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

Sludge 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.     

城市污泥热风与微波干化特性研究

采用热风和微波加热两种方式对城市污泥进行薄层干化实验研究,分别考察污泥薄层厚度和其他操作条件对污泥干燥特性及能耗的影响。研究发现:在热风干燥过程中,由于污泥的塑性,污泥表面容易结壳,从而严重阻碍了水分向外迁移,使得干燥速率呈阶梯状下降。微波干燥污泥具有时间短、干燥速率大,同时节省能源。     

Drying of Sewage Sludge During Composting in an Updraft Dryer with the Concept of Using Self-Energy Generated Within the System

An innovative technology for sewage treatment systems is proposed. A mixture of sewage sludge and char particle is subjected to drying in the progress of composting in an updraft column. Exothermic heat generated during composting is utilized for energy of sludge drying. The char particle in the mixture contributes to enhancement of composting and drying rate. This study presents drying and composting behaviors of sludge to examine the effect of char addition. A fundamental drying experiment was carried out by hot-air heating of the sample in a small vessel. The drying characteristic curve for a mixture of sludge and char was greater than the sample of only sludge over a whole period if the drying rates were compared with the same moisture content. The drying behavior could be analyzed with a reasonable agreement by the water front receding model. The enhancement effect of drying and composting of sludge by char addition was confirmed by examination using a pilot scale of the updraft column whose volume was 50 m³. Sludge with 400% dry base (80% wet base) in moisture content could be dried successfully to lower moisture content than 67% dry base (40% wet base) only by exothermic heat during composting without supplying any auxiliary energy from the outside, and the treatment time was reduced by mixing char in sludge. The behavior was analyzed with a satisfactory agreement by a theoretical model employing moisture diffusion parameters determined from the drying characteristic curve.     

城市污泥快速干燥工艺研究进展

热干化是一种城市污泥安全处置的重要途径。塑化是城市污泥热风干燥常见现象,对污泥干燥过程产生重要负面影响。污泥塑化阻碍了内部水分向外迁移,增加污泥干燥时间和干燥能耗。本文概括了近三年天津科技大学机械工程学院课题组在避免或降低污泥"塑化"影响,发展污泥快速干燥工艺的研究进展。新污泥干化工艺包括城市污泥油炸干燥和脉动燃烧雾化干燥。     

Measurement and simulation of the contact drying of sewage sludge in a Nara-type paddle dryer

Contact drying experiments were carried out in a Nara-type paddle dryer to study the drying kinetics of sewage sludge in the presence of air. In order to have a better understanding of the sewage sludge drying mechanism, a penetration model developed by Tsotsas and Schlünder is used to simulate the drying kinetics of the pasty, lumpy and granular phase which the sludge experiences during the drying process. The pasty phase is assumed to be a saturated particulate phase, and the granular phase is considered as a mono-dispersed particulate phase whose diameter was experimentally determined. In the lumpy phase, a sludge wall sticking still to the paddle-shaft surface was formed, and the heat transfer resistance from the sludge wall is considered during the model calculation. The influence of the drying parameters (temperature, stirrer speed and air flowrate) on the drying process is investigated experimentally and theoretically. The result indicates that the penetration theory is able to describe the sludge drying kinetics of the three distinct phases. Experimental and calculated drying kinetics are in satisfactory agreement for different drying parameters.     

Drying Kinetics of Sludge from a Wastewater Treatment Plant

Drying kinetics of sludge from a municipal sewage treatment plant were determined in a laboratory drying tunnel with parallel airflow at different temperatures and air velocities. The constant drying rate period was identified followed by two falling rate periods. Drying kinetics in these falling rate periods were then satisfactorily modeled using the modified quasi-stationary method and the Fick's second law. The two-period model was used, however, to describe drying kinetics over the entire drying process, and the equation for a generalized drying curve was proposed. The effective diffusivity was also determined from the integrated Fick's equation, and correlated with temperature by an Arrhenius type equation.     

