Concentrating Ammonium

Abstract

This thesis provided an overview of the achievable ammonium concentrations, and the energy used to achieve these concentrations, by ion exchange, electrodialysis and reverse osmosis. The need for this overview arose during research for the N2kWh project, where one step in the process focused on increasing the ammonium concentration of (anaerobic digestion) reject water. The research presented some knowledge gaps. As available reports focused on the ammonium removal from the solution, the concentration in the residual stream was barely mentioned. Even when concentrations were reported, the conditions between the discussed technologies made it hard to make a comparison between the three technologies. Even less was reported on the energy that was needed to achieve the various concentrations.

To compare the three technologies, a 1.5 g NH4+/L solution was used as feed water for all three technologies. The technologies used different techniques to increase the ammonium concentration (using zeolites, electrical current over a membrane or high pressure through a membrane). This made comparing the technologies, very difficult. By using the same feed water for all experiments, the achieved concentrations and the energy that was used in the process could be used to compare the three technologies.

The highest ammonium concentration that was achieved during the experiments (7.1 g NH4+/L) was produced by Electrodialysis. Ion exchange achieved a concentration of 6.0 g NH4+/L, while a laboratory scale Reverse Osmosis achieved 4.5 g NH4+/L. The amount of energy used by the three technologies to produce 6 g NH4+/L was: ion exchange = 0.24 kWh, electrodialysis = 6 kWh, reverse osmosis = 4 kWh (per m3 of produced concentrated NH4+ solution). ROSA software was used to estimate energy use for Reverse Osmosis to achieve 6 g NH4+/L (as the laboratory scale Reverse Osmosis was unable to provide sufficient pressure to achieve this concentration). So ion exchange was the most energy efficient, but the maximum achievable ammonium concentration was the lowest of the three technologies. Electrodialysis was able to achieve the highest ammonium concentration but was less energy efficient and used more energy in the process.