Enhancement of the sludge disintegration and nutrients release by a treatment with potassium ferrate combined with an ultrasonic process

Enhancement of the sludge disintegration and nutrients release by a treatment with potassium ferrate combined with an ultrasonic process

Science of the Total Environment 635 (2018) 699–704 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www...

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Science of the Total Environment 635 (2018) 699–704

Contents lists available at ScienceDirect

Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Enhancement of the sludge disintegration and nutrients release by a treatment with potassium ferrate combined with an ultrasonic process Wei Li, Najiaowa Yu, Qian Liu, Yiran Li, Nanqi Ren, Defeng Xing ⁎ State Key Laboratory of Urban Water Resources and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• K2FeO4 combined with ultrasound process effectively disintegrated waste sludge. • Potassium ferrate offers more safer and cheaper than ozone. • The byproduct of potassium ferrate offered sludge superior flocculation and settleability.

a r t i c l e

i n f o

Article history: Received 12 January 2018 Received in revised form 6 March 2018 Accepted 12 April 2018 Available online xxxx Keywords: Sludge pretreatment Sludge biodegradability Potassium ferrate (K2FeO4) Ultrasound Degree of disintegration (DD) Waste activated sludge (WAS)

a b s t r a c t Sludge disintegration by ultrasound is a promising sludge treatment method. In order to enhance the efficiency of the sludge reduction and hydrolysis, potassium ferrate (K2FeO4) (PF) was used. A novel method was developed to improve the sludge disintegration-sludge pretreatment by using PF in combination with an ultrasonic treatment (PF + ULT). After a short-term PF + ULT treatment, 17.23% of the volatile suspended solids (VSS) were reduced after a 900-min reaction time, which is 61.3% higher than the VSS reduction for the raw sludge. The supernatant soluble chemical oxygen demand (SCOD), total nitrogen (TN), volatile fatty acids (VFAs), soluble protein and polysaccharides increased by 522.5%, 1029.4%, 878.4%, 2996.6% and 801.9%, respectively. The constituent parts of the dissolved organic matter of the sludge products were released efficiently, which demonstrated the positive effect caused by the PF + ULT. The enhanced sludge disintegration process further alleviates environmental risk and offers a more efficient and convenient method for utilizing sludge. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The costs of waste activated sludge (WAS) management has become a dominant environment problems in municipal wastewater treatment plants (WWTP) as the greatly production of WAS continues to increase ⁎ Corresponding author at: School of Environment, Harbin Institute of Technology, P.O. Box 2614, 73 Huanghe Road, Nangang District, Harbin, Heilongjiang Province 150090, China. E-mail address: [email protected]. (D. Xing).

https://doi.org/10.1016/j.scitotenv.2018.04.174 0048-9697/© 2018 Elsevier B.V. All rights reserved.

worldwide along with the increasing demand for urban environmental facilities and water treatment equipment (Kim et al., 2016). Moreover, the WAS treatment accounts for up to 60% of the wastewater treatment costs for sludge disposal (Low and Chase, 1999). The dewatering process is an effective treatment, reduces the sludge volume, and is promoted by the WAS disintegration (Kavitha et al., 2015). Sludge disintegration treatments have been used widely due to their advantages. These include (1) the reduction in the cost of sludge disposal, (2) the elimination of secondary sludge pollution, and (3) the enhancement of efficacy of subsequent processes for WAS recycling such as the

