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Moisture variation associated with water input and evaporation during sewage sludge bio-drying
Bioresource Technology 117 (2012) 13–19
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Moisture variation associated with water input and evaporation during sewage sludge bio-drying Lu Cai, Ding Gao ⇑, Tong-Bin Chen, Hong-Tao Liu, Guo-Di Zheng, Qi-Wei Yang Center for Environmental Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 11A Datun Road, Beijing 100101, PR China
a r t i c l e
i n f o
Article history: Received 14 February 2012 Received in revised form 29 March 2012 Accepted 29 March 2012 Available online 4 April 2012 Keywords: Sewage sludge Bio-drying Moisture content Water evaporation Water generation
a b s t r a c t The variation of moisture during sewage sludge bio-drying was investigated. In situ measurements were conducted to monitor the bulk moisture and water vapor, while the moisture content, water generation, water evaporation and aeration water input of the bio-drying bulk were calculated based on the water mass balance. The moisture in the sewage sludge bio-drying material decreased from 66% to 54% in response to control technology for bio-drying. During the temperature increasing and thermophilic phases of sewage sludge bio-drying, the moisture content, water generation and water evaporation of the bulk initially increased and then decreased. The peak water generation and evaporation occurred during the thermophilic phase. During the bio-drying, water evaporation was much greater than water generation, and aeration facilitated the water evaporation. Ó 2012 Published by Elsevier Ltd.
1. Introduction The moisture content (MC) of dewatered sewage sludge is about 80%, which causes a series of problems in terms of sludge treatment and disposal; therefore, reducing sludge moisture is important to the reduction of sludge volume and quantity (Zhao et al., 2010). Sludge bio-drying is an economical and energy-saving method of simplifying thermophilic aerobic fermentation that utilizes the biological energy produced by microbial fermentation to activate bound water and evaporate moisture (Navaee-Ardeh et al., 2010), resulting in rapid reduction of the moisture in the bio-drying material (Zhang et al., 2008). The main drying mechanism in bio-drying is convective evaporation, which utilizes heat produced from the biodegradation of organic matter and is facilitated by mechanically controlled aeration (Navaee-Ardeh et al., 2006). The process by which the moisture of the bio-drying material is reduced is as follows: water molecules evaporate from the surface of the material into the air, after which the evaporated water (vapor) is transported and removed by airflow (Velis et al., 2009). Air convection and molecular diffusion are the primary approaches to the removal of water from bio-drying material (Frei et al., 2004). Moisture is a critical parameter involved in bio-drying technology that influences the complex biochemical reactions associated with microbial growth and the biodegradation of organic matter
⇑ Corresponding author. Tel./fax: +86 10 64889303. E-mail address: [email protected] (D. Gao). 0960-8524/$ - see front matter Ó 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.biortech.2012.03.092
that occurs during the process (Ryckeboer et al., 2003). In addition, maximizing the removal of moisture present in bio-drying bulk is a crucial pre-treatment step that is beneficial to sludge treatment and disposal (Velis et al., 2009). The water mass balance of bio-drying bulk indicates that variations in bulk moisture are associated with water input and water output. Water input includes: (1) water generation (WG), which is water produced by microbial metabolism during organic matter decomposition (Sole-Mauri et al., 2007; Zhang et al., 2010); and (2) aeration water input (AWI), which is moisture added to the bulk during forced aeration. In a study conducted by Chen (2010), the airflow from the air chamber under the bulk removed moisture from the bottom of the bulk and no leachate was collected; therefore, it is assumed that no leachate is produced during drying of the bulk. As a result, the water is removed by water evaporation (WE) during bio-drying. WE is achieved via the evaporation of free water and primarily removed by air convection (Velis et al., 2009). Accordingly, the water output is actually the water evaporated from the bulk material. In addition, the apparent moisture reduction (AMR) is defined as the difference between two MC values measured at different times. During bio-drying, the degree of drying depends on the ratio of water output to water input (Richard, 2004). Investigation of the WG and WE during sludge bio-drying is beneficial to improving the efficiency of moisture reduction and contributes to reduction of the sludge volume. However, recent studies have focused on the MC of the bulk itself, and few studies have investigated the water mass balance. Accordingly, the water input and evaporation during sludge bio-drying is not fully
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Nomenclature MH2 O;t M0 MC0 VS0 MCt VSt
q0 V e qe ue
total moisture of the bio-drying bulk on day t (kg) initial weight of the bio-drying bulk on day 1 (kg) moisture content of the bio-drying material on day 1 (%) volatile solids content of the bio-drying material on day 1 (%) moisture content of the bio-drying material on day t (%) volatile solids content of the bio-drying material on day t (%) density of the bio-drying bulk on day 1 (kg m3) volume of the bio-drying bulk on day 1 (m3) vapor flux above the bulk surface (kg m2 s1) specific humidity of the airflow above the bulk surface (kg water kg1 air) vertical air velocity of the airflow above the bulk surface (m s1)
understood. Therefore, this study was conducted to investigate the variations in water input and evaporation of sludge bio-drying bulk in terms of a water mass balance developed via in situ moisture and vapor measurement. 2. Methods 2.1. Bio-drying materials Sewage sludge (SS) was collected from the municipal wastewater treatment plant in Qinhuangdao, China. Sawdust (SD) was acquired from wood-working factories in the same city. Bio-dried product (BP) was obtained from Lvgang Municipal Sewage Sludge Treatment Plant, which is a SS bio-drying plant in Qinghuangdao, China. SD and BP were used as bulking agents for bio-drying. Specifically, 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, and this was set by adjusting the rotating speed of the screw conveyors. This ratio was selected based on the initial MC and free air space of bio-drying material that was appropriate for microbial fermentation (Adhikari et al., 2009; Chen et al., 2011). The characteristics of the raw materials and mixture feedstock used in this study are presented in Table 1. 