Study On Operation Characteristics Of A Constructed Wetland With Basalt Fibers

Constructed wetlands（CWs）,as an ecological technology with low cost,simple management and maintenance,and landscape effect,have been widely used in the field of wastewater treatment.The substrate is an vital part of CWs,which plays a decisive role in the removal of pollutants.Traditional substrates often have problems such as small specific surface area,poor adhesion of microorganisms,and poor removal effect.The development of new alternative substrates to make up for the defects of traditional wetland substrates and to improve the system’s decontamination efficiency is one of the focuses of current research on CWs.Basalt fibre（BF）is a high-performance inorganic fiber with large specific surface area,good biological affinity,low cost and no environmental hazards.It has been used as a new filler in SBR,A/O,biological contact oxidation,etc.However,there are few reports on the use of basalt fiber as a new substrate in CWs.Therefore,exploring the feasibility of using basalt fiber as a new substrate to enhance the decontamination performance of CWs is of great significance to develop a low-cost and high-efficiency wetland substrate and broaden the application of basalt fiber in water treatment.The decontamination performances of conventional substrate（gravel）constructed wetland（CW1）and basalt fiber constructed wetland（CW2）under two different pollution loads were investigated.Under the low pollution load（COD=200mg/L,TN=25mg/L,TP=3mg/L）,there were no significant differences in the removal effects of organic matter and phosphorus between the two CWs within the first 60 days of the reaction,but with the extension of running time（60～90d）,the average removal rates of COD and TP by CW2 were increased by 11.62%and 14.08%respectively compared with CW1.In terms of nitrogen removal,the average removal rates of TN,NH₄⁺-N and NO₃^--N by CW2 were all higher than that of CW1,especially the significant advantage of NO₃^--N removal was the main reason for the improvement of its nitrogen removal effect.Under the high pollution load（COD=500mg/L,TN=62.5mg/L,TP=7.5mg/L）,the advantage of CW2 for organic matter removal gradually showed up in the final stage（60～90d）.The average removal rate of NH₄⁺-N by CW2was 29.17%higher than that of CW1,making the overall nitrogen removal effect of CW2better than CW1.CW2 also had a clear advantage of phosphorus removal,whose average removal rate of TP was 37.58%higher than that of CW1.Overall,the decontamination performance of CW2 was better than that of CW1.Therefore,it is feasible to use basalt fiber as a new substrate in CWs to improve the decontamination performance of the system.The effects of basalt fiber filling location（upper layer（CW3）and lower layer（CW2））on the decontamination performance of CWs were investigated.Under the low pollution load,the two CWs did not show significant differences in organic matter removal and phosphorus removal,and the removal effects were good;In the early stage（0～30d）,the average removal rates of NH₄⁺-N and NO₃^--N by CW3 were 11.04%and 13.78%higher than CW2,respectively,and the overall nitrogen removal effect was better than CW2.In the mid-stage（30～60d）and the final stage（60～90d）,the average removal rates of NH₄⁺-N in the two CWs both reached more than 95%,but the average removal rate of NO₃^--N of CW2 was 6.22%and12.04%higher than that of CW3,respectively,making the overall nitrogen removal effect better than CW3.Under the high pollution load,the removal efficiencies of organic matter by the two CWs still showed no significant difference.The average removal effects of NH₄⁺-N and TN by CW2 were significantly better than CW3,but with the extension of operating time,the influence of basalt fiber filling location on the denitrification effect gradually weakened.The average removal rate of TP by CW2 was significantly increased by 17.84%compared to CW3.In summary,the impacts of basalt fiber filling location on the removal efficiency of CWs were related to the influent intensity and operating time.Overall,under the high pollution load,the wetland filled with basalt fibers in the lower layer had obvious advantages in nitrogen and phosphorus removal.Modified basalt fibers were prepared by five different surface modification methods（polydopamine（PDA）,polyacrylamide（PAM）,maleic anhydride（MAH）,silane coupling agent（KH550）and chitosan（CTS））,and their performances were compared.The bioaffinity of these basalt fibers was ranked as follows:PDA-BF>PAM-BF>MAH-BF>BF>KH550-BF>CTS-BF.Scanning electron microscopy showed that the surface roughness of modified basalt fiber（MBF）was significantly higher than that of unmodified basalt fiber（BF）.Fourier infrared spectroscopy showed that the surface of MBF had more active functional groups.When MBF was used as a biological carrier in wastewater treatment,the results showed that MBF was significantly better than BF for organic matter removal,and had strong resistance to organic load.The order of average removal rates of NH₄⁺-N,TN,TP were all as follows:PAM-BF>PDA-BF>MAH-BF>BF.In general,the comprehensive performance of basalt fibers modified by polydopamine and polyacrylamide were better.The effects of modified basalt fiber（PDA-BF and PAM-BF）applied to CWs on the decontamination performance of the system were further investigated.The modified basalt fibers constructed wetlands and the unmodified basalt fiber constructed wetland showed no significant difference in organic matter removal and nitrogen removal.The order of average removal rate of TP was as follows:polyacrylamide modified basalt fiber wetland>polydopamine modified basalt fiber wetland>unmodified basalt fiber wetland.Modified basalt fiber significantly improved the phosphatase activity of wetland substrate.