The sample was the concentrated sludge (water content of about 95%) produced in the sewage treatment process of Second Wastewater Treatment Plant of Everbright Water (Jinan) Ltd. The physical and chemical properties were shown as follows: pH 7.6, water content 98%, ash content 0.54%, and organic matter content 1.25%.

Six-joint stirrer (JJ4A, Changzhou Jintan); Capillary water absorption time tester (DP-304M, British Triton); STOC tester (TOC-L CPN, Japan Shimadzu); ion chromatography (CIC-D100, Qingdao Shenghan); ICP-MS (ICP-MS7500a, Agilent, USA); pH meter (BRCN, Zhejiang Brown); suction filter (VF204A, Sciencetool, USA); jack-type filter press (Homemade); electric heating blast drying box (DHG, Shanghai Binglin).

K₂S₂O₈, Na₂S₂O₈, FeSO₄·7H₂O, CaO were all analytically pure (>99.9%); Cationic polyacrylamide was industrial grade.

Main indicators: CST, Wc, Tc.

The moisture content of the dehydrated cake was measured by gravimetric method; the capillary water absorption time was measured by a capillary water absorption time tester (DP-304M, British Triton); the filtrate after dehydration was analyzed by STOC tester (TOC-L CPN, Japan Shimadzu), Ion Chromatography (CIC-D100, Qingdao Shenghan) and ICP-MS (ICP-MS7500a, Agilent, USA); the sludge dewatering time was defined as the time required to completely filter 500 mL of sludge (suction pressure 0.1 Mpa).

The experimental data is the average data of three parallel experiments.

The main steps were shown in Figure 1. Experimental procedure and main parameters reference [12]: Take 500 mL of experimental sludge → add persulfate → react for 5 min (stirring rate is 300 r·min⁻¹) → add Fe₂SO₄ solution → react for 10

min (stirring rate is 150 r·min⁻¹) → add quicklime → rapid stirring for 30 s (300 r·min⁻¹), slow stirring for 2 min (150 r·min⁻¹) → vacuum filtration dehydration (0.1 Mpa) → jack press filter dehydration (0.5 Mpa). In the single factor experiment, the relevant steps were adjusted according to the different types of conditioning agents.

As mentioned in the literature review, the theoretical dosage ratio of Fe²⁺ to MS₂O₈ was 1:1. When the two dosing ratio exceeds 1:1, the excess Fe²⁺ would react with SO 4 2 −   ⋅ to consume free radicals, which would dreadful to the structure destruction of sludge flocs, and caused unfavorable effectiveness [13] [14] [15] [16]. Several reports had shown that when the dosage ratio was in the range of 0.8:1 to 1.1:1, the effectiveness was almost equivalent. The mechanism was that persulfate could self-decompose or decompose by minerals in the soil to reduce Fe²⁺ consumption [17], and this could result in a significant impact on the cost of SO 4 −   ⋅ advanced oxidation technology. Considering this, the paper had carried out a detailed study on the dosage ratio of Fe²⁺ and MS₂O₈ in the range of 0.6:1 - 1.4:1. Especially, when the ratio was close to 1:1, the ratio gradient scale was reduced to 0.1. The influence of the addition ratio on CST and Wc were shown in Figure 2.

As shown in Figure 2, comparing with no conditioner, CST value dropped rapidly from 68.7 s·L/g SS to 36.3 s·L/g SS with conditioner as a radio of 0.6:1, were an average decline in the proportion of 47.2%. And the moisture content

also declined from 93.0% to 85.0% or less, with a decrease of 8.6%. However, when the ratio of those two additions increased, CST value did not show a trend of decreasing, conversely, the CST value increased when the ratio exceed 0.8:1. the CST value started from 32.9 s·L/g SS continued to increase after the minimum value, and the moisture content of the corresponding dehydrated cake also presented a fluctuating distribution, and Wc fluctuated between 78% and 82%. No significant differences were found when the two dosing ratios are 0.8:1 or 1.2:1, Wc was similar, reaching a minimum of 78% - 79%. The two dosing ratios of 0.8:1 had the most significant effect, when enlarged the ratios, Wc did not have an apparent decrease.

