Waste, Volume 4, Issue 2 (June 2026) – 10 articles

This study examined the scale of informal metal recycling in the King Sabata Dalindyebo (KSD) Municipality and evaluated its socioeconomic and environmental impacts through a circular economy lens. While formal recycling programs have received research attention, informal recycling systems have been less examined despite their critical contribution. A mixed-methods approach was employed, combining quantitative surveys and material measurements with qualitative field observations. Data were collected through focus groups and surveys administered to 48 active recyclers operating along the N2 and R61 highways, supported by systematic field observations and quantification of all recovered materials. The results showed that steel and aluminum were the most recovered metals at 41.7% and 20.8%. Recyclers collected an average of 15.3 kg/day (SD = 4.2) during periods of high material availability and accessibility and the materials are close to market points. The study underscores the socioeconomic and environmental impacts of informal recycling within the circular economy framework, with most participants (85.42%) indicating that recycling was a significant livelihood while 74% cited difficulty transporting bulky materials, often manually. Therefore, the study emphasizes that informal recyclers play a vital role in sustaining the local economy by offering key waste management and resource recovery services. Acknowledge of these informal systems will contribute to a more inclusive understanding of waste management in developing contexts. Full article

Olive pomace is a major side stream originating from olive processing for the production of olive oil. This waste material bears a load of polyphenolic antioxidants, and thus it might serve as a source of precious phytochemicals. This work had as its objective the development of an extraction process for the efficacious recovery of polyphenols from dried olive pomace (dOP), employing eco-friendly extraction media. To this end, environmentally benign solvents were first compared for their efficiency in obtaining increased yields in total polyphenols, and 40% aqueous isopropanol was selected as the best-performing mixture. Further examination of the role of acidification showed that mineral acid addition (sulfuric, hydrochloric) had a rather negative effect on polyphenol yield. To the contrary, incorporation of oxalic acid into the solvent at a 10% level provided significantly higher extraction yield (p < 0.05), which reached 27.1 ± 1.1 mg caffeic acid equivalents (CAE) per g dOP. This solvent system (40% isopropanol/10% oxalic acid) was additionally scrutinized for its effectiveness by studying the role of process severity and response surface optimization. Out of both approaches, it was demonstrated that polyphenol extraction yield, but also antiradical activity, was directly correlated with residence time and temperature, within the limits tested. Moreover, a high correlation between polyphenol concentration and antiradical activity was also revealed. Liquid chromatography-tandem mass spectrometry analyses showed that the extract obtained with the solvent system used (40% isopropanol/10% oxalic acid) was characterized by the presence of both hydroxytyrosol and the flavone luteolin (242.1 and 178.6 μg g⁻¹ dOP, respectively), but, in the absence of isopropanol, the extract produced was largely dominated by hydroxytyrosol (4629.7 μg g⁻¹ dOP). Thus, it was concluded that the solvent system could fundamentally diversify extract composition. It is proposed that, when combined with integrated biorefinery technologies, this approach could effectively contribute to reducing environmental impacts while enabling the production of valuable natural antioxidants or platform chemicals that are vital for the food, pharmaceutical, and cosmetic industries. Within the broader context of sustainable food waste management, such strategies might be key elements of a circular economy framework. Full article

This review synthesizes current advances in the biocatalytic upcycling of plastic waste through microbial and enzymatic systems, emphasizing the transformation of recalcitrant polymers into high-value products. A narrative review methodology was adopted to integrate interdisciplinary findings across microbiology, enzymology, biotechnology, and waste management. Significant progress has been achieved in the depolymerization of plastics such as polyethylene terephthalate (PET), polyurethane, and polyolefins into intermediates, including terephthalic acid and ethylene glycol. These intermediates are subsequently valorized into products such as polyhydroxyalkanoates (PHAs), lipids, terpenoids, organic acids, aromatic compounds, and bacterial cellulose. Quantitative performance metrics demonstrate the potential of these systems. Notably, PHA production from PET-derived substrates has reached up to 1.10 g L⁻¹ (22.7% cell dry weight) and as high as 46% intracellular accumulation, while bacterial cellulose production from PET hydrolysates has achieved ~3.0 g L⁻¹. High conversion efficiencies have been reported in several pathways, including ~90–99% conversion of PET-derived intermediates to catechol, ~91.6% yield of glycolic acid from ethylene glycol (up to 31.4 g L⁻¹), and ~71–79% molar conversion of terephthalic acid to vanillin. Despite these advances, critical limitations persist, including low volumetric productivity in some systems, metabolic imbalances, substrate toxicity, feedstock heterogeneity, and challenges in process integration and scale-up. Future research should prioritize enhancing metabolic flux, improving enzyme efficiency, optimizing microbial consortia, and developing integrated, low-energy depolymerization–bioconversion systems. Full article

