Return Sludge Flow Research Articles

Mechanism insights into the nitrogen removal of bio-augmented AAO: Specifically focusing on the nitrogen metabolic pathways and microbial taxa-functional genes associations

Rapid loss of inoculant microorganisms and the slow-growth of autotrophic bacteria pose significant challenges to bio-augmentation technology. Given the low-cost of coal ash and its greater compatibility with nitrifiers, it was utilized as a carrier to maintain bio-augmentation performance and enhance the AAO's nitrogen removal. Results showed the bio-augmented system operated stably, and immobilized coal ash improved the efficiencies of NH4+-N from 67.6 ± 10.8 % to 94.3 ± 3.0 % and TIN from 63.2 ± 9.2 % to 84.3 ± 4.3 %. An in-depth analysis indicated bio-augmentation altered the microbial community structures and increased the relative abundance of bacterial associated with nitrogen removal, significantly increasing the abundance of Nitrosomonas and Nirospira. In addition, bio-augmentation enhanced the activity of nitrogen removal enzymes, especially amoABC, hao, narGH, nirK, and nosZ transporter genes, according to predicted function profiles. Network analysis between microorganisms and N-metabolic functional genes revealed bio-augmentation improved the complexity of network-structure and the sources of N-metabolic functional genes, thereby enhancing functional stability. Return sludge flow was the most significant factor in regulating the microbial community composition associated with N removal in the bio-augmented AAO system, followed by DO and internal recycle ratio. The aforementioned results obtained from the present study will enable the application of immobilized coal ash in wastewater treatment plants to improve N removal efficiency.

Operator decision support system for integrated wastewater management including wastewater treatment plants and receiving water bodies.

An operator decision support system (ODSS) is proposed to support operators of wastewater treatment plants (WWTPs) in making appropriate decisions. This system accounts for water quality (WQ) variations in WWTP influent and effluent and in the receiving water body (RWB). The proposed system is comprised of two diagnosis modules, three prediction modules, and a scenario-based supporting module (SSM). In the diagnosis modules, the WQs of the influent and effluent WWTP and of the RWB are assessed via multivariate analysis. Three prediction modules based on the k-nearest neighbors (k-NN) method, activated sludge model no. 2d (ASM2d) model, and QUAL2E model are used to forecast WQs for 3days in advance. To compare various operating alternatives, SSM is applied to test various predetermined operating conditions in terms of overall oxygen transfer coefficient (Kla), waste sludge flow rate (Qw), return sludge flow rate (Qr), and internal recycle flow rate (Qir). In the case of unacceptable total phosphorus (TP), SSM provides appropriate information for the chemical treatment. The constructed ODSS was tested using data collected from Geumho River, which was the RWB, and S WWTP in Daegu City, South Korea. The results demonstrate the capability of the proposed ODSS to provide WWTP operators with more objective qualitative and quantitative assessments of WWTP and RWB WQs. Moreover, the current study shows that ODSS, using data collected from the study area, can be used to identify operational alternatives through SSM at an integrated urban wastewater management level.

Sludge reduction in a small wastewater treatment plant by electro-kinetic disintegration.

Sludge reduction in a wastewater treatment plant (WWTP) has recently become a key issue for the managing companies, due to the increasing constraints on the disposal alternatives. Therefore, all the solutions proposed with the aim of minimizing sludge production are receiving increasing attention and are tested either at laboratory or full-scale to evaluate their real effectiveness. In the present paper, electro-kinetic disintegration has been applied at full-scale in the recycle loop of the sludge drawn from the secondary settlement tank of a small WWTP for domestic sewage. After the disintegration stage, the treated sludge was returned to the biological reactor. Three different percentages (50, 75 and 100%) of the return sludge flow rate were subjected to disintegration and the effects on the sludge production and the WWTP operation efficiency evaluated. The long-term observations showed that the electro-kinetic disintegration was able to drastically reduce the amount of biological sludge produced by the plant, without affecting its treatment efficiency. The highest reduction was achieved when 100% return sludge flow rate was subjected to the disintegration process. The reduced sludge production gave rise to a considerable net cost saving for the company which manages the plant.

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; 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 (qcrit) 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 0.4QMAX. However, once the inflow rate exceeds the threshold value the return sludge flow rate should be slowly increased up to 0.6QMAX, 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.

