Thermal Drying Of Sewage Sludge Research Articles

Amplifiers of environmental risk of microplastics in sewage sludge: Thermal drying treatment

Sewage sludge was already identified as an important source of microplastics (MPs) in the environment. Therefore, investigating the effects of sludge treatment processes on sludge-based MPs is essential for understanding the environmental risks and controlling their release. This study investigated the occurrence characteristics and elucidated the fragmentation mechanism of sludge-based MPs before and after the thermal drying treatment of sludge. The results showed that this treatment increased the abundance of sludge-based MPs by about 10-fold, with enhanced fragmentation and fracture parameters, and increased the abundance of <100 μm MPs to >60 %. Remarkably, both polypropylene-microplastics (PP-MPs) and polyethylene terephthalate-microplastics (PET-MPs) did not show significant chemical aging. The structural analysis showed that the molecular chain disorientation and secondary crystallization of PP-MPs and PET-MPs occurred. These transformations caused the contraction of the polymer molecular chains and the generation of micro-mechanical stresses, leading to the formation of warpage structures and stress cracking on the MPs' surface. These phenomena also contributed to the further fragmentation of the MPs and the development of finer MPs particles. The findings of the present investigations emphasize that the thermal drying of sewage sludge amplifies the environmental risk of sludge-based MPs.

Impact of acceleration of agricultural and industrial waste conditioning on thermal drying of sewage sludge.

Improving the dewatering performance of sludge is a necessary advancement for collaborative sludge disposal and energy-efficient utilization in power plants. Herein, the effects of temperature changes and the addition of drying accelerators derived from agricultural and industrial waste on the drying characteristics of sewage sludge (SS) were investigated from the perspectives of drying kinetics and micromorphology. According to the results, the drying time for sludge significantly showed a clear downward trend via medium-low temperature (160°C) thermal drying, which subsequently reduced the energy input substantially. When the drying temperature was 160°C, the ideal addition proportions of rice husk (RH), sludge ash (SA), and coal ash (CA) comprised 15%. The addition of SA prominently boosted the drying rate constant of the original sludge by 48%. Additionally, SEM images along with the spectral dimension obtained via fractal theory explicitly clearly demonstrated that SS + SA had more loose pore channels than other mixed samples. This provided a convenient approach to ensure the evaporation and migration of moisture and thereby shorten the drying time effectively. Consequently, the addition of sludge ash as a drying accelerator can promote the deep dewatering of sludge.

Evaluation and techno-economic analysis of packed bed scrubber for ammonia recovery from drying fumes produced during the thermal drying of sewage sludge

This study investigates the recovery of ammonia from drying fumes during thermal drying of sewage sludge with packed bed acid scrubbers to recover ammonia and to produce ammonium sulfate. The process is modelled for two concentrations, 75 and 100 ppm, and 1000m3/h inlet flowrate of drying fumes containing air and ammonia gas. It results in finding optimal parameters for scale-up of drying fumes during thermal drying of 7700t/a sewage sludge of Lappeenranta city. It is found that a single scrubber, with a 24000 m3/h of inlet gas and an ammonia concentration of 75 ppm, liquid to gas ratio of 1.5, temperature and pH of liquid acid as 100°C and 3 respectively, gives the efficiency of more than 99%, and reduces ammonia concentration in the outlet stream to 0.2 ppm. The capital cost is 290 k€, operating cost is 113k€/a, removal cost with and without revenue of ammonium sulfate is 20 €/t and 18 €/t of sludge. The packed bed acid scrubber would be suitable to remove ammonia in the drying fumes to recover ammonia from the drying fumes, but the initial economic analysis highlights that the production of commercial grade ammonium sulfate fertilizer would be an expensive option.

Technical implications and global warming potential of recovering nitrogen released during continuous thermal drying of sewage sludge

Thermal drying and consequent incineration of sewage sludge result in an absolute loss of an important macronutrient – nitrogen. To fulfill the growing food demand, humanity relies more on industrial fixation of nitrogen, primarily via the Haber-Bosch process. The present paper examines the nitrogen release during continuous thermal drying of municipal sewage sludge and its consequent recovery for fertilization. Furthermore, the possibility of nitrogen recovery from condensate is assessed. Finally, the study assesses the global warming potential of the proposed nitrogen recovery system and compares it with the baseline system manufacturing fertilizers from industrially fixed nitrogen. The results of the drying experiments showed that 0.73–1.03 g N-NH3 kg−1 total solids of sewage sludge was released to off-gases during its continuous thermal drying under 160 °C, which corresponds to 41–58% of ionized nitrogen content in raw sewage sludge subjected to thermal drying. The global warming potential of the nitrogen recovery was 28% lower compared to that of the commercial fertilizer production of equivalent properties: 4.1 kg CO2-Equiv. kg−1 N versus 5.7 kg CO2-Equiv. kg−1 N. Still, the sensitivity analysis showed that the results might traverse and lead to a higher global warming potential of 6.2 kg CO2-Equiv. during the nitrogen recovery process under certain process parameters.

