Improvements on activated sludge settling and flocculation using biomass-based fly ash as activator

Nitrospira dominant pin-point flocs with granule-like settleability in stirred tank reactors with oxic/hypoxic/oxic zones

The characteristics of biomass and microbial community dynamics, in relation to autotrophic nitrification, were studied in two 20 L stirred tank reactors (STR) with oxic/hypoxic/oxic zones. The bioreactors were fed with synthetic wastewater with stepwise increasing ammonia concentrations (50-200 N mg/L) without organic substrate in the first phase (autotrophic phase) for 35 days (R1) and 15 days (R2), followed by a heterotrophic phase (with supplementation of organic substrate). The settling properties of the biomass, represented by pin-point flocs, gradually improved in both reactors during the autotrophic phase. The pin-point flocs of R1 exhibited granule-like settling properties. The SVI30 in RI gradually improved to 29 mL/g MLSS, and the corresponding SVI30/ SVI10 gradually improved to 0.88 during the autotrophic phase. The settling properties of the biomass deteriorated in both bioreactors during the heterotrophic phase. The protein to polysaccharide ratio (PN:PS ratio) gradually increased in the extracted EPS (in both, loosely bound (LB) and tightly bound (TB) EPS) during the autotrophic phase, in both bioreactors. The TB:LB EPS ratio was higher when the pin-point flocs of R1 showed granule-like settling properties, followed by a decline in TB:LB EPS ratio during the heterotrophic phase. A combination of molecular approaches (droplet digital-PCR (dd-PCR) and 16S rRNA gene sequencing) revealed that Nitrospira were the predominant nitrifying bacteria in the pin-point flocs that show granular sludge-like settling properties during autotrophic phase in R1. Comammox Nitrospira was the dominant ammonia oxidizer in seed biomass and at low ammonia concentrations in both bioreactors. The relative abundance of canonical ammonia-oxidizing bacteria increased with an increase in influent-ammonia concentrations.

Effect of different forms and components of EPS on sludge aggregation during granulation process of aerobic granular sludge

The characteristics of three forms of extracellular polymeric substances (EPS) and their component proteins (PN) and polysaccharides (PS) during the granulation process of aerobic granular sludge (AGS) and their effects on the aggregation performance of AGS were studied. Meanwhile, Pearson correlation analysis was conducted to explore the correlations between different forms of EPS and their components and sludge properties in the granulation process. The results showed that the AGS reactor could be started within 40 days by using the strategy of decreasing sedimentation time, and the sludge granulation degree was 90.22%. It was found that the content of tightly bound EPS (TB-EPS) accounted for 72.81-93.11% of the total EPS. Compared with dissolved EPS (S-EPS) and loosely bound EPS (LB-EPS), TB-EPS had a great impact on the zeta potential and hydrophobicity of sludge surface. During the granulation process, the content and relative hydrophobicity of PN in TB-EPS increased from 30.38 ± 0.12 mg/g VSS to 83.99 ± 2.05 mg/g VSS and 37.63%-71.25%, respectively. Through the flocculation experiment, it showed that the contribution rate of TB-EPS to the flocculation capacity of sludge cells increased from 13.30% to 35.37% and that of PN to flocculation capacity was 76.93%, which was 6.8 times that of PS. It indicated that TB-EPS played the most critical role in the formation of AGS and the PN in EPS was the major contributor to promoting the formation of AGS and enhance its aggregation performance. This research could further understand the role of various forms of EPS and its components in the process of sludge granulation, reveal the sludge granulation mechanism, and provide theoretical support for the rapid start-up of AGS reactor.

Comparative study of the effect of rice husk-based powders used as physical conditioners on sludge dewatering

The effects of rice husk flour (RHF), rice husk biochar (RHB), and rice husk-sludge cake biochar (RH-SCB, expresses sludge cake biochar deriving from a sludge that has been previously conditioned with rice husk) used as physical conditioners on sludge dewaterability were compared. The effects of characteristics of physical conditioners on sludge compressibility and zeta potential were analyzed. The optimal rice husk-based powder was RH-SCB, which presented the highest net sludge solid yield (Y_N, expresses the dry mass flow by filtration) at 20.39 kg/(m² h) for 70% dry sludge (DS). Characterization analysis indicates that the hardness and surface Fe content of powders which could influence the compressibility coefficient of sludge cake and sludge zeta potential were the major factors influencing sludge dewaterability. The comparison of feasibility and economic analysis showed that adding RH-SCB improves the quality of the sludge filtrate and reduces the pollution potential of conditioned sludge (the ratio of secondary and primary (RSP) of Cu, Zn, Cd reduces from 43.05, 144.00, 7.25 to 7.89, 14.63, 4.27, respectively), and the costs of using RH-SCB were the lowest (at 88.4% lower than that of the raw sludge). Therefore, it is feasible to use RH-SCB to improve sludge dewaterability.