The leaching potential of sewage sludge and municipal waste incineration ashes in terms of landfill safety and potential reuse

Tracing Gadolinium levels throughout wastewater treatment: Insights from a yearly assessment in northern Spain

Gadolinium (Gd) is a rare earth element (REE) used in the formulation of contrast agents for Magnetic Resonance Imaging (MRI) due to its paramagnetic properties. The growth in population and the improved quality of the healthcare systems over the last years, has promoted the use of MRI as an effective diagnostic tool thus increasing the consumption of gadolinium and its release into the wastewater treatment network. Therefore, the tracking and quantification of this metal in sewage treatment plants and water bodies, is of paramount importance since there are currently no specific rare earth treatment technologies installed in WWTPs, and consequently gadolinium is finally discharged into the environment. In this work, the presence of gadolinium and all other rare earth elements was monitored during a year in three WWTPs in northern Spain (Vuelta Ostrera and San Román in Cantabria and Galindo in País Vasco). These WWTPs are located close to urban centres with hospitals where MRI tests are performed. By tracing Gd throughout the wastewater treatment facilities, its presence was confirmed in water streams, in the order of ng per litter, and in sludge and ashes, in the order of mg per kilogram. A significant human influence was observed, with Gd anomaly values between 3.14 and 79.2 and anthropogenic Gd percentages above 90 %. The presence of Gd in water streams is affected by the sampling period due to the variations of the activity periods of the hospitals nearby the treatment plants. On the contrary, its content in sludge and ashes remains almost constant along the year. The concentration of this metal found in the ashes opens the door to its possible recovery together with other critical raw materials in the context of the circular economy.

Co-vitrification of hazardous waste incineration fly ash and hazardous waste sludge based on CaO-SiO2-Al2O3 system

Based on the CaO-SiO₂-Al₂O₃ system, the feasibility of co-vitrification of hazardous waste incineration fly ash (FA) and hazardous waste sludge (HWS) was verified. In the CaO-SiO₂-Al₂O₃ ternary system diagram, the melting point of the system gradually decreases with an appropriate increase in SiO₂ content when the CaO/Al₂O₃ ratio is determined to be approximately 1. The TG-DSC results revealed that the liquid phase generation temperature in the FA and HWS mixture system was significantly lower than those of FA and HWS individually owing to the different CaO, SiO₂, and Al₂O₃ contents; this is consistent with the results of the theoretical melting characteristics analysis, which show that the melting characteristic temperatures can be reduced by controlling the CaO-SiO₂-Al₂O₃ ratio in the system. The co-vitrification experimental results confirmed that a vitreous content above 92%, a loss ratio on acid dissolution less than 1.74%, and leaching toxicity of heavy metals lower than 0.15 mg/L could be obtained by adjusting the CaO, SiO₂, and Al₂O₃ contents in the FA and HWS system to 20 wt%-32.5 wt%, 35 wt%-61 wt% and 14 wt%-32.5 wt%, respectively, and under a melting temperature of 1350 °C.

Characteristic of phosphorus rich compounds in the incinerated sewage sludge ashes: a case for sustainable waste management

Growing concern over mineral resources supply forces us to search for alternative sources of Phosphorus. The possibility to recover phosphorus from incinerated sewage sludge ashes appears to be an important aspect in anthropogenic phosphorus cycle and sustainable economy. To make phosphorus recovery efficient it is important to learn the chemical and mineral composition of ash and phosphorus speciation. The phosphorus content in the ash was over 7%, what corresponds to medium rich phosphorus ores. The main phosphorus rich mineral phases were phosphate minerals. The most widespread was tri-calcium phosphate Whitlockite with various Fe, Mg and Ca proportions. In minority Fe-PO₄ and Mg-PO₄ were detected. Whitlockite commonly overgrown with hematite, influences negatively mineral solubility and thus recovery potential and indicates low bioavailability of phosphorus. Considerable amount of phosphorus was found in the low crystalline matrix where phosphorus content was around 10 wt% however low crystallinity and dispersed phosphorus also does not strengthen the potential to recover this element.

The assessment of phosphorus recovery potential in sewage sludge incineration ashes - a case study

A sewage sludge incineration ash contains large amounts of phosphorus, which are considered as a novel anthropogenic waste-based substitute for phosphorus natural resources. Phosphorus is accumulated at most in phosphate minerals of whitlockite structure, that contain Fe, Ca, and Mg and in the matrix composed of Si, Al, Fe, Ca, P, Mg, K, Na in various proportions. The goal of this study was to estimate phosphorus recovery potential. A four-stage sequential extraction, following the modified Golterman procedure, was applied. Separation of four independent fractions enabled to understand better the manner of phosphorus occurrence in the studied ash. The results of the extraction indicated the greatest release of phosphorus combined with organic matter using sulfuric acid. The release was on average at the level of 64%. The chelating Na-EDTA compound indicated lower ability to extract phosphorus (at the level of 35%), and the highest ability to extract heavy metals and potentially toxic elements (As, Zn, Mo). The sequential extraction led to the total recovery of phosphorus of around 40-60.