The activated sludge process is pivotal in wastewater treatment, with ongoing research into its process control methods. Modeling treatment plants aids in analyzing relationships among variables, supporting fault detection and operational decision-making. However, datasets from real-world treatment plants often contain outliers and missing values due to sensor faults, maintenance activities, and operational disruptions, making outlier handling and data imputation essential for reliable modeling. Existing studies on data imputation for activated sludge systems are often based on synthetic or short datasets, limited method comparisons, or inconsistent evaluation metrics, which reduces their applicability to full-scale operational settings. This study addresses these limitations by presenting a comprehensive, head-to-head comparison of Kohonen Self-Organising Maps (KSOM) with widely used multiple imputation and tree-based methods, namely Amelia II, MICE, missForest, and missRanger. The methods are applied to a real-world multivariate dataset comprising 19 process variables collected over 8.5 years from a full-scale activated sludge treatment plant, containing 39% overall missing data with highly uneven missingness across variables. A validation framework based on held-out observation data is used, and performance is assessed using complementary metrics, including the coefficient of determination (R²), average absolute error (AAE), relative average absolute error (RAAE), mean squared error (MSE), and root mean squared error (RMSE). Results show that KSOM consistently outperforms the competing methods across most variables and evaluation metrics. KSOM achieves near-perfect R² values (≈1) for many process variables, with lower absolute and relative errors, even for variables with very high (>70%) and irregular missingness. These findings highlight KSOM’s robustness in capturing multivariate relationships and cluster structure in complex, operational WWTP data. Full article

This paper describes the development of an automated solid waste sorting system that integrates advanced computer vision pipelines with a robotic manipulator for real-time classification and actuation. The system consists of a Deep Neural Network (DNN) and a YOLOv8-based perception module. Thedeveloped model is capable of accurately detecting and classifying objects with confidence scores exceeding 0.71, and the overall system attained a sorting accuracy of approximately 81.8% across multiple test batches. From an integration perspective, the coordination among the Intel RealSense camera, Raspberry Pi 5, Arduino Uno, ultrasonic sensors, relay-switching circuit, and SCORBOT-ER 4U robotic arm demonstrated reliable communication and execution, enabling accurate pick-and-place operations. Overall, the results confirm that the proposed system provides a functional and scalable proof of concept for automated waste segregation in controlled environments. The study highlights that while current performance is sufficient for low-speed applications, further improvements in dataset diversity, perception robustness, mechanical gripping, and feedback control are necessary to achieve higher accuracy, reliability, and industrial applicability. Full article

Rambutan peel, an abundant agro-industrial by-product, is a rich source of ellagitannins (ETs) and represents a promising substrate for the sustainable production of ellagic acid (EA) through solid-state fermentation (SSF). This study aimed to optimize EA release from rambutan peel ETs by SSF using Saccharomyces cerevisiae 227 in column reactors. We applied a central composite design (CCD) to evaluate and optimize the effects of temperature, NaCl concentration, and peptone supplementation on EA production. We also used HPLC/ESI/MS to identify and quantify EA. Maximum EA yields were obtained under central experimental conditions (treatments 15 and 16: 35 °C, 0.53 g/L NaCl, and 8 g/L peptone), reaching 8.36 ± 1.58 and 8.23 ± 0.52 mg/g, respectively. The predictive model estimated a maximum EA production of 8.29 mg/g, experimentally validated, at 34.6 °C, 0.58 g/L NaCl, and 8.26 g/L peptone, yielding 8.27 ± 0.47 mg/g. HPLC/ESI/MS analysis further confirmed EA formation and the presence of biodegradation products derived from geraniin and corilagin, indicating effective ET biotransformation. These findings establish optimized conditions for EA production from rambutan peel ETs via SSF and demonstrate the feasibility of implementing a sustainable bioprocess for the valorization of this agro-industrial residue. Full article

The growing generation of household hazardous waste (HHW) presents critical environmental and public health challenges worldwide. This study investigates prevailing trends in HHW management and analyzes the socio-economic and demographic determinants that influence public perceptions, attitudes, and behaviors toward HHW recycling practices. A comparative mass flow analysis is also conducted to evaluate the limitations of current HHW management practices in Greece and outline policy implementation plans. Statistical findings indicate that income significantly influences recycling behavior. Individuals with annual incomes between €10,001 and €30,000 are less likely to engage in HHW recycling, whereas those earning over €70,000 demonstrate higher levels of recycling participation. The public recognizes the need for green collection points for appropriate HHW management. However, no statistically significant correlation is found between income levels and perceived importance of these facilities. This outcome is attributed to the high proportion (46.7%) of dichotomous variables in the χ² independence test, exceeding the recommended threshold of 25%, which limits interpretability. Such findings indicate the complex interplay of behavioral and socio-economic variables in HHW recycling. The study highlights the importance of targeted public policies, educational interventions, and infrastructure improvements to increase recycling participation and promote sustainable HHW management in Greece. Full article

