Preparation and Application of Sewage Sludge Bio-char/Zero Valent Iron (SSBC/ZVI) Composite for Improving the Biodegradability of a Real Chemical Synthesis-Based Pharmaceutical Wastewater

A new kind of micro-electrolysis filler sewage sludge biochar/zero valent iron (SSBC/ZVI) composite was prepared for a real chemical synthesis based pharmaceutical wastewater pretreatment for improving the biodegradation index (BI). The optimal operation condition of micro-electrolysis system was obtained at HRT of 2 h, the initial pH of 3.0 and filler dosage of 100 g/L, with COD removal rate of 30.5%. Characterization analysis of raw/used SSBC/ZVI filler and GC–MS analysis of raw/treated pharmaceutical wastewater were conducted to reveal the pollutant removal mechanisms by micro-electrolysis, and the results suggested that the pollutants removal was mainly attributed to the combination of reduction and oxidation, absorption of SSBC and flocculation of iron sludge. SSBC/ZVI exhibited relative high stability and excellent reusability for COD removal and BI improvement of real pharmaceutical wastewater after 4 runs. Moreover, the leaching tests of heavy metals (HMs) were conducted to investigate their release of the constituents in SS, and the results indicated that HMs leaching into the liquid from the filler was at a relative low level. This study provided new ideas of sewage sludge utilization for real wastewater pretreatment.


Introduction
The pharmaceutical industry is one of the most polluting industries which generating huge amount of wastewater during the production process. The composition of wastewater produced from different pharmaceutical units differs obviously, and the organic content in chemical synthesis based pharmaceutical wastewater (CSPW) usually varies between 800 and 29,000 mg/L [1]. Moreover, pharmaceutical wastewater (PW) containing hazardous and recalcitrant compounds are often non-biodegradable [2,3]. PW treatment is a very challenging task because of its varying composition, recalcitrant nature, high toxicity, and low biodegradability [4]. Various techniques have been developed for PW treatment including physicochemical methods such as adsorption, sedimentation, advanced oxidation processes (AOPs) and biochemical methods [5,6]. Although biochemical processes provide the cheapest option for wastewater treatment, it often exhibits limited pollutants removal efficiency for PW with high toxicity. AOPs provide an efficient option to treat effluents with low biodegradability. Therefore, varieties kinds of AOPs techniques were explored to remove the refractory pollutants and to improve the biodegradability index (BI, BOD 5 /COD) of PW [7].
Micro-electrolysis technique attracted much more attention recently in refractory wastewater pretreatment due to its low operation cost, simple process control and high pollutants removal efficiency [8]. Fe-C micro-electrolysis has been proven to be highly efficient and low-cost in the treatment of various wastewater, including pharmaceutical wastewater [9], petroleum wastewater [10], chemical wastewater [11], etc. In Fe-C micro-electrolysis system, Fe 0 and activated carbon (AC) act as the anode and cathode, respectively, and numerous microscopic galvanic cells will form spontaneously. In anode, Fe 0 loses electrons and dissolves into the solution in the form of Fe 2+ . For the cathode, AC accepts and transfers the electrons to the acceptors. Under anoxic conditions, H + will accept the electrons to form [H] with excellent activity for organic pollutants reduction. If oxygen exists, ‧OH with strong oxidizing property will form and the organic pollutants will be oxidized. [12]. In addition, the pollutants removal by Fe-C micro-electrolysis also includes the adsorption of AC and the flocculation and precipitation effect by ferric and ferrous hydroxides. The pollutants removal mechanisms by micro-electrolysis pretreatment can be summarized as oxidation reduction, coagulation and flocculation, physical adsorption [13]. Previous studies suggested that high COD removal efficiency can be achieved by micro-electrolysis pretreatment and the BI values of refractory wastewater with high organics concentration can be improved obviously [5].
Sewage sludge bio-char (SSBC) is one of the pyrolysis products of sewage sludge, which can be used as an adsorbent for wastewater treatment. In this study, a new kind of micro-electrolysis filler SSBC/ZVI composite was prepared using SSBC as the cathode for pretreating a real chemical synthesis-based pharmaceutical wastewater. Micro-electrolytic fillers preparation using SSBC for PW pretreatment process is expected to realize the waste resources valorization and process pollution emission reduction. To the best of our knowledge, there was no report about Fe-C microelectrolysis filler using SSBC for improving the biodegradability of a real PW. The SSBC/ZVI composites were characterized by scanning electron microscopy coupled to an energy dispersive X-ray spectrometry (SEM-EDX), fouriertransform infrared spectroscopy (FT-IR), x-ray diffraction (XRD) and specific surface area analysis. The pretreatment effect of PW was evaluated by BI improvement.

