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Advanced treatment of bio-treated coal chemical wastewater by a novel combination of microbubble catalytic ozonation and biological process
Separation and Purification Technology 197 (2018) 295–301
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Advanced treatment of bio-treated coal chemical wastewater by a novel combination of microbubble catalytic ozonation and biological process
T
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Chun Liu , Xiao-Xuan Chen, Jing Zhang, Hong-Zheng Zhou, Lei Zhang, Yan-Kai Guo Pollution Prevention Biotechnology Laboratory of Hebei Province, School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Microbubble catalytic ozonation Biological process Bio-treated coal chemical wastewater Advanced treatment Performance evaluation
The advanced treatment performance of real bio-treated coal chemical wastewater (BCCW) by a novel combination of microbubble catalytic ozonation and biological process was investigated. The refractory compounds in BCCW could be degraded effectively by microbubble catalytic ozonation treatment, which resulted in efficient COD removal, inorganic nitrogen release and significant improvement of biodegradability. The dissolved oxygen (DO) supplied by microbubble catalytic ozonation was also enough for aerobic digestion in the following biological treatment even without aeration. The biodegradable COD and ammonia nitrogen released in microbubble catalytic ozonation treatment could be removed further in the following biological treatment efficiently. There was no off-gas ozone required to be treated due to high ozone utilization efficiency of close to 100%. The ratio of ozone dosage to influent COD amount showed a marked impact on the performance of the combination treatment system and its optimal value obtained in this study was 0.44 mg/mg. In this case, for microbubble catalytic ozonation, the COD removal efficiency was 32.16%, the ratio of ozone dosage to COD removed was 1.38 mg/mg and the ozone utilization efficiency was 98.0%. For biological treatment, the COD removal efficiency was 41.93%. For the combination system, the total COD removal efficiency was 60.82%, the average final effluent COD concentration was 91.5 mg/L and the estimated total ratio of ozone dosage to COD removed was 0.68 mg/ mg. Therefore, the combination of microbubble catalytic ozonation and biological process is an effective and economical solution for advanced treatment of BCCW.
1. Introduction Coal chemical wastewater is a complex industrial wastewater generated from coal treatment, including high temperature carbonation, coal gas purification and byproduct recovery processes. Currently, the treatment processes used for coal chemical wastewater generally include pretreatment and biological treatment [1]. Biological treatment process is the main treatment technology for coal chemical wastewater due to the low cost, simple operation and maintenance and maximal mineralization of contaminants [2,3]. Many contaminants in coal chemical wastewater are toxic, mutagene and carcinogenic, including phenols, mono- and poly-cyclic nitrogen-containing aromatics, oxygenand sulfur-containing heterocyclic compounds and polynuclear aromatic hydrocarbons (PAHs) [3–5], which makes coal chemical wastewater much recalcitrant for biodegradation. Thus, the secondary effluent from biological treatment process, namely bio-treated coal chemical wastewater (BCCW), contains certain amount of the above substances and cannot meet corresponding discharge standards. Hence, further removal of remaining refractory organic pollutants in BCCW ⁎
remains of fundamental importance to the environment. Recently, advanced oxidation processes (AOPs) have been given more and more interests in the advanced treatment of industrial wastewater, aiming at the removal of the refractory organics remained in the wastewater. AOPs are considered as effective alternatives for converting the refractory contaminants into less harmful or lower chain compounds which can then be treated biologically [6,7]. The AOPs such as catalytic ozonation [8], Fenton/Fenton-like reaction [9], UV photocatalysis [10] and electrochemical oxidation [11] have been used for advanced treatment of BCCW. Catalytic ozonation is a kind of sludge-free AOPs and becomes increasingly promising because of the limited space and high expense for disposal of sludge. Some kinds of real refractory industrial wastewater have been treated by catalytic ozonation to promote mineralization of refractory organic contaminants [12] and improve the wastewater biodegradability [13]. It is noteworthy that catalytic ozonation for complete eliminating pollutants is expensive because the oxidation intermediates tend to be more and more resistant to their chemical degradation [14]. On the other hand, these oxidation intermediates are generally more biodegradable than
Corresponding author. E-mail address: [email protected] (C. Liu).
https://doi.org/10.1016/j.seppur.2018.01.005 Received 16 August 2017; Received in revised form 11 December 2017; Accepted 3 January 2018 Available online 04 January 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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two layers was supposed to be a remixing zone to facilite the uniform water distribution in the upper catalyst layer, considering the weak mixing effect of microbubbles. The larger amount of activated carbon was used to avoid possible frequent catalyst replacement during the continuous operation. The BR had an effective empty bed volume of 42 L, filling three layers of the same granular activated carbon mentioned above with a total filling ratio of 28.6% as carriers to support biofilm growth. The space between each two layers was also supposed to facilite the uniform water distribution in the upper layer, because of the weak mixing effect without aeration in BR and this made the filling ratio relatively lower than usual. The ozone was generated from pure oxygen by an ozone generator with a maximum capacity of 5 g/h (Guanyu, China). The liquid in MCOR was circulated and mixed with dosing ozone gas in a microbubble generator with an OHR mixer (OHR Laboratory Corporation, Japan) to generate ozone microbubbles. The ozone microbubbles were fed into MCOR for catalytic ozonation from the bottom along with circulating liquid flow. The gas-liquid mixture effluent of MCOR was pressed into the bottom of BR without pumping. There was no aeration for BR and the dissolved oxygen (DO) for aerobic digestion in BR would be provided by remaining oxygen after ozone generation and decomposition.
the original molecules. Therefore, there is a great advantageous of integrating catalytic ozonation with biological process for more efficient and cost-effective treatment of refractory and toxic wastewater. Especially, nitrogen compounds, which were difficult to remove in catalytic ozonation, even the concentration increased, were more suitable for biological treatment. It has been reported that combination of ozonation and biological process was successfully applied to advanced treatment of refractory industrial wastewater [15–17], including BCCW [18,19]. There are some limiting factors for traditional ozonation process such as low ozone dissolution and slow gas-liquid mass transfer rate. The microbubble ozonation could overcome some of these limiting factors. Microbubbles have useful characteristics, such as small bubble size (less than 50 μm), huge interfacial area, long stagnation time, lower bubble rising speed, and high interior pressure so that they have an advantage to dissolve ozone gas into water. Chu et al. [20,21] found that microbubbles could help to improve the ozone mass transfer efficiency and further enhanced the soluble contaminant removal of simulated dyestuff wastewater and practical textile wastewater. Furthermore, hydroxyl radical (%OH) generation can be improved during microbubble ozone intrusion [22]. More recently, it has been found that microbubble collapse in microbubble ozonation also promotes %OH generation [23,24]. The enhanced %OH generation from microbubble ozonation can improve the oxidation effects as %OH possesses a higher oxidation potential than molecular ozone. In this study, a novel combination of microbubble catalytic ozonation and biological process was applied for advanced treatment of a real BCCW. The microbubble catalytic ozonation was expected to be responsible for some organic contaminants removal and more important, biodegradability improvement. Then the biodegradable contaminants and ammonia nitrogen could be removed further by the following biological process. The performance of the combination system was investigated and evaluated during continuous long-term operation. The influence of ratio of ozone dosage to influent COD amount on the performance was also discussed.
