+Efficient dewatering and heavy-metal removal in municipal sewage using oxidants

Journal Pre-proofs Efficient dewatering and heavy-metal removal in municipal sewage using oxidants Jing Yuan, Weining Zhang, Zhihua Xiao, Xihong Zhou, Qingru Zeng PII: DOI: Reference:

S1385-8947(20)30289-8 https://doi.org/10.1016/j.cej.2020.124298 CEJ 124298

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

7 December 2019 28 January 2020 31 January 2020

Please cite this article as: J. Yuan, W. Zhang, Z. Xiao, X. Zhou, Q. Zeng, Efficient dewatering and heavy-metal removal in municipal sewage using oxidants, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/ j.cej.2020.124298

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© 2020 Published by Elsevier B.V.

Efficient dewatering and heavy-metal removal in municipal sewage using oxidants

Jing Yuan a, b, Weining Zhang a, Zhihua Xiao a,b, Xihong Zhou a, Qingru Zeng *a a

College of Resources and Environment, Hunan Agricultural University, Changsha

410128, P. R. China bCollege

of Bioscience and Biotechnology, Hunan Agricultural University, Changsha

410128, P. R. China Correspondence to: Dr. Zeng Qingru Department of Environmental Science College of Resurces and Environment Hunan Agricultural University Changsha, Hunan 410128 P.R. China Tel: 86-0731-4673620; 86-0731-4617972 Fax: 86-0731-4618728 E-mail: [email protected]

Abstract: Municipal sewage sludge (MSS) is widely used in agriculture. In this study, three oxidants (Fenton’s reagent, K2S2O8, and CaO2) were used to simultaneously dehydrate MSS and remove heavy metals, and the heavy metals in the supernatant were separated via the addition of CaO. The respective removal rates of Pb, Zn, Cu, and Cd were 56.8%, 84.9%, 88.3%, and 92.3% using Fenton’s reagent; 66.9%, 86.1%, 88.9%, and 82.8% using K2S2O8; and 53.0%, 58.3%, 64.8% and 2.1% using CaO2. Thus, Fenton’s reagent and K2S2O8 were preferable for heavy metal removal, especially for Cd. The treated sludge was safe for agricultural use based on the standards for pollutants in sludge for agricultural use (GB4284-84). The oxidants also exhibited good dewatering performance for MSS and caused the extracellular polymeric substance （EPS） of MSS to decompose quickly. The water content of the sludge was reduced from 99.08% to 76.1% (Fenton’s reagent), 74.7% (K2S2O8), and 66.5% (CaO2). Hence, the treated sludge was easy to transport and could be safely used in agricultural production. After oxidant treatment, CaO was added to the supernatant, allowing the metals in the supernatant to be separated. This method can reduce Wc and heavy metal content of sludge at low cost. Keywords: Sludge; oxidation; heavy metal removal; dewaterability

1 Introduction In recent years, the large-scale construction of sewage treatment facilities has resulted in the generation of large amounts of municipal sewage sludge (MSS).The treatment of MSS is hindered by substantial operating costs[1] and a lack of treatment policies [2, 3].Sludge treatment involves sludge reduction, stabilization, harmless and resource utilization, especially sludge reduction and harmless[4]. Sludge is primarily treated in sanitary landfills or through agricultural utilization [5]. In agricultural use, MSS can provide nutrients and organic matter to improve the physical properties of the soil and soil fertility [6]. However, the environmental issues associated with MSS disposal are complicated since MSS usually contains pathogens, heavy metals, toxic organic substances, and eutrophication-causing nutrients such as nitrogen and phosphorus [7, 8]. Heavy metals inhibits the absorption and transport of other elements by plants and changes the physiological and ecological characteristics of plants; it can also cause the deterioration of soil quality and enter the food chain, endangering human health [9]. Therefore, how to deal with sludge effectively and reasonably has become an environmental problem throughout the world [10, 11]. In recent years, various advanced oxidation processes have shown good performance in sludge dewatering [12, 13]. The abilities of common inorganic acids such as hydrochloric acid, nitric acid, and sulfuric acid to remove heavy metals in sludge have been investigated under different pH values, acidification times, and sludge types. Inorganic acids show good performance for heavy metal removal [14, 15]. The heavy metal removal efficiencies of organic acids such as oxalic acid, citric acid, and

