Sludge granulation for enhanced biological nitrogen removal in wastewater treatment

Abstract

Nitrogen removal is an important and costly task in biological wastewater treatment. Autotrophic nitrogen removal via partial nitrification and anaerobic ammonium oxidation (Anammox) has been developed to treat ammonium-rich wastewater; however, their slow start-up and strict operating conditions have been the bottlenecks for applications of this process. Heterotrophic nitrogen removal by nitrification and denitrification can be stable but the process has a high demand of oxygen and organic supplies. The present study was carried out to develop innovative biological nitrogen removal systems to improve the treatment performance and reduce the energy consumption and organic demand. Sludge granulation technology was utilized for biological nitrogen removal as it can enrich slow-growing bacteria in bioreactors and improve their resistance to environmental stress. The major developments include: (i) effective start-up strategies with minerals and gel carriers for granulation of partial nitrification sludge, (ii) co-immobilization of ammonia-oxidizing bacteria (AOB) and Anammox sludge in core-shell structured gel beads, and (iii) an innovative step-feed operation for the sequence batch reactor (SBR) system treating low C/N wastewater. Mineral particles and gel beads were used to facilitate the start-up and granulation of partial nitrification sludge. The results showed that both types of the materials could induce aerobic granulation, and sludge dosed with hydroxyapatite achieved the most rapid granulation (4 weeks). Compared to sludge flocs, granular sludge exhibited a higher partial nitrification efficiency (85% vs. 20%) and a stronger resistance to both shear intensity (600 vs. 150 s-1) and pH fluctuation (6.5-8.0 vs. 7.5-8.0). A novel co-immobilization method was developed to entrap AOB sludge and Anammox bacteria in gel beads for autotrophic nitrogen removal in one-stage bioreactors. The AOB-Anammox co-immobilization gel beads exhibited a high capacity and efficiency of nitrogen removal in treating ammonium-rich wastewater. Over 90% of total nitrogen in the influent could be effectively removed via autotrophic denitrification. The biomass in gel beads also showed a strong resistance to pH shocks in a wide pH range of 6-9. The gel beads further served as the carriers to facilitate attached growth of bacteria and sludge granulation in the reactors. The new SBR operating strategy for enhanced nitrogen removal involved the DO control and a feeding modification. Compared to the conventional SBR process, the new SBR process without any external carbon source addition increased the total nitrogen removal efficiency from 31% to 87%. The oxygen-limited aeration facilitated intercellular PHB storage and the stored PHB was used as electron acceptors for nitrate reduction in the subsequent anoxic phase. Sludge granulation was achieved in the SBRs treating actual domestic wastewater with a low C/N ratio. Granular sludge in the new step-feed SBR had a high nitrogen removal efficiency of 90%, whereas that of the conventional SBR remained at around 40%. Microbial population analysis suggested that the abundance of functional nitrifiers, autotrophic and heterotrophic denitrifiers, and PHB-storing bacteria increased in the SBR following the new DO control and feeding modification.