- Email: [email protected]
Conditioning of sewage sludge via combined ultrasonication-flocculation-skeleton building to improve sludge dewaterability
Accepted Manuscript Conditioning of sewage sludge via combined ultrasonication-flocculation-skeleton building to improve sludge dewaterability Cheng Zhu, Panyue Zhang, Hongjie Wang, Jie Ye PII: DOI: Reference:
S1350-4177(17)30332-2 http://dx.doi.org/10.1016/j.ultsonch.2017.07.028 ULTSON 3783
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
18 January 2017 18 July 2017 21 July 2017
Please cite this article as: C. Zhu, P. Zhang, H. Wang, J. Ye, Conditioning of sewage sludge via combined ultrasonication-flocculation-skeleton building to improve sludge dewaterability, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/10.1016/j.ultsonch.2017.07.028
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Conditioning of sewage sludge via combined ultrasonication-flocculation-skeleton building to improve sludge dewaterability
Cheng Zhu1, Panyue Zhang1,*, Hongjie Wang, Jie Ye2,* 1. College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China 2. College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, China
E-mail address of all authors: [email protected], [email protected], [email protected], [email protected] *Corresponding author: Panyue Zhang, Jie Ye Tel.: +86 15001255497; Fax: +86 10 62336900. E-mail address: [email protected] (Panyue Zhang); [email protected] (Jie Ye) Complete postal address: College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China
Abstract Strong water trapping ability of polyelectrolyte flocculated sludge and high compressibility of sludge filter cake greatly hindered the further improvement of sludge dewaterability. Ultrasound (US) was used to strip the water from the sludge flocs through sludge disintegration, traditional cationic polyacrylamide (CPAM) was 1
applied to reconstruct the sludge aggregates through flocculation, and rice husk (RH) was utilized to improve the permeability of sludge filter cake as skeleton builders in this paper. The feasibility of combined US-CPAM-RH sludge conditioning was demonstrated, and single and co-conditioning processes were compared to explore the synergetic effect and mechanisms of US-CPAM-RH sludge conditioning. The results indicated that the sludge dewaterability was considerably improved by the combined US-CPAM-RH conditioning compared with that of raw sludge. The optimal ultrasonication condition was 0.3 W/ml for 12 s with an Ultrasound frequency of 22 kHz, and the RH and CPAM dosage was 50.0 wt. % and 20 mg/L, respectively, which resulted in a shortest time to filter (TTF) of 43 s and a lowest moisture content of sludge filter cake of 62.22%. The US mainly stripped the extracellular polymeric substances and water from the sludge flocs, leading to an obvious increase of concentration of proteins and polysaccharides in sludge filtrate. The CPAM and RH addition resulted in a remarkable sludge floc growth to a mean diameter d(0.5) of 128.85 µm after combined US-CPAM-RH conditioning. Moreover, scanning electron microscopy (SEM) results clearly showed that the combined US-CPAM-RH conditioning formed a more porous structure of sludge filter cake with a porosity increase of 98.80% and a compressibility decrease of 37.81 % compared with that of raw sludge. Thus, the US-CPAM-RH co-conditioning could significantly improve the sludge dewaterability. Keywords: sewage sludge; ultrasound; flocculation; skeleton builder; dewaterability.
2
1. Introduction Activated sludge processes are paramount technologies for treating municipal wastewater [1], but these biological processes generate large amounts of sewage sludge, which is difficult to be dewatered and disposed [2, 3]. The critical issues that prevent sewage sludge from mechanical dewatering are the highly hydrophilic nature of extracellular polymeric substances (EPS), which bind a large amount of water molecules to solid surfaces and trap the water within sludge flocs, forming a high compressibility of sludge matrix [4, 5]. Chemical conditioners, including inorganic coagulants and organic coagulants, are widely applied prior to mechanical dewatering process to improve the performances of sludge dewatering by particle-particle bridging and surface charge neutralization mechanisms [6-10]. Nevertheless, it is difficult to achieve higher solid content of filter cake due to the high sludge compressibility during the compression stage of sludge dewatering [11]. Sørensen et al. [12] investigated the compressibility of filter cake associated with polyelectrolyte conditioning of biological sludge, and found that the filter cake became more compressible and easier to be deformed under pressure during the compression phase of mechanical dewatering. This deformation of sludge cake can cause void closure, which further impedes the sludge dewatering [13]. Physical conditioners, often referred to as skeleton builders or filter aids, are effective to reduce the sludge compressibility, because they commonly help the sludge cake form a permeable and rigid structure and maintain the sludge cake porous, even 3
under a high compression pressure. Some inert minerals and carbon-based solid materials with high porosity and rigid structure have been investigated as skeleton builders, such as cement [14], coal fly ash [15], lime [16], wood chips and wheat dregs [6], gypsum [17], lignite [18], tannery sludge incineration slag [19], and thermal-pretreated phosphogypsum [20]. Rice husk (RH) is a major category of agricultural residues generated in many countries, and need to be properly treated and disposed to avoid sequent environmental concerns. With the advantages of high porosity, light weight, high external surface area and rigid structure, the RH is a promising candidate of physical conditioner for sludge dewatering. However, it is still a great challenge to enhance the sludge dewatering under co-conditioning by chemical and physical conditioner, because strong water affinity of EPS greatly hinders the water from being drained through water passages in sludge filter cake. A number of sludge disintegration methods have been developed, including physical [21], chemical [9, 10, 20, 22-24] and biological disintegration [24, 25], to release the organic matters from solid phase to liquid phase, further enhancing the sludge anaerobic digestion. Simultaneously, the sludge disintegration breaks the floc matrix and releases the sludge water. Ultrasonication is a well known method to disintegrate the sludge through disrupting sludge flocs and lysing microbial cell wall, but the sludge dewaterability may decrease when the applied ultrasonic energy is improper. Li et al. [26] used ultrasound to disintegrate the sludge, and only when sludge disintegration degree was 2-5%, the sludge dewaterability was improved with the co-conditioning of FeCl3. Feng et al. [27] found that the sludge dewaterability was 4
improved after application of a low specific energy dosage (<4400 kJ/kg TS), but significantly dropped when the ultrasonication dosage was higher than 4400 kJ/kg TS. Zhang et al. [28] found that only when the energy input was lower than 1200 kJ/kg DS, the sludge dewaterability was improved, and the sludge dewatering performance was deteriorated when energy input exceeded 1200 kJ/kg DS. Chu et al. [29] found that sonication did not improve the sludge dewaterability because of the obvious increase of bound water content. After studying the relationship between the protein and carbohydrate content in untreated and sonicated waste activated sludge, Hosnani et al. [30] found that sonication generally made the sludge dewatering more difficult due to the increase of protein and carbohydrate as a result of sludge sonication. Cationic polyacrylamide (CPAM) conditioning [31], skeleton builder addition [32], and US disintegration [33] has been separately reported to be able to limitedly improve the sludge dewatering performances. The separate application of either sludge conditioning method greatly has no potential to further improve the sludge dewatering performances. The synergetic effect of these three sludge conditioning methods has not been investigated previously. The goal of this study was to explore the feasibility of US-CPAM-RH conditioning to enhance the sludge dewaterability, to analyze the mechanisms of combined sludge conditioning by US-CPAM-RH, and to optimize the operating conditions for US-CPAM-RH co-conditioning.
2. Materials and methods 2.1. Materials 5
Raw sewage sludge was collected from the thickening tank of a local wastewater treatment plant in Beijing, China. In this wastewater treatment plant the anaerobic-anoxic-oxic process is applied, and the wastewater flow rate is approximately 1.0 × 106 m3/d. The sludge was firstly thickened to a solid content of about 2.5 %, and then transferred to the laboratory as soon as possible. The sludge was stored at 4 ℃ and heated to room temperature before the experiments. All the experiments were conducted in 48 h. The average characteristics of sludge sample were as follows: moisture content of 97.5%, pH of 6.72, specific resistance of filtration (SRF) of 1.84×109 s2/g. Raw RH was collected from Hunan Province, China. The RH was firstly ground into powders of -110 mesh and dried at 110
for 12 h. The RH averagely included
hemicelluloses 24.3 %, cellulose 34.4 %, lignin 19.2 %, ash 18.85 %, and the others 3.25 %. CPAM with a molecular weight of (1.2-1.5)×10 8 and a charge density of 25%-35% was used as the sludge chemical conditioner in this study. The CPAM used was a product of BASF Company, which was obtained from the same local wastewater treatment plant. The CPAM solution of 1000 mg/L was prepared with distilled water and aged for 6-8 h before usage. 2.2. Sludge conditioning and dewatering The sludge conditioning experiments were conducted with a six-paddle stirrers, which can be programmed. The US-CPAM-RH conditioning process was conducted as follows. First, the sludge was ultrasonicated at 22 kHz by an US generator 6
equipped with a probe transducer (SCIENTZ-IID, Xinzhi Inc., China), then the CPAM solution was added to 400 ml ultrosonicated sludge in a 500 ml beaker. The sludge system was stirred at 300 r/min for rapid mixing of 30 s followed by slow mixing of 180 s at 80 r/min. Afterwards, the RH powder was added into the sludge system, which was further stirred at 300 r/min for rapid mixing of 30 s and at 80 r/min for slow mixing of 180 s. After conditioning, 100 ml conditioned sludge sample was poured into 9 cm standard Buchner funnel with pre-wetted 0.45 µm filter paper, and then a constant vacuum pressure of 0.051 MPa [34] was applied until no filtrate came out [5]. The filtrate volume was recorded and the sludge cake moisture content was analyzed for SRF determination. The sludge cake was analyzed by mercury intrusion porosimetry, compressibility test, and scanning electron microscopy (SEM) observation. 2.3. Analytical methods The sludge filter cake was dried at 105
for 24 h and weighed by difference to
determine the moisture content [35]. The sludge SRF was measured using the method described by Qi et al. [5]. TTF is defined as the time when the volume of sludge filtrate reaches the half of sludge volume, and the TTF determination was performed by the method described by Lo [36]. The sludge filtering rate was tested by the method described by Chen [15]. The sludge floc size was measured by a Malvern mastersizer ZS (Malvern Mastersizer 2000, UK). The protein concentration in sludge filtrate was tested by the modified Lowry method using bovine serum albumin as the standard [37], and the 7
polysaccharide concentration in sludge filtrate was analyzed with the anthrone method by using glucose as the standard [38]. The compressibility of sludge filter cake was tested by the method described by Zhao [17]. The sludge samples were tested under five pressures ranging from 0.1-0.5 MPa. The porosity of sludge filter cake was analyzed by a mercury intrusion porosimetry (AutoPore IV 9500, China). The sludge filter cake was dried at 45 ℃ for 24 h before the porosimetry test. The surface structure of sludge filter cake was observed by a SEM (HITACHI S-3400 N, Japan). Each test was performed in triplicate, and the results were reported as average value.
