Removal and recovery of uranium (VI) from aqueous solutions by residual sludge and its biochars

The removal and recovery of uranium (VI) from water solutions are critical for energy and environmental security. In this study, hydrochar at 100, 150, and 190 °C (HC100, HC150, and HC190) and pyrochar at 250 °C (BC250) were prepared from residual sludge (RS). The uranium (VI) adsorption behavior, recovery, and heavy metal risk of RS and its biochars were assessed. The sorption distribution coefficient of RS was higher than those of its biochars within the tested concentration range. The maximum adsorption capacity of uranium (VI) by HC190 was 121.26 mg/g at acidic pH (pH 4.5), which was higher than those of other tested biochars, previously reported unmodified biochars, and activated carbon. The zeta potential, FTIR, and XPS results implied that the adsorption of uranium (VI) by RS and its biochars was regulated by electrostatic attraction and the complexation with oxygen- and phosphorus-containing functional groups. Besides, partial reduction of uranium (VI) into uranium (IV) happened during the process of adsorption. More than 86% of the adsorbed uranium (VI) was recovered by 0.01 M hydrochloric acid and 100% by 0.01 M sodium carbonate. The leaching amount of heavy metals was greatly reduced after the sludge was converted to biochar, indicating that hydrothermal carbonization and pyrolysis can promote the stabilization of heavy metals. This work demonstrates that RS and its biochars can be implemented as low-cost, environment-friendly, and high-efficient materials for the purification of uranium (VI)-containing solutions by means of adsorption and desorption.

Effects of potassium ferrate-walnut shell pretreatment on dehydration performance of residual sludge

Sludge dehydration is not only the first process of sludge reduction but also difficult for sludge treatment and disposal, while the high moisture content of sludge would also limit its resource utilization. In this paper, the residual sludge was treated by the strong oxidizing property of potassium ferrate (PF), and walnut shells were used as a skeleton builder to obtain the recycling dehydrated sludge. It also provides a new solution for the poor stability of PF in the treatment of sludge and waste walnut shells. The experiment results showed that the optimum dosage of PF and walnut shell was 60 mg/gDS and 0.8 g/gDS, respectively, and the water content of the combined PF and walnut shell treatment decreased by 5.2% and 3.7% compared to that of PF conditioning alone and walnut shell conditioning alone, respectively. In addition, scanning electron microscopy and three-dimensional fluorescence spectroscopy revealed a large number of cracks on the sludge surface after the combined treatment, and the sludge floc structure became more loose and dispersed, and the hydrophobic substances humic acid and fulvic acid increased, indicating that the dewatering performance of the sludge was effectively improved, further confirming that the combined PF and walnut shells treatment provides a new idea and method for sludge dewatering.

Metagenomic analysis revealed the microbiota and metabolic function during co-composting of food waste and residual sludge for nitrogen and phosphorus transformation

This paper used bagasse as a composting additive and bulking agent in order to investigate the aerobic composting process of food waste and residual sludge. Accordingly, the variations of nitrogen and phosphorus nutrients, microbiota and metabolic function during the composting process were systematically explored. Three piles with residual sludge, food waste and bagasse mass ratios of 1:1:1, 2:1:1 and 4:1:1 were set. The ammonia nitrogen content in the three compost piles were 3.18 mg/g, 4.68 mg/g and 5.84 mg/g at the end of composting. The final available phosphorus content of the three piles were 3.42 mg/g, 6.70 mg/g and 11.21 mg/g, respectively. X-ray photoelectron spectroscopy (XPS) analysis showed that absorption peaks attributed to amines, amino acids and amides appeared in the 1:1:1 pile. Metagenomic analysis of the glycolysis and ammonia transformation pathways showed that the total relative abundance of key enzyme genes for the conversion of glucose to glucose-6-phosphate in the three plies were 0.326%, 0.213% and 0.248%, respectively. The total relative abundance of 2 glutamate dehydrogenase (GDH2), glud1-2 and E1,4,1,4 dehydrogenases in the three piles was 0.125%, 0.151% and 0.160%, respectively, as the main enzymes for the mutual conversion of ammonia and glutamate.

Protein extraction from excess sludge by alkali-thermal hydrolysis

The protein in excess sludge can be extracted effectively by the alkali-thermal method, and the extracted protein can be used as a foaming agent and in other industrial raw materials to realize its resource utilization. In this paper, the factors influencing sludge protein extraction by the alkali-thermal method were optimized based on the protein extraction rate and the polypeptide content, which determine the foaming performance of the extracted protein. The results showed that the optimal conditions were a pH of 12, a temperature of 120 °C, a reaction time of 4 h, and a sludge moisture content of 92%. Under these optimized conditions, the extraction rate of protein and the concentration of polypeptides were 88.3% and 6599 mg/L, respectively. Additionally, the foaming performance of the extracted protein solution was tested, and the foamability and foam stability were close to 450% and 88.8%, respectively. Therefore, the sludge protein extracted by the alkali-thermal method can meet the relevant standards of foam extinguishing agents and concrete foaming agents in China. In addition, the dewatering performance of the hydrolyzed sludge was improved by 93.1%, which provided favorable conditions for the subsequent separation of the protein solution.

Mesophilic anaerobic co-digestion of residual sludge with different lignocellulosic wastes in the batch digester

Co-digestion of residual sludge (RS) and different lignocellulosic wastes (LWs) including greening waste (GW), decocted Chinese herbs waste (DCHW) and sugarcane bagasse waste (SCBW) was investigated in batch digester. Results show that the application of GW presented the highest specific methane yield (161 mL CH₄/g VS_added) due to its high carbohydrate fraction and more balanced C/N ratio in co-substrate mixture. Buswell equation was applied and it is found that biodegradability index (BI) for co-digestion varied from 68.1% to 74.2% (53.0% for RS mono-digestion) depending on the lignin fractions of the LWs. Variation of pH, VFAs, alkalinity and ammonia throughout the digestion were also examined. The addition of LWs induced VFAs formation, as well as their conversion to methane. The higher microbial diversity in RS/LWs co-digestion further confirmed the positive effect of LWs addition in co-digestion.