Enhancing Sewage Sludge Dewaterability by a Skeleton Builder

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Enhancing sewage sludge dewaterability by a skeleton builder: biochar produced from sludge cake conditioned with rice husk flour and FeCl3 Yan Wu, Panyue Zhang, Guang-ming Zeng, Jie Ye, Haibo Zhang, Wei Fang, and Jianbo Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01654 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016

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ACS Sustainable Chemistry & Engineering

Enhancing sewage sludge dewaterability by a skeleton builder: biochar produced from sludge cake conditioned with rice husk flour and FeCl3 Yan Wua,b, Panyue Zhanga,b,*, Guangming Zenga,b,*, Jie Yea,b, Haibo Zhanga,b, Wei Fanga,b, Jianbo Liua,b a

b

College of Environmental Science and Engineering, Hunan University, Yuelushan, Changsha 410082, P.R. China

Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Yuelushan, Changsha 410082, P.R. China

E-mail address of all authors: [email protected] (Yan Wu), [email protected] (Panyue Zhang), [email protected] (Guangming Zeng), [email protected] (Jie Ye), [email protected] (Haibo Zhang), [email protected] (Wei Fang), [email protected] (Jianbo Liu)

*Corresponding author: *Panyue Zhang E-mail: [email protected] Tel.: +86 15001255497. Fax: +86 731 88823701. *Guangming Zeng E-mail: [email protected] Tel.: +86 13908482238. Fax: +86 731 88823701.

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ACS Paragon Plus Environment


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Abstract: Biochar produced from sludge cake conditioned with rice husk flour and FeCl3 (biochar-conditioned) was used to enhance sewage sludge dewaterability. The pyrolysis temperature and dosage of biochar-conditioned were optimized, and the effect of biochar produced from raw sludge cake (biochar-raw) and biochar-conditioned on the sewage sludge dewaterability was compared. Moreover, the mechanisms of biochar-conditioned improving sludge dewaterability as a skeleton builder were analyzed. The optimal pyrolysis temperature of biochar-conditioned was 400 °C. The biochar-conditioned contained the sludge-based biochar with a high content of iron and rice husk-based biochar. Compared with biochar-raw, biochar-conditioned prepared at 400 °C was more effective to enhance the sludge dewaterability, and the optimal biochar-conditioned dosage was 70% dry sludge (DS). Compared with adding FeCl3 alone, the sludge specific resistance to filtration decreased by 63.9%, the net sludge solids yield increased by 39.2%, and the net percentage sludge water removal increased to 98.36%. Large cracks made the sludge cakes permeable, so that the more sludge moisture was filtered from the sludge cake. In addition, adding biochar-conditioned reduced the turbidity and SCOD of sludge filtrate and adjusted the sludge pH to neutral. Using biochar-conditioned to condition the sewage sludge as a skeleton builder is promising. Keywords: Sludge biochar; Sludge conditioning; Sludge dewaterability; Permeability; Sludge filtrate Introduction With the economy development and rapid urbanization, the sewage amount sharply increased, and huge volumes of excess sludge are produced by wastewater treatment plants (WWTPs).1 Improper sludge treatment may cause serious secondary pollution, so the sludge treatment and disposal is one of the most complex environmental problems in China. The sludge dewatering is a key process in sludge treatment and disposal because of the high moisture content, which goes against further sludge incineration and compost.2 At present, mechanical dewatering is commonly used to increase the solid content of sewage sludge under mechanical pressure. However, when the filtration rate of sludge cake is slow, the cost of sludge dewatering can be expensive.3 Effective improvement of sludge dewatering is still necessary nowadays. Chemical conditioning to improve sludge dewatering is commonly used in WWTPs by adding ferric chloride (FeCl3), cationic polyacrylamide and lime etc. But the chemical conditioning does not always function as expected because of the high compressibility of flocculated sludge.4 To decrease the sludge compressibility under high press, physical conditioners, known as skeleton builders, are commonly used to form more permeable sludge cakes.5 Some skeleton builders have been investigated, such as wood chips, wheat dregs, red mud, gypsum, lignite and coal fly ash etc.6-11 The permeable sludge cakes benefited to sludge dewatering. After sludge conditioning and dewatering, the sludge cake often contains 2