3 Result and discussions 3.1. Dewaterability of sewage sludge conditioned by US-CPAM-RH Sewage sludge was conditioned by different methods: RH (-110 mesh) conditioning with a dosage of 50 wt.%; US conditioning with an ultrasonication density of 0.3 W/ml and an ultrasonication time of 12 s; CPAM conditioning with a CPAM dosage of 30 mg/L; US-CPAM conditioning with an ultrasonication density of 0.3 W/ml, an ultrasonication time of 12 s and a CPAM dosage of 30 mg/L; US-CPAM-RH conditioning with an ultrasonication density of 0.3 W/ml, an ultrasonication time of 12 s, a CPAM dosage of 30 mg/L and a RH (-110 mesh) dosage of 50 wt.%. The corresponding sludge filtering rate profiles are presented in Fig. 1a. All 8
profiles showed a rapid water filtration stage followed by a gentle increase stage. The rapid water filtration stage accorded well with first-order kinetics equation, and the rate constant K is listed in Table 1. The raw sludge showed a lowest water filtration and the slowest filtering rate with K=0.5176. Moreover, the sludge filtering rate profile for raw sludge did not show clear critical point during the whole vacuum filtering process. Meanwhile, the raw sludge showed the highest SRF of 1.39×109 s2/g and longest TTF of 158 s (see Fig. 1b). All single US, CPAM and RH conditioning enhanced the sludge filtration process, resulting in an increase of rate constant K. Correspondingly, the sludge SRF and TTF decreased. The RH conditioning only slightly improved the sludge dewaterability with a filtering rate constant K of 0.5744, and the SRF and TTF of RH conditioning only decreased by 17.27 % and 8.23 %, respectively, compared with that of raw sludge. The US irradiation induced a series of ultrasonic effects including acoustic streaming, local heating, interface instabilities, agitation, sponge effect and cavitation effect. Theses intense physico-chemical reactions disintegrated the sludge matrix and released the water trapped among the sludge flocs [26, 39], leading to a filtering rate constant K of 0.6419. The special macromolecular structure of CPAM could wrap around the sludge colloids into larger and denser flocs through charge neutralization and inter-particle bridging mechanisms, so that the free water and partial interstitial water was more easily released [30, 40-42]. However, the water trapped in the sludge matrix still hindered the improvement of sludge dewaterability due to the high compressibility of sludge filter cake, and the filtering rate constant K increased to 0.7301. The combined US-CPAM 9
conditioning showed a synergistic effect of US and CPAM, leading to a further increase of rate constant K to 0.8379; meanwhile, the sludge SRF and TTF further decreased. The highest filtering rate (K=0.8986) and the highest water removal (34%) were observed for combined US-CPAM-RH conditioning. It took only approximately 3.5 min to reach the equilibrium. The SRF and TTF also reduced to 1.32×108 s2/g and 43 s, respectively, which respectively decreased by 90.50% and 72.78% compared with that of raw sludge. These results were probably attributed to the water passage formation in sludge filter cake and the compressibility reduction of sludge filter cake by introducing RH particles as skeleton builders. Many researchers found similar results about the effect of skeleton builder in sludge conditioning. Ying et al. [18] showed that the time required to same water removal was much less for adding lignite as filtering aid than that only conditioned by polyelectrolyte. Luo et al. [43] found that the sludge co-conditioned by CPAM and sawdust showed obvious filtering rate advantage over the sludge conditioned by single CPAM. Shi et al. [20] observed that a large number of column-shaped skeleton builder filled in the sludge matrix and increased the porosity of the sludge filter cake. Wu et al. [44] found that biochar produced from sludge cake conditioned with RH flour and FeCl3 significantly improved the sludge dewatering performances.
10
Fig. 1. Effect of sludge conditioning on sludge filtration properties (a) Filtering rate, (b) sludge SRF and TTF. Table 1. Rate constant (K) of filtration process under different conditioning methods. 11
Conditioning method
First-order rate constant (K)
Raw sludge
0.5176
RH
0.5744
US
0.6419
CPAM
0.7301
US-CPAM
0.8379
US-CPAM-RH
0.8986
The moisture content of sludge cake conditioned by different methods is shown in Fig. 2. The raw sludge showed poor dewaterability, and the moisture content of sludge filter cake was 91%. For RH conditioning the moisture content of sludge filter cake was 90.55 %, showing only a slight improvement of sludge dewaterability. Single physical conditioner usually could not function as filter aids to reach the same dewatering extent of sludge conditioned with chemical conditioners [32]. After US disintegration, the sludge dewaterability slightly increased because a portion of water could be stripped from the sludge flocs. However, the sludge flocs might become smaller to form more compact sludge filter cake during sludge filtration. The CPAM improved the aggregation of sludge flocs, resulting in the improvement of sludge dewaterability, which is the widely used method currently. The water content of sludge filter cake conditioned by US-CPAM further decreased to 79.91 %, showing a synergistic effect of US or CPAM that the smaller sludge particles stripped from water by US were re-aggregated by the flocculation effect of CPAM. After adding the RH, the moisture content of sludge filter cake significantly dropped to 62.15 %. The 12
improved sludge dewaterability could be further improved by the formation of permeable and rigid skeleton structure of sludge filter cake with a large amount of rigid voids and pores.
Fig. 2. Effect of sludge conditioning on moisture content of sludge cake. 3.2 Mechanisms of sludge conditioning by US-CPAM-RH 3.2.1 Sludge filtrate As the main components of sludge EPS, proteins and polysaccharides have been proven to be the key factors during mechanical dewatering process [45, 46]. The concentration of proteins and polysaccharides in sludge filtrate is presented in Fig. 3. The concentration of proteins and polysaccharides in sludge filtrate of raw sludge was 13.5 and 3.8 mg/L, respectively. The US irradiation resulted in a significant increase of protein and polysaccharide concentration in sludge filtrate, and the concentration of 13
proteins and polysaccharides increased by 73.83 % and 270.18 % compared with that of raw sludge. These results illustrated that the sludge flocs even microorganism cells were disrupted by ultrasonication, and the EPS even the intracellular materials were released from the sludge flocs to sludge liquid. Meanwhile, more internal water among the sludge matrix and inside the bacteria might be released to sludge liquid. Feng et al. [27] reported that a strong quadratic relationship (R=0.9854 and R=0.9972) between the concentration of proteins and polysaccharides and the ultrasonic energy input was found. Si-Kyung et al. [47] observed that the concentration of bound EPS-proteins and carbohydrates decreased during US conditioning, indicating the destruction of sludge flocs increased with increasing the US irradiation energy input. Wang et al. [48] found that the concentration of proteins and polysaccharides increased with the increase of ultrasonication time. However, the water removal in Fig. 1a for single US conditioning was slightly lower than that for single CPAM conditioning, which might be attributed to the breakage of sludge flocs and corresponding compact structure of sludge filter cake. The other conditioning or combined conditioning did not cause obvious change of sludge filtrate. The addition of CPAM and RH could not obviously change the relationship between the internal water and sludge flocs, because the main mechanisms of CPAM and RH conditioning were charge neutralization and bridging, and the sludge floc structure did not change.
14
Fig. 3. Protein and polysaccharide concentration of sludge filtrate under different sludge conditioning processes. 3.2.2 Sludge flocs The sludge floc size distribution based on volume distribution during sludge conditioning is shown in Fig. 4. The mean diameter d(0.5) of raw sludge flocs was 87.37 µm. After US disintegration, the d(0.5) of sludge flocs was 52.63 µm with a reduction of 39.76 % because of the sludge disintegration. The d(0.5) of sludge flocs after CPAM conditioning increased 11.55 % to 97.46 µm compared with that of raw sludge, because the sludge particles were destabilized and agglomerated into a larger sludge matrix by the CPAM through mechanisms of charge neutralization and inter-particle bridging. When the US and CPAM conditioning were combined, the d(0.5) was only 90.20 µm, however it increased by 71.39% compared with that after US disintegration, showing the excellent flocculation efficiency. It was noting that the d(0.5) of sludge flocs after US-CPAM-RH conditioning significantly increased to 15
128.85 µm, which corresponded to an increment of 47.48 % compared with that of raw sludge. The introduction of RH could effectively strengthen the bridging effect between the sludge particles, which might be benefit for mechanical dewatering process [32].
Fig. 4. Particle size distribution of sludge flocs under different sludge conditioning processes. 3.2.3 Sludge filter cake The SEM image of sludge filter cake is shown in Fig. 5. The sludge filter cake after dewatering from raw sludge presented dense and compact without obviously voids or channels. The porosity of filter cake of raw sludge was 15.24 % as shown in Table 2. For US conditioning, some cracks and fractures in sludge filter cake were observed; however, the sludge filter cake seemed denser and more compact. Moreover, from Fig. 6 it can be seen that the porosity of filter cake after US conditioning became 16
lower because of the breakage of sludge flocs with a porosity of 14.44 %. For CPAM conditioning, water channels were obviously observed in Fig. 5c. The sludge CPAM conditioning greatly improved the sludge permeability through bridging the sludge particles together and forming microstructure with pores. However, the porosity of sludge filter cake after CPAM conditioning insignificantly changed as shown in Table 2. For the sludge conditioned by US-CPAM, the surface image of sludge filter cake was rough and full of voids and channels with a increased porosity of 16.51 %. For US-CPAM-RH conditioning, besides the rough surface, voids and huge cracks, the RH fiber was obviously observed (Fig. 5e). The porosity after combined US-CPAM-RH conditioning climbed to 30.30%, corresponding to an increment of 98.82 % than that of raw sludge. Not only the pore volume in the whole pore diameter range of 0-0.37 mm increased, but also there were two peaks around the diameter of 0.045 mm and 0.366 mm. Moreover, the pore volume was almost more than twice of that of raw sludge in the macro and meso pore range of 0.01-0.37 mm according to Thapa et al. [49] . The rigid structure of RH might result in these macro and meso pores along with the skeleton builders, and the link of these pores could create a number of water drainage passages in sludge filter cake. Permeability is the prime impact factor that determined the dewaterability of flocculated suspensions [50].
17
Fig. 5. SEM images of sludge filter cakes (a) Raw sludge, (b) After US conditioning, (c) After CPAM conditioning, (d) After US-CPAM conditioning, (e) After US-CPAM-RH conditioning.