aspartic acid have also been studied in sludge. While organic acids exhibit good removal efficiency for some heavy metals (e.g., Cd by aspartic acid; and Cu, Zn, and Pb by citric acid) [16], organic acid removal must be combined with other methods to further reduce the contents of heavy metals in sludge [17, 18]. Some common chelating agents have also attracted attention for heavy metal removal in sludge. Using ethylenediaminetetraacetic acid as an extraction agent, Wasay [19] reported the removal of approximately 50.7% of Zn and 66.8% of Cu from sludge, sufficient to meet agricultural standards. Fidele Suanonet al.[20] found that glutamic acid can remove heavy metals from sludge while simultaneously fixing P in the sludge, thereby improving the nutritional value of the sludge. Studies on the removal of heavy metals from sludge using Fenton's reagent have shown that Fenton’s reagent can not only increase the dissolution of heavy metals and reduce the content of heavy metals in sludge, it can also significantly affect sludge dewaterability [13, 21]. Zhen et al. [22] studied the dehydration of activated sludge using oxidized by Fe2+ activating persulfate (S2O28−). They found that S2O28−oxidation promoted the degradation of soluble extracellular polymeric substance in sludge along with floc fracturing. Calcium peroxide (CaO2) is economical and has applications in many fields [23], including the chemical and agricultural fields. CaO2can be used as a bactericidal or anticorrosion agent, for bleaching, to increase oxygen, and so on; thus, it could be used to dehydrate sludge [24]. However, the removal of heavy metals in sludge using CaO2 has rarely been studied. In this study, sludge was treated with three antioxidants, Fenton’s reagent,

potassium sulfate (K2S2O8), andCaO2, and the performances for sludge dewatering and heavy metal removal were evaluated. Furthermore, the heavy metals in the sludge were transferred to the supernatant via oxidant conditioning with calcium oxide (CaO), a low-cost and simple process that allows the recycling of resources after sludge treatment. The contents of heavy metals and the changes in the water quality of the supernatant before and after CaO treatment are discussed in this paper. 2 Materials and methods 2.1Sludge samples and chemicals The sludge used in this study was taken from a concentration pond of a sewage treatment plant in Changsha City (Hunan, China). The sludge was retrieved and stored at 4°C. The basic properties of the original sludge are shown in Table 1. Table 1. Characteristics of the activated sludge sample Parameter

Wca

pHa

TSSa

Numerical value

96%–97%

6.6±0.3

4.49±0.38g/L

Parameter

SV30a

CODa

TNa

Numerical value

58.0%±0.5%

15.2±1.8mg/L

6.05±0.43mg/L

Heavy metal

Pbb (mg/kg)

Znb (mg/kg)

Cub (mg/kg)

Concentration

204.5±113.9

521.53±50.58

278.56±40.13

VSSa 2.08±0.56g/L TPa 8.15±0.3mg/L Cdb (µg/kg) 1218．0±162

Wc: water content; pH: hydrogen ion concentration; TSS: total soluble solid; VSS: volatile suspended solid. aAs received. b Dry matter basis.

2.2 Oxidation experiments

One liter of sludge was collected in a glass beaker, and the pH was adjusted with H2SO4 solution (6mol/L). Subsequently, a certain amount of Fenton’s reagent (FeSO4·7H2O was added first followed by 30% H2O2), K2S2O8, or CaO2 was added to the sludge solution, and the solution was stirred for 60 min at 25°C using a magnetic stirrer. Each oxidation experiment was performed in triplicate. An experiment carried out under the same conditions but without the addition of oxidant served as a control. After reaction, the sludge solution was removed, and the sedimentation effect, dehydration performance, and heavy metal contents were evaluated at 25°C. Then, 200 mL of sludge supernatant was placed into a 1-L beaker. Different concentrations of CaO (0.25%, 0.5%, 0.75%, and 1%) were then added to the beaker. After stirring for 1 h, the sample after the reaction was used in the following experiment. 2.3 Analysis methods 2.3.1 Analysis of dehydration performance The water content (Wc, %) was estimated following the standard analysis method (CJ/T 221-2005), while pH was determined using a pH meter (PHS-3C, Shanghai, China). Sludge volume (SV30, %) was measured as follows. The sludge sample (100 mL) was mixed well and transferred to a glass cylinder with a diameter of 40 mm. The height of the sludge–liquid interface was computed every 30 s. The sludge specific resistance (SRF) is a comprehensive index that reflects the sludge filtration performance and is regarded as the resistance to the unit filter area under certain pressure conditions [25]. The method of Zhang [26] was used to calculate SRF in this study.

2.3.2 Heavy metal analysis The oxidant-treated MSS was transferred to an evaporating dish and placed in an oven at 105°C. The dried sample (0.5 g) was placed in a 50-mL digestion tube followed by the addition of 4.5 mL concentrated hydrochloric acid and 1.5 mL concentrated nitric acid, and then the curved neck funnel was covered. After standing for a night, the digestion tube was heated at 90°C for 1 h in a digestion furnace and then reacted at 150°C. After removing the curved neck funnel and allowing the reaction solution to cool, 5 mL perchloric acid was added, and the solution was allowed to react for 1–2 h at 190°C. Finally, the reaction mixture was heated at 220°Cuntil near dryness. At this point, the sludge in the pipe was white ash. The volume was then brought to 25 mL using ultrapure water, and the sample was stored in a PE plastic bottle after filtering. Depending on the metal type and concentration, the heavy metals were determined by atomic absorption spectrophotometry with graphite furnace (FS 240, Agent, USA) for Cd or inductively coupled plasma-optical emission spectrometry (Optima 8300, PerkinElmer, Shelton, USA) for Cu, Zn, and Pb. 2.3.3 Water quality analysis After oxidant treatment, the 200 mL of supernatant was placed in as eparate 1-L beaker. Different concentrations of CaO (0.25%, 0.5%, 0.75%, or 1%) were added to the beaker followed by magnetic stirring for 1 h. The heavy metals in the supernatant were digested using the aqua regia digestion method [27]. The TSS and VSS of the sludge were determined using the determination method for municipal sludge in wastewater treatment plants (CJ /T 221—2005). The COD of the supernatant was