Thermal treatment on sewage sludge by electromagnetic induction heating: Methodology and drying characterization

Thermal drying of sewage sludge is not only an effective way to treat the waste, but also an essential step for further energy utilization. This work focused on drying property of sewage sludge by using electromagnetic induction heating. A novel drying method for sewage sludge was designed. The effects of different electromagnetic-induction media materials, working parameters and conditioning reagents on the efficiency of drying were investigated. Then, the kinetics was analyzed. The change of temperature and heat-transfer was analyzed during the drying process. Experimental results showed that sewage sludge combined with three kinds of induction medias can be efficiently dried by applying electromagnetic induction heating. Fast formation and development of cracks indicated that an increase of drying rates of sludge can be obtained. Considering the release of volatile organic compounds from sludge during drying process, estimated moisture content was used to evaluate the drying effect. A Higher working voltage leaded to a more weight reduction of sludge during a shorter drying time, but a lower voltage prolonged the drying time. It was noted that the estimated moisture rate was very close to the experimental moisture content. Sludge content, forming and induction media significantly affected the drying process. Plate and net media were fitted for thin layer and piled sludge, respectively. However, fiber media seemed to show lower drying rate due to no circuit for induction current. An addition of CaO and sawdust improved the drying process. As a result, few volatile organic compounds released from sludge. For kinetics, three periods (warm-up, constant rate and falling rate period) can be observed and the data fitted linear regression of Lewis drying model very well. The effective moisture diffusivity was influenced by the different induction media and the thickness of sludge. Infrared images showed that outside temperature was higher than central part temperature for all sludge samples. A higher evaporation rate and diffusion of moisture can be obtained from outside part of sludge due to the easy collapse of porous structure.

Effects of thermal drying on phosphorus availability from iron‐precipitated sewage sludge

AbstractThermal drying of sewage sludge implies sanitation and improves practical handling options of the sludge prior to land application. However, it may also affect its value as a fertilizer. The objective of this study was to assess whether thermal drying of sewage sludge, as well as drying temperature, affects plant P availability after application to soil. The experiment included dewatered sewage sludge (20% DM) and thermally dried sewage sludge (95% DM) collected at a Danish wastewater treatment plant, as well as laboratory oven‐dried (70, 130, 190, and 250°C; DM &gt; 95%) subsamples of the dewatered sludge, and a triple superphosphate as a reference. Plant P availability was studied in a 197 d soil incubation experiment, with sampling for Diffusive Gradients in Thin films (DGT) and water extractable P (WEP) analyses over time, and in a pot experiment with spring barley (Hordeum vulgare L.). In both experiments, thermal drying reduced P availability, as shown by 37 and 23% lower DGT and WEP values, respectively, and a 16% lower P uptake by barley in the pot experiment. The specific drying temperature did not appear to have much effect. Overall, our results suggest that thermal drying of iron‐precipitated sewage sludge is not an optimal treatment option if the aim is to optimize plant P availability.

Nitrogen release from mechanically dewatered sewage sludge during thermal drying and potential for recovery

The increasing demand for food has led to heightened demands for fertilizers containing nitrogen, a vital nutrient for efficient crop yields. In the study, a potential for nitrogen recovery during thermal drying of sewage sludge was examined. The mechanically dewatered sewage sludge from a municipal wastewater treatment plant was batch dried under controlled temperature regimes without pH adjustment. The drying temperatures were set to 100 °C, 130 °C, and 160 °C. Fourier transformed infrared (FTIR) spectroscopy analysis was applied to continuously measure the ammonia concentration in the exhaust fumes generated during the batch drying tests. The results indicated that the mass of ammonia released during the drying process ranged between 4700 and 6850 mg/kg total solids (TS), which corresponded to 78–99% of the 6220 mg/kg TS ionized ammonia nitrogen (NH4+N) content determined in the sewage sludge prior to the drying process. No clear relationship between the drying temperature and the amount of ammonia released was identified. Still, out of the total nitrogen contained in the sewage sludge (53,000 mg/kg TS), 81% was present as organic nitrogen, which is bound in cell structure and, therefore, could not be released.

Energetic assessment of air-steam gasification of sewage sludge and of the integration of sewage sludge pyrolysis and air-steam gasification of char

Thermo-chemical treatment of sewage sludge is an interesting option for recovering energy and/or valuable products from this waste. This work presents an energetic assessment of pyrolysis and gasification of sewage sludge, also considering the prior sewage sludge thermal drying and the gasification of the char derived from the pyrolysis stage. Experimental data obtained from pyrolysis of sewage sludge, gasification of sewage sludge and gasification of char (all of these performed in a lab-scale fluidized reactor) were used for the energetic calculations. The results show that the energy contained in the product gases from pyrolysis and char gasification is not enough to cover the high energy consumption for thermal drying of sewage sludge. Additional energy could be obtained from the calorific value of the pyrolysis liquid, but some of its properties must be improved facing towards its use as fuel. On the other hand, the energy contained in the product gas of sewage sludge gasification is enough to cover the energy demand for both the sewage sludge thermal drying and the gasification process itself. Furthermore, a theoretical study included in this work shows that the gasification efficiency is improved when the chemical equilibrium is reached in the process.