Fig. 6. Porosity of sludge filter cake under different sludge conditioning methods. Table 2. Porosity of sludge filter cake under different conditioning methods. Conditioning method
Porosity
18
Raw sludge
15.24
US
14.44
CPAM
15.21
US-CPAM
16.51
US-CPAM-RH
30.30
During the compression filtration stage of sludge dewatering, the permeability of sludge filter cake was significantly influenced by the compressibility (S) of filter cake, which kept the water channels in filter cake from collapsing [51]. Fig. 7 exhibited the effect of sludge conditioning method on the compressibility of sludge filter cake. The compressibility of filter cake of raw sludge was 0.96. The US slightly decreased the compressibility of filter cake, which might be attributed the more hydrophobic structure of sludge particles. The compressibility of filter cake after CPAM conditioning reached 0.83, which decreased by 12.98 % than that of raw sludge. The bridging of CPAM could form the filter cake with bigger sludge aggregates, reducing the compressibility of sludge filter cake. The combination of US and CPAM further decreased the compressibility of sludge filter cake. However, the poor compressibility of sludge filter cake after CPAM conditioning and US-CPAM conditioning still hindered from the improvement of dewatering effectiveness. With the introduction of RH, the compressibility of filter cake after US-CPAM-RH conditioning significantly dropped to 0.60, corresponding to a reduction of 37.50 % compared to that of raw sludge. Introducing of RH effectively reduced the compressibility of filter cake through providing a rigid skeleton structure even under high mechanical compression 19
process [44, 52-54]. Simultaneously reducing the compressibility of sludge filter cake and increasing the permeability of filter cake led to much more water drainage from the channels and voids in filter cake.
Fig. 7. Compressibility of sludge filter cake under different sludge conditioning methods. 3.3 Optimization of combined sludge conditioning of US-CPAM-RH 3.3.1 Ultrasonication density and time Variations of the moisture content of US-CPAM-RH conditioned sludge cake with different ultrasonication densities are presented in Fig. 8. The effect of ultrasonication density on the sludge dewatering performances showed similar trend. The moisture content of sludge cake decreased with increasing the ultrasonication time, and at an ultrasonication time of 12 s the moisture content of sludge cake reached to a minimum, followed by a gradual increase with the further prolonging of 20
ultrasonication time. The ultrasonication time of 12 s was the suitable ultrasonication time. The moisture content of sludge cake decreased with the increase of ultrasonication density. However, the lowest moisture content insignificantly decreased when the ultrasonication density increased from 0.3 to 0.4 W/ml. So, the suitable ultrasonication density should be chosen as 0.3 W/ml to save the energy consumption. The moisture content of sludge filter cake at an ultrasonication density of 0.3 W/ml and an ultrasonication time of 12 s was 85.62 %.
Fig. 8. Effect of ultrasonication density and ultrasonication time on sludge dewatering by US-CPAM-RH conditioning (RH dosage of 10 wt.%, RH particle size of 60 mesh, and CPAM dosage of 10 mg/L). 3.3.2 CPAM dosage Fig. 9 shows that the effect of CPAM dosage on the moisture content of sludge filter cake. The moisture content of sludge filter cake gradually decreased with 21
increasing the CPAM dosage until a CPAM dosage of 30 mg/L, followed a slight increase of moisture content of sludge filter cake with the further increase of CPAM dosage. The minimum moisture content of sludge filter cake reached to 78.19 % at a CPAM dosage of 30 mg/L.
Fig. 9. Effect of CPAM dosage on sludge dewatering by US-CPAM-RH conditioning (RH dosage of 10 wt.%, RH particle size of 60 mesh, ultrasonication density of 0.3 W/ml, and ultrasonication time of 12 s). 3.3.3 RH particle size and dosage Fig. 10 shows that the effect of RH addition on the moisture content of sludge filter cake after US-CPAM-RH conditioning. The moisture content of sludge filter cake declined along with increasing the RH dosage, then the descent trends became insignificant when the RH dosage was over 50 wt.%. For the RH with a particle size -60, -80 and -100 mesh, the moisture content of sludge filter cake was 72.88%, 68.84% 22
and 62.22%, respectively. The bigger RH might better support the formation of voids and water channels in the sludge filter cake. So, the -110mesh RH and 50 wt.% RH dosage should be chosen as the suitable physical conditioner and the corresponding dosage.
Fig. 10. Effect of RH particle size and dosage on sludge dewatering by US-CPAM-RH conditioning (Ultrasonication density of 0.3 W/ml, ultrasonication time of 12 s, CPAM dosage of 30 mg/L). 3.4 Economic analysis of combined sludge conditioning of US-CPAM-RH Economic analysis is necessary for applying the new technology. A preliminary economic analysis of different conditioning methods was conducted by calculating the chemical (CPAM and RH) cost and US energy consumption in this study; moreover, the cost of combined US-CPAM-RH conditioning was compared with that of other similar sludge conditioning methods, as shown in Table 3 [55, 56]. The US energy 23
consumption was estimated according to industrial electricity cost (CNY/t dry solid (DS)). The industrial electricity price in Beijing city is 0.8595 CNY/kW· h, the price of CPAM is 18000 CNY/t, and the price of RH is 472 CNY/t. The total chemical and energy cost of sludge conditioning by US, CPAM, US-CPAM and US-CPAM-RH is 34.38, 28.80, 55.98 and 61.88 CNY/t DS, respectively. The combined sludge conditioning of US-CPAM-RH presented a notable improvement of sludge dewatering performance with a low cost than other sludge conditioning processes [56, 57]. Furthermore, compared with the combined Fenton conditioning, the combined US-CPAM-RH conditioning in this study did not need to re-adjust the sludge pH for further sludge treatment and disposal, saving other extra cost. Therefore, the combined US-CPAM-RH conditioning is an economic and promising sludge conditioning technology.
24
Table 3. Economic analysis of different sludge conditioning methods. Filter press condition
Moisture content
Energy
Material
of sludge cake
consumption
addition
Conditioning
References Total cost
method
(CNY/t DS) (%)
(CNY/t DS)
(CNY/t DS)
Standard Buchner funnel filter: US
with a vacuum pressure of 0.051
In this study 88.7
34.38
34.38
MPa Standard Buchner funnel filter: CPAM
with a vacuum pressure of 0.051
In this study 85.2
28.80
28.80
MPa Standard Buchner funnel filter: US-CPAM
In this study 79.9
34.38
with a vacuum pressure of 0.051
25
21.60
55.98
MPa Standard Buchner funnel filter: US-CPAM-RH
with a vacuum pressure of 0.051
In this study 62.2
34.38
27.50
61.88
MPa Filter press: 40-min pressing phase
[56]
with a pressure of 0.6–0.9 MPa Fenton-lime-Portl and a 5-min filter cloth expanding
699.00
46.5
and cement phase with a pressure of 1.0–1.1 MPa Diaphragm filter press: 40-min Fenton-Red mud
feeding pressing phase with a
[57] 302.72
47.3
pressure of 0.8 MPa and 5-min
26
diaphragm pressing phase with a pressure of 1.1 MPa
27
4. Conclusion In order to improve the sludge dewatering performances, a combined US-CPAM-RH sludge conditioning was applied in this study. After the combined US-CPAM-RH sludge conditioning, the SRF, TTF and moisture content of sludge filter cake significantly reduced by 90.50 %, 72.78 % and 32.31 %, respectively; and the filtering rate increased by more than 73.61 %, compared with that of raw sludge. During the combined US-CPAM-RH sludge conditioning, the sludge flocs were mildly disintegrated by ultrasonic cavitation, leading to an increase of protein polysaccharide concentration of 79.26 % and 292.11 %, respectively; the sludge floc matrix was re-aggregated by CPAM flocculation and RH introduction, resulting in an increase of mean diameter d(0.5) of sludge flocs of 47.48 %; the RH introduction further changed the sludge filter cake to form a permeable and rigid skeleton structure to improve the water release, the porosity of sludge filter cake increased by 83.53 %, and the compressibility of sludge filter cake reduced by 28.92 % compared with that of raw sludge. These changes of sludge flocs and sludge filter cake took a significant positive effect in improving the sludge dewatering performances.
Acknowledgment Authors thank financial supports from National Natural Science Foundation of China (511780479, 51578068).