Wet wastes to bioenergy and biochar: A critical review with future perspectives

The ever-increasing rise in the global population coupled with rapid urbanization demands considerable consumption of fossil fuel, food, and water. This in turn leads to energy depletion, greenhouse gas emissions and wet wastes generation (including food waste, animal manure, and sewage sludge). Conversion of the wet wastes to bioenergy and biochar is a promising approach to mitigate wastes, emissions and energy depletion, and simultaneously promotes sustainability and circular economy. In this study, various conversion technologies for transformation of wet wastes to bioenergy and biochar, including anaerobic digestion, gasification, incineration, hydrothermal carbonization, hydrothermal liquefaction, slow and fast pyrolysis, are comprehensively reviewed. The technological challenges impeding the widespread adoption of these wet waste conversion technologies are critically examined. Eventually, the study presents insightful recommendations for the technological advancements and wider acceptance of these processes by establishing a hierarchy of factors dictating their performance. These include: i) life-cycle assessment of these conversion technologies with the consideration of reactor design and catalyst utilization from lab to plant level; ii) process intensification by integrating one or more of the wet waste conversion technologies for improved performance and sustainability; and iii) emerging machine learning modeling is a promising strategy to aid the product characterization and optimization of system design for the specific to the bioenergy or biochar application.

Comprehensive Estimation of Combustion Behavior and Thermochemical Structure Evolution of Four Typical Industrial Polymeric Wastes

A huge amount of industrial waste will be generated during the industrialization process and their harmless disposal has always been a headache for reducing carbon emissions. In this study, the combustion behaviors and thermal kinetics of four typical industrial polymeric wastes including rubber, leather, plastic and cloth, were systematically studied by using a Thermogravimetric Analysis. The gas emission and structural evolution was comprehensively analyzed using TG-FTIR, 2D-PCIS, ICP and TEM. The results show that the combustibility of leather and cloth are better than the other two samples, while the rubber and plastic have a wider combustion temperature range for higher content of C-H bonds and, the intermediate oxidation process and the stubborn cracking process of C=C bonds. The surface reaction was considered to be the main reaction of rubber and plastic (pre-exponential factor less than 10−9), while both leather and cloth went through a complex procedure during multiple decomposition. The volatiles products are gases (e.g., CO2, CH4) and small molecules (e.g., H2O). The high levels of basic metals in the industrial waste causes serious slagging and fouling tendency (fouling index higher than 4.0), which have a serious adverse influence on the operation of a waste incineration plant.

Waste Classification of Spent Refractory Materials to Achieve Sustainable Development Goals Exploiting Multiple Criteria Decision Aiding Approach

The recycling of used refractory materials in the heavy industry constitutes one of the significant environmental problems in the industry related to environmental and financial issues. This study proposes a multicriteria methodological frame to characterize the refractory material waste and identify the recycling capabilities. Considering the chemical and physical analysis of the refractory material wastes, the proposed methodological frame progresses into a two-phase procedure. The first phase includes an on/off approach that allows discretizing the refractory material wastes to compatible or not compatible as far as their recycling prospects. Then, an additive value model is utilized, including (a) the marginal value functions used for every criterion related to critical environmental factors, and (b) the weight vector reflecting the relative importance of the criteria used. A group of experts concerning the environment and the refractory materials was employed to estimate the additive value model. The assessment of the marginal value function is achieved using the module of the Multicriteria Interactive Intelligence Decision Aiding System (MIIDAS), which is based on a modification of the mid-value split point technique incorporating focused dialogues, artificial intelligence, and visual techniques. The weight vector was assessed using the weight assessment through prioritization method (WAP), which concludes with the estimation of the weights based on the criteria ranking and the pairwise expression of the strength of preferences for the consecutive criteria according to their ranking. The outcome of this approach is to introduce an environmental appropriateness index for refractory materials based on their chemical composition and the judgement of an expert group. The main findings of this research may be useful for engineers, decision-makers, and scientists in the field of circular economy and waste management.

The Reuse of Industrial By-Products for the Synthesis of Innovative Porous Materials, with the Aim to Improve Urban Air Quality

This works concerns the characterization and the evaluation of adsorption capability of innovative porous materials synthesized by using alginates and different industrial by-products: silica fume and bottom ash. Hydrogen peroxide was used as pore former to generate a porosity able to trap particulate matter (PM). These new materials are compared with the reference recently proposed porous SUNSPACE hybrid material, which was obtained in a similar process, by using silica fume. Structural, morphological, colorimetric and porosimetric analyses were performed to evaluate the differences between the obtained SUNSPACE typologies. The sustainability of the proposed materials was evaluated in terms of the Embodied Energy and Carbon Footprint to quantify the benefits of industrial by-products reuse. Adsorption tests were also performed to compare the ability of samples to trap PM. For this aim, titania suspension, with particles size about 300 nm, was used to simulate PM in the nanoparticle range. The results show that the material realized with bottom ash has the best performance.