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.     

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.     

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.     

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.     

A predictive model for the reactor inorganic suspended solids concentration in activated sludge systems

A simple predictive model for the activated sludge reactor inorganic suspended solids (ISS) concentration (excluding that from chemical precipitant dosing) is presented. It is based on the accumulation of influent ISS in the reactor and an ordinary heterotrophic organism (OHO) ISS content (fiOHO) of 0.15 mg ISS/mg OHO organic (volatile) suspended solids (VSS) and a variable phosphate accumulating organism (PAO) ISS content (fiPAO) proportional to their P content (fXBGP). Organism ISS content is conceptualized as the uptake of dissolved inorganic solids by active organisms, which when dried in the total suspended solids (TSS) test procedure, precipitate and manifest as ISS. The model is validated with data from 22 investigations conducted over the past 15 years on 30 aerobic and anoxic-aerobic nitrification-denitrification (ND) systems and 18 anaerobic-anoxic-aerobic ND biological excess P removal (BEPR) systems variously fed artificial and real wastewater, and operated from 3 to 20 days sludge age. The predicted reactor VSS/TSS ratio reflects the observed relative sensitivity to sludge age, which is low, and to BEPR, which is high. To use the model for design, two parameters need to be known: (1) the influent ISS concentration, which is not commonly measured in wastewater characterization analyses and (2) the P content of PAOs (fXBGP), which can vary considerably depending on the extent of anoxic P uptake BEPR that takes place in the system. Some guidance on the measurement of influent ISS concentration and selection of the PAO P content to calculate the mixed liquor VSS/TSS ratio for design is given.     

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.     

Sorption and biodegradation of sulfonamide antibiotics by activated sludge: experimental assessment using batch data obtained under aerobic conditions

This study investigated the adsorption, desorption, and biodegradation characteristics of sulfonamide antibiotics in the presence of activated sludge with and without being subjected to NaN₃ biocide. Batch experiments were conducted and the relative contributions of adsorption and biodegradation to the observed removal of sulfonamide antibiotics were determined. Three sulfonamide antibiotics including sulfamethoxazole (SMX), sulfadimethoxine (SDM), and sulfamonomethoxine (SMM), which had been detected in the influent and the activated sludge of wastewater treatment plants (WWTP) in Taiwan, were selected for this study. Experimental results showed that the antibiotic compounds were removed via sorption and biodegradation by the activated sludge, though biodegradation was inhibited in the first 12 h possibly due to competitive inhibition of xenobiotic oxidation by readily biodegradable substances. The affinity of sulfonamides to sterilized sludge was in the order of SDM > SMM > SMX. The sulfonamides existed predominantly as anions at the study pH of 6.8, which resulted in a low level of adsorption to the activated sludge. The adsorption/desorption isotherms were of a linear form, as well described by the Freundlich isotherm with the n value approximating unity. The linear distribution coefficients (K_d) were determined from batch equilibrium experiments with values of 28.6 ± 1.9, 55.7 ± 2.2, and 110.0 ± 4.6 mL/g for SMX, SMM, and SDM, respectively. SMX, SMM, and SDM desorb reversibly from the activated sludge leaving behind on the solids 0.9%, 1.6%, and 5.2% of the original sorption dose of 100 μg/L. The sorbed antibiotics can be introduced into the environment if no further treatments were employed to remove them from the biomass.     

Characterizing the extracellular and intracellular fluorescent products of activated sludge in a sequencing batch reactor

Three-dimensional excitation-emission-matrix (EEM) fluorescence spectrometry was used to characterize the extracellular and intracellular substances of activated sludge in a sequencing batch reactor (SBR). Parallel factor analysis (PARAFAC) was applied to extract the pure spectra from the overlapped spectra. Three main components, proteins, fulvic- and humic-like substances, were identified from the extracellular substances. Their fluorescence peaks were at an excitation/emission (Ex/Em) of 280/350, 340/400 and 390/450 nm, respectively. The fluorescence of the extracellular proteins had a similar changing pattern with the wastewater chemical oxygen demand, the fulvic-like substance did not vary significantly in a cycle and the humic-like substances accumulated in the substrate uptake phase but decreased later. Proteins and nicotinamide adenine dinucleotide, reduced form (NADH), were identified as the two main intracellular fluorophores, and their fluorescence peaks (Ex/Em) were at 280/340 and 350/450 nm, respectively. The fluorescence intensity scores of the intracellular fluorophores were closely related to the bioreactor performance. Thus, the results of this work provide a foundation for potential utilization of the EEM fluorescence spectroscopy to monitor the activated sludge systems for wastewater treatment.     

