Comparative study of hot air drying and microwave drying for dewatered sludge

The seafood processing industries produce large amounts of wastewater sludge in Thailand. Most of sludge is dewatered and then sent directly to landfills without utilization nor resource recovery. The dried sludge, having a gross calorific value (GCV) higher than 21.9 MJ/kg, might be worthwhile to utilize as refuse-derived fuel. However, the critical obstacle is its high moisture content of 87.4 wt%. This study investigated two techniques for drying sludge: hot air and microwave drying. The effects of hot air temperatures (100 °C–170 °C) and microwave power levels (100–800 W) were studied for their specific energy consumption (SEC), dried product characteristics, and drying kinetics. The results showed that the drying times decreased with the increase in hot air temperatures and microwave power. Under the tested conditions in this study, the application of microwaves reduced the drying time by at least 37.5% compared to using hot air. The reduction of drying times led to decreased energy consumption. Approximately 72.5–80.3 wt% of dried sludge was the volatile matter (VM). As this component was devolatilized, the GCV of the dried sludge decreased. The sludge dried by the microwaves had a GCV of 20.8–21.4 MJ/kg, while by the hot air had a GCV of 19.4–21.8 MJ/kg. The results from product analysis suggested that the microwave technique could be an alternative drying method when one requires a shorter drying time than the conventional hot air technique.


