Characteristics Of Fecal Sewage From Rural Latrines In China And Mechanisms Of Pollutant Migration And Transformation During Synergy-Resource Processes

The collection and treatment of fecal sewage（FS）from rural latrines represents a significant weakness in global environmental health,especially in developing countries where fundamental sanitation infrastructure is lacking.In these locales,FS discharge from rural latrines is typically uncontrolled.Due to the small and dispersed production of latrine FS,there is currently a lack of focus on effective collection and centralized treatment of FS in rural areas,as well as insufficient related research.This leads to the direct exposure of FS to the environment,which can easily cause widespread contamination of water sources and soil.Additionally,the enrichment and transformation of FS pollutants in the environment also increase the risk of various infectious diseases related to feces.Therefore,stemming from the strategic goal of improving the quality of the rural ecological environment in our country,this paper will comprehensively study FS characteristics from rural latrines in China,undertake safe processing and resource utilization of FS,and concurrently track and analyze the migration and transformation patterns of pollutants during treatment processes.This endeavor aims to provide scientific guidance for the resource utilization of latrine FS and disaster risk assessment,which is of significant insights for the formulation of national public health and environmental management policies.This study first conducted a nationwide sampling,analyzing 65 fecal sewage（FS）samples from 27 provincial administrative regions to identify the sources,characteristics,and influencing factors of FS pollutants in rural latrines,in addition to the contamination levels of pathogenic microorganisms.On this basis,mesophilic sequencing batch anaerobic digestion and static aerobic composting techniques were used to simulate the co-treatment of FS,elucidating the mechanisms of pollutant migration and transformation during resource recovery processes.The main conclusions of this study are as follows:（1）Latrine FS is rich in nutrients such as nitrogen,phosphorus,and potassium,offering significant potential for resource utilization.However,FS also contains conventional pollutants（CPs）like heavy metals and organic compounds,as well as emerging pollutants（EPs）such as antibiotics,antibiotic resistance genes（ARGs）,and microplastics（MPs）.Compared to Southern and Northwestern China,pollutant concentrations in rural latrine FS are higher in Northern China.The distribution of FS pollutants is mainly influenced by geographic location,latrine type,local living standards,and dietary habits.FS from biogas digesters shows a high abundance of specific ARGs and plasmids.（2）Using structural equation modeling to investigate driving factors of ARGs in latrine FS,it was found that ARGs changes in septic tank FS were primarily affected by mobile genetic elements（MGEs）（P<0.05）.ARGs were significantly influenced by MGEs,antibiotics,and pathogenic bacteria in biogas digester FS（P<0.01）;and ARGs variations in cesspit FS were notably impacted by MPs（P<0.05）.Research on MPs suggests that feces,rural domestic sewage,and toilet paper are the main sources of MPs in latrine FS.MPs abundance is significantly related with FS physicochemical properties,microbial community composition,and resident medical care spending.FS agricultural reuse can annually transfer approximately 7.8 × 10³ to 5.6 × 10⁴ tons of MPs to cropland.（3）Proteobacteria are the most dominant phylum in latrine FS.Seven core genera were identified in FS samples,accounting for 25.45% of the total genuslevel bacterial abundance.Compared to the northern and northwestern regions,Acinetobacter showed an apparently higher relative abundance（P=0.0025）in southern FS,whereas Bifidobacterium demonstrated a significantly reduced presence（P=0.0017）.At the species level,363 pathogenic species were identified,with an average relative abundance of 17.618%,predominantly composed of animal pathogens（16.664%）.The relative abundance of pathogens,whether affecting animals,plants,or being zoonotic,was higher in FS samples from biogas digesters than in those from septic tanks and cesspits.Furthermore,the microorganisms in biogas digester and cesspit FS were enriched in metabolic pathways associated with human diseases,highlighting potential health risks.（4）When FS and food waste（FW）are mixed in equal proportions,and the dry matter mass of the mixed substrate equals that of inoculated sludge,microbial activity and metabolic processes are enhanced,resulting in optimal performance of the co-anaerobic digestion system.The addition of wheat straw biochar prepared at 350°C significantly enhances the energy efficiency of the anaerobic system.This biochar retains a substantial amount of organic matter and functional groups,exhibiting excellent adsorption properties.By increasing surface redox activity and direct interspecies electron transfer capability,CH₄ yield is boosted to 215.16 m L g^-1 VS^-1,VFA degradation rate reaches 69.13%,and heavy metal adsorption is more sustained.（5）Adding biochar helps stabilize the physicochemical conditions and microbial communities during the co-anaerobic process of FS and FW.An addition of 5% biochar is considered optimal,as it not only enhances the buffering capacity of the anaerobic system but also provides more ecological niches for microorganisms,thereby improving CH₄ yield,VFA degradation,and ARG reduction.By reducing the relative abundance of persistent ARG hosts,the abundance of zoonotic pathogens and ARGs is effectively lowered.After 12 hours of static aerobic treatment post-anaerobic digestion,the slurry exhibited a significant reduction in various CPs and specific ARGs abundance,especially vancomycin ARGs（43.48%-62.72%）.However,aerobic conditions slightly promote an increase in the abundance of zoonotic pathogens（0.34%-0.64%）.（6）Static aerobic composting techniques were employed in the cocomposting of FS and FW,demonstrating that the release of CO₂ and N₂ O was primarily concentrated in the initial composting stage.This phenomenon can be attributed to enhanced microbial carbohydrate metabolism,increased activity of denitrifying bacteria,and high content of nitrate nitrogen in the compost.Metagenomic sequencing revealed the functional contributions of species,indicating that the nos Z gene is more functional in static aerobic composting.Lower nos Z gene abundance or nos Z/nir K ratios lead to higher N₂ O production.Additionally,Spearman correlation analysis showed that N₂ O emissions during the composting were closely related to moisture,p H,temperature,and the abundance of nir K and nos Z denitrifying genes.