2.2. Experimental procedures The mixture for bio-drying was loaded into the fermentation compartment and then flattened using a slope trimmer. The process of sludge bio-drying was conducted by CTB (control technology for bio-drying) auto-control based on a combination of temperature and O2 concentration feedback from temperature sensors and oxygen sensors, which were controlled using CompsoftÒ 3.0 (GreenTech Environmental Engineering Ltd., Beijing, China) (Chen et al., 2011). Air was forced from the air chamber under the bulk to the top of the bulk by an air blower. The volume of forced air was adjusted according to the bulk temperature and O2 consumption rate during different bio-drying phases (Chen et al., 2011). The experimental equipment for SS bio-drying is shown in Fig. 1. An unsealed cylindrical cover made of hydrophobic material was vertically installed into the bulk from top to bottom to minimize the interference caused by external factors. The cylindrical cover had an internal diameter of 1.13 m and a cross-sectional area of 1.0 m2, and the bio-drying bulk was 1.6 m in height; therefore, the volume of the experimental bulk material was about 1.6 m3 (1.0 m2 1.6 m). A water vapor sensor was installed along the
qe Mwater Mair T b E Mi
qi Qi qi I DMH2 O;a MH2 O;t1 DMH2 O;g
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 of the bulk (kg) water vapor input per second (kg s1) density of forced air (kg m3) volume of forced air (m3 s1) specific humidity of air forced into the bulk (kg water kg1 air) aeration water input of the bulk (kg) apparent moisture reduction of the bulk (kg) total moisture of the bulk on day t-1 (kg) water generation of the bulk (kg)
central axis of the cylindrical cover about 0.5 m above the top of the bulk. A moisture sensor and temperature sensor were inserted into the bulk at a depth of 0.6 m (from top to bottom). An airflow meter was located in the horizontal ventilation duct from the air blower to the air chamber. The period of sludge bio-drying was 20 days and the bio-drying bulk was intermittently aerated using an air blower throughout the study period. In addition, the bulk was turned on days 9, 12, 15 and 18. 2.3. Data acquisition and sample analysis Bulk temperature was monitored in real time using a PT100 temperature sensor throughout the study period. The MC of the bio-drying material was measured using a moisture sensor based on time domain reflectometry that consisted of a pulse generator (Soilmoisture Equipment Corp., USA) and probes. The MC was determined by in situ analysis of 15 replicates at the same time every day. A water vapor sensor composed of an ultrasonic anemometer (Gill Instruments, UK), a temperature sensor and a humidity sensor (Rotronic, Switzerland), was used to log data from three replicates at a 2-s interval. Airflow was measured in real time using a thermal flowmeter (Virvo, USA) and data were logged at a 1-min interval based on three replicates. Additionally, bio-drying material was collected from the bulk daily to measure the volatile solids (VS) content, which was determined by oven-drying of the sample and subsequent incineration in a muffle furnace. The collected bio-drying material was also used to determine the bulk density by the cutting ring method. The VS and bulk density were determined based on five replicates analyzed using the methods described by the US Department of Agriculture and US Composting Council (2001). 2.4. Formulas for data computation During sludge bio-drying, the WE accounted for the total water output, while the water input consisted of WG and AWI. The difference between the two values of total moisture is the AMR. 2.4.1. Total moisture of bulk In this study, MC is expressed on a wet weight basis (i.e., weight of moisture/wet weight of sample). Assuming that the change in the weight of the bulk is determined by the degradation of VS and migration of moisture, and that the ash content of the bulk does not change throughout the bio-drying process, the total weight of moisture of the bulk on day t can be obtained from the following formula:
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19 Table 1 Characteristics of the raw materials and mixture feedstock used in the study. Material
Moisture content (%, w.b.a)
Volatile solids (%, d.b.b)
Bulk density (g cm3)
SS SD BP Mixture
82.3 ± 0.09 20.6 ± 0.45 40.8 ± 0.66 66.1 ± 0.08
61.3 ± 0.11 98.3 ± 0.18 73.9 ± 0.39 73.7 ± 0.62
1.04 ± 0.04 0.19 ± 0.01 0.49 ± 0.01 0.80 ± 0.01
Results are the mean ± SD, n = 5. a w.b. refers to a wet basis. b d.b. refers to a dry basis.
The WE of the bulk material during a period of time can be obtained by integrating the vapor flux e as follows: Cylindrical cover
E¼
Z
t1
f ðtÞdt
ð5Þ
t0
Vapour sensor
Computer recorder
where E is the WE of bulk per unit area (kg); t0 is the start time (s); t1 is the end time (s); and f(t) is a function of e (kg m2 s1). 2.4.3. Apparent moisture reduction of bulk AMR is the difference between the two values of total moisture of the bulk, which can be calculated as follows:
DMH2 O;a ¼ MH2 O;t1 MH2 O;t
where DMH2 O;a is the AMR (kg); MH2 O;t1 is the total moisture of the bulk on day t-1 (kg); and MH2 O;t is the total moisture of the bulk on day t (kg).
Moisture sensor Temperature sensor
Flowmeter
Air chamber
ð6Þ
Air blower
2.4.4. Aeration water input into bulk AWI is the water vapor carried into the bulk by forced air, which can be calculated as follows:
M i ¼ qi Q i q i
MH2 O;t
ð1 MC 0 Þð1 VS0 Þ ¼ M0 MCt ð1 MC t Þð1 VSt Þ
ð1Þ
where MH2 O;t is the total moisture of the bulk on day t (kg); M0 is the initial bulk weight on day 1 (kg); MC0 and VS0 are the MC and VS content of the bio-drying material on day 1, respectively (%); and MCt and VSt are the MC and VS content of the bio-drying material on day t, respectively (%). M0 can be calculated using the following equation:
M0 ¼ q 0 V
ð2Þ
where q0 is the bulk density on day 1 (kg m3); and V is the bulk volume on day 1 (m3). 2.4.2. Water evaporation from bulk material The humidity, temperature and air velocity of the airflow above the bulk surface were measured using a vapor sensor, and the vapor flux was calculated using the following formula:
e ¼ qe ue qe
ð3Þ 2
1
where e is the vapor flux above the bulk surface (kg m s ); qe is the specific humidity of the airflow above the bulk surface (kg water kg1 air); ue is the vertical air velocity of the airflow above the bulk surface (m s1); and qe is the density of the air (kg m3). qe can be calculated as follows (Haug,1993): 2238
Mwater 108:896Tþ273 Qe ¼ b 2238 M air 760 j108:896Tþ273
ð4Þ 1
ð7Þ 1
Fig. 1. Schematic of sewage sludge bio-drying experiment (Units: m).
where Mwater is the molecular mass of water (g mol ); Mair is the molecular mass of air (g mol1); T is the temperature of air (°C); and b is the relative humidity of air (%).
