Electrodialysis stacks, systems, and methods for recovering ammonia and monovalent salts from anaerobic digestate

A system for recovering ammonia from anaerobic digestate comprising ammonium ions, other cations, and anions, the system comprising:

• (a) an electrodialysis stack comprising:
• an anode;
• a cathode;
• one or more monovalent-selective cation exchange membranes;
• and
• one or more anion exchange membranes;
• wherein the monovalent-selective cation exchange membranes and the anion exchange membranes are disposed in an alternating arrangement between the anode and the cathode, such that a plurality of cell compartments are defined in the stack, the cell compartments comprising:
• a cathode cell compartment defined between the cathode and an anion exchange membrane;
• an anode cell compartment defined between the anode and a monovalent-selective cation exchange membrane;
• a plurality of diluate cell compartments, each defined between a monovalent-selective cation exchange membrane and its adjacent anion exchange membrane;
• and a plurality of concentrate cell compartments, each defined between an anion exchange membrane and its adjacent monovalent-selective cation exchange membrane;
• (b) an anaerobic digestate source configured to feed a separated anaerobic digestate influent into the diluate cell compartments;
• and
• (c) an ammonia separator configured to accept an effluent from the cathode cell compartment and to separate ammonia from the effluent.

A method for recovering cation salts, monovalent anion salts or both from separated anaerobic digestate comprising cations, bicarbonate anions, other monovalent anions, and multivalent anions using an electrodialysis stack comprising:

• an anode;
• a cathode;
• one or more monovalent-selective anion exchange membranes;
• and
• one or more cation exchange membranes;
• wherein the monovalent-selective anion exchange membranes and the cation exchange membranes are disposed in an alternating arrangement between the anode and the cathode, such that a plurality of cell compartments are defined in the stack, the cell compartments comprising:
• a cathode cell compartment defined between the cathode and a monovalent-selective anion exchange membrane;
• an anode cell compartment defined between the anode and a cation exchange membrane;
• a plurality of diluate cell compartments, each defined between a monovalent-selective anion exchange membrane and its adjacent cation exchange membrane;
• and a plurality of concentrate cell compartments, each defined between an cation exchange membrane and its adjacent monovalent-selective anion exchange membrane;
• the method comprising:
• (a) feeding a separated anaerobic digestate influent into the diluate cell compartments, wherein bicarbonate ions and other monovalent anions selectively pass through the monovalent-selective anion exchange membranes into the concentrate cell compartments to provide a concentrate stream that is concentrated in bicarbonate ions and other monovalent anions, and multivalent anions are selectively retained in the diluate cell compartments to provide a diluate stream;
• (b) passing the diluate stream out of the electrodialysis stack as a diluate effluent;
• (c) feeding at least a portion of the concentrate stream into the anode cell compartment, wherein the bicarbonate ion concentration is reduced via acidification to provide an anode cell effluent that has a reduced bicarbonate ion concentration;
• (d) passing the anode cell effluent out of the electrodialysis stack;
• and
• (e) recovering one or more salts of the cations and other monovalent anions from the anode cell effluent.