Sludge Flow Research Articles

Experimental and theoretical research on agitated contact drying of sewage sludge in a continuous paddle dryer

ABSTRACTSludge drying is an important intermediate process for sewage sludge treatment, usage, and disposal. A series of drying experiments were conducted in a continuous paddle dryer in order to have a better understanding of the sludge drying mechanism. The couple model of penetration model and Markov chain was used to simulate the drying kinetics in a continuous paddle dryer. The drying process has been compared in experimental and theoretical results, which show good agreement. The sewage sludge drying process in a continuous paddle dryer can be divided into three stages: the pasty phase, the sticky phase, and the complete-granular phase. The influences of the drying parameters on the sludge drying kinetics in a continuous paddle dryer were investigated. The sludge flow rate, dryer wall temperature, and dryer slope have remarkable influence on sludge drying in the examined range, but the stirrer speed and steam temperature have little influence on the drying kinetics.

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.

Analysis of the aerotanks efficiency in wastewater treatment system

Тhe problem of wastewater treatment is discussed and the ways of its solution are offered in the given work. We consider the biological method based on the biochemical and physiological laws of self-purification of rivers and other bodies of water. The biological method is promising in wastewater treatment system due to its destructive, because as a result of a partial or complete destruction of the contaminants in the waste water. The intensity and effectiveness of this treatment are investigated in the article. The efficiency of wastewater treatment depends on the degree of maintaining the bacteria in state of physiological activity. For this application cases are compared processes with one, two or more stepwise of purification with differentiated water and sludge movement also structural units for biological purification schemes and the processes that accompany different types of purification. We analyze efficiency in aeration tanks with minimum and maximum regeneration zone. We found that the biological treatment with the maximum regeneration zone is more effective for chemical oxygen demand and oxidation rate higher than mode with the minimum regeneration zone. To solve the problem of efficient wastewater treatment is offered technological scheme of arrangement of aeration with variation of the angle of mixing activated sludge flows from the waste water. This flowsheet complete biological treatment is a complex of sewage treatment plants, which are located in such a way that the liquid waste passing them one after the other, is subject to mechanical, biological treatment and disinfection before lowering it into the reservoir.

Develop and Demonstrate Fundamental Basis for Selectors to Improve Activated Sludge

Successful operation of the activated sludge process relies heavily on the ability to effectively separate mixed liquor into return activated sludge (RAS) and a high quality final effluent. Solids separation efficiency is dictated in large part by the relative population of floc-forming and filamentous microorganisms present in the activated sludge. During normal operation, mixed liquor will be composed primarily of floc-formers with relatively low levels of filamentous organisms. Some population of filamentous organisms is necessary to promote the formation of larger flocs and prevent development of a “pin-floc” condition, which often results in high effluent turbidities (Jenkins, 2004). However, when conditions favoring the growth of filaments over floc-formers occur, a condition commonly referred to as filamentous “bulking” may occur. Filamentous bulking may develop in response to specific wastewater characteristics or secondary process operating conditions. Some facilities may consistently experience bulking as a result of process design and configuration issues that favor the growth of filamentous organisms. Filamentous bulking can severely interfere with proper settling and compaction characteristics in the secondary clarifier, potentially causing high blanket levels, low RAS concentrations, and loss of effluent solids. During bulking episodes, high sludge volume indices (SVIs) may be measured at levels well beyond 150 mL/g, which is a typical measure of a “good settling” sludge. Poor settling sludge requires significantly more aeration basin and secondary clarifier capacity (Parker et al., 2003), in addition to requiring higher RAS rates, higher waste activated sludge (WAS) flow rates and chemical conditioning dose rates at downstream thickening processes, and higher hydraulic flows to downstream digestion processes. This title belongs to WERF Research Report Series ISBN: 9781843397526 (Print) ISBN: 9781780403915 (eBook)

Activated sludge mass reduction and biodegradability of the endogenous residues by digestion under different aerobic to anaerobic conditions: Comparison and modeling

