In situ biological groundwater denitrification: Concepts and preliminary field tests

High nitrate concentration presents the main groundwater quality problem of the Israeli coastal aquifer which supplies 25% of the total water consumption of the country. In about 50% of the coastal wells nitrate concentration exceeds 45 mg/l and in 18% of the wells nitrate concentration is above the maximum permissible concentration of the new Israeli standard - 70 mg/l. Although several protection measures, mainly administrative, were introduced, their impact would be pronounced only after 1-3 decades, thus nitrate removal technologies should be introduced as a mid-term solution. Pilot-plant experiments were conducted in order to develop, demonstrate and compare various in-situ schemes for nitrate removal from groundwater by biological denitrification. Activities were focused towards two schemes: (i) Denitrification in a dual purpose (recharge-pumping) well and (ii) Substrate injection through a battery of small diameter wells surrounding a central production well (the 'Daisy' system). Experiments related to the first scheme indicate that, though nitrate content can be reduced almost to zero, its economic feasibility seems to be unfavorable because of operational difficulties and the apparent requirements for costly supplementary treatment. Experiments related to the 'Daisy' scheme demonstrated a nitrate removal efficiency of approximately 10%. Considering the fact that only one injection well of the three drilled functioned properly, the above mentioned nitrate removal represents the efficiency of a single injection well. It is anticipated that further experiments with the 'Daisy' system consisting of 5-6 injection wells would result in a significant nitrate reduction.