How weak static magnetic field contributes to rapid granulation and better performance of microalgal-bacterial granular sludge?

Microalgal-bacterial granular sludge (MBGS) has received considerable attention as an emerging sustainable process for wastewater treatment, whereas achieving rapid granulation is still challenging. Herein, we applied a static magnetic field (5 mT) to accelerate MBGS granulation in photo-sequencing batch reactors (PSBRs). Results showed that complete granulation of MBGS in the reactor with magnetic field (RM) was achieved 20 days before that in the control reactor (RC). The magnetic field significantly promoted microbial growth, sludge settleability and granule density. The treatment performance for COD (84.1 %–96.7 %), ammonium (91.7 %–100 %) and phosphorus (71.5 %–83.3 %) was efficient and similar in both reactors. However, the volumetric removal rate for total nitrogen (TN) was greatly improved by magnetic field due to increased nitrogen assimilation and denitrification. Moreover, mature MBGS in RM showed higher specific oxygen production rate (SOPR) and specific oxygen uptake rate (SOUR), approximately 90 % and 16 % greater than those in RC. The accumulation of metal ions (iron and calcium), the enhanced production of extracellular polymeric substances (EPS, especially proteins (PN)), and the enrichment of functional bacteria (EPS producers and potential denitrifiers) and phototrophs (Tetradesmus, Monoraphidium and Desmodesmus) contributed to rapid granulation and better performance of MBGS. This study provides an economic, effective and environmental-friendly method for rapid cultivation of MBGS, which would be helpful for large scale applications of MBGS.