2.3. Experimental procedure The BCCW with or without BR effluent was pumped into the liquid circulation pipeline to be mixed with circulating liquid flow of MCOR and then entered MCOR from the bottom with ozone microbubbles, as shown in Fig. 1, to enhance the contact between BCCW and ozone microbubbles. For MCOR, the hydraulic retention time (HRT) was controlled as 1 h at a feeding ozone gas flow rate of 2.0 L/min and its average operation temperature was 26.7 °C. For BR, the activated sludge from the biological contact oxidation tank treating this coal chemical wastewater was inoculated into BR at a concentration of 4.0 g/L to enhance biofilm formation on the carriers in the start-up period. In the stable operation period, the HRT of BR was 6 h and its average operation temperature was 22.2 °C. The stable continuous operation of the combination treatment system included three phases according to ratio of ozone dosage to influent COD amount for MCOR. The BCCW was fed into MCOR directly in Phase I and II, and in Phase III the BCCW mixed with 30% effluent of BR was used as the influent of MCOR to reduce its influent COD concentration. The corresponding operating conditions in these three phases were shown in Table 2. The Influent and effluent water testing for both MCOR and BR were conducted on a daily basis for the duration of the experiment to evaluate the treatment performance of the combination system.
2. Materials and methods 2.1. Wastewater A real bio-treated coal chemical wastewater (BCCW) was obtained from the effluent of an upflow anaerobic sludge bed reactor followed by a biological contact oxidation process in a coal chemical byproducts recovery factory. The BCCW characteristics were shown in Table 1. 2.2. Experimental set-up Fig. 1 shows schematic of the novel combination system, including a microbubble catalytic ozonation reactor (MCOR) and a bioreactor (BR). The combination treatment system was installed in the wastewater treatment station of the factory. The MCOR was a sealed pressure vessel with an effective empty bed volume of 25 L. Three layers of 8–10 mm commercial granular coal-based activated carbon were filled in MCOR as catalyst with a total filling ratio of 28.0%. The space between each
2.4. Analytical methods Total COD, BOD5, ammonia nitrogen, nitrate nitrogen and total nitrogen were measured in accordance with the standard method. The DO concentration was measured with an electrochemical membrane electrode (WTW cellOx 325, Germany) and a digital DO meter. UV254 was measured with a UV–Vis spectrophotometer (Techcomp U-3900, China). The TOC concentration was measured using a TOC analyzer (TOC-VCPN, Shimadzu Corporation, Japan). The ozone concentration in the gas phase was measured by iodometric method with KI solution [25]. The dissolved ozone concentration was measured using indigo colorimetric method [26].
Table 1 Characteristics of bio-treated coal chemical wastewater (BCCW). Index
Average value
COD TOC BOD5/COD Ammonia nitrogen Total nitrogen (TN) pH UV254
283.8 mg/L 99.8 mg/L 0.038 4.8 mg/L 13.4 mg/L 8.8 0.98
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Fig. 1. Schematic diagram of the experimental apparatus.
MCOR Pressure gauge
BR Effluent
Influent pump BCCW
Oxygen Ozone
Ozone generator
Microbubble generator 2.0
Table 2 Operating conditions. Phase I
Phase II
Phase III
Operation days/d Feeding ozone gas flow rate/L/min Ozone concentration/mg/L Reflux ratio of BR effluent/% Average influent COD loading rate of MOCR/kg/ (m3 d) Average influent COD loading rate of BR/kg/ (m3 d) Ratio of ozone dosage to influent COD amount/ mg/mg
0–64 2.0 30.3 0 4.75
65–83 2.0 12.5 0 4.23
84–101 2.0 12.5 30 3.16
0.58
0.57
0.64
0.73
0.33
0.44
MCOR effluent
1.5
UV254
Item
MCOR influent
1.0
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Running time (d) 3. Results and discussion
Fig. 2. UV254 variation of influent and effluent of MCOR with time.
3.1. MCOR performance
microbubble catalytic ozonation treatment, respectively, on compounds with the unsaturated bonds, namely carbon–carbon double bonds or the aromatic rings. The ratio of BOD5 to COD (B/C) is a general indicator for wastewater biodegradability. The initial B/C value of BCCW was only 0.038, indicating its very poor biodegradability. Fig. 3 shows that the B/C value of BCCW increased from initial 0.038 to 0.30 after 60-min microbubble catalytic ozonation treatment, indicating the great improvement of BCCW biodegradability. The degradation of refractory aromatic compounds and subsequent generation of small molecular organic compounds by microbubble catalytic ozonation treatment improved BCCW biodegradability, which would make the following biological treatment more efficient.
3.1.1. UV254 variation and biodegradability improvement UV absorbance at 254 nm, namely UV254 value, represents the existence of unsaturated carbon bonds including aromatic compounds which are generally biorefractory, and the measurement of which was usually used to interpret the changes in the aromatics content [27,28]. Fig. 2 shows the UV254 values of the influent and the effluent of MCOR. The average UV254 values of the influent were 0.90, 1.27 and 0.80 in Phase I, II and III, respectively. The UV254 values of the effluent reduced obviously after microbubble catalytic ozonation treatment. The average UV254 values of the effluent were 0.41, 0.63 and 0.41 in Phase I, II and III, respectively. The corresponding UV254 removal efficiencies were 53.7%, 50.3% and 46.5%. The reductions of UV254 value could be attributed to the indirect attack of HO% mainly generated by collapsing ozone microbubbles [23,24] and catalytic ozone decomposition during
3.1.2. COD removal The adsorption of activated carbon bed for COD removal was 297
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BOD5 /COD
0.30 0.25
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Fig. 3. Variation of ratio of BOD5 to COD of BCCW in microbubble catalytic ozonation treatment.
Fig. 5. Time course of ammonia nitrogen concentration in the influent and the effluent of MCOR.
COD concentration (mg/L)
500 MCOR influent
400
result, the ozone reaction efficiency in Phase I was relatively low although more COD was removed. When this ratio was reduced significantly to 0.33 mg/mg in Phase II by decreasing ozone dosage, the ozone reaction efficiency was improved but COD removal limited by relatively inadequate ozone dosage became less efficient. Then this ratio was increased again to 0.44 mg/mg in Phase III by returning BR effluent to decrease influent COD amount. As a result, the COD removal was improved obviously, compared with Phase II. The ozone reaction efficiency in Phase III was also improved simultaneously because there were less compounds with the unsaturated bonds in the influent of Phase III with a lower average influent UV254 value. Therefore, more ozone might be consumed for COD removal by oxidized-mineralization of organic compounds, not for unsaturated bond degradation.
MCOR effluent
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200 100 0
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Running time (d) Fig. 4. Time course of COD concentration in the influent and the effluent of MCOR.