Stroboscopic determination of settling velocity, size and porosity of activated sludge flocs

A multi-exposure photographic method was developed for the combined measurement of the settling velocity and size of activated sludge flocs. The proposed method mainly differs from the previous stroboscopic tests by introducing a new experimental arrangement which ensures sharp images of flocs on photographs.The relationships between individual floc-settling velocity and the floc size were found to be linear or fractional power functions. All these relationships were well correlated.Based on a simplified floc structure assumption and results of experimental measurements of floc-settling velocity and size, the floc porosity was determined. The porosity increased at two distinct rates as the floc size increased.     

Fate of natural estrogens in batch mixing experiments using municipal sewage and activated sludge

Since natural estrogens such as 17beta-estradiol (E2) and estrone (E1) are excreted daily by humans, E2 and E1, which are classified as inevitable endocrine-disrupting chemicals, are always present in sewage wastewater. For several years, the monitoring and removal of natural estrogens at sewage treatment plants have been examined by many investigators. However, little is known regarding the exact behavior of estrogens in actual sewage when in contact with activated sludge. In this study, the fate of E2 and E1 as a result of adsorption and decomposition in batch mixing experiments using municipal wastewater and activated sludge collected from an actual municipal sewage treatment plant was investigated. Estrogen concentrations were determined using an enzyme-linked immunosorbent assay (ELISA) kit. E2 and E1 in sewage were removed from the liquid phase in contact with activated sludge, and E2 and E1 adsorbed on the sludge were decomposed in 4h. Significant changes in the adsorption and decomposition of E2 and E1 on the sludge were not observed at low temperatures or when different sludge samples were used such as those acclimatized to low-loading and high-loading conditions. In contrast, the processes leading to the removal of estrogens, such as the adsorption and decomposition of estrogens in contact with activated sludge, were inactivated by sterilizing the sludge. Natural estrogens adsorb onto the activated sludge, and are therefore easy to be biodegraded. In a biological reaction process, therefore, the estrogens will be rapidly removed at the initial stage, when the sewage is satisfactorily mixed with the sludge.     

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.     

Quenching effects in the application of multi-channel fluorescence in activated sludge suspended solids

The potential of multi-channel fluorescence as an in-line monitor of the activated sludge process has been investigated. Humic substances are known to be present as an important constituent in most activated sludges, and it was tested whether humic substances influenced the fluorescence signals from protein and NADH/NADPH. A negative correlation between the humic acid (HA) signal and the signals from both protein and NADH/NADPH was found. This phenomenon was investigated in detail by studying the interaction between a standard HA (Janssen) and a standard protein (bovine serum albumin, BSA). When mixing HA and BSA, the fluorescence signal from BSA declined or even disappeared. The quenching of the fluorescence from BSA was shown to relate to an adsorption of HA onto the protein as demonstrated by capillary electrophoresis. This quenching phenomenon was also observed in activated sludge, which was spiked with HA. As 250 mg HA/L was added to activated sludge, the fluorescence signal from both protein and NADH/NADPH was reduced by 20-30%. The results indicate that it is difficult to carry out quantitative analyses of the components in activated sludge by multi-channel fluorescence, and that variations in HA can affect the signals representing the biomass and the biological activity.     

Micro-profiles of activated sludge floc determined using microelectrodes

The microbial activity within activated sludge floc is a key factor in the performance of the activated sludge process. In this study, the microenvironment of activated sludge flocs from two wastewater treatment plants (Mill Creek Wastewater Treatment Plant and Muddy Creek Wastewater Treatment Plant, with aeration tank influent CODs of 60-120 and 15-35 mg/L, respectively) were studied by using microelectrodes. Due to microbial oxygen utilization, the aerobic region in the activated sludge floc was limited to the surface layer (0.1-0.2mm) of the sludge aggregate at the Mill Creek plant. The presence of an anoxic zone inside the sludge floc under aerobic conditions was confirmed in this study. When the dissolved oxygen (DO) in the bulk liquid was higher than 4.0mg/L, the anoxic zone inside the activated sludge floc disappeared, which is helpful for biodegradation. At the Muddy Creek plant, with its lower wastewater pollutant concentrations, the redox potential and DO inside the sludge aggregates were higher than those at the Mill Creek plant. The contaminant concentration in the bulk wastewater correlates with the oxygen utilization rate, which directly influences the oxygen penetration inside the activated sludge floc, and results in redox potential changes within the floc. The measured microprofiles revealed the continuous decrease of nitrate concentration inside the activated sludge floc, even though significant nitrification was observed in the bulk wastewater. The oxygen consumption and nitrification rate analyses reveal that the increase of ammonia flux under aerobic conditions correlates with nitrification. Due to the metabolic mechanisms of the microorganisms in activated sludge floc, which varies from one treatment plant to another, the oxygen flux inside the sludge floc changes accordingly.     