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production of biogas and nutrients including carbohydrates, proteins and volatile fatty acids (VFAs) (Yu et al., 2015). Because of the large disposal costs and the potential risks to the environment of the sludge, it is essential to develop an effective waste sludge predisposal technique that can accelerate the sewage hydrolysis process during anaerobic digestion (Liu et al., 2016); this is urgently needed for the greater amount of sludge production in the future. Cell-wall hydrolysis results in a massive release of all kinds of organic matters (Lu et al., 2016). When disintegration strategies are used for enhancing the hydrolysis, cells of microorganism are destroyed and the intracellular matter is released. The intracellular matter from the cells of the microorganism is utilized again, resulting in the reduction in the overall biomass yield (Chu et al., 2009). The increase in the extracellular polymeric substances (EPS) appears to have an important impact on the floc structure (Basuvaraj et al., 2015). The flocs' constituent variation may be partially responsible for the disintegration promotion (Yu et al., 2014). Many researchers studying disintegration treatments have devoted their efforts to develop potential sludge pretreatment technologies including oxidation(Dytczak et al., 2007), ultrasound pretreatments (Khanal et al., 2007), mechanical disintegration (Kampas et al., 2007), thermal hydrolysis (Bougrier et al., 2008), alkaline treatments (Dogan and Sanin, 2009), and the biological treatment (Ponsa et al., 2008). Furthermore, the increasing degradability results in a higher energy recovery and lower residual energy in the digested sludge. Proteins and polysaccharides and the main organic components in sludge, account for 75–90% of activated sludge (Tsuneda et al., 2003), and are very important for retaining the fundamental property of sludge flocks. Apparently, an improvement in the activated sludge destruction increases the release of proteins and polysaccharides, which then have to be treated. In addition, the potential recovery of nitrogen and phosphorus has been observed consistently (Wang et al., 2010). Potassium ferrate (PF) is a powerful oxidant reagent that can oxidize noxious substances and disintegrate sludge efficiently (He et al., 2009). It is worth noting that PF is an environmental protection reagent that is applied during the waste treatment process and does not produce harmful substances or polluting byproducts. It was observed that a PF pretreatment resulted in negative effects on the ability to filter of the sludge based on capillary suction time (CST) results; the sludge particles and viscosity were reduced. In addition, the settleability of the sludge was enhanced by 17% because a large amount of floc was resized into tight flocs depending on the production of flocculants, Fe(III) (Ye et al., 2012b; Ye et al., 2012a). It was demonstrated that PF (VI) was capable of removing 50% or more color (Vis400-abs) and 30% or more of chemical oxygen demand (COD) (Jiang et al., 2006) and the final biogas production increased by approximately 44% compared with the control (Wu et al., 2015). Moreover, the sludge dewatering was improved after a PF treatment regardless of pH, whereas the sludge conditioning efficiency was enhanced by decreasing the pH value (Zhang et al., 2016b). Specifically, a PF pretreatment at pH 3 is an effective method for enhancing the sludge dewatering ability (Zhang et al., 2012). Previous studies have mainly focused on the changes in the sludge shape and properties and the effects of different acidity/basicity conditions. Nevertheless, less is known about the release and recovery of nutrients during the reduction of waste sludge although this is important for understanding the internal mechanisms of sludge disintegration and for providing nutrients strategies. Therefore, in this study, an intensive pretreatment method using PF (K2FeO4) combined with an ultrasonic treatment (PF + ULT) is investigated, The objective of this study was to investigate (1) the performance of the PF + ULT co-treatment to disintegrate the WAS; (2) the influence of the PF + ULT cotreatment on the release of soluble proteins, and polysaccharides; (3) the influence of the PF + ULT co-treatment on the VFAs concentration and the pH; (4) the influence of the PF + ULT co-treatment on the concentrations of the total nitrogen (TN), NO3_N, and PO− 4 .

2. Materials and methods 2.1. WAS source The WAS used in this study was collected from the secondary sedimentation tank in the Wenchang WWTP in Harbin, China. After an absolute-rest precipitation for 24 h, the sludge supernatant was discarded and the remaining sludge was stored at 4 °C and was used as the raw WAS (R-WAS). The initial index values of the R-WAS and the sludge after co-pretreatment are shown in Table 1. 2.2. Pretreatment with PT + ULT Two liters of the R-WAS was divided evenly into four 500-ml silkmouth bottles, which were numbered 1–4. The sludge in bottle 1 was not pretreated and served as the blank control. The PF was added to bottles 2, 3, and 4 and then the sludge samples were treated ultrasonically using an Ultrasonic Cell Disrupter System (JY92-IIDN ultrasonic cell crusher, Ningbo Scientz Biotechnology Co., Ltd.). The sonication was conducted at frequencies of 25 kHz and the power output was 150 W. The probe diameter was 20 mm and the probe was immersed 150 mm into the sludge during disintegration. After disintegrated for 20 min, the bottles were placed in an incubator shaker with a speed of 180 rpm and were maintained at 35 °C during the entire test period. Ultra-high purity nitrogen gas was purged for 10 min into the bottles to remove the remained air to create an anaerobic environment. The sludge in each bottle was sampled at 0, 10, 20, 30, 40, 50, 60, 90, 120, 180, 240, 480, and 900 min after ultrasonic processing. All the experiments were performed in triplicate. 2.3. Analysis methods and calculation The total chemical oxygen demand (TCOD), soluble chemical oxygen demand (SCOD), total suspended solids (TSS), and volatile suspended solids (VSS) were determined using standard methods (American Public Health Association) (APHA, 1998). The sludge samples were first centrifuged and then filtered through a 0.45-μm sponge filter prior to determining the SCOD. Eq. (1) was used to determine the degree of SCOD disintegration, which indicates the effects of different kinds of pretreatments had on the solubilization of the particle substances. Disintegration degree ¼

