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Influence of forced air volume on water evaporation during sewage sludge bio-drying
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 4 7 6 7 e4 7 7 3
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Influence of forced air volume on water evaporation during sewage sludge bio-drying Lu Cai a,b, Tong-Bin Chen a,*, Ding Gao a, Guo-Di Zheng a, Hong-Tao Liu a, Tian-Hao Pan a a
Center for Environmental Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 11A Datun Road, Beijing 100101, PR China b University of Chinese Academy of Sciences, Beijing 100039, PR China
article info
abstract
Article history:
Mechanical aeration is critical to sewage sludge bio-drying, and the actual water loss
Received 24 December 2012
caused by aeration can be better understood from investigations of the relationship be-
Received in revised form
tween aeration and water evaporation from the sewage sludge bio-drying pile based on in
20 March 2013
situ measurements. This study was conducted to investigate the effects of forced air vol-
Accepted 22 March 2013
ume on the evaporation of water from a sewage sludge bio-drying pile. Dewatered sewage
Available online 18 April 2013
sludge was bio-dried using control technology for bio-drying, during which time the temperature, superficial air velocity and water evaporation were measured and calculated.
Keywords:
The results indicated that the peak air velocity and water evaporation occurred in the
Sewage sludge
thermophilic phase and second temperature-increasing phase, with the highest values of
Bio-drying
0.063 0.027 m s1 and 28.9 kg ton1 matrix d1, respectively, being observed on day 4. Air
Aeration
velocity above the pile during aeration was 43e100% higher than when there was no
Forced air volume
aeration, and there was a significantly positive correlation between air volume and water
Water evaporation
evaporation from day 1 to 15. The order of daily means of water evaporation was thermophilic phase > second temperature-increasing phase > temperature-increasing phase > cooling phase. Forced aeration controlled the pile temperature and improved evaporation, making it the key factor influencing water loss during the process of sewage sludge bio-drying. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Water loss from sewage sludge (SS) bio-drying piles is mainly achieved via the evaporation of free water, and it is primarily removed by air convection (Zhao et al., 2010; Navaee-Ardeh et al., 2010). During SS bio-drying, the main mechanism of water removal is convective evaporation and the actual water loss is the water that has evaporated from the pile (Velis et al., 2009; Cai et al., 2012). Aeration influences the pile temperature and evaporation, thereby affecting the bio-drying process (Adani et al., 2002).
As a main control for SS bio-drying, forced aeration enables water reduction and removal of heat from the pile (NavaeeArdeh et al., 2006) via their transport by airflow (Taban and Movahedi Naeini, 2006). Air convection and molecular diffusion are the primary approaches to water removal from biodrying piles (Frei et al., 2004), and the velocity and volume of forced air are the main operational variables used for process control in bio-drying piles (Sugni et al., 2005; Turan and Ergun, 2008). Studies have shown that high airflow velocity is necessary for fast and effective drying (Adani et al., 2002). Experimental evidence has also indicated that lower pile
* Corresponding author. Tel./fax: þ86 10 64889303. E-mail address: [email protected] (T.-B. Chen). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.03.048
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Nomenclature E Vf va e
water evaporation per ton of bio-drying material per day (kg ton1 matrix d1) volume of forced air per ton of bio-drying material (m3 ton1 matrix) air velocity above the pile surface (m s1) vapor flux above the bulk surface (kg m2 s1)
temperatures achieved by excessive aeration delay fermentation and result in reduced water loss (Finstein et al., 1986; Zhang et al., 2010; Shen et al., 2011). From the perspective of fermentation, during the initial temperature-increasing phase, dewatered SS primarily contains bound water such as vicinal water and water of hydration; therefore, there is little free water that can be evaporated by airflow (Vesilind, 1994). Additionally, in the later cooling phase, large air volume has little effect on water loss when the air convection reaches the hygroscopic limit (Velis et al., 2009). Therefore, during SS bio-drying, the demand for aeration varies based on fermentation phases, especially for commercial bio-drying systems designed for efficient bio-drying with reduced excessive aeration (Zhao et al., 2010; Chen et al., 2011). Previous studies of aeration during bio-drying have focused on the water content of the bio-drying material, however, considering the water generated by microbial metabolism (Zhao et al., 2011) and input by forced air (Cai et al., 2012), results based on water evaporated from the pile rather than analysis of the water content of the bio-drying material are usually more representative for assessment of water loss (Cai et al., 2012; Cai et al., 2013). Thus, the relationship between water evaporation (E ) and forced air volume (Vf) in different phases during SS bio-drying requires greater investigation. This study was conducted to investigate the effects of Vf on E of the bio-drying pile via in situ measurement of water vapor. To accomplish this, the temperature, air velocity above the pile surface (va) and E throughout the SS bio-drying process controlled by control technology for bio-drying (CTB) were determined, after which the evaporation at different values of Vf was calculated and analyzed.
2.
Materials and methods
2.1.
Bio-drying materials
SS was collected from the Qinhuangdao, China municipal wastewater treatment plant. Sawdust (SD) was acquired from wood-working factories in the same city. Bio-drying product (BP) was collected from the Lyugang Sewage Sludge Treatment Plant (Qinghuangdao, China). SD and BP were used as bulking agents for bio-drying. These materials were added to three feed bins and then fed into a mixing machine by screw conveyors. The mixing ratio of the three materials was 3:2:1 (SS:BP:SD) based on volume, which was set by adjusting the rotating speed of the screw conveyors. This ratio was selected based on the initial values of the water content and free
qe re Mwater Mair T b Ep
specific humidity of airflow above the bulk surface (kg water kg1 air) density of the air above the bulk surface (kg m3) molecular mass of water (g mol1) molecular mass of air (g mol1) temperature of air ( C) relative humidity of air (%) water evaporation per ton of bio-drying material during an aeration period (kg ton1 matrix)
air space of bio-drying material that was appropriate for microbial fermentation (Adhikari et al., 2009; Chen et al., 2011; Shen et al., 2012). The water content of the mixture, SS, SD and BP were 66.0 0.1%, 82.4 0.1%, 20.6 0.5% and 40.8 0.7%, respectively.