This study was performed to identify suitable conditions for the in-situ reduction of excess sludge production by intercalated digesters in recycle-activated sludge (RAS) flow. The objective was to compare and model biological sludge mass reduction and the biodegradation of endogenous residues (XP) by digestion under hypoxic, aerobic, anaerobic, and five intermittent-aeration conditions. A mathematical model based on the heterotrophic endogenous decay constant (bH) and including the biodegradation of XP was used to fit the long-term data from the digesters to identify and estimate the parameters. Both the bH constant (0.02–0.05d−1) and the endogenous residue biodegradation constant (bP, 0.001–0.004d−1) were determined across the different mediums. The digesters with intermittent aeration cycles of 12h–12h and 5min–3h (ON/OFF) were the fastest, compared to the aerobic reactor. The study provides a basis for rating RAS-digester volumes to avoid the accumulation of XP in aeration tanks.

Accelerated high-solids anaerobic digestion of sewage sludge using low-temperature thermal pretreatment

High-solids anaerobic digestion (HSAD) of sewage sludge is a promising alternative to conventional anaerobic digestion due to its use of smaller digesters and its reduced energy requirement; however, long digestion time is a key shortcoming. This study investigated whether low-temperature thermal pretreatment of sludge could accelerate high-solids digestion. The results showed that after 30 min treatment at 60, 70 and 80 °C, 9.1, 13.0 and 16.6% of sludge solids, respectively, were disintegrated and sludge flow indices increased from 0 to 0.098, 0.195 and 0.249, respectively. As a consequence, thermal pretreatment at 60, 70 and 80 °C increased the quantity of accessible substrates and decreased sludge viscosity such that, compared to the control, biogas production in the batch anaerobic digestion experiments increased by 7.3, 15.6 and 24.4%, respectively, and the apparent kinetic constants increased from 0.19 d−1 to 0.29, 0.39 and 0.39 d−1, respectively. A high concentration of volatile fatty acids and a low pH resulted from thermal pretreatment, but neither of these exerted a significant negative effect on the subsequent HSAD. The optimal pretreatment condition was a temperature of 70 °C for 30 min. In the semi-continuous pilot scale HSAD experiment with optimal pretreatment, biogas yield increased by 11% compared to that with no pretreatment, and the digestion time decreased from 22 to 15 days. The improved digestion following low-temperature thermal pretreatment confirmed the efficacy of this technique to accelerate high-solids anaerobic digestion, as well as increase biogas yield.

Application of CFD methods to an anaerobic digester: The case of Ontinyent WWTP, Valencia, Spain

In recent years, Computational Fluid Dynamics (CFD) methods have increasingly been used to characterize hydrodynamics and mass-transport in wastewater treatment units. Despite being a well-known and widely applied treatment method to stabilize sludge before final disposal, the hydrodynamics and mass transport in the anaerobic digester have been less studied than those of other treatment units. This paper presents the preliminary results of a 3D numerical study, investigating the characteristics of sludge flow inside the Ontinyent Wastewater Treatment Plant anaerobic digester (Valencia, Spain). An approach based upon the Reynolds Averaged Navier-Stokes (RANS) equation was applied, and closure was obtained using the classical standard k–ϵ model. In this particular case, a single-phase model was applied considering both Newtonian and Non Newtonian behavior for the sludge simulations. These single-phase preliminary results allowed the occurrence of dead zones to be identified as well as possible shortcuts inside the digester. The analysis was carried out considering the velocities and the flow patterns inside the digester as well as the sludge volume in the digester that had lower velocities than a determined settling velocity. The model was calibrated using available experimental pressure and temperature data. Finally, as well-mixed conditions are strongly desirable for effective anaerobic digestion, proposals to reduce the volume of dead zones are suggested and discussed.

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.