Therefore, it could be concluded that Fe²⁺ acted as an initiator to activate MS₂O₈ to produce SO 4 −   ⋅ from Figure 2. In the first place, SO 4 −   ⋅ degraded the organic matter in the flocs, effectively destroyed the structure of the sludge flocs, forced the sludge to release more intracellular water and increase free water in the sludge. The sludge was relatively easier to be filtered than the initial configuration, and the residual moisture in the cake was reduced which had a valid promotion effect on dehydration. Apparently, CST value and Wc showed better conditioning effects than the blank control group. There is one more point, Fe²⁺ would react with SO 4 −   ⋅ , SO 4 −   ⋅ lost the opportunity of partial oxidation to attack the organic matter of sludge flocs when the dosage ratio of Fe²⁺ and MS₂O₈ exceeded 0.8:1, thereby the overall oxidability and damaging were retarded, and the dehydration speed was slower as a result. In summary, these results showed that there was a more pleasurable and accurate chemical dosage ratio, namely, n(Fe²⁺): n(SPS) was 0.8:1, so as to achieve the same conditioning index (CST reduction rate was about 47%). Clearly, the amount of sludge Fe²⁺ reagent could be reduced by 20% with a ton of pollution on average, and the iron loss rate was also reduced. At the same time, the concentration of Fe³⁺ ion in the filtrate was greatly reduced, which could reduce the difficulty and cost of subsequent filtrate wastewater treatment, and improve the economics and practicality of advanced SO₄-·oxidation technology.

A stronger relationship between Fe²⁺ and S 2 O 8 2 − has been reported in many literatures, the dosage of medicament directly affected the destruction degree of sludge flocs and the subsequent physical dehydration treatment effect. The scientific dosage of activated persulfate could further reduce the cost per ton of sludge treatment, and avoid the introduction of excessive SO 4 2 − and other impurity ions in the system at the same time. This similar results were also reported by Zhen et al. [14], who studied the effect of potassium persulfate dosage in the range of 0.1 - 1.5 mmol·g⁻¹ VSS on the sludge CST time, and pointed out that the effect was better at 1.2 mmol·g⁻¹ VSS. Regrettably, the significant decrease interval (0.1 - 0.9 mmol/gVSS) had not been explored in depth by Zhen et al., which would be an appreciable effect on the economics and practicality of advanced SO 4 −   ⋅ oxidation technology. This section made a careful plans based on the determined optimal dosing ratio n(Fe²⁺):n(SPS) = 0.8:1 to further evaluate the reaction system, and effect of activated persulfate dosage on the dewatering performance of sludge was compared with the research results of literature [12]. For this reason, this paper selected 0 - 220 mg·g⁻¹ DS in experiment, and introduced the sludge dewatering time (Tc) as a consideration index.

It could be seen from Figure 3 that with the addition of the Fe 2 + / S 2 O 8 2 − system, the sludge CST decreased rapidly from 73 s·L/g SS to less than 25 s·L/g SS and then tended to slope gently. Wc dropped to 65% when the amount was 161.8 mg·g⁻¹ DS. CST had been reduced to within 86 s when the dosage is 80.9 mg·g⁻¹ DS, and the lowest was 82 s (161.8 mg·g⁻¹ DS), which indicated that the sludge conditioning level represented by this dosage had been completely matched in the later physical dehydration process. Continuous increase addition of persulfate would not increase the dehydration speed significantly. Figure 3 showed that when the dosage of activated persulfate was 80 - 120 mg·g⁻¹ DS, Wc dropped below 70% to reach a relatively ideal moisture content level, and the later change trend for CST and Wc value were basically in agreement, the declines were slowed down significantly. It was noteworthy that when the dosage of activated persulfate was more than 161.8 mg·g⁻¹ DS, Wc would unexpectedly increase instead. These results suggested that when the dosage of activated persulfate increased to a certain extent, the destructive effect of the sludge flocs was enhanced. But there was a critical point, beyond this critical point, the particle size of the solid sludge was too small to compress and deform during the mechanical dehydration process which would have exactly the reverse effect.

According to the existing research conclusions, the conditioning level (water content < 70%) of the sludge under this dosing parameter was equivalent to the performance index of the sludge in the literature [18]. It was recommended that the sludge floc peroxidation should be fully considered in the process of technical application. In view of negative effects and comprehensive technical and economic effects, it was advisable to control the dosage of activated persulfate within 80 - 120 mg·g⁻¹ DS.

Prior studies had noted that Fenton-like oxidation technology had broader requirements for the acidity and alkalinity of the reaction environment than Fenton technology [19]. The scientific pH value of the reaction environment could play a key role in the chemical reaction and sludge dewatering effect. Based on the above-mentioned n(Fe²⁺):n(SPS) and persulfate dosage parameters, this section paid more attention to the influence of the reaction pH on the sludge dewatering performance.

