Low chemical consumption of wastewater ammonium nitrogen recovery through Donnan dialysis-membrane stripping process

Abstract

Wastewater ammonium nitrogen (NH4+-N) recovery is frequently associated with massive and ineffective chemical consumption due to the presence of bicarbonate (HCO3–), resulting in undesirable economic feasibility. In the present study, by incorporating energy-efficient Donnan dialysis (DD) as a pretreatment process for membrane stripping (MS), we developed a low-chemical consumption hybrid DD-MS process with two membranes (cation exchange membrane [CEM] and hydrophobic membrane) and three phases (wastewater, draw solution and acid solution). First, equimolar amount of ammonium was exchanged with sodium ions and passed through CEM into an alkaline draw solution, where it was immediately deprotonated to free ammonia. Finally, it was diffused across a hydrophobic membrane, absorbed by the acid solution, and recovered as an ammonium salt. Meanwhile, HCO3– was confined to wastewater due to electrostatic repulsion from CEM. Owing to the chemical potential difference, NaOH in the draw solution diffused into the wastewater, resulting in additional chemical consumption. Experiments and calculations based on the Donnan equilibrium revealed that using an alkaline draw solution at a slightly higher concentration than that of wastewater ammonium allowed high NH4+-N recovery and low NaOH diffusion simultaneously. In experiments on actual landfill leachate, the hybrid DD-MS process achieved 81.4% NH4+-N recovery from wastewater while reducing alkali and acid consumption by respectively 32.7% and 34.9% compared with the conventional MS process. The hybrid process reduced the cost of chemicals and amount of salt entering the wastewater and environment. Overall, the proposed hybrid process presents excellent application prospects for NH4+-N recovery from wastewater with high-strength HCO3–, such as landfill leachate and livestock wastewater.