3.1.3. Nitrogen variation Fig. 5 shows ammonia nitrogen concentrations of the influent and the effluent of MOCR. There was no ammonia nitrogen removal in MOCR after microbubble catalytic ozonation treatment. On the contrary, the average ammonia nitrogen concentration increased from 4.3 mg/L to 8.8 mg/L in Phase I, from 6.5 mg/L to 7.5 mg/L in Phase II, and from 5.1 mg/L to 5.5 mg/L in Phase III after microbubble catalytic ozonation treatment, respectively. The higher ammonia nitrogen concentration in the effluent of MOCR demonstrates that the nitrogencontaining organic compounds in BCCW were oxidized-degraded by microbubble catalytic ozonation treatment, which resulted in ammonia nitrogen release [13]. Fig. 6 shows nitrate nitrogen concentrations of the influent and the effluent of MOCR. The nitrate nitrogen concentration also increased after microbubble catalytic ozonation treatment. The average nitrate nitrogen concentration increased from 5.6 mg/L to 10.2 mg/L in Phase
Nitrate nitrogen concentration (mg/L)
evaluated using air microbubbles in MOCR before microbubble catalytic ozonation treatment, and its average COD removal efficiency was only 2.7%, indicating little COD adsorption capacity of activated carbon bed. Fig. 4 shows COD concentrations of the influent and the effluent of MOCR in microbubble catalytic ozonation treatment. The average COD removal efficiencies in Phase I, II and III were 26.35%, 20.51% and 32.16% with corresponding average effluent COD concentrations of 201.5, 248.8 and 158.2 mg/L, respectively. The average COD loading rates removed were 1.27, 0.89 and 1.04 kg/(m3 d) in Phase I, II and III, respectively. The ratios of ozone dosage and COD amount removed in each operation phase were calculated to evaluate the ozone reaction efficiency, which were 2.72, 1.64 and 1.38 mg/mg in Phase I, II and III, respectively. Obviously, more efficient ozone reaction for COD removal was achieved in Phase III. In addition, the average TOC concentration decreased from 99.8 mg/L of the influent to 71.9 mg/L of the effluent after microbubble catalytic ozonation treatment in Phase III with an average TOC removal efficiency of 28.96%, which was close to the corresponding COD removal efficiency. This indicated that the COD removal in MCOR was attributed to the mineralization of organic contaminants by microbubble catalytic ozonation treatment. Moreover, when single microbubble ozonation was applied in MOCR without activated carbon bed, the average COD removal efficiency was only 12.3%. On the other hand, when coarse ozone bubbles were applied in MOCR with activated carbon bed, the average COD removal efficiency was only 9.6%. Therefore, both microbubble technology and catalytic activity of activated carbon contributed to efficient COD removal in MOCR. The stable and efficient COD removal achieved in the continuous operation of MCOR for several months also indicated the long-term stability of catalytic activity of activated carbon used in microbubble catalytic ozonation treatment. The ozone dosage might be excessive in Phase I when the ratio of ozone dosage to influent COD amount reached to 0.73 mg/mg. As a
30
MCOR influent
25
MCOR effluent
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Running time (d) Fig. 6. Time course of nitrate nitrogen concentration in the influent and the effluent of MCOR.
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catalytic ozonation treatment might be enough for the following biological treatment where the conventional aeration was not necessary.
8 3.1.6. Ozone utilization efficiency The ozone concentrations in the effluent and the off-gas of MCOR were monitored which were found to be dependent on the ratio of ozone dosage to influent COD amount. The average ozone concentrations in the effluent and the off-gas in Phase I were 2.5 mg/L and 2.3 mg/L at the ratio of ozone dosage to influent COD amount of 0.73 mg/mg, respectively. When the ratio of ozone dosage to influent COD amount reduced to 0.33 mg/mg in Phase II, the average ozone concentrations in the effluent and the off-gas decreased to 0.43 mg/L and 0 mg/L, respectively. Furthermore, the average ozone concentrations in the effluent and the off-gas increased to 0.55 and 0.27 mg/L in Phase III, respectively, after the ratio of ozone dosage to influent COD amount increased to 0.44 mg/mg again. Then the ozone utilization efficiency was estimated based on ozone dosage, ozone concentrations in the effluent and the off-gas [20], which were 94.2%, 99.5% and 98.0% in Phase I, II and III, respectively. This result confirmed that the high efficient ozone utilization could be achieved in microbubble catalytic ozonation treatment due to enhanced ozone mass transfer by microbubble technology [20,21].
pH
6 MCOR influent
4
MCOR effluent
2 0
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100
Running time (d) Fig. 7. Time course of pH value in the influent and the effluent of MCOR.
I, from 3.3 mg/L to 8.3 mg/L in Phase II, and from 10.2 mg/L to 13.9 mg/L in Phase III after microbubble catalytic ozonation treatment, respectively. The increased nitrate nitrogen concentration in the effluent should be attributed to the release of nitrate nitrogen by oxidized-degradation of nitrogen-containing organic compounds and the conversion of ammonia nitrogen by ozone oxidation [19].
3.2. BR performance
3.1.4. pH variation Fig. 7 shows pH values of the influent and the effluent of MOCR. The influent pH values were stable and alkaline. Almost no difference in pH between the influent and the effluent was observed during the whole operation of MCOR. The average pH values of the influent and the effluent were 8.51 and 8.41 in Phase I, 8.48 and 8.49 in Phase II and 8.64 and 8.65 in Phase III. The pH values should decrease after ozonation treatment due to oxidative degradation of refractory organic contaminants to generate small molecule organics, including organic acids [8,29]. However, there was no such a pH variation trend when the BCCW was oxidizing-treated in MCOR although COD and TOC were removed efficiently and the biodegradability was improved. The possible buffer effect of this real BCCW caused by its complex quality might make pH stable in microbubble catalytic ozonation treatment.
3.2.1. COD removal and DO concentration When BCCW was treated directly in BR, only 6.4% of influent COD was removed initially and then the biofilm activity was inhibited completely soon. This indicated that the direct biological treatment of BCCW was almost ineffective due to its very poor biodegradability. However, the COD removal became considerably more efficient in BR after microbubble catalytic ozonation treatment where the biodegradability of BCCW was improved significantly. Fig. 9 shows COD concentrations of the influent and the effluent of BR after microbubble catalytic ozonation treatment. The COD removal became stable after initial 20 d operation in Phase I because of full development of biofilm. After that, the average COD removal efficiencies in Phase I, II and III were 45.53%, 43.85% and 41.93% with corresponding average effluent COD concentrations of 103.5, 138.6 and 91.5 mg/L, respectively. The average COD loading rates removed were 0.26, 0.25 and 0.30 kg/(m3 d) in Phase I, II and III, respectively. As mentioned above, there was a certain amount of residual ozone in the outlet of MCOR which depended on the ratio of ozone dosage to influent COD amount in each operating phase. However, the COD removal capacities of BR in these three phases were almost the same. This implied that such a small amount of residual ozone in the outlet of MOCR at different ratios of ozone dosage to influent COD amount would not cause the considerable influence on the COD removal performance of BR. In fact, the little change in COD removal of BR in these
35
400
30
COD concentration (mg/L)
Effluent DO concentration (mg/L)
3.1.5. Dissolved oxygen (DO) concentration in the effluent Fig. 8 shows DO concentrations of the effluent of MOCR after microbubble catalytic ozonation treatment. The DO concentration of the effluent depended on gas-liquid mass transfer of remaining pure oxygen after ozone generation and oxygen generated after ozone reaction. The stable and high DO concentrations of the effluent were obtained in Phase I with an average DO concentration of 23.1 mg/L. The average DO concentrations of the effluent decreased to 15.0 mg/L in Phase II and 12.6 mg/L in Phase III, respectively, due to reduced ozone dosage. Anyway, the DO concentrations of the effluent after microbubble
25 20 15 10 5 0
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Fig. 8. Time course of dissolved oxygen (DO) concentration in the effluent of MCOR.