污泥干燥机在污泥干化焚烧工艺中的应用

污泥干化焚烧工艺综合考虑了污泥处置的可靠性、先进性、适用性、安全性与经济性等原则,将是现阶段我国一线城市污泥治理的主流工艺路线。本文通过对不同种类的污泥干燥机进行综合性能比较,推荐了作为污泥干化焚烧工艺的主流干燥机。国产干燥机在经过对国外同类设备的消化吸收后,今后将在国内污泥干化领域占据一席之地。     

Effect of sludge thickness on characteristics of ultrasonic assisted hot air drying sludge

Due to the stratification of sludge particles in the ultrasonic field, the acoustic interaction forces the particles to agglomerate on a plane perpendicular to the direction of ultrasonic propagation. Therefore, the thickness of sludge can significantly influence the characteristics of ultrasound-assisted hot air convective drying municipal sewage sludge. In this paper, the stratified aggregation phenomenon of sludge with different thicknesses was observed in the ultrasonic field using the experimental method. It was found that the stratification of the internal structure of sludge became more obvious with the increase of its thickness. The effects of ultrasound on the drying time and the drying rate of sludge with various thicknesses were studied. Meanwhile, the effective moisture diffusivity (D_eff) was analyzed. The experimental results demonstrated that the larger the sludge thickness, the longer the time length of the first falling rate stage and the promotion of the drying rate was worse when the ultrasonic power was less than 135 W. The situation was opposite at the constant rate stage. Among the sludge with the thicknesses of 5, 10, and 15 mm, the total drying time and energy consumption of the sludge with a thickness of 10 mm decreased the most substantially under the condition of ultrasonic power less than 90 W. The smaller the thickness of sludge is, the less obvious the effect of ultrasound on the effective moisture diffusivity, and vice versa.     

污泥厚度对超声波辅助热风干燥污泥特性的影响

由于在超声波声场中污泥微粒会发生分层现象,声互作用力使得微粒于超声传播方向相垂直的平面上发生凝聚,因此污泥厚度大小对超声波辅助热风干燥污泥特性有着重要的影响。通过实验的方法,对不同厚度污泥在超声波声场中的分层凝聚现象进行观察,发现污泥内部结构的分层现象随其厚度的增加而明显。研究了超声波对不同厚度污泥干燥过程中各时期干燥时长、干燥速率的影响效果,以及分析了湿分有效扩散系数（D_eff）随污泥厚度变化的情况。从实验结果中可以发现,在超声波功率小于135 W范围内,污泥厚度越大,干燥过程中第一降速期时间越长,干燥速率提升效果越差,而对恒速干燥期内干燥速率提升效果更明显;在5、10以及15 mm厚度的污泥中,10 mm厚度的污泥在超声波功率小于90 W的条件下总干燥时长降低幅度最大,干燥速率在各阶段提速也较快;污泥厚度越小,超声波功率对污泥湿分有效扩散系数影响越小,反之影响越大。     

Modeling of Sludge Behavior in a Steam Dryer

Steam dryers are in widespread use for sewage sludge treatment. The effectiveness of the steam dryer could be improved by increasing the drying rate, which is influenced by the movement of the sludge in the dryer. Therefore, it is necessary to understand the movement of sludge and water evaporation in the dryer. In this study, tracers were used to elucidate the behavior of sludge in the dryer. The experiment confirmed that sludge did not simply flow in one direction; instead, part of the sludge flowed in the reverse direction (back-mixing). A tanks-in-series model analysis was performed to analyze the back-mixing phenomenon. This analysis provided insight into the behavior of sludge in the dryer, and the drying rate of each segment was determined.     

污水污泥干燥过程中表观形态变化及水分析出特性

The drying mechanism is important to the drying technology of sewage sludge. Using dynamic electronic balance and image acquisition technique, the paper studies the change of surface configuration and characteristics of moisture transference for three kinds of sewage sludge during drying. In the meantime, the paper also analyses the influence of sludge shape on moisture transference and the change of sludge apparent volume and porosity in the course of drying.     