Materials and Reagents
Dewatered anaerobically digested SS with a moisture content of 82.1% (wt%) was obtained from Chongqing Sino French Water Company. The digested SS was oven dried at about 105 °C for 12 h, and then milled and the particles with the size lower than 0.18 mm were collected. CSPW was provided by a chemical synthesis-based pharmacy company in Chongqing manufacturing cephalosporins and anticancer drugs. The wastewater used in this research was mainly generated from products manufacturing and equipment cleaning processes. Relevant characteristics of the CSPW were measured and the results were shown in Table 1. ZVI (300 mesh, 98.0%) was purchased from Nangong Xindun Alloys spraying Co. Ltd. (China). NaHCO 3 (≥ 98.5% purity) was purchased from Tianjin Kaitong Chemical Reagent Co., Ltd (China). Attapulgite clay (ATP) was purchased from Fuchen Chemical Reagents Co. Ltd (China).

Preparation of SSBC/ZVI Composite
SSBC with the yield of 64.3% (wt%) was prepared as described in our previous research [14]. SSBC preparation was carried out on a fixed bed reactor under vacuum condition with − 0.1 MPa by the vacuum pump. The heating program of the reactor was from ambient temperature to 400 °C at a rate of 10 °C/min and then kept at 400 °C for 60 min. The cooling process was from 400 °C to 105 °C at the rate of 1 °C/min, and maintained at 105 °C until taken out from the oven chamber. The obtained SSBC was milled and sieved again, and particles with the size below 0.15 mm were used for SSBC/ZVI composite preparation. The mass ratio of SSBC to ZVI was controlled at 2:1 due to the low carbon content of SSBC, and supplement level of ATP which acts as adhesive was according to the mechanical strength of the filler. In addition, certain amount of sodium bicarbonate was added to increase the porosity of the filler. For composite preparation, SSBC powder, ZVI powder, ATP and NaHCO 3 as main raw material in a mass percentage of 50, 25, 20 and 5 were granulated to particles with the diameter of 10 mm and then sintered at 800 °C for 60 min under an oxygen-free circumstance. NaHCO 3 was added as a pore-forming agent in the raw materials, and the prepared SSBC/ZVI filler contains a certain amount of Na 2 CO 3 after the release of CO 2 and H 2 O under high temperature. In the process of micro-electrolysis, the residual Na 2 CO 3 will consume H + in the liquid phase, which will decrease the effect of micro-electrolysis. Therefore, the filler should be washed for residual Na 2 CO 3 removal before use. The fillers were washed by deionized water with the solid liquid ratio of 1:10 for 12 h, and then the washed fillers were dried at 105 ºC for 120 min under vacuum condition for subsequent batch and repeated tests.

Characterization of SSBC/ZVI Composite
The surface area, porosity and pore volume of fresh and used SSBC/ZVI composite was determined by measuring the adsorption/desorption of N 2 at 77 K according to the Brunauer-Emmett-Teller (BET) principle using a Micromeritics ASAP 2460 (Micromeritics, America). FT-IR was performed to identify the functional groups of SSBC/ ZVI composite. To obtain the FT-IR spectra, samples were milled and mixed with KBr and then pressed into pellets. FT-IR spectra were recorded in the range of 4000-400 cm −1 with the resolution of 32 scans and 4 cm −1 on a Shimadzu IR Prestige-21 Spectrometer. Scanning electron microscopy (SEM, Zeiss Sigma 300, Germany) equipped with an energy dispersive spectrometer (EDX, XFlash 6, Bruker, Germany) detector was used to characterize the microscopic morphology of SSBC/ZVI composite. The crystal structures of SSBC and SSBC/ZVI composite were determined by XRD (X'Pert Pro MPD, Cu Kα radiation). The data in the 2θ range from 5°to 90° were collected with a scanning velocity of 10°/min.