where Mi is the water vapor input per second (kg s ); qi is the density of forced air (kg m3); Qi is the volume of forced air (m3 s1); and qi is the specific humidity of air forced into the bulk (kg water kg1 air). The AWI of the bulk material in a period of time can be obtained by integrating Mi as follows:
I¼
Z
t1
gðtÞdt
ð8Þ
t0
where I is the AWI of bulk per unit area (kg); t0 is the start time (s); t1 is the end time (s); and g(t) is a function of Mi (kg m2 s1). 2.4.5. Water generation in bulk material The generated water can be calculated based on the water mass balance of the bio-drying bulk material. It is assumed that the moisture and vapor concentration in the bulk material only vary in the longitudinal direction as represented in a plug-flow model, which is usually used to describe the moisture distribution in bulk material and the vapor distribution above the bulk (Van Lier et al., 1994; Yamada and Kawase, 2006; Zhang et al., 2010). It is also assumed that all of the forced air is incorporated into the experimental bulk material. The water mass balance of the sludge bio-drying system can be built by employing parameters such as the total moisture of the bulk, WE, AWI and WG:
MH2 O;t1 þ DMH2 O;g þ I ¼ M H2 O;t þ E
ð9Þ
WG can be calculated according to the water mass balance equation from the expression below:
DMH2 O;g ¼ E I ðMH2 O;t1 MH2 O;t Þ
ð10Þ
where DMH2 O;g is the WG (kg); E is the WE (kg); I is the AWI (kg); MH2 O;t1 is the total moisture of the bulk on day t-1 (kg); and MH2 O;t is the total moisture of the bulk on day t (kg).
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19
presented in Fig. 2. From day 1 to day 9, the AMR first increased and then decreased. During the thermophilic phase, the AMR, WE and their difference increased to 28.8 ± 7.90 kg d1, 37.7 ± 7.09 kg d1 and 7.93 ± 2.39 kg d1, respectively. After the first and second turning, the AMR increased and then decreased. The corresponding peak values of AMR lagged by 2 days and 3 days after the first and second turning, respectively. On days 17–20, the difference between the AMR and WE was not remarkable, and the values of both factors leveled off.
3. Results and discussion 3.1. Moisture content and volatile solids content of bio-drying material The bio-drying bulk material entered the thermophilic phase on day 3, and its maximum temperature exceeded 70 °C on day 9. After the first turning on day 9, the bulk entered the second temperature increasing phase. At the end of the bio-drying process, the bulk temperature decreased to that of the surrounding temperature (21.9 °C). The MC of the bio-drying material was reduced from 66.1% ± 0.08% to 54.7% ± 0.12%. During the temperature increasing phase (from day 1 to day 3), the MC increased slightly, was maintained at a relatively high state (66.0% ± 0.31%), then decreased rapidly at a rate of 1.34% d1 on days 4–8. The MC continued to decrease on days 8 and 9, but at a slower rate of 0.92% d1. After turning, the decline rate of MC increased to 1.55% d1. Thereafter, the MC decreased gradually as the sludge bio-drying process continued. The decline rate was 0.51% d1 on days 16–20, during which period it tended to be constant. The VS content of the bio-drying material was also measured, which was used in equation (1) to determine the total moisture of the bulk. The VS content decreased from 73.7% ± 0.62% to 66.7% ± 0.21% during sludge bio-drying. An intense degradation of VS was present on days 1–4 at a decline rate of 1.05% d1, while the decline rate was greatly reduced to 0.17% d1 from day 16 to day 20.
3.2.3. Variation in water evaporation and apparent moisture reduction As shown in Fig. 2, throughout the sludge bio-drying process, the variation in the AMR was similar to that of the WE, even though the value of AMR was lower. This discrepancy in values was because WG accounted for a relatively small portion of the overall WE (discussed in 3.3.2), so the difference between the AMR and WE was not significant. The rapid increase of WE during the temperature increasing phase was a result of the accumulated heat produced by microbial activity. In the initial stage of the thermophilic phase, WE was maximized in response to the following: (1) enhanced evaporation as a result of air with a high temperature being forced through the thermophilic bulk; (2) increased movement of water molecules as a result of increased temperature and air diffusion propelled by aeration at the bottom of the bulk, both of which accelerated the migration of water from inside of the bulk (Velis et al., 2009); and (3) the air velocity above the surface of the aerated bulk being three to four times that of the unaerated bulk, and the uncompacted bulk material having a porous texture that imparted good ventilation, which strengthened the evaporation. During the later stage of the thermophilic phase, the air-filled porosity was reduced by bulk compaction (Richard et al., 2004; Yue et al., 2008) and the bulk surface was hardened, which weakened the aeration efficiency and decreased the WE. During the second temperature increasing phase, WE increased again owing to the air convection being increased by mechanically enhanced ventilation, alleviation of bulk compaction after turning, and re-establishment of the high bulk temperature. During the cooling phase, bound water was converted into free water and removed by air convection during the initial stage of cooling. In the later stage of cooling, the bulk temperature dropped gradually, and the thermodynamic equilibrium between the bio-drying material and the air flowing through the material was maintained in a
3.2. Water output 3.2.1. Water evaporation from bulk material WE from the bulk material was calculated by integrating the daily vapor flux of the bio-drying process (Fig. 2). From day 2, the WE began to increase dramatically, reaching the peak value of 47.8 kg d1 on day 4. The WE then gradually decreased to 29.0 kg d1 on day 9. After the first and second turning, the WE increased and then decreased. The peak values of the WE were observed 2 and 3 days after the first and second turning, respectively. The mean WE decreased to 5.49 ± 0.28 kg d1 during the last 3 days. 3.2.2. Apparent moisture reduction in bulk The AMR of the bulk material was the difference between the two moisture values determined at two different times. A comparison of the AMR and the actual water removal (WE) are
Water evaporation Apparent moisture reduction
50
Water mass (kg)
40
30
20
10
0 0
2
4
6
8
10
12
14
16
18
20
Time (d) Fig. 2. Water evaporation and apparent moisture reduction during the sewage sludge bio-drying process. Error bars show the standard deviations of the means (n = 3).
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19
et al., 2010; Xu et al., 2011). As a result, microbial metabolism was weakened and less water was generated. As shown in Fig. 2, WG accounted for a relatively small amount of the overall WE throughout the sludge bio-drying process (3.8% to 35.4% with a mean of 13.2% ± 9.49%); therefore, the WE was far greater than WG, resulting in drying of the bulk material.
constant state. Consequently, convection reached the hygroscopic limit, resulting in a low WE (Velis et al., 2009). 3.3. Water input 3.3.1. Aeration water input into bulk The amount of water input by aeration was calculated by integrating the daily inputted water vapor (Fig. 3). From day 2, the AWI increased rapidly as the forced air volume increased, reaching a mean of 0.73 ± 0.04 kg d1 during the period from day 2 to day 5. The AWI then decreased to 0.38 kg d1 as the air volume was mechanically mitigated. After the first turning, the air volume increased mechanically, resulting in an increased AWI of 1.41 kg d1. The forced air volume was mechanically decreased on day 18, leading to a decreasing value of AWI. Generally, high AWI values were maintained from day 10 to day 20, with a mean of 1.16 ± 0.22 kg d1.
