Study On Rural Domestic Wastewater Treatment In Soil Infiltration Treatment Systems By Biochar And Intermittent Aeration Strengthening Measures

In recent years,the eutrophication of rivers and lakes have still very serious in China.Especially in rural areas,the domestic sewage is discharged into the nearby lake or stream without any treatment which lead to deterioration of water quality.Therefore,it is very important to explore a suitable technology for domestic wastewater treatment.At present,Soil Infiltration Treatment system?SIT?,an artificially enhanced sewage land-treatment technology,has been applied to domestic sewage treatment in rural areas.Nevertheless,an effective and sustainable nitrogen removal is still a major challenge for conventional SIT.Therefore,the aim of this study was to access the influences of the enhancing strategies including intermittent aeration and biochar addition on the nitrogen transformation in SIT.Four parallel laboratory-scale SITs were constructed,including conventional,intermittent aeration with non-biochar,non-aerated with biochar,intermittent aeration with biochar.we studied the long-term decontamination efficiency and N₂O emission characteristics of different SITs.Meanwhile,we explored the effect of different process parameters including aeration,hydraulic loading rate and COD/TN ratios on removal performance.The decontamination mechanism of biochar combined intermittent aeration SITs was further studied.The main conclusions are as follows:?1?Biochar combined intermittent aeration SITs could effectively removal COD,NH₄⁺-N,TP in this reaearch.The average removal efficiencies of COD,NH₄⁺-N and TP exceeded 90%,95%and 98%in all SITs.The removal efficiencies of TN were79.14%,68.74%,92.72%,and 85.44%in the non-aerated and non-biochar SIT,intermittent aeration SIT,non-aerated with biochar SIT and intermittent aeration with biochar SIT,respectively.The average N₂O emission fluxes in the non-aerated and non-biochar was 99.00 ug m^-2 h^-1,and obviously higher than other systems(30.56-44.17 ug m^-2 h^-1).In addition,there was no significant difference in N₂O emission flux between non-aerated with biochar SIT and intermittent aeration with biochar SIT?P>0.05?.The removal of TN in with biochar addition SIT was significantly better than other SITs at 40-80 cm soil depth,indicating that the addition of biochar could promote the TN removal.When the depth was 60 cm,the SITs for removal TP are the best.?2?The removal efficiencies of COD,NH₄⁺-N and TN were 82.90%,98.32%and85.44%at 6 h d^-1 in the intermittent aeration with biochar SIT,respectively.The removal efficiencies of COD,NH₄⁺-N and TN were 90.89%?99.53%and 62.95%at12 h d^-1 in the intermittent aeration with biochar SIT,respectively.The average removal efficiencies of TP exceeded 98%at 6 h d^-1 and 12 h d^-1 in all SITs.When the hydraulic loading rate increased from 2 cm d^-1 to 6 cm d^-1 in the intermittent aeration with biochar SIT,the removal rates of COD,NH₄⁺-N increased from 82.90%to93.90%,98.32%to 99.38%.While the removal rates of TN decreased from 85.44%to73.00%.In addition,the efficiencies of COD,NH₄⁺-N in other SITs increased gradually with the increasing hydraulic loading rate.However,the hydraulic loading rate has little change in the removal of TP in SITs.The average removal efficiencies of COD exceeded 98%under different C/N conditions in non-biochar addition SITs.When the C/N rate was 5,the COD removal rates in biochar addition SITs varied in73.61-82.90%and the lowest removal efficiency.The average removal efficiencies of NH₄⁺-N exceeded 95%under different C/N conditions in all SITs.When the C/N rate was 7,the TN removal rates in non-biochar addition SITs varied in 74.07-77.37%and the best removal efficiency.What's more,when the C/N rate was 7,intermittent aeration with biochar SIT also had the best TN removal rates.The average removal efficiencies of TP exceeded 95%under different C/N conditions in all SITs.The average N₂O emission fluxes of the non-aerated and non-biochar SIT always higher than other systems,indicating intermittent aeration and biochar addition in SITs could reduce N₂O emissions.Moreover,when the C/N rate was 3,the N₂O emissions are the highest in the four systems,indicating the lower of C/N,the greater of N₂O emissions.?3?In this experiment,the microorganisms could not only utilize the nitrogen of the sewage,but also utilize the nitrogen of the soil.In addition,The phosphorus could be far better adsorped and immobilized by the addition of biochar and aeration in SITs.We can conclude that the aeration strategy has little effect on microbial activities by comparaing the changes of DOC content in system A and B at 25-85cm.Moreover,the aeration strategy has no effect on microbial activities with the increase of soil depth.In system C,the DOC content in the biochar layer was significantly decreased,which indicated the addition of biochar could promote the action of microorganisms.Comparing the sources of DOM in the matrix before and after the experiment,it could be concluded that intermittent aeration and biochar addition improve the the diversity of sources of dissolved organic matter in SITs.