BR influent BR effluent
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Fig. 9. Time course of COD concentration in the influent and the effluent of BR.
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BR influent BR effluent
15 10
5 0
0
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40 50 60 70 Running time (d)
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Fig. 10. Time course of ammonia nitrogen concentration in the influent and the effluent of BR.
Fig. 11. Time course of UV254 value in the influent and the effluent of BR.
BR. The average influent UV254 values of BR were 0.41, 0.63 and 0.41 in Phase I, II and III, respectively. The corresponding average effluent UV254 values of BR were 0.28, 0.40 and 0.30, respectively. Therefore, the remaining compounds with the unsaturated bonds after microbubble catalytic ozonation treatment could be removed further in the biological treatment although the removal efficiency declined.
three phases was attributed to almost the same biodegradable COD generated in MOCR mainly, considering that the effluent B/C values of MCOR after microbubble catalytic ozonation treatment were detected as 0.30, 0.28 and 0.31 in Phase I, II and III, respectively. The reason for these results might be that ozone was preferentially used to destroy the structures of refractory compounds and the biodegradability of BCCW was almost fully improved to a certain limitation in each phase after microbubble catalytic ozonation treatment. The average effluent DO concentrations of BR without aeration were still as high as 26.5, 19.1 and 14.2 mg/L in Phase I, II and III, respectively, which were even higher than the corresponding influent DO concentrations although COD removal in BR by aerobic degradation consumed a certain amount of DO. The remaining oxygen gas in the effluent mixture of MCOR was considered to supply additional DO in BR. How to make full use of the excess DO to increase COD removal capacity should be investigated further.
3.3. Performance evaluation of the combination treatment system The combination of microbubble catalytic ozonation and biological process was effective for advanced treatment of BCCW and the ratio of ozone dosage to influent COD amount showed a marked impact on the performance of the combination treatment system. The overall treatment performance in Phase III at a ratio of ozone dosage to influent COD amount of 0.44 mg/mg seemed better. In MCOR, the average COD removal efficiency reached to 32.16%, the average ratio of ozone dosage to COD removed was 1.38 mg/mg and the average ozone utilization efficiency was 98.0%. In BR, the average COD removal efficiency reached to 41.93% and the ammonia nitrogen was removed effectively. The total COD removal efficiency of 60.82% and the total ratio of ozone dosage to COD removed of 0.68 mg/mg were achieved in the combination treatment system. Furthermore, no residual ozone was detected in the effluent and the off-gas of BR, which means that all dosing ozone was dissolved and decomposed in the combination treatment system and there was no unused ozone in the off-gas required to be treated. The operation of BR effluent reflux in Phase III could also provide a potential to improve the final effluent quality by increasing the effluent reflux ratio for further treatment of the refractory organic pollutants remaining in the BR effluent, but the treatment capacity would decrease as a cost. Overall, this combination treatment system would be an effective and economical solution for advanced treatment of BCCW, due to low ozone consumed, high ozone utilization efficiency, no residual ozone to be treated and no aeration required for biological process. On the other hand, the B/C values of MCOR influent, MCOR effluent (i.e. BR influent) and BR effluent in Phase III were determined as 0.069, 0.31 and 0.098, respectively. This demonstrated that the BCCW biodegradability was improved after treatment in MCOR and became worse again after treatment in BR. Furthermore, when doubling the influent COD loading rate of BR in Phase III, the COD loading rate removed also almost doubled to 0.50 kg/(m3 d) but the COD removal efficiency was similar compared with the case of low influent COD loading. This result imply that the biodegradable COD generated in MCOR could be removed easily and efficiently in BR, but the remaining refractory COD was still difficult to be removed in the biological treatment and made the biodegradability worse again after treatment in BR. Therefore, not COD removal but biodegradability improvement should be paid more attention in microbubble catalytic ozonation treatment of this combination system or other AOPs, and thus more COD could be removed in the following biological process at a much
3.2.2. Nitrogen removal More ammonia nitrogen was generated after microbubble catalytic ozonation treatment of BCCW, which could be converted to nitrate nitrogen in BR by nitrification. Fig. 10 shows ammonia nitrogen concentrations of the influent and effluent of BR. The average ammonia nitrogen concentrations of the influent and the effluent in Phase I were 8.8 mg/L and 4.0 mg/L with an average removal efficiency of 52.9%. In Phase III, the average ammonia nitrogen concentrations of the influent and the effluent were 7.5 mg/L and 3.9 mg/L with an average removal efficiency of 47.2%. In Phase III, the average ammonia nitrogen concentrations of the influent and the effluent were 5.1 mg/L and 3.3 mg/L with an average removal efficiency of 35.5%. Therefore, ammonia nitrogen removal should not be a main concern for this combination system when treating this BCCW because its initial ammonia nitrogen concentration was low and BR was effective to removal ammonia nitrogen. In addition, there was also no total nitrogen (TN) removal in MOCR after microbubble catalytic ozonation treatment since the TN concentrations of the influent and the effluent of MOCR were almost same. For BR, its average influent TN concentrations were 13.3, 14.6 and 7.9 mg/L in Phase I, II and III, respectively and the corresponding average effluent TN concentrations were 11.2, 12.3 and 7.7 mg/L, respectively. The TN removal in the biological treatment depends on nitrification and denitrification process generally. The high DO concentration in BR could inhibit denitrification process and resulted in little TN removal. The biofilm assimilation should be the main reason for the slightly decreased TN concentration in BR. 3.2.3. UV254 variation Fig. 11 shows the UV254 values of the influent and the effluent of BR. The effluent UV254 values reduced further after biological treatment in 300
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lower cost. [7]
4. Conclusions
[8]
The novel combination of microbubble catalytic ozonation and biological process was proven to be an effective and economical solution for advanced treatment of BCCW. The microbubble catalytic ozonation treatment was responsible for efficient degradation of the remaining refractory compounds in BCCW to remove COD partially and improve biodegradability significantly. The following biological treatment was responsible for further removal of degradable COD and ammonia nitrogen which were generated in microbubble catalytic ozonation treatment. The oxygen mass transfer and ozone decomposition in microbubble catalytic ozonation treatment also supplied enough DO for the following biological treatment without aeration. The ratio of ozone dosage to influent COD amount was proven to be an important influencing factor for the performance of the combination treatment system. The better performance of the combination system could be achieved when the ratio of ozone dosage to influent COD was 0.44 mg/mg with a BR effluent reflux ratio of 30%. The COD removal efficiency was 32.16% and the ratio of ozone dosage to COD removed was 1.38 mg/mg in microbubble catalytic ozonation treatment under this condition. The COD removal in the following biological treatment was also enhanced with the average removal efficiency of 41.93% and COD loading rates removed of 0.30 kg/(m3 d). The total COD removal efficiency in the combination system reached to 60.82% with the average final effluent COD concentration of 91.5 mg/L and the total ratio of ozone dosage to COD removed of 0.68 mg/mg. Moreover, the ozone utilization efficiency was as high as 98.0% in microbubble catalytic ozonation treatment and the very small amount of residual ozone could not affect the following biological treatment.
