Ammonia recovery from anaerobic digester effluent through direct aeration

• Solely injecting air into digester effluent can elevate pH and remove ammonia.
• Smaller bubble size favors the process.
• Carbonate and ammonium concentrations first increase then decrease during aeration.

A direct aeration strategy was developed for ammonia recovery without alkali addition. The main chemical buffer system of digested effluent is made up of high concentration of acidic species, which lead to a hypothesis that elevation of pH for ammonia release could be achieved by removing supersaturated CO2, dissolved carbonate, and bicarbonate solely. The concept was tested in lab scale reactors with digested dairy manure focusing on temperature, bubble size, liquid depth and airflow rate. It has demonstrated that simple air stripping without alkali chemical input is an effective way to elevate pH of the digested effluent due to intriguing chemical shifts strongly related to the high levels of carbon dioxide, bicarbonates and carbonates present in digested effluent. These chemical shifts, ultimately release carbon dioxide and raise the pH of the effluent to levels near 10, which with combined elevated operating temperatures from waste engine heat, can lead to 70–90% shift from ionic to free, gaseous form of ammonia and subsequent recovery of ammonia through acid contact. The chemical relationships and equilibrium shifts associated with the aeration process and its subsequent release of gases were further investigated by chemical equilibrium model.

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