Column Study On Evaluating A Multi-media Permeable Reactive Barrier To Remove Ammonium From Groundwater

In China, the problem of ammonium pollution in rivers is becoming more and more seriousin recent years. Groundwater may be contaminated due to the recharge of ammoniumcontaminated river water. Permeable reactive barrier （PRB） has shown a promising in-situremediation technique of groundwater. but most PRBs use a single media or a singlemechanism to achieve the purpose of ammonium-nitrogen removal, so study on differentnitrogen removal mechanism and their combined appliacation is of great siginificance. In thisstudy, different media were chosen and studied by batch and column experiment. Two setsof multi-media columns were designed to modify the PRB in this research, which described astrategy that combined zeolite adsorption and microbial processes （nitrification-denitrification）to remove ammonium and finally transferred it to atoxic nitrogen. The research results weresummarized as follows:（1） During the column operation of300pore volumes, under different ammoniumconcentration loads （10mg/L&20mg/L） and groundwater flow velocity （0.5m/d&0.8m/d）,the two sets of composite media columns all had an ammonium nitrogen removal rate of98%and didn’t have the nitrite and nitrate accumulation.（2） Cement has a good package effect on CaO2. High proportion of cement inoxygen-releasing materials can delay the oxygen releasing rate and avoid high pH of watercaused by the fast reaction of oxygen-releasing materials. The new developed oxygen releasingmaterial has good oxygen-releasing property, it can ensure the dissolved oxygen （DO）condition of column that needed by nitrifying bacteria, the DO value increased from2mg/L to10mg/L after the water flow through oxygen releasing column.（3） Bio-ceramsite can promote nitrifying bacteria to grow on it’s surface and ensure astable nitrification intensity in the reaction column. Besides ammonium adsorption, zeolite canalso be a carrier, on which the microorganisms can grow. The zeolite could be biologicallyregenerated due to further removal of ammonium by microbial nitrification. The column filledwith bio-ceramsite and zeolite can ensure the column keeping high ammonium removalefficiency through the bio-chemical effect. The bio-ceramsite had a greater bio-film effect thanzeolite, the contribution of ammonium removed by nitrification was55%in the column filledwith bio-ceramsite and zeolite （mass ration is1.8:1），it was higher than the column filled with zeolite, in which biological removal efficiency was only30%.（4） The cation exchange ability of zeolite depends on the cation radius and it’sconcentration in solid-liquid phase. K⁺has a closer exchange order than NH4+, so it wasadsorbed first. Na⁺occupied the major part of the ammonium exchange amount in this process,because it’s cation radius is small, it would be easier to overcome the diffusion resistance ofzeolite and be exchanged into liquid-phase. Though the exchange order of Mg²⁺was behindNH4+, it was also preferentially adsorbed, because it’s concentration in zeolite is much lowerthan that in solution.（5） Sponge iron can create anaerobic environment for microbial denitrification and realizepartial removal of nitrate by chemical reduction. Pine bark, as a solid carbon source, its’ slowlydecomposition can provide enough nutrient for microorganism and enhance the denitrificationof heterotrophic bacteria.