Treatment effect and carbon emission of rural domestic sewage by biochar-enhanced biofilter

Biochar can be expected to serve as a biofilter filler for wastewater treatment. But it is still stayed in the laboratory stage at present. Only a few reports on the biochar and other filter materials combined to strengthen the pilot biofilter for the treatment of rural domestic sewage. A single removal rate is only considered to evaluate the performance and carbon emission. However, this assessment cannot fully meet the requirements of the new "dual carbon" targets. Taking the biochar-enhanced biological aerated filter as the research object, this study aims to investigate the biofilm start-up time, treatment ability of rural domestic sewage, stability, and daily carbon emission under medium temperature environment (25-30℃). The results showed that the hanging time of the filter was 14 days with the activated sludge inoculation, and the removal rate of the chemical oxygen demand (COD) and ammonia nitrogen (NH₄⁺-N) in the aerated biofilter was stable at more than 90%, while the removal rate of NH₄⁺-N was stable at more than 80%. At the same time, 1-2 mm transparent yellowish-brown biofilm was observed on the suspended sphere filler. When the hydraulic load and influent flow rate were 0.045 m³/(m²·h) and 0.9 m³/d, the optimum influent hydraulic retention time of the system was 8 h, and the average removal rates of chemical oxygen demand, ammonia nitrogen, total nitrogen, and total phosphorus were 95.0%, 79.1%, 62.5%, and 78.4%, respectively; When the influent hydraulic retention time and influent flow rate were 8 h and 1.2 m³/d. respectively, the optimum hydraulic load was 0.030 m³/(m²·h), and the average removal rates of chemical oxygen demand , ammonia nitrogen, total nitrogen, and total phosphorus were 95.3%, 87.4%, 68.1%, and 79.0%, respectively. The effluent concentrations of COD, NH₄⁺-N, TN, and TP fully met the second-level discharge standards in Jiangxi Province's "Discharge Standards for Water Pollutants in Rural Domestic Sewage Treatment and Disposal Facilities" (DB 36/1102-2019). Extracellular polymeric substances (EPS) in different periods were extracted from the suspended spheres and ceramic particles. The contents of EPS were analyzed by UV-vis full-wavelength scanning with a UV-visible spectrophotometer (DR6000). It was found that there was an increase in the EPS content on the suspended spheres and ceramic particles in the different periods, compared with D0. It infers that the biofilter was stable for better sludge activity in the actual operation. The COD was monitored to collect and sort the energy consumption data. The carbon balance equation was established to calculate the overall carbon emission by total organic carbon (TOC). Therefore, the carbon emission of the wastewater discharged by the biological aerated filter was 9.08 kg/d under the optimal hydraulic retention time of 8 h and an inlet flow rate of 0.9 m³/d. Therefore, the biochar-enhanced biological aerated filter can provide a high application prospect to treat rural domestic sewage.