SCODafter −SCOD0 TCOD0 −SCOD0

ð1Þ

where SCODafter is the SCOD of the pre-treated sludge at each sampling point, SCOD0 is the SCOD of the untreated sludge, and TCOD0 is the TCOD of the untreated sludge. The pH was measured using a Shang Hai Lei Ci PHS-2F type pH meter and the VFAs were analyzed using a gas chromatograph (GC4890, Agilent, America) (Lu et al., 2012). The polysaccharides and proteins were also examined in this experiment. The phenol sulfuric acid colorimetric method was used to test the carbohydrate concentration

Table 1 Characteristics of the buffered R-WAS and the pretreatment sludge. Parameter

0

0.5

1.0

1.5

Volatile suspended solids (VSS) (g L−1) Soluble chemical oxygen demand (SCOD) (mg/ L) Soluble protein (mg/ L) Soluble carbohydrate (mg/ L) TN (mg/ L) PO− 4 (mg/ L) NO3−-N (mg/ L) pH

9.38 280 34.9 44.7 75 380 15 6.66

9.16 2580 645.8 269.4 890 347 169 8.26

8.7 4210 1114.5 386.6 986 220 594 9.76

8.11 5290 1435.7 408.6 1062 196 806 10.6

0.5, 1.0, and 1.5 represent 0.5 g, 1.0 g, and 1.5 g potassium ferrate added to a sludge concentration of 1 g VSS.

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(Herbert et al., 1971) and a Bicinchoninic Acid Protein Assay Kit (Sigma– Aldrich) (Smith et al., 1985) was used to determine the protein concen− tration. Ion chromatography was used to determine the NO− 3 , NO2 , and PO− concentrations. A total organic carbon (TOC) analyzer was used to 4 measure the concentration of the TN. 3. Results 3.1. Performance of the PF + ULT co-treatment to disintegrate the WAS The PF + ULT co-treatment releases the sludge organics into the liquid phase and decreases the solid content of the WAS, which results in decrease in the TSS and VSS of the sludge (Zhang et al., 2009). The efficiency of the sludge disintegration can be evaluated by the rate of increase in the SCOD value in the WAS. During the process, there were significant increases in the degree of disintegration (DDSCOD) and the SCOD (Fig. 1(A)). As the dosage increased from 0.5 g/g VSS to 1.5 g/g VSS, the SCOD increased from 4950 mg/L to 6350 mg/L during the 180-min co-treatment. After that, the SCOD stabilized, which might be due to the balance of the sludge lysis and the oxidation of the soluble substances, suggesting that the period of disintegration is short. The increase in the VFAs, proteins, and polysaccharides in the supernatant result in SCOD growth (Zhang et al., 2016a). The SCOD of the raw sludge

Fig. 1. (A) SCOD of sludge at different PF concentration during the 900-min test. (B) Degree of disintegration of the PF dosages initially and after 900 min. The error bars represent the standard deviation based on measurements from three duplicate reactors.

