| The nitrogen pollution of rural domestic sewage seriously restricts the improvement of rural water environment.In the actual treatment process,rural domestic sewage often has insufficient carbon sources in the influent,resulting in poor denitrification effect.Therefore,an external carbon source is often required for efficient biological denitrification.However,both liquid carbon sources and single solid carbon sources have their own limitations,such as the rate of carbon release is too fast,difficult to control the dosage,prone to cause secondary pollution and high cost,etc.For these reasons,slow-release carbon sources have become a research focus both in China and abroad.Since economics,operation and maintenance,etc.are limited in rural areas,it is more urgent to develop an economical and effective slow-release carbon source for rural areas.In this study,the raw materials were firstly compared and optimized;secondly,the preparation parameters were optimized to prepare a new composite slow-release carbon source;thirdly,the physical properties and carbon release mechanism of the composite carbon source were analyzed;finally,the static nitrification effect of the composite carbon source was investigated,and the composite carbon source was used as a filler in the denitrification biological filter to observe its enhanced denitrification effect on simulated rural domestic sewage.The main results are concluded as follows:（1）Through multi-angle comparative analysis,it was found that corncob,rice husk,and reed stalk were superior to other natural materials in terms of carbon release,nitrogen and phosphorus release,leachate C/N,leachate color,and biomass composition;polyvinyl alcohol-sodium alginate（PVA-SA）,an artificial composite framework,met the requirements of denitrification carrier for the compressive strength,and its carbon release and mass transfer properties were better than other framework materials.（2）By combining the Plackett-Burman method and the response surface method（RSM）,with the maximum carbon release as the target value,the Design-Expert software was used to obtain the optimal preparation parameters:8.64 g PVA and 2.41 g SA were dissolved in 100 m L of pure water,and heated and melt at 95℃for 3 h.After cooling,PVA-SA was mixed with 3.82 g rice husk powder,4.47 g corncob powder and6.06 g reed stalk powder,and frozen at-10℃for 19.11 h for molding.The mold was put into 2%Ca Cl2-H3BO4 saturated solution,permitting to cross-link at 4℃12 h to prepare a composite slow-release carbon source.（3）Measuring the physical properties of the composite carbon source indicated that the bulk density,the apparent density and the porosity were 0.42 g/cm~3,1.63 g/cm~3and 74.54%,respectively.Fitting the carbon release mechanism of the composite carbon source with the model revealed that the carbon release process was conformed to the first-order model and also to the Higuchi model,with diffusion and skeleton dissolution in the fast stage and Fick diffusion in the flat stage.The average carbon release from the composite carbon source was 6.20 mg/g·d,which was between natural materials and framework materials.（4）Through the static denitrification test,it was found that the maximum nitrate（NO3--N）removal rate of the composite carbon source was 90.8%,much higher than30.8%in the sludge control group and 43.9%in the PVA-SA group.The maximum denitrification rate of the composite carbon source was 0.079 mg/g·h,while the sludge control group was only 0.026 mg/g·h and the PVA-SA group was only 0.037 mg/g·h.These results also confirm that microbes are able to utilize carbon source to reinforce denitrification effect.The enhanced denitrification effect of the denitrification filter used the composite carbon source as a filler was further analyzed in the simulated rural domestic sewage.When the influent NO3--N was 20 mg/L and the hydraulic retention time（HRT）=3 h,the NO3--N removal rate reached the maximum value of 89.1%.At this time point,the accumulation of nitrous nitrogen（NO2--N）was 0.10 mg/L,and the total nitrogen（TN）in the effluent was 2.40 mg/L,demonstrating that the denitrification was relatively complete.In conclusion,by the reasonable selection of raw materials and scientific optimization of the preparation parameters,a new natural-artificial composite slow-release carbon source for water denitrification was developed.Also,it should be noted that the preparation cost is very low,only 6.33 yuan/kg.This will greatly reduce the cost resulted from addition carbon sources.The high denitrification efficacy and low cost demonstrate that the composite slow-release carbon source can be widely used in rural domestic sewage to solve poor denitrification effect due to insufficient carbon sources.Also provides a new idea and practical significance for solidification of added carbon sources and denitrification of rural domestic sewage. |