Introduction
Thailand is one of the world's leading seafood producers. For the tuna business alone, Thailand contributes 33.2% of the world's export value. In 2019, exported products consisted of about 710,962 tons, generating 2826 million dollars (Thai tuna industry association 2020). This business has also created jobs for local communities. With support from government policies, the food processing industry continues to expand and remain vital. Unfortunately, most food processing industries generate a considerable amount of wastewater and sludge from their treatment plants. Common sludge management in Thailand begins with thickening and dewatering processes. After that, the dewatered sludge is sent to landfills. The landfilling of sludge is widely used in Thailand because it is simple and cheap to construct the system. However, it can be costly due to the transportation and tipping fees. Also, it is not the best option for a circular economy as the sludge becomes non-value-added waste.
Generally, dried sludge has a gross calorific value (GCV) of about 14-23 MJ/kg (He et al. 2013;Mawioo et al. 2017;Poudel et al. 2015), comparable to lignite 12-25 MJ/kg (Arlabosse et al. 2011;Syed-Hassan et al. 2017;Zhang et al. 2016), and sub-bituminous coal 18-23 MJ/kg (Breeze 2015), and even reach the lower level of bituminous coal 21-29 MJ/ kg (ASTM International 2019; Syed-Hassan et al. 2017). The calorific value reveals that dried sludge utilization as a solid fuel in thermochemical processes is an attractive management solution. Refuse-derived fuel (RDF) produced from sludge can be used as a single fuel, or co-fired with fossil fuels commonly feeds the industry's boilers (Lu et al. 2013). Callegari et al. (2019) categorized wastewater sludge as a feedstock for biofuels with several benefits, including (I) significant reductions in particulate matter and CO 2 emissions when compared with fossil fuels, (II) addition of new energy sources and energy security, and (III) promotion of circular economy through waste recovery and fuel production. In addition to the direct use of dried sludge, Callegari and Capodaglio (2018) produced biochar from sewage sludge by using pyrolysis process. The biochar contained colorific values around 18 MJ/kg, which can be fed for combustion with coal. A significant impediment to utilizing sludge as RDF and biochar is its high moisture content. Kijo-Kleczkowska et al. (2016) recommends using sludge with a moisture content of less than 10% to produce sludge-derived fuel pellets. Bolognesi et al. (2019) dried sludge to a moisture content of less than 10% prior to pyrolyzing it. Generally, dewatered sludge retains a high moisture content of 73-89 wt% (Syed-Hassan et al. 2017). Therefore, efficient drying processes are required for preparing dried sludge for use as an alternative solid fuel. Drying not only improves fuel properties but also helps reduce the sludge volume and lower the cost of storing, transporting, and handling of the dried material. Conventional sludge drying is usually performed by a convective technique using hot air and a solar greenhouse technique. Solar drying is cost-efficient and requires low energy inputs, about four times lower than hot air drying. Boguniewicz-Zablocka et al. (2020) studied solar greenhouse drying of sludge from an urban wastewater treatment plant. The results showed that the method could reduce the moisture content by 79% within two weeks. However, the longer operation time than thermal drying also required a large load area of about 35 kg/m 2 /year. This land requirement could be a problem for factories located in a limited area or industrial estate. Weather condition is also a vital factor controlling the yield of dried sludge produced from the solar drying technique. Countries with a high level of precipitation such as Thailand, where the average annual rainfall is 1528 mm and the rainy days are 131 days (Thai meteorological department 2021), could not rely solely on the solar drying to produce dried sludge in large quantities.
Hot air drying is a common convective drying method that requires much smaller land space and is less weather dependent compared to solar drying. It is widely used in many industrial sectors due to its uncomplicated operation. It is applied in different configurations such as belt dryers, flash dryers, and drum dryers. In general, hot air dryers have a specific energy consumption (SEC) varies between 2.5 and 5.0 MJ/kg of evaporated water (Arlabosse et al. 2011). The hot air temperature for sludge drying could be varied between 60 °C and 180 °C according to the thermal degradation characteristics of the raw material (Tańczuk et al. 2016;Zhang et al. 2016). Unfortunately, the conventional hot air drying of sludge is energy-intensive and time-consuming because of sludge's high moisture content (Syed-Hassan et al. 2017). The moisture content gradually decreases during the hot air drying because both water evaporation and heating occur at the material's surface (Mounir et al. 2019). To overcome these drawbacks, microwave drying could be an alternative technique for reduced drying time, saving energy consumption, and increasing loading capacity.
The key difference between microwave and hot air drying is the heating mechanism (Mounir et al. 2019). Microwaves produce heat for both the surface and internal materials by oscillatory electric fields. When applying microwave to moist materials, the energy is absorbed by the dielectric properties of water and then converted into heat within the material volume. The material is heated rapidly, resulting in an increase in the evaporation rate during the drying period (Gaukel et al. 2017). The volume of raw sludge can be reduced by more than 60% (Mawioo et al. 2017). The final temperature of a material is a crucial factor that affects the product properties in microwave heating. In the study by Wang et al. (2015), microwave pretreated sludge at 100℃ effectively released nitrogen up to 4.61 mg/g (on a dry basis). Liu et al. (2017) reported that the microwave treatment at a temperature higher than 220℃ resulted in the dechlorination of the organochloride-waste mixture with a chlorine removal efficiency up to 87%. Due to different heating mechanisms between microwave and hot air, the properties of the final products from the two drying techniques might vary. Vongpradubchai and Rattanadecho (2009) dried biomaterials such as wood by microwave and hot air. The microwave dried product exhibited a better arrangement of microstructure and less shrinkage due to uniform energy absorption. Domínguez et al. (2004) dried four different types of sewage sludge using 1000 W of microwave output power versus 150 °C of hot air temperature. The results reported no significant qualitative differences between both techniques regarding the dried sludge's heating value and proximate and ultimate properties.
Hot air and microwave drying have been widely studied mainly in food science and postharvest technology. Palamanit et al. (2019) studied hot air and microwave-assisted hot air drying for instant parboiled rice using volumetric air flow rate at 12.6 m 3 /min. The microwave-assisted hot air technique reduced the drying time by 50% compared to the hot air technique. Zohrabi et al. (2019) studied hot air drying of wood chips in a pilot-scale dryer using a volumetric air flow rate at 6-7.5 m 3 /min. The drying time decreased about 25% when the volumetric air flow rate increased due to the increasing mass transfer coefficients between the sample and hot air. However, studies on sludge and waste treatment are still very limited, especially on microwave drying. Bennamoun et al. (2016b) studied convective drying of sewage sludge under air temperatures of 80 °C, 140 °C, and 200 °C. The increase in hot air temperature results in improved drying kinetics. The optimum drying condition is 140 °C due to exergy efficiency reaching 90%. Zhang et al. (2016) studied the drying of sludge and sludge mixed with lignite briquettes under hot air temperatures between 60 ℃ and 180 ℃. The average drying rates of mixed ingredients were ≥ 2.6% higher than the raw sludge. Drying temperatures over 120 ℃ indicated the optimal moisture diffusion of the briquettes. Li et al. (2015) studied drying of sludge mixed with sawdust under 110 °C, 80 °C, and 50 °C. The drying process of sludge mixed with sawdust showed a negative result, indicating that the drying rate for the mixed material was lower than for the raw sludge. The maximum drying time was about 5.5 h with a drying temperature of 50 °C. Bennamoun et al. (2016a) investigated microwave drying of sewage sludge under power levels between 480 W and 1080 W. The results showed that microwave drying reduced the drying time by around 93% compared to the convective technique. The drying rate decreased with an increased power level. The maximum temperature of the sludge reached 140 °C during drying. The shrinkage of material reduced its volume up to 42%, which was less than the shrinkage from convective drying. Mawioo et al. (2017) studied drying in a pilot-scale microwave reactor at a power of 3.4 kW using four different types of sludge. The results indicated that the moisture content reduction was inversely related to the sludge temperature increment. The maximum temperature of sludge during the process was 102 °C. The dried products have the potential to produce solid fuel with a GCV of 16-23 MJ/kg. Each trial was started in a cold chamber that caused higher SEC (10-16 MJ/kg of evaporated water). Several studies have investigated the efficiency of drying methods and the characteristics of dried products. However, the study on comparison of the fuel characteristics of sludge dried by hot air and microwave methods is very limited. Therefore, the objective of this study was to investigate sludge drying under microwave and hot air techniques and to compare the characteristics of the dried sludge and the drying kinetics.

