Analysis of the physical properties of an in-vessel composting matrix

Efficient compost production requires a thorough understanding of the process dynamics in terms of the feedstock materials used and the interactions of the physical properties involved. The main properties affecting the composting process are temperature, moisture content, bulk density, porosity and oxygen availability. In this study, the correlations between a selected set of physical properties of a batch-composting matrix were determined. The key physical changes in the composting materials for a blend of woodchips, chicken manure and mixed green vegetables have been monitored during a 36-day composting period in a 200-L rotary drum. The daily measurements conducted on the solid samples included temperature, pH, volatile solids, bulk density, moisture content, free airspace and substrates particle density while the carbon dioxide release was monitored weekly using jar respiration tests. The results from the compost process monitoring were a maximum temperature rise to 66.3 °C over the first 3 days, a marked decrease in free airspace from 76.3% to 40.0% at the end of the process, a variation in average composting material particle density from 1097 kg/m³ to 2325 kg/m³, and an increase in wet bulk density from 255 kg/m³ to 628 kg/m³. Correlations developed among free airspace, wet bulk density, dry bulk density and wet moisture content were in agreement with previously determined equations from literature. Free airspace varied linearly with both dry and wet bulk densities (R² value of 0.89 and 0.95, respectively) while the free airspace-wet moisture content profile followed a fourth degree polynomial trend with a correlation coefficient of 0.63.     

Attribution of sewage sludge and sludge-based char in a fluidized bed reactor

Fluidized bed pyrolysis has been recognized as an innovative technology for sewage sludge treatment. The physical and attrition properties of sewage sludge are changed through the fluidized bed pyrolysis. The minimum fluidization velocities and attribution rate constants for sewage sludge and sludge based-char were obtained from pressure drop and attribution tests. As a result, sewage sludge with 20% moisture content and char were classified as Geldart B solids and the superficial gas velocity for bubbling fluidization was 0.2142-0.8755 m/s. In addition, attribution of the sewage sludge and char was more affected by particle size than by material type. The equations for the overall attrition rate constants are K _a × 10⁵ = 1.09U − 14.82 for sludge and ln k _a = 0.1(U−U _mf )− 13.63 for char, respectively.     

Effect of air temperature and aeration strategy on water removal during sewage sludge composting

Water removal is one of the main objectives of sewage sludge composting to reduce the volume and mass of sewage sludge. In this study, the effect of aeration strategy and ambient temperature (summer and winter) on composting pile temperature, moisture content, and volatile solid (VS) change were studied during sewage sludge composting processes in a well-operated sludge composting plant. The results showed that the temperatures of the composting pile could remain above 55°C for more than 3 days under all conditions, which was sufficient to meet the requirement of harmless treatment. Moisture content of composting material decreased from 65%–69% to 39.2%–52.6% within 20 days, and VS decreased to about 77%. In the end of the composting process, cumulative water removal was 0.45–0.47?kg/kg in summer, which was significantly higher than that in winter (0.35–0.36?kg/kg). When the average aeration rate was fixed, aeration strategy with a smaller aeration rate and higher on/off time ratio showed more water removal than that with a higher aeration rate and lower on/off time ratio in summer, but showed reverse results in winter.     

不同热解条件下制备的污泥炭低温还原NO

以市政污泥为原料热解制备污泥炭，开展了污泥炭催化还原NO的实验研究。考察了不同热解温度（400℃、600℃和800℃）和污泥初始含水率（0、66%和80%，质量分数）条件下热解制备的污泥炭的脱硝性能。研究表明，污泥炭中包含大量铁元素（41.1mg/g），提高热解温度可促进污泥炭中亚铁化合物（Fe₂P和FeS）的生成，使铁元素具备催化还原NO的能力，从而显著提高污泥炭的脱硝效率。提高污泥热解初始含水率可显著提高污泥炭比表面积，污泥炭对NO的低温还原能力也随初始含水率提升而显著提高。通过对污泥炭的比表面积、X射线衍射（XRD）和傅里叶红外（FTIR）分析表征，结果表明污泥炭中亚铁化合物的生成是影响NO转化的关键影响因素，而比表面积和表面官能团类型对污泥炭脱硝反应并无明显影响。     

Drying kinetics of cork planks in a cork pile in the field

Moisture content is one important parameter in the trading of raw cork planks after harvesting. This study presents a mathematical modeling of the drying curve of raw cork planks in a cork pile in the field, under natural sun drying conditions. Experimental data were obtained by following the water loss (i.e. by daily weighing) of 97 cork planks positioned in nine points within a cork pile. Immediately after harvesting, the raw cork planks had a mean moisture content in a dry basis of 40.4% and after 20-day drying 16.6%. The drying process of the cork planks showed three phases: drying was very fast in the first 2 days; in the next 2–15 days there was a decreasing drying rate; and a final phase, after 15 days in the cork pile, with a slightly decreasing drying rate. Mathematical modeling provided a direct relation between moisture content and drying time. After comparing sixteen empirical drying models, the Modified Henderson and Pabis model showed the best fit. According to this model, the cork planks are commercial dry (14% moisture content in a wet basis) 15 days after harvest.     