Effect of solids retention time on structure and characteristics of sludge flocs in sequencing batch reactors

The effect of solids retention time (SRT) (4-20 d) on sludge floc structure, size distribution and morphology in laboratory-scale sequencing batch reactors receiving a glucose-based synthetic wastewater was studied using image analysis in a long-term experiment over one year. Floc size distribution (>10 microm) could be characterized by a log-normal model for no bulking situations, but a bi-modal distribution of floc size was observed for modest bulking situations. In each operating cycle of the SBRs, the variation in food /microorganisms ratio (0.03-1.0) had no significant influence on floc size distribution and morphology. The results from a long-term study over one year showed that no clear relationship existed between SRT and median floc size based on frequency. However, sludge flocs at the lower SRTs (4-9 d) were much more irregular and more variable in size with time than those at higher SRTs (16 and 20 d). The level of effluent-suspended solids at lower SRTs was higher than that at higher SRTs.     

Relationship between floc composition and flocculation and settling properties studied at a full scale activated sludge plant

The variation in activated sludge floc composition, flocculation and settling properties was studied at a full scale plant over a 2-year period. A comprehensive set of process parameters was analysed and related to the floc properties to increase the understanding of the factors affecting floc formation. The composition of the activated sludge showed a seasonal change with higher concentrations of extractable extracellular polymeric substances (EPS) during the winter months. The protein content of the total sludge and EPS increased significantly during the winter. This coincided with higher effluent suspended solids concentrations and increased shear sensitivity of the sludge flocs. Only poor correlations between EPS contents and stirred sludge volume index (SSVI) could be observed. High iron concentrations in the sludge due to dosage of iron salt to precipitate phosphorus were found to have a negative impact on the settling and compaction properties of the sludge, whereas it had a positive impact on floc stability. Higher organic loading due to by-passed primary settlers leads to improved settling and compaction properties.     

Dynamic response of nitrifying activated sludge batch culture to increased chloride concentration

Dynamic response of nitrifying activated sludge batch cultures to increased chloride concentration was studied in this paper, which focused upon the changes in the specific nitrification rate (SNR) and nitrifier population when the chloride level was gradually or stepwise increased to 30,000 mg Cl L-1. The dominant species of ammonia-oxidizers and nitriteoxidizers in the population were examined by Fluorescent in situ hybridization technique with 16S rRNA-targeted oligonucleotide probes. It was found that neither chloride increasing approaches affected the SNR of the batch cultures before the chloride concentration exceeded 10,000 mg Cl L-1, after which the stepwise increase approach reduced the SNR more significantly than the gradual increase approach. From 10,000 to 18,000 mg Cl L-1 a down-and-up pattern of the SNR variation appeared in both approaches, which was associated with the change in the dominant species of ammonia-oxidizers from non-saline-resistant species such as Nitrosomonas europaea-lineage and Nitrosomonas eutropha to saline-resistant species, such as the Nitrosococcus mobilis-lineage. Nitrobacter was the only dominant species when the chloride concentration was below 10,000 mg Cl L-1, where no nitrite-oxidizers survived. Therefore, the 10,000 mg Cl L-1 chloride level is a critical level for the shift of the nitrifier population in the nitrifying activated sludge batch cultures.     

Waste activated sludge fermentation: Effect of solids retention time and biomass concentration

Laboratory scale, room temperature, semi-continuous reactors were set-up to investigate the effect of solids retention time (SRT, equal to HRT hydraulic retention time) and biomass concentration on generation of volatile fatty acids (VFA) from the non-methanogenic fermentation of waste activated sludge (WAS) originating from an enhanced biological phosphorus removal process. It was found that VFA yields increased with SRT. At the longest SRT (10 d), improved biomass degradation resulted in the highest soluble to total COD ratio and the highest VFA yield from the influent COD (0.14 g VFA-COD/g TCOD). It was also observed that under the same SRT, VFA yields increased when the biomass concentration decreased. At a 10 d SRT the VFA yield increased by 46%, when the biomass concentration decreased from 13 g/L to 4.8 g/L. Relatively high nutrient release was observed during fermentation. The average phosphorus release was 17.3 mg PO₄-P/g TCOD and nitrogen release was 25.8 mg NH₄-N/g TCOD.     

Removal and degradation characteristics of natural and synthetic estrogens by activated sludge in batch experiments

The removal and degradation characteristics of natural and synthetic estrogens by activated sludge were investigated by a series of batch experiments using the activated sludge samples of four actual wastewater treatment plants and synthetic wastewater spiked with estrogen. The rapid removal and degradation of 17β-estradiol (E2) and estrone (E1) were observed by the activated sludge samples of the oxidation ditch process which operated at higher solids retention time (SRT). On the other hand, E1 tended to remain both in the water phase and the sludge phase in the activated sludge samples of the conventional activated sludge process which operated at lower SRT. The anoxic condition was considered to be not favorable to the effective removal of estrogens as compared with the aerobic condition. The removal and degradation of EE2 showed the lag phase, which neither E2 nor E1 showed, but EE2 was finally removed and degraded completely after 24 h. The removal of estrogens in the water phase did not follow the first-order-rate reaction because a large part of the spiked estrogen was immediately removed from the water phase to the sludge phase by adsorption.