75:25 w/w). The results showed that when applied as a post-treatment of digestate, ultrasonication can positively affect the yield of anaerobic digestion, mainly due to the dissolution effect of complex organic molecules that have not been hydrolyzed by biological degradation. A good correlation was found between the CH4 production yield and the amount of soluble organic matter at the start of digestion tests. The maximum gain in biogas production was 30% compared to that attained with the unsonicated substrate, which was tentatively related to the type and concentration of the metabolic products.

A factorial study was conducted on basic oxygen furnace slag from a steelmaking industry with the aim of systematically identifying the individual and joint effects of the operating parameters (total pressure, CO2 concentration in the gas phase and temperature) on the CO2 sequestration yield of a direct aqueous carbonation process. Each operating parameter was varied over a range of three levels according to a 3(3) factorial design, resulting in 27 carbonation experiments. The carbonation performance and the changes in particle size and mineralogical characteristics of the slag were investigated in detail. The analysis of the experimental results indicated large effects of the operating factors on CO2 uptake, which was observed to span the range 6.7-53.6 g CO2/100 g slag. The best carbonation performance achieved was particularly significant compared to previous studies, even more considering the relative mild operating conditions adopted (P = 5 bar, C = 40% vol. CO2, T = 50 °C, t = 4 h). The analysis of the solid and liquid phases at the end of the carbonation treatment evidenced significant changes in the physical, chemical and mineralogical composition of the material. In particular, evidence was gained of other elements (Mg, Fe, Mn, Zn) in addition to Ca being intensively involved in the carbonation reactions, with a variety of carbonate phases being produced in addition to calcium carbonate forms.

TREATMENT AND DISPOSAL OF INCINERATION RESIDUES

In the present work, a lab-scale experimental campaign on dark fermentation of milk powder (used as a surrogate of cheese whey) was conducted in order to assess the bio-hydrogen production potential of cheese whey. The experiments were performed in automated semi-continuous reactors and were aimed at estimating the effect of the Organic Loading Rate (OLR) and Hydraulic Retention Time (HRT) on the H2 yield. Hydrogenogenic biomass was harvested from aerobic wastewater sludge by applying a thermal pre-treatment (105 °C, 30 min) and used as the initial inoculum. The experiments were conducted at a set-point pH of 6.5, which was automatically adjusted by the control system. In total, 20 fermentation tests were run, with HRT ranging from 4 to 20 h and OLR from 16 to 129 g TOC/(L·d). The biogas composition was evaluated by periodic gas sampling. The evolution of digestate composition was monitored through periodic measurements of volatile fatty acids, total organic carbon and soluble carbohydrates in order to derive information about the prevalent metabolic pathways and draw the carbon mass balance.

Batch dark fermentation tests were performed on sheep cheese whey without inoculum addition at different operating pHs, relating the type and production yields of the observed gaseous and liquid by-products to the evolution of fermentation. Cheese whey fermentation evolved over time in two steps, involving an initial conversion of carbohydrates to lactic acid, followed by the degradation of this to soluble and gaseous products including short-chain fatty acids (mainly acetic, butyric and propionic acids) and hydrogen. The operating pH affected the production kinetics and yields, as well as the fermentation pathways. By varying the duration of the fermentation process, different cheese whey exploitation strategies may be applied and oriented to the main production of lactic acid, hydrogen or other organic acids.

In the present study, PHA production from sheep cheese whey (SCW) through a 3-stage process (dark fermentation, PHA-biomass selection and PHA accumulation process) was investigated. To this aim, batch dark fermentation (DF) tests were performed on raw SCW without any addition of biomass inoculum nor any pre-treatment of the substrate and adopting different pH operative conditions. The DF effluents, rich in organic acids, were characterized and used for selection and enrichment of a PHA-storing microorganisms and in PHA accumulation tests. The impact of adopting different operating pH in the DF tests was evaluated in terms of organic acids and biohydrogen yields, PHA composition and yields in order to evaluate the feasibility of the waste biorefinery process here proposed. Results showed that the production of PHA from SCW is a promising valorisation approach. Operating pH adopted during the fermentation step affected the biohydrogen production yield as well as the organic acid type and, in turn, the yield and the composition of the obtained biopolymer. An overall yield of 5.3 L H2 and 16.1 g PHA per litre of SCW could be obtained by setting the fermentation operative pH = 6.

