Physical and chemical properties of activated sludge floc

Physical and chemical characteristics of activated sludge such as floc size, density, specific surface, carbohydrate content, dehydrogenase activity and settleability were investigated by seven parallel bench scale activated sludge units operated under different sludge ages (1.1–17.4 days). The analytical methods used included a dye adsorption technique for specific surface area determinations, the Coulter Counter method for floc size measurements and interference microscopy for floc density determinations. The typical floc sizes were found to be in the range 10–70 μm with floc densities in the range 1.015–1.034 g cm⁻³. A strong correlation between floc density and size was obtained. The specific surface areas measured (typically 100–200 m² g⁻¹ dry sludge) were found to be one to two orders of magnitude higher than the corresponding geometric floc surface areas, indicating a porous floc structure. Sludge settleability, for non-filamentous sludges, was well correlated to both floc size, density and specific surface area, but not to the sludge carbohydrate content, which was found to vary between 6 and 18%.     

Density effects on activated sludge zone settling velocities

A recently developed method to manipulate biomass density based on the addition of synthetic microspheres was used to isolate density effects on zone (hindered) settling velocities in samples from full-scale activated sludge plants over a range biomass densities and concentrations. Settling velocities increased by as much as a factor of five as density was increased by microsphere addition. The effects of density on the Vesilind sedimentation model parameters (V=V(o)e(-kX) , where V is the settling velocity, X is the biomass concentration, and V(o) and k are constants) were evaluated. The parameter V(o) was positively correlated with density for values greater than approximately 1.02 g/mL, while k values were approximately constant in this range. New models were developed for settling velocity as a function of both density and biomass concentration, and these may be useful for future incorporation with clarifier models to improve predictions of system performance, particularly when biomass characteristics known to affect density are variable, such as polyphosphate and nonvolatile suspended solids content.     

A method for characterizing the complete settling process of activated sludge

Based on batch settling tests, a model describing the compression settling process of activated sludge was developed and validated by experiments. Furthermore, a theoretical equation for determining the critical point when the compression settling stage commences, and a new velocity function for the compression settling were deduced from the model. By combining the new model and the conservation of mass, it was proved that the Vesilind function was also capable of describing the compression settling velocity on condition that the appropriate parameters were estimated. Dividing the complete settling process of activated sludge into the zone settling and compression settling stages, and describing them by the Vesilind function with different parameter sets was more reasonable for characterizing the complete settling process of activated sludge. The method was applied to predict the sludge blanket height during batch settling tests, and the results showed that the settling processes could be simulated well.     

An experimental study of low concentration sludge settling velocity under turbulent condition

Particle Image Velocimetry (PIV) was used to study the settling of activated sludge flocs under turbulent flow conditions. Experimental results showed that a larger particle diameter led to a higher settling velocity while the higher turbulence intensity led to lower settling velocity. Based on the measurements a mathematical relation has been derived which correlates the settling velocity for individual sludge flocs under turbulent conditions through a modified Vesilind equation. Settling velocity shows a power-type relation to sludge particle diameter and an exponential-type relation with turbulence intensity and sludge concentration.     

絮凝体沉速在线测量方法及其应用研究

提出了在絮凝池在线测量絮凝体沉速的方法,通过连续采集絮凝体图像和计算机数据处理,跟踪絮凝体运动轨迹,从而计算每一个絮凝体的沉速,同时介绍了数据处理的过程和基本算法。将基于该方法的絮凝体沉速测量设备(FVD)应用于临江水厂,对试验结果进行的分析表明,絮凝体沉速既可作为评价絮凝效果的指标,又可作为控制混凝剂加注量的依据。     

Control of activated sludge settleability using preaeration and preprecipitation

This paper deals with the addition of a preaeration and a preprecipitation stage to a conventional activated sludge process. The experiments were carried out in a pilot plant using wastewater from the city of Valencia (Spain) as feed. The effect of the F/M ratio and sludge age on sludge settleability was established. In both cases, process stability was observed to increase with respect to the conventional process. Good sludge settleability was thus assured in a greater F/M ratio range. A comparative economic study was also carried out.     