[9]
[10] [11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
Acknowledgment The authors acknowledge the financial supports from special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control, China (17K05ESPCT), and Natural Science foundation of Hebei province, China (E2015208140).
[21]
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Advanced treatment of bio-treated coal chemical wastewater by a novel combination of microbubble catalytic ozonation and biological process
T
⁎
Chun Liu , Xiao-Xuan Chen, Jing Zhang, Hong-Zheng Zhou, Lei Zhang, Yan-Kai Guo Pollution Prevention Biotechnology Laboratory of Hebei Province, School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Microbubble catalytic ozonation Biological process Bio-treated coal chemical wastewater Advanced treatment Performance evaluation
The advanced treatment performance of real bio-treated coal chemical wastewater (BCCW) by a novel combination of microbubble catalytic ozonation and biological process was investigated. The refractory compounds in BCCW could be degraded effectively by microbubble catalytic ozonation treatment, which resulted in efficient COD removal, inorganic nitrogen release and significant improvement of biodegradability. The dissolved oxygen (DO) supplied by microbubble catalytic ozonation was also enough for aerobic digestion in the following biological treatment even without aeration. The biodegradable COD and ammonia nitrogen released in microbubble catalytic ozonation treatment could be removed further in the following biological treatment efficiently. There was no off-gas ozone required to be treated due to high ozone utilization efficiency of close to 100%. The ratio of ozone dosage to influent COD amount showed a marked impact on the performance of the combination treatment system and its optimal value obtained in this study was 0.44 mg/mg. In this case, for microbubble catalytic ozonation, the COD removal efficiency was 32.16%, the ratio of ozone dosage to COD removed was 1.38 mg/mg and the ozone utilization efficiency was 98.0%. For biological treatment, the COD removal efficiency was 41.93%. For the combination system, the total COD removal efficiency was 60.82%, the average final effluent COD concentration was 91.5 mg/L and the estimated total ratio of ozone dosage to COD removed was 0.68 mg/ mg. Therefore, the combination of microbubble catalytic ozonation and biological process is an effective and economical solution for advanced treatment of BCCW.
1. Introduction Coal chemical wastewater is a complex industrial wastewater generated from coal treatment, including high temperature carbonation, coal gas purification and byproduct recovery processes. Currently, the treatment processes used for coal chemical wastewater generally include pretreatment and biological treatment [1]. Biological treatment process is the main treatment technology for coal chemical wastewater due to the low cost, simple operation and maintenance and maximal mineralization of contaminants [2,3]. Many contaminants in coal chemical wastewater are toxic, mutagene and carcinogenic, including phenols, mono- and poly-cyclic nitrogen-containing aromatics, oxygenand sulfur-containing heterocyclic compounds and polynuclear aromatic hydrocarbons (PAHs) [3–5], which makes coal chemical wastewater much recalcitrant for biodegradation. Thus, the secondary effluent from biological treatment process, namely bio-treated coal chemical wastewater (BCCW), contains certain amount of the above substances and cannot meet corresponding discharge standards. Hence, further removal of remaining refractory organic pollutants in BCCW ⁎
remains of fundamental importance to the environment. Recently, advanced oxidation processes (AOPs) have been given more and more interests in the advanced treatment of industrial wastewater, aiming at the removal of the refractory organics remained in the wastewater. AOPs are considered as effective alternatives for converting the refractory contaminants into less harmful or lower chain compounds which can then be treated biologically [6,7]. The AOPs such as catalytic ozonation [8], Fenton/Fenton-like reaction [9], UV photocatalysis [10] and electrochemical oxidation [11] have been used for advanced treatment of BCCW. Catalytic ozonation is a kind of sludge-free AOPs and becomes increasingly promising because of the limited space and high expense for disposal of sludge. Some kinds of real refractory industrial wastewater have been treated by catalytic ozonation to promote mineralization of refractory organic contaminants [12] and improve the wastewater biodegradability [13]. It is noteworthy that catalytic ozonation for complete eliminating pollutants is expensive because the oxidation intermediates tend to be more and more resistant to their chemical degradation [14]. On the other hand, these oxidation intermediates are generally more biodegradable than
Corresponding author. E-mail address: [email protected] (C. Liu).
https://doi.org/10.1016/j.seppur.2018.01.005 Received 16 August 2017; Received in revised form 11 December 2017; Accepted 3 January 2018 Available online 04 January 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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two layers was supposed to be a remixing zone to facilite the uniform water distribution in the upper catalyst layer, considering the weak mixing effect of microbubbles. The larger amount of activated carbon was used to avoid possible frequent catalyst replacement during the continuous operation. The BR had an effective empty bed volume of 42 L, filling three layers of the same granular activated carbon mentioned above with a total filling ratio of 28.6% as carriers to support biofilm growth. The space between each two layers was also supposed to facilite the uniform water distribution in the upper layer, because of the weak mixing effect without aeration in BR and this made the filling ratio relatively lower than usual. The ozone was generated from pure oxygen by an ozone generator with a maximum capacity of 5 g/h (Guanyu, China). The liquid in MCOR was circulated and mixed with dosing ozone gas in a microbubble generator with an OHR mixer (OHR Laboratory Corporation, Japan) to generate ozone microbubbles. The ozone microbubbles were fed into MCOR for catalytic ozonation from the bottom along with circulating liquid flow. The gas-liquid mixture effluent of MCOR was pressed into the bottom of BR without pumping. There was no aeration for BR and the dissolved oxygen (DO) for aerobic digestion in BR would be provided by remaining oxygen after ozone generation and decomposition.