Sludge from the drinking water treatment plant at Ggaba III, located in Kampala (Uganda), was tested to evaluate the feasibility of two valorization routes, for building material and Solid Recovered Fuel (SRF) production. The aim of the research was to divert the huge amount of sludge produced every year, approximately equal to 2,140 metric tons of TSS/year, from landfilling. The average high heating value of the sludge was 8.44 MJ/kg TS, corresponding to the lower value of the interval of variation typically reported for other biosolids (8.0-23 MJ/kg). Different bricks were prepared at sludge to clay ratios of 0, 0.05, 0.1, 0.3 and 0.5 by weight. For each mixture composition, bricks of nominal size 215 x 102.5x 65mm were prepared by hand and fired for 6hrs in a Hoffman kiln at temperatures: 850°C, 900°C, 950°C, 1000°C and 1050°C. The bricks produced with a sludge to clay ratio of 0.1 fired at temperatures of ≥980°C met the compressive strength of 3N/mm2 for common bricks according to Ugandan Standard (US) 102:1995. These results suggest that water treatment sludge at Ggaba is more suitable for the production of common bricks than using it as an energy source. Given the encouraging results that make the studied valorization route applicable in an emerging economy country as Uganda, further investigations are required to assess the leaching behaviour and stability of the mechanical properties over time.

