Sludge Flow Research Articles

Improved Monitoring Protocol for Evaluating the Performance of a Sewage Treatment Works Based on Sensitivity Analysis of Mathematical Modelling

Extensive historical data of a sewage treatment works are required by numerical models in order to simulate the biological processes accurately. However, the data are recorded mostly for daily operational purpose. They are basically not comprehensive enough to meet the modelling’s requirements. A comprehensive sampling protocol to accurately characterise the influent is required in order to determine all model components, which is very time-consuming and expensive. In a project of evaluating a sewage treatment works in Chongqing by using BioWin 4.1 for mathematical modelling, sensitivity analysis was conducted to determine the most critical parameters for process monitoring. It was found that influent characteristics, wasted sludge flow rate, water temperatures, DO levels of the biological tanks and five bio-kinetic parameters were the most influential parameters governing the plant performance. Therefore, apart from monitoring the effluent quality, regular checking of the afore-mentioned influential parameters can help examine the performance of a sewage treatment works. Moreover, operators of the sewage treatment works can conduct “what-if” analysis to determine how these most influential parameters can be adjusted to improve the treatment performance of the sewage treatment works.

Evaluation of three turbulence models in predicting the steady state hydrodynamics of a secondary sedimentation tank

The secondary sedimentation tank (SST) is a sensitive and complicated process in an activated sludge process. Due to the importance of its performance, computational fluid dynamics (CFD) methods have been employed to study the underflow hydrodynamics and solids distribution. Unlike most of the previous numerical studies, in the present investigation, the performance of three different types of turbulence models, standard k-ε, RNG k-ε and Realizable k-ε, are evaluated. Firstly, two-dimensional axisymmetric CFD models of two circular SSTs are validated with the field observations. Next, comprehensive comparisons are presented of the model predictions of the key physical quantities, such as the concentration of effluent suspended solids (ESS), and returned activated sludge (RAS), sludge blanket height (SBH), turbulent properties and flow and concentration patterns.A surprising result shows that the prediction of the ESS concentration is not sensitive to the change of turbulence models; while remarkable prediction difference can be observed in the inlet zone and near-field of sludge hopper and SBH. The results suggest that more observations inside the inlet zone are needed to achieve better model calibration and correct application of the turbulence model, which can be crucial to optimizing the geometry of inlet structure and sludge hopper as well as changing return solids concentration for the operation.

Control of hydrogen sulphide in full-scale anaerobic digesters using iron (III) chloride: performance, origin and effects

The study presents the results of sulphide control using iron (III) chloride in full-scale anaerobic digesters (ADs) at a large-scale municipal wastewater treatment plant (WWTP). Iron (III) (‘ferric’) chloride was applied at a range of 24–105 mg FeCl3/L with and without alkali solution using different strategies. Introduction points were implemented at the feeding line and the sludge thickener unit. Response of the ADs in terms of biogas H2S reduction over time, solid loading rates (SLR), feed sludge flow rate and pH level were investigated. Reduction of H2S in the biogas reached 4 035 μg/L in directly-dosed AD versus a 1 345 μg/L drop in non-dosed ADs, as a result of internal recirculation among the digesters, where actual values were possibly higher as volatile solid (VS) degradation increased in all ADs during the dosing period. No noticeable effect on biogas production and pH was observed. The degree of H2S production was found to be correlated to the volatile SLR, where primary sludge solids contributed mostly to the organic and proteinaceous content of the thickened sludge and presented a high variation and a strong relationship with H2S production. Correlation analysis based on data for the 17-month period that followed supported the significant role of primary SLR in H2S production.