determined by the potassium dichromate method with ammonium sulfate titration. The TP of the supernatant was determined by potassium persulfate (K2S2O8) digestion [28], while the TN of the supernatant was determined by ultraviolet spectrophotometry[29]. 2.4 Data analysis All data were processed using SPSS 19.0, and graphs were created using Origin 8.0. All experiments were performed in triplicate. 3 Results and Discussion 3.1 Sludge dewatering performances of the three oxidants 3.1.1 Effects of different oxidant concentrations on Wc and SV30 The oxidant can destroy EPS [30], the main constituent of sludge [13]. Therefore, oxidizer can be used to improve the dewatering capacity of sludge and reduce the Wc and SV30.Some studies have applied Fenton reagent and similar approaches to sludge dewatering; however, there is no consensus on the optimal reagent concentration and solution pH [31, 32]. As shown in Fig. 1, the Wc and SV30values of the sludge decreased significantly with increasing oxidant concentration. Moreover, the Wc and SV30values of the sludge changes tended to slowly at the high concentrations of oxidants in Fenton's and S2O28− treatment. For treatment with Fenton’s reagent, Wc and SV30 decreased to 75.7% and 28%, respectively, as the Fe2+ concentration increased from 0 to 1.0 g/L. Wc and SV30 decreased slowly with increasing Fe2+ concentration when the Fe2+concentration exceeded 9 g/L, and the lowest values obtained were 76.4% and 24%, respectively. As a reaction catalyst, Fe2+ directly affects the rate of the Fenton reaction. Increasing the Fe2+ dosage enhances the sludge dewatering performance (Fig. 1 and 2)

and the removal rates of heavy metals (Fig. 3). At the Fe2+ concentration of 1.0 g/L, sludge Wc, SV30, and SRF decreased the faster when the H2O2 concentration is lower than 3 g/L, the increase of the removal rate of heavy metals is also faster. When the H2O2 dosage continued to increase, Wc, SV30, SRF, and the heavy metal removal rates changed slowly. Wc and SV30 decreased rapidly before the dosage of H2O2 content exceeded 12 g/L than after. When the amount of H2O2 is 12g/L, the Wc and SV30 are 76.9% and 28%, respectively. Since H2O2 is the main source of ·OH in the Fenton system, increasing the H2O2 dosage will increase the concentration of ·OH in solution, improving the oxidation capacity of the system. The increase in H2O2 concentration increased the amount of ·OH in the solution, thereby increasing the oxidation capacity of the Fenton system; however, as the amount of H2O2 continued to increase, the reaction rate may have been reduced because the excess H2O2 caused the Fe2+ in solution to be rapidly oxidized to Fe3+. That's going to block the formation of ·OH in virtue of the Fe3+ produced in the solution is fast against H2O2. At present, the optimal concentrations of H2O2 and Fe2+ to treat sludge with different properties are not clear, even though the H2O2 and Fe2+ concentrations are key factors that directly affect oxidation [33]. The results indicate that the most suitable mass ratio of H2O2 to Fe2+ is between 8 and 12. The oxidation/reduction potential (E0 = +2.60 V) of the free radical of sulfuric acid (SO4-·) produced by K2S2O8 is similar to that of ·OH (E0 = +2.80 V). Both of them have good oxidation capacity [34]. For treatment with K2S2O8, The values of Wc, SV30, and SRF decreased rapidly as the K2S2O8concentration increased from 0 to 3g/L and then

