Corncob Biochar Substrates Used In Constructed Wetland: Pollutants Removal Efficiencies And Disposal

Constructed wetland is a kind of sewage treatment technology which is environmentally friendly,low energy consumption and good effluent quality.However,the shortages such as large area occupation,low pollutants load,easy saturation,clogging and difficult disposal after use greatly restrict its popularization and application.To solve these problems,corncob biochar has been adopted as a kind of new substrate in constructed wetland with little artificial aeration,trying to improve the purification efficiency of constructed wetland(compared with the traditional inorganic ceramic substrate),clarify the purification mechanisms.After purification test,corncob biochar substrates can be burned to ashes,and then phosphorus could be recovered from the ashes.In this process,not only the final disposal problem(a kind of solid waste)of substrates is solved,but also achieves the purpose of phosphorus recovery.To investigate the purification efficiencies of constructed wetlands using raw corncob substrate,corncob biochar substrate and ceramic substrate under different aeration conditon,four lab-scale vertical flow constructed wetlands(VFCW)have been built.The results show that both corncob biochar and ceramsite substrate achieved good pollutants removal efficiency under 2.9m3/m2·d artificial aeration during the start-up of VFCWs(batch mode influent).After 60 days,the effluent water quality of VFCW using corncob biochar substrates became stable with over 90%COD,NH4+-N removal rates,and about 45%TP removal rate,thus corncob biochar may be a suitable substrate in constructed wetland.As for VFCW using raw corncob substrates,effluent pollutants concentration were much higher than influent.After the start-up period,influent has been switch to continuous flow,and when the effluent became stable the single factor tests on hydraulic load,aeration rate and influent water quality began.Throuhg the tests,the optimum operation condition has been determined:hydraulic load lower than 1m3/(m2·d),lower influent pollutants concentration(NH4+-N<50,TP<4mg/L),and the aeration rate should be 1.8-3.6m3/m2·d(air to influent ratio between 1.8 and 3.6).The effects of C/N on nitrogen removal efficiency and nitrogen removal pathways were studied in both VFCW modles and corncob biochar substrates taken from VFCWs.The results showed that VFCW using corncob biochar substrate achieved better nitrogen removal efficiency in treating low C/N waste water than the traditional inorganic substrate ceramsite,this can be attributed to the organic sustance released by corncob biochar substrate which may be used by denitrifiers.Simultaneous nitrification and denitrification(SND)occurred in VFCWs because of different oxygen concentration in different spaces which are created by the piling up of substrates and holes spread in the substrates.And the efficiency of SND increased with the increase of C/N,whitch can be explained by a more balanced nitrification rate and denitrification rate.Nitrogen removal pathway was one of the main work of this study,the different denitrification rate via nitrate and nitrite under different C/N ratio suggests that the traditional nitrification and denitrification process may be the main pathway of nitrogen removal under high C/N,while partial nitrification process may be the main pathway under low C/N.By high-throughput sequencing method,microbial community structure of VFCW using corncob substrates in different height were analysed.The results showed that microbial communities changed with the height in phylum,class and genus level.The dominant bacteril phylum is Proterbacteria?and class is Gammaproteobacteria and genus is Thiothrix.Bacterials which were reported to play a important role in nitrogen removal includes Nitrosomonas?Nitrospira?Azospira?Tauera?Zoogloea?Denitratisoma?unclassified_f_Comamonadaceae?Acinetobacter?Dechloromonas.After VFCW lab-scale model tests,corncob biochar substrates were taken out from the columns,mixed,dried and bumed(under 500? with air)to ashes.XRF test of the ashes showed that the content of phosphorus(counted by P2O5)is about 13.3%,much higher than other raw biomass ashes.In the extrcation of phosphorus from the ashes,80mL 0.05mol/L HCl solution per 0.1 g with 10min's reaction time is the optimum extraction condition in which most phosphorus(about 67%)are diluted to HCI solution and the least comsuption of HCl solution.As for the recovery of phosphorus,pH and P/N/Mg have great influence on the recovery rate of phosphorus.The best reaction condition is pH=9?9.5 and P:N:Mg= 1:1.4:1.6,and the recovery rate of phosphorus is above 90%under this condition.