2.2.
Experimental procedures
The mixture used for bio-drying was loaded into the compartment, flattened using a slope trimmer, and then subjected to bio-drying by CTB auto-control, which was controlled using Compsoft 3.0 (GreenTech Environmental Engineering Ltd., Beijing, China) (Chen et al., 2001). During the 20-day bio-drying period, the pile was aerated intermittently from the air chamber under the pile by an air blower and turned once on days 9, 12, 15 and 18. The experimental equipment for SS bio-drying is shown in Fig. 1. An unsealed cylindrical cover made of hydrophobic material with an internal diameter of 1.13 m and a crosssectional area of 1.0 m2 was vertically installed. The biodrying pile was 1.6 m in height and the volume and weight of the experimental pile was 1.6 m3 and 1280 kg, respectively. A water vapor sensor was installed along the central axis of the cylindrical cover and 0.5 m above the pile surface. An airflow meter was located in the horizontal ventilation duct from the air blower to the air chamber.
Cylindrical cover
Vapour sensor
Data logger
Temperature sensor
Air chamber
Flowmeter Air blower
Fig. 1 e Structure of sewage sludge bio-drying experiment (unit: m).
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During SS bio-drying, Vf was controlled by CTB according to the pile temperature and O2 consumption rate (Chen et al., 2011). The pile temperature, va and E were determined or calculated during bio-drying. Next, several values of Vf were set based on the SS bio-drying according to the aeration strategy used for bio-drying engineering (Chen et al., 2001; de Guardia et al., 2008; Zhang et al., 2010; Shen et al., 2012). The aeration period was 40 min, consisting of 10-min of aeration time and 30-min of unaerated time. During each phase (temperature-increasing phase, thermophilic phase, second temperature-increasing phase and cooling phase), different values of Vf were set and the values of water evaporation during the 40-min aeration period (Ep) were calculated.
2.3.
Data acquisition
Bulk temperature was monitored in real time using a Pt100 temperature sensor throughout the study period. A water vapor sensor was composed of an ultrasonic anemometer (Gill Instruments Corp., Lymington, UK), a temperature sensor and a humidity sensor (Rotronic Corp., Bassersdorf, Switzerland), which were used to determine the parameters of water vapor in real time. Data were logged at a 2-s interval based on three replicates using a computer. Airflow was measured continuously using a thermal flowmeter (Virvo Corp., Madison, Wisconsin, USA) and data were logged at a 1-min interval based on three replicates.
2.4.
Calculation
2.4.1.
Water evaporation
The humidity, temperature and va were measured during SS bio-drying, and the vapor flux was calculated as follows: e ¼ qe $ue $re
(1)
where e is the vapor flux above the pile surface (kg m2 s1); qe is the specific humidity of the airflow above the pile surface (kg water kg1 air); ue is the vertical air velocity of the airflow above the pile surface (m s1); and re is the density of air (kg m3). qe was calculated as follows (Haug, 1993): 2238
qe ¼ b$
Mwater 108:896Tþ273 $ 2238 Mair 760 108:896Tþ273
(2)
where Mwater is the molecular mass of water (g mol1); Mair is the molecular mass of air (g mol1); T is the temperature of air ( C); and b is the relative humidity of air (%). Ee can be calculated by integrating e using the following equation: Zt1 Ee ¼
f ðtÞdt
(3)
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2.4.2. Relationship between air volume and water evaporation In the initial temperature-increasing phase and the later cooling phase, the relationship between Vf and E is not linear owing to comprehensive factors (Vesilind, 1994; Velis et al., 2009). Additionally, this relationship is complicated because aeration impacts the pile temperature and free air space (Wang et al., 2011), which affects fermentation (Yu et al., 2009) and therefore evaporation. As a result, it is usually not possible to distinguish separate effects of each factor. To enable better application of bio-drying engineering, a black box model was adopted (Zhang et al., 2009) and an inductive strategy associated with an empirical second order equation was used (Hamelers, 2004) to describe the relationship between air volume and water evaporation: Ep ¼ a þ bVf þ cVf2
(4)
where Ep is the water evaporation per ton of bio-drying material during an aeration period (kg ton1 matrix); Vf is the volume of forced air per ton of bio-drying material (m3 ton1 matrix); and a, b and c are the fitting coefficients.
3.
Results and discussion
3.1.
Control of the pile temperature
The pile had an initial temperature of 22.5 C, then increased above 55 C on day 3 and reached its peak value of 72.3 C on day 9. After the first turning on day 9, the pile temperature decreased to 47.2 C, then entered the second temperatureincreasing phase, during which the temperature increased to 57.8 C. After day 15, the pile temperature decreased gradually, finally reaching 21.7 C on day 20. As shown in Table 1, during the temperature-increasing phase (days 1e2), the Vf was lower, with a mean value of 19.2 m3 ton1 matrix d1, ensuring the rapid self-heating of the bio-drying pile. During the thermophilic phase (days 3e9), the Vf increased to a mean value of 29.3 m3 ton1 matrix d1 to promote evaporation; however, to maintain the optimal temperature range during the thermophilic period, the Vf was restrained. During the second temperature-increasing phase (days 10e15), the Vf was reduced to enable self-heating of the pile since the temperature decreased after turning. When the temperature of the pile was sufficient, the Vf was enhanced to the highest level of the entire bio-drying process, reaching a mean value of 41.7 m3 ton1 matrix d1. Once entering the cooling phase (days 16e20), to evaporate the free water from the bio-drying material and cool the pile (Vesilind, 1994; Cai et al., 2012), the Vf was set at a mean of 27.8 m3 ton1 matrix d1, which reduced the pile temperature effectively at a rate of 4.44 C d1.
t0
where Ee is the water evaporation (kg m2); t0 is the start time (s); t1 is the end of time (s); and f(t) is a function of e (kg m2 d1). The water evaporation per square meter per day was calculated based on Eq. (3) and the weight of the experimental pile was 1.28 ton per square meter, therefore, E was used to represent the water evaporation per ton of bio-drying material per day (kg ton1 matrix d1).