the original molecules. Therefore, there is a great advantageous of integrating catalytic ozonation with biological process for more efficient and cost-effective treatment of refractory and toxic wastewater. Especially, nitrogen compounds, which were difficult to remove in catalytic ozonation, even the concentration increased, were more suitable for biological treatment. It has been reported that combination of ozonation and biological process was successfully applied to advanced treatment of refractory industrial wastewater [15–17], including BCCW [18,19]. There are some limiting factors for traditional ozonation process such as low ozone dissolution and slow gas-liquid mass transfer rate. The microbubble ozonation could overcome some of these limiting factors. Microbubbles have useful characteristics, such as small bubble size (less than 50 μm), huge interfacial area, long stagnation time, lower bubble rising speed, and high interior pressure so that they have an advantage to dissolve ozone gas into water. Chu et al. [20,21] found that microbubbles could help to improve the ozone mass transfer efficiency and further enhanced the soluble contaminant removal of simulated dyestuff wastewater and practical textile wastewater. Furthermore, hydroxyl radical (%OH) generation can be improved during microbubble ozone intrusion [22]. More recently, it has been found that microbubble collapse in microbubble ozonation also promotes %OH generation [23,24]. The enhanced %OH generation from microbubble ozonation can improve the oxidation effects as %OH possesses a higher oxidation potential than molecular ozone. In this study, a novel combination of microbubble catalytic ozonation and biological process was applied for advanced treatment of a real BCCW. The microbubble catalytic ozonation was expected to be responsible for some organic contaminants removal and more important, biodegradability improvement. Then the biodegradable contaminants and ammonia nitrogen could be removed further by the following biological process. The performance of the combination system was investigated and evaluated during continuous long-term operation. The influence of ratio of ozone dosage to influent COD amount on the performance was also discussed.
2.3. Experimental procedure The BCCW with or without BR effluent was pumped into the liquid circulation pipeline to be mixed with circulating liquid flow of MCOR and then entered MCOR from the bottom with ozone microbubbles, as shown in Fig. 1, to enhance the contact between BCCW and ozone microbubbles. For MCOR, the hydraulic retention time (HRT) was controlled as 1 h at a feeding ozone gas flow rate of 2.0 L/min and its average operation temperature was 26.7 °C. For BR, the activated sludge from the biological contact oxidation tank treating this coal chemical wastewater was inoculated into BR at a concentration of 4.0 g/L to enhance biofilm formation on the carriers in the start-up period. In the stable operation period, the HRT of BR was 6 h and its average operation temperature was 22.2 °C. The stable continuous operation of the combination treatment system included three phases according to ratio of ozone dosage to influent COD amount for MCOR. The BCCW was fed into MCOR directly in Phase I and II, and in Phase III the BCCW mixed with 30% effluent of BR was used as the influent of MCOR to reduce its influent COD concentration. The corresponding operating conditions in these three phases were shown in Table 2. The Influent and effluent water testing for both MCOR and BR were conducted on a daily basis for the duration of the experiment to evaluate the treatment performance of the combination system.
2. Materials and methods 2.1. Wastewater A real bio-treated coal chemical wastewater (BCCW) was obtained from the effluent of an upflow anaerobic sludge bed reactor followed by a biological contact oxidation process in a coal chemical byproducts recovery factory. The BCCW characteristics were shown in Table 1. 2.2. Experimental set-up Fig. 1 shows schematic of the novel combination system, including a microbubble catalytic ozonation reactor (MCOR) and a bioreactor (BR). The combination treatment system was installed in the wastewater treatment station of the factory. The MCOR was a sealed pressure vessel with an effective empty bed volume of 25 L. Three layers of 8–10 mm commercial granular coal-based activated carbon were filled in MCOR as catalyst with a total filling ratio of 28.0%. The space between each
2.4. Analytical methods Total COD, BOD5, ammonia nitrogen, nitrate nitrogen and total nitrogen were measured in accordance with the standard method. The DO concentration was measured with an electrochemical membrane electrode (WTW cellOx 325, Germany) and a digital DO meter. UV254 was measured with a UV–Vis spectrophotometer (Techcomp U-3900, China). The TOC concentration was measured using a TOC analyzer (TOC-VCPN, Shimadzu Corporation, Japan). The ozone concentration in the gas phase was measured by iodometric method with KI solution [25]. The dissolved ozone concentration was measured using indigo colorimetric method [26].
Table 1 Characteristics of bio-treated coal chemical wastewater (BCCW). Index
Average value
COD TOC BOD5/COD Ammonia nitrogen Total nitrogen (TN) pH UV254
283.8 mg/L 99.8 mg/L 0.038 4.8 mg/L 13.4 mg/L 8.8 0.98
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Fig. 1. Schematic diagram of the experimental apparatus.
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Table 2 Operating conditions. Phase I
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0–64 2.0 30.3 0 4.75
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3.1. MCOR performance
microbubble catalytic ozonation treatment, respectively, on compounds with the unsaturated bonds, namely carbon–carbon double bonds or the aromatic rings. The ratio of BOD5 to COD (B/C) is a general indicator for wastewater biodegradability. The initial B/C value of BCCW was only 0.038, indicating its very poor biodegradability. Fig. 3 shows that the B/C value of BCCW increased from initial 0.038 to 0.30 after 60-min microbubble catalytic ozonation treatment, indicating the great improvement of BCCW biodegradability. The degradation of refractory aromatic compounds and subsequent generation of small molecular organic compounds by microbubble catalytic ozonation treatment improved BCCW biodegradability, which would make the following biological treatment more efficient.
3.1.1. UV254 variation and biodegradability improvement UV absorbance at 254 nm, namely UV254 value, represents the existence of unsaturated carbon bonds including aromatic compounds which are generally biorefractory, and the measurement of which was usually used to interpret the changes in the aromatics content [27,28]. Fig. 2 shows the UV254 values of the influent and the effluent of MCOR. The average UV254 values of the influent were 0.90, 1.27 and 0.80 in Phase I, II and III, respectively. The UV254 values of the effluent reduced obviously after microbubble catalytic ozonation treatment. The average UV254 values of the effluent were 0.41, 0.63 and 0.41 in Phase I, II and III, respectively. The corresponding UV254 removal efficiencies were 53.7%, 50.3% and 46.5%. The reductions of UV254 value could be attributed to the indirect attack of HO% mainly generated by collapsing ozone microbubbles [23,24] and catalytic ozone decomposition during
3.1.2. COD removal The adsorption of activated carbon bed for COD removal was 297
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BOD5 /COD
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Fig. 5. Time course of ammonia nitrogen concentration in the influent and the effluent of MCOR.
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result, the ozone reaction efficiency in Phase I was relatively low although more COD was removed. When this ratio was reduced significantly to 0.33 mg/mg in Phase II by decreasing ozone dosage, the ozone reaction efficiency was improved but COD removal limited by relatively inadequate ozone dosage became less efficient. Then this ratio was increased again to 0.44 mg/mg in Phase III by returning BR effluent to decrease influent COD amount. As a result, the COD removal was improved obviously, compared with Phase II. The ozone reaction efficiency in Phase III was also improved simultaneously because there were less compounds with the unsaturated bonds in the influent of Phase III with a lower average influent UV254 value. Therefore, more ozone might be consumed for COD removal by oxidized-mineralization of organic compounds, not for unsaturated bond degradation.
