Converting biowaste into biochar and incorporating it into soil is recognized as an effective method for carbon (C) sequestration. However, biochar inevitably undergoes an aging process in soil, which influences its stability, and ultimately threatens its carbon sequestration ability. This study selected CaCl₂ as an exogenous additive of sewage sludge and bone dreg for producing Ca-rich biochars, and both surface C and bulk C stability in them were investigated under three aging processes (dry–wet aging, freeze–thaw aging, and natural aging in farmland soil). The results showed that the above three aging processes resulted in oxidized surface C in Ca-rich biochar decreasing by 10~23%, 28~41%, and 0~74%, respectively, compared with that in pristine biochar, while oxidized bulk C decreased by 6~10%, 0~1%, and 0~35%, respectively. This was attributed to the “protective effect” of Ca-containing crystals on the biochar surface, including CaO, Ca₅(PO₄)₃Cl, Ca₅(PO₄)₃(OH), Ca₈H₂(PO₄)₆·H₂O, and Ca₁₀(PO₄)₆(OH)₂, which intervened in the oxidation of C–C/C=C in biochar but also reduced the generation of a fragmented C structure. By comprehensively assessing surface C and bulk C stability under three aging processes, final C sequestration in Ca-rich biochar increased to 27~80%, compared to that in pristine biochar (23~74%). Therefore, Ca-rich biochar is more dominant than pristine biochar, considering C sequestration potential during long-term aging in soil.

In this study, urban wastewater was treated by flocculation, ozone pre-oxidation and denitrification for efficient purification. Polymeric aluminum chloride (PAC) and polymeric aluminum ferric sulfate (PAFS) were added to the wastewater at different levels to remove the COD, turbidity, TP and TN of the wastewater. A better flocculant was selected and its optimum ozone pre-oxidation concentration was determined by changing the ozone concentration and measuring the effluent quality. Denitrification was further enhanced by varying the C/N ratio of the wastewater. The results show that, with the increase in flocculant dosage, the removal rates of COD, turbidity, TP and TN by PAC and PAFS were improved. The purification effect of PAC was better than that of PAFS and the optimum removal of COD, turbidity, TP and TN was obtained at a dosage of 80 mg L⁻¹ by PAC, at 55.9%, 55.6%, 90.0% and 13.3%, respectively. Ozone pre-oxidation enhanced the removal of COD, turbidity and TN by PAC, and the optimal ozone dosage was 1.2 mg L⁻¹, which resulted in 64.8%, 57.1% and 24.8% removal of COD, turbidity and TN, respectively. With the increase in the C/N ratio from 2.0 to 4.0, the NO₃⁻-N concentration of PAC-treated water gradually decreased, but when the C/N ratio was 4.0, the COD concentration increased, so the optimal C/N ratio should be 3.5. Overall, the combination of ozone pre-oxidation, denitrification and flocculation was an effective method to treat urban wastewater, which has a strong application prospect.

With the increasing frequency of desilting in urban drainage systems, the safe disposal and resource reuse of sewer sediments have become a prominent practical challenge. Screened sand, the most promising component for resource recovery from sewer sediments, still lacks systematic insight into its pollution risks and the necessity of pretreatment. In this study, 120 raw sewer sediment samples were collected from sanitary, storm, and illicitly connected (IC) storm sewers in Shanghai, alongside seasonal screened sand samples. We systematically characterized their physicochemical properties and heavy metal and antibiotic pollution profiles, and evaluated the purification performance of ultrasonic treatment, sodium hexametaphosphate (SHMP) washing, and their coupled processes. Results revealed significant differences in sediment properties across pipeline types. Screened sand, dominated by SiO₂ and CaO, shows preliminary potential for reuse as a low-grade bulk building material, but its organic loss on ignition (LOI) of 5.29–13.42% exceeded the reuse limit. Concentrations of heavy metals and antibiotics were generally higher in screened sand than in raw sediments, with further enrichment in the fine sand fractions, indicating that screening only redistributed contaminants rather than eliminated them. The coupled ultrasonic–SHMP process, applied for the first time to screened sand from sewer sediments, achieved optimal performance, with a maximum LOI reduction and over 85% removal of certain antibiotics, without damaging the sand’s mineral skeleton. This study provides a scientific basis for the safe resource reuse of screened sand.