Comparison and evaluation of empirical zone settling velocity parameters based on sludge volume index using a unified settling characteristics database

Over the past few decades classic and contemporary research on the process of secondary clarification in activated sludge plants has illuminated several aspects of the solids-flux theory which actually require a closer examination. Inter alia the most challenging and controversial part in the field has been the development of reliable models for the settling properties of the activated sludge in the settler. Numerous studies have been performed aiming at the evolvement of reliable mathematical formulas that would satisfactorily describe this behaviour, but no universally accepted solution seems to exist to date. That is mostly because different experimental conditions, sludge types and instrumentation have been employed, thus complicating the process of reaching a conclusive result. In order to bring theoretical and practical developments of secondary settling tank design and simulation closer together, a number of related tasks are addressed in this study by the use of an integrated and unified settling characteristics database. Several drawbacks and advantages of the methodologies published hitherto are examined on a universal basis and under the same assumptions in order to reveal artifacts that complicate the procedure of settling velocity estimation. It is suggested that universally accepted solutions may be feasible especially for design purposes. For simulation analysis real-time data of settling velocity should be tested rather than values derived from laboratory experiments which are shown to produce different results depending on the applied approach. In conclusion, an integrated database is proposed as a means for a more robust and universally accepted design procedure.     

Gravity drainage of activated sludge: new experimental method and considerations of settling velocity, specific cake resistance and cake compressibility

A laboratory scale setup was used for characterization of gravitational drainage of waste activated sludge. The aim of the study was to assess how time of drainage and cake dry matter depended on volumetric load, SS content and sludge floc properties. It was demonstrated that activated sludge forms compressible cakes, even at the low pressures found in gravitational drainage. The values of specific cake resistance were two to three orders of magnitude lower than those obtained in pressure filtration. Despite the compressible nature of sludge, key macroscopic parameters such as time of drainage and cake solid content showed simple functional dependency of the volumetric load and SS of a given sludge. This suggests that the proposed method may be applied for design purposes without the use of extensive numerical modeling. The possibilities for application of this new technique are, among others, the estimation of sludge drainability prior to mechanical dewatering on a belt filter, or the application of surplus sludge on reed beds, as well as adjustments of sludge loading, concentration or sludge pre-treatment in order to optimize the drainage process.     

On the stability of activated sludge flocs with implications to dewatering

It was shown that Ca²⁺ can be extracted from activated sludge from a plant with biological nitrogen and phosphorus removal either by an ion exchange process, where H⁺, Na⁺, K⁺ or Mg²⁺ served as counter ions, or by chelation by EGTA. The extraction of Ca²⁺ led to an increase in the number of small particles and subsequently an increase in the specific resistance to filtration.

It was argued that approx. half of the total Ca²⁺ pool was associated with the exopolymers and thereby formed an alginate-like gel, which constituted the backbone of the floc structure. This notion was further emphasized by the observation that addition of Cu²⁺ which, along with releasing Ca²⁺, improved the dewaterability—probably because of the Cu²⁺ ions ability to maintain the three-dimensional structure of the exopolymers.     

Temperature and salt effects on settling velocity in granular sludge technology

Settling velocity is a crucial parameter in granular sludge technology. In this study the effects of temperature and salt concentrations on settling velocities of granular sludge particles were evaluated. A two-fold slower settling velocity for the same granules was observed when the temperature of water decreases from 40 °C to 5 °C. Settling velocities also decreased with increasing salt concentrations. Experiments showed that when granules were not pre-incubated in a solution with increased salt concentration, they initially floated. The time dependent increase in mass and hence in settling speed of a granule due to salt diffusion into the granule was dependent on the granule diameter. The time needed for full salt equilibrium with the bulk liquid took 1 min for small particles from the top of the sludge bed and up to 30 min for big granules from the bottom of the sludge bed. These results suggest that temperature and salt concentration are important parameters to consider in the design, start-up and operation of granular sludge reactors and monitoring of these parameters will aid in a better control of the sludge management in anaerobic and aerobic granular sludge technology. The observations also give an explanation for previous reports which were suggesting that a start-up of granular sludge reactors is more difficult at low temperatures.     

Temperature and salt effects on settling velocity in granular sludge technology

Settling velocity is a crucial parameter in granular sludge technology. In this study the effects of temperature and salt concentrations on settling velocities of granular sludge particles were evaluated. A two-fold slower settling velocity for the same granules where observed when the temperature of water decreases from 40 °C to 5 °C. Settling velocities also decreased with increasing salt concentrations. Experiments showed that when granules were not pre-incubated in a solution with increased salt concentration, they initially floated. The time dependent increase in mass and hence in settling speed of a granule due to salt diffusion into the granule was dependent on the granule diameter. The time needed for full salt equilibrium with the bulk liquid took 1 min for small particles from the top of the sludge bed and up to 30 min for big granules from the bottom of the sludge bed. These results suggest that temperature and salt concentration are important parameters to consider in the design, start-up and operation of granular sludge reactors and monitoring of these parameters will aid in a better control of the sludge management in anaerobic and aerobic granular sludge technology. The observations also give an explanation for previous reports which were suggesting that a start-up of granular sludge reactors is more difficult at low temperatures.     

