| Constructed Rapid Infiltration（CRI）is a new ecological sewage treatment technology improved on the basis of land rapid infiltration system.In comparison to conventional sewage biological treatment techniques,it boasts the benefits of low cost of operation,straightforward process,robust resistance to shock load,and so forth.It is a suitable technology for small and medium-sized towns in our country.However,compared with traditional biological sewage treatment,the CRI system has a large land occupation and poor removal effeciency on total nitrogen（often only 10%-30%）.In view of this,in order to improve the TN removal effeciency of CRI system,this paper will optimize the traditional CRI system in terms of the filler operational conditions.Considering that the paddy soil is rich in microorganisms and easily available in rural areas,a certain proportion of the paddy soil was introduced to the traditional filler of the quartz sand in this study.Furthermore,in addition to focusing on operational conditions such as hydraulic load and carbon-nitrogen ratio,we have noticed that some studies in recent years found that the addition of exogenous iron can improve the efficiency of biological nitrogen removal.Therefore,this study intended to add a certain concentration of Fe3+to improve the denitrification effect of CRI.The whole experiment lasted for 197 days,and the main research results are as follows:（1）Four packing columns were set,namely sole quartz sand（C1）,quartz sand:paddy soil=4:1 mixed filler（C2）,quartz sand:paddy soil=2:1 mixed filler（C3）and sole paddy soil（C4）.The temperature was kept at 25±1℃.The penetration experiments employed Na Cl as the tracer,and the water dispersion coefficient（D）and pore water velocity（U）were ascertained for various packing systems.Then,according to the convection-diffusion equation describing nitrogen transport and transformation,the first order reaction rate constant（Kt）of the nitrification reaction was obtained by combining with the nitrification data.The Kt values of C1,C2 and C3 were 14.27 h-1,21.76 h-1 and 27.23 h-1,respectively.C4 could not carry out the experiment normally due to the rapid blockage.Under the experimental conditions of wet to dry ratio of 1:5,hydraulic load of 0.20 m3/m2/d and 25±1℃,the simulated wastewater was used for influent.After 50 days of uninterrupted operation,the NH4+-N removal rates of C1,C2,and C3 were respectively 55.6%,78.2%,and 86%.The TN removal rates were 50.08%,61.75%,and 69.36%.The experimental results show that the optimized filler CRI system has significant improvement in nitrogen removal efficiency compared with the traditional CRI system.（2）The filler composition with the best nitrogen removal effeciecy was selected.Exploring the influence of hydraulic load and C/N on CRI nitrogen removal was investigated.L1,L2 and L3 columns,respectively corresponding to the hydraulic load of 0.2,0.6 and 1.0 m3/m2/d were set.and run for 50 days.The experimental results show that the average removal efficiencies of NH4+-N in L1,L2 and L3 were 86.00%,75.47%and 71.60%,respectively,and the average TN removal efficiencies were 46.75%,43.66%and 39.03%,respectively.Generally speaking,reducing hydraulic load is beneficial to improve the nitrogen removal.The influent TN concentration was constant,and the influent COD concentration was adjusted to maintain the influent C/N as 10:1,7:1 and 5:1,respectively.Experimental results revealed that the NH4+-N removal rates in R1,R2,and R3 were,on average,85.76%,92.04%and 94.76%,respectively,with corresponding filling columns and running times of 50 days.The average TN removal effficiencies were 57.01%,63.50%and 70.74%,respectively.In general,the higher the influent C/N,the worse the nitrogen removal.It’s speculated that the higher influent C/N condition is conducive to the growth of heterotrophic bacteria with organic matter as substrate,and is not conducive to the nitrification functioned by autotrophic nitrobacteria.The influent Fe3+concentrations were 0 mg/L,5 mg/L and 10 mg/L,respectively.The filler columns T1,T2,and T3 were used,with a continuous running time of 40 days.The average removal efficiencies of NH4+-N and TN for T1,T2,and T3 were 75.02%and 68.67%,87.64%and 72.25%,and 94.94%and 75.95%,respectively.Generally,the addition of a suitable concentration of Fe3+increased the enrichment of denitrification microorganisms in the system,as well as the removal efficiencies of NH4+-N and TN in CRI systems.（3）The samples of whole quartz sand and sand mixture were selected for high-throughput sequencing after periodic operation.The results showed that after continuous operation,the microbial diversity,community richness and evenness of the sand and soil mixed filler were significantly improved compared with the traditional whole quartz sand system.Proteobacteria（Proteobacteria）,Planctomycetes and Nitrospirae were the dominant bacteria in the sand samples.The relative abundance of Nitrospirae,Sphingomonas,Gemmatimonas and other functional bacteria participating in the process of biological nitrogen removal is also improved compared with that of the whole quartz sand system,which also provides a microbial explanation for the difference in nitrogen removal effect between the two systems.（4）At room temperature of 25±1℃,the nitrogen removal performance of the CRI system with optimized filler compostion and operating parameters was verified by treating real sewage.The sole quartz sand system yielded NH4+-N and TN removal efficiencies of 66.35%and 36.38%,respectively.C2 and C3,however,had respective averages of 87.45%and 50.77%,93.81%and 59.24%.The removal efficiency of NH4+-N and TN in treating real wastewater by C3 is more than 20%higher than that of C1.This paper can provide reference for the optimization design of CRI’s filler,and also provide guidance for determining the appropriate operational conditions of CRI. |
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