decreased more slowly as the K2S2O8 concentration increased from 6 to 9g/L. Wc decreased from 82.6% to 76.5% as the K2S2O8concentration increased from 0 to 3 g/L, while SV30 decreased from 65% to 44%. When the K2S2O8 concentration increased beyond 6 g/L, Wc began to decrease more slowly. The minimum Wc and SV30 values were 74.7%and 39% at K2S2O8 concentrations of 9 and 19g/L, respectively. Persulfate is a nonselective oxidant that can improve sludge dewaterability through oxidation. However, S2O28−is stable at room temperatures, and it is often necessary to adopt different activation methods to promote sulfate decomposition and produce SO4-·. However, excess K2S2O8 will not increase the reaction rate of the system [35]; instead, excess K2S2O8 will serve as a quenching agent of SO4-· [36, 37]. The excess SO4-· in solution will react with existing SO4-· to produce persulfate (S2O28−), which will further react with SO4-· in solution to produce sulfate, leading to decreased oxidation efficiency. CaO2 also significantly reduced the Wc and SV30 values of sludge. The fastest decreases in Wc and SV30 were observed as CaO2 concentration increased from 0.25% to 1.5%, when Wc decreased from 76.3% to 60.2%, and the decreases slowed when the CaO2 concentration exceeded 1.5%. Overall, Wc and SV30 decreased linearly with increasing CaO2 dosage. The minimum Wc and SV30 obtained by treatment with CaO2 were 59% and 28%, respectively. In acidic media, CaO2 can degrade pollutants in sewage by releasing ·OH and help improve water quality [38]. In sludge, CaO2 regulates the sludge solution pH and produces Ca(OH)2, resulting in the formation of a porous grid skeleton structure. This structure destroys the stability of sludge colloids.

70

Wc SV30

84

63

82

84

70

Wc SV30

82

63

SV30 / %

56

SV30 / % Wc / %

Wc / %

56 80

49

78

80

49

42

42

78

76

35

74

28

0.0

1.2

0.8

0.4

1.6

2.8

2.4

2.0

3.2

35

76

-2

2+

28

0

2

Fenton reagent Fe （g/L ）

8

10

12

14

16

Wc / %

SV30 / %

56 80

49

78

42

80

28

0

2

4

6

8

10

12

14

16

Potassium persulfate（g/L ）

18

20

63 56

75

49

70

42

65

35

76

20

SV30 / %

63

Wc SV30

Wc / %

Wc SV30

18

70

70

82

-2

6

85

84

74

4

Fenton reagent H2O2（g/L ）

35

60

28 55

0

CaO2 （% ）

2

Fig.1.Sludge Wc and SV30 as functions of the concentrations of different oxidants. 3.1.2 Effects of oxidant concentration on SRF SRF is one of the main parameters determining sludge dewatering performance. A larger SRF corresponds to worse filtration performance. In this study, the SRF of the original sludge was 6.75×108 S2·g-1. As shown in Figure 2, SRF first declined rapidly with increasing oxidant concentration, but the change in SRF slowed when the oxidant concentration became too high. For treatment with Fenton’s reagent (Table 2), when the amount of Fe2+ was fixed, SRF decreased by 26.7% as the H2O2concentration increased from 0 to 3 g/L; SRF further decreased by 36.7% as the H2O2 concentration increased to 18 g/L. When the H2O2 concentration was fixed, SRF also decreased with increasing Fe2+ concentration; SRF reached a minimum of 3.79×108 S2·g−1at the Fe2+ concentration of 3 g/L,

corresponding to a decrease of 43.9%. These results are similar to those reported by Song [39]. Fenton’s reagent acted not only an oxidant, but also an effective coagulant for sludge fragments [40]. For treatment with K2S2O8, the values of SRF decreased rapidly as the K2S2O8 concentration increased from 0 to 3 g/L and then decreased more slowly as the K2S2O8 concentration increased from 6 to 9 g/L. SRF decreased with increasing K2S2O8concentration from 0 to 9 g/L, when the minimum SRF of 5.19×108 S2·g−1 was observed. Further increases in K2S2O8 concentration caused SRF to increase. And that also shows the excess sulfate radical (SO4-·) in solution leading to decreased oxidation efficiency [36]. Persulfate is stable and needs to be activated to generate SO4-·, while excessive K2S2O8 will become quenching agent of SO4-·, which will not only reduce the reaction rate of the system, but also reduce the oxidation of sludge by SO4-·. For treatment with CaO2, SRF decreased significantly as the CaO2concentration increased from 0% to 0.25%. SRF decreased significantly only when the CaO2 concentration increased to 0.25%; it changed very little upon further increases in concentration. SRF reached the minimum value of 2.52×108 S2·g−1when the concentration of CaO2 was 1%, corresponding to a decrease of 62.7% compared to the initial value. When the CaO2 concentration increased beyond 1%, SRF increased slightly. The Ca2+ produced after the addition of CaO2 to the sludge solution neutralized the negatively charged colloidal particles, thereby reducing the repulsion between particles and promoting the formation and settlement of the floc in the sludge [41]. Excessive CaO2 produced excess calcium hydroxide in the sludge, and a large number

of inorganic particles gradually blocked the micropores of the filter paper during filtration. This reduced the dewatering capacity of the sludge and increased the SRF. The results indicate that Fenton’s reagent and CaO2 have better effects on SRF than K2S2O8. In this study, after SRF determination, the sludge appeared to float upward, and some of the sludge sunk after shaking. This may be attributed to the agglomeration of floc particles in the sludge, because the surface area increased by the addition of CaO2. Meanwhile, CaO2 dissolved in water, increasing the oxygen content in the sludge. The sludge flocs adhered to the many bubbles, causing the sludge to rise. After shaking, the bubbles adsorbed by the sludge were broken, and the sludge sunk. 7.0