NaHCO3-enhanced sewage sludge thin-layer drying: Drying characteristics and kinetics

Drying sewage sludge is a highly energy-extensive process. For this reason, this work seeks to identify a reagent that can enhance the effectiveness of the drying process. In this study, drying experiments of sewage sludge were conducted at drying temperatures ranging from 100 to 160°C. NaHCO₃ was selected as the drying reagent, which was added to the sludge before drying. The thin-layer drying characteristics of the sludge and sludge/NaHCO₃ mixtures were later investigated and compared. Various mathematical models were used to simulate the sludge drying curves. It was found that adding 2 and 6% (wet basis) of NaHCO₃ to the sludge was effective in improving the moisture diffusion during the drying process, whereas the drying rate of the sludge/NaHCO₃ mixtures decreased when the addition of NaHCO₃ was further increased to 10% (wet basis). When the addition ratio was 2%, the increase in the maximum drying rate was the largest. With coefficients of determination (R²) over 0.9999, the modified Midilli model proposed in this study was observed to be the most suitable model to describe thin-layer drying of sludge relative to the other models examined in terms of R², reduced χ², root mean square error, and residual sum of squares. The values of the diffusion coefficients at each temperature were obtained using Fick’s second law of diffusion, which varied from 3.700?×?10^?9 to 1.085?×?10^?8?m²/s over the temperature range (i.e., 100–160°C). The activation energy of moisture diffusion was determined to be 27.57?kJ/mol. Scanning electron microscope images of the dried sludge and sludge/NaHCO₃ mixtures indicated that the porosity of the sludge after drying increased with an increase in the NaHCO₃ addition ratio. Overall, the results suggested that NaHCO₃ is a suitable reagent to improve the drying efficiency of the sludge.     

市政污泥干化焚烧工艺探讨

随着城市污水处理量的增大,污泥的产量也在增加。针对污泥自身的特点,介绍了市政污泥的常用的处理方法,重点介绍了干化焚烧的处理方法和工艺。污泥干化使污泥的处理方法多元化、简便化,污泥焚烧工艺被世界各国认为是污泥处理中最佳实用技术之一,以处理速度快、减量化程度高、能源再利用的突出特点而著称。     

Convective drying of sawdust-sludge mixtures: The effect of the sludge matrix

Thermal drying is an important treatment method for sewage sludge. Sludges from Oupeye, Grosses Battes, and Embourg wastewater treatment plants (WWTPs) were chosen for study. The effect of the mass fraction of sawdust (10%, 20%, 30%, and 40%) on sludge drying in a fixed bed dryer was investigated. A universal testing machine was used to determine the stiffness of the samples. X-ray tomography was employed to measure the structural properties of sample beds including the volume, total exchange surface, and void fraction. Both a lower moisture content of sludge and a higher mass fraction of sawdust lead to higher stiffness. The initial void fraction, bed volume, and total exchange surface of the sample bed are all related to sludge matrix and sawdust addition; however, the maximum drying rate is proportional to the initial total exchange surface, which is independent of the sludge matrix. This proves that sawdust addition is a positive means of sludge drying because of the increase in the initial total exchange surface.     

市政污泥热力干化过程中黏滞特性研究进展

黏滞现象是城镇污泥脱水干化过程中面临的主要问题之一,污泥在干化设备中的黏附和结团,导致干化设备运行效率大幅降低,给污泥干化系统的经济性和安全性带来显著的负面影响。本文从污泥黏附力和结团力入手,简要介绍了污泥黏滞现象的形成机理以及低含水率条件下污泥黏滞性的主要测量方法,包括搅拌法、平板法、黏附-结团失效特性法、阻抗法、黏附量法和滑板黏度法等,并对比分析了各种分析方法的优缺点。综述了包括胞外聚合物（EPS）、温度和污泥含水率等主要因素对污泥黏滞特性的影响规律,并简要探讨了各因素的影响机理。概述了国内外污泥降黏方法的研究进展,讨论了各种降黏方法的工业应用前景,同时探讨了污泥黏滞特性研究领域未来值得深入研究的方向。     

流化床干化法在污水处理厂污泥处置中的应用

针对大伙房水库上游污水处理厂污泥处理过程中存在的问题,提出采用流化床污泥干化法对污水厂脱水后的污泥进一步处理,实现减量和无害化。文中给出了详细的工艺过程和参数,对污泥干化工程的实施有一定的参考价值。     