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pollutants such as pathogens, heavy metals and organic pesticides that may have an adverse impact on environmental quality, human health and agriculture.12 Therefore, the dewatered sludge cake treatment and disposal is of particular concern. In our previous study, the sewage sludge was effectively conditioned with rice husk flour and FeCl3. The specific resistance to filtration (SRF) of sludge decreased by 96.2%, the net sludge solids yield (YN) increased nearly tenfold, and the moisture content of sludge cake decreased from 94.7% to 74.0%. The conditioned sludge cake needs to be further treated and disposed. The treatment and disposal of dewatered sludge cake normally includes incineration, landfill and agricultural application as soil conditioners or fertilizers. Apart from the conventional methods, lately there is increasing interest in regarding thermal processing of the dewatered sludge.13-14 A carbon-rich, solid by-product, which named sludge biochar, can be produced by pyrolysis of the dewatered sludge cake. At present, the sludge biochar intended for adsorption of toxic material such as heavy metals, pyrene and phenanthrene etc.12, 15 However, there were few researches on using the sludge biochar for sludge conditioning and dewatering. In this study, the possibility to improve sewage sludge dewaterability with biochar produced from sludge cake conditioned with rice husk flour and FeCl3 (biochar-conditioned) as a skeleton builder was explored. The preparation temperature and conditioning dosage of biochar-conditioned were optimized, and the effect of raw sludge biochar (biochar-raw) and biochar-conditioned on the sludge dewaterability was compared. The properties of biochar-raw and biochar-conditioned were investigated to analyze the mechanisms to improve the sludge dewaterability. Additionally, other advantages using biochar-conditioned for sewage sludge conditioning were investigated through the change of turbidity and SCOD of sludge filtrate. Materials and method Materials Sewage sludge was taken from a local municipal WWTP in Changsha, Hunan, China, and stored at 4 °C.16-17 Before experiments the sewage sludge was kept in a water bath at 20 °C for 30 min.18 The main sludge characteristics were as follows: moisture content of 98.6%-99.01%, dry sludge (DS) content of 9.52-13.97 g/L, SRF of 9.87×1012-26×1012 m/kg, and YN of 1.38-2.42 kg/(m2·h). Skeleton builders are normally combined with chemical conditioning for effective enhancement on sludge dewaterability because of the weak effect of adding skeleton builders alone.3, 7 In this study, FeCl3 solution with a concentration of 5 g/L, a common inorganic conditioner, was used together with biochar-conditioned to condition the sewage sludge. The biochar-conditioned was prepared from sludge cake conditioned with rice husk flour and FeCl3. Preparation of biochar-conditioned According to our preliminary study, the sludge was conditioned with 138.09 g/kg 3