References 28
[1] J. Houghton, J. Quarmby, T. Stephenson, Municipal wastewater sludge dewaterability and the presence of microbial extracellular polymer, Water Science & Technology, 44 (2001) 373-379. [2] X. Yin, P. Han, X. Lu, Y. Wang, A review on the dewaterability of bio-sludge and ultrasound pretreatment, Ultrasonics Sonochemistry, 11 (2004) 337-348. [3] A. Elliott, T. Mahmood, Pretreatment technologies for advancing anaerobic digestion of pulp and paper biotreatment residues, Water Research, 41 (2007) 4273-4286. [4] G. Sheng, H. Yu, X. Li, Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: a review, Biotechnology Advances, 28 (2010) 882-894. [5] Y. Qi, K.B. Thapa, A.F.A. Hoadley, Application of filtration aids for improving sludge dewatering properties – A review, Chemical Engineering Journal, 171 (2011) 373-384. [6] J. Benítez, A. Rodríguez, A. Suárez, Optimization technique for sewage sludge conditioning with polymer and skeleton builders, Water Research, 28 (1994) 2067-2073. [7] K.B. Thapa, Y. Qi, A.F.A. Hoadley, Interaction of polyelectrolyte with digested sewage sludge and lignite in sludge dewatering, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 334 (2009) 66-73. [8] H. Yuan, X. Cheng, S. Chen, N. Zhu, Z. Zhou, New sludge pretreatment method to improve dewaterability of waste activated sludge, Bioresource Technology, 102 (2011) 5659-5664. [9] W. Zhang, B. Cao, D. Wang, T. Ma, H. Xia, D. Yu, Influence of wastewater sludge treatment using combined peroxyacetic acid oxidation and inorganic coagulants re-flocculation on characteristics of extracellular polymeric substances (EPS), Water Research, 88 (2016) 728-739. [10] Z. Chen, W. Zhang, D. Wang, T. Ma, R. Bai, D. Yu, Enhancement of waste activated sludge dewaterability using calcium peroxide pre-oxidation and chemical re-flocculation, Water Research, 103 (2016) 170-181. [11] S. Agarwal, M. Abu-Orf, J.T. Novak, Sequential polymer dosing for effective dewatering of ATAD sludges, Water Research, 39 (2005) 1301-1310. [12] P.B. Sørensen, J.A. Hansen, Extreme solid compressibility in biological sludge dewatering, Water Science & Technology, 28 (1993) 133-143. [13] K.B. Thapa, Y. Qi, S.A. Clayton, A.F. Hoadley, Lignite aided dewatering of digested sewage sludge, Water Research, 43 (2009) 623-634. [14] C. Liu, L. Lai, X. Yang, Sewage sludge conditioning by Fe (II)‐activated persulphate oxidation combined with skeleton builders for enhancing dewaterability, Water and Environment Journal, 30 (2016) 96-101. [15] C. Chen, P. Zhang, G. Zeng, J. Deng, Y. Zhou, H. Lu, Sewage sludge conditioning with coal fly ash modified by sulfuric acid, Chemical Engineering Journal, 158 (2010) 616-622. [16] J. Zall, N. Galil, M. Rehbun, Skeleton builders for conditioning oily sludge, Journal (Water Pollution Control Federation), (1987) 699-706. [17] Y. Zhao, Enhancement of alum sludge dewatering capacity by using gypsum as skeleton builder, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 211 (2002) 205-212. [18] Y. Qi, K.B. Thapa, A.F. Hoadley, Benefit of lignite as a filter aid for dewatering of digested sewage sludge demonstrated in pilot scale trials, Chemical Engineering Journal, 166 (2011) 504-510. [19] X. Ning, H. Luo, X. Liang, M. Lin, X. Liang, Effects of tannery sludge incineration slag pretreatment on sludge dewaterability, Chemical Engineering Journal, 221 (2013) 1-7. [20] Y. Shi, J. Yang, W. Yu, S. Zhang, S. Liang, J. Song, Q. Xu, N. Ye, S. He, C. Yang, Synergetic conditioning of sewage sludge via Fe 2+/persulfate and skeleton builder: Effect on sludge 29
characteristics and dewaterability, Chemical Engineering Journal, 270 (2015) 572-581. [21] A. Tiehm, K. Nickel, M. Zellhorn, U. Neis, Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization, Water Research, 35 (2001) 2003-2009. [22] P. Kampas, S.A. Parsons, P. Pearce, S. Ledoux, P. Vale, J. Churchley, E. Cartmell, Mechanical sludge disintegration for the production of carbon source for biological nutrient removal, Water Research, 41 (2007) 1734-1742. [23] K. Xiao, Y. Chen, X. Jiang, Q. Yang, W.Y. Seow, W. Zhu, Y. Zhou, Variations in physical, chemical and biological properties in relation to sludge dewaterability under Fe (II)–Oxone conditioning, Water Research, 109 (2017) 13-23. [24] Z. Chen, W. Zhang, D. Wang, T. Ma, R. Bai, Enhancement of activated sludge dewatering performance by combined composite enzymatic lysis and chemical re-flocculation with inorganic coagulants: kinetics of enzymatic reaction and re-flocculation morphology, Water Research, 83 (2015) 367-376. [25] A. Guellil, M. Boualam, H. Quiquampoix, P. Ginestet, J. Audic, J. Block, Hydrolysis of wastewater colloidal organic matter by extracellular enzymes extracted from activated sludge flocs, Water Science and Technology, 43 (2001) 33-40. [26] H. Li, Y. Jin, B. M. Rasool, Z. Wang, Y. Nie, Effects of ultrasonic disintegration on sludge microbial activity and dewaterability, Journal of Hazardous Materials, 161 (2009) 1421-1426. [27] X. Feng, J. Deng, H. Lei, T. Bai, Q. Fan, Z. Li, Dewaterability of waste activated sludge with ultrasound conditioning, Bioresource Technology, 100 (2009) 1074-1081. [28] G. Zhang, T. Wan, Sludge conditioning by sonication and sonication-chemical methods, Procedia Environmental Sciences, 16 (2012) 368-377. [29] Y. Chen, H. Yang, G. Gu, Effect of acid and surfactant treatment on activated sludge dewatering and settling, Water Research, 35 (2001) 2615-2620. [30] E. Hosnani, M. Nosrati, S. Shojasadati, Role of extracellular polymeric substances in dewaterability of untreated, sonicated and digested waste activated sludge, Iranian Journal of Environmental Health Science & Engineering, 7 (2010) 395. [31] Q. Chen, Y. Wang, Influence of single-and dual-flocculant conditioning on the geometric morphology and internal structure of activated sludge, Powder Technology, 270 (2015) 1-9. [32] Y. Qi, K.B. Thapa, A.F. Hoadley, Application of filtration aids for improving sludge dewatering properties–a review, Chemical Engineering Journal, 171 (2011) 373-384. [33] M. Ruiz-Hernando, E. Cabanillas, J. Labanda, J. Llorens, Ultrasound, thermal and alkali treatments affect extracellular polymeric substances (EPSs) and improve waste activated sludge dewatering, Process Biochemistry, 50 (2015) 438-446. [34] X. Li, H. Zheng, B. Gao, Y. Sun, X. Tang, B. Xu, Optimized preparation of micro-block CPAM by response surface methodology and evaluation of dewatering performance, RSC Advances, 7 (2017) 208-217. [35] APHA, Standard methods for the examination of water and wastewater, American Public Health Association (APHA): Washington, DC, USA, (2005). [36] I.M. Lo, K.C. Lai, G. Chen, Salinity effect on mechanical dewatering of sludge with and without chemical conditioning, Environmental Science & Technology, 35 (2001) 4691-4696. [37] B. Frølund, R. Palmgren, K. Keiding, P.H. Nielsen, Extraction of extracellular polymers from activated sludge using a cation exchange resin, Water Research, 30 (1996) 1749-1758. [38] M. Dubois, K.A. Gilles, J.K. Hamilton, P. Rebers, F. Smith, Colorimetric method for 30
determination of sugars and related substances, Analytical Chemistry, 28 (1956) 350-356. [39] E.R. de Sarabia, Franco, J.A. Gallego Juárez, G. Rodríguez Corral, L. Elvira Segura, I. González Gómez, Application of high-power ultrasound to enhance fluid/solid particle separation processes, Ultrasonics, 38 (2000) 642-646. [40] S. Pilli, P. Bhunia, S. Yan, R. LeBlanc, R. Tyagi, R. Surampalli, Ultrasonic pretreatment of sludge: a review, Ultrasonics Sonochemistry, 18 (2011) 1-18. [41] J. Wang, C. Chen, Q. Gao, T. Li, F. Zhu, Relationship between the characteristics of cationic polyacrylamide and sewage sludge dewatering performance in a full-scale plant, Procedia Environmental Sciences, 16 (2012) 409-417. [42] L. Lu, Z. Pan, N. Hao, W. Peng, A novel acrylamide-free flocculant and its application for sludge dewatering, Water Research, 57 (2014) 304-312. [43] H. Luo, X.A. Ning, X. Liang, Y. Feng, J. Liu, Effects of sawdust-CPAM on textile dyeing sludge dewaterability and filter cake properties, Bioresource Technology, 139 (2013) 330-336. [44] Y. Wu, P. Zhang, G. Zeng, J. Ye, H. Zhang, W. Fang, J. Liu, Enhancing sewage sludge dewaterability by a skeleton builder: biochar produced from sludge cake conditioned with rice husk flour and FeCl3, ACS Sustainable Chemistry & Engineering, 4 (2016) 5711-5717. [45] M.-F. Dignac, V. Urbain, D. Rybacki, A. Bruchet, D. Snidaro, P. Scribe, Chemical description of extracellular polymers: implication on activated sludge floc structure, Water Science and Technology, 38 (1998) 45-53. [46] B. Jin, B.-M. Wilén, P. Lant, Impacts of morphological, physical and chemical properties of sludge flocs on dewaterability of activated sludge, Chemical Engineering Journal, 98 (2004) 115-126. [47] S.-K. Cho, H.-S. Shin, D.-H. Kim, Waste activated sludge hydrolysis during ultrasonication: two-step disintegration, Bioresource Technology, 121 (2012) 480-483. [48] F. Wang, S. Lu, M. Ji, Components of released liquid from ultrasonic waste activated sludge disintegration, Ultrasonics Sonochemistry, 13 (2006) 334-338. [49] C. Bergins, J. Hulston, K. Strauss, A.L. Chaffee, Mechanical/thermal dewatering of lignite. Part 3: Physical properties and pore structure of MTE product coals, Fuel, 86 (2007) 3-16. [50] R. Buscall, L.R. White, The consolidation of concentrated suspensions. Part 1.—The theory of sedimentation, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 83 (1987) 873-891. [51] J.T. Novak, M.L. Agerbæk, B.L. Sørensen, Hansen, J. Aa, Conditioning, filtering, and expressing waste activated sludge, Journal of Environmental Engineering, 125 (1999) 816-824. [52] N. Soltani, A. Bahrami, M. Pech-Canul, L. González, Review on the physicochemical treatments of rice husk for production of advanced materials, Chemical Engineering Journal, 264 (2015) 899-935. [53] Y. Wu, P. Zhang, H. Zhang, G. Zeng, J. Liu, J. Ye, W. Fang, X. Gou, Possibility of sludge conditioning and dewatering with rice husk biochar modified by ferric chloride, Bioresource Technology, 205 (2016) 258-263. [54] Y. Wu, P. Zhang, G. Zeng, J. Liu, J. Ye, H. Zhang, W. Fang, Y. Li, Y. Fang, Combined sludge conditioning of micro-disintegration, floc reconstruction and skeleton building (KMnO 4/FeCl 3/Biochar) for enhancement of waste activated sludge dewaterability, Journal of the Taiwan Institute of Chemical Engineers, 74 (2017) 121-128. [55] H. Liu, J. Yang, N. Zhu, H. Zhang, Y. Li, S. He, C. Yang, H. Yao, A comprehensive insight into the combined effects of Fenton's reagent and skeleton builders on sludge deep dewatering performance, Journal of Hazardous Materials, 258-259 (2013) 144-150. 31
[56] H. Zhang, J. Yang, W. Yu, S. Luo, L. Peng, X. Shen, Y. Shi, S. Zhang, J. Song, N. Ye, Y. Li, C. Yang, S. Liang, Mechanism of red mud combined with Fenton's reagent in sewage sludge conditioning, Water Research, 59 (2014) 239-247. [57] H. Liu, J. Yang, Y. Shi, Y. Li, S. He, C. Yang, H. Yao, Conditioning of sewage sludge by Fenton's reagent combined with skeleton builders, Chemosphere, 88 (2012) 235-239.