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increased very slowly and reached 1050 mg/L after 900 min; the DDSOD value, raw sludge is just about 16.5% of 1.5 g/g VSS potassium ferrate dose conditions (Fig. 1(B)). The DDSCOD values increased was almost linearly (14.6%, 25.0%, and 31.8%, respectively) with the PF dosage immediately after the co-treatment. After 900 min, the DDSCOD was only 4.7% without the PF; in contrast, the DDSCOD of 0.5 g/g VSS increased rapidly to 29.1%, which was almost double the value of the initial condition. Although more reagent was added, the DDSCOD load still was limited to 40%. 3.2. Effect of the PF + ULT co-treatment on the protein and polysaccharide concentrations The concentrations of the soluble protein and polysaccharide increased considerably in the filtrate, indicating that the sludge released a large number of proteins and polysaccharides in the aqueous phase (Fig. 2). The soluble protein concentration reached 1435.7 mg/L with the largest amount of PF and increased slowly as the reaction time increased. After 900 min, the concentrations of the soluble protein first increased to 1003.9 mg/L when a small amount of PF was added and then reached almost 1700 mg/L. The concentrations of the soluble

Fig. 2. (A) Soluble protein concentrations for different PF dosages initially and after 900 min. (B) Polysaccharide concentrations of the PF dosages initially and after 900 min. The error bars represent the standard deviation based on measurements from three duplicate reactors.

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polysaccharides exhibited a similar trend to that of the soluble protein initially and after 900 min. 3.3. Effect of the PF + ULT co-treatment on the VFAs, VSS and pH The production of the VFAs, including acetic, butyric, propionic, isobutyric, isovaleric and valeric acids exhibited different trends (Fig. 3). The gap between the smaller dose and larger dose of the VFAs concentration is approximately 300 mg/L in the initial stage and the co-treatments all result in rapid increases in the VFAs after 200 min. Except for the raw sludge, all the VFAs values increase rapidly after 4 h and the highest amount of VFAs (2255 mg/L) is observed after 900 min for the dosage of 1.0 g/g VSS. The VFAs amounts generated by the cotreated sludge are 5.67–8.78 times higher than those generated by the untreated raw sludge. The highest removal rate of the VSS was 17.23% after the pretreatment of 1.5 g/g VSS, which indicated the PF + ULT co-treatment destroyed the cell walls, which resulted in a change from the solid to the liquid phase for parts of the organic substances (Fig. 4). At the same time, because of the strong oxidizing power, parts of the inorganic substances were adsorbed by the interior and surface of the sludge particles that separated and became part of the solid phase of the sludge. The more PF was added, the higher the pH value was (Fig. 5). Over time, the pH approached the pH value of the raw sludge.

Fig. 4. Reduction of VSSs for different PF dosages initially and after 900 min. The error bars represent the standard deviation based on measurements from three duplicate reactors.

The organic compounds and many nitrogen species were released during the pretreatment of the WAS samples. Similar to the degree of disintegration in the WAS, the cumulative TN release increased gradually and a higher dosage of PF resulted in a greater TN concentration in the liquid phase. The concentrations of nitrate nitrogen and total nitrogen exhibited fluctuations over time in the filtered solution after the PF + ULT co-treatment with different PF doses and the raw sludge (Fig. 6(A)). During the initial time after the pretreatment, the liquid phase TN concentrations of all PF + ULT co-treatment samples were in the range of 890–1100 mg/L compared to the initial TN of b100 mg/L. In terms of the percentages of the supernatant releasing the soluble TN, the pretreatment with 1.5 g/g VSS PF performed the best (1029%). In general, the highest levels of TN-N (1095 mg/L after 240 min) were extracted using the dosage of 1.5 g/g VSS. The extraction of the TN-N reached a peak between 180 min and 240 min and then gradually decreased. At a dosage of 1.5 g/g VSS, nitrogen was present

at almost 9.73 times the amount compared with the extraction of the raw sludge; similar extraction levels were observed for all PF dosages after about 200 min and ranged from 8.66 to 7.72 times the amount with values of 986 mg/L and 890 mg/L, respectively. The highest concentration of nitrate nitrogen was observed for the dosage of 1.5 g/g VSS (Fig. 6(B)). Generally, the levels of nitrogen increased with the PF dosages but reaction time had little influence. The release of inorganic phosphorus from the sludge is clearly affected by the PF dosage initially and after 900 min (Fig. 6(C)). A higher extraction of phosphate anions (PO− 4 ) was achieved for the raw sludge. The highest concentration of phosphorus (380 mg/L) was observed for the initial time for the raw sludge. The phosphorus concentration of the untreated raw sludge was 54.7% higher than that obtained by the PF treatment of 1.5 g/g VSS. The lowest concentration of phosphorus (172 mg/L) was observed after 10 min of PF being extracted into solution. The PO− 4 began to decrease as a result of the formation of ferrous phosphate (FePO4) by the phosphate anions and ferric ion. The PO− 4 content of the first two dosages was considerably lower after 10 min and achieved 29.5% and 42.4%; for the last two dosages, the level of the final grade and the PO4−concentration hardly changed as more PF was added to the bottle. Due to the production of ferrous phosphate, a

Fig. 3. Concentrations of total VFAs of the sludge for different concentrations of PF during the 900-min test.