污泥堆肥处理影响因素研究

通过试验,考察了城市污水处理厂污泥中水分和pH值对污泥堆肥过程的影响。试验结果显示:污泥可堆肥的水分质量分数上限为80%,下限为30%,最佳含水率为60%;污泥可堆肥pH值为5.2~8.8,而最佳pH值为7.6~8.7。     

Importance of Initial Moisture Content and Bulking Agent for Biodrying Sewage Sludge

For the final disposal and reutilization of organic wastes, reduction of moisture content (MC) is essential because water contained in the organic waste generates leachate, lowers energy content, and indirectly causes odor problems. MC also directly determines the drying cost. In this study, MC of sewage sludge was effectively decreased by applying a biodrying process in which the microbial metabolic heat evaporated the water. By controlling the initial MC through air drying and its subsequent biodrying, it was found that 50–70 wt% was the optimal initial MC range for the sludge biodrying process. In this range, 33.7–47.1% of the initial water was removed by consuming 12.3–21.2% of the volatile solids (VS) initially contained in the sludge during 10 days of biodrying. When treating the same amount of raw sludge, air-dried and biodried sludge showed better performance as bulking agents than rubber and sawdust. About 55.1% of the initial water was removed from the mixture with air-dried sludge by consuming 23.8% of the initial VS and 38.3% of the initial water was lost from the sludge mixture with biodried sludge by consuming 14.0% of the initial VS during 12.1 days of biodrying. When rubber and sawdust were used as the bulking agents, only 18.2 and 16.5% of the initial water was removed, respectively. It was thought that the easily biodegradable VS contained in the air-dried sludge produced more heat and consequently removed more water. Although air-dried sludge showed better performance than biodried sludge, the use of biodried sludge as a bulking agent was thought to be more practical than air-dried sludge because biodried sludge can be used as bulking agent for the next round of biodrying in a repeated operation of the biodrying reactor. After biodrying, the lower heating value of the biodried sludge (51.5 wt% of MC) was 14.644 MJ kg^?1, which was much higher than that of the original wet sludge.     

Heat and Mass Transfer During Fry-Drying of Sewage Sludge

Deep-frying, which consists of immersing a wet material in a large volume of hot oil, presents a process easily adaptable to dry rather than cook materials. A suitable material for drying is sewage sludge, which may be dried using recycled cooking oils (RCO) as frying oil. One advantage is that this prepares both materials for convenient disposal by incineration.

This study examines fry-drying of municipal sewage sludge using recycled cooking oil. The transport processes occurring during fry-drying were monitored through sample weight, temperature, and image analysis. Due to the thicker and wetter samples than the common fried foods, high residual moisture is observed in the sludge when the boiling front has reached the geometric center of the sample, suggesting that the operation is heat transfer controlled only during the first half of the process followed by the addition of other mechanisms that allow complete drying of the sample. A series of mechanisms comprising four stages (i.e., initial heating accompanied by a surface boiling onset, film vapor regime, transitional nucleate boiling, and bound water removal) is proposed. In order to study the effect of the operating conditions on the fry-drying kinetics, different oil temperatures (from 120 to 180°C), diameter (D = 15 to 25 mm), and initial moisture content of the sample (4.8 and 5.6 kg water·kg^?1 total dry solids) were investigated.     

A Simulation of Wet Pocket Lumber Drying

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D_eff) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D_eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.     

花生壳作为调理剂在城市污泥堆肥中的应用

研究不同类型调理剂与污泥在堆肥过程中温度、pH值、氨氮、总磷、含水率、有机质等物质的变化特征,结果表明:花生壳具有良好的污泥堆肥调理效果,好氧堆肥使污泥达到无害化和稳定化,当花生壳添加量为20%左右时,堆肥升温较快,全磷含量高,氨氮损失小。     

Heat and Mass Transfer During Fry-Drying of Sewage Sludge

Deep-frying, which consists of immersing a wet material in a large volume of hot oil, presents a process easily adaptable to dry rather than cook materials. A suitable material for drying is sewage sludge, which may be dried using recycled cooking oils (RCO) as frying oil. One advantage is that this prepares both materials for convenient disposal by incineration.

This study examines fry-drying of municipal sewage sludge using recycled cooking oil. The transport processes occurring during fry-drying were monitored through sample weight, temperature, and image analysis. Due to the thicker and wetter samples than the common fried foods, high residual moisture is observed in the sludge when the boiling front has reached the geometric center of the sample, suggesting that the operation is heat transfer controlled only during the first half of the process followed by the addition of other mechanisms that allow complete drying of the sample. A series of mechanisms comprising four stages (i.e., initial heating accompanied by a surface boiling onset, film vapor regime, transitional nucleate boiling, and bound water removal) is proposed. In order to study the effect of the operating conditions on the fry-drying kinetics, different oil temperatures (from 120 to 180°C), diameter (D = 15 to 25 mm), and initial moisture content of the sample (4.8 and 5.6 kg water·kg^-1 total dry solids) were investigated.     