3.4. Process of moisture variation The daily means of the WE, AMR and WG in each phase are presented in Table 2. During the sludge bio-drying process, the order of daily means of WE, AMR and WG in the four phases was as follows: thermophilic phase > second temperature increasing phase > temperature increasing phase > cooling phase. Therefore, the thermophilic phase is the peak of both WG and moisture removal in the bulk material. The bio-drying material had a high MC during the temperature increasing phase. This was because SS primarily contains bound water such as vicinal water and water of hydration after mechanical dewatering (Vesilind, 1994), which is actually abundant in biodrying material during the initial phase (temperature increasing phase). Therefore, there is little free water that can be removed by forced air or convection. In addition, the microbial activity increased notably during this phase (Hassen et al., 2001), resulting in the generation of more metabolic water (Fig. 4) and the WG reaching as high as 12.0 kg on day 3. Consequently, the MC of the bio-drying material increased moderately instead of decreasing during the initial phase of sludge bio-drying. From day 4, the high temperature of the bio-drying material destroyed the vicinal water boundary water layers, which increased the effectiveness of sludge dewatering during the thermophilic phase (Vesilind, 1994). Moreover, the abundant heat produced by the microbial metabolism and the forced aeration both facilitated WE, leading to efficient removal of moisture from the bulk from day 4. On days 8 and 9, the decline rate of MC slowed. This decrease was likely a result of the air channels between the solid particles being reduced by compaction of the bulk (Richard et al., 2004) and hardening of the bulk surface after one week of fermentation (Luo et al., 2008; Yue et al., 2008), which subsequently reduced the efficiency of WE. After mechanical turning on day 9, the compaction of the bulk was modified, which improved the efficiency of aeration
3.3.2. Water generation in bulk material The WG in the bulk material was determined by calculating the water mass balance, and the results are presented in Fig. 4. The metabolic WG increased dramatically during the first 3 days, reaching its peak value of 12.0 kg d1 on day 3, after which it decreased. The mass of water generated on days 2–9 was generally abundant, with a mean value of 7.77 ± 2.75 kg d1. After the first turning, the WG increased to 10.7 kg d1. Once the cooling phase started, the WG decreased gradually to a mean of 0.60 ± 0.44 kg d1, eventually trending toward zero (0.20 kg d1). The variation of WG in the bulk material indicates that a large amount of metabolic water was generated from day 2 to day 9 of the sludge bio-drying period. This was attributed to the high content of organic matter and the increasing microbial metabolism in the bulk, which was dependant on organic matter and generate a large amount of metabolic water (Zhao et al., 2011). During this period, the WG first increased and then decreased. After turning, the bulk entered the second temperature increasing phase (from day 9 to day 15), and a second WG peak occurred. This second peak was due to the modified bulk condition after turning, which promoted secondary microbial fermentation (Léonard et al., 2008). During the cooling phase (from day 15 to day 20), WG decreased, primarily because the readily biodegradable organic matter that the microbes fed on was almost completely degraded (Zhang
4.0
12000
Aeration water input Forced air volume
3.5
10000
8000 2.5 2.0
6000
1.5 4000 1.0
Aeration water input (kg)
3
Forced air volume (m )
3.0
2000 0.5 0.0
0 0
2
4
6
8
10
12
14
16
18
20
Time (d) Fig. 3. Volume of forced air and aeration water input during the sewage sludge bio-drying process. Error bars show the standard deviations of the means (n = 3).
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19
15
Water generation (kg)
12
9
6
3
0 0
2
4
6
8
10
12
14
16
18
20
Time (d) Fig. 4. Water generation during the sewage sludge bio-drying process.
Table 2 Daily means of WG, WE and AMR in each phase. Phase
Daily mean of WG (kg d1)
Daily mean of WE (kg d1)
Daily mean of AMR (kg d1)
Temperature increasing phase (0 h–40 h) Thermophilic phase (40 h–208 h) Second temperature increasing phase (208 h–352 h) Cooling phase (352 h–480 h)
2.46 ± 1.97 7.93 ± 2.39 3.82 ± 3.47 0.60 ± 0.44
24.2 ± 17.95 37.7 ± 7.09 29.0 ± 12.59 9.56 ± 6.08
21.3 ± 15.60 28.8 ± 7.90 25.6 ± 11.92 7.80 ± 5.72
Results are the mean ± SD.
and enabled a second fermentation of the undegraded material in the bulk, resulting in an increase in WE (Fig. 2) and a rapid decrease of MC. On day 18 to day 20, the decline rate of MC slowed gradually, indicating that sludge bio-drying was accomplished. 4. Conclusions The moisture in SS bio-drying material could be rapidly decreased from 66% to 54% by CTB. During the temperature increasing phase and the thermophilic phase, the MC, WG and WE of the bulk material all increased and then decreased. The peak values of WG and WE were attained in the thermophilic phase. Bio-drying bulk entered the second temperature increasing phase after turning, resulting in an increased WG and WE, which accelerated the rate of water removal. Throughout the SS bio-drying process, aeration facilitated WE and the WE was much greater than WG. Acknowledgements This project was financially supported by the National Hightech Research and Development Program of China (863 Program) (No. 2009AA064703) and the National Water Pollution Control and Management Technology Major Project of China (No. 2009ZX07318-008-007). References Adhikari, B.K., Barrington, S., Martinez, J., King, S., 2009. Effectiveness of three bulking agents for food waste composting. Waste Manage. 29, 197–203. Chen, J., Chen, T.B., Gao, D., Lei, M., Zheng, G.D., Liu, H.T., Guo, S.L., Cai, L., 2011. Reducing H2S production by O2 feedback control during large-scale sewage sludge composting. Waste Manage. 31, 65–70.
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19 Xu, H., He, P., Wang, G., Shao, L., Lee, D., 2011. Anaerobic storage as a pretreatment for enhanced biodegradability of dewatered sewage sludge. Bioresour. Technol. 102, 667–671. Yamada, Y., Kawase, Y., 2006. Aerobic composting of waste activated sludge: Kinetic analysis for microbiological reaction and oxygen consumption. Waste Manage. 26, 49–61. Yue, B., Chen, T.B., Gao, D., Zheng, G.D., Liu, B., Lee, D.J., 2008. Pile settlement and volume reduction measurement during forced-aeration static composting. Bioresour. Technol. 99, 7450–7457. Zhang, D.Q., He, P.J., Jin, T.F., Shao, L.M., 2008. Bio-drying of municipal solid waste with high water content by aeration procedures regulation and inoculation. Bioresour. Technol. 99, 8796–8802.
