Ammonia is a pollutant that is present in many waste streams. When employing wet air oxidation at elevated temperatures for wastes containing nitrogen components, ammonia can be generated in concentrations that negatively impact downstream wastewater treatment facilities. For this reason, upstream segregation of these nitrogenous wastes is required and alternative treatment options, such as incineration, are used.
SCFI ran trials using the AquaCritox® SCWO technology to show that supercritical water oxidation (SCWO) could effectively treat ammonia-containing streams. Results indicate that ammonia destruction efficiencies up to 99.7% can be achieved, oxidising ammonia to nitrogen gas and water. The technology eliminates the need to use incineration and provides a more sustainable solution at a lower cost.
The Application
To date biological treatment has been the main method of removing ammonia from waste inventory. This biological oxidation method is sensitive and is easily inhibited by process contaminants. Wet Air Oxidation (WAO) is an oxidation process that can destroy organics but is known to generate ammonia at high levels when proteins or nitrogenous components are present. This case study aimed at evaluating the ability of supercritical water oxidation to destroy ammonia.
Site Operations
SCFI undertook trials using its AquaCritox demonstration facility in Cork, Ireland, to prove or disprove SCWO’s ability to destroy ammonia with and without organic material present in the feed.
Synthetic streams of various ammonia concentrations were made up. In some of the trials, methanol was used as a source of carbon and in other trials, ammonia was fed to the reactor without the presence of any carbon. This was done to evaluate whether the presence of carbon contributes to the destruction efficiencies achieved.
Ammonia concentrations varied from 1.7 to 3.5 wt% and methanol concentrations were varied between 0 and 2.4 wt%. The waste was injected into the reactor along with 120% of the stoichiometric oxygen requirement. In the various trials, the main reactor temperature ranged between 455°C and 530°C. In all cases, analyses of the treated effluent indicated that a COD and ammonia destruction of more than 98%. Effluent analyses further indicated that the ammonia is converted to gaseous nitrogen (N2) with the maximum nitrite and nitrate concentrations recorded being 2.7 ppm and 182 ppm respectively.
The Success
Ammonia has a very high auto-ignition temperature of 651°C; however, these trials indicate that SCWO can effectively be used to achieve near-complete destruction of ammonia in wastewater at temperatures ranging from 450 – 530°C. The efficiency of the destruction is not related to the presence of organics in the waste stream.
This work proves that SCWO can be used to treat nitrogenous wastewaters containing significant amounts of recalcitrant COD. This process completely destroys all contaminants resulting in a treated effluent that meets discharge specifications.