Leaching-induced migration and compositional form change of Cu, Zn, and Cd from sludge to loess

Land utilization of sewage sludge has become one of the major issues in environmental mitigation in China. This is particularly important in the Loess Plateau of Northwest China. Sludge enriched with organic matter and nutrients might effectively help to resolve the problems of silty loess soils as evidenced by porosity defect, structural alterations and absence of fertility. After sludge flows into the loess, irrigation water contains heavy metals that leach and migrate into sludge and consequently related human health risks may occur that raise concerns; and this situation needs to be rectified. The characteristics of the vertical migration and composition form change of Cu, Zn, and Cd from composted sludge to loess, and corresponding influencing factors, were investigated by performing a soil column simulation test under the leaching treatment of a one-year irrigation water capacity. Results demonstrated that: (1) composted sludge significantly improved loess fertility, and irrigation leaching transported only a small quantity of organic matter in sludge in the plough layer; (2) although some of the Cu, Zn, and Cd in composted sludge migrated to and concentrated in the middle and upper layers of the soil column during leaching, these metals were mostly retained in the plough layer; and (3) after the leaching treatment of the one-year irrigation water capacity, the compositions of Cu, Zn, and Cd (particularly Cu and Zn) in both plough layer and loess began to stabilize with low concentrations in the exudate. These findings confirmed the applicability of composted sludge in loess regions. This study provided a new insight into the sludge reuse in alkaline soils in arid and semi-arid region.

Influence of Operating Parameters on Sewage Sludge Drying in a Paddle Dryer: Design of Experiments for the Determination of Hold-Up and Water Content Profiles

In previous works, solids hold-up proved to be the most influential parameter of a stochastic flow model of a continuous paddle dryer; we thus aimed at correlating it with operating parameters. A fractional factorial Design of Experiments (DoE) was implemented in which dryer slope, stirrer speed, overflow level, and sludge flow rate were initially selected as factors. The DoE was then completed after reducing the number of influent parameters and their interactions, using surface response methodology (RSM), to get a predictive relationship for solids hold-up determination. The nonlinear influence of overflow height and dryer slope was confirmed by the different water content profiles obtained during the experiments. Finally, the key role of dry solids residence time on final sludge water content was highlighted, with an almost linear relationship. These results emphasize the importance of hold-up for modeling purposes; this parameter has an effect on sludge flow but also on the contact area between sludge and dryer walls.

Effects of rising biogas bubbles on the hydrodynamic shear conditions around anaerobic granule

Anaerobic reactor is one of gas–liquid–solid three-phase bio-reactors and has attracted high attentions worldwide due to biogas, a renewable energy. As is well-known, hydrodynamic conditions play crucial roles in the performance of the three-phase reactors. But the knowledge about the hydrodynamic conditions around sludge (microbial aggregates) is limited. The hydrodynamic shear force exerted on sludge derives from the liquid flow and rising biogas bubbles nearby. The in situ investigation in a two-dimensional micro anaerobic reactor shows that 56.8–96.6% shear rate exerted on one piece of granular sludge (major axis of 2mm and minor axis of 1.5mm) in typical motions stems from the rising bubbles when biogas production rate is greater than 0.1m3kgVSS−1d−1 and the superficial liquid velocity of reactor was fixed at 6.48mh−1 in this study. Furthermore it linearly correlates with specific bubble population with R2 of superior to 0.95. The specific bubble population plays more important roles in the shear rate on granules than the bubble diameter. Liquid flow is also important for the shear rate exerted on moving sludge in term of the relative velocity between sludge and liquid flow rather than the superficial liquid velocity. Thus the shear rate derived from liquid flow would be significantly lower than that part originated from bubbles. A high-speed digital camera and a particle image velocimetry (PIV) system were used to in situ quantify the bubbles and their behavior in reactor. This study could facilitate understanding and improving the hydrodynamic conditions in three-phase bio-reactors.

Modeling and monitoring cyclic and linear volatile methylsiloxanes in a wastewater treatment plant using constant water level sequencing batch reactors

The fate of cyclic and linear volatile methylsiloxanes (VMSs) was evaluated in a wastewater treatment plant (WWTP) using constant water level sequencing batch reactors from Dalian, China. Influent, effluent, and sewage sludge samples were collected for seven consecutive days. The mean concentrations of cyclic VMSs (cVMSs) in influent and effluent samples are 1.05μgL−1 and 0.343μgL−1; the total removal efficiency of VMSs is >60%. Linear VMS (lVMS) concentration is under the quantification limitation in aquatic samples but is found in sludge samples with a value of 90μgkg−1. High solid-water partition coefficients result in high VMS concentrations in sludge with the mean value of 5030μgkg−1. No significant differences of the daily mass flows are found when comparing the concentration during the weekend and during working days. The estimated mass load of total cVMSs is 194mgd−11000inhabitants−1 derived for the population. A mass balance model of the WWTP was developed and derived to simulate the fate of cVMSs. The removal by sorption on sludge increases, and the volatilization decreases with increasing hydrophobicity and decreasing volatility for cVMSs. Sensitivity analysis shows that the total suspended solid concentration in the effluent, mixed liquor suspended solid concentration, the sewage sludge flow rate, and the influent flow rate are the most influential parameters on the mass distribution of cVMSs in this WWTP.