Experimental and mathematical characterisation of the rheological instability of concentrated waste activated sludge subject to anaerobic digestion

For a proper operational performance assessment of excess sewage sludge digesters in practice, a better understanding of waste activated sludge (WAS) rheological behaviour is important, especially regarding the low-shear and poor mixing zones in anaerobic digesters. The potential rheological instability of WAS with different total solids (TS) concentrations was studied in this research. The obtained yield-pseudoplastic behaviour showed varying patterns in a wide shear rate range, of which characterisation depended on hybrid model fitting of defined shear rate segments. Although a new mathematical expression improved the fitting quality, limited applicability of the empiric models reflected the samples’ transient rheological behaviour rather than intrinsic properties, challenging the included parameters definition. Characterised by the distinct flow status and transitions, the observed rheological instability gives more insight in viscoelastic and thixotropic effects on sludge flow and mixing behaviour in full-scale WAS treatment systems. Recommendations for developing a rheological measurement protocol were also formulated.

Prediction of filtrate suspended solids and solids capture based on operating parameters for belt filter press

The control and optimization of belt filter presses in the wastewater treatment industry is still largely dependent on operator intervention. This is mainly due to the lack of predictive models for belt filter press performance. This paper presents correlations determined via a factorial trial for the determination of filtrate suspended solids and solids capture as a function of individual process parameters as well as their interactions. A Box–Behnken experimental based design methodology was used to examine the relationship between the operating parameters and the response variables. The process parameters investigated were polymer concentration, polymer dosing, sludge flow rate and the belt speed. The measured responses were the filtrate suspended solids and sludge cake solids. The measured responses obeyed a second order polynomial relationship and their respective interaction effects of the operating parameters were illustrated on three-dimensional surface graphs. The sludge cake solids concentration was found to be constant over the wide range of experimental conditions. The interaction between the sludge flow rate and polymer dosing rate was the second most influential operating parameter on the filtrate suspended solids following sludge flow rate. The belt speed had an insignificant influence on the filtrate suspended solids and solids capture. The correlation is useful to determine the optimum amount of polymer required if the sludge flow rate changes due to production requirements for constant feed solids concentrations and can be used to develop automated control systems for a particular WWTP.

Effect of total suspended solids and various treatment on rheological characteristics of municipal sludge

Rheological characteristics play an important role in the design and optimization of sludge treatment processes that are dependent on the total suspended solids (TSS). The present study systematically investigated the impact of various treatment processes on sludge rheology. The effect of TSS on rheological properties was comprehensively explored. Rheological tests were performed at (20 ± 0.1) °C, physicochemical parameters of the sludge were measured for compositional analysis. The composition of sludge is altered when subjected to both heat treatment and AD. All sludges exhibit shear-thinning behavior, the results of a t test (with significance level of 0.05) indicated that the Herschel–Bulkley model provides a valid description of sludge flow behaviors. The anaerobically digested sludge (ADS) is the least viscous while thermal hydrolyzed sludge (THS) shows slightly higher viscosity than that of fresh mixed sludge (FMS) at high shear rates. The yield stress and thixotropy decrease gradually as the treatment proceeds. Both infinite viscosity and yield stress of THS increases dramatically when the TSS exceeds 7.30%, hence 7.30% may serve as a critical level for THS to preliminarily determine whether it is economically viable to implement a thermal pretreatment. The TSS threshold for ADS is determined to be 7.12% given the sharp increase in infinite viscosity and yield stress, which are undesirable for dewatering. The thixotropy increases with increasing TSS, and TSS only has a slight influence on the thixotropic properties of THS relative to FMS, which demonstrates the advantage of thermal pretreatment for the purpose of achieving high solid sludge AD.

Characterization of particulate matter formed during sewage sludge pyrolysis

Particulate matter (PM) formation during sludge pyrolysis was investigated under different operating conditions. The impact of inert gas flow rate, heating rate, and sludge types on PM generation was examined. PM generated during sewage sludge pyrolysis varied with tested conditions and ranged from 1.14 wt% to 6.33 wt%. A PM characterization study revealed that carbon content decreased, but oxygen, sulfur, and phosphorus contents increased with increasing temperature. It was considered that the PM formation mechanism varied with different temperature ranges. The increase of inert gas flow and volatile content of sewage sludge promoted PM generation. The formation of PM could be reduced by a low inert gas flow rate, and with lower volatile content of sewage sludge.