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3.1.3. Nitrogen variation Fig. 5 shows ammonia nitrogen concentrations of the influent and the effluent of MOCR. There was no ammonia nitrogen removal in MOCR after microbubble catalytic ozonation treatment. On the contrary, the average ammonia nitrogen concentration increased from 4.3 mg/L to 8.8 mg/L in Phase I, from 6.5 mg/L to 7.5 mg/L in Phase II, and from 5.1 mg/L to 5.5 mg/L in Phase III after microbubble catalytic ozonation treatment, respectively. The higher ammonia nitrogen concentration in the effluent of MOCR demonstrates that the nitrogencontaining organic compounds in BCCW were oxidized-degraded by microbubble catalytic ozonation treatment, which resulted in ammonia nitrogen release [13]. Fig. 6 shows nitrate nitrogen concentrations of the influent and the effluent of MOCR. The nitrate nitrogen concentration also increased after microbubble catalytic ozonation treatment. The average nitrate nitrogen concentration increased from 5.6 mg/L to 10.2 mg/L in Phase
Nitrate nitrogen concentration (mg/L)
evaluated using air microbubbles in MOCR before microbubble catalytic ozonation treatment, and its average COD removal efficiency was only 2.7%, indicating little COD adsorption capacity of activated carbon bed. Fig. 4 shows COD concentrations of the influent and the effluent of MOCR in microbubble catalytic ozonation treatment. The average COD removal efficiencies in Phase I, II and III were 26.35%, 20.51% and 32.16% with corresponding average effluent COD concentrations of 201.5, 248.8 and 158.2 mg/L, respectively. The average COD loading rates removed were 1.27, 0.89 and 1.04 kg/(m3 d) in Phase I, II and III, respectively. The ratios of ozone dosage and COD amount removed in each operation phase were calculated to evaluate the ozone reaction efficiency, which were 2.72, 1.64 and 1.38 mg/mg in Phase I, II and III, respectively. Obviously, more efficient ozone reaction for COD removal was achieved in Phase III. In addition, the average TOC concentration decreased from 99.8 mg/L of the influent to 71.9 mg/L of the effluent after microbubble catalytic ozonation treatment in Phase III with an average TOC removal efficiency of 28.96%, which was close to the corresponding COD removal efficiency. This indicated that the COD removal in MCOR was attributed to the mineralization of organic contaminants by microbubble catalytic ozonation treatment. Moreover, when single microbubble ozonation was applied in MOCR without activated carbon bed, the average COD removal efficiency was only 12.3%. On the other hand, when coarse ozone bubbles were applied in MOCR with activated carbon bed, the average COD removal efficiency was only 9.6%. Therefore, both microbubble technology and catalytic activity of activated carbon contributed to efficient COD removal in MOCR. The stable and efficient COD removal achieved in the continuous operation of MCOR for several months also indicated the long-term stability of catalytic activity of activated carbon used in microbubble catalytic ozonation treatment. The ozone dosage might be excessive in Phase I when the ratio of ozone dosage to influent COD amount reached to 0.73 mg/mg. As a
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catalytic ozonation treatment might be enough for the following biological treatment where the conventional aeration was not necessary.
8 3.1.6. Ozone utilization efficiency The ozone concentrations in the effluent and the off-gas of MCOR were monitored which were found to be dependent on the ratio of ozone dosage to influent COD amount. The average ozone concentrations in the effluent and the off-gas in Phase I were 2.5 mg/L and 2.3 mg/L at the ratio of ozone dosage to influent COD amount of 0.73 mg/mg, respectively. When the ratio of ozone dosage to influent COD amount reduced to 0.33 mg/mg in Phase II, the average ozone concentrations in the effluent and the off-gas decreased to 0.43 mg/L and 0 mg/L, respectively. Furthermore, the average ozone concentrations in the effluent and the off-gas increased to 0.55 and 0.27 mg/L in Phase III, respectively, after the ratio of ozone dosage to influent COD amount increased to 0.44 mg/mg again. Then the ozone utilization efficiency was estimated based on ozone dosage, ozone concentrations in the effluent and the off-gas [20], which were 94.2%, 99.5% and 98.0% in Phase I, II and III, respectively. This result confirmed that the high efficient ozone utilization could be achieved in microbubble catalytic ozonation treatment due to enhanced ozone mass transfer by microbubble technology [20,21].
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I, from 3.3 mg/L to 8.3 mg/L in Phase II, and from 10.2 mg/L to 13.9 mg/L in Phase III after microbubble catalytic ozonation treatment, respectively. The increased nitrate nitrogen concentration in the effluent should be attributed to the release of nitrate nitrogen by oxidized-degradation of nitrogen-containing organic compounds and the conversion of ammonia nitrogen by ozone oxidation [19].
3.2. BR performance
3.1.4. pH variation Fig. 7 shows pH values of the influent and the effluent of MOCR. The influent pH values were stable and alkaline. Almost no difference in pH between the influent and the effluent was observed during the whole operation of MCOR. The average pH values of the influent and the effluent were 8.51 and 8.41 in Phase I, 8.48 and 8.49 in Phase II and 8.64 and 8.65 in Phase III. The pH values should decrease after ozonation treatment due to oxidative degradation of refractory organic contaminants to generate small molecule organics, including organic acids [8,29]. However, there was no such a pH variation trend when the BCCW was oxidizing-treated in MCOR although COD and TOC were removed efficiently and the biodegradability was improved. The possible buffer effect of this real BCCW caused by its complex quality might make pH stable in microbubble catalytic ozonation treatment.
3.2.1. COD removal and DO concentration When BCCW was treated directly in BR, only 6.4% of influent COD was removed initially and then the biofilm activity was inhibited completely soon. This indicated that the direct biological treatment of BCCW was almost ineffective due to its very poor biodegradability. However, the COD removal became considerably more efficient in BR after microbubble catalytic ozonation treatment where the biodegradability of BCCW was improved significantly. Fig. 9 shows COD concentrations of the influent and the effluent of BR after microbubble catalytic ozonation treatment. The COD removal became stable after initial 20 d operation in Phase I because of full development of biofilm. After that, the average COD removal efficiencies in Phase I, II and III were 45.53%, 43.85% and 41.93% with corresponding average effluent COD concentrations of 103.5, 138.6 and 91.5 mg/L, respectively. The average COD loading rates removed were 0.26, 0.25 and 0.30 kg/(m3 d) in Phase I, II and III, respectively. As mentioned above, there was a certain amount of residual ozone in the outlet of MCOR which depended on the ratio of ozone dosage to influent COD amount in each operating phase. However, the COD removal capacities of BR in these three phases were almost the same. This implied that such a small amount of residual ozone in the outlet of MOCR at different ratios of ozone dosage to influent COD amount would not cause the considerable influence on the COD removal performance of BR. In fact, the little change in COD removal of BR in these
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3.1.5. Dissolved oxygen (DO) concentration in the effluent Fig. 8 shows DO concentrations of the effluent of MOCR after microbubble catalytic ozonation treatment. The DO concentration of the effluent depended on gas-liquid mass transfer of remaining pure oxygen after ozone generation and oxygen generated after ozone reaction. The stable and high DO concentrations of the effluent were obtained in Phase I with an average DO concentration of 23.1 mg/L. The average DO concentrations of the effluent decreased to 15.0 mg/L in Phase II and 12.6 mg/L in Phase III, respectively, due to reduced ozone dosage. Anyway, the DO concentrations of the effluent after microbubble
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Fig. 11. Time course of UV254 value in the influent and the effluent of BR.
