Effect of substrate pretreatment on <i>in situ</i> heterotrophic denitrification of nitrate-contaminated groundwater

Intensive use of nitrogen fertilizers in Algeria has caused significant nitrate pollution of groundwater, with concentrations reaching 218 mg L^-1 in the Khemis-el-Khechena region, well above the permissible limit of 50 mg L^-1. This study investigates an economical and sustainable biological treatment using date pedicels, an abundant agricultural by-product, as both a carbon source and microbial support for heterotrophic denitrification. Date pedicels were pretreated with 0.5% sodium hydroxide for two hours to enhance biodegradability. Batch experiments showed optimal nitrate removal with 10 g L^-1 of treated biomass, neutral pH, and a substrate-to-nitrate ratio of 67 g L mg^-1. Applied to real groundwater (212 mg per L NO₃ ^-, pH 7.3), nitrate concentrations decreased to 15.3 mg L^-1 within seven days, with 4.3 mg per L nitrites detected. A pilot-scale continuous system simulating an <i>in situ</i> bioreactor achieved nearly complete nitrate removal from the first day, with minor nitrite accumulation (0.8 mg L^-1 decreasing to 0.3 mg L^-1 by day five). Secondary treatment is still required to meet drinking standards, although natural processes such as oxygenation and filtration could further improve water quality.