Experimental and Theoretical Study of Agitated Contact Drying of Sewage Sludge under Partial Vacuum Conditions

Agitated contact drying experiments were carried out in a cylindrical lab-scale paddle dryer to study the drying kinetics of sewage sludge under partial vacuum conditions. To explore the drying mechanisms, the penetration model for vacuum contact drying of agitated particulate materials, developed by Schlünder and Mollekopf, was applied to simulate the experimental results. The influence of the drying parameters (system pressure, heating temperature, stirrer speed, and dryer load) on the drying kinetics was studied both experimentally and theoretically. The results indicate that pressure and temperature significantly influence the drying rates of sewage sludge, whereas stirring speed and dryer load have no obvious effects on drying rates in the examined range. The experimental and simulated drying rates were in acceptable agreement as long as the temperature difference between dryer and sludge was small. A crust is likely to form on the dryer surface, however, when the temperature difference becomes large; this crust leads to markedly lower drying rates than the calculated ones. Furthermore, a comparison between partial vacuum contact drying and atmospheric contact drying is presented. The results indicate that the mass transfer resistance increases with rising drying potential in the examined experimental range.     

Effect of moisture on sewage sludge combustion temperature profile and heavy metal leaching

Moisture is the most important parameter affecting sewage sludge treatment efficiency and cost. In this article, the effect of moisture on sewage sludge combustion temperature profile and heavy metal leaching behavior has been experimentally studied. The temperature profiles inside the sludge pellet were recorded, and the volume shrinkage was obtained by image processing method. Results revealed that combustion rate and burnout of sludge strongly depends on moisture content and there exists an optimal moisture content for sludge incineration with respect to best combustion performance and minimum heavy metal leaching. The enlarged pore size may account for this moisture rate, that is, 48% for the sludge studied. During combustion, the shrinkage rate varied from 15 to 25%, and it was found to be proportional with moisture content. The cross-section image of burning sludge pellet illustrated that during the drying and combustion stage, the moisture evaporation was running parallel with the decomposition of volatile that was burned out in the vicinity of the sludge surface. Measured temperature profiles indicate that effect of volatile combustion on inner temperature profile was ignorable. A mathematic model taking sludge volume shrinkage into consideration has been developed to predict sludge central and surface temperature during combustion.     

Atomization and Drying Characteristics of Sewage Sludge inside a Helmholtz Pulse Combustor

Sewage sludge of pasty consistency was atomized and dried directly using a high-temperature, highly turbulent gas flow in the tailpipe generated by a Helmholtz gas-fired pulse combustor (PC). Parametric studies were carried out to investigate the effect of drying pipe length and sludge feed tube size on the PC sludge drying process. Experimental results showed that the pulsating gas stream can disperse the sludge into small particles with a narrow size distribution between 0.01 and 4 mm and the granular sludge was dried rapidly due to the increased particle surface area and the high temperature of the drying medium. PC drying of sewage sludge was compared to convective- and microwave-drying processes. The application potential of the PC sludge-drying process is discussed based on the experimental results.     

VOLUME REDUCTION AND BIOLOGICAL STABILIZATION OF SLUDGE IN SMALL SEWAGE PLANTS BY SOLAR DRYING

High investments, high dumping costs and high electrical energy consumption turn waste water treatment in small sewage plants an expensive process. Therefore, the University of Hohenheim and the company Thermo-System have developed a fully automatic low temperature solar drying plant allowing further stabilization and volume reduction of sewage sludge by continuous mixing and aerating. To test the potential of the technology, the drying process was investigated in a commercially operated municipal sewage treatment plant with aerobic sludge digestion. During experiments, the changes of the dry solids concentration and the volatile solids, the climatic conditions and the energy consumption were constantly measured and documented. It was found, that the sludge could be dried from a dry solids concentration of 3 to 93% w/w in 64 (flocculated) or 83 days (non-flocculated). At the same time, the volatile solids content was reduced from 65 to 34% w/w, respectively. The electrical energy consumption was only 22 to 28 kWh per ton of evaporated water, compared to 70 to 110 kWh required for conventional drying processes. The thermal energy for evaporation was fully covered by solar energy.