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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 Manage. 30, 1931–1938. 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 Res. 44, 6144–6152. 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. Bioresour. Technol. 102, 10745–10749.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Moisture variation associated with water input and evaporation during sewage sludge bio-drying Lu Cai, Ding Gao ⇑, Tong-Bin Chen, Hong-Tao Liu, Guo-Di Zheng, Qi-Wei Yang Center for Environmental Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 11A Datun Road, Beijing 100101, PR China
a r t i c l e
i n f o
Article history: Received 14 February 2012 Received in revised form 29 March 2012 Accepted 29 March 2012 Available online 4 April 2012 Keywords: Sewage sludge Bio-drying Moisture content Water evaporation Water generation
a b s t r a c t The variation of moisture during sewage sludge bio-drying was investigated. In situ measurements were conducted to monitor the bulk moisture and water vapor, while the moisture content, water generation, water evaporation and aeration water input of the bio-drying bulk were calculated based on the water mass balance. The moisture in the sewage sludge bio-drying material decreased from 66% to 54% in response to control technology for bio-drying. During the temperature increasing and thermophilic phases of sewage sludge bio-drying, the moisture content, water generation and water evaporation of the bulk initially increased and then decreased. The peak water generation and evaporation occurred during the thermophilic phase. During the bio-drying, water evaporation was much greater than water generation, and aeration facilitated the water evaporation. Ó 2012 Published by Elsevier Ltd.
1. Introduction The moisture content (MC) of dewatered sewage sludge is about 80%, which causes a series of problems in terms of sludge treatment and disposal; therefore, reducing sludge moisture is important to the reduction of sludge volume and quantity (Zhao et al., 2010). Sludge bio-drying is an economical and energy-saving method of simplifying thermophilic aerobic fermentation that utilizes the biological energy produced by microbial fermentation to activate bound water and evaporate moisture (Navaee-Ardeh et al., 2010), resulting in rapid reduction of the moisture in the bio-drying material (Zhang et al., 2008). The main drying mechanism in bio-drying is convective evaporation, which utilizes heat produced from the biodegradation of organic matter and is facilitated by mechanically controlled aeration (Navaee-Ardeh et al., 2006). The process by which the moisture of the bio-drying material is reduced is as follows: water molecules evaporate from the surface of the material into the air, after which the evaporated water (vapor) is transported and removed by airflow (Velis et al., 2009). Air convection and molecular diffusion are the primary approaches to the removal of water from bio-drying material (Frei et al., 2004). Moisture is a critical parameter involved in bio-drying technology that influences the complex biochemical reactions associated with microbial growth and the biodegradation of organic matter
⇑ Corresponding author. Tel./fax: +86 10 64889303. E-mail address: [email protected] (D. Gao). 0960-8524/$ - see front matter Ó 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.biortech.2012.03.092
that occurs during the process (Ryckeboer et al., 2003). In addition, maximizing the removal of moisture present in bio-drying bulk is a crucial pre-treatment step that is beneficial to sludge treatment and disposal (Velis et al., 2009). The water mass balance of bio-drying bulk indicates that variations in bulk moisture are associated with water input and water output. Water input includes: (1) water generation (WG), which is water produced by microbial metabolism during organic matter decomposition (Sole-Mauri et al., 2007; Zhang et al., 2010); and (2) aeration water input (AWI), which is moisture added to the bulk during forced aeration. In a study conducted by Chen (2010), the airflow from the air chamber under the bulk removed moisture from the bottom of the bulk and no leachate was collected; therefore, it is assumed that no leachate is produced during drying of the bulk. As a result, the water is removed by water evaporation (WE) during bio-drying. WE is achieved via the evaporation of free water and primarily removed by air convection (Velis et al., 2009). Accordingly, the water output is actually the water evaporated from the bulk material. In addition, the apparent moisture reduction (AMR) is defined as the difference between two MC values measured at different times. During bio-drying, the degree of drying depends on the ratio of water output to water input (Richard, 2004). Investigation of the WG and WE during sludge bio-drying is beneficial to improving the efficiency of moisture reduction and contributes to reduction of the sludge volume. However, recent studies have focused on the MC of the bulk itself, and few studies have investigated the water mass balance. Accordingly, the water input and evaporation during sludge bio-drying is not fully
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19
Nomenclature MH2 O;t M0 MC0 VS0 MCt VSt
q0 V e qe ue
total moisture of the bio-drying bulk on day t (kg) initial weight of the bio-drying bulk on day 1 (kg) moisture content of the bio-drying material on day 1 (%) volatile solids content of the bio-drying material on day 1 (%) moisture content of the bio-drying material on day t (%) volatile solids content of the bio-drying material on day t (%) density of the bio-drying bulk on day 1 (kg m3) volume of the bio-drying bulk on day 1 (m3) vapor flux above the bulk surface (kg m2 s1) specific humidity of the airflow above the bulk surface (kg water kg1 air) vertical air velocity of the airflow above the bulk surface (m s1)