6.8 SRF

6.0

SRF/ 10 S g

2 -1

6.0

8

8

5.5 5.0

5.6 5.2

4.5

4.8

4.0 3.5

4.4 0.0

0.5

1.0

1.5

2+

2.0

2.5

4.0 -2

3.0

Fenton reagent Fe （g/L ）

7.0 6.8

SRF/ 10 S g

2 -1

2 -1 8

6.2

8

SRF/ 10 S g

8

2

SRF/ 10 S g-1

6.0

0

2

4

6

8

10

12

14

16

Fenton reagent H2O2（g/L ）

18

7

SRF

6.6 6.4

SRF

6.4

SRF/ 10 S g

2 -1

6.5

5.8 5.6 5.4

20

SRF

6 5 4 3

5.2 5.0 -2

0

2

4

6

8

10

12

14

16

Potassuim persultfate（g/L ）

18

20

2

0.0

0.5

1.0

1.5

Calcium peroxide（% ）

Fig.2. Sludge SRF as a function of the concentrations of the three oxidants. Table.2 The effect of Fe2+ and H2O2 on SRF decreasing.

2.0

Fe2+( H2O2=9g/L)

SRF

0 0.2 0.4 0.6 0.8 1 1.2 1.8 2.25 3

6.7 6.56 6.43 5.72 5.36 4.67 4.49 4.39 3.94 3.79

H2O2(Fe2+=1.0g/L ) 0 3 6 9 12 15 18

\ \ \

SRF 6.7 4.91 4.74 4.67 4.63 4.53 4.23 \ \ \

3.2 Effects of oxidant concentration on heavy metal removal in sewage sludge Some researchers have evaluated the use of oxidants in sludge treatment[42, 43]. Their papers have focused mainly on sludge dewatering, primarily using transition metal ions in deep dewatering; few studies have been related to the removal of heavy metals in sludge [44]. In addition, there is little existing research on the use of CaO2 as a strong oxidant for sludge treatment. Therefore, it is important to explore the removal of heavy metals from sludge so that clean sludge can be applied in agriculture. In treatment using Fenton reagent, the reaction between H2O2 and Fe2+ produces highly oxidizing free radicals (·OH), which can decompose and destroy some organic refractory pollutants [45]. The difference fractionation of heavy metals affects the removal efficiency. In sludge, Pb, Zn, Cu and Cd mainly exist in organic and sulfides fraction, carbonate-bound fraction [46], that show a great migration ability. It is beneficial to remove heavy metals from sludge. In this paper, the reagents with different concentrations had little effect on the morphology of heavy metals (Fig.S1 and S2). Pb mainly exists as F3 and F4. Zn exists as F1 and F4 more than F2 and F3. Cu and exists

at most as F3. Figure 3 shows the removal efficiencies for different heavy metals (Pb, Zn, Cu, and Cd) in sludge treated by different concentrations of oxidants for 1 h at pH 2 and 25°C. For treatment with Fenton’s reagent, the highest metal removal rates were obtained at the highest Fe2+or H2O2 concentration. The rate of Pb removal was most affected by oxidant concentration; the removal rates of Zn, Cu and Cd did not change significantly with Fe2+or H2O2 concentration. When the Fe2+ concentration was fixed at 1g/L, the removal rate of Pb increased from 36.6% to 50.6% as H2O2concentration increased from 3 to 6 g/L, whereas the removal rates of Zn, Cu and Cd changed little (from 84.55% to 85.75%, 88.5% to 91.05%, and 90.7% to 91.65%, respectively). At constant H2O2 concentration (3 g/L), the removal rates of Pb, Zn, Cu, and Cd increased by 42.8%, 9.26%, 11.32%, and 7.79%, respectively, as the Fe2+concentration increased from 0.2 g/L to 3 g/L. In general, Fenton’s reagent was less efficient at removing Pb compared to the other metals. The appropriate conditions for the removal of heavy metals is the dosage of H2O2: Fe2+ was 9: 1 to 12:1. The removal rate of Pb, Zn, Cu, Cd reached 51.04%, 85.20%, 89.94%, 92.41%, respectively. For treatment with K2S2O8, the heavy metal removal rates increased with K2S2O8 concentration, and among metals, the removal rate of Pb was most strongly affected by oxidant concentration. At the K2S2O8 concentration of 9 g/L, the removal rates of Pb, Zn, Cu, and Cd were 63.90%, 87.10%, 86.40%, and 84.25% respectively. As for treatment with Fenton’s reagent, the removal rates of Zn, Cu, and Cd obtained using and K2S2O8 were much higher than that of Pb.