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。     

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.     

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。     

Experimental Study on Thermal Hydrolysis and Dewatering Characteristics of Mechanically Dewatered Sewage Sludge

After mechanical dewatering, sewage sludge has a moisture content of around 80 wt% and further disposal is required. A new sewage sludge semi-drying (dewatering) process is proposed and verified. It combines thermal hydrolysis and subsequent mechanical dewatering, with less energy consumption than traditional thermal drying. Sludge treated using this new process satisfies further disposal requirements (e.g., landfill or autothermal incineration). In the present study, a high-pressure test reactor was used to study the thermal hydrolysis of dewatered sludge. Thermally hydrolyzed sludge was subsequently dewatered by centrifugal sedimentation or by pressure filtration. The amount of organic compounds returning to the water phase was also measured. According to the results from centrifugal settling tests, the optimal thermal hydrolysis treatment temperature was 180°C. The moisture content then dropped to 1.44 kg/kg dry solids (DS; 59 wt%) after dewatering under relative centrifugal force of 9,000 × g from 5.67 kg/kg DS (85 wt%). Pressure filtration further reduced the moisture content of filter cakes to only 0.5 kg/kg DS (33 wt%, hydrolysis temperature 180°C). After thermal hydrolysis, the heating value of sludge (moisture-free basis) was about 80% that of the untreated sludge.     

Sewage Sludge Solar Drying: Experiences from the First Pilot-Scale Application in Greece

Increasingly strict regulations governing sludge management have raised interest in drying technologies. The feasibility of sewage sludge solar drying was experimentally evaluated in a 66-m² pilot-scale greenhouse plant under typical weather conditions in Greece. The greenhouse was equipped with ventilation fans to maximize the drying process efficiency and a turning drum for efficient sludge mixing. The obtained results proved the applicability and the high performance of the solar drying technology. The time necessary to achieve a dry product with a dry solids content up to 95% ranged between 8 and 31 days, depending on the weather conditions. During drying, sludge organic matter was reduced by 5–21%, and total and fecal coliform content was also decreased up to three orders of magnitude. By taking into consideration the sludge content in heavy metals, the final product can partially or totally replace commercially available inorganic fertilizers in agricultural applications, in accordance with the restrictions imposed by national and European regulations. Based on a preliminary cost analysis concerning the construction of a solar drying facility covering a sum of 80,000 population equivalent (PE), a corresponding capital cost of 24 €/PE is anticipated.     

A Simulation of Wet Pocket Lumber Drying

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D _eff ) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D _eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.     

Sewage Sludge Dewatering Process Using Liquefied Dimethyl Ether as Solid Fuel

This article describes the dewatering of sewage sludge using liquefied dimethyl ether (DME), which clarified the effect of sludge type and its particle size. Changes in the heating value of the dewatered sludge were also examined. In addition, the change in dewaterability upon reuse of the liquefied DME was investigated experimentally.

Regardless of the nature of the sludge, its dewatering behavior was similar. The amount of dewatered water could be increased by decreasing the diameter of the sludge cake. Complete drying of the sludge cake via DME dewatering resulted in retention of approximately 96% of the heating value (17.7 MJ/kg-sludge) of the original sludge cake dry base (18.4 MJ/kg db). The DME could be recovered after extraction, and reuse experiments showed that liquefied DME dewatering performance was unchanged after five reuses.     

QUANTIFICATION OF THROUGH DRYING RATE DATA

ABSTRACT

For through drying there are three distinct drying rate periods, increasing rate, constant rate and falling rate. The increasing rate period is so important that nearly half of the drying is completed in this period only. A drying rate - moisture content relationship for this period was obtained based on theoretical analysis. It was verified with experimental data. A quantitative representation of the complete drying rate curve was established using this relationship and a modified power law equation for the falling rate period drying rate - moisture content relation. It needs five parameters to quantify the through drying from wet to dry: moisture content at the end of the increasing rate period; exponent for the drying rate - moisture relationship during the increasing rate period; constant drying rate; critical moisture content and the power-law exponent for the falling rate period.     

基于微波诱导定向加热的污泥新型热解方法能耗分析

针对现有污泥热解技术耗时耗能、炭性能受限等难题，提出微波诱导协同热解的新型技术思路，即仅先用常规初级热解获得微波强化吸收的热解基体，再用微波诱导其高能位点效应，以期低能耗制备较高性能的污泥炭。对样品进行介电特性、工业分析等多种测试，在简要分析并验证该思路可行的基础上，探寻其能耗机制，以期为实际应用提供参考。结果表明，通过常规700℃热解10 min的热解基体，介电特性提高约22%，可在微波900 W中5 min升高到平均900℃；不仅提高炭性能，而且比常规700℃热解60 min节能省时达50%以上，这主要归因于对热解过程整体用时的显著缩减与微波能的高效利用。研究思路为低能耗制备高附加值污泥炭奠定工艺应用基础，有望实现污泥大规模资源化处置。