Batch and Repeated Experiments for CSPW Treatment
Batch CSPW treatment was conducted in 250 mL of glass bottles placed in a water bath shaker at 25 °C and 50 rpm. Each bottle contained 100 mL supernatant of CSPW after centrifugation at 2770 g RCF for 10 min. The initial pH was adjusted into desired values by sulfuric acid, and then the washed SSBC/ZVI composite was added into the reactor with different solid/liquid ratio. 1 mL of sample was taken at certain time intervals for analysis. After reaction, the SSBC/ ZVI composite filler was taken out and dried at 105 ºC for 120 min under vacuum condition for materials characterization. Repeated CSPW treatment was also conducted in 250 mL of glass bottles placed in a water bath shaker at 25 °C and 50 rpm. The conditions of reaction time, initial pH, fillers dosage level were based on the results of batch experiments. For reusability tests of SSBC/ZVI composite fillers, the fillers used in the preliminary experiment were directly used in the subsequent tests after soaking up surface water using the filter paper without drying.

Wastewater Analysis
Chemical oxygen demand (COD) of CSPW was measured by COD analyzer (Lianhua Technology Company, China) according to the users instructions. Total phosphorus (TP) in CSPW was measured by ammonium molybdate spectrophotometric method. Ammonia nitrogen (NH 4 + -N) concentration was measured by spectrophotometry method based on the Chinese standard method (HJ 535-2009 China). pH value was measured by a pH meter (Leichi Instrument Factory, Shanghai, China). Total organic carbon (TOC) and total nitrogen (TN) of CSPW was analyzed using a TOC analyzer (Shimadzu TOC-V CPH, Japan). Qualitative analysis of pollutants in CSPW was conducted on gas chromatographymass spectrometry (Shimadzu GCMS-QP 2010 plus, Japan). Before analysis, the sample was centrifuged for 10 min at 8110 g RCF, and the supernatant was filtered through a 0.45 μm membrane filter for analysis. The injection volume of each analysis was controlled at 1 μL with split ratio of 10:1. The compounds were separated by Agilent DB-17 MS column (30 m × 0.25 μm × 0.25 mm) with helium as the carrier gas. The injector was kept at 300ºC. The oven heating program was from 60 ºC (holding for 2 min) to 300 ºC (holding for 5 min) at a rate of 10 ºC/min. MS analysis was performed in full scan acquisition mode, in the mass range 29-500 m/z at 1 scan/s. The main pollutants were identified by comparing the mass spectra with the NIST library and similarity index calculation results by the software. In addition, the qualitative analysis results are also confirmed by comparing with the pollutant types in the environmental assessment reports.

Heavy Metals Analysis Leached from the SSBC/ZVI Composites
The concentrations of six kinds of heavy metals (HMs) leached from the SSBC/ZVI were measured using coupled plasma optical emission spectrometer, Agilent ICP-OES 720 (Agilent Technologies).

Characterization of SSBC/ZVI Composite
In order to characterize the characteristics of SSBC/ZBI composite filler and identify the mechanism of pollutant removal, fresh and used SSBC/ZVI composite was characterized by SEM-EDX, FT-IR, XRD and specific surface area analysis. The results and discussion about the characterization of materials were presented as follows.

SEM-EDX Characterization
The microstructure of fresh SSBC/ZBI composite with 200 and 10,000 times magnifications was presented in the SEM results in Fig. 1a. It can be found that the filler surface was heterogeneous due to the inconsistency of the particle size and density of the raw materials. The lamellar structure with relative smooth surface and short rod crystal pieces should be assigned to ferron particles and ATP, respectively. Previous research suggested that the surface of Fe 0 in the fresh Fe-C micro-electrolytic filler is relatively smooth [15]. The ATP acts as the adhesive is short rod with 2 μm in length and about 100 nm in diameter [16]. The rest amorphous material of the filler which contains rough, irregular structures with very few pores is SSBC [17]. EDX element contents analysis (shown in Fig. 1b) made clear to us that the filler contains iron, carbon and oxygen derived from the organic matter of SS, aluminum and potassium derived from the inorganic components of SS, and magnesium, silicon, and oxygen in ATP. Approximately, 32.15% iron and 13.56% carbon were measured in the SSBC/ZVI composites by EDX.