3.2.
Increase in air velocity above the pile
The va was monitored during the SS bio-drying process (Fig. 2). As shown in Fig. 2, the va on day 1 was lower, with a mean of 0.042 0.022 m s1, after which it increased gradually to a peak value during the thermophilic phase of a mean velocity of 0.063 0.027 m s1 and the highest velocity of 0.176 m s1
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Table 1 e Forced air volume, water evaporation and air velocity above the pile in each phase. Temperatureincreasing phase
Thermophilic phase
Second temperatureincreasing phase
Cooling phase
19.2 9.8 15.8 5.3
29.3 6.0 23.5 4.7
41.7 5.2 17.0 4.6
27.8 6.2 5.4 2.9
0.037 0.019 0.068 0.021 0.041 0.012
0.043 0.021 0.098 0.042 0.056 0.037
0.038 0.017 0.096 0.031 0.053 0.029
0.035 0.013 0.066 0.017 0.036 0.017
Forced air volume (m3 ton1 matrix d1) Water evaporation (kg ton1 matrix d1) Air velocity above the pile (m s1) Before aeration During aeration After aeration Results are the means SD.
on day 4. On day 7, the va decreased, but it then increased again after the first turning on day 9. After turning, the mean va was 0.057 0.023 m s1 with a peak velocity of 0.175 m s1 being observed on day 10. During the cooling phase, the va decreased, with the mean being 0.036 0.016 m s1 and the highest velocity only 0.101 m s1 on day 20. The means of va before aeration, during aeration and after aeration during each phase are presented in Table 1, which shows the variation of va during an aeration period. On day 1, the values of va before aeration, during aeration and after aeration were 0.037 0.019 m s1, 0.068 0.021 m s1, and 0.041 0.012 m s1, respectively, while on day 4 they were 0.043 0.021 m s1, 0.098 0.042 m s1, and 0.056 0.037 m s1, respectively, on day 10 they were 0.038 0.017 m s1, 0.096 0.031 m s1, and 0.053 0.029 m s1 and on day 20 they were 0.035 0.013 m s1, 0.066 0.017 m s1, and 0.036 0.017 m s1, respectively. As shown in Table 1, during aeration the va increased significantly to levels 43e100% higher than observed when there was no aeration (P < 0.05). After aeration, the va returned to the levels observed before aeration. The Vf and va during each phase were analyzed (data see Figs. 2 and 3), and va was found to be highly dependent on Vf during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase (P < 0.05), while no significant dependence was observed during the cooling phase
(P > 0.05). The factors that directly affected the va were aeration intensity, pile temperature and free air space (Yu et al., 2009). In this study, the blower frequency was constant; thus, the pile temperature and the free air space had a strong effect on the va. When the pile had a high free air space and good air permeability, aeration promoted its fermentation and self-heating, enhancing the air convection above the pile (Richard et al., 2004; Huet et al., 2012). In addition, the high pile temperature facilitated the convection and evaporation. Taken together, these effects led to increased va with Vf during the temperatureincreasing phase, thermophilic phase and second temperatureincreasing phase. Conversely, during the cooling phase, pile temperature decreased and readily biodegradable material was degraded (Bernal et al., 2009); therefore, larger Vf had little effect on fermentation and no significant impact on va. During the temperature-increasing phase, thermophilic phase and second temperature-increasing phase, the larger airflow volume significantly promoted the air convection above the SS bio-drying pile, while it did not during the cooling phase.
3.3.
Effects on water evaporation
The water content of bio-drying material decreased from 66.0% to 48.7%, during which time the vapor flux was integrated and the E from the bio-drying pile was calculated as shown in Fig. 3. From day 2, E increased dramatically, reaching
0.10
50
40
30
30
20
20
10
10
0.04
0.02
3
0.06
-1
-1
40
Forced air volume (m ton matrix)
Water evaporation (kg ton matrix)
-1
Air velocity (m s )
0.08
Turning
Water evaporation Forced air volume
50
0.00
0 0
2
4
6
8
10
12
14
16
18
20
Time (d)
Fig. 2 e Air velocity above the pile during the process of sewage sludge bio-drying. Temperature-increasing phase: days 1e2, thermophilic phase: days 3e9, second temperature-increasing phase: days 10e15 and cooling phase: days 16e20.
0 0
2
4
6
8
10
12
14
16
18
20
Time (d)
Fig. 3 e Forced air volume and evaporation during the process of sewage sludge bio-drying. Temperatureincreasing phase: days 1e2, thermophilic phase: days 3e9, second temperature-increasing phase: days 10e15 and cooling phase: days 16e20.