As Shown in Figure 4, the first and foremost, the influence of pH on Wc presented an arc-shaped bowl shape, which appeared the neutral point as the symmetry axis. Wc decreased with the increase of pH, and the lowest Wc was 70.9%. With the increase of the alkaline of the reaction, t Wc gradually increased, which showed the chemical pretreatment was completely invalid. This outcome was contrary to the literature [8] and that [18], the positive correlation between water content and pH under the conditions of HSO 5 − reagent ( HSO 5 − concentration 0.9 mmol/gVSS, n(Fe²⁺)/n( HSO 5 − ) = 0.9) was given [8], and the literature [18] kept behind (CaO: 70 mg·g⁻¹ DS, Fe²⁺: 50 mg·g⁻¹ DS; H₂O₂: 30 mg·g⁻¹ DS). This section obtained the relatively best cost-effective n(Fe²⁺):n(SPS) and dosage and other parameters which were especially suitable for neutral reaction environment without pH adjustment. Thus indicated more significant superiority to further

simplify the complexity and cost of the sludge pretreatment process. There is one more point, there was a significant difference between the change rule of CST and Wc. Under acidic and neutral conditions, CST value remained relative stable, with a fluctuation range of less than 6%, when pH exceeded 8, CST value gradually increased to 205 s·L/g SS (pH = 8), and the sludge dewatering rate dropped rapidly. It could be inferred that: 1) the oxidation performance of SO 4 −   ⋅ radicals were weakened in a strong acid reaction environment (pH = 2 - 4), and Tc and Wc were relatively higher; 2) the process parameters we recommended were especially suitable for a neutral reaction environment (pH = 6 - 8), the sludge treatment effect was the most ideal, CST value and Wc reduction rates are 53.5% and 16.5% respectively. A possible explanation for those might be the introduction of race amounts of OH·system where OH· and SO 4 −   ⋅ existed simultaneously. The combination of the two could trigger a more violent oxidation reaction and destroy the extracellular polymer of sludge. This phenomenon had also been preliminary showing by Liang, Su et al. [20]; 3) the process parameters we recommended were not suitable for alkaline reaction environment (pH = 10 - 12). The strong alkaline reaction environment would significantly inhibit the activity of Fe²⁺, which was related to the rapid precipitation of Fe²⁺ ions at higher pH. It also deflected the excitation of free radicals, reduced the cracking effect on the extracellular polymer of the sludge, and weakened the flocculation effect of the sludge itself, so the sludge had a poor dewatering effect and high water content. Wc, CST and Tc could not give us the desired value.

In fact, the pH value of the sludge produced by municipal sewage treatment plants was usually between 6.5 and 8, which was almost close to a neutral reaction environment and very consistent with the process parameters recommended by this research. The conclusion of this paper was beneficial to reduce the unit sludge treatment cost and reduce excess anions and cations. It was of significance to practical guiding significance for effective reducing of the cost and complexity of leachate treatment. However, in some special industries, such as electroplating, mining, etc., the sludge produced by the treatment was particular acidic, and a small amount of alkaline substance was essential to be appropriately adjust the pH value of the sludge reaction environment. These would be helpful to reduce the moisture content of the dehydrated filter cake and reduce the time for sludge dehydration, ensuring the satisfactory effect of sludge regulation.

The addition of skeleton constructs (quicklime, fly ash, phosphogypsum, etc.) could effectively solve the problem of high compressibility of activated persulfate oxidation conditioning sludge. Compared with other physical conditioning agents, the amount of CaO was considered to be carried out at minimal cost, which effectively reduced the specific resistance of sludge and the moisture content of sludge [10] [21]. Coupled with quicklime in the Fe 2 + / S 2 O 8 2 − oxidation system, as a dual conditioning method, the dewatering channel of sludge could be more unobstructed which was beneficial to promote sludge dewatering speed, and the moisture content of the sludge cake was expected to drop below 60% to meet the future of China’s municipal sludge deep dewater Key needs. In this section, a detailed study on the amount of CaO added was studied to effectively alleviate the increase in mud cake caused by the addition of the skeleton construct.

As shown in Figure 5: 1) when the CaO addition amount was in the small dose range (0 - 100 mg·g⁻¹ DS), with the CaO addition amount gradually increasing, the sludge dehydration time decreased rapidly, the dehydration velocity increases rapidly, and the time required for dehydration changes from 105 s (0 mg·g⁻¹ DS) to 78 s (100 mg·g⁻¹ DS), and the dehydration velocity increased by nearly 26%; 2) when CaO addition was at the conventional research dose (100 - 400 mg·g⁻¹ DS), with the increase of CaO addition, the decreasing trend of sludge dewatering time and dehydrated cake moisture content were obviously weakened, the minimum dehydration time was 40s and the most desirable Wc was 55.34% (400 mg·g⁻¹ DS); 3) When the amount of CaO exceeded 400 mg·g⁻¹ DS, Tc increases slightly, and Wc was no longer significantly decreasing; 4) the change trends of CST and Wc corresponding to Tc were similar. In the early stage, the two indexes decreased rapidly with the increase of CaO addition. When the amount of CaO added exceeded 200 mg·g⁻¹ DS, the CST value basically fluctuated around 38 s, the minimum was 36s (300 mg·g⁻¹ DS), and the reduction of conditioning effect was no longer remarkable. These showed that, on the one hand, calcium ions in quicklime could be combined with negative charged groups in extracellular polymers to increase the strength of sludge flocs, which was beneficial to the removal of bound water during post-treatment. On the other hand, the inorganic components in the quicklime could be used as the skeleton of the sludge, reducing the compressibility of the sludge, greatly shortening the dewatering time of the sludge, and improving the dewatering efficiency of the sludge.

