The High-efficiency Slow Release Carbon Source Filter For The Advanced Nitrogen Removal Of The Wastewater Treatment Plant Effluent

The effluent of wastewater treatment plant has the characteristics of low carbon to nitrogen ratio and high NO₃^--to total nitrogen ratio,and additional carbon is required as electron donor during its biological denitrification.Inexpensive agricultural wastes,which can be used as the carbon source and carrier of microorganisms,have some disadvantages as fast initial release of organic carbon,poor biodegradability,and the unclear release characteristic of secondary pollutants.Six kinds of agricultural wastes,i.e.,rice straw,wheat straw,corn stalk,corncob,soybean stalk,and soybean hull were investigated in this study to select slow-release carbon source with the highest efficiency and the lowest secondary pollution risk.The influences of different pretreatment methods on the physical and chemical properties of the carbon source and the following advanced denitrification performances were compared.The main conclusions are as follows:（1）The corncob could be utilized and adhered easily due to its rough surface and abundant effective carbon source（cellulose and hemicellulose）.Static release experiments showed that the carbon release of 6 kinds of agricultural wastes followed the second-order kinetic equation and Ritger-Peppas equation.The corncob had the best nitrate removal performance and the lowest secondary pollution risk.（2）After freeze-thaw,acid,alkali,and freeze-thaw-alkali pretreatments,the surface structure of corncob was damaged to some extent,and the pore structures and the specific surface area increased.The small molecular organic matters in the corncob were removed to different degrees which might avoid the excessive COD release.Alkali and freeze-thaw-alkali pretreatment can effectively increase the proportion of cellulose and hemicellulose in the corncob.The lignocellulose structure and its link bonds were destroyed by all of pretreatment methods,and the damage caused by acid,alkali,and freeze-thaw-alkali treatments was severer.The crystallinity indices of the treated corncob increased due to the partially removal of the amorphous structure.（3）Rapid start-up of the denitrification biofilter could be achieved by carbon source pretreatment,and the start-up stages of alkali and freeze-thaw-alkali treated corncobs were shortened from 15 days to 11 and 10 days respectively.The biofilm attachment and growth on the filter and the initial nitrate removal performance were enhanced effectively.During the stable operation period,the biofilter with alkali-treated corncob had the best nitrate removal performance.At 2 h HRT,the effluent NO₃^--N and TN were（0.52±0.11）mg/L and（1.34±0.29）mg/L,which were lowest of all biofilters.Meanwhile,the COD was（30.08±7.63）mg/L and the effluent quality satisfied the class V standard for surface water.At 1.5 h HRT,the effluent NO₃^--N and TN were（1.73±0.13）mg/L and（3.32±0.24）mg/L,which were still the lowest of all biofilters.At1 h HRT,the effluent NO₃^--N and TN of all biofilters except the one filled with freeze-thaw-alkali-treated corncob increased significantly.High-throughput sequencing results showed that the microbial communities on the carbon source surface were influenced by the pretreatments,and the dominated microbes were denitrifiers and cellulose hydrolytic bacteria.