The progressive implementation of the circular economy concepts, the population growth and associated concerns in terms of availability of non-renewable resources, the will of many countries to diversify their strategic sources and to free themselves from the supply of materials and energy resources from areas politically and socially unstable, the fight against climate change, the need to favor delocalization of production systems and promote regional and rural development, the improvement of the knowledge of the factors that govern biological processes, entail that the context of biowaste management currently looks at more ambitious and articulated targets which find the most appropriate and complete synthesis in the concept of waste biorefinery. The potential inherent in biorefineries is huge. The global industrial production of organic chemicals accounts for a major share of the overall global chemicals industry and is estimated to amount, excluding fuels, to more than 300 Mt/year; the associated market was worth over 6 billion $ in 2014 and grew at an average of 8% per year from 2009 to 2014. The primary outputs of the chemical industrial activity are represented by a relatively limited number of building blocks used to produce a plethora of end products such as food and beverages, pharmaceuticals, pesticides, agrochemicals, water treatment, crop protection, personal care products and cosmetics, fertilizers, automotive industry, gasoline additives, polymers and chemicals, etc. Each building block can also be obtained from biomass, enabling the supply of raw materials at the local level, releasing the industrial activity from expensive and risky supplies, and opening the door to economic sustainability even in disadvantaged contexts such as, for instance, the insular ones. The demand for bioproducts from renewable sources is estimated to reach, depending on more or less favorable market conditions, 26-113 Mt/year in 2050, which would correspond to 38 and 17% of the total organic chemicals production, respectively; the associated market should account for some 7-8 billion $, with a growth rate of 15%/year which could further benefit from the increasing demand for biopolymers (IEA Bioenergy - Task 42 Biorefinery, 2012). The concept of biorefinery is not new in its more traditional meaning, and has evolved over time driven by three pivotal aspects (Akhlaghi et al., 2016): - cascade approach; - environmental sustainability; - economic sustainability. The cascade approach involves the flexible integration of different processes aimed at producing a mix of biofuels and bioproducts. The integration of processes and products according to the traditional or inverse cascade, is basically linked to economic sustainability, which requires an appropriate mix of products characterized either by significant market sizes - typical of biofuels - or high added values, but also to environmental aspects. In fact, as the number of usable and marketable outputs increases, this would logically correspond to less waste production, thus approaching the zero waste concept. The improvement in environmental sustainability is the main element underlying the hypothesis of transition towards a new generation of biorefineries: waste biorefineries. The environmental sustainability of the first and current biorefinery generations was, and still is, linked mainly to benefits related to the reduction of the consumption of non-renewable resources and CO2 emissions. To this respect, it is estimated that the production of a large share of synthetic organic compounds from renewable resources could lead to a global reduction of CO2 emissions ranging between 400 and 1000 Mt/year. The use of residual biomass would bring further environmental benefits: - first, the environmentally sound management of residues through their valorisation; - waste biomass should not be grown/bred, leading to a reduction in production costs; no biomass for food use would be treated nor areas that could be dedicated to other uses would be occupied; - the economic budget would benefit also from the waste treatment fees and short supply chain, besides the sale of the obtained bioproducts; - the different environmental and economic dynamics that would characterize waste biorefineries could make sustainable process schemes characterized by greater simplicity and smaller plant size as compared to traditional biorefineries. However, despite the potential advantages highlighted above, it is not conceivable that all the technological and economic perspectives associable to traditional biorefineries could be fully extended to waste biorefineries, if only due to the nature of the residual biomass, which would be, in most cases, qualitatively more heterogeneous and quantitatively less controllable. Therefore, the following challenge awaits environmental researchers and technicians: is the biorefinery concept feasible for waste management? and to what extent? A univocal answer probably does not exist. In contexts where large traditional biorefineries were available, the search for synergies could probably lead to consider the waste biomass as a secondary inflow that may contribute mainly to the recovery of products characterised by lower added value (energy, energy carriers, biofuels). Conversely, where the organic waste is the main biomass to be treated, the question would be whether and under what circumstances (availability and type of residues, market conditions, etc.) process schemes were applicable which, though lacking the complexity and the articulation of treatment steps/final products achievable in a traditional biorefinery, move away from current relatively simple valorization options, such as the recovery of biogas/biomethane and composting, to approach the concept of industrial biorefinery. Indeed, though some types of organic waste contain appreciable quantities of substances whose value may reach even 15,000 EUR/g - or are suitable to be converted in valuable building blocks (e.g. lactate) - thus being worth exploiting in relatively small dedicated plants where extraction of compounds and energy recovery from the resulting residues are performed, the minimum size that is considered sustainable from an economic point of view, the qualitative/quantitative characteristics of the waste biomass to be treated, the applicable processes, and the recoverable products, are still subjects of debate. Traditional biorefineries are meant to require rather large plants; it is well aknowledged that the minimum size to ensure economic sustainability is about 500,000-700,000 t/year (Kuchta, 2016). The use of waste biomass may lead to lowering this threshold, provided that the benefits in terms of simpler process schemes and revenues from waste treatment fees exceed the drawbacks related to the anticipated poorer quality of the final outputs and less ambitious market targets. However, under most of the circumstances, the less stringent requirements in terms of minimum plant size would not be such as to allow for a process based on the treatment of a single type of residue. Therefore, in the current state of affairs waste biorefineries should probably co-treat different types of residues, either of municipal origin or deriving from production activities, with all the advantages and problems which are associated to co-treatment, as pointed out by the experience gained with reference to anaerobic digestion. As for the processes to be applied, it is important to emphasize the pivotal role that fermentation would play in a waste biorefinery scheme, due to its ability to hydrolyze and simplify the organic substance and convert it to marketable products or building blocks. Indeed, the number of building blocks attainable through fermentation is remakable and current global production of fermentation products accounts to more than 8 Mt/year with an associated market of more than 20 billions $ (IEA Bioenergy - Task 42 Biorefinery, 2012). However, fermentation is a complex process, in particular when applied to substrates which are heterogenous and contain indigeneous microorganisms, and strongly depends on numerous and interconnected factors such as substrate chemical composition, concentration and pretretament methods, presence/type of inoculum and eventual pretreatment, inoculum- to substrate- ratio, reactor type and operation regime, applied operating conditions (e.g., pH, hydraulic and cell residence time, temperature, organic loading rate, etc.) (Alibardi and Cossu, 2015; De Gioannis et al., 2013; Dionisi and Silva, 2016; Ghimire et al., 2016). Therefore, there is still a strong claim for better understanding the complex interrelations among the relevant factors and, in turn, predicting the evolution of the process and optimizing its performance when has to be applied to residual biomass. As far as the obtainable products are concerned, if one looks at the current conditions in several European countries, characterized by strong incentives to the production of biomethane, a possible simplified and readily applicable waste biorefinery scheme could consist of an anaerobic process performed according to two stages, with the first one properly managed in order to recover H2 + CO2 from fermentation, besides the CH4 + CO2 mixture produced in the second stage. Both mixtures should be refined to recover biohydrogen and biomethane which could be then used individually or as a mixture (hythane). The separated CO2 could be reused or marketed or, alternatively, if the H2 use were not well established yet, both could be fed to reactors where carbon dioxide is biologically reduced in order to increase the overall biomethane production. Possible alternatives could include the commercialization of the organic acids-rich solution produced in the fermentative hydrogenogenic stage, or its further and innovative valorization as readily biodegradable substrate for biopolymers production or to be fed to microbial electrochemical systems (MES), known also as bioelectrochemical systems (BES), with production of electric energy (microbial fuel cells - MFC), or further biohydrogen or hydrogen peroxide or caustic solutions (microbial electrolysis cells - MEC), or even synthesis of organic compounds (microbial electrosynthesis - MES).

