Evaluation of plasma generated reactive nitrogen species in digestates: exploring the potential of acid-base titration as a proxy indicator of nitrate formation

ABSTRACT Workflow and key results for DBD plasma treatment of anaerobic digestate, showing NOx uptake, TON-N, and FOS/TAC monitoring. Ammonia accumulation reduces the stability and fertiliser value of anaerobic digestate, while conventional anaerobic digestion monitoring tools may not fully capture chemically driven nitrogen transformations. This study evaluated dielectric barrier discharge (DBD) plasma as a post-digestion treatment for ammonium-rich digestate and assessed whether acid–base titration parameters could serve as proxy indicators of plasma-induced nitrogen chemistry. Digestate was treated using an indirect gas-phase air–argon (GAAG) plasma configuration, and temporal changes in pH, FOS, TAC, the FOS/TAC ratio, ammonium nitrogen (NH4-N), total oxidised nitrogen (TON-N), and nitrogen oxides (NOx) uptake were measured. Plasma treatment produced a staged acid–base response, comprising of initial alkalinisation, buffer-controlled stability, and delayed acidification after depletion of the digestate's acid-neutralising reserve. NH4-N decreased while TON-N increased, indicating the conversion of reduced nitrogen into oxidised, dissolved nitrogen forms. Total alkalinity capacity (TAC) showed the strongest response to plasma exposure and was significantly correlated with NOx uptake, whereas the volatile organic acids-to-total alkalinity capacity ratio (FOS/TAC) showed weaker, non-significant relationship. These findings demonstrate that DBD plasma can promote ammonia stabilisation and nitrogen retention in anaerobic digestate through buffer-regulated reactive nitrogen chemistry. TAC has potential as a practical titration-based proxy indicator of early plasma treatment performance in digestate-management systems.