Nitrogen recovery from wastewater using gas-permeable membranes: Impact of inorganic carbon content and natural organic matter

Abstract

Gas-permeable membranes coupled with low-rate aeration is useful to recover ammonia (NH4+) from livestock effluents. In this study, the role of inorganic carbon (bicarbonate, HCO3−) to enhance the N recovery process was evaluated using synthetic effluents with various NH4+ to HCO3− molar ratios of 0.5, 1.0, 1.5 and 2.0. The study also evaluated the effect of increased organic matter on the NH4+ recovery using humic acids (3000–6000 mg L−1), and the N recovery from high-strength swine manure. The release of hydroxide from the HCO3− with aeration increased the wastewater pH and promoted gaseous ammonia formation and membrane uptake. At the same time, the recovery of gaseous ammonia (NH3) through the membrane acidified the wastewater. Therefore, an abundant inorganic carbon supply in balance with the NH4+ is needed for a successful operation of the technology. NH4+ removal efficiencies >96% were obtained with NH4+ to HCO3− ratios ≤1. However, higher molar ratios inhibited the N recovery process resulting in lower efficiencies (<65%). Fortunately, most swine manures contain ample supply of endogenous inorganic carbon and the process can be used to more economically recover the ammonia using the natural inorganic carbon instead of expensive alkali chemicals. In 4 days, the recovered NH4+ from swine manure contained 48,000 mg L−1. Finally, it was found the process was not inhibited by the increasing levels of organic matter in the wastewater evaluated.