Activated Sludge Morphology Research Articles

Impact of cationic polyelectrolytes on activated sludge morphology and biological wastewater treatment in a Sequential Batch Reactor (SBR)

The variability of the sedimentation characteristics of activated sludge is a phenomenon that often occurs in biological wastewater treatment plants. Deterioration of sedimentation properties requires the application of polyelectrolytes. They affect the morphology of sludge flocs, facilitating and accelerating their deposition. On the other hand, the type of reagent used and its dose may affect the quality of the treated wastewater. This study investigates the effect of dosing two different cationic polyelectrolytes on the properties of activated sludge and treated wastewater parameters in lab-scale sequencing batch reactors (SBRs). During studies, doses of polyelectrolytes that significantly changed activated sludge flocs' morphology were applied. It caused an improvement of the sludge volume index (SVI) from 42 to even 80 % in relation to the control reactor. However, polymers overdose resulted in the SVI increase by 10–14 % and obtaining sludge flocs with a highly branched structure, difficult to thicken. The applied polymers Praestol 855BS and Superfloc C-18530 did not affect the chemical characteristics of the treated wastewater. The only significant effect was observed for the turbidity of the wastewater. Polymers doses reduced the value of that parameter by 54–64 %. In addition, a simulation of a technological failure for several days was introduced to assess the durability of the produced flocs and their impact on the effectiveness of wastewater treatment. The experiment showed that Praestol formed stable and durable flocs, while for Superfloc polymer, a slight temporary deterioration of wastewater quality was observed due to the lower cohesion of the activated sludge.

Long-term effects of hydrocyclone operation on activated sludge morphology and full-scale secondary settling tank wet-weather operation in long sludge age WWTP

This paper presents the study concerning long-term effects of a full scale hydrocyclone unit implemented in a continuous flow long sludge age system, on sedimentation, treatment efficiency and sludge morphology. The research concentrates on identifying the mechanisms of sludge behaviour within the system. The gravimetric selection of activated sludge via a hydrocyclone is a recent development for enhancing sludge separation, where heavier flocs are retained in the system, and lighter ones are discarded as waste sludge. The effects of implementing hydroclyclones were analysed with the use of SEM imagining and fractal dimensioning through the frequent assessment of sludge settling capabilities, effluent quality, and floc properties. Over the course of 60 weeks of hydrocyclone operation, sedimentation efficiency varied significantly. Sludge volume index values of 40 mL/g, achieved during the warm season, were not sustained when the temperature decreased and an overgrowth of filamentous bacteria occurred. Good settling efficiency was also observed in batch tests, where settling velocity of experimental sludge was app. 1 m/h higher than for the reference train at the same concentrations. This was confirmed during wet weather, as the experimental train sustained safe sludge blanket height in secondary clarifiers. SEM imaging and fractal dimension analysis revealed that the underflow that returned to the system had a more compact and spherical shape, which led to an increased content of granule-like particles in the reactor. The presence of flocs with a diameter exceeding 900 μm in the underflow, which is not observed in the feed, indicated agglomeration within the hydrocyclone. This is contradictory to most of the literature data coming from laboratory experiments. This phenomenon was attributed to differences in the size and geometry of the used hydrocyclones, and the potential process mechanism was presented.

Filamentous organisms degrade oxygen transfer efficiency by increasing mixed liquor apparent viscosity: Mechanistic understanding and experimental verification

