Modeling and optimizations of mixing and aeration processes in bioreactors with activated sludge

Mixing aimed at homogenization of the volume of bioreactors with the activated sludge is of great importance for the proper course of the wastewater treatment process. It affects both the efficiency of pollutants removal and the properties of the activated sludge related to its sedimentation. The mixing process in bioreactors can be carried out in different ways. In batch bioreactors in the aeration phase or flow bioreactors in aerobic chambers, mixing is carried out through aeration systems. These systems should aerate the activated sludge flocs for efficient biological treating of wastewater, as well as effectively homogenize the volume of the bioreactor. Hence, it is important to choose such a design of the aeration system and its operation settings that provide the amount of air ensuring the exact amount of oxygen for the implementation of technological processes, counteract sedimentation of sludge at the bottom of the reactor, are reliable as well as economical in operation (demand of electric energy). The paper presents the model studies aimed at optimization of the design and settings of aeration and mixing systems used in active sludge bioreactors.

Influence of spent drilling mud on sedimentation time of activated sludge flocs

Co-treatment of drilling muds with municipal wastewater in the reactors operating on the activated sludge principle constitutes a potentially safe method of their disposal. The method is based on the process of biological degradation of pollutants by assemblages of activated sludge organisms (prokaryotic and eukaryotic), which include different species described as functional and trophic groups. When the ecosystem in the bioreactor is in equilibrium, high wastewater treatment efficiency and process stability can be achieved. Analysis of qualitative and quantitative changes occurring in assemblages of activated sludge organisms may facilitate understanding the causes and mechanisms involved in the observed processes. In such a context, using a model of an SBR wastewater treatment plant, a study was performed to assess the feasibility of co-treating spent drilling mud with municipal wastewater using the activated sludge method. The floc constitutes the basic structural and physiological unit forming activated sludge. In this study, the sedimentation velocity of activated sludge flocs was analysed, and the obtained results were subjected to statistical analysis.

Comparison of Segmentation Performance of Activated Sludge Flocs Using Bright-Field and Phase-Contrast Microscopy at Different Magnifications

Activated sludge (AS) is a type of process which is commonly used for the treatment of sewage and industrial wastewater. In this treatment process, the settling of the sludge flocs is important to ensure the normal functioning of the system, while sludge bulking has become a common and long-term problem that greatly affects floc settleability. Thus, methods based on image processing and analysis are introduced for monitoring AS wastewater treatment plants. However, the effectiveness of using image processing methods heavily depends on the performance of segmentation algorithms. The AS wastewater plant can be monitored through microscopic images of the flocs and filaments. Water samples are taken from the aeration tank of the wastewater plants and then observed using bright field and phase-contrast microscopy to compare the segmentation accuracy at different magnifications i.e., 4x, 10x, 20x, 40x. In this paper, three methods to segment and quantify the flocs in bright field and phase-contrast microscopy images have been analyzed. The first method is image segmentation using Bradley local thresholding method, the second method is texture segmentation using range filtering and Otsu’s thresholding and the third method is Gaussian Mixture Method based segmentation. The experimental results show that Gaussian Mixture Model Method gives the best segmentation accuracy for bright-field microscopy and 10x magnification gives the best results.

Evaluation of hydrophobically associating cationic starch-based flocculants in sludge dewatering

Two series of binary graft cationic starch-based flocculants (CS-DMCs and CS-DMLs) with different hydrophilicity and charge density (CD) were prepared by graft copolymerization of acrylamide with 2-(Methacryloyloxy)-N,N,N-trimethylethanaminium chloride and methacrylic acid 2-(benzyldimethylaminio) ethyl chloride, respectively, on the starch (St) backbone. The sludge dewatering performance of CS-DMCs and CS-DMLs were evaluated and compared based on the changes in filter cake moisture content (FCMC), specific resistance of filtration (SRF), fractions and components of extracellular polymeric substances, and various physiochemical characteristics of sludge flocs and cakes. Increase in CD of the St-based flocculants caused improved sludge dewaterability. Under the similar CD, CS-DML with relatively high hydrophobicity exhibited lower FCMC and SRF, larger and denser sludge flocs, and better permeability of sludge cakes than CS-DMCs due to the synergistic effects of charge neutralization, bridging flocculation and hydrophobic association. Furthermore, a second-order polynomial model on the basis of phenomenological theory was successfully applied to quantitatively evaluate the influences of the two important structural factors of these St-based flocculants, i.e., hydrophobicity and CD, on the sludge dewaterability. The structure–activity relationship of the St-based flocculants in sludge dewatering was obtained according to the theoretic simulation. The dewatering mechanisms was discussed in depth on the basis of the experimental and simulated results; besides, the FCMC and optimal dose can be predicted by the established structure–activity relationship. This current work offered a novel and valuable way to exploit and design of low-cost and high-performance graft natural polymeric flocculants applied in efficient conditioning of sludge.