This work reports the results of a combined accelerated carbonation and wet granulation treatment applied to Basic Oxygen Furnace (BOF) steel slag with the aim of producing secondary aggregates for civil engineering applications and of storing CO2 in a solid and thermodynamically stable form. The tests were carried out in a laboratory scale granulation device equipped with a lid and CO2 feeding system. In each test, humidified slag (liquid/solid ratio of 0.12 l/kg) was treated for reaction times varying between 30 and 120 min under either atmospheric air or 100% CO2. Under both conditions, the particle size of the treatment product was observed to increase progressively with reaction time; specifically, the d50 values obtained for the products of the combined granulation and carbonation treatment increased from 0.4 mm to 4 mm after 30 min and to 10 mm after 120 min. Significant CO2 uptake values (between 120 and 144 g CO2/kg) were measured even after short reaction times for granules with diameters below 10 mm and for the coarser particle size fractions after reaction times of 90 min. The density, mineralogical composition and leaching behavior of the obtained granules were also investigated, showing that the combined granulation–carbonation process may be a promising option for BOF slag valorization, particularly in terms of decreasing the Ca hydroxide content of the slag. Another interesting finding was that the leaching behavior of the product of the combined treatment appeared to be significantly modified with respect to that of the untreated slag only for coarse uncrushed granules, an indication that the carbonation reaction occurs mainly on the outer layer of the formed granules.

Hydrogen is believed to play a potentially key role in the implementation of sustainable energy production, particularly when it is produced from renewable sources and low energy-demanding processes. In the present paper an attempt was made at critically reviewing more than 80 recent publications, in order to harmonize and compare the available results from different studies on hydrogen production from FW and OFMSW through dark fermentation, and derive reliable information about process yield and stability in view of building related predictive models. The review was focused on the effect of factors, recognized as potentially affecting process evolution (including type of substrate and co-substrate and relative ratio, type of inoculum, food/microorganisms [F/M] ratio, applied pre-treatment, reactor configuration, temperature and pH), on the fermentation yield and kinetics. Statistical analysis of literature data from batch experiments was also conducted, showing that the variables affecting the H2 production yield were ranked in the order: type of co-substrate, type of pre-treatment, operating pH, control of initial pH and fermentation temperature. However, due to the dispersion of data observed in some instances, the ambiguity about the presence of additional hidden variables cannot be resolved. The results from the analysis thus suggest that, for reliable predictive models of fermentative hydrogen production to be derived, a high level of consistency between data is strictly required, claiming for more systematic and comprehensive studies on the subject.

Summarization: The present study evaluates the influence of alkaline (NaOH) pretreatment on anaerobic digestion of olive pomace. Batch hydrolysis experiments with different NaOH dosages, process durations and temperatures were conducted, in which the variation of olive pomace solubilization in the liquid phase was investigated. The effect of pretreatment on anaerobic digestion was studied through biochemical methane potential assays. The results demonstrated the effectiveness of the NaOH pretreatment in improving olive pomace solubilization as well as its biodegradability. Maximum specific methane yields were achieved at different NaOH dosages depending on the pretreatment temperature. Consequently, it was concluded that the two operating parameters of the pretreatment stage (NaOH dosage and temperature) may exert a joint effect on substrate biodegradability and methane yields. The highest methane yield (242 NmL CH4/gVS) was obtained for the material pretreated at 90 °C, at a dosage of 1 mmol/gVS (4% of VS). Presented on: Waste Management

Laboratory experiments on a lead-contaminated soil were carried out to test the effects of chelant addition on metal leachability using column tests. Tests were aimed at studying metal mobilisation upon application of two different chelating agents (ethylenediaminetetraacetic acid [EDTA] and ethylenediaminedisuccinic acid [EDDS]). Column operation was arranged to simulate a flushing treatment in which the chelating agent is applied in a pulse mode to the soil for one bed volume, while deionised water was continuously introduced for the rest of the experiment. Two different concentrations (3 and 5 mmol·kg −1 respectively) of the two chelating agents and a control solution (deionised water) were tested in separate experiments; pH, total organic carbon and the total concentrations of Pb, Zn, Fe, Cd, Cu and Ni were monitored during each run. A seven-step sequential extraction procedure was used to evaluate metal partitioning and concentration in the contaminated soil after treatment as a function of depth. Th...

