Ghg emissions for nitrogen removal in wastewater treatment
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Answer:
Traditional technologies, such as selective catalytic reduction and selective noncatalytic reduction, are currently used to control NOx emissions from power plants . However, both processes require operating at high temperatures or using catalysts, which revert to high installation and maintenance costs. These total costs become prohibitive in large-scale facilities treating air flows containing low-to-moderate concentrations of NOx . Recently, many different bioprocesses, using nitrifying and denitrifying bacteria or microalgae, have been developed to control NOx gas emissions. Technologies based on the denitrification process have been successfully used to remove N2O with efficiencies of 75–99% . However, the low aqueous solubility of this greenhouse gas limits the mass transfer rate from the air flow to the liquid phase and, therefore, high hydraulic retention times (HRT) are required to achieve high N2O removal efficiencies. These long applied HRT result in large bioscrubber (or biofilter) volumes, with the subsequent increase in capital costs . Another alternative is to collect the outlet gaseous stream from the top of the nitrifying unit, containing N2O, and use it as oxidizer to burn the methane produced in the anaerobic sludge digester .
Answer:
The operation of wastewater treatment plants results in direct emissions, from the biological processes, of greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), as well as indirect emissions resulting from energy generation. In this study, three possible ways to reduce these emissions are discussed and analyzed: minimization through the change of operational conditions, treatment of the gaseous streams, and prevention by applying new configurations and processes to remove both organic matter and pollutants. In current WWTPs, to modify the operational conditions of existing units reveals itself as possibly the most economical way to decrease N2O and CO2 emissions without deterioration of effluent quality. Nowadays the treatment of the gaseous streams containing the GHG seems to be a not suitable option due to the high capital costs of systems involved to capture and clean them. The change of WWTP configuration by using microalgae or partial nitritation-Anammox processes to remove ammonia from wastewater, instead of conventional nitrification-denitrification processes, can significantly reduce the GHG emissions and the energy consumed. However, the area required in the case of microalgae systems and the current lack of information about stability of partial nitritation-Anammox processes operating in the main stream of the WWTP are factors to be considered.
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