Considering the above research conclusions, combined the growth of sludge weight caused by the addition of skeleton constructs, it was recommended that the most suitable for practical application of the scale for CaO and activated persulfate was less than 200 mg·g⁻¹ DS. For the acidic sludge, the amount of quicklime added could be increased as appropriate. On the one hand, the pH value of the sludge liquid could be effectively adjusted to a neutral point, which was conducive to floc destruction and recombination. On the other hand, the addition of quicklime could also greatly shorten the sludge dewatering time, but it was recommended to no more than 400 mg·g⁻¹ DS.

Based on the current research of sludge conditioning, a set of data of the typical Fe 2 + / S 2 O 8 2 − composite CaO conditioning method, Fenton oxidation method and CPAM conditioning method were summarized and analyzed. Among them, the Fenton oxidation method quoted from Liu Peng et al. [22], and the CPAM conditioning method quoted from Liu Lirong et al. [23]. The reaction conditions were shown in Table 1.

After different conditioning reactions, CST and Wc were measured. It could be seen from Figure 6, the Fe 2 + / S 2 O 8 2 − composite CaO conditioning method had obvious advantages over the Fenton oxidation method and the CPAM conditioning method, especially in the CST value and moisture content index. The Fenton oxidation method had relatively stringent requirements for the pH of the reaction (pH = 3 - 4). Compared with the Fe 2 + / S 2 O 8 2 − + CaO compound conditioning process, Fenton oxidation had a limited application prospects due to the potential safety hazards of storage and transportation caused by the usage of hydrogen peroxide and the corrosion of acid filtrate [24]. The commonly CPAM method improved the sedimentation performance and dewatering performance of the sludge, but still had problems such as larger dosage, higher cost and secondary pollution [18]. Clearly, it was impossible to add a pure coagulant to meet the key needs of China’s municipal sludge deep dewatering in the future.

Table 2 showed the typical research results of different advanced oxidation technologies for sludge conditioning. It could be summarized: 1) in the categories of reaction type, activated persulfate was better than Fenton oxidation method for deep dewatering of sludge. The CST reduction rate was highest, and it had the advantages of high efficiency and simple operation, would have more research potential and application prospects; 2) in the reaction conditions, the conditions determined by this research were milder, suitable for most of the original conditions of municipal sludge, and could effectively reduce the cost of reagents and the input of excess ions, and reduce the filtrate treatment pressure; 3) in the following influence, the time required for the dehydration in this research was significantly reduced and the moisture content was distinctive more pleasurable .

The Fe 2 + / S 2 O 8 2 − composite CaO sludge conditioning method could destroy the sludge floc structure and reduce the organic matter content in the dewatered sludge. The STOC index detection could effectively reflect the solubility of total organic carbon in filtrate and the degree damage for sludge EPS. So the filtrate after Fe 2 + / S 2 O 8 2 − composite CaO conditioning was subjected to STOC detection and partial anion and cation analysis. The test results were shown in Figure 7.

From Figure 7, the STOC of the filtrate from the blank control group and CPAM method were nearly identical (105 mg/L), and the filtrate STOC content of Fe 2 + / S 2 O 8 2 − composite CaO sludge conditioning method (sodium persulfate and potassium persulfate seemed similar) was surprisingly 580.5 mg/L, the Fenton filtrate STOC content was 305 mg/L, the activated persulfate compound CaO conditioning method significantly improved the total organic carbon concentration in the sludge filtrate and he degree damage for sludge EPS. After measurement, the content of iron ions in the sludge dewatering filtrate was 1.9 mg/L (equivalent to 0.095 mg/gDS), and the iron loss rate was 0.5%, which was better than the lowest value of 0.6% in the literature [18], 99.5% iron stayed in the filter cake. These shows that: 1) the activated persulfate compound CaO conditioning method had a strong destructive effect on the extracellular polymer in the sludge, dissolving a large amount of organic matter, and the efficiency of EPS cracking was greatly higher; 2) Fenton oxidation method could crack the sludge EPS by oxidation as well but the weaker effect indicated that the application of this oxidation system to sludge conditioning still needed further improvement; 3) the higher STOC value indicated the higher total organic carbon concentration in the filtrate, which further proved that the activated persulfate compound CaO conditioning method synergistically improved dewatering performance of the sludge; 4) in view of the better inherent effect of the filtrate itself [29] [30], it was recommended that the filtrate should be returned to the sludge thickening tank to give full use of the chemical effects of the residual cell breaker and activator to improve the sewage treatment economical, and reduce the operating cost of sewage treatment plants.