Recent findings have demonstrated that activated sludge morphology significantly impacts oxygen transfer efficiency (OTE) in the activated sludge process. In this study, we developed a mechanistic understanding of this impact. Mixed liquor samples collected from a domestic wastewater treatment plant (WWTP) were blended with a bulking activated sludge from a bench scale reactor (BSR) cultured on synthetic wastewater to manipulate various morphological parameters such as the settled sludge volume (SV), the sludge volume index (SVI), and the specific filament length (SFL). The filaments that were present in the blended sludges consisted largely of Type 0041 and Type 021N, which are commonly found in WWTPs that treat domestic wastewater. Variations in sludge morphology, as quantified by settled sludge volume after 30 min (SV30), SVI, and SFL, systematically affected the mixed liquor apparent viscosity (μapp), which consequently impacted OTE. An increase in the SFL from 9.61 × 106 μm g−1 to 6.88 × 107 μm g−1 resulted in a 41.4% increase in apparent viscosity and a 24.6% decrease in volumetric mass transfer coefficient (KLa). A new parameter, named the ultimate settleability (SVULT), was developed by curve fitting the SV versus time data and found to relate with μapp through an expanded form of the Einstein Equation for the viscosity. Therefore, SVULT is a corollary for the particle volume fraction that incorporates effects of both the sludge morphology and mass concentration on μapp. Theoretical derivation revealed that an increase in SVULT resulted in an increase in μapp, which reduced oxygen transfer by increasing the air bubble size and reducing refreshment of the liquid at the gas-liquid interface. The KLa was found to be inversely proportional to μapp0.75 through fitting the experimental data with the theoretical model. Using a variance-based global sensitivity analysis, three operating parameters that have the most impact on oxygen transfer were identified: the power input per unit volume, the superficial gas flowrate, and the μapp.

Assessing activated sludge morphology and oxygen transfer performance using image analysis

The morphology of the microbial communities can have dramatic impacts on not only the treatment performance, but also the energy use performance of an activated sludge process. In this research, we developed and calibrated an image analysis technique to determine key morphological parameters such as the floc diameter and the specific filament length (SFL) and discovered that the SFL has significant impacts on sludge floc size, the specific extracellular polymeric substances production, the settleability, mixed liquor viscosity, and oxygen transfer efficiency. When the SFL increased from 2.5 × 109 μm g−1 to 6.0 × 1010 μm g−1, the apparent viscosity normalized by the mixed liquor suspended solids concentration increased by 67%, and the oxygen transfer efficiency decreased by 29%. A long solids retention time (SRT) of 40 day reduced SFL, improved sludge settling performance, and improved oxygen transfer efficiency as compared to shorter SRTs of 10 and 20 day. The findings underscore the need to assess microbial morphology when quantifying the treatment performance and energy performance of activated sludge processes.

Activated sludge morphology significantly impacts oxygen transfer at the air-liquid boundary.

Oxygen transfer is a key process determining the energy use of a biological wastewater treatment process. In this research, we investigated the effect of sludge morphology, especially the role of filamentous microorganisms, on oxygen transfer using bench-scale complete-mix activated sludge reactors with solids retention times (SRTs) of 10-, 20-, and 40-days, respectively. Results indicated 5%-10% reduced aeration need in the 40-day SRT reactor, compared with 10- and 20-day SRT reactors to maintain the same dissolved oxygen level, due to the improvement in sludge settleability and oxygen transfer efficiency (OTE). Filamentous microorganisms adversely impacted OTE via an increase in apparent viscosity of the mixed liquor, which resulted in an increase in the air bubble size and liquid film thickness and, therefore, limited oxygen transfer at the air-liquid boundary. A statistical analysis also confirmed that the mixed liquor viscosity is a statistically significant parameter links to OTE. PRACTITIONER POINTS: Filamentous organisms reduce oxygen transfer via increasing mixed liquor viscosity, which increases air bubble size and liquid film thickness at the air-liquid boundary. Increasing solids retention time reduces filament density. As a result, very long solids retention time promotes oxygen transfer.

Assessing activated sludge morphology by laser and image analysis

An excess of filamentous bacteria is a common settlement problem affecting the activated sludge process. In this study activated sludge morphology was monitored by image analysis and laser scattering. These were compared with the conventional sludge volume index. Good correlations between the two optical techniques for floc size were found when the small and interfering particles (< 50 μm) were removed from the analysis (activated sludge size 100–500 μm). A simpler image analysis procedure was developed using common software and equipment available in most laboratories. This should enable reproducible microbial morphology measurements to be used more widely routinely to predict the onset of poor settlement. The image analysis was used to determine the standard index of filament-induced poor settlement, namely the total extended filament length. The results in common with some previously noted work did not show a simple link between either floc size or total extended filament length and sludge volume index. A new indicator of excess filaments is suggested, based on the frequency of occurrence of just the long extended filaments (> 100 μm) which was shown to give better prediction of poor settlement (bulking).