For treatment with CaO2, the removal efficiency of heavy metals in sludge was not ideal, and the removal rate of Cd was very low at all CaO2 concentrations. At the CaO2 concentration of 0.25%, the Cd removal rate was only 10.3%, while those of Pb, Zn, and Cu were 48.2%, 43.3%, and 47.7%, respectively. The removal rates of Pb, Zn, and Cu were maximized at the CaO2 concentration of 1% (53%, 58.3%, and 63.8%, respectively). When the CaO2 concentration increased beyond 1g/L, the removal rates of the heavy metals decreased significantly. The mixture of solid substances produced by different concentrations of calcium peroxide and sludge was added, which resulted in the increase of the weight of dry matter in the sludge, and the heavy metal content of the mud cake under the dosage was generally low and the removal rate was high. For all three oxidants, the removal rate of Pb was lower than those of Zn, Cu, and Cd. Fenton regent and K2S2O8 exhibited better removal performance than CaO2. 120

105 Pb Zn Cu Cd

95

100

removal rate(%)

90 85 60 55 50 45 40 35 30

90 80 50 40 30 20

2

4

6

8

10

12

14

16

Fenton reagent H2O2 (g/L）

105

18

Pb Zn Cu Cd

100 95

Removal rate （%）

Pb Zn Cu Cd

110

90

10

20

0.0

0.5

1.0

1.5

2+ 2.0

Fenton reagent Fe g/L

80

2.5

3.0

Pb Zn Cu Cd

70

Removal rate （%）

Removal rate （%）

100

60 50

85

40

80 65

10

60 55 2

4

6

8

10

12

14

16

Potassium persulfate (g/L）

18

20

0

0.0

0.5

1.0

CaO2 (%)

1.5

2.0

Fig.3. Removal rates of Pb, Zn, Cu, and Cd in sludge as functions of the concentrations of three oxidants. 3.3 Effects of pH on the removal of heavy metals in sewage sludge The initial pH is a key parameter affecting heavy metal removal. Figure 4 shows the removal rates of Pb, Zn, Cu, and Cd under the treatment of the three oxidants at different pH. For treatment with Fenton’s reagent, the best removal rates were observed when the initial pH was between 1 and 3, and the maximum removal rates of Pb, Zn, Cu, and Cd were 59.9%, 83.1%, 84.5%, and 91.2%, respectively. The removal rates decreased as the pH increased from 3 to 5. Similarly, K2S2O8 exhibited good performance for heavy metal removal at pH 1– 3. Among the metals, the effect of pH on Pb removal was greater than the effects on Zn, Cu, and Cd removal. As pH increased from 3 to 5, the removal efficiency decreased for each heavy metal, as observed for treatment with Fenton’s reagent. For treatment with CaO2, the removal rates increased slightly as pH increased from 1 to 4. The maximum removal rates were observed at pH 4 (52.2%, 57%, 64.5%, and 2% for Pb, Zn, Cu, and Cd, respectively). When pH increased further to 5, the removal rates of the heavy metals decreased. The heavy metal removal efficiency of CaO2 was not ideal, and the removal rate of Cd at each tested CaO2 concentration was very low. When the initial pH was 2, adding a certain amount of CaO2 increased the content of H2O2 in the system and increased the heavy metal removal efficiency. As the CaO2 dosage increased, pH increased rapidly. When the CaO2 concentration exceeded 1%, the pH exceeded 10. In this strongly alkaline environment, the large amount of OH− in solution

reacted directly with metal ions to produce insoluble metal hydroxide precipitates, thereby reducing the heavy metal removal efficiency. Compared to Fenton’s reagent and K2S2O8, the heavy metal removal performance of CaO2 was worse. The results indicate that the three oxidants are most effective for heavy metal removal when the initial sludge pH is in the range of 1–3. 110

Pb Zn Cu Cd

Pb Zn Cu Cd

90

Removal rate （%）

Removal rate （%）

100

80

90 80 60

70

60

50

50

40 0.5

1.0

1.5

2.0

2.5

3.0

pH

3.5

4.0

4.5

5.0

5.5

2+

80

0.5

1.0

1.5

2.0

2.5

3.0

pH

3.5

4.0

4.5

5.0

5.5

Potassium persulfate (9g/L)

Fenton reagent H2O2:Fe =9:1

Pb Zn Cu Cd

Removal rate （%）

70 60 50 40 10

0

0.0

0.5

1.0

pH CaO2 (1%)

