Addition of sodium alginate as a nucleus shortens granulation of aerobic sludge

Abstract

To date, only a handful of studies have described application of organic materials as carriers (nuclei) in the aerobic granular sludge (AGS) system, compared to inorganic materials. Here, we evaluated the feasibility of sodium alginate (SA) as a nucleus (SAN) to enhance the aerobic granulation process. Varying dosages of SAN were mixed with activated sludge in lab-scale sequencing batch reactors, and the system operated for 15 days. Results from batch experiments showed that addition of SAN not only effectively accelerated aerobic granulation but also improved biomass retention, while overdosing would enhance the hydrophilicity of sludge and eventually affect the granulation process. The optimal dosage was found to range between 0.04 and 0.08 mg g⁻¹ MLSS. Addition of SAN to P2-R1 resulted in AGS that had an average size of 1.9 mm after around ten days, while no particles were observed in the absence of SAN. Moreover, mature granules following addition of SAN had channels and pores, with a dense and rough surface. With regards to the efficiency of pollutant removal, P2-R1 exhibited markedly higher removal rates of chemical oxygen demand (COD) (80%), ammonia nitrogen (99%), total nitrogen (60%), and total phosphorus (70%), than P2-BK without SAN which exhibited 40%, 60%, 40%, and 40%, respectively. Further, extracellular polymeric substances (EPS) in P2-R1 were enhanced during granulation, especially extracellular proteins (PN), indicating that PN plays an important role in connecting microbial cells to form AGS. Results from microbial community analysis showed that P2-R1 was mainly enriched with bacteria that promoted EPS secretion and nitrogen removal. Overall, these results indicate that SAN application is an efficient and eco-friendly method for improving aerobic granulation.