Detailed spatio-temporal solids concentration profiling during batch settling of activated sludge using a radiotracer

In building and tuning good settling models for secondary clarifiers of wastewater treatment plants, there is a need for measured continuous solids concentration profiles during batch settling. Conventional measuring techniques have difficulties in recording this kind of data, either because they are invasive, or because of the low solids concentration and/or solids density of activated sludge. This paper investigates a novel non-invasive measurement technique borrowed from nuclear medicine, using a solids radiotracer and gamma cameras, to obtain solids concentration profiles during the batch settling of activated sludge, in a pilot-scale column with a height of 1m. The technique does not disturb the settling process, does not alter the settling characteristics, gives profiles every minute and every few millimeters, and is capable of measuring in a range of 0-25 g/l with high accuracy. Dynamic solids concentration profile measurements were performed for sludges of different wastewater treatment plants, and at different initial concentrations. The results show a quantitative representation of the settling process, and reveal hindered and compression settling.     

Effects of integrated fixed film activated sludge media on activated sludge settling in biological nutrient removal systems

Integrated fixed film activated sludge (IFAS) is an increasingly popular modification of conventional activated sludge, consisting of the addition of solid media to bioreactors to create hybrid attached/suspended growth systems. While the benefits of this technology for improvement of nitrification and other functions are well-demonstrated, little is known about its effects on biomass settleability. These effects were evaluated in parallel, independent wastewater treatment trains, with and without IFAS media, both at the pilot (at two solids residence times) and full scales. While all samples demonstrated good settleability, the Control (non-IFAS) systems consistently demonstrated small but significant (p < 0.05) improvements in settleability relative to the IFAS trains. Differences in biomass densities were identified as likely contributing factors, with Control suspended phase density > IFAS suspended phase density > IFAS attached phase (biofilm) density. Polyphosphate content (as non-soluble phosphorus) was well-correlated with density. This suggested that the attached phases had relatively low densities because of their lack of anaerobic/aerobic cycling and consequent low content of polyphosphate-accumulating organisms, and that differences in enhanced biological phosphorus removal performance between the IFAS and non-IFAS systems were likely related to the observed differences in density and settleability for the suspended phases. Decreases in solids retention times from 8 to 4 days resulted in improved settleability and increased density in all suspended phases, which was related to increased phosphorus content in the biomass, while no significant changes in density and phosphorus content were observed in attached phases.     

Influence of secondary settling tank performance on suspended solids mass balance in activated sludge systems

Secondary settling is the final step of the activated sludge-based biological waste water treatment. Secondary settling tanks (SSTs) are therefore an essential unit of producing a clear effluent. A further important function of SSTs is the sufficient thickening to achieve highly concentrated return sludge and biomass within the biological reactor. In addition, the storage of activated sludge is also needed in case of peak flow events (Ekama et al., 1997). Due to the importance of a high SST performance the problem has long been investigated ( [Larsen, 1977], [Krebs, 1991], [Takács et?al., 1991], [Ekama et?al., 1997], [Freimann, 1999], [Patziger et?al., 2005] and [Bürger et?al., 2011]), however, a lot of questions are still to solve regarding e.g. the geometrical features (inflow, outflow) and operations (return sludge control, scraper mechanism, allowable maximum values of surface overflow rates). In our study we focused on SSTs under dynamic load considering both the overall unsteady behaviour and the features around the peaks, investigating the effect of various sludge return strategies as well as the inlet geometry on SST performance. The main research tool was a FLUENT-based novel mass transport model consisting of two modules, a 2D axisymmetric SST model and a mixed reactor model of the biological reactor (BR). The model was calibrated and verified against detailed measurements of flow and concentration patterns, sludge settling, accompanied with continuous on-line measurement of in- and outflow as well as returned flow rates of total suspended solids (TSS) and water.As to the inlet arrangement a reasonable modification of the geometry could result in the suppression of the large scale flow structures of the sludge-water interface thus providing a significant improvement in the SST performance. Furthermore, a critical value of the overflow rate (q_crit) was found at which a pronounced large scale circulation pattern develops in the vertical plane, the density current in such a way hitting the outer wall of the SST, turning then to the vertical direction accompanied with significant flow velocities. This phenomenon strengthens with the hydraulic load and can entrain part of the sludge thus resulting in unfavourable turbid effluent.As a representative case study an operating circular SST most commonly used in practice was investigated. Focusing on the sludge return strategies, it was found that up to a threshold peak flow rate the most efficient way is to keep the return sludge flow rate constant, at 0.4Q_MAX. However, once the inflow rate exceeds the threshold value the return sludge flow rate should be slowly increased up to 0.6Q_MAX, performed in a delayed manner, about 20-30 min after the threshold value is exceeded. For preserving the methodology outlined in the present paper, other types of SSTs, however, need further individual investigations.     

