Nitrate Removal In Groundwater By A Sawdust-Fe~0 Coupling Denitrification System

In the recent years, the presence of elevated levels of nitrates in groundwater, due to excessive use of fertilizers, domestic sewage and industrial wastewater discharges, has become a serious worldwide concern, as it directly or indirectly contribute to serious human health problems, especially infants and young children. Nitrate is most commonly removed from drinking water using ion ex-change, reverse osmosis, electrodialysis, chemical reduction and biological denitrification. Among these methods, the rear two are widely studied and cost-effctive methods for in-situ Groundwater pollution remediation. Permeable reactive barriers（PRBs） packed with Fe⁰ or natural solid organic carbon are attracted widespread attention in the United States and Europe for nitrate pollution Remediation. The main problem of Fe⁰ chemical reduction is the product of ammonia which must be removed. Biological denitrification comprises heterotrophic denitrification and autotrophic denitrification, the former requires the support of organic matter, which may cause secondary pollution problems, while the latter requires sufficient electron donor to reduce nitrate. Thus, the present study examined the nitrate removal in a coupling denitrification system（CDS） which coexist t hree methods.In this paper, sawdust, Fe⁰ were used as reaction medium, activated sludge as denitrifying bacteria sources, to investigate the effect of p H, dissolved oxygen（DO）, sawdust pretreated, iron particle size, groundwater common anions and cations and other factors on nitrate removal and products in CDS. We compared 4 denitrification systems and used SEM, XPS spectroscopy analysis to elucidate the nitrate removal mechanism by CDS. By means of high-throughput sequencing（HTS） analysis, the relative abundance of dominant strain and community diversity were examined the coupling system. The results can be summarized as below:（1）When the p H changed within 6⁹, the effect of p H on nitrate removal and nitrite, ammonia accumulation were not significant in CDS. However, the nitrate removal rate increased at p H 4.5, while nitrite and ammonia accumulation appeared. When DO value varied in the range of 0-6.5, the effect of DO on nitrate removal, nitrite and ammonia accumulation were inapparent. At the same time, N2 O conversion and the dissolution of iron were significant effected by p H and DO.（2）Nitrate removal rate was reduced due to sawdust pretreated, and Na OH-H2O2 treating method had a least effect on nitrate removal rate. Compared to untreated sawdust, pretreated sawdust led to the nitrite, ammonia accumulation concentration increase. The nitrite concentration was as high as 4.9mg/L in Na OH-H2O2-treated sawdust system, and the ammonia concentration was up to 20.9mg/L in H2SO4- ultrasound- treated sawdust system. By sawdust pretreated, secondary pollution of organic matter can be avoid.（3）The diameter of the iron had little impact on nitrate removal in Fe⁰ reduction system, while in CDS when the iron particle size greater than 1,000 mesh, with the particle size decreasing, nitrate removal rate increased, but when the particle size was reduced to 3,000 mesh, the nitrate removal rate decreased. Moreover, the accumulation of nitrite and ammonia were significantly affected by the particle size of the iron in CDS, overall, the concentration of the two forms nitrogen rised with the decrease of particle size. In CDS, 100 mesh of iron is an ideal choice.（4）Common inorganic anions, to various extents, promoted the nitrate removal in CDS, and the promoting extent were: HCO3- >SO42- >PO43-> Cl-. HCO3- and high concentration of SO42- reduced Nir activity, lead to nitrite accumulation, and also promoted chemical reduction pathway in CDS, resulting in high ammonia conversion rate. PO43- could reduce the conversion rate of ammonium, increase the total nitrogen removal rate. Anion will lead to the increase of DOC and the reduction of soluble iron concentration.（5）In CDS, the promotion of common cations on nitrate removal was as follows: Ca2+（200mg/L） > Ca2+（400mg/L） > Mg2+（400mg/L） > Mg2+（200mg/L） > Fe²⁺（50,100mg/L） > Fe³⁺（50mg/L） > control = Na+（100,200mg/L） > Cu2+（200mg/L） > Fe³⁺（100mg/L） > Cu2+（100mg/L）. Results showed that the effect of cations on CDS was not only about cation species, but also its concentration.（6）Fe³⁺ and Fe²⁺ could inhibit Nar and Nir, resulting in high concentrations of nitrite accumulation. Except Na+, other cations, to various extents, promoted the translation of ammonia, especially Cu2+, Fe³⁺ and Fe²⁺. In addition, Cu2+ would result in high TOC and soluble iron concentration, but at the final stage of reaction the soluble iron would be reduced to a low level.（7）Cations had no significant impact on total nitrogen removal. Fe³⁺, Fe²⁺, Cu2+ would reduce the removal rate of total nitrogen, especially Cu2+ which caused the nitrate removal no more than 20%. The highest TN removal rate was only 80% in the system that including Fe³⁺or Fe²⁺, while in other systems itwas close to 100%.（8）Biological denitrification was the main mechanism for nitrate removal in CDS, it included heterotrophic denitrification and hydrogen autotrophic denitrification, the former played a leading role. The main role of Fe⁰ in the system was to produce hydrogen, which provided energy for autotrophic denitrification. CO2 generated in heterotrophic denitrification process not only provided as inorganic carbon for autotrophic denitrifying bacteria, but also promoted Fe⁰ corrosion. The nitrate removal rate was 99.1% in CDS and almost no nitrite, ammonia nitrogen accumulation during the reaction process. Moreover, N2 O conversion rate was in a very low level and less than 1%. High-throughput sequencing results showed that comparing with sawdust supported biological denitrification system, Fe⁰ in CDS influenced on microbial relative abundance, while it was no significant impact on the community structure.