Stable nutrient removal from wastewater with fluctuating seawater content ensured by an adaptable aerobic granular sludge microbiome

Seawater intrusion in coastal regions can alter the wastewater composition, threatening the microbial communities in wastewater treatment processes. An aerobic granular sludge (AGS) system was challenged by fluctuations in wastewater salinity levels promoted by seawater intrusion events for 286 days, divided into two stages. During stage I, the seawater content in wastewater increased stepwise, and over stage II the seawater content in wastewater oscillated throughout the day. Most of the time, the AGS effectively removed COD during the anaerobic phase, regardless of the wastewater salt content. Ammonium removal was slightly unstable (ca. 75 ± 19 %) during stage I, with nitrite release in the effluent. Over stage II, the ammonium content in the wastewater was fully removed. The nitrite content in the effluent decreased, and nitrate became the main nitrogen form released. Phosphate removal was quite unstable at the beginning, improving over time with complete removal achieved during stage II (ca. 98.4 ± 1.1 %). Taxa involved in nitrogen and phosphorous removal were identified in the AGS microbiome at both stages but with superior abundance in the latter stage. A diverse core microbiome, mainly composed of extracellular polymeric substances-producing bacteria (Thauera, Flavobacterium, Paracoccus) and denitrifying bacteria (Thiotrix, Azoarcus, Aequorivita) was identified in stage II. The AGS system efficiently managed daily oscillating seawater levels in wastewater, corroborated by the effective removal performance that seemed to be sustained by an adaptable AGS microbiome.