In Situ Remediation Of Contaminated Sediment Using Combined Biological And Physicochemical Technology In The Urban Rivers And Lakes

With the rapid development of industrialization and urbanization, eutrophicationof the urban rivers and lakes is increasingly more serious in China. Nitrogen （N） andphosphorus （P） concentrations are the main limiting factors for eutrophication of therivers and lakes, and sediment plays an important role in eutrophication because it wasregarded as a source or a sink for N and P of the rivers and lakes. Therefore, in situreducing N and P released from sediment will be an effective solution for controllingeutrophication of the rivers and lakes. Currently, the capping material of in situcontaminated sediment capping may have the following issues:（1） in situ regeneration;（2） secondary release of nitrogen from active adsorbent;（3） limited utilization rate ofactive adsorbent. In order to resolve these issues, in situ combined biological andphysicochemical technology with biozeolite as main capping material was proposed toreduce N, P and organics released from sediment in the study. The efficiencies,mechanisms and the main influences of controlling contaminant released fromsediment using the technology were examined through laboratory and field scaleincubation experiments. The feasibility of in situ biological regenerating theammonium （NH4+-N） adsorption capacity of zeolite was investigated. The successivechanges in microbial community structure, nitrifiers and denitrifiers number on thebiozeolite were analyzed. The composition methods of capping layer and the methodsof large-scale preparation of biozeolite were optimized. The study mainly carried out as the follows:（1） The preparation of biozeolite: To obtain the bacterial consortium, twoheterotrophic nitrifiers WGX10and WGX18（Bacillus sp.） and two aerobic denitrifierHF3and HF7（Acinetobacter sp.） isolated from contaminated sediment were enrichedrespectively, and then mixed in equal proportion. The natural zeolite of a certainamount were put in mixed liquor of the bacterial consortium suspension and sterilizedraw water at1:9in volume for2³days under the condition of25³0℃anddissolved oxygen （DO） above2³mg L^-1via artificial aeration until the biofilmformation process was finished.（2） The efficiencies of the technology: The laboratory sediment incubationexperiment results showed that the reduction efficiency of total nitrogen （TN） ofoverlying water using capping with biozeolite of the dose rates of10kg m^-2was morethan90%, the TN reduction efficiency of sediment core （0¹8cm） was10%, and thetotal phosphorus （TP） fixation efficiency of sediment core （0⁸cm） was13%. Thefield sediment incubation experiment results showed that the average reductionefficiencies of TN and chemical oxygen demand （COD） of overlying water by cappingwith biozeolite dose rates of2kg m^-2were36%and41%, respectively, and the TPaverage fixation efficiency of overlying water and sediment core （0²0cm） was35%and13%, respectively. Therefore, biozeolite capping could not only effectively inhibitN, P and organics released from sediment, but also can further reduce some of N fromsediment, and is an effective technology for in situ remediating contaminated sediment.（3） The key factors of the technology: The DO concentrations of overlying water,the dosage of biozeolite and carbon source were the main factors of the technology.The efficiencies of biological nitrogen removal were influenced by too high or low DOconcentrations, and the optimal concentration range of DO was between1⁴mg L^-1.The higher the dosages of biozeolite were, the better efficient reduction of contaminantreleasd from sediment were, and in order to design the dosage of biozeolite, pollutionlevel of sediment, requirement of reduction efficiency and technology cost should betaken into consideration. The reduction of carbon source was the major reason for thedecrease of biological nitrogen removal.（4） The mechanisms of the technology: The N removal mainly depends on biological, physical and chemical processes. NH4+-N is quickly adsorbed by biozeolitethrough physical adsorption and chemical ion exchange. The adsorbed NH4+-N isgradually desorbed from biozeolite and transformed to NO3--N through biologicalnitrification, indicating that in situ biological regenerating the ammonium （NH4+-N）adsorption capacity of zeolite is feasible. The P fixation mainly depends on physicaland chemical processes.（5） The successive changes in microbial community structure, nitrifiers anddenitrifiers number on the biozeolite: The reduction of carbon source was the majorreason for the decrease of nitrifiers number. The reduction of carbon source and toohigh DO concentration (more than3mg L^-1) were the primary cause for the decreaseof denitrifiers number. Therefore, the DO concentration and carbon source are the mainfactors for the growth of nitrifiers and denitrifiers. Bacillus sp. and Acinetobacter sp.were still present on the biozeolite and were dominant bacteria. There was competitionbetween indigenous bacteria and isolated bacteria, and the indigenous dominantbacteria progressed continually. Moreover, two indigenous aerobic denitrificationstrains （Delftia sp. and Serratia sp.） were found on the biozeolite. The bacterial shapesvaried from sphere bacilli form to rod-shaped.（6） The optimization of the composition methods of capping layer: The optimalcombination method of biozeolite and sand was to put biozeolite under the fine sand.The purpose of sand on the upper layer are: to improve the NH4+-N adsorption capacityof biozeolite capping layer; to strengthen biological denitrification of biozeolitecapping layer; to cover the sediment that missed the coverage by biozeolite. Thecombination capping of biozeolite and sand could decrease the dose rate of biozeoliteand reduce the cost.（7） The large-scale preparation method of biozeolite was established. Thecombination capping technology of biozeolite and sand was applied for controllingcontaminant released from sediment in Hongtai River at Yangzhou City. Thecomposition method of capping layer from top to bottom was sand layer （the thicknessof5mm）, biozeolite layer （the thickness of20mm） and sand layer （the thickness of10mm）. The results of the average reduction efficiencies of TN, TP and COD ofoverlying water using the combination capping of biozeolite and sand were61%and 34%, respectively, and the TP average fixation efficiencies was37%. Therefore, thelarge-scale preparation method of biozeolite is feasible.