Fig. 5. pH of sludge for different concentrations of PF during the 900-min test.

− 3.4. Effect of the PF + ULT co-treatment on the TN, NO− 3 -N and PO4 concentrations

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4. Discussions It is very difficult to digest raw sludge rapidly and anaerobically due to the rate-limiting cell lysis; this results in a slow hydrolysis speed and higher disposal costs (Appels et al., 2008). Therefore, pretreatments are essential to increase the hydrolysis speed and they are effective because they use the oxidation potential to destroy the cell walls. Similarly, ozone, a strong oxidant, can oxidize the undissolved matters, disintegrate the biomass particulates, and transform them into soluble compositions in the WAS. At the same time, part of the WAS is mineralized to H2O and CO2. Moreover, ozone does not only accelerate the cell decay process, but also increase the cell dissolution rate (Dytczak et al., 2007). When it comes to the WAS disintegration mechanism, PF and ozone are that different but PF is safer, cheaper, and easily available because it is a commonly used chemical reagent. PF also provides high sludge disposal efficiency. At a pH of 10, the SCOD of the WAS reached 6000 mg/L after alkaline pretreatment and nearly 8 days of fermentation (Yuan et al., 2006). In the contrary, we found that the SCOD of the WAS reached 6000 mg/L after only 20 min by adding PF at the dosage of 1.0 g/g VSS combined with the ultrasonic process; a larger amount of soluble organics can be released after a longer-time fermentation. Moreover, compared with previous studies of sludge pretreatment with PF, a large amount of TN was released (He et al., 2017) and a much larger amount of SCOD was obtained (Li et al., 2018). This study provides information on physicochemical indices and offers a deeper understanding of the PF pretreatment process (Ye et al., 2012a; Ye et al., 2012b; Zhang et al., 2012). The byproduct of potassium ferrate, Fe(III), is a red precipitate that provides superior flocculation and settleability of the sludge. Compared with traditional flocculants, such as aluminum sulfate or ferric chloride, the use of Fe (OH) 3 provides a better result and more economic benefits. In addition, Fe(III) could also contributes to improve the methane generation (Speece, 1983). Fe (VI) limits the odor problem originating from residual putrescible matter. Fe(VI) has much greater oxidizing power and capacity than Fe(III) chemicals, results in a much shorter reaction time, and requires a much lower dosage to oxidize the same amount of odorous compounds such as H2S(He et al., 2009). These advantages may allow an online application by using Fe (VI) as an effective odor-reducing reagent for sludge odor control during sewage treatment works. Furthermore, Fe(VI) can oxidize endocrine disruptors and pharmaceuticals present in the sludge to relatively nontoxic byproduct (Li et al., 2008). Above all, it was demonstrated that PF had a strong ability to disintegrate and dispose of the WAS. Hence, a conclusion can be drawn that it is feasible and convenient to obtain the abundant nutrients from sludge using the PF + ULT technique. This approach can provide broad prospects for the practical application due to the advantages of the process that include the reduction of the sludge hydrolysis retention time and the sludge volume, as well as the release of the nutrients. In addition, this pretreatment process can also be combined with others drainage processes to achieve great efficiency (An et al., 2017). An indepth microbial community analysis will be performed in a future study. 5. Conclusions

− Fig. 6. Concentrations of TN, NO− 3 N, and PO4 of the sludge for different concentrations of PF during the 900-min test.

large quantity of dark yellow powders appeared in the bottle. The concentration fluctuated during the first 2 h and subsequently stabilized. A similar PO− 4 concentration was observed using the PF + ULT cotreatment regardless of the PF dosage after 900 min. However, the PF + ULT co-treatment decreased the phosphorus concentration.

