As a by-product of lignocellulosic depolymerization for furfural production, furfural residue (FR) is composed of residual cellulose, lignin, humic acid, and other small amounts of materials, which have high reuse value. However, due to the limitation of furfural production scale and production technology, the treatment of FR has many problems such as high yield, concentrated stacking, strong acidity, and difficult degradation. This leads to the limited treatment methods and high treatment cost of furfural residue. At present, most of the furfural enterprises can only be piled up at will, buried in soil, or directly burned. The air, soil, and rivers are polluted and the ecological balance is destroyed. Therefore, how to deal with furfural residue reasonably needs to be solved. In this review, value-added products for furfural residue conversion are described in detail in the fields of soil culture, catalytic hydrolysis, thermal decomposition, and porous adsorption. The future studies reporting the FR to convert value-added products could find guidance from this review to achieve specific goals.

With the rapid development of large and medium-sized biogas projects, the high-value utilization of anaerobic fermentation residues has become a hot spot in recent years. In this study, biogas residue from biogas engineering was used as composting raw material, and 0 (CK), 2.5% (T1), 5.0% (T2), 7.5% (T3), and 10.0% (T4) biochar was added to investigate its effects on physico-chemical properties, microbial populations, and maturity degree during the aerobic composting process. Results show that the addition of biochar shortens the time (3 days) to reach the high-temperature period, increases the composting temperature (63.8 °C) and germination index (GI), decreases the electrical conductivity (EC), reduces the loss of C and N elements, and increases the microbial population during composting. These results suggest that biochar can improve the maturity and fertility of compost products, and significantly regulate the structure and function of microbial communities during the composting process.

In this study, biogas residue fermented by biogas engineering was used as compost raw material, and different quality biochar was added in the composting process to explore the effect of biochar on the transformation of heavy metals in the composting process. The composting process was comprehensively analyzed with the potential ecological risk assessment of heavy metals and redundancy analysis. The addition of 10.0% biochar during composting had a strong passivation effect on exchangeable Cu and Cd, with passivation rates of 11.75 and 63.89%, respectively; the addition of 2.5 and 7.5% biochar had strong passivation ability for exchangeable Zn and Pb, and the passivation rates were 15.26 and 45.02%, respectively. At the end of composting, the potential ecological risk indexes of each treatment were T4 (10.0% biochar) > T3 (7.5% biochar) > T2 (5.0% biochar) > T1 (2.5% biochar) > CK (no biochar added). The risk of heavy metal pollution during the aerobic composting of biogas residue was low, which significantly reduced secondary pollution during the composting process.

Reflux of biogas slurry is an effective way to reduce the discharge of wastewater. In order to improve the utilization efficiency of reflux fluid and reduce ammonia inhibition in an anaerobic digestion (AD) system, biogas slurry was pretreated by microfiltration membrane and biochar adsorption. In this study, the suspension solid (SS), biochemical oxygen demand (COD) and ammonia nitrogen (NH₄⁺-N) were investigated, as well as the gas production effect of the reflux of biogas slurry in AD, so as to evaluate the filtration effect of the microfiltration membrane with different pore sizes and the adsorption effect of biochar with different dosages. The results showed that 0.65 μm microfiltration had the best interaction effect and 7 g/L biochar had the best adsorption effect. The results of anaerobic co-digestion of the biogas slurry and corn stover showed the peak gas production of the pretreated reflux fluid was advanced by 1 day, and the maximum daily methane production and maximum cumulative methane production reached 39.20 mL·g⁻¹ VS and 137.14 mL·g⁻¹ VS, respectively. These results indicated that the combined treatment of biogas slurry by microfiltration membrane and biochar could have potential applications for the treatment and recycling of biogas slurry.

Development and verification of the fluid model for organic degradable maize straw seedling raising bowl