Increased performance of secondary clarifiers using dynamic distribution of minimum return sludge rates

Most wastewater treatment plants have several secondary clarifiers or even more sets of clarifiers including several secondary clarifiers, and in practice it is a well known problem that equal distribution of the load to the single clarifier (or set of clarifiers) is very difficult-not to say impossible-to obtain. If the problem is neglected, quite a big percentage of the total clarifier capacity-measured as the max. allowed hydraulic load-can be lost. Further, return sludge rates are seldom controlled by any other means than as a (typically too high) percentage of the inlet to the wastewater treatment plant-giving a varying and too low suspended solids concentration in the return sludge, which again can lead to an unnecessary use of polymer in the pre-dewatering of the surplus sludge taken from the return sludge. A control of the return sludge rate divided into two parts - control of the total return sludge flow and control of how the total flow shall be distributed between the secondary clarifiers - is able to solve the mentioned problems. Finally, as shall be demonstrated on full scale wastewater treatment plants, a considerable increase of the hydraulic capacity of the treatment plants can be obtained.

Model-based evaluation of control strategies for phosphorus removal in a full-scale advanced phase isolation ditch process

A model-based evaluation of operation conditions and control strategies was conducted for phosphorus removal in a full-scale Advanced Phase Isolation Ditch (APID) process. The APID process is an alternating type and does not have a separated anaerobic reactor. We suggested that it would be a suitable operational option for robust phosphorus removal by having a different input point for the influent and return sludge flow at specific modes. For evaluation of control strategies, three cases of influent disturbance were assumed, and five manipulated variables were selected for controlling the cases of disturbance. In the case of an increased influent flow rate, a combination of four manipulated variables is proposed through our simulation results as the best control strategy. The optimal k(L)a value was found to be 250/d when pollutants loading kept increasing without variations in the flow rate. When both the pollutants loading and the influent flow rates were increased simultaneously, the robust control strategy is to combine the return sludge inflow point, the exclusive operation modes which have a relatively long hydraulic retention time (HRT), operation period of 30 minutes, and the increase of the return sludge flow rate in proportion to the influent flow rate added to 300/d of k(L)a value.

Analysing sludge balance in activated sludge systems with a novel mass transport model

In activated sludge systems the mechanically treated wastewater is biologically cleaned by biomass (activated sludge). The basic requirement of an efficient biological wastewater treatment is to have as a high biomass concentration in the biological reactor (BR) as possible. The activated sludge balance in activated sludge systems is controlled by the settling, thickening, scraper mechanism in the secondary settling tank (SST) and sludge returning. These processes aim at keeping maximum sludge mass in the BR and minimum sludge mass in the SST even in peak flow events (storm water flow). It can be, however, only reached by a high SST performance. The main physical processes and boundary conditions such as inhomogeneous turbulent flow, geometrical features of the SST, wastewater treatment plant (WWTP) load, return sludge flow, sludge volume index etc. all influence settling thickening and sludge returning. In the paper a novel mass transport model of an activated sludge system is presented which involves a 2-dimensional SST model coupled with a mixed reactor model of the biological reactor. It makes possible to investigate different sludge returning strategies and their influence on the sludge balance of the investigated activated sludge system, furthermore, the processes determining the flow and concentration patterns in the SST. The paper gives an overview on the first promising model results of a prevailing peak flow event investigation at the WWTP of Graz.

Simple Rule-Based Algorithm for Optimizing Volatile Fatty Acids Production in Primary Sludge Fermentation Schemes

In this paper, a model-based algorithm for optimizing the primary sludge fermentation process design and operation is described and evaluated. This is a recently used method to obtain the volatile fatty acid (VFA), needed for the enhanced biological phosphorus removal process, directly from raw wastewater. The proposed algorithm consists of a heuristic reasoning algorithm based on the expert knowledge of the process. Only effluent VFA and the sludge blanket height have to be set as design criteria, and the optimization algorithm obtains the minimum return sludge and waste sludge flow rates which fulfill those design criteria. It has been evaluated by simulating different temperature and sludge volume index (SVI) influent conditions in order to evaluate the algorithm performance under different conditions, as well as its sensitivity with respect to temperature and SVI, which are the most important parameters in these systems. The same tendencies observed by simulation were observed afterward in pilot plant experiments.