FT-IR Characterization
FT-IR characterization was conducted to obtain the chemical structure and functional groups information of SSBC/ ZVI composite, and the result was presented in Fig. 1c. The strong peak at about 3437 cm −1 was caused by O-H stretching vibration [18]. The peak at 1632 cm −1 was preferentially assigned to the C=O stretching vibration of carbonyl groups. The peak at 1063 cm −1 was probably caused by C-O-C stretching vibrations. The broad band at about 1035 cm −1 was mainly related to C-O stretching vibration [19]. The weak peak at 770 cm −1 was probably associated with C-H bending vibration, while the peak at 779 cm −1 was correspond to the stretching vibration of aromatic C-H groups [20]. Diversity of functional groups explains that SSBC and SSBC/ZVI can be used as adsorbent for multiple pollutants removal. c d Fig. 1 Characterization of fresh SSBC/ZVI composite filler a SEM image, b Energy dispersive X-ray spectroscopy, c FT-IR spectra, and d XRD spectra

XRD Characterization
XRD characterization was conducted to investigate the crystal structures of Fe 0 and carbon on the surface of SSBC/ZVI composites. As the results shown in Fig. 1d, the SSBC/ZVI showed obvious peaks at 2θ of 44.9°, which corresponds to the reflection of Fe 0 [21]. The weak bands at 2θ of 25.9°indicated the carbonaceous structure presented in SSBC/ZVI [22]. The diffraction peaks of SiO 2 were often appeared in SSBC as previous reports [23]. XRD analysis showed that SiO 2 existed in both SSBC and SSBC/ZVI, and their content and morphology remained relatively stable.

Specific Surface Area Analysis
The evolution of the BET surface area of SSBC/ZVI filler can be observed in Table 2. As can be seen, the BET surface area of fresh SSBC/ZVI was 32.35 m 2 g −1 , indicating the development of pores at the stage of carbonization and filler calcination. However, the BET surface area was smaller than commercially available Fe-C filler (165.638 m 2 g −1 ) [24], which was due to the relative low specific surface area of SSBC [25].

Evaluation of CSPW Treatment Effect Under Different Condition
The pollutants removal efficiency of micro-electrolysis was closely related to operation conditions, such as reaction time, initial pH, filler dosage In this study, hydraulic retention time (HRT), initial pH, and filler dosage were chosen as operational factors to discuss their influences on COD removal efficiency, and the results were illustrated in Fig. 2.

Pollutants Removal Efficiency with Different HRT
HRT determines the reaction time of micro-electrolysis, and the influence of HRT on COD removal efficiency of CSPW was depicted in Fig. 2a. The COD remove process could be divided into three stages. In the first stage  (0-20 min), 15.2% of COD was removed within the 20 min and the COD removal rate was much higher than the residual two stages, which was due to the porous structure of SSBC/ZVI composite could adsorb the pollutants on its surface [26]. In the second stage (20-120 min), the growth of COD removal efficiency slowed down gradually. On the one hand, the adsorption capacity of SSBC/ZVI filler was limited, and the pollutants absorbed on the filler surface made a mass transfer resistance of electrons [5]. On the other hand, Fe 2+ released in the anode might gathered on the filler surface, which further increased the mass transfer resistance [12]. In the third stage (120-180 min), the COD removal efficiency kept at relative constant value. As the reaction time extended, the pH value of the reaction system will rise to neutral or even alkaline. The microelectrolysis reaction will halt at the neutral or alkaline condition due to the precipitated iron in the form of hydroxides attached on the filling surface, and the accessibility of active [H] and ‧OH will decrease. Base on the results above, a satisfied COD removal efficiency (30.5%) can be achieved at HRT of 2 h.