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DS FeCl3 (138.09 g FeCl3 was added to 1 kg sludge DS) and 70% DS rice husk flour (700 g rice husk flour was added to 1 kg sludge DS). Then the conditioned sludge was dewatered by pressure filtration, and the dewatering sludge cake was dried in an oven and milled. The milled sludge was filled into 100 ml airtight crucible and compacted. Then the airtight crucible was put into a muffle furnace (XMT-7000, China). The nitrogen gas was introduced into the chamber for 10 min to replace the air. Then the sludge was pyrolyzed at different temperatures, and the pyrolysis gas was led into an alkaline liquid. The pyrolysis temperatures were 250, 400, 550 and 700 °C, respectively. The biochar yield at different temperatures was calculated. The pyrolyzed sludge was the biochar-conditioned, which was milled and sieved. The biochar-conditioned within a particle size range of 80-250 µm was used. The characteristics of biochar-conditioned were measured by environmental scanning electron microscope (ESEM) (Quanta 200, America) and energy-dispersive spectrometry (EDS) (EDAX genesis xm-2, USA). As a control, the biochar-raw was prepared with the dewatered sludge cake from raw sludge under the same pyrolysis conditions. Sludge conditioning and dewatering The biochar-conditioned and FeCl3 were sequentially added in 100 ml sewage sludge. The FeCl3 dosage was set as the optimal dosage of 115.07 g/kg DS according to our preliminary study. The biochar dosage was ranged from 0 to 90% DS. After a rapid mixing at 350 r/min for 30 s and a slow agitation at 40 r/min for 3 min, the conditioned sludge was put into a 9 cm Buchner funnel and filtered under a pressure of 0.03 MPa for 6 min. The filtration time required for the filtrate volume to increase up to half of the sludge volume (the time to filter, TTF) was recorded.4 The sludge filterability was evaluated with SRF and YN. The sludge cake produced by the Buchner funnel filtration process was dried at 105 °C to determine the moisture content of sludge cake.9 Meanwhile, the mass of filtrate was measured. Then the net percentage sludge water removal after filtration time of t (SWR(t)) was measured. Additionally, the microstructure of sludge cake was observed by ESEM and coefficient of compressibility of sludge cakes was measured.19 Meanwhile, the sludge pH, the SCOD and turbidity of sludge filtrate were tested. The average values in this paper were calculated from the twice or more repeated experiments. Analytical methods The SRF was measured according to Huang et al.20 In each filtration dewatering experiment, 100 ml sewage sludge was firstly conditioned with the biochar and FeCl3, and then put into a 9 cm Buchner funnel for the sludge filtration. The dosage of biochar was 0%-90% DS, and the FeCl3 dosage was 115.07 g/kg DS. The SRF is normally used as a criterion to evaluate the sludge dewaterability, when the sludge solids remain relatively constant, independent of conditioner dosage. Adding the biochar-conditioned as skeleton builders obviously increased the sludge solid content, 4


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so the YN was combined to evaluate the sludge filterability. The YN expresses the amount of sludge solids filtered per unit filtration area and unit time,21 and is calculated by Eq. (1) according to Ning et al.,4 1/ 2

 2Pω 1  YN = F ⋅  ⋅   µ ⋅ t SRF 

(1) 2

where P is pressure (N/m ), ω is the weight of cake solids per volume of filtrate and is assumed to be constant (kg/m3), µ is the filtrate viscosity (N·s/m2), t is the filtration time required for the filtrate volume to increase up to half of the sludge volume (s), and F is a correction factor as Eq. (2), which is used to incorporate the effect of added conditioners, F=

SSoriginal SSoriginal + SSconditioner

(2)

where SSoriginal and SSconditioner is the mass of original sludge solids and conditioner solids per unit volume sludge, respectively (g). To improve YN is one main objective of sludge conditioning. The moisture content of sludge cake after was determined by gravimetric method. The SWR(t) is determined by Eq. (3),22 to further indicate the effect of biochar addition for dewatering, SWR (t) (%) = (

m F ( t ) − mW m S − m SS

) × 100

(3)

where mF(t) is the filtrate mass at time t (in this paper the filtration time t was 6 min) (kg), mW is the total extra water added mass (including that in FeCl3 solution) (kg), mS is the total sludge mass at the start (kg), and mSS is the sludge solids mass (kg). The biochar yield was determined by gravimetric method. The SCOD of sludge filtrate was determined by the potassium dichromate method and the turbidity of sludge filtrate was determined by spectrophotometry. Results and discussion Sludge conditioning with FeCl3 Preliminary experiments demonstrated that the sludge SRF firstly decreased with the increase of FeCl3 dosages when the sludge was conditioned with FeCl3 alone, and the lowest SRF was 20.10×1011 m/kg with a FeCl3 dosage of 115.07 g/kg DS (as shown in Supporting Information Figure S1); when the FeCl3 dosage exceeded 115.07 g/kg DS, the sludge SRF slightly increased again. Therefore, the FeCl3 dosage was set as 115.07 g/kg DS in this study. Effect of pyrolysis temperature of biochar-conditioned preparation on sludge dewaterability Pyrolysis