Materials
The dewatered sludge was collected from a belt filter press machine of a wastewater treatment system located in a seafood processing plant at Samut Sakhon province, Thailand. To prepare a sufficient amount of sample with constant properties throughout the study, the sludge sample was thoroughly mixed and stored in closed plastic containers at − 2 °C. Frozen temperatures encouraged the physical and chemical preservation of materials (ASTM International 2020) and maintained both the alpha and beta diversity of microbial composition in materials (Rubin et al. 2013). Before being tested, the sludge was removed from the freezer and placed under the room temperature (30 °C) for at least 1 h. Throughout this process, the sludge is kept within a closed container to preserve the original level of moisture content.

Experimental setup and procedures
The sludge drying was performed under two different methods of hot air and microwave techniques. The influences of hot air and microwave on the sludge characteristics and drying kinetics were investigated under different experimental conditions. During the drying period, the relative humidity and ambient temperature measured by a thermo-hygrometer (Testo, 605i, USA), were around 67% and 30 °C, respectively. A drying experiment of the hot air technique was conducted using a laboratory hot air oven (Scientific series 9000, 973, Norway). Downdraft ventilation of the oven was set to promote moisture reduction from the sample surface. Due to the oven limitations, the volumetric airflow rate was fixed at 3 m 3 /min, which was measured by a vane anemometer (Testo, 410i, USA). The experimental conditions were studied at air temperatures of 100 °C, 125 °C, and 150 °C in two different thicknesses of the sludge layer as 2.5 and 5 cm. Each experimental condition was performed in quadruplicate to ensure data quality, and the average values were reported. The sludge drying by microwave technique was conducted in 23 L domestic microwave oven (Electrolux, EMM2301W, China). The microwave cavity was rectangular with interior dimensions of 31.4 cm × 22.1 cm × 34.7 cm. The microwave generator had a frequency of 2450 MHz and a maximum power output of 800 W. The microwave experiments varied the powers of 100, 400, 600, and 800 W with the microwave power density of 0.25, 1, 1.5, and 2 W/g, respectively. The sludge was placed in a ceramic dish 20 cm in diameter. The thickness of the sample in the dish was approximately 2.5 cm. A penetration depth of microwaves at 2,450 MHz was 1-4.8 cm when applied on wastewater sludge (Hong et al. 2006;Karlsson et al. 2019). A hot air drying of 170 °C at 2.5 cm thickness was completed to bridge the gap between the two different heating mechanisms of hot air and microwave. This was done because the hot air oven at 170 °C and the microwave oven of 800 W were operated in the same power level range. The initial sample weight was 400 g for each experiment. The mass monitoring of samples followed the weighing technique outlined by Villagracia et al. (2016). The instantaneous weight of the sludge was measured using a precision balance (Sartorius, BP4100S, Germany), which has an accuracy of ± 0.01 g, every 10 min for the microwave experiment and 60 min for the hot air experiment until it reached the constant weight. At the same time, the surface temperature of the sludge was determined under ambient air by an infrared thermometer (Testo, 805i, USA) with an accuracy of ± 1.5 °C. The thermometer was connected to a smartphone via Testo Smart Probes for data recording. The sludge was dried until it reached the bone-dried mass. The dried samples were collected in plastic zip bags and kept in a desiccator before property analyses.