Pollutants Removal Efficiency Under Different Initial pH Condition
The pH value can affect the availability of active [H] and ‧OH in the reaction system, which in turn affects the pollutants removal efficiency by iron-carbon micro-electrolysis. Five different pH levels (1, 3, 5, 7 and 9) with HRT of 2 h and filler dosage of 100 g/L were other condition, were operated to survey COD removal efficiency in batch experiments. The influence of pH on the pollutants removal efficiency was illustrated in Fig. 2b. With the increase of the pH from 1 to 3, COD removal efficiency was marginally increased from 24.6 to 26.7%. When the pH increased from 3 to 9, the COD removal efficiency decreased gradually from 26.7 to 3.0%. The micro-electrolysis was acid promoted reaction and mainly occurred between anode and cathode materials with the electron transfer. During the micro-electrolysis, the Fe 0 was gradually corroded under the acid condition with electron lose. The electron transfer will be enhanced by increasing the H + concentration and the accessibility of active [H] and ‧OH will be improved [27]. Large amount of Fe 2+ will be generated gradually from Fe0 corrosion, and then the precipitate Fe(OH)x will be created as the Fe 2+ oxidization and H + consumption. The iron hydroxides acts as the mass transfer resistance could be dissolved in the acidic condition, resulting in more reactive sites exposed. In neutral or alkaline condition, the micro-electrolysis reaction will halt. It was generally accepted that the electron transfer rate would be inhibited remarkably in the alkaline condition, thus the COD removal efficiency continued to decline when the initial pH increased from 3.0 to 9.0. In order to achieve an effective COD removal efficiency, the initial pH of CPSW was adjusted to 3 before treatment.

Pollutants Removal Efficiency with Different SSBC/ZVI Filler Dosage
The COD removal efficiency under different filler dosage was studied and the results were shown in Fig. 2c. As illustrated in Fig. 2c, COD removal efficiency increased gradually when the filler dosage increased from 50 to 150 g/L, and the maximum COD removal rate of 33.9% was achieved with 150 g/L filler dosage. This could be explained by the increased surface area, reactive sites and [H] formation when the filler dosage increased [27]. Improved pollutants removal rate will be achieved when the electrode materials dosage increased because the pollutants degradation generally occurs at the active sites of the anode surface [28]. However, the pollutant removal efficiency will decrease when the electrode material dosage is too high, as the contacting area between electrode materials particles and wastewater would decrease. Moreover, micro-electrolysis reactions would be weakened when excess iron reacted with H + [29]. The appropriate dosage of SSBC/ZVI composites was 100 g/L with corresponding COD removal rate of 26.7%.

Characterization of Pollutants Removal by GC-MS
To clarify the pollutants degradation pathways, GC-MS analysis was used to examine the pollutants and derived intermediates in CSPW. The pollutants species and derived intermediates in CSPW were listed in Table 3. Total of 17 main organic pollutant species were detected in raw CSPW, which were the major contributors to COD. For the untreated CSPW, 17 main organic pollutants could be divided into 7 types including alcohols (No.  18,20). After microelectrolysis treatment for 120 min, the number of main pollutant species increased to 23. Specifically, there was no pollutant species were completely removed and 6 new pollutant species were generated after treatment. The above results suggested that the COD of CSPW could be partially removed owing to the abatement of organic pollutant species in SSBC/ZVI treatment. It was obvious that most pollutants presented in CSPW were degraded into smaller molecules after micro-electrolysis treatments, and the above discoveries supported the COD removal by SSBC/ZVI treatment.