temperature

is

an

important

influencing

5


factor

for

the


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biochar-conditioned preparation, which further affects sludge dewaterability.23 The effect of pyrolysis temperature for biochar-conditioned preparation is shown in Figure 1. The biochar-conditioned dosage was 50% DS; by contract, the biochar-raw was set the same dosage. The SRF and YN of sludge conditioned with FeCl3 alone were 20.10×1011 m/kg and 14.15 kg/(m2 h), respectively, which was regarded as control experiment. When the biochar-conditioned or biochar-raw was added, the sludge SRF further remarkably decreased while the sludge YN further increased, which meant that the sludge dewaterability was significantly enhanced.24 When the sludge was conditioned with the biochar-conditioned prepared at 400 °C, the sludge SRF was the lowest and the YN reached the highest; the sludge SRF decreased by 67.6% and the YN increased by 62.6%, compared with that conditioned with FeCl3 alone. Hence, the optimal pyrolysis temperature for biochar-conditioned preparation was chosen as 400 °C. Meanwhile, the sludge SRF conditioned with the biochar-conditioned was obviously lower than that with the biochar-raw, and the sludge YN with the biochar-conditioned was obviously higher than that with the biochar-raw. The yield of biochar-conditioned was 58.92% at 400 °C, which was 13.36% higher than that of biochar-raw. XRD analysis demonstrated that there were no significant differences between the biochar-raw and biochar-conditioned pyrolyzed at different temperatures.

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Figure 1 Effect of pyrolysis temperature for biochar-conditioned preparation on sludge dewaterability (a) SRF, (b) YN (Control with 115.07 g/kg DS FeCl3 alone, experimental groups with 115.07 g/kg DS FeCl3 and 50% DS biochar). Characteristics of biochar-conditioned prepared at 400 °C Microstructure analysis Supporting Information Figure S2d showed the microstructure of biochar-conditioned prepared at 400 °C tested by ESEM. As a control, the microstructure of biochar-raw and raw rice husk biochar prepared at 400 °C (as shown in Supporting Information Figure S2a and 2b) and biochar-conditioned prepared at 250, 550 and 700 °C (as shown in Supporting Information Figure S2c, 2e and 2f) were also analyzed. Compared with the biochar-raw prepared at 400 °C, the biochar-conditioned prepared at 400 °C contained rice husk-based biochar besides the sludge-based biochar (as shown in Supporting Information Figure S2a and 2b). The rice husk-based biochar is more rigid than sludge-based biochar, because the rice husk-based biochar contained a high content of silica.25 A skeleton builder with a rigid structure was more beneficial during mechanical dewatering.19 Thus the biochar-conditioned was superior to biochar-raw for sludge dewatering. The microstructure of biochar-conditioned prepared at 400 °C was also very different from biochar-conditioned prepared at 250, 550 and 700 °C. It is reported that the feedstock was incompletely decomposed and carbonized within low depth at low temperature. As the temperature increased, the feedstock decomposition increased and the underlying layers became more exposed, resulting in improved porosity of the biochar.26 Therefore, the surface of biochar-conditioned prepared at 400 °C was 7



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rougher and showed more pores than that of biochar-conditioned prepared at 250 °C, thus causing the better adsorption of fine sludge particles onto the biochar-conditioned to improve the sludge dewaterability.9 The ESEM images indicated that the amount of tiny particles on the surface of the sludge-based biochar in biochar-conditioned prepared at 400 °C was less than that of the biochar-conditioned prepared at 550 and 700 °C. More tiny particles might block the water channels of sludge cakes thus leading to the decline of sludge dewaterability.