In this study, the influence of the inoculum-to-substrate ratio (ISR) on dark fermentative hydrogen production from food waste (FW) was evaluated. ISR values ranging from 0.05 to 0.25 g VSinoculum/g VSsubstrate were investigated by performing batch tests at T = 39 °C and pH = 6.5, the latter being the optimal value identified based on a previous study. The ISR was found to affect the fermentation process, clearly showing that an adequate ISR is essential in order to optimise the process kinetics and the H2 yield. An ISR of 0.14 proved to optimum, leading to a maximum H2 yield of 88.8 L H2/kg VSFW and a maximum production rate of 10.8 L H2/kg VSFW∙h. The analysis of the fermentation products indicated that the observed highest H2 production mostly derived from the typical acetate/butyrate-type fermentation.

In the present work, the application of an assisted electrokinetic process for the removal of heavy metals from real contaminated sediments was investigated. The process made use of both chemical and physical methods, including addition of chelating and acid agents, as well as application of a hydraulic gradient. Lab-scale electrokinetic runs were applied on two different dredged sediments varying the applied voltage gradient and the treatment duration. The use of EDTA significantly improved the overall performance of the electrokinetic treatment for sediment V (more than 60% mobilized for each metal), while only aggressive acid conditioning with nitric acid was able to remove significant amounts of heavy metals (up to 40.5% for Pb) from sediment S due to the strong buffering capacity of this material. This clearly assesses that the specific characteristics of the materials under concern and the reactions occurring at the electrodes must be carefully evaluated when applying an electroremediation process.

Considerable technical advances in the cleaning of flue gases from waste incineration have increasingly made solid residues from the process the main emission route for inorganic contaminants. Appropriate strategies to manage waste incineration residues are therefore required and have driven scientific research toward finding environmentally sound solutions. Among the various types of solid residues from waste incineration, bottom ash (BA) is generated in the largest amounts and is recognized as the most suited for use. Several techniques have been developed for processing incinerator BA in view of reuse in various applications. Generally, valuable components of BA include ferrous and nonferrous metals as well as the mineral fraction. This chapter provides, on the basis of documented scientific literature studies and information about technical applications, an overview of the physical, chemical, mineralogical, and leaching properties of incinerator BA, along with relevant techniques proposed for BA processing. The chapter was compiled by members of the pHOENIX working group on Management of Municipal Solid Waste Incineration Residues, which was established in 2002 in Vienna with a focus on promoting sustainable solutions with respect to the treatment, use, and disposal of incineration residues. In 2002, the working group was included as a task group under the International Waste Working Group.

The paper reports the results of slurry-phase accelerated carbonation experiments using basic oxygen surface steel slag as the source of alkalinity for CO2 sequestration. In particular, the effect of particle size distribution on the carbon sequestration performance was investigated in detail. To this aim, different size classes of the slag were separately subjected to accelerated carbonation, and the CO2 uptake, the mineralogical changes, the final particle size distribution and the chemical equilibria in the liquid phase were studied. While the total content of potentially reactive elements in the different size fractions of the slag exerted only a minor influence on the carbonation performance, particle size was found to have a dramatic role during the process, in particular during the dissolution stage. Changes by up to two orders of magnitude in CO2 sequestration (which varied in the range 4.7–465 g of CO2 sequestered per kg of slag) were observed for the different size classes investigated. Geochemical modeling of the liquid phase after the carbonation treatment showed that the system was dominated by carbonate minerals of several elements including Ca, Mn, Mg, Fe and Zn, indicating the involvement of several components in the carbonation reactions. The presence of carbonate minerals containing not only Ca, but also Mn, Mg and Fe was also detected through XRD analyses. The analysis of the pore size distribution of the slag before and after carbonation revealed the competing effects of particle erosion/dissolution and pore clogging as a result of carbonate precipitation, the relative contribution of which was found to be related to the carbonation degree attained.

Il presente lavoro riporta i risultati ottenuti attraverso la valorizzazione di sedimenti dragati da piccoli porti. Il recupero di tali sedimenti, e in particolare della frazione sabbiosa in essi contenuta, rappresenta un'importante risorsa nelle attività di ripascimento delle aree marino-costiere soggette ad erosione. Le attività sperimentali, volte a verificare il potenziale recupero della frazione sabbiosa nei sedimenti dragati all'interno di piccoli porti della Regione Emilia-Romagna, sono state condotte nell'ambito del progetto Coast- Best "Approccio coordinato per il trattamento e il riutilizzo di sedimenti di dragaggio in una rete di piccoli porti" sovvenzionato dallo strumento finanziario europeo Life+. Obiettivo principale del progetto è stato quello di sviluppare una gestione integrata dei sedimenti derivanti dal dragaggio di piccoli porti, prevedendo un loro recupero e riutilizzo e rendendo così tale attività sostenibile dal punto di vista ambientale, tecnico ed economico. In particolare, vengono riportati nel presente articolo i risultati analitici di alcune prove di separazione fisica per il recupero delle frazioni grossolane dei sedimenti dragati nelle diverse aree di studio, svolte a scala pilota che hanno evidenziato, dopo la corretta regolazione delle condizioni operative del processo, l'ottenimento di una frazione di sabbia con caratteristiche idonee al successivo riutilizzo e, in alcuni casi, la separazione di frazioni fini ad alta qualità.

