Alkali-treated Lignocellulosic Carbon Sources Enhancing Denitrification Of AAO-MBR

When treating domestic wastewater with low C/N ratio,AAO-MBR wastewater treatment plants often add liquid carbon sources such as methanol to improve denitrification efficiency.However,these liquid carbon sources have the disadvantages of being costly,posing safety hazards,and presenting difficulties in dosage control.Alkali-modified lignocellulosic carbon sources possess the characteristics of being lowcost,transportable,and storable,while also having stable carbon release.Therefore,they are suitable for replacing traditional liquid carbon sources.However,the process of modifying lignocellulosic carbon sources produces a large amount of waste alkaline liquid,and the effect and mechanism of modified lignocellulosic carbon sources in AAO-MBR are not yet fully understood.In this study,we modified lignocellulosic waste materials from agriculture(corn cob(CC),corn stover(CS),)and from constructed wetlands(giant reed(GR))with alkali.After screening for optimum alkalitreated lignocellulosic carbon sources based on their substrate release performance,we studied the denitrification performance of the modified lignocellulosic carbon sources,and added them as additional carbon sources to a small-scale AAO-MBR to achieve the coupling of AAO-MBR and solid-phase denitrification technology.This provided a low-cost denitrification solution for urban wastewater treatment plants using the AAOMBR process.Additionally,this study investigated the in-situ disposal of waste alkaline liquid.The main research findings are as follows:1)The alkali treatment of lignocellulosic materials with sodium hydroxide(Na OH)at different concentrations yielded promising results for their use as lignocellulosic carbon sources for denitrification.CS treated with 4 % Na OH(4-CS),CC treated with2 % Na OH,and GR treated with 4 % Na OH(4-GR)exhibited the best denitrification performance,with average denitrification rates of 1.53±0.04 mg TN/(g material·d),1.39±0.12 mg TN/(g material·d),and 0.55±0.06 mg TN/(g material·d),respectively,after microbial acclimation.Three kinds of investigated alkali-treated lignocellulosic carbon sources exhibited stable and excellent denitrification capacity.2)The fluid-wastes generated during alkali treatment could be effectively recovered in situ by the AAO-MBR,adding fluid-wastes into the influent of the AAOMBR system enhanced the removal efficiency of TN and TP without adversely affecting the removal of other pollutants,the TN concentration of the effluent of AAOMBR with fluid-waste of 2-CC addition was reduced to 8.58±0.65 mg/L,and the TP concentration was reduced to 1.06±0.10 mg/L by 4-GR fluid-waste addition.3)The AAO-MBR system with the addition of alkali-treated lignocellulosic carbon sources(4-GR,2-CC,and 4-CS)exhibited excellent denitrification and phosphorus removal capabilities while maintaining high removal rates of ammonia nitrogen and chemical oxygen demand(COD).Among the three types of alkali-treated lignocellulosic carbon sources,2-CC showed the best TN removal efficiency(7.09±0.40 mg/L)during phase 3(when the hydraulic retention time was extended to15 hours),while 4-GR showed the best TP removal efficiency(0.14±0.08 mg/L)during phase 5(with anaerobic tank 4-GR addition).In addition to the heterotrophic denitrification pathway,the AAO-MBR system with the addition of alkali-treated lignocellulosic carbon sources also exhibited multiple TN removal pathways,leading to significantly higher denitrification efficiency than the theoretical denitrification efficiency of AAO.4)Alkali treatment of lignocellulosic materials improved their bioavailability by degrading semi-lignocellulosic and breaking down the cellulose-hemicellulose-lignin network.When added to the AAO-MBR,the alkaline-modified lignocellulosic carbon source formed tightly-packed biofilms that primarily decompose cellulose and semicellulose.Nitrospira dominated nitrification,while Dechloromonas and Aeromonas dominated phosphorus removal in the AAO-MBR system with the addition of alkalitreated lignocellulosic carbon sources.Denitrification was dominated by Paludibacter,Sphingomonas,Flavobacterium,Trichococcus,and other bacterial genera.