Evaluation of Novel Ammonia Recovery from the Anaerobic Effluent by Integrating Biogas Stripping and Gypsum Absorption

Abstract

<b><sc>Abstract.</sc></b> Ammonia recovery from the anaerobic digestion (AD) effluent is one of the sustainable processes since it not only mitigates potential environmental impacts but recycles nitrogen from organic wastes to fertilizer for agricultural fields. Most ammonia recovery processes use sulfuric acid to convert ammonia into ammonium sulfate fertilizer because of fast absorption and easy operation in full-scale applications. However, the high acidity causes safety and environmental concerns. It requires the equipment to resist corrosion during operation, which often limits the use of the strong acid on small municipalities and farms. In order to recover the ammonia more efficiently and cost-effectively, this study aimed to develop a novel technology by combining stripping and recovery using biogas and gypsum (CaSO₄·2H₂O) as a stripping agent and a recovery agent, respectively. With the use of biogas as a stripping gas, the stripping gas can supply carbon dioxide (CO₂) without additional input to the gypsum solution, which can recover the stripped ammonia into the fertilizer as ammonium bicarbonate (NH₄HCO₃) and ammonium sulfate ((NH₄)₂SO₄). The experiments mainly investigated ammonia removal and recovery under different levels of gas-liquid ratio for an ammonia stripping, the CO₂ ratio of the stripping gas, and the recovery temperature of the gypsum solution. As a result, the efficiency of ammonia recovery using gypsum had a reversed relation with the CO₂ ratio in biogas and the recovery temperature. When biogas contained 10% CO₂ at the 8-hour operation with 5 l/l∙min, the ammonia removal efficiency reached 75%, resulting in low total ammonia nitrogen (TAN) of 443.86 mg-N/L remained in AD effluent. The gypsum solution recovered around 95% of the stripped ammonia when the stripping gas contained 10% CO₂. This figure rose to 100% when the stripping gas had over 30% CO₂. In terms of the recovery temperature ranging from 20 ℃ to 50 ℃, the recovery efficiency gradually decreased from 100 % to 81 % as temperatures increased. These results indicated that combining biogas stripping with gypsum recovery can effectively eliminate ammonia emission from the AD effluent.