32
Highlights
Ultrasound (US) mildly disintegrated sludge matrix and released trapped water
Cationic polyacrylamide (CPAM) reconstructed sludge aggregates through flocculation
Rice husk (RH) particles helped form rigid skeleton structure of sludge filter cake
US-CPAM-RH co-conditioning effectively improved sludge dewaterability
33
S1350-4177(17)30332-2 http://dx.doi.org/10.1016/j.ultsonch.2017.07.028 ULTSON 3783
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
18 January 2017 18 July 2017 21 July 2017
Please cite this article as: C. Zhu, P. Zhang, H. Wang, J. Ye, Conditioning of sewage sludge via combined ultrasonication-flocculation-skeleton building to improve sludge dewaterability, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/10.1016/j.ultsonch.2017.07.028
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Conditioning of sewage sludge via combined ultrasonication-flocculation-skeleton building to improve sludge dewaterability
Cheng Zhu1, Panyue Zhang1,*, Hongjie Wang, Jie Ye2,* 1. College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China 2. College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, China
E-mail address of all authors: [email protected], [email protected], [email protected], [email protected] *Corresponding author: Panyue Zhang, Jie Ye Tel.: +86 15001255497; Fax: +86 10 62336900. E-mail address: [email protected] (Panyue Zhang); [email protected] (Jie Ye) Complete postal address: College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China
Abstract Strong water trapping ability of polyelectrolyte flocculated sludge and high compressibility of sludge filter cake greatly hindered the further improvement of sludge dewaterability. Ultrasound (US) was used to strip the water from the sludge flocs through sludge disintegration, traditional cationic polyacrylamide (CPAM) was 1
applied to reconstruct the sludge aggregates through flocculation, and rice husk (RH) was utilized to improve the permeability of sludge filter cake as skeleton builders in this paper. The feasibility of combined US-CPAM-RH sludge conditioning was demonstrated, and single and co-conditioning processes were compared to explore the synergetic effect and mechanisms of US-CPAM-RH sludge conditioning. The results indicated that the sludge dewaterability was considerably improved by the combined US-CPAM-RH conditioning compared with that of raw sludge. The optimal ultrasonication condition was 0.3 W/ml for 12 s with an Ultrasound frequency of 22 kHz, and the RH and CPAM dosage was 50.0 wt. % and 20 mg/L, respectively, which resulted in a shortest time to filter (TTF) of 43 s and a lowest moisture content of sludge filter cake of 62.22%. The US mainly stripped the extracellular polymeric substances and water from the sludge flocs, leading to an obvious increase of concentration of proteins and polysaccharides in sludge filtrate. The CPAM and RH addition resulted in a remarkable sludge floc growth to a mean diameter d(0.5) of 128.85 µm after combined US-CPAM-RH conditioning. Moreover, scanning electron microscopy (SEM) results clearly showed that the combined US-CPAM-RH conditioning formed a more porous structure of sludge filter cake with a porosity increase of 98.80% and a compressibility decrease of 37.81 % compared with that of raw sludge. Thus, the US-CPAM-RH co-conditioning could significantly improve the sludge dewaterability. Keywords: sewage sludge; ultrasound; flocculation; skeleton builder; dewaterability.
2
1. Introduction Activated sludge processes are paramount technologies for treating municipal wastewater [1], but these biological processes generate large amounts of sewage sludge, which is difficult to be dewatered and disposed [2, 3]. The critical issues that prevent sewage sludge from mechanical dewatering are the highly hydrophilic nature of extracellular polymeric substances (EPS), which bind a large amount of water molecules to solid surfaces and trap the water within sludge flocs, forming a high compressibility of sludge matrix [4, 5]. Chemical conditioners, including inorganic coagulants and organic coagulants, are widely applied prior to mechanical dewatering process to improve the performances of sludge dewatering by particle-particle bridging and surface charge neutralization mechanisms [6-10]. Nevertheless, it is difficult to achieve higher solid content of filter cake due to the high sludge compressibility during the compression stage of sludge dewatering [11]. Sørensen et al. [12] investigated the compressibility of filter cake associated with polyelectrolyte conditioning of biological sludge, and found that the filter cake became more compressible and easier to be deformed under pressure during the compression phase of mechanical dewatering. This deformation of sludge cake can cause void closure, which further impedes the sludge dewatering [13]. Physical conditioners, often referred to as skeleton builders or filter aids, are effective to reduce the sludge compressibility, because they commonly help the sludge cake form a permeable and rigid structure and maintain the sludge cake porous, even 3
under a high compression pressure. Some inert minerals and carbon-based solid materials with high porosity and rigid structure have been investigated as skeleton builders, such as cement [14], coal fly ash [15], lime [16], wood chips and wheat dregs [6], gypsum [17], lignite [18], tannery sludge incineration slag [19], and thermal-pretreated phosphogypsum [20]. Rice husk (RH) is a major category of agricultural residues generated in many countries, and need to be properly treated and disposed to avoid sequent environmental concerns. With the advantages of high porosity, light weight, high external surface area and rigid structure, the RH is a promising candidate of physical conditioner for sludge dewatering. However, it is still a great challenge to enhance the sludge dewatering under co-conditioning by chemical and physical conditioner, because strong water affinity of EPS greatly hinders the water from being drained through water passages in sludge filter cake. A number of sludge disintegration methods have been developed, including physical [21], chemical [9, 10, 20, 22-24] and biological disintegration [24, 25], to release the organic matters from solid phase to liquid phase, further enhancing the sludge anaerobic digestion. Simultaneously, the sludge disintegration breaks the floc matrix and releases the sludge water. Ultrasonication is a well known method to disintegrate the sludge through disrupting sludge flocs and lysing microbial cell wall, but the sludge dewaterability may decrease when the applied ultrasonic energy is improper. Li et al. [26] used ultrasound to disintegrate the sludge, and only when sludge disintegration degree was 2-5%, the sludge dewaterability was improved with the co-conditioning of FeCl3. Feng et al. [27] found that the sludge dewaterability was 4
improved after application of a low specific energy dosage (<4400 kJ/kg TS), but significantly dropped when the ultrasonication dosage was higher than 4400 kJ/kg TS. Zhang et al. [28] found that only when the energy input was lower than 1200 kJ/kg DS, the sludge dewaterability was improved, and the sludge dewatering performance was deteriorated when energy input exceeded 1200 kJ/kg DS. Chu et al. [29] found that sonication did not improve the sludge dewaterability because of the obvious increase of bound water content. After studying the relationship between the protein and carbohydrate content in untreated and sonicated waste activated sludge, Hosnani et al. [30] found that sonication generally made the sludge dewatering more difficult due to the increase of protein and carbohydrate as a result of sludge sonication. Cationic polyacrylamide (CPAM) conditioning [31], skeleton builder addition [32], and US disintegration [33] has been separately reported to be able to limitedly improve the sludge dewatering performances. The separate application of either sludge conditioning method greatly has no potential to further improve the sludge dewatering performances. The synergetic effect of these three sludge conditioning methods has not been investigated previously. The goal of this study was to explore the feasibility of US-CPAM-RH conditioning to enhance the sludge dewaterability, to analyze the mechanisms of combined sludge conditioning by US-CPAM-RH, and to optimize the operating conditions for US-CPAM-RH co-conditioning.
2. Materials and methods 2.1. Materials 5
Raw sewage sludge was collected from the thickening tank of a local wastewater treatment plant in Beijing, China. In this wastewater treatment plant the anaerobic-anoxic-oxic process is applied, and the wastewater flow rate is approximately 1.0 × 106 m3/d. The sludge was firstly thickened to a solid content of about 2.5 %, and then transferred to the laboratory as soon as possible. The sludge was stored at 4 ℃ and heated to room temperature before the experiments. All the experiments were conducted in 48 h. The average characteristics of sludge sample were as follows: moisture content of 97.5%, pH of 6.72, specific resistance of filtration (SRF) of 1.84×109 s2/g. Raw RH was collected from Hunan Province, China. The RH was firstly ground into powders of -110 mesh and dried at 110
for 12 h. The RH averagely included
hemicelluloses 24.3 %, cellulose 34.4 %, lignin 19.2 %, ash 18.85 %, and the others 3.25 %. CPAM with a molecular weight of (1.2-1.5)×10 8 and a charge density of 25%-35% was used as the sludge chemical conditioner in this study. The CPAM used was a product of BASF Company, which was obtained from the same local wastewater treatment plant. The CPAM solution of 1000 mg/L was prepared with distilled water and aged for 6-8 h before usage. 2.2. Sludge conditioning and dewatering The sludge conditioning experiments were conducted with a six-paddle stirrers, which can be programmed. The US-CPAM-RH conditioning process was conducted as follows. First, the sludge was ultrasonicated at 22 kHz by an US generator 6
equipped with a probe transducer (SCIENTZ-IID, Xinzhi Inc., China), then the CPAM solution was added to 400 ml ultrosonicated sludge in a 500 ml beaker. The sludge system was stirred at 300 r/min for rapid mixing of 30 s followed by slow mixing of 180 s at 80 r/min. Afterwards, the RH powder was added into the sludge system, which was further stirred at 300 r/min for rapid mixing of 30 s and at 80 r/min for slow mixing of 180 s. After conditioning, 100 ml conditioned sludge sample was poured into 9 cm standard Buchner funnel with pre-wetted 0.45 µm filter paper, and then a constant vacuum pressure of 0.051 MPa [34] was applied until no filtrate came out [5]. The filtrate volume was recorded and the sludge cake moisture content was analyzed for SRF determination. The sludge cake was analyzed by mercury intrusion porosimetry, compressibility test, and scanning electron microscopy (SEM) observation. 2.3. Analytical methods The sludge filter cake was dried at 105
for 24 h and weighed by difference to
determine the moisture content [35]. The sludge SRF was measured using the method described by Qi et al. [5]. TTF is defined as the time when the volume of sludge filtrate reaches the half of sludge volume, and the TTF determination was performed by the method described by Lo [36]. The sludge filtering rate was tested by the method described by Chen [15]. The sludge floc size was measured by a Malvern mastersizer ZS (Malvern Mastersizer 2000, UK). The protein concentration in sludge filtrate was tested by the modified Lowry method using bovine serum albumin as the standard [37], and the 7
polysaccharide concentration in sludge filtrate was analyzed with the anthrone method by using glucose as the standard [38]. The compressibility of sludge filter cake was tested by the method described by Zhao [17]. The sludge samples were tested under five pressures ranging from 0.1-0.5 MPa. The porosity of sludge filter cake was analyzed by a mercury intrusion porosimetry (AutoPore IV 9500, China). The sludge filter cake was dried at 45 ℃ for 24 h before the porosimetry test. The surface structure of sludge filter cake was observed by a SEM (HITACHI S-3400 N, Japan). Each test was performed in triplicate, and the results were reported as average value.