Characterization of activated sludge flocs by confocal laser scanning microscopy and image analysis

The purpose of this study was to characterize microbial floc structure and properties under phosphorus (P) limiting and non-limiting regimes. The P-limitation applied to the biomass did not significantly impact on reactor performance in terms of COD removal and MLSS. The composition of EPS was affected by the P-limitation with significantly increased accumulation of carbohydrates, uronic acids and proteins. CLSM and glycoconjugate mapping revealed that the relative abundance of α and β- N -acetylgalactosaminyl/galactopyranosyl and N -acetylglucosaminyl residues was affected by P-limitation, suggesting changes in microbial populations within the floc structure, which in turn could cause the compositional changes of EPS. The image analysis performed on CLSM images indicated that under non-limiting conditions the cell clumps within the floc were significantly smaller as compared to P-limiting conditions. The fractal dimension and porosity under limiting conditions were either significantly higher or lower than under P-rich conditions.

Characterization of activated sludge flocs by confocal laser scanning microscopy and image analysis

The purpose of this study was to characterize microbial floc structure and properties under phosphorus (P) limiting and non-limiting regimes. The P-limitation applied to the biomass did not significantly impact on reactor performance in terms of COD removal and MLSS. The composition of EPS was affected by the P-limitation with significantly increased accumulation of carbohydrates, uronic acids and proteins. CLSM and glycoconjugate mapping revealed that the relative abundance of α and β- N -acetylgalactosaminyl/galactopyranosyl and N -acetylglucosaminyl residues was affected by P-limitation, suggesting changes in microbial populations within the floc structure, which in turn could cause the compositional changes of EPS. The image analysis performed on CLSM images indicated that under non-limiting conditions the cell clumps within the floc were significantly smaller as compared to P-limiting conditions. The fractal dimension and porosity under limiting conditions were either significantly higher or lower than under P-rich conditions.

Freeze–thaw combined with activated carbon improves electrochemical dewaterability of sludge: Analysis of sludge floc structure and dewatering mechanism

Freeze–thaw (F/T) and electrochemistry are environment-friendly and efficient sludge treatment technologies. In this study, F/T and electrochemistry were combined in the pretreatment of sludge dewatering in the laboratory, and activated carbon (AC) was added to improve the electrochemical dewatering performance of sludge. During the experiment, the effect of F/T on the floc structure was analyzed by a laser particle analyzer and scanning electron microscope. F/T treatment not only improved the dewatering performance of sludge, but also promoted the aggregation of sludge flocs into larger particles. The median diameter (D50) increased from 45.27 µm to 128.94 µm. Then, the intracellular polymer of large-particle sludge was analyzed by three-dimensional excitation–emission matrix (3D-EEM). The tightly bound extracellular polymeric substances (TB-EPS) still contained a large amount of protein substances, which hindered the improvement of sludge dewatering performance. AC was added to the thawed sludge solution before electrochemical treatment (EP). The conductivity of AC enhanced the effect of EP, thereby cracking the sludge flocs. Thus, the light intensity of TB-EPS in the 3D-EEM fluorescence spectroscopy was decreased, and the D50 was also reduced to 105.3 µm. The final specific resistance of filtration and water content were reduced by 96.39% and 32.17%, respectively. Element analysis of the sludge cake after dehydration showed that the addition of AC significantly improved the combustion efficiency of the sludge cake. Moreover, preliminary economic analysis showed that the cost of this research was low, which indicated the potential application value of combined treatment.

Improvement of phosphorus release from sludge by combined electrochemical-EDTA treatment

In this paper, combined with the addition of ethylenediaminetetraacetic acid (EDTA), the electrochemical treatment of waste activated sludge (WAS) was investigated to explore its effect on the release of phosphorus (P) from WAS. The results showed that during the electrochemical treatment, the addition of EDTA could significantly promote the release of P from the WAS to the supernatant, the optimal amount of EDTA was 0.4 g/g total suspended solids (TSS), when the release of total dissolved phosphorus (TDP), organic phosphorus (OP) and molybdate reactive phosphorus (PO43−-P) were 187.30, 173.84 and 13.46 mg/L, respectively. OP was the most likely form of P to be released during this process. Moreover, combined electrochemical-EDTA treatment could promote the release of P and metal ions from extracellular polymeric substances (EPSs) to the supernatant, and increase the solubility and disintegration of sludge. EDTA chelated the metal ions of sludge flocs and phosphate precipitates to cause sludge floc decomposition, thereby promoting the release of P from WAS.