Preparation And Water Treatment Of Controlled-released Carbon Source Composites For Biological Denitrification

Groundwater is an essential water source for both domestic water and industrial and agricultural production.But because of the pollution of domestic wastewater,industrial effluents and over fertilization,the concentration of nitrate in groundwater is increasing which leads an attention for researchers.Since in-situ remediation is an efficient method for contaminated groundwater,researchers are focusing on using the external solid carbon sources,such as straw,nutshell,agarophyte and sawdust,for biological denitrification.However,for the long run,this natural solid carbon sources tends to mildewed which might result to secondary pollution.In this case,this paper was involving into preparing a series of novel carbon source composites which could release organic carbon source slowly in long-term operation.Fixed bed biofilm reactor(FBBR)was used for water treatment.The nitrate and nitrite concentration of effluents was monitored to investigate the nitrogen removal ability of FBBR.The morphology of the composite and biofilm were observed by optical microscope and SEM.Hemp fiber,nutshell,brown coal,charcoal,degreasing cotton fiber,coconut fiber,straw,poly(butylene succinate)(PBS)and polylactic acid(PLA)were investigated through static experiment.The carbon source composites were fabricated by twin-screw extruder with PBS,PLA as controlled release component,hemp fiber,powder of nutshell as natural solid carbon component and HDPE as a skeleton body.Cross-linking agent,desoxidant and the pH regulator were also added into the composites.After microbionation by activated sludge in set-up period,the FBBR was operating for several months with hydraulic retention time of 24 h.During the operation,the concentration of nitrate and nitrite,COD and pH were monitored to characterize the nitrogen removal and organic carbon release ability.The results indicated that when the single nutshell granules as carbon source,the nitrogen removal rate was about 85%to 90%,then decreased to 24%at 290th day.However,the FBBR filled with HF/PBS composite remained a high nitrogen removal rate at 90%with an effluent NO3-N concentration below 10 mg/L.During a 290-day sequential reaction,the nitrogen removal rate was fluctuating slightly which indicated that the HF/PBS composite was an efficient and long lasting carbon source.The NS/PBS and NS/PLA composites were fabricated with foaming agent which increased the pore canals in the granules.The denitrifying experiments shown that the FBBR filled with NS/PBS composite remained a stable nitrogen removal rate of 83%with effluent NO3-N concentration below 10 mg/L.In the same time,the NS/PLA composite exhibited excellent bacteria compatibility which,however,resulted to the propagation of blue-green algae in the FBBR.The optical microscope and SEM images showed that the denitrifying bacteria attached on the composites were usually coccus and bacillus.The size of composite granules was changed slightly,but there were many pore canals in them.That was because of the decrease of degradable polymer and the exposure of HDPE.