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3.4. Dependence of the water evaporation on the forced air volume
Temperature increasing phase Thermophilic phase Second temperature increasing phase Cooling phase
1.5
1.2
-1
Water evaporation (kg ton matrix)
its peak value of 28.9 kg ton1 matrix d1 on day 4, then decreasing gradually. After turning on day 9, the pile entered its second fermentation period, during which time the E increased and reached its highest value of 22.8 kg ton1 matrix d1 on day 10. Finally, the E decreased to 2.6 kg ton1 matrix d1 on day 20. As shown in Table 1, the daily means of E in each phase were 15.8 5.3 kg ton1 matrix d1, 23.5 4.7 kg ton1 matrix d1, 17.0 4.6 kg ton1 matrix d1 and 5.4 2.9 kg ton1 matrix d1, respectively, indicating that the daily mean E during the thermophilic phase was highest and the one during the cooling phase was lowest. The results revealed that there was a significant positive dependence between the daily E and the daily Vf from day 1 to 15 (P < 0.05). To enable sufficient self-heating of the pile, the Vf was set at the lowest level of 12.2 m3 ton1 matrix d1 on day 1 during the temperature-increasing phase, when the vapor flux was lower and the E was only 12.1 kg ton1 matrix d1. When the pile temperature was higher than 55 C, the Vf was enhanced to 37.0 m3 ton1 matrix d1, which increased the evaporation. As shown in Fig. 3, from the temperatureincreasing phase to the thermophilic phase the Vf first increased then decreased, with its peak value of 37.4 m3 ton1 matrix d1 occurring on day 4; accordingly, the E first increased then decreased, with its peak of 28.9 kg ton1 matrix d1 also being observed on day 4. After the first turning, the Vf was lower on day 9 to support the second self-heating. When the pile reached the required temperature on day 10, the Vf became larger and increased to 47.2 m3 ton1 matrix d1 on day 11. The variation of E was similar to that of Vf, which increased to 22.8 kg ton1 matrix d1 on day 10. Based on these findings, the forced aeration dominated the water evaporation during the temperature-increasing phase and thermophilic phase. From day 16 to the end of bio-drying, Vf was maintained at a high level to remove free water and reduce the pile temperature after sufficient fermentation (Luo et al., 2008). The fermentation was to be achieved during the later stage of cooling phase, at which time the E was low, with a mean of 5.4 2.9 kg ton1 matrix d1, even though the Vf was 27.8 m3 ton1 matrix d1. The Vf did not impact E significantly (P > 0.05), indicating its influence became weak in the later stage of the SS bio-drying process.
0.9
0.6
0.3
0.0
0.5
1.0
1.5 3
2.0
2.5
-1
Forced air volume (m ton matrix)
Fig. 4 e Relationship between water evaporation and forced air volume during aeration periods in each phase of sewage sludge bio-drying.
bio-drying process and power utilization efficiency. During the thermophilic phase and second temperature-increasing phase, Ep increased with Vf; however, excessive airflow led to a decline in temperature (Velis et al., 2009; Bernal et al., 2009), which had a negative effect on evaporation (Wang et al., 2011). This negative effect was caused by a combination of airflow and temperature; therefore, both should be considered when investigating water removal (Haug, 1983; Bittelli et al., 2008). In the present study, the recommended maximum Vf values were set at 1.8 m3 ton1 matrix and 2.2 m3 ton1 matrix during the thermophilic phase and second temperature-increasing phase, respectively. This resulted in desirable water loss, which reduced the water content of the SS bio-drying material from 66.0% to 48.7% and produced the optimal pile temperature. During the cooling phase, the larger Vf contributed little to the Ep because of the hygroscopic limit of air convection and the decline of microbiological reaction during this last fermentation phase (Yamada and Kawase, 2006). A black box model was employed to fit Vf and Ep during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase: Temperature increasing phase : Ep ¼ 0:148 þ 0:538Vf 0:135Vf2 R2 ¼ 0:998 (5)
The Ep at different Vf values was determined during each phase of SS bio-drying. As shown in Fig. 4, during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase, Ep increased with Vf and there was a significant positive dependence (P < 0.05), while during the cooling phase, Ep did not significantly depend on Vf (P > 0.05). As shown in Fig. 4, during the temperature-increasing phase, Ep increased with Vf until Vf was increased to 1.44 m3 ton1 matrix, after which the Ep did not vary obviously. It is worth considering decreasing the Vf during the temperature-increasing phase to facilitate self-heating of the pile. Specifically, keeping the Vf at 1.44 m3 ton1 matrix is an appropriate level for favorable evaporation based on the
Thermophilic phase : Ep ¼ 0:427 þ 0:799Vf 0:158Vf2 R2 ¼ 0:998 Second temperature increasing phase : Ep ¼ 0:114 þ 0:555Vf 0:103Vf2 R2 ¼ 0:996
(6)
(7)
In engineering applications of CTB, these empirical equations could be used for determination of E in different air volumes to optimize the aeration strategy and improve the efficiency of water loss, which requires further debugging. Based on the results presented in Sections 3.3 and 3.4, during the SS bio-drying process, the order of daily means of E in the four phases was as follows: thermophilic phase > second
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temperature-increasing phase > temperature-increasing phase > cooling phase, showing that peak evaporation occurred in the thermophilic phase and second temperatureincreasing phase, during which time forced aeration dominated the evaporation process.
4.
Conclusions
The peak air velocity above the pile and water evaporation were observed during the thermophilic phase, with the highest values of 0.063 0.027 m s1 and 28.9 kg ton1 matrix d1 being observed on day 4, respectively. Additionally, the mean water evaporation was 23.5 4.7 kg ton1 matrix d1 during the thermophilic phase. Air velocity and water evaporation were significantly dependent on the forced air volume and increased with air volume during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase. During the process of SS bio-drying, forced aeration controlled the pile temperature and improved the evaporation, dominating the water loss.
Acknowledgments This project was financially supported by the National Key Technology R&D Program (2011BAZ03160) and the National High-tech Research and Development Program of China (863 Program) (No. 2009AA064703).