understood. Therefore, this study was conducted to investigate the variations in water input and evaporation of sludge bio-drying bulk in terms of a water mass balance developed via in situ moisture and vapor measurement. 2. Methods 2.1. Bio-drying materials Sewage sludge (SS) was collected from the municipal wastewater treatment plant in Qinhuangdao, China. Sawdust (SD) was acquired from wood-working factories in the same city. Bio-dried product (BP) was obtained from Lvgang Municipal Sewage Sludge Treatment Plant, which is a SS bio-drying plant in Qinghuangdao, China. SD and BP were used as bulking agents for bio-drying. Specifically, 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, and this was set by adjusting the rotating speed of the screw conveyors. This ratio was selected based on the initial MC and free air space of bio-drying material that was appropriate for microbial fermentation (Adhikari et al., 2009; Chen et al., 2011). The characteristics of the raw materials and mixture feedstock used in this study are presented in Table 1. 2.2. Experimental procedures The mixture for bio-drying was loaded into the fermentation compartment and then flattened using a slope trimmer. The process of sludge bio-drying was conducted by CTB (control technology for bio-drying) auto-control based on a combination of temperature and O2 concentration feedback from temperature sensors and oxygen sensors, which were controlled using CompsoftÒ 3.0 (GreenTech Environmental Engineering Ltd., Beijing, China) (Chen et al., 2011). Air was forced from the air chamber under the bulk to the top of the bulk by an air blower. The volume of forced air was adjusted according to the bulk temperature and O2 consumption rate during different bio-drying phases (Chen et al., 2011). The experimental equipment for SS bio-drying is shown in Fig. 1. An unsealed cylindrical cover made of hydrophobic material was vertically installed into the bulk from top to bottom to minimize the interference caused by external factors. The cylindrical cover had an internal diameter of 1.13 m and a cross-sectional area of 1.0 m2, and the bio-drying bulk was 1.6 m in height; therefore, the volume of the experimental bulk material was about 1.6 m3 (1.0 m2 1.6 m). A water vapor sensor was installed along the
qe Mwater Mair T b E Mi
qi Qi qi I DMH2 O;a MH2 O;t1 DMH2 O;g
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 of the bulk (kg) water vapor input per second (kg s1) density of forced air (kg m3) volume of forced air (m3 s1) specific humidity of air forced into the bulk (kg water kg1 air) aeration water input of the bulk (kg) apparent moisture reduction of the bulk (kg) total moisture of the bulk on day t-1 (kg) water generation of the bulk (kg)
central axis of the cylindrical cover about 0.5 m above the top of the bulk. A moisture sensor and temperature sensor were inserted into the bulk at a depth of 0.6 m (from top to bottom). An airflow meter was located in the horizontal ventilation duct from the air blower to the air chamber. The period of sludge bio-drying was 20 days and the bio-drying bulk was intermittently aerated using an air blower throughout the study period. In addition, the bulk was turned on days 9, 12, 15 and 18. 2.3. Data acquisition and sample analysis Bulk temperature was monitored in real time using a PT100 temperature sensor throughout the study period. The MC of the bio-drying material was measured using a moisture sensor based on time domain reflectometry that consisted of a pulse generator (Soilmoisture Equipment Corp., USA) and probes. The MC was determined by in situ analysis of 15 replicates at the same time every day. A water vapor sensor composed of an ultrasonic anemometer (Gill Instruments, UK), a temperature sensor and a humidity sensor (Rotronic, Switzerland), was used to log data from three replicates at a 2-s interval. Airflow was measured in real time using a thermal flowmeter (Virvo, USA) and data were logged at a 1-min interval based on three replicates. Additionally, bio-drying material was collected from the bulk daily to measure the volatile solids (VS) content, which was determined by oven-drying of the sample and subsequent incineration in a muffle furnace. The collected bio-drying material was also used to determine the bulk density by the cutting ring method. The VS and bulk density were determined based on five replicates analyzed using the methods described by the US Department of Agriculture and US Composting Council (2001). 2.4. Formulas for data computation During sludge bio-drying, the WE accounted for the total water output, while the water input consisted of WG and AWI. The difference between the two values of total moisture is the AMR. 2.4.1. Total moisture of bulk In this study, MC is expressed on a wet weight basis (i.e., weight of moisture/wet weight of sample). Assuming that the change in the weight of the bulk is determined by the degradation of VS and migration of moisture, and that the ash content of the bulk does not change throughout the bio-drying process, the total weight of moisture of the bulk on day t can be obtained from the following formula:
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19 Table 1 Characteristics of the raw materials and mixture feedstock used in the study. Material
Moisture content (%, w.b.a)
Volatile solids (%, d.b.b)
Bulk density (g cm3)
SS SD BP Mixture
82.3 ± 0.09 20.6 ± 0.45 40.8 ± 0.66 66.1 ± 0.08
61.3 ± 0.11 98.3 ± 0.18 73.9 ± 0.39 73.7 ± 0.62
1.04 ± 0.04 0.19 ± 0.01 0.49 ± 0.01 0.80 ± 0.01
Results are the mean ± SD, n = 5. a w.b. refers to a wet basis. b d.b. refers to a dry basis.
The WE of the bulk material during a period of time can be obtained by integrating the vapor flux e as follows: Cylindrical cover
E¼
Z
t1
f ðtÞdt
ð5Þ
t0
Vapour sensor
Computer recorder
where E is the WE of bulk per unit area (kg); t0 is the start time (s); t1 is the end time (s); and f(t) is a function of e (kg m2 s1). 2.4.3. Apparent moisture reduction of bulk AMR is the difference between the two values of total moisture of the bulk, which can be calculated as follows:
DMH2 O;a ¼ MH2 O;t1 MH2 O;t
where DMH2 O;a is the AMR (kg); MH2 O;t1 is the total moisture of the bulk on day t-1 (kg); and MH2 O;t is the total moisture of the bulk on day t (kg).
Moisture sensor Temperature sensor
Flowmeter
Air chamber
ð6Þ
Air blower
2.4.4. Aeration water input into bulk AWI is the water vapor carried into the bulk by forced air, which can be calculated as follows:
M i ¼ qi Q i q i
MH2 O;t
ð1 MC 0 Þð1 VS0 Þ ¼ M0 MCt ð1 MC t Þð1 VSt Þ
ð1Þ
where MH2 O;t is the total moisture of the bulk on day t (kg); M0 is the initial bulk weight on day 1 (kg); MC0 and VS0 are the MC and VS content of the bio-drying material on day 1, respectively (%); and MCt and VSt are the MC and VS content of the bio-drying material on day t, respectively (%). M0 can be calculated using the following equation:
M0 ¼ q 0 V
ð2Þ
where q0 is the bulk density on day 1 (kg m3); and V is the bulk volume on day 1 (m3). 2.4.2. Water evaporation from bulk material The humidity, temperature and air velocity of the airflow above the bulk surface were measured using a vapor sensor, and the vapor flux was calculated using the following formula:
e ¼ qe ue qe
ð3Þ 2
1
where e is the vapor flux above the bulk surface (kg m s ); qe is the specific humidity of the airflow above the bulk surface (kg water kg1 air); ue is the vertical air velocity of the airflow above the bulk surface (m s1); and qe is the density of the air (kg m3). qe can be calculated as follows (Haug,1993): 2238
Mwater 108:896Tþ273 Qe ¼ b 2238 M air 760 j108:896Tþ273
ð4Þ 1
ð7Þ 1
Fig. 1. Schematic of sewage sludge bio-drying experiment (Units: m).
where Mwater is the molecular mass of water (g mol ); Mair is the molecular mass of air (g mol1); T is the temperature of air (°C); and b is the relative humidity of air (%).
