| Aerobic activated sludge process has been a major technology for the treatment of domestic and industrial wastewater. However, several disadvantages such as complex process for phosphorus and nitrogen removal, poor stability and high footprint requirement have limited its renovation and application. Aerobic sludge granulation is known as an emerging technology for domestic and industrial wastewater treatment in recent years. Compared with activated sludge, granular sludge has multiple advantages such as dense structure, high biomass retention, and high pollutant removal efficiency. Previous studies mainly focused on affecting factors during granulation process, performance optimization, model simulation of pollutants removment, granular structure and microbial community, while the mechanism of granulation, as well as stable operation strategy for granular process remained unclear, which have become the bottleneck for full-scale application. This paper was aimed to enhance the granulation and stability of aerobic granular sludge process by using hydraulic control strategy. Research on reactor multiphase simulation, configuration optimization, and interaction mechanism between flow field and granules were conducted, and the results are as follow:1. A novel high-speed Charge-Coupled Device (CCD) visualization technology was used to discern the effect of superficial upflow air velocity on flow field, dead region and aggregation process of flocs. Turbulence intensity (IR (total)) were used to quantify the flow field. Results showed that IR (total) increased significantly with the increase of superficial air velocity. Stable and homogenized flow field was achieved under low superficial upflow air velocity.The lifecycle (Tcluster) and fraction (Fratio) were used to quantify aggregation effects of flocs aggregates. It's notable that with the increase of IR(total), PRatio and TCluster decreased gradually, hindering aggregation process and finally causing suppression of aerobic sludge granulation. Results showed that excessive superficial upflow air velocity inhibited the aggregation effects, while too low superficial upflow air velocity led to a dead region in the bottom of reactor, affecting the performance and stability of the system. A moderate superficial upflow air velocity of 1.0 cm s-1 have the smallest area of dead region and relatively low turbulence intensity, and was considered to be the optimized superficial upflow air velocity for aerobic sludge granulation.2. A sludge retention time (SRT) control strategy was proposed to facilitate the growth of dense biomass and aggregation effects treating low-strength wastewater. The novel variable sludge discharge (predetermined by SRT) of slow-settling flocs prevented excessive sludge discharge treating low-strength wastewater. In this study, MLSS and SRT in were successfully stabilized at 11.6±0.24 g/L and 12±1.5 days, respectively, and granulation was achieved after 43 days. While in the conventional reactor, MLSS and SRT fluctuated due to significant washout of sludge, and no granulation was achieved. Meanwhile, TN removal efficiency in R1 increased gradually as the granulation process developed, and stabilized around 65.4±4.3%, while in R2 it was only 48.9±3.1%, and sludge were mainly in floc form.The SRT control strategy led to long SRT, high MLSS and low F/M (food-to-microorganism) ratio. Microbial aggregation and settling ability then increased due to the enrichment of slow growing biomass. High biomass retention was then achieved, which created stronger alkaline condition, and had accumulative effects on both carbon dioxide and calcium. Calcium carbonate particles then formed and performed as primary nucleus, which enhanced the biomass attachment under low-strength wastewater, and then achieved complete granulation. However, with a high MLSS and low F/M, the granules had relatively small size, causing a decline in nitrogen removal efficiency.3. The augment of carriers was believed to promote granulation treating low strength wastewater. In this study, three parallel aerobic granular sludge reactors treating low-strength wastewater were established using granular activated carbon (GAC) of different sizes as the nucleating agent. A novel visual quantitative evaluation method was used to discern how GAC size affects velocity field differences (GAC versus flocs) and aggregation behavior during granulation. Results showed that sludge granulation was significantly enhanced by addition of 0.2 mm GAC. However, there was no obvious improvement in granulation in reactor amended with 0.6 mm GAC.Hydraulic analysis revealed that increase of GAC size enhanced velocity field difference between flocs and GAC, which decreased the lifecycle and fraction of flocs-GAC aggregates. Based on analysis of aggregation behavior and velocity field, flocs and GAC with size of 0.2 mm displayed a relatively uniform velocity field, while the velocity field difference between flocs and GAC with size of 0.6 mm was 7.62 times higher than that of 0.2 mm GAC. The values of Tcluster and Fratio in reactor augmented with 0.6 mm GAC only accounted for 38.5% and 55.6% in reactor augmented with 0.2 mm GAC, which inhibited the flocs-GAC coaggregation. GAC with size of 0.2 mm had similar velocity field to that of flocs, and served as an effective nucleating agent to accelerate microbial coaggregation and aerobic granule formation. 4. Total shear rate (ttotal) combined with shear rate fields and the holdupdistribution fields were used to characterize the overall hydraulic shear stress on the granules. Effects of different superficial upflow air velocities on hydraulics parameters were investaged, which provided theoretical basis for further study on hydraulic control strategy. Reactor with a novel funnel-shaped internal was then developed basedon multiphase simulation of reactor.A high hydrodynamic shear stress increases the attrition of granules, suppressing the overgrowth of large granules, which enhanced stability of aerobic granular sludge process. Results showed that hydraulic shear stress increased with superficial upflow air velocity, and the trend gradually slow down. When superficial upflow air velocity increased from 1.0 cm s-1 to 4 cm s-1, aeration intensity increased 4 times, while ttotai only increased 1.53±0.08 times, causing the waste of energy. Moreover, excessive high superficial upflow air velocity suppressed the aggregation effects of floes, hindering aerobic granulation process. Results showed increasing superficial upflow air velocity was not an effective strategy to improve the stability of reactor, and led to extra energy consumption.Basing on multiphase simulation, a novel funnel-shaped internals was proposed to enhance the stability and pollutant removal performance of an aerobic granular process by optimizing granule size distribution under low superficial upflow air velocity. Results showed up to 68.3±1.4%of the granules in novel reactor (R1) were situated in the optimal size range (700-1900 ?m) compared with less than 29.7±1.1% in conventional reactor (R2), and the overgrowth of large granules was effectively suppressed without requiring additional energy. Total nitrogen (TN) removal efficiency was 81.6±2.1%in R1, which was much higher than that in R2 (48.1±2.7).Results showed the total shear rate (rtotoal) on large granules was selectively increased in R1, which were 3.07±0.14 times higher than that of R2, while ttotai of small granules in Rl was 70.7±4.6% in R2. Further analysis on granular structure shown that the large granules in R1 had compact apparent structure and inorganic inner core (Ca5(PO4)3OH), which reinforces the stability of the large granules in R1. In contrast, the connected irrigated channels and pores structure in granules from R2 result in weak structures, and disintegrated before being washed out of the reactor.The paper comes to conclusions as follows:A moderate superficial upflow air velocity of 1.0 cm s-1 have the smallest area of dead region and relatively low turbulence intensity, and reduce energy consumption of reactor. Optimizing strategies like SRT control, carrier augmentation and novel internal enhance hydraulic conditions in the reactor, which facilitated the formation of aerobic granules and enhanced the stability of the aerobic granular sludge process, providing theoretical support for the full-scale application of aerobic granular sludge process. |
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