Optimum design and operation of primary sludge fermentation schemes for volatile fatty acids production

This paper presents a model-knowledge based algorithm for optimising the primary sludge fermentation process design and operation. This is a recently used method to obtain the volatile fatty acids (VFA), needed to improve biological nutrient removal processes, directly from the raw wastewater. The proposed algorithm consists in a heuristic reasoning algorithm based on the expert knowledge of the process. Only effluent VFA and the sludge blanket height (SBH) have to be set as design criteria, and the optimisation algorithm obtains the minimum return sludge and waste sludge flow rates which fulfil those design criteria. A pilot plant fed with municipal raw wastewater was operated in order to obtain experimental results supporting the developed algorithm groundwork. The experimental results indicate that when SBH was increased, higher solids retention time was obtained in the settler and VFA production increased. Higher recirculation flow-rates resulted in higher VFA production too. Finally, the developed algorithm has been tested by simulating different design conditions with very good results. It has been able to find the optimal operation conditions in all cases on which preset design conditions could be achieved. Furthermore, this is a general algorithm that can be applied to any fermentation-elutriation scheme with or without fermentation reactor.

Phosphorus removal under anoxic conditions in a continuous-flow A2N two-sludge process

The Anaerobic-Anoxic/Nitrification (A2N) system is a continuous-flow, two-sludge process in which Poly-P bacteria are capable of taking up phosphate under anoxic conditions using nitrate as an electron acceptor. The process is very efficient because it maximizes the utilization of organic substrate for phosphorus and nitrogen removal. An experimental lab-scale A2N system fed with domestic sewage was tested over a period of 260 days. The purpose of the experiment was to examine phosphorus removal capacity of a modified A2N two-sludge system. Factors affecting phosphorus and nitrogen removal by the A2N system were investigated. These factors were the influent COD/TN ratio, Sludge Retention Time (SRT), Bypass Sludge Flow rate (BSF) and Return Sludge Flow rate (RSF). Results indicated that optimum conditions for phosphorus and nitrogen removal were the influent COD/TN ratio around 6.49, the SRT of 14 days, and the BSF and RSF were fixed at about 26-33% of influent flow rate.

Denitrifying Phosphorus Removal in a Continuously-Flow A2N Two-Sludge Process

The Anaerobic-Anoxic/Nitrification (A2N) system is a continuous-flow, two-sludge process in which Poly-p bacteria are capable of taking up phosphate under anoxic conditions using nitrate as an electron acceptor. The process is very efficient because it maximizes the utilization of organic substrate for phosphorus and nitrogen removal. Further, the process solves the competition for organic substrate among Poly-p organisms and denitrifies as well as the problem of overgrowing of slow nitrifiers by fast organotrophs. An experimental lab-scale A2N system fed with domestic sewage was tested over a period of 260 days. The purpose of the experiment was to examine phosphorus removal capacity of a modified A2N two-sludge system. Factors affecting phosphorus and nitrogen removal by the A2N system were investigated. These factors were the influent COD/TN ratio, Sludge Retention Time (SRT), Bypass Sludgy Flow rate (BSF), and Return Sludge Flow rate (RSF). Results indicated that optimum conditions for phosphorus and nitrogen removal were the influent COD/TN ratio around 6.49, the SRT of 14 days, and the BSF and RSF were fixed about 26%–33% of influent flow rate.

878 3-year prospective patient evaluated urinary and intestinal side effects after stereotactic dose-escalated radiotherapy of prostate cancer with the beamcath(R) technique

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, 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 (qcrit) 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 0.4QMAX. However, once the inflow rate exceeds the threshold value the return sludge flow rate should be slowly increased up to 0.6QMAX, 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.

Computer aided model analysis and dynamic simulation of a wastewater treatment plant

A nitrogen removal benchmark was analyzed using the Activated Sludge Models No. 1 (ASM1) and No. 3 (ASM3) in order to establish a basis for designing an experimental comparison of the two model types. Differences in steady state effluent concentrations predicted by both models could to a large extent be explained by different model concepts. The steady state system performance was analyzed by evaluating the Monod factor values, and through a sensitivity analysis of the kinetic model parameters. Both methods complement each other. Analysis of the Monod factor values can lead to determination of parameters to be estimated during model calibration. The steady state system response to manipulation of the potential actuators for control was evaluated via a sensitivity analysis. The concept of relative sensitivity was introduced to compare the relative effect of each actuator in both models. The negative relative sensitivities of X S to four of the five control handles analyzed imply an opposite response of both models, which can be important for control structure design. The analysis of the process behavior to different disturbances showed different dynamics of both models. ASM3 simulation results are easier to interpret because the model structure is more transparent, mainly due to the simpler cell decay model principle considered in ASM3. An inverse response was obtained for the return sludge and nitrate recycle flow rate, indicating that multivariable control design is required.

Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge.

One of the most important functions of the settling tanks in an activated sludge process is thickening the sludge to a high enough suspended solids concentration so that the return sludge flow will maintain the desired mixed liquor suspended solids concentration. The settling phenomena which result in the thickening are zone settling and sludge compression. Both zone settling and sludge compression have been studied extensively. However, the transition from zone settling to compression has previously received only superficial treatmenThe objective of this study was to characterize the transition of activated sludge from zone settling to compression by using simple laboratory methods. Batch settling tests were conducted using samples of activated sludge from one treatment plant. The samples were obtained over a period of fourteen month and the sludge volume index (SVI) values ranged from 80 ml/g to 213 ml/g. A model for predicting a first order rate constant for the transition from zone settling to compression was developed by analyzing the experimental data By classifying the sludges into six groups according to the SVI range, the transition from zone settling to compression was successfully characterized with respect to the initial suspended solids concentratfon

Recent Progress in the Numerical Modelling of Wastewater Sedimentation Tanks

The recent progress in numerical modelling techniques applied to sedimentation tanks in wastewater treatment is reviewed to determine their usefulness. The models reviewed range from empirical models to sophisticated computational fluid dynamics (CFD) models and the conclusions are as follows. Empirical models are still widely used today to predict mainly the characteristics of the effluent and return sludge from sedimentation tanks but cannot model the flow pattern or solids distribution within the tank. The solids mass flux model is used to perform a mass balance on secondary sedimentation tanks for new designs or for the audit of a poor process and to determine the return sludge flow. It can sometimes be used to monitor the height of the sludge blanket. The lumped parameter model is usually used to determine the characteristics of the effluent from primary sedimentation. CFD models are used to predict the flow pattern and suspended solids distributi on with in the tank and are normally applied in research to find the relationship between the tank hydraulics and process performance. Modelling using CFD in the water industry has not been widespread because of the associated costs and the unfamiliarity with the mathematical models. The paper suggests that the important CFD modelling criteria for the settling of suspended solids in sedimentation tanks are the velocity distribution, settling velocity distribution of suspended solids, turbulent mass diffusion of suspended solids, resuspension of settled solids from tank base, temperature effects, flow variation, effect of flow on floc growth or breakup, wind effects on the water surface and movement of scrapers. Gaps in the literature on CFD modelling can be identified for the particle density, particle size, particle flocculation, the turbulent mass diffusion of solids, the effect of the flow on particle growth or breakup, denitrification, wind on the water surface, baffle designs, inlet pipe geometries, the side wall depth, the slope of the tank, the hopper design, peripheral weirs and in-board launders. Where possible experimental data should be entered in these numerical models to give realistic parameters and be used to validate these models.

On-line estimations of settling capacity in secondary clarifier

In a biological nitrogen removal system, efficient performance of the secondary clarifier is essential since the system depends on a high concentration of active biological solids being available in the biological reactor. Estimations of the current capacity of the clarifier can be made from observing maximum feasible solids flux to the clarifier which does not cause escape of sludge. The aim of this work is to demonstrate a method to establish maximum feasible solids flux, the limiting solids flux, from on-line operational data. In this method the solids flux into the clarifier is calculated from on-line measurements of effluent flow and of suspended solids in the biological reactor, and a known value of return sludge flow. From relations between solids flux and sludge blanket height, the value of limiting flux is estimated. This method is based on the assumption that the movement of sludge blanket height is directly related to changes in the solids flux. When the limiting solids flux is estimated from on-line measurements a measurement of the capacity of the clarifier is retrieved. The suggested method also gives an early warning to prevent sludge escape from the clarifier.