where Mi is the water vapor input per second (kg s ); qi is the density of forced air (kg m3); Qi is the volume of forced air (m3 s1); and qi is the specific humidity of air forced into the bulk (kg water kg1 air). The AWI of the bulk material in a period of time can be obtained by integrating Mi as follows:
I¼
Z
t1
gðtÞdt
ð8Þ
t0
where I is the AWI of bulk per unit area (kg); t0 is the start time (s); t1 is the end time (s); and g(t) is a function of Mi (kg m2 s1). 2.4.5. Water generation in bulk material The generated water can be calculated based on the water mass balance of the bio-drying bulk material. It is assumed that the moisture and vapor concentration in the bulk material only vary in the longitudinal direction as represented in a plug-flow model, which is usually used to describe the moisture distribution in bulk material and the vapor distribution above the bulk (Van Lier et al., 1994; Yamada and Kawase, 2006; Zhang et al., 2010). It is also assumed that all of the forced air is incorporated into the experimental bulk material. The water mass balance of the sludge bio-drying system can be built by employing parameters such as the total moisture of the bulk, WE, AWI and WG:
MH2 O;t1 þ DMH2 O;g þ I ¼ M H2 O;t þ E
ð9Þ
WG can be calculated according to the water mass balance equation from the expression below:
DMH2 O;g ¼ E I ðMH2 O;t1 MH2 O;t Þ
ð10Þ
where DMH2 O;g is the WG (kg); E is the WE (kg); I is the AWI (kg); MH2 O;t1 is the total moisture of the bulk on day t-1 (kg); and MH2 O;t is the total moisture of the bulk on day t (kg).
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19
presented in Fig. 2. From day 1 to day 9, the AMR first increased and then decreased. During the thermophilic phase, the AMR, WE and their difference increased to 28.8 ± 7.90 kg d1, 37.7 ± 7.09 kg d1 and 7.93 ± 2.39 kg d1, respectively. After the first and second turning, the AMR increased and then decreased. The corresponding peak values of AMR lagged by 2 days and 3 days after the first and second turning, respectively. On days 17–20, the difference between the AMR and WE was not remarkable, and the values of both factors leveled off.
3. Results and discussion 3.1. Moisture content and volatile solids content of bio-drying material The bio-drying bulk material entered the thermophilic phase on day 3, and its maximum temperature exceeded 70 °C on day 9. After the first turning on day 9, the bulk entered the second temperature increasing phase. At the end of the bio-drying process, the bulk temperature decreased to that of the surrounding temperature (21.9 °C). The MC of the bio-drying material was reduced from 66.1% ± 0.08% to 54.7% ± 0.12%. During the temperature increasing phase (from day 1 to day 3), the MC increased slightly, was maintained at a relatively high state (66.0% ± 0.31%), then decreased rapidly at a rate of 1.34% d1 on days 4–8. The MC continued to decrease on days 8 and 9, but at a slower rate of 0.92% d1. After turning, the decline rate of MC increased to 1.55% d1. Thereafter, the MC decreased gradually as the sludge bio-drying process continued. The decline rate was 0.51% d1 on days 16–20, during which period it tended to be constant. The VS content of the bio-drying material was also measured, which was used in equation (1) to determine the total moisture of the bulk. The VS content decreased from 73.7% ± 0.62% to 66.7% ± 0.21% during sludge bio-drying. An intense degradation of VS was present on days 1–4 at a decline rate of 1.05% d1, while the decline rate was greatly reduced to 0.17% d1 from day 16 to day 20.
3.2.3. Variation in water evaporation and apparent moisture reduction As shown in Fig. 2, throughout the sludge bio-drying process, the variation in the AMR was similar to that of the WE, even though the value of AMR was lower. This discrepancy in values was because WG accounted for a relatively small portion of the overall WE (discussed in 3.3.2), so the difference between the AMR and WE was not significant. The rapid increase of WE during the temperature increasing phase was a result of the accumulated heat produced by microbial activity. In the initial stage of the thermophilic phase, WE was maximized in response to the following: (1) enhanced evaporation as a result of air with a high temperature being forced through the thermophilic bulk; (2) increased movement of water molecules as a result of increased temperature and air diffusion propelled by aeration at the bottom of the bulk, both of which accelerated the migration of water from inside of the bulk (Velis et al., 2009); and (3) the air velocity above the surface of the aerated bulk being three to four times that of the unaerated bulk, and the uncompacted bulk material having a porous texture that imparted good ventilation, which strengthened the evaporation. During the later stage of the thermophilic phase, the air-filled porosity was reduced by bulk compaction (Richard et al., 2004; Yue et al., 2008) and the bulk surface was hardened, which weakened the aeration efficiency and decreased the WE. During the second temperature increasing phase, WE increased again owing to the air convection being increased by mechanically enhanced ventilation, alleviation of bulk compaction after turning, and re-establishment of the high bulk temperature. During the cooling phase, bound water was converted into free water and removed by air convection during the initial stage of cooling. In the later stage of cooling, the bulk temperature dropped gradually, and the thermodynamic equilibrium between the bio-drying material and the air flowing through the material was maintained in a
3.2. Water output 3.2.1. Water evaporation from bulk material WE from the bulk material was calculated by integrating the daily vapor flux of the bio-drying process (Fig. 2). From day 2, the WE began to increase dramatically, reaching the peak value of 47.8 kg d1 on day 4. The WE then gradually decreased to 29.0 kg d1 on day 9. After the first and second turning, the WE increased and then decreased. The peak values of the WE were observed 2 and 3 days after the first and second turning, respectively. The mean WE decreased to 5.49 ± 0.28 kg d1 during the last 3 days. 3.2.2. Apparent moisture reduction in bulk The AMR of the bulk material was the difference between the two moisture values determined at two different times. A comparison of the AMR and the actual water removal (WE) are
Water evaporation Apparent moisture reduction
50
Water mass (kg)
40
30
20
10
0 0
2
4
6
8
10
12
14
16
18
20
Time (d) Fig. 2. Water evaporation and apparent moisture reduction during the sewage sludge bio-drying process. Error bars show the standard deviations of the means (n = 3).