1.5

2.0

Fig.4. Effects of pH on the removal rates of Pb, Zn, Cu, and Cd in sewage sludge using Fenton’s reagent (H2O2/Fe2+ = 9:1; 9 g/LH2O2and 1 g/LFe2+), K2S2O8 (9 g/L), and CaO2 (1%). Briefly, the sludge treated with oxidants in this study had a small volume along with a small amount of heavy metals. Table 3 shows that the heavy metal contents in the sludge after oxidation treatment at the optimal oxidant concentrations were all lower than the national standards. Thus, the treated sludge can be used safely in agricultural

production. Table 3. Comparison of the control standards for pollutants in sludge for agricultural use and the heavy metal concentrations in sludge treated with the optimal oxidant concentrations (mg/kg). Treatment

Control Standards for Pollutants in Sludge for Agricultural Use (GB4284-84)

Fenton

K2S2O8

CaO2

Pb

300

88.44

67.62

96.12

Zn

500

78.86

72.40

218.00

Cu

250

32.59

30.80

98.05

Cd

5

0.09

0.21

1.19

pH=3; temperature=25°C; Fenton: Fe2+=1g/L, H2O2=9g/L; K2S2O8=9g/L; CaO2=1%.

3.4 Removal rates of heavy metals from sludge supernatant via CaO treatment As shown in Fig. 5, the addition of CaO to the sludge supernatant after Fenton oxidation resulted in the precipitation of the heavy metals removed from the sludge. For all metals, the removal rates increased with increasing CaO concentration. When the CaO concentration was 0.5%, the removal rate of Pb was 52.9%, while those of Zn, Cu, and Cd were only 19.2%, 10.4%, and 34.1%, respectively. When the CaO concentration was further increased to 0.75%, the removal rates of Pb, Zn, Cu, and Cd increased significantly to 81.8%, 86.8%, 68.8%, and 82.7%, respectively. When the supernatant was treated with 0.25% CaO after oxidation with K2S2O8, the removal rates of Zn (79.9%) and Cu (73.9%) were good, whereas those of Pb (25.15%) and Cd (33%) were low. After increasing the CaO concentration to 1%, the removal rates of Pb, Zn, Cu, and Cd reached 77.1%, 89.4%, 86.5%, and 88%, respectively. It was difficult to detect Cu and Cd in the supernatant after CaO2 treatment,

potentially because the addition of CaO2 (1%) caused the solution to become strongly alkaline. In the alkaline environment, the heavy metal ions produced hydroxide, which precipitated before adding CaO. Although Pb and Zn in the supernatant could be detected before treatment, their concentrations were low (62 and 16 μg/L, respectively). 110

90

Pb Zn Cu Cd

Removal rate (%)

70

90

60 50 40 30 20

80 70 60 50 40 30

10 0.2

0.3

0.4

0.5

0.6

0.7

CaO (%)

0.8

0.9

1.0

1.1

20

2+

Fenton reagent H2O2:Fe =9:1

35

0.2

0.3

0.4

0.5

0.6

0.7

CaO (%)

0.8

0.9

1.0

1.1

Potassium persultfate （9g/L)

Pb Zn

30

Removal rate （%）

Pb Zn Cu Cd

100

Removal rate (%)

80

25 20 15 10 5 0 0.2

0.3

0.4

0.5

0.6

CaO%

0.7

0.8

0.9

1.0

1.1

Calcium peroxide 1%

Fig.5. Effects of CaO concentration on the removal rates of Pb, Zn, Cu, and Cd from sludge supernatant treated with Fenton’s reagent (H2O2/Fe2+ = 9:1; 9 g/LH2O2and 1 g/LFe2+), K2S2O8 (9 g/L), and CaO2 (1%).

3.5 Effect of CaO treatment on the water quality of sludge supernatant Table 4. Water quality indices of sludge supernatant before treatment with CaO

Fenton

K2S2O8

CaO2

H2O2/Fe2+ =9:1 (g/L)

9 g/L

1.0%

COD

149

216.2

163

TN

14.3

25.6

13.2

TP

13.0

12.8

1.8

Content

Table 5. Water quality indices of sludge supernatant after treatment with different concentrations of CaO Fenton

K2S2O8

CaO2

Content 0.2 (%)

0.7 0.5

1.0

5

5

85.

54.

COD

0.2 0.25

0.5

0.75

82.

219.

196.

219.

7

3

1

TN

5.2

187 2

8

4.1

5.1

14.3

12.0

14.1

5.8

5.7

5.6

4.8

1.2

1.1

1.0

7 0.2

TP

0.2 0.3

8

18

13.