DEVELOPMENT OF A COMPUTER PROGRAMME FOR THE DESIGN OF MUNICIPAL WASTEWATER TREATMENT FACILITIES Part 6: Sludge Thickening and Blending Facility

The first sludge proc essing unit in a municipal wastewater treatment plant is sludge blending tank and sludge thickener before the sludge flow into sludge stabilization unit. A computer programme was developed using FORTRAN to design sludge blendin g tank and sludge thickening facility. The design include determining the size and hydraulic profile in the sludge thickener and in sludge blending tank. The influent and effluent structure is also designed using the same method in the secondary clarifier. The' input data to this computer programme is mainly the quantity of sludge effluent from the primary sedimentation bas ins and from the secondary or final clarifiers. These values are computed in a separate subroutine which analyze the material mass balance in the treatment plant. The thickened sludge withdrawal rate and the effluent quality from the sludge thickener are also computed . The results are easy to interpret and automatically appear on the computer screen or in an output file.

Simulation and Analysis of Flow Field in Sludge Anaerobic Digestion Reactor based on Computational Fluid Dynamics

Abstract Because of the complexity of flow and the opacity of sludge, usually we can’t gain a precise and comprehensive sight of sludge flow type and its associated flow characteristics in the anaerobic digestion (AD) reactor. In the present study, we focused on the sludge rheological properties as well as the flow behavior in the digester. The viscosity decreased with the increase of shear rate, and sludge as a kind of pseudo-plastic fluid was proved. Based on computational fluid dynamics (CFD), taking sludge rheological index and rotational speed into consideration, then the flow field distribution in the digester was obtained. The fluid velocity raised with increase in rotational speed, moreover, fluid near blades had higher velocity while it was almost stagnant in the areas near reactor bottom and top as well as reactor wall and stirring shaft. The effect of rheological index on improving the velocity of fluid farther from impeller exceeded the influence on fluid at the impeller installation height. Regarding dead zone fraction as an indicator of the mixing effect, it was recommended that the suitable rotational speed for AD of 96 % moisture content sludge is 40 r/min. Finally, the reactor performance was optimized respectively from impeller form and reactor configuration, the results showed that both combined impellers and oval reactor can reduce dead zone volumes and produce a better mixing effect.

Computational fluid dynamics analysis of a high throughput viscous heater to process feces and a fecal simulant using temperature and shear rate-dependent viscosity model.

Open defecation and poor fecal management facilitates the spread of disease. Viscous heating can pasteurize fecal sludge by creating a high shear field in the annular gap between a stationary, cylindrical outer shell and a rotating inner core. As sludge flows axially through the annular gap, thorough mixing and frictional heating eliminate cool spots where microbes may survive. A viscous heater (VH) compares favorably to a conventional heat exchanger, where cool slugs may occur. Computational fluid dynamics (CFD) was used to determine the effects of geometry and fluid rheology on VH performance over a range of conditions. A shear-rate and temperature-dependent rheological model was developed from experimental data, using a sludge simulant. CFD of an existing VH used the model to improve the original naïve design by including temperature and shear rate-dependent viscosity. CFD results were compared to experimental data at 132 and 200 L/hr to predict design and operating conditions for 1,000 L/hr. Subsequent experimentation with fecal sludge indicated that the CFD approach was valid for design and operation.

Bacterial inactivation and regrowth in ozonated activated sludges

Ozonation of the return activated sludge (AS) flow is an emerging option for excess-sludge reduction. This study aimed to evaluate the potential changes suffered by some kinetic parameters of the activated sludge models (ASMs) in the combined ozone-AS process. The heterotrophic maximum specific growth rate (μHmax) was determined by respirometry in three model-sludges (S1 to S3) treated in batch with different O3 doses. S1 was a fresh synthetic biosolid composed by only two particulate fractions. S2 was a digestate of S1 almost made by the endogenous residues. S3 was from a municipal wastewater treatment plant. μHmax increased significantly from 3.5 d−1 originally, to more than 10 d−1 in the ozonated sludges. Ozonation promoted the selection of fast-growing bacteria in the activated sludges, after transitory inactivation and long lag times. Some microorganisms survived to 3 months of digestion and subsequent ozonation, and then regrow faster than before, once fed again with acetate. The research is of interest from the point of view of the application of the ASM models to the ozone-AS process, but also for wastewater disinfection in general.

