Identifying Bacteria and Sludge Characteristics of Foaming Sludge in Four Full-Scale Wastewater Treatment Plants in Fujian Province, China

Biological foaming is a major problem in activated sludge (AS) wastewater treatment systems. In this study, four wastewater treatment plants (WWTPs) (a total of six AS treatment systems) were investigated. The microscopic examination shows that foaming was mainly caused by gram-positive short branch microorganisms, sludge fragments, and/or other microorganisms, while the long unbranched filamentous was easy to cause bulking. The high throughput sequencing (HTS) and Linear discriminant analysis effect Size (LEfSe) identified the significant discrepancy of bacteria in foams compared to normal AS. Mycobacterium, Mycobacteriaceae, Nocardiaceae, Actinomycetales, Chryseobacterium, Flavobacterium, Ormithobacterium, Flavobacteriaceae, and Portibacter were considered as the dominant foaming-potential bacteria but not the most abundant bacteria in the foams. The excessive growth of foaming bacteria (including Haliscomentbacter, Saprospiraceae, and Tetrasphra) directly led to bulking with a high sludge volume index and was positively correlated with sludge retention time (SRT) and negatively correlated with dissolved oxygen (DO), which means long SRT and low DO may lead bulking instead of foaming. It also found that the foaming bacteria (including Skermania, Comamonadaceae, Cloacibacterium, Flavobacterium, and Chryseobacterium) had significant positive correlations with suspended solids and mixed liquid suspended solids, and negative correlations with temperature and DO concentration.