Minimizing greenhouse gas emission from wastewater treatment process by integrating activated sludge and microalgae processes

Abstract

A novel integrated microalgae and activated sludge (MA/AS) system was constructed to minimize greenhouse gas emission from traditional wastewater treatment plants. Its removal properties for aqueous pollutants were assessed as well. The ratio of microalgae-to-activated sludge volatile suspended solids of 1.3 and an incident light intensity of 12 W/m2 provided the best performance: COD, NH4+, and total phosphorus (TP) removals were up to 100%, 99.6% and 100%, respectively. Even without illumination, COD, NH4+, and TP removal efficiencies were as high as 95.1%, 96.5% and 100%, respectively. In both cases, nutrient uptake by MA was proved to play an important role in nutrients removal. And no CH4 or N2O emissions were detected during the whole experimental period of the MA/AS system (mass ratio of 1.3:1). Only negligible CO2 was detected up to 45 μmol with illumination and 130 μmol without illumination in the headspace of the serum bottles, which merely accounted for 2.0% and 5.8% of the initial total carbon equivalent (glucose serving as organic carbon source). Since photosynthesis by microalgae could provide oxygen to heterotrophs or nitrifying bacteria, extra energy demand (mainly from aeration units) could be greatly cut down, which would heavily reduce the total energy demands and further indirect CO2 emission from wastewater treatment plants. Our integrated system is demonstrated to be a sustainable approach for contaminants removal from aqueous phase, restraining greenhouse gas emission and saving energy consumption contemporaneously.