Sensitivity of Effluent Variables in Activated Sludge Process

Abstract The quality of a treated effluent changes when there is a sudden variation in the influent flow to the wastewater treatment plant during dry, rain, and storm weather conditions. In this study, various influent flow conditions in an activated sludge process are considered that affect the sensitivity of effluent variables such as chemical oxygen demand (COD), biological oxygen demand (BOD), nitrate nitrogen (SNO), ammonical nitrogen (SNH), and total nitrogen (TN) with respect to varying internal recycle flow rate (Qa), sludge recycle flow rate (Qr), sludge wastage flow rate (Qw) and oxygen transfer rate co-efficient of aerobic tanks (KLa(3,4,5)). The analysis has been carried out based on benchmark simulation model no.1 (BSM 1) plant layout which comprises of two models namely activated sludge model no.1 (ASM 1) and simple one dimensional (Simple 1-D) Takacs model. Based on the present analysis, it is observed that the changes in influent flow rate have larger impact on the effluent variables. This variation can be subdued by introducing additional tanks to smoothen the perturbations or using internal recycle rate from the fifth tank in order to maintain the flow around the optimal influent flow rate. The sludge wastage rate has a greater impact on all effluent variables except nitrogenous variables during maximum flow conditions.

Dual-Control of Autothermal Thermophilic Aerobic Digestion Using Aeration and Solid Retention Time

Autothermal thermophilic aerobic digestion (ATAD) is an advanced sewage sludge treatment which allows compliance with increasingly demanding regulations. Concerning sludge pasteurization, a certain average temperature must be assured in the digester during batch treatment. Aeration flow is the variable most manipulated to regulate the digester temperature. Additionally, the manipulation of the batch sludge flow—which is related to the solid-retention-time—is considered to improve temperature regulation despite variations in air and sludge temperatures and the variability of raw sludge organic content. Thus, a dual-input control structure was provided where the aeration and solid-retention-time contributed as faster and slower inputs, respectively. Two controllers intervened, and the set-point for the batch average temperature was chosen to meet the minimum effluent quality established by the US regulations or European recommendations, considering that lower set point temperatures save aeration costs. A set-point for the aeration allowed us to achieve an extra goal, which aimed at either reducing operation costs or increasing production rates. The two feedback controllers were designed following the robust control methodology known as quantitative feedback theory (QFT). Improvements were compared with single-input (aeration-flow) control strategy and open-loop control strategy. Simulations were performed on a benchmark non-linear simulation model for ATAD.

The role of extracellular polymeric substances on carbon capture in a high rate activated sludge A-stage system

This paper quantifies the effect of varying solids retention time (SRT), hydraulic retention time (HRT) and dissolved oxygen (DO) concentrations on extracellular polymeric substances (EPS) production and subsequently effluent quality, carbon capture (bioflocculation) and carbon redirection (settling) in a high rate activated sludge A-stage system treating domestic wastewater. Two pilot-scale A-stage reactors were set-up with HRTs of 30 and 60min. Cascade DO control was used to maintain 3 DO set-points of 0.5, 1.0 and 1.5mg/L. A mixed liquor suspended solids (MLSS) concentration of 3000mg/L was maintained and the waste activated sludge (WAS) flow was varied to achieve SRTs of 0.28 and 0.56day. EPS fractions and the protein and polysaccharide concentrations of the mixed liquor were measured. Operation at the 0.56day SRT and 1.0mg/L DO resulted in the highest total suspended solids (TSS), total COD (tCOD), particulate COD (pCOD), and colloidal (cCOD) removal. The best overall performance in terms of bioflocculation (cCOD removal) and carbon capture (percent COD in the WAS) occurred at the 0.56day SRT and coincided with decreasing total EPS concentrations but the settling characteristics of the sludge were better at the 0.28day SRT. Overall, low correlations were found between EPS production and system performance. It is likely that at the high loading rate of the A-stage system, EPS production did not play a major role compared to the influence of operating parameters on effluent quality, carbon capture and redirection.