Several review studies have addressed the implications of improper waste management on urban livability conditions at large, but we still do not have an overall picture of the link between poor waste management in Sub-Saharan countries and short- and long-term health impacts. Considering that Sub-Saharan Africa is the location of 19 of the 50 biggest dumpsites in the world, it is important to better understand what we do and do not know so far about this public health–waste management link. This study, therefore, provides an overall understanding of health risks associated with improper waste disposal in Sub-Saharan Africa, with a focus on air, water and soil pollution. Employing a systematic review approach, this study utilized academic databases, including PubMed, ScienceDirect, and Google Scholar, to identify and analyze 27 relevant articles, covering eight Sub-Saharan countries. The review was undertaken by categorizing trends and characteristics under themes of solid waste disposal practices, pollution consequences, and reported health problems. The results showed that air pollution, which was the most widely studied in Sub-Saharan Africa, accounted for 155 deaths/100,000 people. Water pollution has led to outbreaks of cholera, typhoid, and diarrhea, especially in communities near waste sites, while contaminated soil poses long-term risks, including for cancer and developmental harm. The findings also revealed that children, waste workers, and communities living near dumpsites are the most vulnerable. Despite growing evidence of harm, gaps remain in our understanding of chronic and long-term effects due to a lack of longitudinal data and inconsistent methodologies to measure health effects. The study also identified inconsistency in distance-based exposure metrics, as studies used varying distances of residents from waste sites to measure health outcomes. Finally, it highlights the urgent need for improved waste infrastructure, clear landfill siting guidelines, and long-term epidemiological studies to inform health-focused waste policies in Sub-Saharan Africa. Full article

Crude oil tank bottoms, a mix of oil, sediment, and water from crude oil collection tanks, are a significant waste stream associated with global oil production. In benchtop testing, as an intermediate evaluative step in progressive scaling toward industrial-scale treatment to recover oil from tank bottoms and to reduce waste, sonication was applied to samples from three California oilfields using only added water and mechanical mixing. The 2.2 kg sample size, over 40 times that used in most of the testing found in literature, utilized no chemicals during treatment. The tests employed a 1:4 ratio of sludge to added water in an acrylic tank with two 500-watt drop-in plate sonicators. Trial variables included sonication frequencies of 28 or 40 kilohertz, mixing speed, treatment duration, and oil collection methods. Oil recoveries up to 98.2% with mean recoveries of 27.4% were achieved using 0.2 kilowatt-hours per kilogram sample. This study indicated sonication treatment of crude oil tank bottoms with only added water and mixing was a potentially viable approach for waste oil recovery and waste minimization and warrants further investigation at even larger scale. Full article

Sewage sludge management remains a critical challenge in Greece, where increasing regulatory pressure, environmental constraints, and limited stakeholder participation complicate regional decision-making. In particular, the revision of regional Waste Management Plans requires decision-support approaches that are both technically robust and socially legitimate. This study develops and applies a participatory, data-driven multi-criteria decision analysis framework to evaluate sustainable sewage sludge management strategies in the Region of Eastern Macedonia and Thrace. The framework combines structured stakeholder participation with quantitative performance assessment, enabling transparent, reproducible, and systematic comparison of alternative sewage sludge management options. Four realistic sludge management alternatives—composting fr agriculture, forestry use, land restoration, and thermal drying with energy recovery were assessed against fifteen economic, environmental, and social sub-criteria. Data were collected through structured questionnaires administered to forty-four representatives from five stakeholder groups: utilities (water and sewerage service providers), local authorities, scientists/experts, end-users, and citizens. Group preferences were aggregated using equal group weighting to ensure balanced representation. The results show that environmental and economic criteria outweigh social aspects. The highest mean weights were assigned to compliance with environmental requirements for products derived from the disposal method (0.105) and compliance with stricter national environmental legislation (0.104), followed by energy intensity (0.097), installation cost (0.065), and operation and maintenance (O&M) cost (0.061). Overall rankings identified composting and thermal drying as the most preferred options, followed by land restoration and forestry use; sensitivity analysis (±10% variation in sub-criterion weights) confirmed ranking stability. The proposed framework enhances decision transparency by embedding measurable criteria and stakeholder inputs within a structured analytical process. From a policy perspective, it addresses participation gaps in Greek waste planning and offers a transferable decision-support tool for future regional planning. Further extensions may include integration with life cycle assessment and cost–benefit analysis to support adaptive updates under circular economy objectives. Full article