BR. The average influent UV254 values of BR were 0.41, 0.63 and 0.41 in Phase I, II and III, respectively. The corresponding average effluent UV254 values of BR were 0.28, 0.40 and 0.30, respectively. Therefore, the remaining compounds with the unsaturated bonds after microbubble catalytic ozonation treatment could be removed further in the biological treatment although the removal efficiency declined.
three phases was attributed to almost the same biodegradable COD generated in MOCR mainly, considering that the effluent B/C values of MCOR after microbubble catalytic ozonation treatment were detected as 0.30, 0.28 and 0.31 in Phase I, II and III, respectively. The reason for these results might be that ozone was preferentially used to destroy the structures of refractory compounds and the biodegradability of BCCW was almost fully improved to a certain limitation in each phase after microbubble catalytic ozonation treatment. The average effluent DO concentrations of BR without aeration were still as high as 26.5, 19.1 and 14.2 mg/L in Phase I, II and III, respectively, which were even higher than the corresponding influent DO concentrations although COD removal in BR by aerobic degradation consumed a certain amount of DO. The remaining oxygen gas in the effluent mixture of MCOR was considered to supply additional DO in BR. How to make full use of the excess DO to increase COD removal capacity should be investigated further.
3.3. Performance evaluation of the combination treatment system The combination of microbubble catalytic ozonation and biological process was effective for advanced treatment of BCCW and the ratio of ozone dosage to influent COD amount showed a marked impact on the performance of the combination treatment system. The overall treatment performance in Phase III at a ratio of ozone dosage to influent COD amount of 0.44 mg/mg seemed better. In MCOR, the average COD removal efficiency reached to 32.16%, the average ratio of ozone dosage to COD removed was 1.38 mg/mg and the average ozone utilization efficiency was 98.0%. In BR, the average COD removal efficiency reached to 41.93% and the ammonia nitrogen was removed effectively. The total COD removal efficiency of 60.82% and the total ratio of ozone dosage to COD removed of 0.68 mg/mg were achieved in the combination treatment system. Furthermore, no residual ozone was detected in the effluent and the off-gas of BR, which means that all dosing ozone was dissolved and decomposed in the combination treatment system and there was no unused ozone in the off-gas required to be treated. The operation of BR effluent reflux in Phase III could also provide a potential to improve the final effluent quality by increasing the effluent reflux ratio for further treatment of the refractory organic pollutants remaining in the BR effluent, but the treatment capacity would decrease as a cost. Overall, this combination treatment system would be an effective and economical solution for advanced treatment of BCCW, due to low ozone consumed, high ozone utilization efficiency, no residual ozone to be treated and no aeration required for biological process. On the other hand, the B/C values of MCOR influent, MCOR effluent (i.e. BR influent) and BR effluent in Phase III were determined as 0.069, 0.31 and 0.098, respectively. This demonstrated that the BCCW biodegradability was improved after treatment in MCOR and became worse again after treatment in BR. Furthermore, when doubling the influent COD loading rate of BR in Phase III, the COD loading rate removed also almost doubled to 0.50 kg/(m3 d) but the COD removal efficiency was similar compared with the case of low influent COD loading. This result imply that the biodegradable COD generated in MCOR could be removed easily and efficiently in BR, but the remaining refractory COD was still difficult to be removed in the biological treatment and made the biodegradability worse again after treatment in BR. Therefore, not COD removal but biodegradability improvement should be paid more attention in microbubble catalytic ozonation treatment of this combination system or other AOPs, and thus more COD could be removed in the following biological process at a much
3.2.2. Nitrogen removal More ammonia nitrogen was generated after microbubble catalytic ozonation treatment of BCCW, which could be converted to nitrate nitrogen in BR by nitrification. Fig. 10 shows ammonia nitrogen concentrations of the influent and effluent of BR. The average ammonia nitrogen concentrations of the influent and the effluent in Phase I were 8.8 mg/L and 4.0 mg/L with an average removal efficiency of 52.9%. In Phase III, the average ammonia nitrogen concentrations of the influent and the effluent were 7.5 mg/L and 3.9 mg/L with an average removal efficiency of 47.2%. In Phase III, the average ammonia nitrogen concentrations of the influent and the effluent were 5.1 mg/L and 3.3 mg/L with an average removal efficiency of 35.5%. Therefore, ammonia nitrogen removal should not be a main concern for this combination system when treating this BCCW because its initial ammonia nitrogen concentration was low and BR was effective to removal ammonia nitrogen. In addition, there was also no total nitrogen (TN) removal in MOCR after microbubble catalytic ozonation treatment since the TN concentrations of the influent and the effluent of MOCR were almost same. For BR, its average influent TN concentrations were 13.3, 14.6 and 7.9 mg/L in Phase I, II and III, respectively and the corresponding average effluent TN concentrations were 11.2, 12.3 and 7.7 mg/L, respectively. The TN removal in the biological treatment depends on nitrification and denitrification process generally. The high DO concentration in BR could inhibit denitrification process and resulted in little TN removal. The biofilm assimilation should be the main reason for the slightly decreased TN concentration in BR. 3.2.3. UV254 variation Fig. 11 shows the UV254 values of the influent and the effluent of BR. The effluent UV254 values reduced further after biological treatment in 300
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lower cost. [7]
4. Conclusions
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The novel combination of microbubble catalytic ozonation and biological process was proven to be an effective and economical solution for advanced treatment of BCCW. The microbubble catalytic ozonation treatment was responsible for efficient degradation of the remaining refractory compounds in BCCW to remove COD partially and improve biodegradability significantly. The following biological treatment was responsible for further removal of degradable COD and ammonia nitrogen which were generated in microbubble catalytic ozonation treatment. The oxygen mass transfer and ozone decomposition in microbubble catalytic ozonation treatment also supplied enough DO for the following biological treatment without aeration. The ratio of ozone dosage to influent COD amount was proven to be an important influencing factor for the performance of the combination treatment system. The better performance of the combination system could be achieved when the ratio of ozone dosage to influent COD was 0.44 mg/mg with a BR effluent reflux ratio of 30%. The COD removal efficiency was 32.16% and the ratio of ozone dosage to COD removed was 1.38 mg/mg in microbubble catalytic ozonation treatment under this condition. The COD removal in the following biological treatment was also enhanced with the average removal efficiency of 41.93% and COD loading rates removed of 0.30 kg/(m3 d). The total COD removal efficiency in the combination system reached to 60.82% with the average final effluent COD concentration of 91.5 mg/L and the total ratio of ozone dosage to COD removed of 0.68 mg/mg. Moreover, the ozone utilization efficiency was as high as 98.0% in microbubble catalytic ozonation treatment and the very small amount of residual ozone could not affect the following biological treatment.
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Acknowledgment The authors acknowledge the financial supports from special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control, China (17K05ESPCT), and Natural Science foundation of Hebei province, China (E2015208140).
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