Optimization de la producción de lodos en un sistema de lodos activados a través de la calibración del modelo ASM1

Recently several protocols have been proposed for the calibration the ASM1 model and each one of them emphasizes obtaining a good adjustment in the production of sludge. Precisely here is where the discrepancies are evaluated and adjustments made to data, through a mass balance. The objective of this work was to conduct first a balance of flows in the wastewater treatment plant “Dulces Nombres” in Monterrey, Nuevo Leon, Mexico, and then adjust them until closing properly the mass balance for the total phosphorus, total suspended solids, and inorganic suspended solids (ISS). On might say that to calibrate the biological model was important to determine reliably the flow of sludge (QWAS) since on it depends the cell retention mean time (θC) in the reactor; this was accomplished successfully through the balance of the ISS and using of GPS-X software for the simulation of scenarios of optimization.

Design methodology for submerged anaerobic membrane bioreactors (AnMBR): A case study

The main objective of this study is to propose guidelines for designing submerged anaerobic MBR (AnMBR) technology for municipal wastewater treatment. The design methodology was devised on the basis of simulation and experimental results from an AnMBR plant featuring industrial-scale hollow-fibre membranes. The proposed methodology aims to minimise both capital expenditure and operating expenses, and the key parameters considered were: hydraulic retention time, solids retention time, mixed liquor suspended solids concentration in the membrane tank, 20°C-standardised critical flux, specific gas demand per square metre of membrane area, and flow of sludge being recycled from the membrane tank to the anaerobic reactor. An AnMBR WWTP operating at 15 and 30°C with both sulphate-rich (5.7mgCODmg−1 SO4-S) and low-sulphate (57mgCODmg−1SO4-S) municipal wastewater was designed. The minimum cost of the designed plant was €0.097 and €0.070 per m3 when treating sulphate-rich and low-sulphate wastewater, respectively.

Fluid dynamic analysis of a continuous stirred tank reactor for technical optimization of wastewater digestion

Continuous stirred tank reactors (CSTR) are widely used in wastewater treatment plants to reduce the organic matter and microorganism present in sludge by anaerobic digestion. The present study carries out a numerical analysis of the fluid dynamic behaviour of a CSTR in order to optimize the process energetically. The characterization of the sludge flow inside the digester tank, the residence time distribution and the active volume of the reactor under different criteria are determined. The effects of design and power of the mixing system on the active volume of the CSTR are analyzed. The numerical model is solved under non-steady conditions by examining the evolution of the flow during the stop and restart of the mixing system. An intermittent regime of the mixing system, which kept the active volume between 94% and 99%, is achieved. The results obtained can lead to the eventual energy optimization of the mixing system of the CSTR.

Modeling of Sewage Sludge Flow in a Continuous Paddle Dryer

A model based on the theory of Markov chains has been developed to represent the residence time distribution (RTD) of municipal sewage sludge in a continuous paddle dryer. The flow of dry solids is described by a chain of n perfectly mixed cells, n corresponding to the number of paddles attached to the shaft. The transition probabilities between the cells are governed by two parameters: the parameter of internal recirculation, R, and the solids hold-up, Hu. In the absence of available correlation, both parameters are identified by fitting the model to experimental RTD data. The model demonstrates its ability to describe the sludge flow in a continuous lab-scale paddle dryer. A sensitivity analysis highlights that R is critical for the treatment uniformity while Hu controls the mean residence time and thus the final moisture content.