8 0.1

5

191

3

4.2

1.0

21 210

4

0.5 5

106 9

1.0

0.7

5

0.1 0.15

5

0.1

1.3

0.13 3

5

As shown in Table 4 and 5, the changes in COD, TN, and TP in the three supernatants resulting from treatment with Fenton reagent (H2O2 / Fe2+ = 9:1), K2S2O8 (9 g/L), and CaO2 (1%) differed after treatment with different concentrations of CaO. Because of the Fe2+ ions present in solution after Fenton treatment, the color of the

sludge supernatant changed significantly after adding CaO. After the supernatant resulting from treatment with Fenton’s reagent (H2O2 / Fe2+ = 9:1) was treated with 0.75% CaO, COD was reduced from 149 to 54.4 mg/L (63.9% removal rate), TN was reduced from 14.3 to 4.1 mg/L (72% removal), and TP was reduced from 13.0 to 0.3 mg/L (97.8% removal). The addition of Ca （ OH ） 2 (1250 mg/L) to river water reduced COD and TP by 64.3% and 96.7%, respectively [47]. But the removal rates were not strongly affected by the CaO2 concentration. In this study, the best removal performance was obtained by adding 0.5% CaO to the supernatant resulting from treatment with K2S2O8 (9 g/L). Under this condition, TN decreased from 25.6 to 12.0 mg/L (53.1% removal), and TP decreased from 12 to 0.1 mg/L (99.2% removal). When pH > 10, P mainly exists in the forms of HPO42− and PO43−, and Ca2+ from CaO can combine with P in water to form Ca3(PO4)2, removing P from solution [48]. In this study, treating the supernatant with 1% CaO resulted in a COD removal rate of 9.3%, a TN removal rate of approximately 50%, and a negligible change in TP content (1.8 to 1.0 mg/L). This may be because the solution contained large amounts of Ca2+ and OH−; thus, adding CaO to the supernatant resulting from treatment with CaO2 increased the ionic concentration of the reaction system and further improved the pH. The dissolved CaO2 also had dephosphorization and decolorization effects. Therefore, adding CaO did not have a strong effect on the quality of the supernatant, and COD increased slightly. 3.6 Cost estimation of oxidants and CaO. Fenton treatment has the lowest cost, and removed 3.88 g Pb, 16.52 g Zn, 9.32 g

Cu and 41.87 mg Cd for every 1 CNY. And then there's the cost of K2S2O8, which is 1.45g Pb, 5.05g Zn, 2.67g Cu and 11.40 mg Cd was removed for 1 CNY. In the CaO2 treatment, 1.35g Pb, 3.80g Zn, 2.22g Cu and 0.33 mg Cd can be removed for every 1 CNY. In the study of Jialin Liang’s [49], Fenton treatment is also a low-cost and efficient method for dewatering sludge. The cost of treating sludge with Fenton reagent is 121.67 CNY/ton DS (Dry Solid). K2S2O8 treatment has high removal efficiency for heavy metals, but the cost (355.73 CNY/ton DS) is higher than Fenton reagent treatment. The removal efficiency of heavy metals by CaO2 is obviously weaker than that of the other two oxidants. And the cost of CaO2 is 238.81 CNY/ton DS. CaO can effectively remove heavy metals from the supernatant of sludge treated by oxidant in this paper. The cost can be saved and heavy metal removal rate is high when CaO is 0.75%. After the sludge was treated with Fenton reagent (Fe2+ = 1 g/L, H2O2 = 9g/L), 0.75% CaO was added to recover heavy metals in the supernatant, and the cost of sludge treatment was 47.25 CNY/ton DS (table S1 and S2). And the cost of K2S2O8 treatment and CaO2 are 107.79 CNY/ton DS and 93.43 CNY/ton DS, respectively (table S1 and S2). 4 Conclusions In this study, three strong oxidizing agents (Fenton, K2S2O8, CaO2) were evaluated for their sludge dewatering and heavy metal removal abilities, and the changes in the supernatant water quality after treatment were investigated. The three oxidants effectively reduced the moisture content of the sludge cake and significantly reduced the SRF value, which is used to characterize sludge filtration performance. Treatment

with Fenton reagent (Fe2+: 1.0 g/L, H2O2: 9 g/L) or K2S2O8 (9 g/L) for 1 h at 25°Ceffectively removed most of the Pb, Zn, Cu, and Cd in the sludge, while the heavy metal removal performance of CaO2was poor. The addition of CaO to the sludge supernatant allowed the recovery of most of the heavy metals removed from sludge by treatment with Fenton reagent or K2S2O8, and CaO addition significantly reduced TN and TP. The results demonstrate that Fenton reagent and K2S2O8can be used to dewater and remove heavy metals from sludge, and the heavy metals in the supernatant can be easily and economically separated using CaO. This method has the characteristics of high efficiency and low cost to dispose sludge. The findings are expected to promote industrial-scale sludge treatment. Acknowledgements The study was financially supported by the National Key Research and Development Program of China （2016YFD0800304）and the Changsha plan project of science and technology (kq1801025). The authors declare no conflicts of interest.

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Declaration of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

Highlights 

Fenton’s reagent, K2S2O8, and CaO2 can effectively remove heavy metals.



Fenton’s reagent, K2S2O8, and CaO2 can be used to dehydrate MSS.



CaO has positive influence on separating the heavy metals in the supernatant.



The treated sludge was could be safely used in agricultural production.