Skin effect in the heat and mass transfer model for sewage sludge drying

The thermal drying of sewage sludge was studied in the 30–65 °C temperature range with two types of sewage sludge, showing different behaviour in the formation of a skin during the drying process. Twenty-one runs were carried out at different temperatures and air flows in an apparatus that provided a continuous flow of air. Small sewage sludge spheres and cylindrical tablets were produced to study the kinetics. During each run, the variation of moisture content vs. operation time was monitored, and during some tests, the temperature inside the sample as well. A kinetic model was developed to correlate the experimental data, both moisture content and temperature, vs. time. The resulting kinetic parameters (mass transfer coefficient, heat transfer coefficient, skin effect parameter, diffusion coefficient and activation energy) are discussed with reference to correlations and kinetic parameters presented in the literature. In general, the model seemed to suitably reproduce the experimental moisture and temperature data. For some runs there were significant deviations between the experimental and calculated temperatures, which could be explained by an evaporation effect taking place at the surface of the thermocouple wire placed inside the sludge spheres. The kinetic parameters have been used for simulation of continuous dryers. Finally, an analysis of the influence of the different parameters on the drying process has been carried out in order to obtain a general model valid for different types of sewage sludges.

Immersion frying for the thermal drying of sewage sludge: An economic assessment

This paper presents an economic study of a novel thermal fry-drying technology which transforms sewage sludge and recycled cooking oil (RCO) into a solid fuel. The process is shown to have significant potential advantage in terms of capital costs (by factors of several times) and comparable operating costs. Three potential variants of the process have been simulated and costed in terms of both capital and operating requirements for a commercial scale of operation. The differences are in the energy recovery systems, which include a simple condensation of the evaporated water and two different heat pump configurations. Simple condensation provides the simplest process, but the energy efficiency gain of an open heat pump offset this, making it economically somewhat more attractive. In terms of operating costs, current sludge dryers are dominated by maintenance and energy requirements, while for fry-drying these are comparatively small. Fry-drying running costs are dominated by provision of makeup waste oil. Cost reduction could focus on cheaper waste oil, e.g. from grease trap waste.

Two examples of thermal drying of sewage sludge

The increasing amount of sewage sludge, legal requirements and a change in the assessment of common disposal paths lead more and more to thermal drying of sewage sludge. During the last years a lot of sludge drying plants in Germany were planned, built and brought into service. This paper describes two different kinds of sludge drying plants with their determining and operating values, the costs and the reasons for the choice of the drying method. The examples given are the sludge drying plant at Darmstadt (2 lines with a 2 stage dryer) and in the surrounding area of Frankfurt (1 line with a drum dryer).

Two examples of thermal drying of sewage sludge

The increasing amount of sewage sludge, legal requirements and a change in the assessment of common disposal paths lead more and more to thermal drying of sewage sludge. During the last years a lot of sludge drying plants in Germany were planned, built and brought into service. This paper describes two different kinds of sludge drying plants with their determining and operating values, the costs and the reasons for the choice of the drying method. The examples given are the sludge drying plant at Darmstadt (2 lines with a 2 stage dryer) and in the surrounding area of Frankfurt (1 line with a drum dryer).

Developments in the Thermal Drying of Sewage Sludge

ABSTRACTThe application of heat to dried sewage sludge has been practised by the water industry for many years. Early technologies were effective but their utilization of energy was not so efficient. Other industries have faced up to this problem and, as a result, have developed thermal‐drying techniques which are more efficient and therefore more cost effective. In recent years, the transfer of this technology to sewage‐sludge drying has been taking place, and some companies have designed dryers specifically for this purpose. Commercial and legal pressures have also focused attention on the final product and its use in agriculture, horticulture, land reclamation, landfill, incineration, and as a substitute for fuel.This paper reviews the work which has been undertaken to determine the changes which occur as sewage sludge is dried, and discusses the technology which is available at present. Factors influencing capital and operating costs and product choice are also included.

Thermal Drying of Sewage Sludge

AbstractOver the next decade the UK will experience significant, imposed changes in the quantity of sewage sludge produced and in the means available for its disposal. Existing practices are being reappraised and new technologies are being explored to cope with a predicted 40% increase in the quantity for disposal. Thermal drying of sludge undoubtedly has a role to play in coping with this demand.The two principal process types, i.e. direct and indirect heat application, are appraised in depth and a comparison is drawn. A case study is identified to examine the energy demands of the process, including the potential for heat recovery and for resource reuse. The environmental impact of a typical process is also examined.