A new methodology for measurement of sludge residence time distribution in a paddle dryer using X-ray fluorescence analysis

Drying is a necessary step before sewage sludge energetic valorization. Paddle dryers allow working with such a complex material. However, little is known about sludge flow in this kind of processes. This study intends to set up an original methodology for sludge residence time distribution (RTD) measurement in a continuous paddle dryer, based on the detection of mineral tracers by X-ray fluorescence. This accurate analytical technique offers a linear response to tracer concentration in dry sludge; the protocol leads to a good repeatability of RTD measurements. Its equivalence to RTD measurement by NaCl conductivity in sludge leachates is assessed. Moreover, it is shown that tracer solubility has no influence on RTD: liquid and solid phases have the same flow pattern. The application of this technique on sludge with different storage duration at 4 °C emphasizes the influence of this parameter on sludge RTD, and thus on paddle dryer performances: the mean residence time in a paddle dryer is almost doubled between 24 and 48 h of storage for identical operating conditions.

Quantification method of N2O emission from full-scale biological nutrient removal wastewater treatment plant by laboratory batch reactor analysis

This study proposes a simplified method for the quantification of N2O emission from a biological nutrient removal wastewater treatment plant (WWTP). The method incorporates a laboratory-scale batch reactor which had almost the same operational (wastewater and sludge flow rates) condition of a unit operation/process of the WWTP. Cumulative N2O emissions from the batch reactor at the corresponding hydraulic retention times of the full-scale units (primary and secondary clarifiers, pre-anoxic, anaerobic, anoxic and aerobic basins) were used for the quantification of N2O emission. The analysis showed that the aerobic basin emitted 95% of the total emission and the emission factor (yield) reached 0.8% based on the influent nitrogen load. The method successfully estimated N2O emission from the WWTP and it has shown advantages in measurement time and cost over the direct field measurement (floating chamber) method.

PERSPECTIVES FOR ENERGETIC VALORIZATION OF WASTEWATER SEWAGE SLUDGE: THE TURIN CASE

The right individuation of the more convenient solution for final treatment and destination of sewage sludge deriving from wastewater plants, normally after anaerobic digestion process, is a very interesting problem, in accounting of urgent needs for acceptable solutions. The produced sludge, primarily after thermal drying, can be sent to energetic valorization processes as, for example, incineration, gasification and also pyrolysis. In this work we examined the thermal solutions (direct combustion in incineration plant first of all, but also gasification), for the sewage sludge deriving from the main Italian wastewater plants (ubicated in the metropolitan area of Turin), from both technological and environmental point of view. The plant actually produces about 100,000 t/y of sludge (38.9% d.s. (dry sludge)), in three main streams: centrifuged sludge, filtered sludge and thermal dried sludge. The water content of the streams is obviously different, while the indicated humidity is a mean value. In order to propose a solution for sludge valorization, a first analysis was carried out in order to define the main characteristics of the produced sludge (flow rates and composition); afterwards we analyzed in detail the potential plant design solutions, from the technological and the environmental point of view, achieving some perspectives in terms odd design data

Nitrogen and Phosphorus Removal of Modified A<sup>2</sup>/O Process on Low-Carbon Domestic Sewage under Low Temperature

A modified compound multiple-influent A2/O process was used to treat sewage with a low C/N under low temperature, the nitrogen and phosphorus removal was investigated. Raw water was distributed into anaerobic tank and anoxic tank by the proportion of 5:5 to rationally distribute the carbon sources required by anaerobic phosphorus release and anoxic denitrification. And some carriers were added into aerobic tank and anoxic tank separately to stabilize the effect of nitrification and denitrification and improve the denitrification efficiency. The effluent of anaerobic tank and anoxic tank flowed into aerobic tank to reduce the sludge flow path. In order to increase microbial biomass, adjust sludge return ratio was ranged from 50% to 75%. The results showed that when the water temperature is14°C, the concentration of COD, NH3-N, TN and TP in influent were 69.5 mg/L, 33.74 mg/L, 42.31 mg/L and 4.94 mg/L, the effluent quality of the above indexes were 40.1 mg/L, 1.45 mg/L, 12.54 mg/L and 0.46 mg/L, and the average removal of COD, NH3-N, TN, TP were 85.10%, 95.7%, 70.37% and 90.73%, respectively.