Efficient nitrogen removal and recovery from real digested sewage sludge reject water through electroconcentration

Reject waters from the dewatering of anaerobically digested municipal sewage sludge are nitrogen-rich (ca. 1 g_NH4-N L⁻¹) wastewater streams. They account for up to 25% of the total nitrogen load of wastewater treatment due to their internal recirculation within treatment plants. In this study, nitrogen was effectively removed and recovered from real reject water using a novel electrochemical setup combining electroconcentration and stripping. High nitrogen removal (≤ 94 ± 0.7%) and recovery (≤ 87 ± 8.5%) efficiencies from real reject water were obtained while simultaneously reducing the influent nitrogen concentration of 913 ± 14 mg_NH4-N L⁻¹ to 57 ± 6.7 mg_NH4-N L⁻¹ in the effluent. Most of the nitrogen recovery took place via electroconcentration into a liquid concentrate (≤ 82 ± 5.7%), while stripping contributed little to the removal and recovery (≤ 5 ± 2.8%). The reported removal and recovery efficiencies are the highest to date for a system utilising three-chamber electroconcentration. Furthermore, the concept of cation load ratio (the ratio between applied current density and cation loading rate) was introduced as a more precise parameter than the widely used and simpler NH₄-N load ratio for predicting the performance of a (bio)electrochemical nutrient removal and recovery system.