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Zhang, J., Gao, D., Chen, T.B., Zheng, G.D., Chen, J., Ma, C., Guo, S.L., Du, W., 2010. Simulation of substrate degradation in composting of sewage sludge. Waste Management 30, 1931e1938. Zhao, L., Gu, W.M., He, P.J., Shao, L.M., 2010. Effect of air-flow rate and turning frequency on bio-drying of dewatered sludge. Water Research 44, 6144e6152. Zhao, L., Wang, X.Y., Gu, W.M., Shao, L.M., He, P.J., 2011. Distribution of C and N in soluble fractionations for characterizing the respective biodegradation of sludge and bulking agents. Bioresource Technology 102, 10745e10749.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Influence of forced air volume on water evaporation during sewage sludge bio-drying Lu Cai a,b, Tong-Bin Chen a,*, Ding Gao a, Guo-Di Zheng a, Hong-Tao Liu a, Tian-Hao Pan a a
Center for Environmental Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 11A Datun Road, Beijing 100101, PR China b University of Chinese Academy of Sciences, Beijing 100039, PR China
article info
abstract
Article history:
Mechanical aeration is critical to sewage sludge bio-drying, and the actual water loss
Received 24 December 2012
caused by aeration can be better understood from investigations of the relationship be-
Received in revised form
tween aeration and water evaporation from the sewage sludge bio-drying pile based on in
20 March 2013
situ measurements. This study was conducted to investigate the effects of forced air vol-
Accepted 22 March 2013
ume on the evaporation of water from a sewage sludge bio-drying pile. Dewatered sewage
Available online 18 April 2013
sludge was bio-dried using control technology for bio-drying, during which time the temperature, superficial air velocity and water evaporation were measured and calculated.
Keywords:
The results indicated that the peak air velocity and water evaporation occurred in the
Sewage sludge
thermophilic phase and second temperature-increasing phase, with the highest values of
Bio-drying
0.063 0.027 m s1 and 28.9 kg ton1 matrix d1, respectively, being observed on day 4. Air
Aeration
velocity above the pile during aeration was 43e100% higher than when there was no
Forced air volume
aeration, and there was a significantly positive correlation between air volume and water
Water evaporation
evaporation from day 1 to 15. The order of daily means of water evaporation was thermophilic phase > second temperature-increasing phase > temperature-increasing phase > cooling phase. Forced aeration controlled the pile temperature and improved evaporation, making it the key factor influencing water loss during the process of sewage sludge bio-drying. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Water loss from sewage sludge (SS) bio-drying piles is mainly achieved via the evaporation of free water, and it is primarily removed by air convection (Zhao et al., 2010; Navaee-Ardeh et al., 2010). During SS bio-drying, the main mechanism of water removal is convective evaporation and the actual water loss is the water that has evaporated from the pile (Velis et al., 2009; Cai et al., 2012). Aeration influences the pile temperature and evaporation, thereby affecting the bio-drying process (Adani et al., 2002).
As a main control for SS bio-drying, forced aeration enables water reduction and removal of heat from the pile (NavaeeArdeh et al., 2006) via their transport by airflow (Taban and Movahedi Naeini, 2006). Air convection and molecular diffusion are the primary approaches to water removal from biodrying piles (Frei et al., 2004), and the velocity and volume of forced air are the main operational variables used for process control in bio-drying piles (Sugni et al., 2005; Turan and Ergun, 2008). Studies have shown that high airflow velocity is necessary for fast and effective drying (Adani et al., 2002). Experimental evidence has also indicated that lower pile
* Corresponding author. Tel./fax: þ86 10 64889303. E-mail address: [email protected] (T.-B. Chen). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.03.048
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Nomenclature E Vf va e
water evaporation per ton of bio-drying material per day (kg ton1 matrix d1) volume of forced air per ton of bio-drying material (m3 ton1 matrix) air velocity above the pile surface (m s1) vapor flux above the bulk surface (kg m2 s1)
temperatures achieved by excessive aeration delay fermentation and result in reduced water loss (Finstein et al., 1986; Zhang et al., 2010; Shen et al., 2011). From the perspective of fermentation, during the initial temperature-increasing phase, dewatered SS primarily contains bound water such as vicinal water and water of hydration; therefore, there is little free water that can be evaporated by airflow (Vesilind, 1994). Additionally, in the later cooling phase, large air volume has little effect on water loss when the air convection reaches the hygroscopic limit (Velis et al., 2009). Therefore, during SS bio-drying, the demand for aeration varies based on fermentation phases, especially for commercial bio-drying systems designed for efficient bio-drying with reduced excessive aeration (Zhao et al., 2010; Chen et al., 2011). Previous studies of aeration during bio-drying have focused on the water content of the bio-drying material, however, considering the water generated by microbial metabolism (Zhao et al., 2011) and input by forced air (Cai et al., 2012), results based on water evaporated from the pile rather than analysis of the water content of the bio-drying material are usually more representative for assessment of water loss (Cai et al., 2012; Cai et al., 2013). Thus, the relationship between water evaporation (E ) and forced air volume (Vf) in different phases during SS bio-drying requires greater investigation. This study was conducted to investigate the effects of Vf on E of the bio-drying pile via in situ measurement of water vapor. To accomplish this, the temperature, air velocity above the pile surface (va) and E throughout the SS bio-drying process controlled by control technology for bio-drying (CTB) were determined, after which the evaporation at different values of Vf was calculated and analyzed.
2.
Materials and methods
2.1.
Bio-drying materials
SS was collected from the Qinhuangdao, China municipal wastewater treatment plant. Sawdust (SD) was acquired from wood-working factories in the same city. Bio-drying product (BP) was collected from the Lyugang Sewage Sludge Treatment Plant (Qinghuangdao, China). SD and BP were used as bulking agents for bio-drying. These materials were added to three feed bins and then fed into a mixing machine by screw conveyors. The mixing ratio of the three materials was 3:2:1 (SS:BP:SD) based on volume, which was set by adjusting the rotating speed of the screw conveyors. This ratio was selected based on the initial values of the water content and free
qe re Mwater Mair T b Ep
specific humidity of airflow above the bulk surface (kg water kg1 air) density of the air above the bulk surface (kg m3) molecular mass of water (g mol1) molecular mass of air (g mol1) temperature of air ( C) relative humidity of air (%) water evaporation per ton of bio-drying material during an aeration period (kg ton1 matrix)
air space of bio-drying material that was appropriate for microbial fermentation (Adhikari et al., 2009; Chen et al., 2011; Shen et al., 2012). The water content of the mixture, SS, SD and BP were 66.0 0.1%, 82.4 0.1%, 20.6 0.5% and 40.8 0.7%, respectively.