3 Result and discussions 3.1. Dewaterability of sewage sludge conditioned by US-CPAM-RH Sewage sludge was conditioned by different methods: RH (-110 mesh) conditioning with a dosage of 50 wt.%; US conditioning with an ultrasonication density of 0.3 W/ml and an ultrasonication time of 12 s; CPAM conditioning with a CPAM dosage of 30 mg/L; US-CPAM conditioning with an ultrasonication density of 0.3 W/ml, an ultrasonication time of 12 s and a CPAM dosage of 30 mg/L; US-CPAM-RH conditioning with an ultrasonication density of 0.3 W/ml, an ultrasonication time of 12 s, a CPAM dosage of 30 mg/L and a RH (-110 mesh) dosage of 50 wt.%. The corresponding sludge filtering rate profiles are presented in Fig. 1a. All 8
profiles showed a rapid water filtration stage followed by a gentle increase stage. The rapid water filtration stage accorded well with first-order kinetics equation, and the rate constant K is listed in Table 1. The raw sludge showed a lowest water filtration and the slowest filtering rate with K=0.5176. Moreover, the sludge filtering rate profile for raw sludge did not show clear critical point during the whole vacuum filtering process. Meanwhile, the raw sludge showed the highest SRF of 1.39×109 s2/g and longest TTF of 158 s (see Fig. 1b). All single US, CPAM and RH conditioning enhanced the sludge filtration process, resulting in an increase of rate constant K. Correspondingly, the sludge SRF and TTF decreased. The RH conditioning only slightly improved the sludge dewaterability with a filtering rate constant K of 0.5744, and the SRF and TTF of RH conditioning only decreased by 17.27 % and 8.23 %, respectively, compared with that of raw sludge. The US irradiation induced a series of ultrasonic effects including acoustic streaming, local heating, interface instabilities, agitation, sponge effect and cavitation effect. Theses intense physico-chemical reactions disintegrated the sludge matrix and released the water trapped among the sludge flocs [26, 39], leading to a filtering rate constant K of 0.6419. The special macromolecular structure of CPAM could wrap around the sludge colloids into larger and denser flocs through charge neutralization and inter-particle bridging mechanisms, so that the free water and partial interstitial water was more easily released [30, 40-42]. However, the water trapped in the sludge matrix still hindered the improvement of sludge dewaterability due to the high compressibility of sludge filter cake, and the filtering rate constant K increased to 0.7301. The combined US-CPAM 9
conditioning showed a synergistic effect of US and CPAM, leading to a further increase of rate constant K to 0.8379; meanwhile, the sludge SRF and TTF further decreased. The highest filtering rate (K=0.8986) and the highest water removal (34%) were observed for combined US-CPAM-RH conditioning. It took only approximately 3.5 min to reach the equilibrium. The SRF and TTF also reduced to 1.32×108 s2/g and 43 s, respectively, which respectively decreased by 90.50% and 72.78% compared with that of raw sludge. These results were probably attributed to the water passage formation in sludge filter cake and the compressibility reduction of sludge filter cake by introducing RH particles as skeleton builders. Many researchers found similar results about the effect of skeleton builder in sludge conditioning. Ying et al. [18] showed that the time required to same water removal was much less for adding lignite as filtering aid than that only conditioned by polyelectrolyte. Luo et al. [43] found that the sludge co-conditioned by CPAM and sawdust showed obvious filtering rate advantage over the sludge conditioned by single CPAM. Shi et al. [20] observed that a large number of column-shaped skeleton builder filled in the sludge matrix and increased the porosity of the sludge filter cake. Wu et al. [44] found that biochar produced from sludge cake conditioned with RH flour and FeCl3 significantly improved the sludge dewatering performances.
10
Fig. 1. Effect of sludge conditioning on sludge filtration properties (a) Filtering rate, (b) sludge SRF and TTF. Table 1. Rate constant (K) of filtration process under different conditioning methods. 11
Conditioning method
First-order rate constant (K)
Raw sludge
0.5176
RH
0.5744
US
0.6419
CPAM
0.7301
US-CPAM
0.8379
US-CPAM-RH
0.8986
The moisture content of sludge cake conditioned by different methods is shown in Fig. 2. The raw sludge showed poor dewaterability, and the moisture content of sludge filter cake was 91%. For RH conditioning the moisture content of sludge filter cake was 90.55 %, showing only a slight improvement of sludge dewaterability. Single physical conditioner usually could not function as filter aids to reach the same dewatering extent of sludge conditioned with chemical conditioners [32]. After US disintegration, the sludge dewaterability slightly increased because a portion of water could be stripped from the sludge flocs. However, the sludge flocs might become smaller to form more compact sludge filter cake during sludge filtration. The CPAM improved the aggregation of sludge flocs, resulting in the improvement of sludge dewaterability, which is the widely used method currently. The water content of sludge filter cake conditioned by US-CPAM further decreased to 79.91 %, showing a synergistic effect of US or CPAM that the smaller sludge particles stripped from water by US were re-aggregated by the flocculation effect of CPAM. After adding the RH, the moisture content of sludge filter cake significantly dropped to 62.15 %. The 12
improved sludge dewaterability could be further improved by the formation of permeable and rigid skeleton structure of sludge filter cake with a large amount of rigid voids and pores.
Fig. 2. Effect of sludge conditioning on moisture content of sludge cake. 3.2 Mechanisms of sludge conditioning by US-CPAM-RH 3.2.1 Sludge filtrate As the main components of sludge EPS, proteins and polysaccharides have been proven to be the key factors during mechanical dewatering process [45, 46]. The concentration of proteins and polysaccharides in sludge filtrate is presented in Fig. 3. The concentration of proteins and polysaccharides in sludge filtrate of raw sludge was 13.5 and 3.8 mg/L, respectively. The US irradiation resulted in a significant increase of protein and polysaccharide concentration in sludge filtrate, and the concentration of 13
proteins and polysaccharides increased by 73.83 % and 270.18 % compared with that of raw sludge. These results illustrated that the sludge flocs even microorganism cells were disrupted by ultrasonication, and the EPS even the intracellular materials were released from the sludge flocs to sludge liquid. Meanwhile, more internal water among the sludge matrix and inside the bacteria might be released to sludge liquid. Feng et al. [27] reported that a strong quadratic relationship (R=0.9854 and R=0.9972) between the concentration of proteins and polysaccharides and the ultrasonic energy input was found. Si-Kyung et al. [47] observed that the concentration of bound EPS-proteins and carbohydrates decreased during US conditioning, indicating the destruction of sludge flocs increased with increasing the US irradiation energy input. Wang et al. [48] found that the concentration of proteins and polysaccharides increased with the increase of ultrasonication time. However, the water removal in Fig. 1a for single US conditioning was slightly lower than that for single CPAM conditioning, which might be attributed to the breakage of sludge flocs and corresponding compact structure of sludge filter cake. The other conditioning or combined conditioning did not cause obvious change of sludge filtrate. The addition of CPAM and RH could not obviously change the relationship between the internal water and sludge flocs, because the main mechanisms of CPAM and RH conditioning were charge neutralization and bridging, and the sludge floc structure did not change.
14
Fig. 3. Protein and polysaccharide concentration of sludge filtrate under different sludge conditioning processes. 3.2.2 Sludge flocs The sludge floc size distribution based on volume distribution during sludge conditioning is shown in Fig. 4. The mean diameter d(0.5) of raw sludge flocs was 87.37 µm. After US disintegration, the d(0.5) of sludge flocs was 52.63 µm with a reduction of 39.76 % because of the sludge disintegration. The d(0.5) of sludge flocs after CPAM conditioning increased 11.55 % to 97.46 µm compared with that of raw sludge, because the sludge particles were destabilized and agglomerated into a larger sludge matrix by the CPAM through mechanisms of charge neutralization and inter-particle bridging. When the US and CPAM conditioning were combined, the d(0.5) was only 90.20 µm, however it increased by 71.39% compared with that after US disintegration, showing the excellent flocculation efficiency. It was noting that the d(0.5) of sludge flocs after US-CPAM-RH conditioning significantly increased to 15
128.85 µm, which corresponded to an increment of 47.48 % compared with that of raw sludge. The introduction of RH could effectively strengthen the bridging effect between the sludge particles, which might be benefit for mechanical dewatering process [32].
Fig. 4. Particle size distribution of sludge flocs under different sludge conditioning processes. 3.2.3 Sludge filter cake The SEM image of sludge filter cake is shown in Fig. 5. The sludge filter cake after dewatering from raw sludge presented dense and compact without obviously voids or channels. The porosity of filter cake of raw sludge was 15.24 % as shown in Table 2. For US conditioning, some cracks and fractures in sludge filter cake were observed; however, the sludge filter cake seemed denser and more compact. Moreover, from Fig. 6 it can be seen that the porosity of filter cake after US conditioning became 16
lower because of the breakage of sludge flocs with a porosity of 14.44 %. For CPAM conditioning, water channels were obviously observed in Fig. 5c. The sludge CPAM conditioning greatly improved the sludge permeability through bridging the sludge particles together and forming microstructure with pores. However, the porosity of sludge filter cake after CPAM conditioning insignificantly changed as shown in Table 2. For the sludge conditioned by US-CPAM, the surface image of sludge filter cake was rough and full of voids and channels with a increased porosity of 16.51 %. For US-CPAM-RH conditioning, besides the rough surface, voids and huge cracks, the RH fiber was obviously observed (Fig. 5e). The porosity after combined US-CPAM-RH conditioning climbed to 30.30%, corresponding to an increment of 98.82 % than that of raw sludge. Not only the pore volume in the whole pore diameter range of 0-0.37 mm increased, but also there were two peaks around the diameter of 0.045 mm and 0.366 mm. Moreover, the pore volume was almost more than twice of that of raw sludge in the macro and meso pore range of 0.01-0.37 mm according to Thapa et al. [49] . The rigid structure of RH might result in these macro and meso pores along with the skeleton builders, and the link of these pores could create a number of water drainage passages in sludge filter cake. Permeability is the prime impact factor that determined the dewaterability of flocculated suspensions [50].
17
Fig. 5. SEM images of sludge filter cakes (a) Raw sludge, (b) After US conditioning, (c) After CPAM conditioning, (d) After US-CPAM conditioning, (e) After US-CPAM-RH conditioning.