Surface element content analysis The carbon content of the biochar-conditioned surface was shown in Supporting Information Table S1. The sludge-based biochar and rice husk-based biochar are shown respectively because the EDS only determines the surface element content of particles. The carbon content of sludge-based biochar in biochar-conditioned increased with the increase of pyrolysis temperature, demonstrating the incomplete decomposition and low carbonized depth of conditioned sludge at low temperature, which meant that the biochar-conditioned prepared at 250 °C had less pores than that prepared at 400 °C. The carbonized depth of conditioned sludge increased with the increase of pyrolysis temperature and the carbon content increased with the increase of carbonized depth of biochar feedstock,27 causing the appearance of more tiny particles on the surface of sludge-based biochar prepared at high temperatures (550 and 700 °C). These tiny particles might block the water release channels and further lead to a worse sludge dewaterability. The iron content on the surface of biochar-conditioned prepared at 400 °C is shown in Supporting Information Table S2. As controls, the iron content of biochar-raw and raw rice husk biochar prepared at 400 °C was also analyzed. Compared with the biochar-raw and raw rice husk biochar, the iron content of both sludge-based biochar and rice husk-based biochar in biochar-conditioned increased, because the sludge cake conditioned with rice husk and FeCl3 contained much more iron. The iron on biochar-conditioned surface could improve sludge dewaterability by charge neutralization in certain extent,28-29 thus leading to a better sludge dewatering performance of the biochar-conditioned prepared at 400 °C. Effect of biochar-conditioned dosage on sludge dewaterability Figure 2 shows the effect of biochar-conditioned (prepared at 400 °C) dosage on the sludge filtration dewaterability. The effect of biochar-raw (prepared at 400 °C) dosages was considered as a comparison. With the biochar-conditioned addition, both the sludge SRF and moisture content of sludge cake decreased. The YN and SWR(t) increased with the increase of biochar-conditioned dosage, and the highest YN and SWR(t) achieved with a biochar-conditioned dosage of 70% DS; when the biochar-conditioned dosage was more than 70% DS, the YN and SWR(t) slightly decreased again. So the optimal biochar-conditioned dosage was 70% DS. The highest YN and SWR(t) were 28.6 kg/(m2h) and 98.36%, respectively. Compared with adding FeCl3 alone, when further adding the optimal biochar-raw of 40% DS, the sludge SRF decreased by 35.10%, the YN increased by 19.97%, the TTF decreased from 17.0 to 8


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12.3 s, the SWR(t) increased from 94.31% to 96.33%, and the moisture content of sludge cake decreased from 76.93% to 76.12%; when adding the optimal biochar-conditioned of 70% DS, the sludge SRF decreased by 63.9%, the YN increased by 39.20%, the TTF decreased from 17.0 to 11.5 s, the SWR(t) increased to 98.36%, and the moisture content of sludge cake decreased to 70.27%. The total weight of sludge cake conditioned with FeCl3 (115.07 g/kg DS) and biochar-conditioned (70% DS) was the lowest. Ning et al. used tannery sludge incineration slag to enhance sludge dewaterability with an optimal dosage of 150% DS;4 Qi et al. used lignite to improve sludge dewaterability with an optimal dosage of 100% DS.22 These results illustrated that using biochar-conditioned for sludge dewatering was superior to using biochar-raw. Therefore, it is significant that using biochar-conditioned to enhance the sludge dewaterability.

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Figure 2 Effect of biochar-conditioned dosage on sludge filtration dewatering (a) SRF, (b) YN and TTF, (c) the moisture content of sludge cake and (d) SWR(t) (Filtration time of 6 min, FeCl3 dosage of 115.07 g/kg DS, biochar-raw was as a control). Change of microstructure of sludge cakes About the sludge conditioned with skeleton builders, some reports only tested the change of sludge flocs by ESEM;3, 5, 8 another report analyzed the change of sludge cakes only by speculation without any experimental results.22 The surface and longitudinal sections of sludge cakes were analyzed by ESEM (as shown in Supporting Information Figure S3) in this study to investigate the change of sludge cake microstructures and analyze the function mechanisms of biochar-conditioned. Supporting Information Figure S3a shows the change of sludge cake surfaces. The surface of raw sludge cake showed typically compact and less porous. Being conditioned with FeCl3 alone, only several small pores were observed on the sludge cake surface. Some large cracks, however, appeared on the sludge cake surface with adding biochar-raw and biochar-conditioned. The rice husk-based biochar in biochar-conditioned, which had special microstructure, was found along the large cracks easily. Supporting Information Figure S3b shows the change of longitudinal section of sludge cakes. The longitudinal section of raw sludge cake and sludge conditioned with FeCl3 was very compact, and it was very hard to found the cracks in the ESEM images (magnified 500 times). The cracks were found in the ESEM image (magnified 500 times), when the sludge was conditioned with biochar-raw and FeCl3. When adding the biochar-conditioned, broader cracks were easily found in the ESEM image (only magnified 100 times), which meant that the sludge cake conditioned with biochar-conditioned and FeCl3 was more permeable than that with biochar-raw and 11



