Heterotrophic/autotrophic denitrification (HAD) of drinking water: prospective use for permeable reactive barrier

This work aims to explore the use of an innovative denitrification process developed by our group for groundwater remediation. This process is coupling heterotrophic-autotrophic denitrification processes (HAD) supported by cotton and zero-valent iron (ZVI). In the experimental part, the effect of two amounts of ZVI (150 and 300 g), two nitrate (100 and 220 mg/l) and phosphate (3 and 6 mg/l) inlet concentrations on nitrate removal performance is investigated in two parallel continuous fed plug-flow reactors. The possible in-situ application of the proposed system in permeable reactive barriers (PRB) is further discussed. The HAD showed higher volumetric nitrate removal ratio (VNR) than the cotton supported heterotrophic denitrification one, and VNR increased with the amount of ZVI packed in the reactors. Ammonium production by the reductive action of iron was kept on acceptable level adjusting the contact time between water and ZVI. Iron release from ZVI decreased with time to negligible value (<0.5 mg/l) thanks to the formation of iron green rust compounds (GR). The HAD seems to be adequate for PRB systems, because both cellulose-based material and ZVI have been used in reactive trench for site remediation. Moreover, the proposed process could have the function for removal of nitrate and priority pollutants, such as chlorinated ethenes, simultaneously.

A nitrate removal process previously developed by our group [17] was tested in continuous up-flow reactors to investigate the effect of ZVI amount, inlet nitrate and phosphate concentrations on the reactor performance and the formation of by-products, such as iron and bacterial releases, TOC increase, ammonia production, and nitrite accumulation. Moreover potential use of the HAD in permeable reactive barrier (PRB) is discussed. The HAD showed higher volumetric nitrate removal ratio (VNR) than cotton supported denitrification alone. This increase in VNR is attributed to the contribution of autotrophic denitrification, which occurred here in the HAD differently due to iron amounts in the tested reactors. Ammonium pro-duced by iron-inducted nitrate reduction was the main parameter which determines process performance. The ammonia production varied due to inlet nitrate concentration. For 100 mg/l inlet nitrate concentration maximum ZVI-water contact time resulted in around 1 h for Table 1 Specific nitrate removal ratios obtained in different solid carbon supported denitrification studies Carbon source System description – NO3 inlet (mg/l) VNR* (kg N/m3 d)