Assessment of activated sludge, membrane bioreactors and vertical flow wetlands for upgrading sewage treatment works

ABSTRACTThis paper demonstrates that utilising a vertical flow (VF) wetland after a conventional activated sludge (CAS) delivers equivalent or better effluent quality to a membrane bioreactor (MBR) based on a side-by-side pilot trial. The CAS was operated under the solids retention times (SRT) of 6, 12, and 20 days, with the effluent from each pilot plant fed onto a soil aquifer treatment column to better understand their water reuse application potential. Results showed an upgraded CAS + VF system could deliver effluents with median values of 34 mgO2.L–1, 7 mg.L–1 and 1.9 mg.L–1 for organics, solids and ammonia nitrogen, respectively, which were statistically similar to those from the MBR. Water reuse standards were achieved by the upgraded system for most parameters, with the exception of total coliform removal. The upgraded system delivered superior metal removal when compared to the CAS. An economic analysis showed upgrading a CAS with a VF wetland was more favourable than investing in an MBR system for example works of 5000 and 50,000 population equivalents if the VF system was operated at hydraulic loading rates of 0.03 m.d–1 and 0.08 m.d–1, respectively. This was delivered for a tenth of the carbon footprint of the MBR treatment.

Concentration of Nitrate in Main Anoxic Stage and PHA, TP Metabolism for Nitrogen and Phosphorus Removal in Single Sludge System with Continuous Flow

Based on test results and mass balance, PHA, TP metabolic regularity was revealed under different nitrate nitrogen concentrations in main anoxic stage [c(NO3)] for nitrogen and phosphorus removal in single sludge system with continuous flow, then the effectiveness of using c(NO3) as control parameter was proved from the perspective of the reaction mechanism. During experiment period, the influent COD, total nitrogen (TN), and total phosphorus (TP) concentrations were stabilized at (285.78±18.19), (58.13±3.79), and(7.14±0.51) mg·L-1, respectively. The experiment was carried out under the condition that the c(NO3) values were 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mg·L-1 based on the feedback control structure using PLC automatic control system to control the nitrifying liquid flow with the water quality. The sludge load of COD was (0.253±0.071)kg·(kg·d)-1, the sludge load of TP in anaerobic stage was (0.006±0.001) kg·(kg·d)-1, the sludge load of TN in aerobic stage was (0.049±0.006) kg·(kg·d)-1, the hydraulic retention time (HRT) in bioreactor was 9h, the sludge recycle flow was 0.5, and the mixed liquor recycle was 1.0. The results showed that effect of c(NO3) value on PHA synthesis and storage rate in the ANS was conspicuous, and the percentage of PHA storage occupied 74% of COD removal when c(NO3) value was 2.5 mg·L-1.The impact of c(NO3) value on PHA degraded in the main anoxic stage was great, and the percentage of PHA degradation in the main anoxic stage occupied 55% of total PHA degradation when c(NO3) value was 2.5 mg·L-1. The phosphorus released in anaerobic stage changed along with increasing c(NO3), and the amount of phosphorus released obtained the maximum value 6.16 g·d-1 when c(NO3) value was 2.5 mg·L-1. In addition, under c(NO3) value of 2.5 mg·L-1, the amount of total phosphorus uptake and anoxic phosphorus uptake obtained the maximum values of 8.04 g·d-1 and 3.67 g·d-1, respectively. Then it was confirmed thatc(NO3) could serve as a run controlling parameter with the best value of 2.5 mg·L-1 from the perspective of PHA and TP metabolic mechanism.