Structural, physicochemical and microbial properties of flocs and biofilms in integrated fixed-film activated sludge (IFFAS) systems

Integrated fixed-film activated sludge systems (IFFAS) may achieve year-round nitrification or gain additional treatment capacity due to the presence of both flocs and biofilms, and the potential for multiple redox states and long solids retention time. Flocs and biofilms are distinctive microbial structures and characterization of the physicochemical and structural properties of these may provide insight into their respective roles in wastewater treatment and contaminant removal in IFFAS. Flocs and biofilms were examined from five different pilot media systems being evaluated for potential full scale implementation at a large municipal wastewater treatment plant. Flocs and biofilms within the same system possessed different surface characteristics; flocs were found to have a higher negative surface charge (−0.35 to −0.65 meq./g VSS) and are more hydrophobic (60%-75%) than biofilms (−0.05 to −0.07 meq/g VSS; 19-34%). The EPS content of flocs was significantly higher (range of 2.1-4.5 folds) than that of biofilms. In floc-derived extracellular polymeric substances (EPS), protein (PN) was clearly dominant; whereas in biofilm-derived EPS, PN and polysaccharide (PS) were present in approximately equal proportions. Biofilm EPS had a higher proportion of DNA when compared to flocs. Biofilm growth was preferential on the protected internal surfaces of the media. Colonization of the external surfaces of the media was evident by the presence of small microcolonies. The structural heterogeneity of the biofilms examined was supported by observed differences in biomass content, thickness and roughness of biofilm surface. The biofilm on the interior surface of media was found to be patchy with clusters of cells connected by an irregular arrangement of interconnecting EPS projections. Biofilm thickness ranged between 139 μm and 253 μm. The pattern of oxygen penetration is expected to be complex. Nitrifiers and denitrifiers were predominantly associated with the biofilms, and the latter were found to be dispersed throughout the film and arranged in micro-clusters, suggesting partial oxygen penetration.     

Diffusivity, tortuosity and pore structure of activated sludge

This paper describes new techniques in studying and defining pore morphology of activated sludge. Diffusion measurements on activated sludge are carried out in a diffusion cell. The average diffusivity in activated sludge is found to be 7.4 × 10⁻¹⁰ m² s⁻¹. Surface area determinations are used to calculate the mean pore radius of activated sludge flocs. This value is found to be between 108 and 130 Å. The tortuosity of the pores in activated sludge is also calculated and found to have an average value of 2.73.     

An improved method using respirography for the design of activated sludge aeration systems

For the computation of the oxygen uptake for the aeration tank of the biological treatment plant several methods are available such as the OC/L method, Emde's method with fixed coefficients, Eckenfelder's method, and the method of levels of dissolved oxygen. The method for the determination of oxygen uptake for the treatment of industrial wastewaters was developed. The advantage of this method is a simple calculation of oxygen uptake for industrial biological treatment plant due to independent measurement of oxygen uptake for individual phases. The method is rapid and allows easy determination of individual phases of oxygen uptake.     

An examination of the surface characteristics of activated sludge in relation to bulking during the treatment of paper mill wastewater

The addition of mineral talc, Aquatal^®(Toulouse, France), to activated sludge treating paper mill effluents and its effect on settlement characteristics has been investigated. One laboratory study and a full‐scale investigation on a large waste water treatment plant (WWTP) with a capacity of 500.000 population equivalent (p.e.) have been carried out using this mineral. The sludge in the full‐scale plant was filamentous and had specific sludge volume index (SSVI) values in excess of 250. The talc dosing which was adapted to sludge settleability reduced SSVI values by 38%, improved the total suspended solids' concentrations by 86% and reduced the specific sludge load by 34%. In the laboratory‐scale plant, the sludge had SSVI values of 200–260 before treatment and was hydrophilic. The use of Aquatal^® at a dose of 0.7 g/g mixed liquor suspended solids (MLSS) took 2 weeks to reduce the SSVI to 78. It also increased the hydrophobicity to a balanced 50%. The use of mineral converted the somewhat diffuse flocs into a compact structure.     

Ecological study of activated sludge settling property in the completely mixed system

The bulking problem associated with both nitrogen-rich and nitrogen-deficient activated sludges was thoroughly investigated under a DO sufficient condition in the completely-mixed continuous flow system. The reactor temperature was controlled at 27 ± 1°C and the pH was maintained at nearly equal to 7.0. The activated sludge cultures were acclimated under various organic loadings and their settling characteristics were determined by sludge volume index and zone settling velocity. The contents of sludge protein and carbohydrate along with the cell surface charge were examined for understanding the correlation between sludge composition and sludge settleability. Also, the major types of long-length filaments existed in the bulky activated sludge were identified. Moreover, the effect of sludge settleability on effluent quality was determined.