Analytical methods
The initial moisture content of the sludge was analyzed based on ASTM D3173/D3173M-17a (2017). The samples were dried at 105 °C using a hot air oven (Scientific series 9000, 973, Norway) until a constant weight was achieved (around 24 h). The dried sludge from the standard method was analyzed for ignition temperature, burnout temperature, thermal degradation, and combustion behaviors using thermogravimetric analysis (TGA). The TGA was conducted by a thermogravimetric analyzer (Mettler Toledo, TGA 2, UK). The testing condition was performed in a nitrogen atmosphere at 50 ml/min of the flow rate. The heating steps were set by (I) heating from 30 to 110 °C with 10 °C/min of heating rate, (II) holding for 10 min at 110 °C, (III) heating from 110 to 900 °C with 10 °C/min of heating rate, and (IV) switching the gas from nitrogen to air under 50 ml/min of the flow rate, and holding at 900 °C for 20 min. Furthermore, the dried sludges from the microwave and hot air techniques and the standard method were analyzed for GCV and proximate and elemental characteristics. The GCV analysis was done by a bomb calorimeter (LECO, model AC350) using benzoic acid as a reference standard. Proximate analysis for fixed carbon (FC), volatile matter (VM), and ash content was analyzed by a thermogravimetric analyzer (PerkinElmer, Pyris 1 TGA, USA). The testing condition was set as the same as the mentioned TGA. Elemental analysis for C, H, and N content was based on the standards of WI-03 and ASTM D5373-16 (2016) by using an elemental analyzer (J-science, JM10 Micro, Japan). The content of S in the sludge was measured by a carbon-sulfur analyzer (Horiba, Emia-220 V2, Japan) under the conditions of a high-frequency induction furnace. The content of O was calculated from 100% minus by the total amount of C, H, N, and S content.

Calculation of moisture ratio
The sludge's moisture ratio (MR) during the processes was calculated as follows Eq. (1). The ratio of moisture content (on a dry basis) and drying time is usually presented to compare the various drying kinetics curves (Devahastin 2012).
where M t denotes the moisture content (g water/g dry solid) measured at drying time t, M e is the equilibrium moisture content (g water/g dry solid), and M i is the initial moisture content (g water/g dry solid). M e was supposed to be constant as zero in each drying condition because M e of the dried sample was very lower than the initial moisture content (Palamanit et al. 2019).

Specific energy consumption
The energy consumption of the microwave and hot air systems was defined in the electric energy consumption using an energy meter (Electan, ET-MP01U, China). The energy efficiency of experimental conditions was computed in SEC as follows in Eq.
(2) (Darvishi et al. 2013). The SEC was expressed in MJ of total supplied electrical energy per kg of the total amount of water evaporated from sludge.
where E t is the electrical power consumption (kWh) of the dryer system at drying time t, that the moisture content of sludge reached a target of the final moisture content, and m s is the mass of the dry solid.

Statistical analysis
Statistical analyses were calculated by applying Microsoft Excel software. The significant difference of the investigated parameters was analyzed by one-way analysis of variance (ANOVA) at a confidence level of 95% (P-value < 0.05). In each experimental condition, the data was computed mean values and standard deviations (SD).

Characteristics of the raw sludge
Thermal and chemical characteristics of the sludge are important for studying the characteristic variation that reflects the performance of the drying and the solid fuel product. Characteristics of the sludge sample, including moisture content, proximate analysis, elemental analysis, and GCV, are shown in the main decomposition process is reasonably held on the devolatilization and combustion phase (Yu and Li 2014).
The fuel characteristic results have demonstrated that the sludge has the advantages of looking forward to the solid fuel conversion as RDF. However, the ash content of sludge could be problematic, because it is higher than bituminous at 6.1-8.3 wt% (Jong and Ommen 2015;Phongphiphat et al. 2020). Kijo-Kleczkowska et al. (2016) compared combustion characteristics of sewage sludge, coal, and biomass under the thermal conditions of 800-900 °C. The sludge behaves similarly to biomass. The VM combustion of both fuels consumed 23-27% of the total process time, while char combustion consumed 68-72% of the total process time. In comparison to coal, the sludge ignited more rapidly and at a lower temperature than the coal. The processing time for VM combustion and char combustion were, respectively, 9% and 88% for anthracite, and 20% and 77% for lignite.