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FeCl3. Large amount of huge cracks in the sludge cakes were caused by biochar-conditioned, demonstrating that the biochar-conditioned played as a skeleton builder in sludge cakes. With the presence of these cracks, the sludge moisture could easily flow through the sludge cakes. During the filtration stage, for the raw sludge cake and sludge cake conditioned with FeCl3, the high sludge compressibility hindered the moisture to be removed out because the sludge cakes became more and more compact. However, for the sludge conditioned with biochar-conditioned and FeCl3, the sludge moisture could easily passed through the sludge cakes because the sludge cakes maintained not only as a certain skeleton structure but also permeable with the huge holes as water channels. As shown in Figure 3, the coefficient of compressibility of raw sludge cake and sludge cake conditioned with FeCl3 alone were 1.06 and 0.90, respectively. When adding the biochar-raw (40% DS) and FeCl3, the coefficient of compressibility of sludge cake decreased to 0.86. When adding the biochar-conditioned (70% DS) and FeCl3, the coefficient of compressibility of sludge cake further decreased to 0.81, which meant that the sludge cakes was more incompressible and permeable.

Figure 3 Coefficient of compressibility of sludge cakes (FeCl3 dosage of 115.07 g/kg DS, biochar-raw dosage of 40% DS, biochar-conditioned dosage of 70% DS). Other advantages of sludge conditioning with biochar-conditioned The change of turbidity and SCOD of sludge filtrate, and sludge pH were investigated to show other advantages of sludge conditioning with biochar-conditioned, and the results are shown in Figure 4. The filtrate turbidity of sludge conditioned by FeCl3 alone was 27.2 NTU. When biochar-conditioned was 12


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added, the turbidity of sludge filtrate decreased rapidly with the increase of biochar-conditioned dosage, and the lowest turbidity of 11.2 NTU reached with a biochar-conditioned dosage of 50% DS; when the biochar-conditioned dosage was more than 50% DS, the turbidity of sludge filtrate increased again. The filtrate turbidity of sludge with the optimal dosage of biochar-conditioned (70% DS) was 15.8 NTU. The filtrate SCOD of sludge conditioned by FeCl3 alone was 752.1 mg/L. The filtrate SCOD decreased rapidly with the increase of biochar-conditioned dosage, and the lowest SCOD was 197.8 mg/L with a biochar-conditioned dosage of 50% DS; when biochar-conditioned dosage was more than 50% DS, SCOD increased slightly again. When the biochar-conditioned dosage was optimal (70% DS), the SCOD slightly increased to 214.2 mg/L. Adding the biochar-conditioned effectively reduced the turbidity and SCOD of sludge filtrate because the biochar-conditioned was also functioned as absorbents.15, 30 The low turbidity and SCOD of sludge filtrate benefited to further treatment of sludge filtrate, which might significantly reduce the difficulty and cost of wastewater treatment. The raw sludge pH was 6.85. The pH of sludge conditioned with FeCl3 decreased to 4.63 due to the hydrolyzation of FeCl3.31 When the biochar-conditioned was added, the sludge pH increased with the increase of biochar-conditioned dosage. With the optimal biochar-conditioned dosage of 70% DS, the sludge pH increased to 6.61. The biochar-conditioned addition adjusted the sludge pH, which was beneficial to sludge final disposal.32

Figure 4 Effect of biochar-conditioned dosage on turbidity and SCOD of sludge filtrate and sludge pH (FeCl3 dosage of 115.07 g/kg DS). The iron content in the filtrate and dewater sludge was detected as shown in Supporting Information Figure S4. The iron content in filtrate increased with the 13



