Biological nitrogen removal from petroleum refinery wastewater: A review of microbial processes, operational challenges and enhancement strategies.

Biological nitrogen removal (BNR) from petroleum refinery wastewater (PRWW) is challenging due to presence of recalcitrant compounds and inherent characteristics of this wastewater. Nitrification inhibition, microbial washout due to toxic shock loads, and limited denitrification resulting from low COD/TN ratios have been reported to prevent sustained nitrogen removal from PRWW, even when COD removal is adequate. This review highlights that controlled carbon supplementation can improve BNR to ≥90%. Novel strategies that can be directly implemented in treatment plants to mitigate nitrification inhibition include aerobic granular sludge (AGS) and bioaugmentation with heterotrophic nitrifiers and aerobic denitrifiers (HNAD). AGS offers protection to sensitive microbes, while HNAD-based systems take advantage of the higher growth rates of heterotrophic bacteria compared to slow-growing nitrifiers, and may aid in quick recovery after nitrification inhibition following shock loads. Moreover, these technologies enable simultaneous nitrification and denitrification along with carbon removal in a single reactor. These technologies have demonstrated good performance in refinery wastewater, with COD removal of 85-98% and total nitrogen (TN) removal of 65-90%. To address low COD/TN ratios in refineries, novel technologies such as anammox based technologies and autotrophic denitrification have shown promising TN removal i.e., 70-85% and >90%, respectively in recalcitrant wastewater. Conditioning of PRWW through pretreatment may further improve downstream BNR processes. However, integration and implementation of these novel technologies need to be evaluated in real refinery wastewater and in pilot-scale systems based on their long-term performance.