2.2.
Experimental procedures
The mixture used for bio-drying was loaded into the compartment, flattened using a slope trimmer, and then subjected to bio-drying by CTB auto-control, which was controlled using Compsoft 3.0 (GreenTech Environmental Engineering Ltd., Beijing, China) (Chen et al., 2001). During the 20-day bio-drying period, the pile was aerated intermittently from the air chamber under the pile by an air blower and turned once on days 9, 12, 15 and 18. The experimental equipment for SS bio-drying is shown in Fig. 1. An unsealed cylindrical cover made of hydrophobic material with an internal diameter of 1.13 m and a crosssectional area of 1.0 m2 was vertically installed. The biodrying pile was 1.6 m in height and the volume and weight of the experimental pile was 1.6 m3 and 1280 kg, respectively. A water vapor sensor was installed along the central axis of the cylindrical cover and 0.5 m above the pile surface. An airflow meter was located in the horizontal ventilation duct from the air blower to the air chamber.
Cylindrical cover
Vapour sensor
Data logger
Temperature sensor
Air chamber
Flowmeter Air blower
Fig. 1 e Structure of sewage sludge bio-drying experiment (unit: m).
w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 4 7 6 7 e4 7 7 3
During SS bio-drying, Vf was controlled by CTB according to the pile temperature and O2 consumption rate (Chen et al., 2011). The pile temperature, va and E were determined or calculated during bio-drying. Next, several values of Vf were set based on the SS bio-drying according to the aeration strategy used for bio-drying engineering (Chen et al., 2001; de Guardia et al., 2008; Zhang et al., 2010; Shen et al., 2012). The aeration period was 40 min, consisting of 10-min of aeration time and 30-min of unaerated time. During each phase (temperature-increasing phase, thermophilic phase, second temperature-increasing phase and cooling phase), different values of Vf were set and the values of water evaporation during the 40-min aeration period (Ep) were calculated.
2.3.
Data acquisition
Bulk temperature was monitored in real time using a Pt100 temperature sensor throughout the study period. A water vapor sensor was composed of an ultrasonic anemometer (Gill Instruments Corp., Lymington, UK), a temperature sensor and a humidity sensor (Rotronic Corp., Bassersdorf, Switzerland), which were used to determine the parameters of water vapor in real time. Data were logged at a 2-s interval based on three replicates using a computer. Airflow was measured continuously using a thermal flowmeter (Virvo Corp., Madison, Wisconsin, USA) and data were logged at a 1-min interval based on three replicates.
2.4.
Calculation
2.4.1.
Water evaporation
The humidity, temperature and va were measured during SS bio-drying, and the vapor flux was calculated as follows: e ¼ qe $ue $re
(1)
where e is the vapor flux above the pile surface (kg m2 s1); qe is the specific humidity of the airflow above the pile surface (kg water kg1 air); ue is the vertical air velocity of the airflow above the pile surface (m s1); and re is the density of air (kg m3). qe was calculated as follows (Haug, 1993): 2238
qe ¼ b$
Mwater 108:896Tþ273 $ 2238 Mair 760 108:896Tþ273
(2)
where Mwater is the molecular mass of water (g mol1); Mair is the molecular mass of air (g mol1); T is the temperature of air ( C); and b is the relative humidity of air (%). Ee can be calculated by integrating e using the following equation: Zt1 Ee ¼
f ðtÞdt
(3)
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2.4.2. Relationship between air volume and water evaporation In the initial temperature-increasing phase and the later cooling phase, the relationship between Vf and E is not linear owing to comprehensive factors (Vesilind, 1994; Velis et al., 2009). Additionally, this relationship is complicated because aeration impacts the pile temperature and free air space (Wang et al., 2011), which affects fermentation (Yu et al., 2009) and therefore evaporation. As a result, it is usually not possible to distinguish separate effects of each factor. To enable better application of bio-drying engineering, a black box model was adopted (Zhang et al., 2009) and an inductive strategy associated with an empirical second order equation was used (Hamelers, 2004) to describe the relationship between air volume and water evaporation: Ep ¼ a þ bVf þ cVf2
(4)
where Ep is the water evaporation per ton of bio-drying material during an aeration period (kg ton1 matrix); Vf is the volume of forced air per ton of bio-drying material (m3 ton1 matrix); and a, b and c are the fitting coefficients.
3.
Results and discussion
3.1.
Control of the pile temperature
The pile had an initial temperature of 22.5 C, then increased above 55 C on day 3 and reached its peak value of 72.3 C on day 9. After the first turning on day 9, the pile temperature decreased to 47.2 C, then entered the second temperatureincreasing phase, during which the temperature increased to 57.8 C. After day 15, the pile temperature decreased gradually, finally reaching 21.7 C on day 20. As shown in Table 1, during the temperature-increasing phase (days 1e2), the Vf was lower, with a mean value of 19.2 m3 ton1 matrix d1, ensuring the rapid self-heating of the bio-drying pile. During the thermophilic phase (days 3e9), the Vf increased to a mean value of 29.3 m3 ton1 matrix d1 to promote evaporation; however, to maintain the optimal temperature range during the thermophilic period, the Vf was restrained. During the second temperature-increasing phase (days 10e15), the Vf was reduced to enable self-heating of the pile since the temperature decreased after turning. When the temperature of the pile was sufficient, the Vf was enhanced to the highest level of the entire bio-drying process, reaching a mean value of 41.7 m3 ton1 matrix d1. Once entering the cooling phase (days 16e20), to evaporate the free water from the bio-drying material and cool the pile (Vesilind, 1994; Cai et al., 2012), the Vf was set at a mean of 27.8 m3 ton1 matrix d1, which reduced the pile temperature effectively at a rate of 4.44 C d1.
t0
where Ee is the water evaporation (kg m2); t0 is the start time (s); t1 is the end of time (s); and f(t) is a function of e (kg m2 d1). The water evaporation per square meter per day was calculated based on Eq. (3) and the weight of the experimental pile was 1.28 ton per square meter, therefore, E was used to represent the water evaporation per ton of bio-drying material per day (kg ton1 matrix d1).