Thermogravimetric analysis
The thermal behavior of the sludge can be identified from thermogravimetric (TG) analysis. The TG and derivative thermogravimetric (DTG) curves of the sludge are shown in Fig. 1. As offered by Kijo-Kleczkowska et al. (2016), the curve of weight loss by temperature variation can be divided into 3 zones of (I) evaporation, (II) devolatilization, and (III) combustion. The TG and DTG curves displayed that devolatilization began at 120 °C, and at a temperature around 180 °C showed the inception of intensive thermal degradation. The results indicated that the initial degradation phase, attributable to the dehydration and drying process, occurred up to 180 °C (Záleská et al. 2017). The ignition temperature (T i ) and burnout temperature (T b ) of the sludge were 200 °C and 510 °C, respectively, which were in the range of T i and T b of sludge at 230-291 °C and 455-664 °C, respectively (Parshetti et al. 2013;Yu and Li 2014). When sludge was ignited, the main decomposition processes began with VM combustion and complex organic structures. The DTG curve showed the major peaks at the temperature interval of 265-340 °C. The peak values indicated two maximum-degradation temperatures of the sludge at 270 °C and 310 °C. The maximum-degradation temperatures are due to the volatile combustion (Lu and Chen 2015). The high devolatilization indicated that VM was a major factor in the thermal behavior of sludge. The thermal treatment at a higher temperature of the sludge results in devolatilization and combustion. Therefore, the experimental conditions

Drying kinetics of sludge
The drying kinetics of sludge during drying with hot air and microwave techniques are displayed in Figs. 2 and 3 (2018), microwave radiation rapidly heats the material from the inside out, accelerating the material's moisture transfer and drying rate. Therefore, the MR under the microwave technique decreased the final moisture content faster than the hot air technique. Bennamoun et al. (2016a) mentioned that the heat is transferred from air to the material surface by convection during hot air drying. The temperature of materials is gradually increased. The moisture from the internal layer slowly moves to evaporate at the surface. Therefore, the drying phases are relatively longer than microwave drying.

Effect of hot air temperature and layer thickness on drying kinetics of sludge
The drying curves of sludge during the hot air drying are presented in Fig. 2. The results indicated that the increasing temperature caused a decrease in drying time. The drying time at 2.5 and 5 cm of thickness decreased by 4 and 2 h, respectively, when the drying temperatures were increased from 100 °C to 150 °C. Increasing in drying temperature enhances the material heat transfer, accelerating evaporation, mass transfer, and drying rate. Therefore, the moisture of the high-temperature conditions is evaporated more rapidly than in the low-temperature conditions (Zhang et al. 2016).
The results of varied thicknesses of the sludge layer showed that the MR of 2.5 cm thickness was reduced faster than the MR of 5 cm thickness. The drying time of the 2.5 cm thickness of 150 °C, 125 °C, and 100 °C decreased by 45, 38, and 31%, respectively, compared with the 5 cm thickness. In convection drying, heat is slowly transferred (Paengkanya et al. 2015). The high layer thickness gradually increased in temperature, and water was slowly transferred from the inside to the sample's surface. Therefore, the thinner layer provides a higher drying rate than the thicker layer (Jafari et al. 2017).
The effect of the temperatures was insignificant with a P-value of 0.88, which means that the variation of temperature between 100 and 150 °C was not much different in the drying characteristics of the sludge. The thickness of the sludge layers was significant with a P-value lower than 0.05, which means that reducing layer thickness improved the drying characteristics of the sludge. The results suggested that the layer thickness had a more significant impact on the drying characteristics than the hot air temperature. Nevertheless, in terms of the change of chemical and physical properties, the sludge temperature is more significant than the heating time of sludge (Poudel et al. 2015). For largescale applications, the mixing condition of the material in the dryer is an essential factor. Appropriate mixing for a stationary bed or using a moving bed, such as a rotary dryer, encourages drying rate and process efficiency. For instance, drying fertilizer using a rotary dryer reduced the residence time by 48% and increased the drying rate by 3.1 to 4.9 times when compared with the conventional dryer (Delele et al. 2014).