Pollutants Removal Mechanisms Analysis
In order to clarify the pollutants removal mechanism by the SSBC/ZVI micro-electrolysis system, the used SSBC/ZVI filler and iron sludge were characterized by EDX and FTIR firstly, and the results were shown in Fig. 3. The EDX analysis was carried out for used SSBC/ZVI composite to show the change of the filler after CSPW treatment (Fig. 3a). It is shown in EDX spectra that after SCPW treatment, the content of Fe in the composite decreased obviously while the content of carbon increased correspondingly, indicating that a great amount of Fe 0 was consumed after micro-electrolysis treatment.
The FTIR analysis of used SSBC/ZVI composite was also conducted to show the change of the filler after CSPW treatment (Fig. 3b). Compared with the FTIR spectrum of fresh SSBC/ZVI (Fig. 1c), several new absorption peaks were found in the FTIR spectrum of used SSBC/ZVI. The peak at 1420 cm −1 was caused by C=O symmetric stretching vibration. The peak at 1320 cm −1 corresponds to C-N stretching vibration of amines. In addition, the peak at 1063 cm −1 covered by the two strong peaks at 1035 and 986 cm −1 . The peak at 986 cm −1 corresponds to C-O-C vibrations of proteins or polysaccharides. The peak at 689 cm −1 was ascribed to C-Cl stretching vibration. The peaks at 513 cm −1 and 472 cm −1 were associated with M-X (M-metal, X-halogen) stretching vibration. Diversity of functional groups in SSBC/ ZVI explains SSBC/ZVI can be utilized for the adsorption removal of multiple pollutants in CSPW. These new absorption peaks in FTIR spectra of SSBC/ZVI indicated that the pollutants in CSPW can be removed through adsorption. In addition, in order to investigate the pollutants removal by SSBC adsorption, separate adsorption experiment was also conducted. The results of adsorption experiments showed that 5.2% of COD could be removed within 2 h under neutral pH conditions and 100 g/L SSBC/ZVI dosage. SSBC/ ZVI showed limited adsorption removal rate of pollutants in CSPW due to the low absorption capacity of SSBC [30].
Iron sludge containing the ferrous and ferric hydroxides was formed by oxidation and precipitation of Fe 2+ released from anode. Pollutants can be removed by iron sludge with the combinational effects of enmeshment, co-precipitation, adsorption and interparticle bridging in the process of coagulation [13]. FTIR characterization of iron sludge (Fig. 3c) showed that there were some new absorption peaks in addition to the infrared absorption peaks produced by the detrital materials exfoliated from SSBC/ZVI. The new FTIR peak at 3055 cm −1 caused by the C-H vibration of ethyl and methylene groups suggested that the pollutants with aliphatic chain could be removed by iron sludge flocculation. The new peak at 1617 cm −1 suggested that compounds containing benzene ring could be also removed by iron sludge flocculation. However, some pollutants may volatilize during the drying process of sample preparation for FTIR characterization and cannot be detected.  Figure 4 proposed the removal mechanisms of pollutants in CSPW treated by SSBC/ZVI micro-electrolysis system. The mechanism of pollutants removal includes the adsorption of SSBC and the coagulation as discussed above, as well as micro-electrolysis [13,31]. In SSBC/ZVI microelectrolysis system, SSBC acts as the cathode in which H + obtain electrons to generate [H] for pollutants reduction. Fe 0 acts as the anode of micro-electrolysis system, and Fe 0 loses electrons to form Fe 2+ . OH radical formed in anode can oxide the pollutants to intermediates or CO 2 and H 2 O. In terms of newly generated organic species as shown in Table 3, the possible formation pathway can be illustrated as follows. Ethylamine (No. 16) was formed by de-isopropyl and de-ethyl of diisopropylethylamine (No. 13) and triethylamine (No. 14), respectively. Formamide

Reusability of SSBC/ZVI Composite Filler
Previous researches reported that the micro-electrolysis fillers can be gradually hardened after the repeated running process owing to the passivation, blockage, and mass transport limitation of the filler surface [5,32]. Long-term run of SSBC/ZVI system for CSPW treatment was conducted to verify whether the fillers could become harden. The operation conditions for 6 runs within 720 min were set as follows with the initial pH of 3, HRT of 2 h, filler dosage of 100 g/L. The COD removal efficiency and BI values were presented in Fig. 5. The COD removal efficiency in SSBC/ZVI was stable and performed well within the first 5 runs. While for the sixth run, COD removal efficiency decreased gradually from 29.1 to 23.9%, owing to the surface passivation, blockage, and mass transport limitation. The results suggested that SSBC/ZVI systems had relative high stability and excellent reusability for COD removal from CSPW. The COD of SSBC/ZVI treated CSPW was still high and the effluent cannot be discharged directly. We explore the feasibility using SSBC/ZVI as pretreatment technology before biochemical treatment, since micro-electrolysis pretreatment could increase the BI value of wastewater, which is helpful for subsequent biological processes [33]. The BI value is normally used to express the biodegradability of wastewater. When BI value is higher than 0.3, the wastewater usually exhibits a better biodegradability [34]. The BI values after different operation runs were analyzed and the results were shown in Fig. 5. Raw CSPW cannot be biodegraded according to the low BI value of 0.17. Improvement in biodegradability was achieved with SSBC/ZVI micro-electrolysis system in the first 4 run operation, and BI value increased from initial 0.17 to 0.39, 0.36, 0.36 and 0.32, respectively. The results indicated the biodegradability of SSBC/ZVI system treated CSPW was enhanced. However, the BI value decreased to 0.29 and 0.27 after the 5th and 6th run. Therefore, after b c Fig. 3 Characterization of used SSBC/ZVI and micro-electrolytic sludge a Energy dispersive X-ray spectroscopy of used SSBC/ZVI, b FT-IR spectra of used SSBC/ZVI, and c FT-IR spectra of microelectrolytic sludge 1 3 4 run operation using SSBC/ZVI for CSPW pretreatment, necessary measures should be taken to ensure the stability of the system. Table 4 shows a comparison of different advanced oxidation processes (AOPs) as pretreatment methods of real chemical synthesis-based pharmaceutical wastewater, which mainly includes ozone oxidation, inner electrolysis, Fenton oxidation, catalytic oxidation and the combination technologies. The pretreatment effect of SSBC/ZVI system on real pharmaceutical wastewater is equivalent to the reported methods such as CoMnAl-CMO-Al 2 O 3 /H 2 O 2 oxides catalyst process [35], microwave assisted Fenton like process [36], but higher than the process using an Fe 2 O 3 / SBA-15 nano composite catalyst [37]. It should be emphasized that the raw material of SSBC/ZVI composites filler is waste sewage sludge, and the reaction process does not require the addition of chemical reagents such as H 2 O 2 . These results indicated that SSBC/ZVI micro electrolysis was suitable for CSPW pretreatment.