Fig. 6. Porosity of sludge filter cake under different sludge conditioning methods. Table 2. Porosity of sludge filter cake under different conditioning methods. Conditioning method
Porosity
18
Raw sludge
15.24
US
14.44
CPAM
15.21
US-CPAM
16.51
US-CPAM-RH
30.30
During the compression filtration stage of sludge dewatering, the permeability of sludge filter cake was significantly influenced by the compressibility (S) of filter cake, which kept the water channels in filter cake from collapsing [51]. Fig. 7 exhibited the effect of sludge conditioning method on the compressibility of sludge filter cake. The compressibility of filter cake of raw sludge was 0.96. The US slightly decreased the compressibility of filter cake, which might be attributed the more hydrophobic structure of sludge particles. The compressibility of filter cake after CPAM conditioning reached 0.83, which decreased by 12.98 % than that of raw sludge. The bridging of CPAM could form the filter cake with bigger sludge aggregates, reducing the compressibility of sludge filter cake. The combination of US and CPAM further decreased the compressibility of sludge filter cake. However, the poor compressibility of sludge filter cake after CPAM conditioning and US-CPAM conditioning still hindered from the improvement of dewatering effectiveness. With the introduction of RH, the compressibility of filter cake after US-CPAM-RH conditioning significantly dropped to 0.60, corresponding to a reduction of 37.50 % compared to that of raw sludge. Introducing of RH effectively reduced the compressibility of filter cake through providing a rigid skeleton structure even under high mechanical compression 19
process [44, 52-54]. Simultaneously reducing the compressibility of sludge filter cake and increasing the permeability of filter cake led to much more water drainage from the channels and voids in filter cake.
Fig. 7. Compressibility of sludge filter cake under different sludge conditioning methods. 3.3 Optimization of combined sludge conditioning of US-CPAM-RH 3.3.1 Ultrasonication density and time Variations of the moisture content of US-CPAM-RH conditioned sludge cake with different ultrasonication densities are presented in Fig. 8. The effect of ultrasonication density on the sludge dewatering performances showed similar trend. The moisture content of sludge cake decreased with increasing the ultrasonication time, and at an ultrasonication time of 12 s the moisture content of sludge cake reached to a minimum, followed by a gradual increase with the further prolonging of 20
ultrasonication time. The ultrasonication time of 12 s was the suitable ultrasonication time. The moisture content of sludge cake decreased with the increase of ultrasonication density. However, the lowest moisture content insignificantly decreased when the ultrasonication density increased from 0.3 to 0.4 W/ml. So, the suitable ultrasonication density should be chosen as 0.3 W/ml to save the energy consumption. The moisture content of sludge filter cake at an ultrasonication density of 0.3 W/ml and an ultrasonication time of 12 s was 85.62 %.
Fig. 8. Effect of ultrasonication density and ultrasonication time on sludge dewatering by US-CPAM-RH conditioning (RH dosage of 10 wt.%, RH particle size of 60 mesh, and CPAM dosage of 10 mg/L). 3.3.2 CPAM dosage Fig. 9 shows that the effect of CPAM dosage on the moisture content of sludge filter cake. The moisture content of sludge filter cake gradually decreased with 21
increasing the CPAM dosage until a CPAM dosage of 30 mg/L, followed a slight increase of moisture content of sludge filter cake with the further increase of CPAM dosage. The minimum moisture content of sludge filter cake reached to 78.19 % at a CPAM dosage of 30 mg/L.
Fig. 9. Effect of CPAM dosage on sludge dewatering by US-CPAM-RH conditioning (RH dosage of 10 wt.%, RH particle size of 60 mesh, ultrasonication density of 0.3 W/ml, and ultrasonication time of 12 s). 3.3.3 RH particle size and dosage Fig. 10 shows that the effect of RH addition on the moisture content of sludge filter cake after US-CPAM-RH conditioning. The moisture content of sludge filter cake declined along with increasing the RH dosage, then the descent trends became insignificant when the RH dosage was over 50 wt.%. For the RH with a particle size -60, -80 and -100 mesh, the moisture content of sludge filter cake was 72.88%, 68.84% 22
and 62.22%, respectively. The bigger RH might better support the formation of voids and water channels in the sludge filter cake. So, the -110mesh RH and 50 wt.% RH dosage should be chosen as the suitable physical conditioner and the corresponding dosage.
Fig. 10. Effect of RH particle size and dosage on sludge dewatering by US-CPAM-RH conditioning (Ultrasonication density of 0.3 W/ml, ultrasonication time of 12 s, CPAM dosage of 30 mg/L). 3.4 Economic analysis of combined sludge conditioning of US-CPAM-RH Economic analysis is necessary for applying the new technology. A preliminary economic analysis of different conditioning methods was conducted by calculating the chemical (CPAM and RH) cost and US energy consumption in this study; moreover, the cost of combined US-CPAM-RH conditioning was compared with that of other similar sludge conditioning methods, as shown in Table 3 [55, 56]. The US energy 23
consumption was estimated according to industrial electricity cost (CNY/t dry solid (DS)). The industrial electricity price in Beijing city is 0.8595 CNY/kW· h, the price of CPAM is 18000 CNY/t, and the price of RH is 472 CNY/t. The total chemical and energy cost of sludge conditioning by US, CPAM, US-CPAM and US-CPAM-RH is 34.38, 28.80, 55.98 and 61.88 CNY/t DS, respectively. The combined sludge conditioning of US-CPAM-RH presented a notable improvement of sludge dewatering performance with a low cost than other sludge conditioning processes [56, 57]. Furthermore, compared with the combined Fenton conditioning, the combined US-CPAM-RH conditioning in this study did not need to re-adjust the sludge pH for further sludge treatment and disposal, saving other extra cost. Therefore, the combined US-CPAM-RH conditioning is an economic and promising sludge conditioning technology.
24
Table 3. Economic analysis of different sludge conditioning methods. Filter press condition
Moisture content
Energy
Material
of sludge cake
consumption
addition
Conditioning
References Total cost
method
(CNY/t DS) (%)
(CNY/t DS)
(CNY/t DS)
Standard Buchner funnel filter: US
with a vacuum pressure of 0.051
In this study 88.7
34.38
34.38
MPa Standard Buchner funnel filter: CPAM
with a vacuum pressure of 0.051
In this study 85.2
28.80
28.80
MPa Standard Buchner funnel filter: US-CPAM
In this study 79.9
34.38
with a vacuum pressure of 0.051
25
21.60
55.98
MPa Standard Buchner funnel filter: US-CPAM-RH
with a vacuum pressure of 0.051
In this study 62.2
34.38
27.50
61.88
MPa Filter press: 40-min pressing phase
[56]
with a pressure of 0.6–0.9 MPa Fenton-lime-Portl and a 5-min filter cloth expanding
699.00
46.5
and cement phase with a pressure of 1.0–1.1 MPa Diaphragm filter press: 40-min Fenton-Red mud
feeding pressing phase with a
[57] 302.72
47.3
pressure of 0.8 MPa and 5-min
26
diaphragm pressing phase with a pressure of 1.1 MPa
27
4. Conclusion In order to improve the sludge dewatering performances, a combined US-CPAM-RH sludge conditioning was applied in this study. After the combined US-CPAM-RH sludge conditioning, the SRF, TTF and moisture content of sludge filter cake significantly reduced by 90.50 %, 72.78 % and 32.31 %, respectively; and the filtering rate increased by more than 73.61 %, compared with that of raw sludge. During the combined US-CPAM-RH sludge conditioning, the sludge flocs were mildly disintegrated by ultrasonic cavitation, leading to an increase of protein polysaccharide concentration of 79.26 % and 292.11 %, respectively; the sludge floc matrix was re-aggregated by CPAM flocculation and RH introduction, resulting in an increase of mean diameter d(0.5) of sludge flocs of 47.48 %; the RH introduction further changed the sludge filter cake to form a permeable and rigid skeleton structure to improve the water release, the porosity of sludge filter cake increased by 83.53 %, and the compressibility of sludge filter cake reduced by 28.92 % compared with that of raw sludge. These changes of sludge flocs and sludge filter cake took a significant positive effect in improving the sludge dewatering performances.
Acknowledgment Authors thank financial supports from National Natural Science Foundation of China (511780479, 51578068).