On-line estimations of settling capacity in secondary clarifier

In a biological nitrogen removal system, efficient performance of the secondary clarifier is essential since the system depends on a high concentration of active biological solids being available in the biological reactor. Estimations of the current capacity of the clarifier can be made from observing maximum feasible solids flux to the clarifier which does not cause escape of sludge. The aim of this work is to demonstrate a method to establish maximum feasible solids flux, the limiting solids flux, from on-line operational data. In this method the solids flux into the clarifier is calculated from on-line measurements of effluent flow and of suspended solids in the biological reactor, and a known value of return sludge flow. From relations between solids flux and sludge blanket height, the value of limiting flux is estimated. This method is based on the assumption that the movement of sludge blanket height is directly related to changes in the solids flux. When the limiting solids flux is estimated from on-line measurements a measurement of the capacity of the clarifier is retrieved. The suggested method also gives an early warning to prevent sludge escape from the clarifier.

Denitrification in secondary clarifiers

Denitrification in secondary clarifiers is primarily due to hydrolysis of particulate degradable COD and decay of biomass. The reduction factor of these processes under anoxic as compared to aerobic conditions was investigated in the treatment plants of the City of Zurich. The Zürich-Glatt plant has circular clarifiers with central inlet. The treatment plant Zürich-Werdhölzli has rectangular clarifiers with cross flow and scrapers with vacuum removal of the sludge. At Zürich-Glatt denitrification capacity in the clarifiers was 30% of total denitrification. whereas at Zürich-Werdhölzli only 15 % was observed. The sludge mass in the secondary clarifier depends on inlet flow, return sludge flow, activated sludge concentration, sedimentation properties of the sludge and scraper intervals. During dry weather and due to good sedimentation properties the activated sludge blanket in the thickening zone of the rectangular clarifiers at Zörich-Werdhölzli is almost completely removed after passage of the scraper. Denitrification volume in the secondary clarifier can therefore be modelled in relation to the ratio of inlet to return sludge flow and scraper interval. With the aid of control of return sludge flow and scraper intervals denitrification capacity in the secondary clarifiers may be improved. In order to control denitrification in the secondary clarifier the model must include a variable sludge volume in the clarifier related to scraper interval under the assumption of constant sludge mass in the entire system.

Denitrification in secondary clarifiers

Denitrification in secondary clarifiers is primarily due to hydrolysis of particulate degradable COD and decay of biomass. The reduction factor of these processes under anoxic as compared to aerobic conditions was investigated in the treatment plants of the City of Zurich. The Zürich-Glatt plant has circular clarifiers with central inlet. The treatment plant Zürich-Werdhölzli has rectangular clarifiers with cross flow and scrapers with vacuum removal of the sludge. At Zürich Glatt denitrification capacity in the clarifiers was 30% of total denitrification, whereas at Zürich-Werdhölzli only 15 % was observed. The sludge mass in the secondary clarifier depends on inlet flow, return sludge flow, activated sludge concentration, sedimentation properties of the sludge and scraper intervals. During dry weather and due to good sedimentation properties the activated sludge blanket in the thickening zone of the rectangular clarifiers at Zürich-Werdhölzli is almost completely removed after passage of the scraper. Denitrification volume in the secondary clarifier can therefore be modelled in relation to the ratio of inlet to return sludge flow and scraper interval. With the aid of control of return sludge flow and scraper intervals denitrification capacity in the secondary clarifiers may be improved. In order to control denitrification in the secondary clarifier the model must include a variable sludge volume in the clarifier related to scraper interval under the assumption of constant sludge mass in the entire system.

A one-dimensional model for a secondary settling tank including density current and short-circuiting

This paper presents a dynamic one-dimensional flux model for the secondary settling tank which is suitable for use with the latest innovations in models for activated sludge tanks, and which takes into account observed effects of density current and short-circuiting. The components of the influent to the settling tank are divided into three fractions. Soluble components, non-settleable particulate components (primary particles), and settleable particulate components. (macroflocs). Soluble components and primary particles are considered to follow the hydraulic flow in the settling tank. The transport of macroflocs in the settling tank is modelled according to the traditional flux theory on a layer model of the settling tank extended with a model for density current and short-circuiting. For modelling of the density current in the inlet region of the settler a dynamic inlet height is introduced. The short-circuiting is modelled by the introduction of a factor which accounts for the dilution in the suspended solids concentration at the bottom of the settling tank down to the concentration in the return sludge flow. Settling velocities of the macroflocs for both free and hindered sedimentation are measured, and a new model for the settling velocity is proposed. The model is validated with data from the wastewater treatment plant Lynetten, Copenhagen, Denmark. It was found that the suspended sludge concentration profile and the suspended sludge concentration in the return sludge were predicted well with the model.