Effect of Selected Nonionic Surfactants on the Activated Sludge Morphology and Activity in a Batch System

AbstractIn this study, two nonionic surfactants, one alcohol ethoxylate (AE) and one alkylphenol ethoxylate (APE) were investigated with regard to their influence on the morphology of activated sludge flocs, microbial activity and wastewater treatment efficiency in a laboratory batch system. The experiments were carried out for a range of nonionic surfactants concentrations in wastewater from 5 to 500 mg L−1. Additionally, these results were compared to the data obtained in previous experiments on anionics and performed under the same conditions. Both nonionics tested caused a decrease in the size of activated sludge flocs but they did not affect the shape of the flocs. The circularity index and convexity of flocs remained similar to the control run, containing no surfactant. The presence of nonionic surfactants within the tested concentrations range caused a decrease in biomass activity. In spite of morphological changes of activated sludge flocs and a decrease in microbial activity, only higher concentrations of nonionics in wastewater starting with the level of 50 mg L−1 can induce pinpoint flocs and decrease wastewater treatment efficiency. APE showed a stronger impact on the decrease in floc size and microbial activity than alcohol ethoxylate did. APE was also more difficult to biodegrade than AE. Comparing the efficiency of wastewater treatment (in terms of COD removal) in the presence of nonionic and anionic surfactants at the same concentration of 50 mg L−1, the degree of organic pollutant removal was found to be higher by about 10% for anionics than for nonionics.

The influence of the selected nonionic surfactants on the activated sludge morphology and kinetics of the organic matter removal in the flow system

Synthetic surfactants are believed to act on the structure of activated sludge flocs and some of them are toxic towards aquatic organisms. Nevertheless, the number of studies concerning their effect on the activity of sludge biomass and flocs morphology is limited. In this work, the removal of two different nonionic surfactants: alcohol ethoxylate and alkyl phenol ethoxylate in the activated sludge wastewater treatment processes was investigated. In contrast to our previous study, in which the influence of anionic surfactants on flocs morphology was the main aim, in this paper also the chosen aspects of the kinetics of the tested processes were included. Moreover, the influence of two most common groups of synthetic surfactants: anionics and nonionics, on the activated sludge flocs and biomass is compared and discussed. The presence of nonionic surfactants in wastewater at the concentration of 50 mg L −1 in the influent causes a decrease of flocs size when the dilution rate is equal to or higher than 0.102 h −1. The removal rate of organic pollutants for both nonionics tested is lower than in the control run for this range of dilution rates and so is the degree of COD removal, which then does not exceed 80%. The affinity of substrate to the biomass, expressed as a COD half-saturation constant, decreases in the presence of synthetic surfactants, as this constant is elevated in all surfactant runs. The strongest effect was exerted by the anionic surfactants: sodium alkyl benzene sulphonate, while nonionics acted moderately on the affinity of the substrate.

Activated sludge morphology characterization through an image analysis procedure

This work deals with the development of a digital image analysis procedure to characterize microbial flocs obtained in three different WWTP: a bench-scale Sequencing Batch Reactor (SBR) dealing with phenol and nitrogen biological removal, a municipal treatment unit (Ilha do Governador, Rio de Janeiro, Brazil) and an industrial wastewater treatment plant (Ciba - Estrada do Colégio, Rio de Janeiro, Brazil). The developed procedure permits to obtain its morphological parameters like equivalent diameter, compactness, roundness and porosity properties as well as the fractal dimension. This procedure was validated and lead to identify the major relationships between the analysed morphological parameters. A minimum of 300 flocs should be included in the image analysis and a significant influence of the sample dilution step on the mean size of the flocs was verified. The porosity parameter positively correlated with the fractal dimension of microbial aggregates indicating the that highly porous flocs are very irregular.

Relationships between activated sludge morphology and activity

Relationships between activated sludge morphology and activity