This work presents the results of carbonation experiments performed on Basic Oxygen Furnace (BOF) steel slag samples employing gas mixtures containing 40 and 10% CO2 vol. simulating the gaseous effluents of gasification and combustion processes respectively, as well as 100% CO2 for comparison purposes. Two routes were tested, the slurry-phase (L/S = 5 l/kg, T = 100°C and Ptot = 10 bar) and the thin-film (L/S = 0.3–0.4 l kg, T = 50°C and Ptot = 7–10 bar) routes. For each one, the CO2 uptake achieved as a function of the reaction time was analyzed and on this basis, the energy requirements associated with each carbonation route and gas mixture composition were estimated considering to store the CO2 emissions of a medium size natural gas fired power plant (20 MW). For the slurry-phase route, maximum CO2 uptakes ranged from around 8% at 10% CO2, to 21.1% (BOF-a) and 29.2% (BOF-b) at 40% CO2 and 32.5% (BOF-a) and 40.3% (BOF-b) at 100% CO2. For the thin-film route, maximum uptakes of 13% (BOF-c) and 19.5% (BOF-d) at 40% CO2, and 17.8% (BOF-c) and 20.2% (BOF-d) at 100% were attained. The energy requirements of the two analyzed process routes appeared to depend chiefly on the CO2 uptake of the slag. For both process route, the minimum overall energy requirements were found for the tests with 40% CO2 flows (i.e., 1400−1600 MJ/tCO2 for the slurry-phase and 2220 – 2550 MJ/tCO2 for the thin-film route).

The increasing volumes of municipal solid waste produced worldwide are encouraging the development of processes to reduce the environmental impact of this waste stream. Combustion technology can facilitate volume reduction of up to 90%, with the inorganic contaminants being captured in furnace bottom ash, and fly ash/APC residues. The disposal or reuse of these residues is however governed by the potential release of constituent contaminants into the environment. Accelerated carbonation has been shown to have a potential for improving the chemical stability and leaching behaviour of both bottom ash and fly ash/APC residues. However, the efficacy of carbonation depends on whether the method of gas application is direct or indirect. Also important are the mineralogy, chemistry and physical properties of the fresh ash, the carbonation reaction conditions such as temperature, contact time, CO(2) partial pressure and relative humidity. This paper reviews the main issues pertaining to the application of accelerated carbonation to municipal waste combustion residues to elucidate the potential benefits on the stabilization of such residues and for reducing CO(2) emissions. In particular, the modification of ash properties that occur upon carbonation and the CO(2) sequestration potential possible under different conditions are discussed. Although accelerated carbonation is a developing technology, it could be introduced in new incinerator facilities as a "finishing step" for both ash treatment and reduction of CO(2) emissions.

In the present work the influence of accelerated mineral carbonation on the leaching behaviour of basic oxygen furnace steel slag was investigated. The environmental behaviour of the material as evaluated through the release of major elements and toxic metals under varying pH conditions was the main focus of the study. Geochemical modelling of the eluates was used to derive a theoretical description of the underlying leaching phenomena for the carbonated material as compared to the original slag. Among the investigated elements, Ca and Si were most appreciably affected by carbonation. A very clear effect of carbonation on leaching was observed for silicate phases, and lower-Ca/Si-ratio minerals were found to control leaching in carbonated slag eluates as compared to the corresponding untreated slag sample as a result of Ca depletion from the residual slag particles. Clear evidence was also gained of solubility control for Ca, Mg and Mn by a number of carbonate minerals, indicating a significant involvement of the original slag constituents in the carbonation process. The release of toxic metals (Zn, V, Cr, Mo) was found to be variously affected by carbonation, owing to different mechanisms including pH changes, dissolution/precipitation of carbonates as well as sorption onto reactive mineral surfaces. The leaching test results were used to derive further considerations on the expected metal release levels on the basis of specific assumptions on the relevant pH domains for the untreated and carbonated slag.