Determining stoichiometric parameters of detached biomass from a HSSF-CW using respirometry

This study investigated the possibility of measuring yield coefficient values for the biomass of a laboratory-scale horizontal sub-surface flow constructed wetland (HSSF-CW) using a static respirometer. The respirometric tests used 300mL bulk volume of biomass-laden gravel samples from the HSSF-CW, from which the biomass was detached by a vigorous washing with 1L of tap water. The presented tests were performed using sodium acetate as standard substrate and had a duration of 6–21h. The obtained value ranges for initial yield and storage yield were, respectively, 0.41–0.67 and 0.75–0.83 (gCODgCOD−1), comprising the literature ranges reported for activated sludge and vertical flow constructed wetlands.

The viscoelastic characterisation of thermally-treated waste activated sludge

Compositional change in sewage sludge due to thermal treatment drives rheological changes in the sludge. In this study, the viscoelastic characteristics of waste activated sludge (WAS) was investigated by measuring viscoelastic properties of sludge, the storage (G′) and loss (G″) moduli over 1h thermal treatment at different temperatures ranging from 20 to 80°C as well as after treatment when sludge cooled down to 20°C (thermal history effect). The soluble chemical oxygen demands (COD) was also measured at the same thermal treatment conditions.The results showed a distinctive difference between thermally-treated sludge and raw sludge in transitions from solid-like to liquid-like behaviour. It was observed that thermally induced compositional change in sludge impacted on both viscoelastic properties (G′ and G″) and viscous properties of sludge. Modified Cox-Merz rule (or Rutgers-Delaware) was successfully used to determine the sludge flow curve from the linear viscoelastic measurements of sludge.It was also found that the effect of short-time thermal treatment at high temperature on viscoelastic properties of sludge is similar to the effect of long-time thermal treatment at low temperature. This indicates that the amount of supplied thermal energy was the key controlling parameter of the evolution of rheological properties and the degree of organic matter solubilisation. A linear proportionality was also established between a dimensionless form of the released soluble COD and a dimensionless form of both G′ and G″. This suggests a possibility of controlling thermal treatment processes via rheological measurements.

Modeling of a continuous sewage sludge paddle dryer by coupling Markov chains with penetration theory

Sewage sludge can be dried in paddle dryers and turned into an interesting material for energetic valorization. This costly operation generally relies on manufacturer's know how and is not very flexible. Setting up a descriptive model of such dryers could lead to a better energy integration of this technology and make sludge drying more competitive by adjusting the operating conditions in order to reach the desired water content depending on subsequent applications. This paper presents the development of such a model adapted to a continuous pilot-scale sludge paddle dryer. The model combines sewage sludge flow description by means of a homogeneous Markov chain and drying kinetics thanks to the penetration theory, leading to the simulation of water content and temperature profiles along the dryer during steady-state operation. The principle of coupling these models is presented and the approach is validated against experimental data in various operating conditions. A parametric study emphasizes the crucial role of wall temperature and sludge residence time on the final water content, while stirring speed or sludge initial water content are less influent.

A secondary sludge flow hazard induced by shallow-source seismic activity in karst mining area, Guangxi, South China: localized karstification in anticline

On the 16th of June 2013, a small-magnitude, shallow-source seismic activity just occurred at a karst mining area. No serious ground damage occurred, but it induced a secondary geological hazard. The wastewater and sludge leaked from the mining water reservoir which caused a great economic loss and groundwater contamination through multilevel underground karst channels. The seismic swarm was located along the Youjiang fault, an active fault zone whose kinematics is controlled by the regional tectonic stress field with NW300° compression and NE30° extension. The tectonic fractures are sealed by calcite, and stylolites have permeabilities of less than 0.01 mD. The localized intensive karstification occurred at the core of Yalang anticline, which is the result of a local extension stress environment and hydrogeological conditions. The seismic activity in the study area is not strong enough to cause ground destruction, but did alter the flow dynamics of groundwater. The seismic activity may have caused a temporary high pressure fluid in the fault zone. The karst system blocked by sediments can be dredged by the fluid pressure. Then, the karstification of limestone massif could be re-activated.