Effect of microwave power level on drying kinetics of sludge
The drying curves of sludge during microwave drying are presented in Fig. 3. The MR of the sample decreased with a greater slope when increasing the microwave power to 400-800 W. The drying time at 400, 600, and 800 W decreased more than 83% compared to the drying time at 100 W. Thus, increasing the microwave power level contributed to a higher drying rate than lower power levels. Unlike conventional heating, the microwave has a high penetration into moist materials, which comprise polar and/or ionic molecules (Jafari et al. 2017). The high moisture content inside the material could absorb a greater amount of microwave energy at the high level of output power, which led to volumetric heating generation in the sample. This enhanced the evaporation of the water inside the sludge and reduced the drying time (Paengkanya et al. 2015).

Effect of microwave power level on sludge temperature
The surface temperatures of sludge during the microwave experiment are shown in Fig. 4. The results of the temperature variations are shown in the three main drying periods, namely the adaptation, constant rate, and falling rate periods. Related results of drying periods were found by Bennamoun et al. (2016a). Their works studied sludge drying by varying microwave power levels. However, regarding surface temperature, it should be noted that the measured temperature was probably expected to be lower than that of the entire material mass (Fennell and Boldor 2014). This is because the microwave energy is uniformly heated and absorbed from the center of the material, resulting in a higher temperature in the interior than the exterior (Vongpradubchai and Rattanadecho 2009). Karlsson et al. (2019) comparatively measured the sludge temperature during the microwave heating experiment, which included (I) surface temperature using infrared temperature sensors and (II) internal temperature using thermocouple probes. The difference in temperature between the infrared sensor and thermocouple probe measurement was 12-21 °C when the microwave power density was ranged from 0.75 to 1.20 W/g with the final drying temperature of 90 °C.
The sludge surface temperature rapidly increased up to 80 °C within the first 10 min, especially at 400, 600, and 800 W of microwave power. The high amounts of heat were generated by the interaction between the microwaves and dipolar molecule, which is the high concentration of water in the initial wet sludge (Lima et al. 2016). The sludge temperature at 400-800 W of microwave power increased faster than 100 W. In agreement with Apinyavisit et al. (2017), the sample absorbed more microwave energy when using the higher power level. However, the variation between 400 and 800 W reflected almost no different effect during the adaptation period, as shown by almost overlapping curves.
In the constant rate period, most of the energy is supplied to the evaporation of the water in the material, and the material temperature is almost constant (Mawioo et al. 2017). During this period, constant temperature lines were around 10, 20, 30, and 190 min for 800, 600, 400, and 100 W, respectively. The length of the period varied at different microwave power levels. The higher power levels showed a shorter time, which indicated the simple trend of the power level effect.
In the falling drying rate period, the retained moisture in the material was slowly evaporated while it was still capable of absorbing the intensive microwave energy (Bennamoun et al. 2016a). Consequently, the temperature increased to the end of the drying method. The sludge temperature at 100, 400, 600, and 800 W of microwave levels increased up to 114 °C, 142 °C, 215 °C, and 204 °C, respectively. The sharp increase in the material temperature might be due to the phenomenon of thermal runaway (Ao et al. 2018). This phenomenon enhanced thermal degradation and the charring in the dried material. As presented in Fig. 1, the sludge temperatures beyond 120 °C and 200 °C indicated organic matter decomposition and ignition point, respectively. The sludge dried at 600 and 800 W had the final temperatures a little over 200 °C which was the sludge's ignition point. The sludge dried at 600 W had a slightly higher temperature than that dried at 800 W possibly because the longer time for drying was required at a lower microwave power. The longer time may lead to a larger quantity of volatile matter released and a higher temperature of the remaining substances. The related results of temperature variation were also reported by Gaukel et al. (2017).

