Removal of ammonia (nitrification) in conventional and microbial fuel cell type bioreactors

Abstract

The challenge of access to clean water is apparent throughout the world as it experiences overpopulation, the resulting pollution, and struggle with various manifestations of global climate change. In addition, as the first world countries are able to enjoy clean drinking water, they are increasing the global energy demand thereby exacerbating the pollution of limited water resources and global climate change. Nitrogenous compounds such as ammonia, nitrate and nitrite are among the major water pollutants. In this work, biological removal of ammonia and nitrite from contaminated waters and potential for producing electricity were studied using conventional bioreactors and microbial fuel cell (MFC) type bioreactors. Performance of the microbial fuel cell (MFC) has been compared with bioreactors which are conventionally used for nitrification in wastewater treatment plants. Specifically, effects of the contaminant concentration and the types of nitrogenous contaminants (ammonia and nitrite) on the removal rate of ammonia and nitrite, as well as generation of electricity have been investigated. Findings of the present work in the conventional reactors with free cells and biofilms revealed that oxidation of ammonia to nitrite and produced nitrite to nitrate takes place sequentially. The removal rates of both ammonia and nitrite are directly related to feed concentration up to 60 mM. The biofilm reactor was able to handle much higher loading rates of contaminants and led to much higher removal rates at shorter residence times when compared with the continuous reactor (CSTR) with free cells. This was probably due to higher biomass concentration in the biofilm system. The continuous operation of CSTR and biofilm reactors demonstrated that removal rates of both ammonia and nitrite were dependent on the reactor loading rates and that loading rate (residence time) can be used to control the composition of end products during the nitrification process. Finally, results obtained in the microbial fuel cell bioreactors have shown that efficient removal of ammonia and nitrite (nitrification) and generation of electricity can be successfully achieved in this system. Nitrite removal rates obtained in MFC without a mediator are comparable to that obtained in the conventional reactor for certain concentration ranges. While with ammonia, comparable rates are only achieved in the presence of mediator (resazurin).