This research experimentally determined the influence of dissolved organics on the disintegration of waste sludge after the novel pretreatment process of PF + ULT. After a short-term PF + ULT treatment, 17.23% of the VSS was reduced after a 900-min reaction time, which is 61.3% higher than the VSS reduction for the untreated raw sludge. The SCOD, TN, VFAs, soluble proteins and polysaccharides increased by 522.5%, 1029.4%, 878.4%, 2996.6%, and 801.9%, respectively compared to the untreated raw sludge. The constituent parts of the dissolved organic matter of the sludge products were released efficiently, which demonstrated the positive effect caused by the PF + ULT. Larger doses

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of PF or a longer treatment time do not result in a greater disintegration of sludge and are, therefore, not necessary. Acknowledgements This study was supported by National Natural Science Foundation of China (No. 31470233), the Science Fund for Distinguished Young Scholars of Heilongjiang Province (Grant No. JC201407), and State Key Laboratory of Urban Water Resources and Environment (Harbin Institute of Technology) (No. 2016DX10). References An, Y., Zhou, Z., Yao, J., Niu, T., Qiu, Z., Ruan, D., Wei, H., 2017. Sludge reduction and microbial community structure in an anaerobic/anoxic/oxic process coupled with potassium ferrate disintegration. Bioresour. Technol. 245 (Pt A), 954–961. American Public Health Association (APHA), 1998. American water works association and water environment federation. Standard Methods for the Examination of Water and Wastewater, twentieth ed. (Washington, DC). Appels, L., Baeyens, J., Degrève, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 34 (6), 755–781. Basuvaraj, M., Fein, J., Liss, S.N., 2015. Protein and polysaccharide content of tightly and loosely bound extracellular polymeric substances and the development of a granular activated sludge floc. Water Res. 82, 104–117. Bougrier, C., Delgenès, J.P., Carrère, H., 2008. Effects of thermal treatments on five different waste activated sludge samples solubilisation, physical properties and anaerobic digestion. Chem. Eng. J. 139 (2), 236–244. Chu, L., Yan, S., Xing, X.H., Sun, X., Jurcik, B., 2009. Progress and perspectives of sludge ozonation as a powerful pretreatment method for minimization of excess sludge production. Water Res. 43 (7), 1811–1822. Dogan, I., Sanin, F.D., 2009. Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method. Water Res. 43 (8), 2139–2148. Dytczak, M.A., Londry, K.L., Siegrist, H., Oleszkiewicz, J.A., 2007. Ozonation reduces sludge production and improves denitrification. Water Res. 41 (3), 543–550. He, C., Li, X.Z., Sharma, V.K., Li, S.Y., 2009. Elimination of sludge odor by oxidizing sulfurcontaining compounds with ferrate(VI). Environ. Sci. Technol. 43 (15), 5890–5895. He, Z.W., Liu, W.Z., Gao, Q., Tang, C.C., Wang, L., Guo, Z.C., Zhou, A.J., Wang, A.J., 2017. Potassium ferrate addition as an alternative pre-treatment to enhance short-chain fatty acids production from waste activated sludge. Bioresour. Technol. 247, 174–181. Herbert, D., Philipps, P.J., Strange, R.E., 1971. Chemical analysis of microbial cells. Academic Press, New York. Jiang, J.Q., Panagoulopoulos, A., Bauer, M., Pearce, P., 2006. The application of potassium ferrate for sewage treatment. J. Environ. Manag. 79 (2), 215–220. Kampas, P., Parsons, S.A., Pearce, P., Ledoux, S., Vale, P., Churchley, J., Cartmell, E., 2007. Mechanical sludge disintegration for the production of carbon source for biological nutrient removal. Water Res. 41 (8), 1734–1742. Kavitha, S., Kaliappan, S., Adish, K.S., Yeom, I.T., Rajesh, B.J., 2015. Effect of NaCl induced floc disruption on biological disintegration of sludge for enhanced biogas production. Bioresour. Technol. 192, 807–811. Khanal, S.K., Grewell, D., Sung, S., van Leeuwen, J., 2007. Ultrasound applications in wastewater sludge pretreatment: a review. Crit. Rev. Environ. Sci. Technol. 37 (4), 277–313. Kim, M.S., Lee, K.M., Kim, H.E., Lee, H.J., Lee, C., Lee, C., 2016. Disintegration of waste activated sludge by thermally-activated persulfates for enhanced dewaterability. Environ. Sci. Technol. 50 (13), 7106–7115.

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