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increase of FeCl3 dosage, while the iron content in dewatered sludge cake firstly increased with the increase of FeCl3 dosage, and then kept at a stable level when the FeCl3 dosage was more than 115.07 g/kg DS. The maximum iron content in dewatered sludge was nearly constant and the excess iron was solubilized in the filtrate. Furthermore, the iron in biochar-conditioned functioned through charge neutralization,33 which was beneficial to the sludge dewatering. So the biochar-conditioned could be cyclically prepared and used for sludge conditioning with stable characteristics. When all excess sludge was used to produce the biochar-conditioned, besides sludge conditioning, the excess biochar-conditioned can be used for wastewater treatment or soil improvement etc.15, 34 Conclusion Compared with the biochar-raw, the biochar-conditioned prepared at 400 °C contained the sludge-based biochar with high content of iron and the rice husk-based biochar with a lot of pores and drapes, and was more effectively and significantly enhance sewage sludge dewaterability. The optimal biochar-conditioned dosage was 70% DS. As a skeleton builder, the biochar-conditioned built more porous skeleton structure in sludge cakes to improve sludge dewaterability. Compared with adding FeCl3 alone, the sludge SRF decreased by 63.9%, the YN increased by 39.2%, the SWR(t) at 6 min increased from 94.31% to 98.36%, the moisture content of sludge cake decreased from 76.9% to 70.3% with a biochar-conditioned dosage of 70% DS. In addition, the turbidity and SCOD of sludge filtrate reduced, which was beneficial to further treatment. Using biochar-conditioned for sludge conditioning and dewatering cyclically provides a green approach for the waste recycling and the sustainable waste management. Acknowledgements This research was funded by the National Natural Science Foundation of China (51178047, 51521006) and Furong Scholar of Hunan Province. Supporting Information Detailed results on effect of FeCl3 dosage on sludge dewaterability, effect of pyrolysis temperature on biochar-conditioned microstructure, change of microstructure of sludge cakes, effect of FeCl3 dosage on iron distribution in filtrate and dewatered sludge cake, as well as carbon and iron content of biochar-conditioned surface tested by EDS. Notes The authors declare no competing financial interest. References (1) Liu, F.; Zhou, J.; Wang, D.; Zhou, L., Enhancing sewage sludge dewaterability by bioleaching approach with comparison to other physical and chemical conditioning methods. J. Environ. Sci. 2012, 24, 1403-1410. 14


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Nomenclature

Biochar-raw DS WWTPs FeCl3 SRF YN

Biochar produced from sludge cake conditioned with rice husk flour and FeCl3 Biochar produced from raw sludge cake Dry sludge Wastewater treatment plants Ferric chloride Specific resistance to filtration Net sludge solids yield

TTF

The time to filter

MC

Moisture content of sludge cake

138.09 g/kg DS FeCl3 70% DS rice husk flour ESEM EDS

F SSoriginal

138.09 g FeCl3 was added to 1 kg sludge DS 700 g rice husk flour was added to 1 kg sludge DS Environmental scanning electron microscope Energy-dispersive spectrometry Net percentage sludge water removal after filtration time of t Pressure Weight of cake solids per volume of filtrate Filtrate viscosity Filtration time required for the filtrate volume to increase up to half of the sludge volume Correction factor Mass of original sludge solids per unit volume sludge

SSconditioner

Mass of conditioner solids per unit volume sludge

mF(t)

Filtrate mass at time t

mW

Total extra water added mass

mS

Total sludge mass at the start

mSS

Sludge solids mass

Biochar-conditioned

SWR(t) P ω µ t

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Enhancing sewage sludge dewaterability by a skeleton builder: biochar produced from sludge cake conditioned with rice husk flour and FeCl3 Yan Wua,b, Panyue Zhanga,b,*, Guangming Zenga,b,*, Jie Yea,b, Haibo Zhanga,b, Wei Fanga,b, Jianbo Liua,b Table of Contents (TOC) Graphic Other treatment and disposal

Adding FeCl3 and rice husk flour Sewage sludge

Conditioning and dewatering

Sludge cake

Dried and milled Adding for sludge conditioning Anaerobically pyrolyzed Milled and sieved

Dried sludge

Biochar-conditioned

Synopsis Biochar produced from sludge cake conditioned with rice husk flour and FeCl3 (biochar-conditioned) was reused for effective sludge conditioning and dewatering.

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Enhancing Sewage Sludge Dewaterability by a Skeleton Builder

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