3.2.
Increase in air velocity above the pile
The va was monitored during the SS bio-drying process (Fig. 2). As shown in Fig. 2, the va on day 1 was lower, with a mean of 0.042 0.022 m s1, after which it increased gradually to a peak value during the thermophilic phase of a mean velocity of 0.063 0.027 m s1 and the highest velocity of 0.176 m s1
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Table 1 e Forced air volume, water evaporation and air velocity above the pile in each phase. Temperatureincreasing phase
Thermophilic phase
Second temperatureincreasing phase
Cooling phase
19.2 9.8 15.8 5.3
29.3 6.0 23.5 4.7
41.7 5.2 17.0 4.6
27.8 6.2 5.4 2.9
0.037 0.019 0.068 0.021 0.041 0.012
0.043 0.021 0.098 0.042 0.056 0.037
0.038 0.017 0.096 0.031 0.053 0.029
0.035 0.013 0.066 0.017 0.036 0.017
Forced air volume (m3 ton1 matrix d1) Water evaporation (kg ton1 matrix d1) Air velocity above the pile (m s1) Before aeration During aeration After aeration Results are the means SD.
on day 4. On day 7, the va decreased, but it then increased again after the first turning on day 9. After turning, the mean va was 0.057 0.023 m s1 with a peak velocity of 0.175 m s1 being observed on day 10. During the cooling phase, the va decreased, with the mean being 0.036 0.016 m s1 and the highest velocity only 0.101 m s1 on day 20. The means of va before aeration, during aeration and after aeration during each phase are presented in Table 1, which shows the variation of va during an aeration period. On day 1, the values of va before aeration, during aeration and after aeration were 0.037 0.019 m s1, 0.068 0.021 m s1, and 0.041 0.012 m s1, respectively, while on day 4 they were 0.043 0.021 m s1, 0.098 0.042 m s1, and 0.056 0.037 m s1, respectively, on day 10 they were 0.038 0.017 m s1, 0.096 0.031 m s1, and 0.053 0.029 m s1 and on day 20 they were 0.035 0.013 m s1, 0.066 0.017 m s1, and 0.036 0.017 m s1, respectively. As shown in Table 1, during aeration the va increased significantly to levels 43e100% higher than observed when there was no aeration (P < 0.05). After aeration, the va returned to the levels observed before aeration. The Vf and va during each phase were analyzed (data see Figs. 2 and 3), and va was found to be highly dependent on Vf during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase (P < 0.05), while no significant dependence was observed during the cooling phase
(P > 0.05). The factors that directly affected the va were aeration intensity, pile temperature and free air space (Yu et al., 2009). In this study, the blower frequency was constant; thus, the pile temperature and the free air space had a strong effect on the va. When the pile had a high free air space and good air permeability, aeration promoted its fermentation and self-heating, enhancing the air convection above the pile (Richard et al., 2004; Huet et al., 2012). In addition, the high pile temperature facilitated the convection and evaporation. Taken together, these effects led to increased va with Vf during the temperatureincreasing phase, thermophilic phase and second temperatureincreasing phase. Conversely, during the cooling phase, pile temperature decreased and readily biodegradable material was degraded (Bernal et al., 2009); therefore, larger Vf had little effect on fermentation and no significant impact on va. During the temperature-increasing phase, thermophilic phase and second temperature-increasing phase, the larger airflow volume significantly promoted the air convection above the SS bio-drying pile, while it did not during the cooling phase.
3.3.
Effects on water evaporation
The water content of bio-drying material decreased from 66.0% to 48.7%, during which time the vapor flux was integrated and the E from the bio-drying pile was calculated as shown in Fig. 3. From day 2, E increased dramatically, reaching
0.10
50
40
30
30
20
20
10
10
0.04
0.02
3
0.06
-1
-1
40
Forced air volume (m ton matrix)
Water evaporation (kg ton matrix)
-1
Air velocity (m s )
0.08
Turning
Water evaporation Forced air volume
50
0.00
0 0
2
4
6
8
10
12
14
16
18
20
Time (d)
Fig. 2 e Air velocity above the pile during the process of sewage sludge bio-drying. Temperature-increasing phase: days 1e2, thermophilic phase: days 3e9, second temperature-increasing phase: days 10e15 and cooling phase: days 16e20.
0 0
2
4
6
8
10
12
14
16
18
20
Time (d)
Fig. 3 e Forced air volume and evaporation during the process of sewage sludge bio-drying. Temperatureincreasing phase: days 1e2, thermophilic phase: days 3e9, second temperature-increasing phase: days 10e15 and cooling phase: days 16e20.
