Application of membrane contactor for ammonia recovery from anaerobic digestate

Abstract

Ammonia is the second most-produced inorganic chemical

however, the production of ammonia through the Haber-Bosch process is an energy-intensive process, which uses up to 1% of the world’s energy production. Thus, the recovery of ammonia from waste streams would be beneficial for not only saving energy but also decreasing the nutrient loading to the natural ecosystem. Among various physical and chemical processes to recover ammonia, membrane contactors (MCs) can provide high efficiency with a compact design compared to conventional wet scrubbers. For the practical application of MC for the recovery of ammonia from anaerobic digestate of activated sludge

parameter studies, fouling, and cleaning strategy were examined, and finally, the process was modeled using physicochemical theories. In Chapter 2, a series of experiments were performed to understand the impacts of operational parameters such as pH, flow rate, temperature, feed ammonium concentration, and type of absorbents. The results exhibited that ammonia recovery follows the physicochemical theories, while feed ammonium concentration gives negligible impact. There are no noticeable differences in ammonia recovery for absorbents ($H_3PO_4$, $H_2SO_4$, and HCOOH) tested, but $H_3PO_4$ exhibited slightly better ammonia recovery. At the optimal condition obtained from the experiment, the recovery of ammonia in semi-conductor wastewater was tested. Chapter 3 contains a physicochemical model to simulate and predict MC processes in various operating conditions. The physicochemical models on the balance between ammonium ions and ammonia, phase conversion from the aqueous phase to gas phase expressed by Henry’s law, and gas transport through MC membrane pores explained by Fick’s law, are considered in series at various pH, temperature, contact time, and feed concentration. The results show that combined physicochemical models can successfully match and explain the ammonia recovery in the MC process. Among the operational parameters, pH and temperature are the most influential factors. In Chapter 4, ammonia recovery from anaerobic digestate by MC was investigated. MC is proven to be useful for ammonia recovery

however, during the operation, ammonia recovery was significantly reduced after the deposition of particulates and organic matters. The major foulants attached to the MC were high molecular weight (MW) microorganism oriented organic matter (MOOM). Chemical cleaning using 2% NaCl followed by 2% HCl was proven to be effective on the MC. Related to chapter 3, the pore shrinkage due to the deposition of organic matters could successfully explain the decrease in ammonia removal in MC. In this study, the feasibility of the MC process for the recovery of ammonia has been evaluated, it is suggested and concluded that the three-stage of MC could remove more than 90% of ammonia regarding the initial concentration of ammonia with proper pre-treatment and cleaning mechanism. Future studies may involve different types of cleaning solutions, spatial structure change of the membrane contactor, and more advanced pre-treatment techniques to improve the applicability of MC for ammonia recovery.