References 28
[1] J. Houghton, J. Quarmby, T. Stephenson, Municipal wastewater sludge dewaterability and the presence of microbial extracellular polymer, Water Science & Technology, 44 (2001) 373-379. [2] X. Yin, P. Han, X. Lu, Y. Wang, A review on the dewaterability of bio-sludge and ultrasound pretreatment, Ultrasonics Sonochemistry, 11 (2004) 337-348. [3] A. Elliott, T. Mahmood, Pretreatment technologies for advancing anaerobic digestion of pulp and paper biotreatment residues, Water Research, 41 (2007) 4273-4286. [4] G. Sheng, H. Yu, X. Li, Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: a review, Biotechnology Advances, 28 (2010) 882-894. [5] Y. Qi, K.B. Thapa, A.F.A. Hoadley, Application of filtration aids for improving sludge dewatering properties – A review, Chemical Engineering Journal, 171 (2011) 373-384. [6] J. Benítez, A. Rodríguez, A. Suárez, Optimization technique for sewage sludge conditioning with polymer and skeleton builders, Water Research, 28 (1994) 2067-2073. [7] K.B. Thapa, Y. Qi, A.F.A. Hoadley, Interaction of polyelectrolyte with digested sewage sludge and lignite in sludge dewatering, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 334 (2009) 66-73. [8] H. Yuan, X. Cheng, S. Chen, N. Zhu, Z. Zhou, New sludge pretreatment method to improve dewaterability of waste activated sludge, Bioresource Technology, 102 (2011) 5659-5664. [9] W. Zhang, B. Cao, D. Wang, T. Ma, H. Xia, D. Yu, Influence of wastewater sludge treatment using combined peroxyacetic acid oxidation and inorganic coagulants re-flocculation on characteristics of extracellular polymeric substances (EPS), Water Research, 88 (2016) 728-739. [10] Z. Chen, W. Zhang, D. Wang, T. Ma, R. Bai, D. Yu, Enhancement of waste activated sludge dewaterability using calcium peroxide pre-oxidation and chemical re-flocculation, Water Research, 103 (2016) 170-181. [11] S. Agarwal, M. Abu-Orf, J.T. Novak, Sequential polymer dosing for effective dewatering of ATAD sludges, Water Research, 39 (2005) 1301-1310. [12] P.B. Sørensen, J.A. Hansen, Extreme solid compressibility in biological sludge dewatering, Water Science & Technology, 28 (1993) 133-143. [13] K.B. Thapa, Y. Qi, S.A. Clayton, A.F. Hoadley, Lignite aided dewatering of digested sewage sludge, Water Research, 43 (2009) 623-634. [14] C. Liu, L. Lai, X. Yang, Sewage sludge conditioning by Fe (II)‐activated persulphate oxidation combined with skeleton builders for enhancing dewaterability, Water and Environment Journal, 30 (2016) 96-101. [15] C. Chen, P. Zhang, G. Zeng, J. Deng, Y. Zhou, H. Lu, Sewage sludge conditioning with coal fly ash modified by sulfuric acid, Chemical Engineering Journal, 158 (2010) 616-622. [16] J. Zall, N. Galil, M. Rehbun, Skeleton builders for conditioning oily sludge, Journal (Water Pollution Control Federation), (1987) 699-706. [17] Y. Zhao, Enhancement of alum sludge dewatering capacity by using gypsum as skeleton builder, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 211 (2002) 205-212. [18] Y. Qi, K.B. Thapa, A.F. Hoadley, Benefit of lignite as a filter aid for dewatering of digested sewage sludge demonstrated in pilot scale trials, Chemical Engineering Journal, 166 (2011) 504-510. [19] X. Ning, H. Luo, X. Liang, M. Lin, X. Liang, Effects of tannery sludge incineration slag pretreatment on sludge dewaterability, Chemical Engineering Journal, 221 (2013) 1-7. [20] Y. Shi, J. Yang, W. Yu, S. Zhang, S. Liang, J. Song, Q. Xu, N. Ye, S. He, C. Yang, Synergetic conditioning of sewage sludge via Fe 2+/persulfate and skeleton builder: Effect on sludge 29
characteristics and dewaterability, Chemical Engineering Journal, 270 (2015) 572-581. [21] A. Tiehm, K. Nickel, M. Zellhorn, U. Neis, Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization, Water Research, 35 (2001) 2003-2009. [22] P. Kampas, S.A. Parsons, P. Pearce, S. Ledoux, P. Vale, J. Churchley, E. Cartmell, Mechanical sludge disintegration for the production of carbon source for biological nutrient removal, Water Research, 41 (2007) 1734-1742. [23] K. Xiao, Y. Chen, X. Jiang, Q. Yang, W.Y. Seow, W. Zhu, Y. Zhou, Variations in physical, chemical and biological properties in relation to sludge dewaterability under Fe (II)–Oxone conditioning, Water Research, 109 (2017) 13-23. [24] Z. Chen, W. Zhang, D. Wang, T. Ma, R. Bai, Enhancement of activated sludge dewatering performance by combined composite enzymatic lysis and chemical re-flocculation with inorganic coagulants: kinetics of enzymatic reaction and re-flocculation morphology, Water Research, 83 (2015) 367-376. [25] A. Guellil, M. Boualam, H. Quiquampoix, P. Ginestet, J. Audic, J. Block, Hydrolysis of wastewater colloidal organic matter by extracellular enzymes extracted from activated sludge flocs, Water Science and Technology, 43 (2001) 33-40. [26] H. Li, Y. Jin, B. M. Rasool, Z. Wang, Y. Nie, Effects of ultrasonic disintegration on sludge microbial activity and dewaterability, Journal of Hazardous Materials, 161 (2009) 1421-1426. [27] X. Feng, J. Deng, H. Lei, T. Bai, Q. Fan, Z. Li, Dewaterability of waste activated sludge with ultrasound conditioning, Bioresource Technology, 100 (2009) 1074-1081. [28] G. Zhang, T. Wan, Sludge conditioning by sonication and sonication-chemical methods, Procedia Environmental Sciences, 16 (2012) 368-377. [29] Y. Chen, H. Yang, G. Gu, Effect of acid and surfactant treatment on activated sludge dewatering and settling, Water Research, 35 (2001) 2615-2620. [30] E. Hosnani, M. Nosrati, S. Shojasadati, Role of extracellular polymeric substances in dewaterability of untreated, sonicated and digested waste activated sludge, Iranian Journal of Environmental Health Science & Engineering, 7 (2010) 395. [31] Q. Chen, Y. Wang, Influence of single-and dual-flocculant conditioning on the geometric morphology and internal structure of activated sludge, Powder Technology, 270 (2015) 1-9. [32] Y. Qi, K.B. Thapa, A.F. Hoadley, Application of filtration aids for improving sludge dewatering properties–a review, Chemical Engineering Journal, 171 (2011) 373-384. [33] M. Ruiz-Hernando, E. Cabanillas, J. Labanda, J. Llorens, Ultrasound, thermal and alkali treatments affect extracellular polymeric substances (EPSs) and improve waste activated sludge dewatering, Process Biochemistry, 50 (2015) 438-446. [34] X. Li, H. Zheng, B. Gao, Y. Sun, X. Tang, B. Xu, Optimized preparation of micro-block CPAM by response surface methodology and evaluation of dewatering performance, RSC Advances, 7 (2017) 208-217. [35] APHA, Standard methods for the examination of water and wastewater, American Public Health Association (APHA): Washington, DC, USA, (2005). [36] I.M. Lo, K.C. Lai, G. Chen, Salinity effect on mechanical dewatering of sludge with and without chemical conditioning, Environmental Science & Technology, 35 (2001) 4691-4696. [37] B. Frølund, R. Palmgren, K. Keiding, P.H. Nielsen, Extraction of extracellular polymers from activated sludge using a cation exchange resin, Water Research, 30 (1996) 1749-1758. [38] M. Dubois, K.A. Gilles, J.K. Hamilton, P. Rebers, F. Smith, Colorimetric method for 30
determination of sugars and related substances, Analytical Chemistry, 28 (1956) 350-356. [39] E.R. de Sarabia, Franco, J.A. Gallego Juárez, G. Rodríguez Corral, L. Elvira Segura, I. González Gómez, Application of high-power ultrasound to enhance fluid/solid particle separation processes, Ultrasonics, 38 (2000) 642-646. [40] S. Pilli, P. Bhunia, S. Yan, R. LeBlanc, R. Tyagi, R. Surampalli, Ultrasonic pretreatment of sludge: a review, Ultrasonics Sonochemistry, 18 (2011) 1-18. [41] J. Wang, C. Chen, Q. Gao, T. Li, F. Zhu, Relationship between the characteristics of cationic polyacrylamide and sewage sludge dewatering performance in a full-scale plant, Procedia Environmental Sciences, 16 (2012) 409-417. [42] L. Lu, Z. Pan, N. Hao, W. Peng, A novel acrylamide-free flocculant and its application for sludge dewatering, Water Research, 57 (2014) 304-312. [43] H. Luo, X.A. Ning, X. Liang, Y. Feng, J. Liu, Effects of sawdust-CPAM on textile dyeing sludge dewaterability and filter cake properties, Bioresource Technology, 139 (2013) 330-336. [44] Y. Wu, P. Zhang, G. Zeng, J. Ye, H. Zhang, W. Fang, J. Liu, Enhancing sewage sludge dewaterability by a skeleton builder: biochar produced from sludge cake conditioned with rice husk flour and FeCl3, ACS Sustainable Chemistry & Engineering, 4 (2016) 5711-5717. [45] M.-F. Dignac, V. Urbain, D. Rybacki, A. Bruchet, D. Snidaro, P. Scribe, Chemical description of extracellular polymers: implication on activated sludge floc structure, Water Science and Technology, 38 (1998) 45-53. [46] B. Jin, B.-M. Wilén, P. Lant, Impacts of morphological, physical and chemical properties of sludge flocs on dewaterability of activated sludge, Chemical Engineering Journal, 98 (2004) 115-126. [47] S.-K. Cho, H.-S. Shin, D.-H. Kim, Waste activated sludge hydrolysis during ultrasonication: two-step disintegration, Bioresource Technology, 121 (2012) 480-483. [48] F. Wang, S. Lu, M. Ji, Components of released liquid from ultrasonic waste activated sludge disintegration, Ultrasonics Sonochemistry, 13 (2006) 334-338. [49] C. Bergins, J. Hulston, K. Strauss, A.L. Chaffee, Mechanical/thermal dewatering of lignite. Part 3: Physical properties and pore structure of MTE product coals, Fuel, 86 (2007) 3-16. [50] R. Buscall, L.R. White, The consolidation of concentrated suspensions. Part 1.—The theory of sedimentation, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 83 (1987) 873-891. [51] J.T. Novak, M.L. Agerbæk, B.L. Sørensen, Hansen, J. Aa, Conditioning, filtering, and expressing waste activated sludge, Journal of Environmental Engineering, 125 (1999) 816-824. [52] N. Soltani, A. Bahrami, M. Pech-Canul, L. González, Review on the physicochemical treatments of rice husk for production of advanced materials, Chemical Engineering Journal, 264 (2015) 899-935. [53] Y. Wu, P. Zhang, H. Zhang, G. Zeng, J. Liu, J. Ye, W. Fang, X. Gou, Possibility of sludge conditioning and dewatering with rice husk biochar modified by ferric chloride, Bioresource Technology, 205 (2016) 258-263. [54] Y. Wu, P. Zhang, G. Zeng, J. Liu, J. Ye, H. Zhang, W. Fang, Y. Li, Y. Fang, Combined sludge conditioning of micro-disintegration, floc reconstruction and skeleton building (KMnO 4/FeCl 3/Biochar) for enhancement of waste activated sludge dewaterability, Journal of the Taiwan Institute of Chemical Engineers, 74 (2017) 121-128. [55] H. Liu, J. Yang, N. Zhu, H. Zhang, Y. Li, S. He, C. Yang, H. Yao, A comprehensive insight into the combined effects of Fenton's reagent and skeleton builders on sludge deep dewatering performance, Journal of Hazardous Materials, 258-259 (2013) 144-150. 31
[56] H. Zhang, J. Yang, W. Yu, S. Luo, L. Peng, X. Shen, Y. Shi, S. Zhang, J. Song, N. Ye, Y. Li, C. Yang, S. Liang, Mechanism of red mud combined with Fenton's reagent in sewage sludge conditioning, Water Research, 59 (2014) 239-247. [57] H. Liu, J. Yang, Y. Shi, Y. Li, S. He, C. Yang, H. Yao, Conditioning of sewage sludge by Fenton's reagent combined with skeleton builders, Chemosphere, 88 (2012) 235-239.
32
Highlights
Ultrasound (US) mildly disintegrated sludge matrix and released trapped water
Cationic polyacrylamide (CPAM) reconstructed sludge aggregates through flocculation
Rice husk (RH) particles helped form rigid skeleton structure of sludge filter cake
US-CPAM-RH co-conditioning effectively improved sludge dewaterability
33