Specific energy consumption of drying processes
The SEC of different drying techniques for 2.5 cm of sludge layer thickness is presented in Fig. 5. The SEC of the microwave technique at 100, 400, 600, and 800 W were 10.39, 5.23, 4.82, and 4.62 MJ/kg of evaporated water, respectively, while the SEC of the hot air technique of 170 °C, 150 °C, 125 °C, and 100 °C were 4.33, 4.78, 5.04, and 6.03 MJ/kg of evaporated water, respectively. The SEC of hot air and microwave techniques was reduced with the drying temperature and power level increases, respectively. It can be seen that the 800 W condition saved the SEC about 56% compared to that required by using the 100 W drying. The 170 °C condition also saved the SEC about 28% compared to that required by using the 100 °C drying. This is because increasing both the temperature and the power enhanced the drying rate and reduced the drying time. Short drying time resulting in less SEC was found in Paengkanya et al. (2015). The SEC of sludge drying of other studies in Table 2 shows that the current results are still higher than the reviewed SEC for the conductive and convective commercial driers. The reason for these high SEC could be that the ovens used in this study was a small own-adapted instrument. Its efficiency is therefore lower compared with equipment used in other studies or those of commercial scale. The energy Fig. 4 Temperature of sludge during drying with microwave technique by using a domestic microwave oven at power levels of 100, 400, 600, and 800 W in a thickness of the sludge layer as 2.5 cm consumption was measured only during the experiment, but not the entire drying process of full-scale drying plants. Tańczuk et al. (2016) presented the performance data of a sludge drying plant, which had the capacity of input sludge about 40 tons per day. The SEC of a hot-air belt-dryer for the entire process was 3.9 MJ/kg of evaporated water. About 92% was the energy consumption in drying procedure.

Proximate analysis and energy values of dried sludge
The dried sludge characteristics in terms of proximate analysis and energy values are presented in Table 3. The proximate analysis results clearly show that the VM of dried sludge under microwave and hot air techniques decreased compared to that of the dried raw sludge under the standard condition at 105 °C for 24 h. The VM reached the lowest level at 72.4 wt% for 170 °C of hot air temperature, whereas the FC increased to 27.6 wt% at the same condition. The decrease in the VM was attributed to an increase in the FC, that occurred due to the influence of sludge temperature during the processes (Kannan et al. 2017).
Presenting TG and DTG curves, Fig. 1 shows that devolatilization and combustion phases are, respectively, located at the temperature range of 120-200 °C and 200-900 °C. The material's high temperature probably affected the loss of product quality (Szadzińska et al. 2019). For hot air drying at 170 °C and 150 °C, the decrease in VM indicated that the sludge temperature reached the devolatilization zone.   Following theoretical hot air drying, the temperature of materials finally increased to reach the dry-bulb temperature of the hot air (Devahastin 2012), which was about 170 °C and 150 °C in this case. The sludge temperatures of microwave techniques (Fig. 4) reached the devolatilization zone, especially at the microwave power of 600 W that showed the highest temperature at 215 °C. Besides the temperature effect, Yu and Li (2014) showed that the release of volatile content was significantly influenced by microwave irradiation because of increased in sludge surface porosity during microwave drying. The fuel characteristics of the dried sludge could be indicated by the "Fuel ratio", which is the ratio of FC per VM (Kannan et al. 2017). The fuel ratio of dried sludge under both techniques was slightly higher than that of raw sludge. The increase in fuel ratio normally represents improved fuel characteristics of the product, due to the decrease of VM. Nevertheless, in the case of the sludge, it affected the decrease in calorific value. GCV of the dried sludge of microwave and hot air techniques decreased up to 11% compared with raw sludge. The GCV exhibited a relative change with VM content. For instance, the raw sludge revealed the highest VM and highest GCV, while the dried sludge of 170 °C revealed the lowest VM and lowest GCV. This is because VM represented the main combustible content in the sludge. The proportion of VM was higher than FC by about three times. When the sludge temperature increased, the VM easily decomposed, and the GCV decreased. Similar trends were observed in the studies of Mawioo et al. (2017). The energy densification in the dried product could be calculated in terms of the "Energy enrichment factor", which is the ratio of the calorific value of dried sludge per calorific value of the raw sludge (Kannan et al. 2017). All dried sludge showed the value of energy enrichment factor lower than 1, indicating the decrease in energy densification of the product. This decrease in the energy enrichment factor was attributed to the decrease in GCV. However, the lowest GCV obtained in this study (19.4 MJ/kg) is comparable to the GCV of other sludge as 14-23 MJ/kg (He et al. 2013;Mawioo et al. 2017;Poudel et al. 2015 The calorific values of the dried product were comparatively higher than the SEC during the processes. Mawioo et al. (2017) suggest that the energy from the sludge could be recovered to be used as RDF. The combustion properties of dried sludge need to be further studied to prove whether the dried sludge could be an alternative solid fuel. Furthermore, the drying cost needs to be further assessed in the pilot and industrial scales. The primary challenges in implementing microwave techniques on a large scale are the energy efficiency and economic aspects (Bundhoo 2018). The short lifetime of magnetrons, approximately 1 year for continuous operation, is the essential high cost of equipment for the microwave drying process scale-up. Radoiu (2020) reported that the capital investment and operating expenditure of industrial-scale microwave dryer for 400 kW were 2.5 million USD and 0.2 million USD per year, respectively. However, dried sludge in co-combustion with fossil fuels has commonly fed industry boilers, an application with significant benefits, such as cost saving for sludge disposal and minimizing the cost of substituted fossil fuel. Phongphiphat et al. (2020) reported that mixing dried sludge and bituminous coal for use in industry's boilers avoided disposal fees for the sludge landfilling (40 USD/ton) and reduced the purchased cost of the coal (130 USD/ton) by about 5%.