Leaching Characters of HMs from SSBC/ZVI Composites
Since the raw material SS contains a certain amount of HMs, HMs leaching occurred during CSPW pretreatment by SSBC/ZVI. Leaching tests of HMs from SSBC/ZVI composites were conducted. The pH values of wastewater will increase during the micro-electrolysis process due to H + consumption by Fe 0 , therefore the initial pH value was controlled at 1, 3, 5, 7 and 9, respectively. Five groups of tests were carried out at room temperature with flask bottles placed on an oscillating shaker for 12 h. The concentrations of six kinds of HMs were measured by ICP-OES, and the test results were shown in Fig. 6. It can be seen from Fig. 6, HMs leaching was greatly affected by the initial pH values. Under acidic conditions, HMs were more likely to be leached from SSBC/ZVI composites. While under neutral or basic conditions, HMs were not easily to be leached. This phenomenon is consistent with previously reported literature [38]. Overall, the concentrations of HMs leached in to the liquid phase were at a relative low level. On the one hand, SSBC preparation by pyrolysis led to better immobilization of HMs [39]. On the other hand, APT used as the adhesive for SSBC/ZVI preparation probably enhanced the Fig. 4 Proposed mechanism of CSPW treatment by SSBC/ZVI composite filler Fig. 5 The reusability of SSBC/ZVI composite filler for COD removal and BI improvement at initial pH 3.0 and filler dosage of 100 g/L 1 3 combination of HMs with mineral matter to decrease their leachability. In the discharge standard of water pollutants for pharmaceutical industry chemical synthesis products category for existed enterprise, the limited concentration of Cd, Cr(VI), Cu, Ni, Pb, Zn were 0.1, 0.5, 0.5, 1.0, 1.0, and 0.5 mg/L, respectively. Therefore, HMs leaching will not affect SSBC/ZVI application for CSPW pretreatment.

Conclusions
In this study, SSBC was used to establish micro-electrolysis systems for real pharmaceutical wastewater pretreatment. The optimal parameters of SSBC/ZVI micro electrolysis system were HRT of 2 h, initial pH of 3 and the filler dosage of 100 g/L. The COD removal rate was 30.5%. According to the characterization results of SEM-EDS, FTIR, XRD analysis of raw and used SSBC/ZVI composites, and the GC-MS analysis of raw and treated CSPW, the removal of pollutants mainly includes adsorption, micro electrolysis and flocculation. Besides, SSBC/ZVI possessed stability in repeated runs for BI improvement of CSPW. Leaching tests results of HMs suggested that the concentrations of HMs leached in to the liquid phase were at a relative low level. Therefore, SSBC/ZVI micro-electrolysis could be considered as a cost-effective, feasible, and promising method for real pharmaceutical wastewater pretreatment.

Data Availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflict of interest
The authors declare no competing interests.

Ethical Approval and Consent to Participate
No animals or human subjects were used in this research.

Consent for Publication
Our manuscript does not contain any individual data in any form.