Nitrogen recovery from anaerobic digestate via ammonia stripping and absorbing with a nitrified solution

2021 Spring.Includes bibliographical references.Animal wastes cause environmental pollution, including contamination of air and water, when not managed properly. For example, stored livestock manure releases greenhouse gasses, which contribute to air pollution and global warming. Anaerobic digesters have been used for animal waste treatment in order to reduce the environmental impacts of animal wastes. However, current anaerobic digestion systems have serious economical and operational challenges such as high capital cost, low byproduct price, and ammonia toxicity. Therefore, more research is needed to increase the benefits of anaerobic digestion and reduce its challenges. The goal of this project was to improve the cost and performance of anaerobic digesters by enhancing their byproducts, biogas and fertilizer, while reducing one of their serious operational challenges, ammonia toxicity. To achieve these goals, this project investigated an integrated anaerobic digestion nitrogen recovery process that includes anaerobic digestion, nitrogen recovery and nitrification. The nitrogen produced during anaerobic digestion is volatilized in a stripper, captured in an absorber, and converted to nitrogen certified organic fertilizer in the nitrification process. Recovering the ammonia in anaerobic digesters not only produces organic fertilizer but also reduces ammonia toxicity, enhancing biogas production. Experiments and modeling were used to identify appropriate operating conditions for the stripper and absorber units of the proposed process. The objective of the nitrogen recovery system experiments was to find the best operational conditions as well as to evaluate the performance of the nitrification solution as an ammonia absorbent. Stripping and absorption columns were designed to measure the ammoniacal nitrogen recovery. The ammonia stripping and absorption extents were calculated for several operational conditions: stripping and absorption feed pH, stripping temperature and absorbent nitrogen concentration. The experimental results showed that a feed pH of 10 was optimal for ammonia stripping in the pH range 8.5–10.5, providing an ammonia stripping extent of 77%, while the optimal stripping temperature was 50 °C since it provides the highest extent of ammonia stripping in the tested range of 35–65 °C. An Aspen Plus simulation model was also developed for the ammonia stripping process to calculate the effects of the number of equilibrium stages, feed pH, and the amount of CO2 in the stripping gas. The model showed that the use of three equilibrium stages, a feed pH of 10, and having no CO2 in the stripping gas provides the most feasible operational conditions considering the stripping performance and economics. Moreover, the data suggested that the stripping units will require pH control for effective ammonia recovery since the pH of the stripper decreases with the ammonia removal. For the ammonia absorption unit, the experimental data showed that ammonia absorption was not greatly impacted by the feed pH nor by the concertation of nitrogen in the liquid feed. With a low concentration of nitrogen in the liquid feed (2 g/L NH4NO3 as N), the extents of ammonia absorption for feed pH values of 7 and 2 were 82% and 92, respectively. However, the extents of ammonia absorption using a high concentration nitrogen liquid feed (7 g/L NH4NO3) for feed pH values of 7 and 2 decreased to 70% and 85%, respectively. However, a Two-Factor ANOVA test with replication has a p-value >0.05, so there is no statistically significant difference in the ammonia absorption due to the feed pH nor in the concentration of nitrogen in the absorbent. Consequently, it can be concluded that nitrified solution can be used as an ammonia absorbent because it can affectively absorb ammonia over a wide range of its pH and its nitrogen concentration. This project demonstrated that it is possible to recover nitrogen in an integrated anaerobic digestion process and determined recommended operational conditions for the nitrogen recovery system. The novel integrated anaerobic digestion system proposed in this work decreases ammonia toxicity for anaerobic digestion, while increasing potential for revenue from increased biogas yield and recovery of ammonia fertilizer. increasing the biogas yield, producing organic fertilizer and decreasing ammonia toxicity