Elemental analysis of dried sludge
The results of the elemental analysis are presented in Table 4. Inferring the high values of GCV and energy enrichment factor, which are advantages of using the dried sludge as solid fuel, the elemental analysis was done for the dried sludge at 800 W of microwave power and 150 °C of hot air temperature. The results of dried sludge at 150 °C showed an increase in the proportions of C and N compared with those of the raw sludge. In contrast, the proportions of H and O were decreased. As usual, the decrease of hydrogen and oxygen contents is influenced by the dehydration of biomass (Poudel et al. 2015). The C and N contents were relatively increased due to the fact that the other elements were decreased. The dried sludge at 170 °C and 800 W showed an apparent decrease in C proportion because of the high drying temperature. Especially with the overheating problem at 800 W, the sludge temperature increased to 204 °C indicating combustion (Fig. 1) at the falling period of the The proportions of O and S followed an opposite tendency compared with the carbon content change. H/C and O/C ratios were calculated to understand the occurred reactions that might affect solid fuel production from the sludge. The H/C ratio of dried sludge at 800 W, 170 °C, and 150 °C were lower than that of the raw sludge because of the dehydration reaction (He et al. 2013). The O/C ratio slightly decreased for the hot air temperature of 150 °C. The O/C ratio change was described by Poudel et al. (2015). The reduction of the O/C ratio is related to the devolatilization of volatiles rich in oxygen.

Conclusions
This study investigated the characteristics of dewatered sludge and the drying using hot air and microwave techniques. The raw sludge showed high initial moisture content of 87 wt%, but the GCV was higher than 21 MJ/kg. The high amount of VM of the sludge influenced the low ignition temperature at 200 °C. The TG and DTG curves of raw sludge indicated the devolatilization phase around a temperature above 120 °C. The drying time of the hot air technique was reduced by more than 30% by decreasing the sludge layer thickness. The microwave technique showed positive effects on the drying kinetics and accelerated the evaporation of water. The application of microwaves reduced the drying times by more than 37.5% compared to that required with the hot air technique. The drying time decreased with the increase of the microwave power level or hot air temperature due to higher drying rates. The reduction of drying time led to the saving SEC of the processes. The low SEC were in the same range as 4.3-4.8 MJ/kg of evaporated water for hot air condition at 150-170 °C and microwave condition at 600-800 W. The loss of VM in the dried products mainly influenced the decrease in GCV at 0.95-0.99 times of the raw sludge. The microwave technique showed the GCV lower than the hot air technique due to the overheating problem, which exhibited the maximum temperature of the sludge up to 215 °C.
According to the studied results, the dried sludge of both techniques was almost in the same range of product characteristics. The microwave and hot air techniques can produce the dried sludge with high GCV comparable to sub-bituminous coal. For fuel characteristic aspects, the dried sludge has the potential for recovery and reuse for energy applications. Considering the drying time savings and productivity, the microwave technique offered an attractive benefit for drying sludge compared to conventional drying by hot air technique. Under the conditions of this experiment, the lowest drying time, low SEC, and high GCV of the dried sludge were observed in the drying condition of 800 W. However, the prevention or reduction of thermal degradation during microwave drying should be studied further to improve the quality of dried products.