Important advantages, including reductions in fuel consumption and labour cost, arise from the optimal design of solid waste (SW) collection routes. Further, optimal design can reduce vehicle maintenance expenditures and improve traffic conditions in urban areas. To date, optimal routes have been developed according to intuitive methodologies and field experience. However, increasing attention is being devoted to innovative approaches, such as those able to simulate complex collection systems. To analyse these complexities, operational research applications are used. They are typically based on the implementation of heuristic procedures allowing for high quality solutions to the problem at hand. From a computational point of view, however, heuristic procedures have a complexity which is o(n3), where n is the number of points which have to be visited during each route. This is a limit for an accurate representation of urban areas and for the quality of the calculated solutions. An alternative methodology, which is the subject of this paper, is based on a genetic algorithm. Also, an ad hoc algorithm, developed in the framework of a wider research, is illustrated. Results of a preliminary field test conducted for verification are also presented.

This paper presents the results of a wider experimental programme conducted in the framework of the NNAPICS ("Neural Network Analysis for Prediction of Interactions in Cement/Waste Systems") project funded by the European Commission and a number of industrial partners under Brite-EuRamIII. Based on the fact that bottom ashes from waste incineration are classified as non-hazardous wastes according to the European Waste Catalogue, the aim of the present work was to investigate the feasibility of addressing the potential use of such residues in cement-based mixtures. This issue was suggested by the analysis of the properties of different bottom ashes coming from Italian municipal and hospital solid waste incinerators, which showed a chemical composition potentially suitable for such applications. Different mixes were prepared by blending bottom ash with ordinary Portland cement in different proportions and at different water dosages. The solidified products were tested for setting time and bulk density, unconfined compressive strength and evaporable water content at different curing times. The results of the experimental campaign were analysed through a statistical procedure (analysis of variance), in order to investigate the effect of mixture composition (waste replacement level and water dosage) on the product properties.

The results of direct aqueous accelerated carbonation of three types of steel manufacturing residues, including an electric arc furnace (EAF) slag and two basic oxygen furnace (BOF) slags, are reported. Batch accelerated carbonation tests were conducted at different temperatures and CO2 pressures applying the thin-film route (liquid to solid, L/S, ratio = 0.3 L/kg) or the slurry-phase route (L/S ratio = 5 L/kg). The CO2 uptake strongly depended on both the slag characteristics and the process route; maximum yields of 280 (EAF), 325 (BOF1) and 403 (BOF2) g CO2/kg slag were achieved in slurry phase at T = 100 °C and pCO2 = 10 bar. Differently from previous studies, additional carbonates (other than Ca-based phases) were retrieved in the carbonated BOF slags, indicating that also Mg-, Fe- and Mn-containing phases partially reacted with CO2 under the tested conditions. The results hence show that the effects of accelerated carbonation in terms of CO2 uptake capacity, yield of mineral conversion into carbonates and mineralogy of the treated product, strongly rely on several factors. These include, above all, the mineralogy of the original material and the operating conditions adopted, which thus need specific case-by-case optimization to maximize the CO2 sequestration yield.

This paper discusses the effects of accelerated carbonation on the leaching behaviour of two types of stainless steel slags (electric arc furnace and argon oxygen decarburisation slag). The release of major elements and toxic metals both at the natural pH and at varying pH conditions was addressed. Geochemical modelling of the eluates was used to theoretically describe leaching and derive information about mineralogical changes induced by carbonation. Among the investigated elements, Ca and Si were most appreciably affected by carbonation. A very clear effect of carbonation on leaching was observed for silicate phases; geochemical modelling indicated that the Ca/Si ratio of Ca-controlling minerals shifted from ∼ 1 for the untreated slag to 0.5-0.67 for the carbonated samples, thus showing that the carbonation process left some residual Ca-depleted silicate phases while the extracted Ca precipitated in the form of carbonate minerals. For toxic metals the changes in leaching induced by carbonation appeared to be mainly related to the resulting pH changes, which were as high as ∼ 2 orders of magnitude upon carbonation. Depending on the specific shape of the respective solubility curves, the extent of leaching of toxic metals from the slag was differently affected by carbonation.