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3.4. Dependence of the water evaporation on the forced air volume
Temperature increasing phase Thermophilic phase Second temperature increasing phase Cooling phase
1.5
1.2
-1
Water evaporation (kg ton matrix)
its peak value of 28.9 kg ton1 matrix d1 on day 4, then decreasing gradually. After turning on day 9, the pile entered its second fermentation period, during which time the E increased and reached its highest value of 22.8 kg ton1 matrix d1 on day 10. Finally, the E decreased to 2.6 kg ton1 matrix d1 on day 20. As shown in Table 1, the daily means of E in each phase were 15.8 5.3 kg ton1 matrix d1, 23.5 4.7 kg ton1 matrix d1, 17.0 4.6 kg ton1 matrix d1 and 5.4 2.9 kg ton1 matrix d1, respectively, indicating that the daily mean E during the thermophilic phase was highest and the one during the cooling phase was lowest. The results revealed that there was a significant positive dependence between the daily E and the daily Vf from day 1 to 15 (P < 0.05). To enable sufficient self-heating of the pile, the Vf was set at the lowest level of 12.2 m3 ton1 matrix d1 on day 1 during the temperature-increasing phase, when the vapor flux was lower and the E was only 12.1 kg ton1 matrix d1. When the pile temperature was higher than 55 C, the Vf was enhanced to 37.0 m3 ton1 matrix d1, which increased the evaporation. As shown in Fig. 3, from the temperatureincreasing phase to the thermophilic phase the Vf first increased then decreased, with its peak value of 37.4 m3 ton1 matrix d1 occurring on day 4; accordingly, the E first increased then decreased, with its peak of 28.9 kg ton1 matrix d1 also being observed on day 4. After the first turning, the Vf was lower on day 9 to support the second self-heating. When the pile reached the required temperature on day 10, the Vf became larger and increased to 47.2 m3 ton1 matrix d1 on day 11. The variation of E was similar to that of Vf, which increased to 22.8 kg ton1 matrix d1 on day 10. Based on these findings, the forced aeration dominated the water evaporation during the temperature-increasing phase and thermophilic phase. From day 16 to the end of bio-drying, Vf was maintained at a high level to remove free water and reduce the pile temperature after sufficient fermentation (Luo et al., 2008). The fermentation was to be achieved during the later stage of cooling phase, at which time the E was low, with a mean of 5.4 2.9 kg ton1 matrix d1, even though the Vf was 27.8 m3 ton1 matrix d1. The Vf did not impact E significantly (P > 0.05), indicating its influence became weak in the later stage of the SS bio-drying process.
0.9
0.6
0.3
0.0
0.5
1.0
1.5 3
2.0
2.5
-1
Forced air volume (m ton matrix)
Fig. 4 e Relationship between water evaporation and forced air volume during aeration periods in each phase of sewage sludge bio-drying.
bio-drying process and power utilization efficiency. During the thermophilic phase and second temperature-increasing phase, Ep increased with Vf; however, excessive airflow led to a decline in temperature (Velis et al., 2009; Bernal et al., 2009), which had a negative effect on evaporation (Wang et al., 2011). This negative effect was caused by a combination of airflow and temperature; therefore, both should be considered when investigating water removal (Haug, 1983; Bittelli et al., 2008). In the present study, the recommended maximum Vf values were set at 1.8 m3 ton1 matrix and 2.2 m3 ton1 matrix during the thermophilic phase and second temperature-increasing phase, respectively. This resulted in desirable water loss, which reduced the water content of the SS bio-drying material from 66.0% to 48.7% and produced the optimal pile temperature. During the cooling phase, the larger Vf contributed little to the Ep because of the hygroscopic limit of air convection and the decline of microbiological reaction during this last fermentation phase (Yamada and Kawase, 2006). A black box model was employed to fit Vf and Ep during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase: Temperature increasing phase : Ep ¼ 0:148 þ 0:538Vf 0:135Vf2 R2 ¼ 0:998 (5)
The Ep at different Vf values was determined during each phase of SS bio-drying. As shown in Fig. 4, during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase, Ep increased with Vf and there was a significant positive dependence (P < 0.05), while during the cooling phase, Ep did not significantly depend on Vf (P > 0.05). As shown in Fig. 4, during the temperature-increasing phase, Ep increased with Vf until Vf was increased to 1.44 m3 ton1 matrix, after which the Ep did not vary obviously. It is worth considering decreasing the Vf during the temperature-increasing phase to facilitate self-heating of the pile. Specifically, keeping the Vf at 1.44 m3 ton1 matrix is an appropriate level for favorable evaporation based on the
Thermophilic phase : Ep ¼ 0:427 þ 0:799Vf 0:158Vf2 R2 ¼ 0:998 Second temperature increasing phase : Ep ¼ 0:114 þ 0:555Vf 0:103Vf2 R2 ¼ 0:996
(6)
(7)
In engineering applications of CTB, these empirical equations could be used for determination of E in different air volumes to optimize the aeration strategy and improve the efficiency of water loss, which requires further debugging. Based on the results presented in Sections 3.3 and 3.4, during the SS bio-drying process, the order of daily means of E in the four phases was as follows: thermophilic phase > second
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temperature-increasing phase > temperature-increasing phase > cooling phase, showing that peak evaporation occurred in the thermophilic phase and second temperatureincreasing phase, during which time forced aeration dominated the evaporation process.
4.
Conclusions
The peak air velocity above the pile and water evaporation were observed during the thermophilic phase, with the highest values of 0.063 0.027 m s1 and 28.9 kg ton1 matrix d1 being observed on day 4, respectively. Additionally, the mean water evaporation was 23.5 4.7 kg ton1 matrix d1 during the thermophilic phase. Air velocity and water evaporation were significantly dependent on the forced air volume and increased with air volume during the temperature-increasing phase, thermophilic phase and second temperature-increasing phase. During the process of SS bio-drying, forced aeration controlled the pile temperature and improved the evaporation, dominating the water loss.
Acknowledgments This project was financially supported by the National Key Technology R&D Program (2011BAZ03160) and the National High-tech Research and Development Program of China (863 Program) (No. 2009AA064703).
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