The Yi River, the second longest river in Shandong Province, China, flows through Linyi City and is fed by three tributary rivers, Beng River, Liuqing River, and Su River in the northeastern part of the city. In this study, we determined the concentrations of five heavy metals (Cr, Ni, Cu, Zn, and Pb) in water, sediment, and aquatic macrophyte samples collected from the junction of the four rivers and evaluated the potential ecological risk of heavy metal pollution. Most of the heavy metals in water were in low concentrations with the water quality index (WQI) below 1, suggesting low metal pollution. The sediments showed low heavy metal concentrations, suggesting a low ecological risk based on the potential ecological risk index (RI) and the geo-accumulation index (I_geo). The aquatic plant species Potamogeton crispus accumulated considerable amounts of heavy metals, which were closely related to the metal concentrations of the sediment. The plant species Salvinia natans also showed an excellent metal accumulation capability. Based on our results, the junction of the four rivers is only slightly polluted in terms of heavy metals, and the plant species P. crispus is a suitable bioindicator for sediment heavy metal pollution.

Two floating treatment wetlands (FTWs) in experimental tanks were compared in terms of their effectiveness on removing nutrients. The results showed that the FTWs were dominated by emergent wetland plants and were constructed to remove nutrients from simulated urban stormwater. Iris pseudacorus and Thalia dealbata wetland systems were effective in reducing the nutrient. T. dealbata FTWs showed higher nutrient removal performance than I. pseudacorus FTWs. Nitrogen (N) and phosphorous (P) removal rates in water by T. dealbata FTWs were 3.95 ± 0.19 and 0.15 ± 0.01 g/m²/day, respectively. For I. pseudacorus FTWs, the TN and TP removal rates were 3.07 ± 0.15 and 0.14 ± 0.01 g/m²/day, respectively. The maximum absolute growth rate for T. dealbata corresponded directly with the maximum mean nutrient removal efficiency during the 5th stage. At harvest, N and P uptak of T. dealbata was 23.354 ± 1.366 g and 1.489 ± 0.077 g per plant, respectively, approximate twice as high as by I. pseudacorus.

In order to improve the performance of methane production from agro-waste, a batched four-stage–two-phase anaerobic digestion (4S2P-AD) system was designed to combine the advantages of both anaerobic co-digestion (co-AD) and two-phase AD. The initial separation of two phases was performed using rice straw (RS) as a feedstock in acidogenic phase and cow manure (CM) in methanogenic phase at low inoculum/substrate (I/S) ratios of 0.5 and 0.2 and a high organic loading of 60 g volatile solid (VS)/L. The periodic round-trip reflux of leachate during the 4S2P-AD process facilitated re-inoculation throughout the four stages. The results showed that this round-trip reflux also dispersed toxic ammonia, balanced the carbon/nitrogen ratio, unified the microbial community structure, and led to the selection of Methanosarcina (relative abundance > 80%) as the dominant methanogens. With the abilities to overcome volatile fatty acid accumulation, shorten lag times, improve biodegradability, and foster synergistic effects, it was verified that the 4S2P-AD process can maintain efficient and stable methanogenesis from high-solid lignocellulosic feedstock. The averaged methane production throughout the four stages of 4S2P-AD was 234 mL/g VS. This result is 96% higher than the averaged methane production obtained from the four one-step AD groups using mono-feedstock, and 91% higher than that obtained using co-feedstock. This study provides a scientific reference for the development of new processes of bio-methane production from agro-waste with a high fermentation capacity and stability in the future.

In order to alleviate the inhibition of sodium ions and phenols on methanogenesis by anaerobic digestion of sweet sorghum bagasse pretreated with alkaline hydrogen peroxide, zeolite and trace elements (TEs) were added to sweet sorghum bagasse slurry (PSSBS) for biogas production. The influence of zeolite dosage and TE on the anaerobic digestion performance was revealed from the perspective of biogas production, as well as the cellulase and dehydrogenase activities. The results showed that zeolite and TE increased methane production and shorten the lag time. The maximum methane yield of 274.5 mL/gVS from PSSBS + 5 g/L zeolite + 1 mL TE could be obtained and 58.4% higher than that of untreated sweet sorghum bagasse. Addition of zeolite and TE was beneficial to increase the activities of cellulase and dehydrogenase of the digestate. This work would provide a theoretical reference for the resource utilization of sweet sorghum bagasse for clean industrial application in future.

The global fish industry is an essential contributor to food security and the economy; however, it produces considerable quantities of fish waste. Depending on species and processing methods, 20–80% of fish biomass is discarded as waste, amounting to almost 64 million tons annually. Conventional disposal methods such as landfilling, incineration, and wastewater discharge cause environmental pollution and greenhouse gas emissions. Therefore, for sustainable development, it is important to valorize fish waste into renewable energy and value-added products. Hence, this review aims to encourage the technical development of fish waste into bioenergy and bioresources by thoroughly studying key technologies for converting it into biogas, biodiesel, fertilizer, animal feed, and biochar. Although several studies have explored these technologies, most remain disjointed and lack a systematic assessment. Therefore, this work attempted to critically evaluate the technological principles, advantages, limitations, and optimization strategies of current valorization approaches. Furthermore, several recommendations have been proposed, including conducting pilot-scale trials, producing higher value-added products within a biorefinery system. Overall, this review provides a holistic outlook on the energy and resource conversion of fish waste, supporting sustainable waste management.