Study On Performance Of Solide Carbon Sources And Phosphorus Source For Biological Denitrification In In-situ Groundwater Remediation

Groundwater is an important freshwater resource accessible for human use.However, nitrate contamination of groundwater aquifers has been an increasingproblem. Recently, biological denitrification is considered to be the best option forremoving nitrate from groundwater because of its efficiency and moderate cost. Thepurpose of this study was to solve the problem of the lack of carbon source andphosphorus source for biological denitrification in in-situ groundwater remediation. Inthis study, wheat straw, sawdust and biodegradable plastic were selected as carbonsources and phosphate rock was selected as phosphorus source to evaluate theircapacity for the promotion of denitrification. And then, the effect of nitrateconcentrations, influent flow and temperature on denitrification rate was investigated.The species composition of denitrifying bacteria in denitrification system was alsoinvestigated by molecular biological methods.From this study, it was concluded that biodegradable plastic, sawdust and wheatstraw can be used as carbon source for biological denitrification in groundwaterremediation, and biodegradable plastic showed the best effect for the promotion ofdenitrification owing to the lower amount of nitrogen compounds released, higherdenitrification efficiency and lower accumulation of nitrite. The ambient temperaturehas a great influence on denitrification.When the ambient temperature decreased from25±2°C to16±2°C, nitrate breakthrough occurred, and the nitrite accumulatedsignificantly. Additionally, the influent nitrate concentration appeared to have someinfluence on denitrification. When the influent nitrate concentrations were50,60,70,80,90mgNO₃^--N/L, complete nitrate reduction was achieved. However, when theinfluent nitrate concentration increased to100mgNO3--N/L, a breakthrough of nitratewas observed and nitrite accumulation became serious. Phosphate rock has thepotential to provide phosphorus for denitrifying bacteria. The nitrate removalefficiency of the columns containing1500g or750g phosphate rock was over97%when temperature was20±2°C, at the same time, the nitrite concentrations were lowerthan0.1mgNO₂^--N/L. On the contrary, the nitrate removal efficiency in columnscontaining500g or0g phosphate rock was low, and the nitrite accumulation wasserious. Consequently, phosphate rock can provide sufficient phosphorous fordenitrifying bacteria and support denitrification thoroughly. When the ambienttemperature increased to25±2°C, complete denitrification was achieved in thecolumns containing1500g,750g,500g,0g phosphate rock. Additionally, the influent flow appeared to have some influence on denitrification. When the influent flowincreased to3.6ml/min, the effluent nitrate concentration increased sharply withseriously nitrite accumulation. However, nitrate removal efficiency of columnscontaining1500g or750g phosphate rock was better than the columns containing500g or0g phosphate rock. So it can be concluded that phosphate rock wasapplicable for further use as a filling phosphorus source for in-situ nitrate-pollutedgroundwater remediation.Biological samples were collected from the columns and analysised for theirbacteria phase. The results indicated that proteobacteria, especially-proteobacteria,were the predominant microorganisms.