Efficient nitrogen removal and recovery from real digested sewage sludge reject water through electroconcentration
Veera Koskue, Stefano Freguia, Pablo Ledezma, Marika Kokko
Abstract
Reject waters from the dewatering of anaerobically digested municipal sewage sludge are nitrogen-rich (ca. 1 gNH4-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 mgNH4-N L−1 to 57 ± 6.7 mgNH4-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.