One of the most appropriate methods of utilizing different types of waste is using these wastes as alternative fuels in cement factories and similar enterprises. In this paper, the impacts of the use of waste mixtures as alternative fuels in cement plants on process parameters, technological availability, costs, kiln design, quality of alternative fuels, calorific value, clinker quality, and the environment were reviewed based on the available literature. To contribute to the environmentally sensitive and feasible activities of cement enterprises and other stakeholders affected by waste, this study analyzed the sustainable contributions of using different waste mixtures as alternative fuels in cement factories through their combined processing. For this purpose, sustainability criteria for the use of wastes as alternative fuels were established, and the contribution of these waste mixtures to these criteria was determined. The literature reviewed supports the use of these wastes as alternative fuels in cement factories due to their environmental benefits, especially through waste, greenhouse gas, and fossil fuel reduction, as well as their benefits such as cost reduction and feasibility in enterprises in a sustainable scope. The results show that, compared to other waste groups, plastic waste, biomass waste, wood waste, tire waste, and wastewater sludge, in combination with other waste mixtures, provide more evidence of sustainability criteria. Furthermore, the evidence indicates that, compared to other criteria, the feasibility of using waste groups as alternative fuel is assessed by focusing on different percentages of mixtures, environmental remediation, the physical and chemical conditions of the waste and factors affecting its combustion, and the energy content of the waste. This study serves as a guide for investors, waste providers, municipal and industrial operators, all stakeholders affected by waste, countries, and policymakers targeting the use of alternative fuels in cement factories.
Abstract This study investigates the optimization of microalgae harvesting through an ultra-fine bubble (UFB) flotation system. Microalgae possess high potential for carbon capture and biofuel production, but the harvesting stage remains a critical bottleneck in large-scale commercialization due to high costs and inefficiencies in traditional techniques. This research develops and evaluates a UFB-assisted flotation method aimed at improving microalgae recovery efficiency. The novelty of this study lies in the integration of nanometric ultra-fine bubbles (10–250 nm), systematic chemical screening, and response surface methodology (RSM)-based optimization for microalgae harvesting. Unlike conventional flotation systems that typically employ microbubbles in the micrometer range, the developed UFB system generates highly stable nanobubbles, thereby enhancing bubble-microalgae interactions and harvesting performance. A custom-designed UFB generator was used to produce nanobubbles with an average diameter of 30–70 nm, demonstrating strong stability and uniform distribution. The freshwater microalga Chlorella sp . was cultivated and subjected to flotation tests under varying conditions of pH and activator (polyaluminum chloride, PACl) dosage. Key operational parameters were screened through jar tests and further optimized using a central composite design integrated with RSM. Results indicated that PACl outperformed alum in enhancing microalgal recovery, regardless of whether collectors (sodium oleate (NaOl) or cetyltrimethylammonium bromide) was co-applied. Regression analyses yielded second-order polynomial models that accurately predicted the recovery efficiency for OD₆₈₀, turbidity, chlorophyll-a, and biomass, with R² values exceeding 0.80. Optimal recovery was achieved at 23 mg L -1 PACl and pH 5.3, with observed efficiencies reaching up to ≈ 100%. Validation tests confirmed model reliability, with predicted values within 10% of experimental results. The study concludes that UFB-assisted flotation is a promising, cost-effective approach for efficient microalgae harvesting and can facilitate the development of microalgae-based carbon capture and biofuel production systems.
Abstract Digestate, the solid by‑product of anaerobic digestion, can cause nutrient leaching, greenhouse‑gas release and heavy‑metal accumulation. Slow pyrolysis offers a promising route for upgrading digestate into biochar, yet its high energy demand and uncertain economics must be carefully weighed. This study evaluates the functional unit of 1 t solid digestate, the conversion of digestate to biochar at a pig farm in Pingtung, Taiwan, and compares the results with composting and incineration. Life‑Cycle Assessment and Techno‑Economic Assessment covered five scenarios: composting (S1); solid digestate pyrolysis at 500 °C (S2), 600 °C (S3) and 700 °C (S4); and incineration (S5). Because S2 required the least process energy, it yielded the lowest global-warming potential of 67.7 kg CO₂-eq t⁻¹. In contrast, S4 produced higher-value biochar and showed the strongest economic performance, with a positive Net Present Value of USD 371,480 and a payback period of 11 year under the scenario without carbon fees. Sensitivity tests (± 20% capital expenditure, operating expenditure and benefits) confirmed S4’s economic resilience, while S2–S3 were vulnerable to market shifts. A Technique for Order Preference by Similarity to an Ideal Solution multi-criterion ranking placed S4 first under the stakeholder-derived weighting scheme, mainly because of its stronger economic performance and product value. These results suggest that digestate-to-biochar conversion can improve digestate management, but the preferred pyrolysis temperature depends on the decision priority. In this case, 500 °C showed stronger environmental performance, whereas 700 °C provided greater economic potential when high-value biochar use and energy recovery were assumed.
Abstract Wastewater treatment plants are emission hotspots of airborne antibiotic resistance genes, yet the physicochemical drivers of their amplification remain unclear. Here, we quantified airborne antibiotics and oxidative potential (OP) in particulate matter above an aeration tank and a nearby reference site, linking these measurements to resistome profiles. Among the eleven targeted antibiotics, only erythromycin was detected in aerosols (250 ± 64 pg m− 3). Rapid droplet evaporation can concentrate dissolved antibiotics within aerosol microenvironments; however, the resulting aqueous-equivalent concentrations depend on aerosol water content and were not directly measured in this field campaign. Volume-normalized OP showed no site differences, but erythromycin abundance showed an exploratory positive co-variation with mass-normalized OP, suggesting co-variation between antibiotic loading and particle redox activity. Correspondingly, macrolide resistance genes were enriched in aerosols, while sulfonamide resistance genes showed no enrichment, reflecting the absence of sulfonamide partitioning. The dominance of stress-tolerant taxa such as Pseudomonas and Streptococcus is consistent with survival under atmospheric stressors at the aeration area. Together, these findings provide an exploratory framework in which aeration-driven aerosolization can concentrate dissolved constituents and co-occur with redox-active particulate matter, potentially shaping the airborne resistome. These results motivate expanded monitoring and mechanistic studies that explicitly include the air compartment when assessing environmental transmission of antibiotic resistance.
Practical guidance for selecting plant-substrate combinations in constructed wetlands treating polluted urban waters is limited. This study employed a simulated pollution experiment to assess the purification capacity and net contribution of total nitrogen (TN), total phosphorus (TP), ammonia (NH₄⁺), and chemical oxygen demand (COD) in polluted waters using five commonly used aquatic plants in Beijing, namely Phragmites australis, Typha orientalis, Iris pseudacorus, Scirpus validus, Lythrum salicaria, combined with five substrates including crushed stone, gravel, volcanic rock, ceramic granule, and zeolite. The results showed that: (1) The 25 plant-substrate combined systems effectively removed pollutants, with average removal efficiencies of TN, TP, NH₄⁺ and COD exceeding 63, 21, 79 and 82
