Effect of substrate pretreatment on in situ heterotrophic denitrification of nitrate-contaminated groundwater

Abstract

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 in situ 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.