This paper presents the results of an extensive set of laboratory experiments performed to design a demonstrative electrokinetic plant for extracting heavy metals from marine sediments dredged from the Livorno marine harbour. The investigated sediments displayed a high salinity, a high acid neutralization capacity, a low electrical resistivity (0.5 Ωm), a high alkalinity (pH ≈ 8) and a large fraction of fine particles. The target metals were Cd, Cr, Cu, Ni, Pb and Zn at relatively weak and inhomogeneous concentrations with high non-mobile fractions. After an accurate characterization, several screening and full electrokinetic tests were performed using cells of two different sizes, several conditioning agents (HNO 3 , HCl, H 2 SO 4 , citric acid, oxalic acid, ascorbic acid, EDTA), different applied current intensities and durations. The tests highlighted the need for long treatment times in order to obtain a significant pH reduction, with some appreciable metal removal being attained only after several weeks. The best results were obtained with strong acids used as the conditioning agents, with significant specific effects of each acid, including pronounced resistivity increase (from 0.5 up to 10 Ωm) and a high electroosmotic flow (EOF) with H 2 SO 4 , or a reversed EOF (electroendosmosis), and minor resistivity changes with HNO 3 . The use of the obtained data to design a demonstrative plant is also presented in the paper, with considerations on operating parameters such as energy and reagent consumption, characteristics of plant components and required safety measures.

The present paper deals with the use of incineration bottom ash in cementitious systems. The results of the physical–mechanical characterisation of the solidified products were discussed elsewhere, while the present work focuses on the acid neutralisation capacity (ANC) of the investigated mixtures as a means to evaluate the type of hydration products and their relative amounts in the solid matrix. The approach consists of differential acid neutralisation analysis, which was validated using traditional X-ray diffraction (XRD) methods.

Site selection for waste disposal is a complex task that should meet the requirements of communities and stakeholders. In this article, three decision support methods (Boolean logic, index overlay and fuzzy gamma) are used to perform land suitability analysis for landfill siting. The study was carried out in one of the biggest metropolitan regions of Italy, with the objective of locating suitable areas for waste disposal. Physical and socio-economic information criteria for site selection were decided by a multidisciplinary group of experts, according to state-of-the-art guidelines, national legislation and local normative on waste management. The geographic information systems (GIS) based models used in this study are easy to apply but require adequate selection of criteria and weights and a careful evaluation of the results. The methodology is arranged in three steps, reflecting the criteria defined by national legislation on waste management: definition of factors that exclude location of landfills or waste treatment plants; classification of the remaining areas in terms of suitability for landfilling; and evaluation of suitable sites in relation to preferential siting factors (such as the presence of quarries or dismissed plants). The results showed that more than 80% of the provincial territory falls within constraint areas and the remaining territory is suitable for waste disposal for 0.72% or 1.93%, according to the model. The larger and most suitable sites are located in peripheral areas of the metropolitan system. The proposed approach represents a low-cost and expeditious alternative to support the spatial decision-making process.

Batch dark fermentation experiments were performed on food waste and mixtures of food waste and wastewater activated sludge to evaluate the influence of pH on biological H2 production and compare the process performance with and without inoculum addition. The effect of a preliminary thermal shock treatment of the inoculum was also investigated as a means to harvest the hydrogenogenic biomass. The best performance in terms of both H2 generation potential and process kinetics was observed at pH=6.5 under all experimental conditions (no inoculum, and untreated or thermally treated inoculum added). H2 production from food waste was found to be feasible even without inoculum addition, although thermal pre-treatment of the inoculum notably increased the maximum production and reduced the lag phase duration. The analysis of the fermentation products indicated that the biological hydrogen production could be mainly ascribed to a mixed acetate/butyrate-type fermentation. However, the presence of additional metabolites in the digestate, including propionate and ethanol, also indicated that other metabolic pathways were active during the process, reducing substrate conversion into hydrogen. The plateau in H2 generation was found to mirror the condition at which soluble carbohydrates were depleted. Beyond this condition, homoacetogenesis probably started to play a role in the degradation process.

Hydrogen production from cheese whey through dark fermentation was investigated in this study in order to systematically analyse the effects of the operating pH. The effluents from pecorino cheese and mozzarella cheese production were the substrates used for the fermentation tests. Either CW only or a mixture of CW and heat-shocked activated sludge were used in mesophilic pH-controlled batch fermentation experiments. The results indicated that hydrogen production was strongly affected by multiple factors including the substrate characteristics, the addition of an inoculum as well as the pH. The process variables were found to affect to a varying extent numerous interrelated aspects of the fermentation process, including the hydrogen production potential, the type of fermentation pathways, as well as the process kinetics. The fermentation products varied largely with the operating conditions and mirrored the H2 yield. Significant fermentative biohydrogen production was attained at pHs of 6.5–7.5, with the best performance in terms of H2 generation potential (171.3 NL H2/kg TOC) being observed for CW from mozzarella cheese production, at a pH value of 6.0 with the heat-shocked inoculum.