Ammonia recovery from simulated anaerobic digestate using a two‐stage direct contact membrane distillation process
Bing Xu, Zhen He
Abstract
Abstract Ammonia is a key inorganic contaminant in wastewater and an important nutrient element for agriculture. Herein, a two‐stage direct contact membrane distillation (DCMD) system was developed and investigated for ammonia recovery from a synthetic anaerobic digestate. In the 1st stage DCMD (DCMD‐1), both ammonia and water moved across MD membrane to realize ammonia separation, while in the 2nd stage (DCMD‐2), only water migrated and as a result ammonia was concentrated. It was found that increasing the initial feed solution pH could enhance ammonia removal in the DCMD‐1 from 16.0 ± 2.0% (no pH adjustment) to 84.2 ± 1.9% (pH 12). A higher feed solution temperature increased both ammonia flux and water flux. The optimal condition was determined as an initial feed pH of 12, a feed temperature of 60°C, and the 0.6 M H 2 SO 4 adsorption solution. With the addition of the DCMD‐2, the ammonia concentration was improved from 3 g L −1 to 7.8 ± 0.2 g L −1 , which was further enhanced to 26.3 ± 3.0 g L −1 after five batches of operation. These results have demonstrated the feasibility of a two‐stage DCMD system for ammonia recovery from anaerobic digestate and warrant further investigation of several key issues that may advance this technology. Practitioner points A two‐stage membrane distillation system is developed to remove and recover ammonia from anaerobic digester effluents. The system uses ammonia/ammonium equilibrium to separate ammonia in the 1st stage and then concentrate it in the 2nd stage. A high initial pH of the feed solution plays a key role in achieving high ammonia removal. Minimizing the volume of permeate solution can increase the ammonia concentration.