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19
et al., 2010; Xu et al., 2011). As a result, microbial metabolism was weakened and less water was generated. As shown in Fig. 2, WG accounted for a relatively small amount of the overall WE throughout the sludge bio-drying process (3.8% to 35.4% with a mean of 13.2% ± 9.49%); therefore, the WE was far greater than WG, resulting in drying of the bulk material.
constant state. Consequently, convection reached the hygroscopic limit, resulting in a low WE (Velis et al., 2009). 3.3. Water input 3.3.1. Aeration water input into bulk The amount of water input by aeration was calculated by integrating the daily inputted water vapor (Fig. 3). From day 2, the AWI increased rapidly as the forced air volume increased, reaching a mean of 0.73 ± 0.04 kg d1 during the period from day 2 to day 5. The AWI then decreased to 0.38 kg d1 as the air volume was mechanically mitigated. After the first turning, the air volume increased mechanically, resulting in an increased AWI of 1.41 kg d1. The forced air volume was mechanically decreased on day 18, leading to a decreasing value of AWI. Generally, high AWI values were maintained from day 10 to day 20, with a mean of 1.16 ± 0.22 kg d1.
3.4. Process of moisture variation The daily means of the WE, AMR and WG in each phase are presented in Table 2. During the sludge bio-drying process, the order of daily means of WE, AMR and WG in the four phases was as follows: thermophilic phase > second temperature increasing phase > temperature increasing phase > cooling phase. Therefore, the thermophilic phase is the peak of both WG and moisture removal in the bulk material. The bio-drying material had a high MC during the temperature increasing phase. This was because SS primarily contains bound water such as vicinal water and water of hydration after mechanical dewatering (Vesilind, 1994), which is actually abundant in biodrying material during the initial phase (temperature increasing phase). Therefore, there is little free water that can be removed by forced air or convection. In addition, the microbial activity increased notably during this phase (Hassen et al., 2001), resulting in the generation of more metabolic water (Fig. 4) and the WG reaching as high as 12.0 kg on day 3. Consequently, the MC of the bio-drying material increased moderately instead of decreasing during the initial phase of sludge bio-drying. From day 4, the high temperature of the bio-drying material destroyed the vicinal water boundary water layers, which increased the effectiveness of sludge dewatering during the thermophilic phase (Vesilind, 1994). Moreover, the abundant heat produced by the microbial metabolism and the forced aeration both facilitated WE, leading to efficient removal of moisture from the bulk from day 4. On days 8 and 9, the decline rate of MC slowed. This decrease was likely a result of the air channels between the solid particles being reduced by compaction of the bulk (Richard et al., 2004) and hardening of the bulk surface after one week of fermentation (Luo et al., 2008; Yue et al., 2008), which subsequently reduced the efficiency of WE. After mechanical turning on day 9, the compaction of the bulk was modified, which improved the efficiency of aeration
3.3.2. Water generation in bulk material The WG in the bulk material was determined by calculating the water mass balance, and the results are presented in Fig. 4. The metabolic WG increased dramatically during the first 3 days, reaching its peak value of 12.0 kg d1 on day 3, after which it decreased. The mass of water generated on days 2–9 was generally abundant, with a mean value of 7.77 ± 2.75 kg d1. After the first turning, the WG increased to 10.7 kg d1. Once the cooling phase started, the WG decreased gradually to a mean of 0.60 ± 0.44 kg d1, eventually trending toward zero (0.20 kg d1). The variation of WG in the bulk material indicates that a large amount of metabolic water was generated from day 2 to day 9 of the sludge bio-drying period. This was attributed to the high content of organic matter and the increasing microbial metabolism in the bulk, which was dependant on organic matter and generate a large amount of metabolic water (Zhao et al., 2011). During this period, the WG first increased and then decreased. After turning, the bulk entered the second temperature increasing phase (from day 9 to day 15), and a second WG peak occurred. This second peak was due to the modified bulk condition after turning, which promoted secondary microbial fermentation (Léonard et al., 2008). During the cooling phase (from day 15 to day 20), WG decreased, primarily because the readily biodegradable organic matter that the microbes fed on was almost completely degraded (Zhang
4.0
12000
Aeration water input Forced air volume
3.5
10000
8000 2.5 2.0
6000
1.5 4000 1.0
Aeration water input (kg)
3
Forced air volume (m )
3.0
2000 0.5 0.0
0 0
2
4
6
8
10
12
14
16
18
20
Time (d) Fig. 3. Volume of forced air and aeration water input during the sewage sludge bio-drying process. Error bars show the standard deviations of the means (n = 3).
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L. Cai et al. / Bioresource Technology 117 (2012) 13–19
15
Water generation (kg)
12
9
6
3
0 0
2
4
6
8
10
12
14
16
18
20
Time (d) Fig. 4. Water generation during the sewage sludge bio-drying process.
Table 2 Daily means of WG, WE and AMR in each phase. Phase
Daily mean of WG (kg d1)
Daily mean of WE (kg d1)
Daily mean of AMR (kg d1)
Temperature increasing phase (0 h–40 h) Thermophilic phase (40 h–208 h) Second temperature increasing phase (208 h–352 h) Cooling phase (352 h–480 h)
2.46 ± 1.97 7.93 ± 2.39 3.82 ± 3.47 0.60 ± 0.44
24.2 ± 17.95 37.7 ± 7.09 29.0 ± 12.59 9.56 ± 6.08
21.3 ± 15.60 28.8 ± 7.90 25.6 ± 11.92 7.80 ± 5.72
Results are the mean ± SD.
and enabled a second fermentation of the undegraded material in the bulk, resulting in an increase in WE (Fig. 2) and a rapid decrease of MC. On day 18 to day 20, the decline rate of MC slowed gradually, indicating that sludge bio-drying was accomplished. 4. Conclusions The moisture in SS bio-drying material could be rapidly decreased from 66% to 54% by CTB. During the temperature increasing phase and the thermophilic phase, the MC, WG and WE of the bulk material all increased and then decreased. The peak values of WG and WE were attained in the thermophilic phase. Bio-drying bulk entered the second temperature increasing phase after turning, resulting in an increased WG and WE, which accelerated the rate of water removal. Throughout the SS bio-drying process, aeration facilitated WE and the WE was much greater than WG. Acknowledgements This project was financially supported by the National Hightech Research and Development Program of China (863 Program) (No. 2009AA064703) and the National Water Pollution Control and Management Technology Major Project of China (No. 2009ZX07318-008-007). References Adhikari, B.K., Barrington, S., Martinez, J., King, S., 2009. Effectiveness of three bulking agents for food waste composting. Waste Manage. 29, 197–203. Chen, J., Chen, T.B., Gao, D., Lei, M., Zheng, G.D., Liu, H.T., Guo, S.L., Cai, L., 2011. Reducing H2S production by O2 feedback control during large-scale sewage sludge composting. Waste Manage. 31, 65–70.
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