Journal of Hazardous Materials 295 (2015) 1–8
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Influence of powdered activated carbon addition on water quality, sludge properties, and microbial characteristics in the biological treatment of commingled industrial wastewater Qing-Yuan Hu, Meng Li, Can Wang ∗ , Min Ji School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
h i g h l i g h t s • • • •
PAC–AS system shows better COD removal efficiencies in the long-term operation. PAC facilitates the removal of hydrophobic matter and metabolic acidic products. PAC shows bioaugmentation through enhancing biomass, sludge settleability and SOUR. PAC enhances microbial ability but does not change microbial community structures.
a r t i c l e
i n f o
Article history: Received 3 November 2014 Received in revised form 28 February 2015 Accepted 31 March 2015 Available online 1 April 2015 Keywords: Powdered activated carbon Commingled industrial wastewater Water quality Microbial characteristics
a b s t r a c t A powdered activated carbon–activated sludge (PAC–AS) system, a traditional activated sludge (AS) system, and a powdered activated carbon (PAC) system were operated to examine the insights into the influence of PAC addition on biological treatment. The average COD removal efficiencies of the PAC–AS system (39%) were nearly double that of the AS system (20%). Compared with the average efficiencies of the PAC system (7%), COD removal by biodegradation in the PAC–AS system was remarkably higher than that in the AS system. The analysis of the influence of PAC on water quality and sludge properties showed that PAC facilitated the removal of hydrophobic matter and metabolic acidic products, and also enhanced the biomass accumulation, sludge settleability, and specific oxygen uptake rate inside the biological system. The microbial community structures in the PAC–AS and AS systems were monitored. The results showed that the average well color development in the PAC–AS system was higher than that in the AS system. The utilization of various substrates by microorganisms in the two systems did not differ. The dissimilarity index was far less than one; thus, showing that the microbial community structures of the two systems were the same. © 2015 Published by Elsevier B.V.
1. Introduction The powdered activated carbon–activated sludge (PAC–AS) is a wastewater treatment method that combines powdered activated carbon (PAC) and biological treatment, which is also called PACT (powdered activated carbon treatment) process [1,2]. PAC–AS technology integrates adsorption and biodegradation of organic matter with activated sludge (AS) in one unit [3,4]. Activated carbon adsorption has the disadvantages of high price and the biological treatment is not effective for refractory contaminants [5–7]. The PAC–AS process overcomes the shortcomings of the stand-alone
∗ Corresponding author. Tel.: +86 22 27406057; fax: +86 22 27406057. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.jhazmat.2015.03.070 0304-3894/© 2015 Published by Elsevier B.V.
technology. The addition of PAC enhances the biological stability and improves the effectiveness of the AS system with respect to the removal of the refractory compounds with simple operation and low price [4,8]. That was mainly because PAC has large surface area, acting as a supporting medium and supplying bacteria with suitable living micro-environment [8,9]. PAC–AS has been extensively used in wastewater treatment and has also been the subject of an in-depth study around the world [5]. Aziz et al. [10] compared the treatment performance for landfill leachate using conventional sequencing batch reactor (SBR) and PAC–SBR processes. The result showed that the PAC–SBR displayed superior performance in term of removal efficiencies when compared to SBR and also improved the sludge settleability. A municipal sewage treatment plant in Nebraska, USA, obtained high BOD removal efficiencies and improved nitrification performance with
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Fig. 1. Schematic diagram of the experimental apparatus.
Approximately, 10 g of PAC was added to the first system and the system was replenished with 1 g of PAC every 3 days (PAC–AS system). The second system was operated without PAC addition (AS system). In the third system, 10 g of PAC was added and the AS was inactivated by 120 ◦ C autoclave to prevent biodegradation (PAC system). During the operation, 1 mg/L HgCl was added as a biological inhibitor every 3 days to inhibit the growth of active microorganisms in the PAC system. The MLSS of the AS in the three systems was 4000 mg/L. The three systems were operated for 70 days. The hydraulic retention time was 40 h, and the sludge reflux ratio was 200%. Testing water was collected from a chemical industrial park in Tianjin, China. The COD of the influent reached 800–1000 mg/L with a BOD5 /COD ratio of less than 0.2. The AS was obtained from an SBR treating municipal wastewater and was then cultivated with wastewater. The PAC was produced by a company in Zhejiang and dried for 2 h before use. 2.2. Wastewater fraction analysis
the addition of 95–470 mg/L PAC [11]. The technology of powdered activated carbon-membrane bioreactor (PAC–MBR) has also been widely applied in the treatment of polluted water. The addition of PAC not only improves the removal performance and stability of MBR, but also helps to control membrane fouling [12–14]. These studies showed that the addition of PAC significantly improved the treatment performance of wastewater. However, only a few studies have focused on the influence of PAC on subsequent biological processes. A few studies indicated that PAC eliminated the inhibition effects of toxic contaminants on microorganisms. Lee et al. [15] investigated the effect of PAC on the removal of Cr(VI) and COD. The result showed that the average COD and Cr(VI) removal efficiencies with PAC addition were 96% and 41%, respectively, while those without PAC addition were 85% and 9%, respectively. Lim et al. [16] treated wastewater with Cu(II) and Cd(II) using the PAC–SBR process, and the removal efficiencies were improved from 60% to 85%. Martin et al. [17] obtained higher removal efficiencies of phenol and organic compounds using the PAC–AS process to treat highly concentrated organic wastewater containing phenol compared with the traditional AS process. The removal of aqueous Hg(II) and microcystin toxins by PAC is also proven to be effective [18–20]. In this study, a PAC system, a traditional AS system, and a PAC–AS system were established to treat refractory comprehensive industrial wastewater from a chemical industrial park in Tianjin, China. The main pollutants in the water were refractory biodegradable organic compounds, i.e., aniline compounds and benzene serial materials, which was analyzed using GC–MS technology (Aglient 7890A–5975C). The effects of PAC on water quality characteristics, sludge properties, and metabolic characteristics were analyzed to examine the insights into the effects and influence of PAC addition on the biological treatment of commingled industrial wastewater.
2. Materials and methods 2.1. Experimental setup and operation Three sets of parallel operational biological systems were established in this study (Fig. 1). The reactor was made of organic glass with an effective volume of 10 L and a size of 250 mm × 100 mm × 600 mm. A mixer and a pump were used to maintain the concentration of dissolved oxygen at less than 0.5 mg/L in the anaerobic tank and at approximately 2 mg/L in the aerobic tank. The temperature was maintained relatively constant at 20 ◦ C by a heating rod.
The hydrophilicity and hydrophobicity of the influent and effluent were analyzed by the resin fractionation method [21]. Jar tests of three groups were carried out to make resin fractionation of the stand-alone AS, PAC, and PAC–AS systems. The content of each component was characterized by TOC. Amberlite XAD-8 and XAD-4 resin adsorption columns were used in series to adsorb and elute different organic matters in wastewater. On the basis of the adsorption characteristics of components with different resin contents, organic compounds in the water samples were divided into strong hydrophobic acid compounds (HPO-A), strong hydrophobic neutral compounds (HPO-N), transition of hydrophilic acidic compounds (TPI-A), transition of hydrophilic neutral compounds (TPI-N), and hydrophilic compounds (HPI). 2.3. Sludge fraction analysis 2.3.1. Biomass analysis MLSS in the PAC–AS system consists of mixed liquid biomass suspended solids (MLBSS), mixed liquid carbon suspended solids (MLCSS), and inorganic matters. Differential heating method [22] was used to measure biomass, which took advantage of the material balance equation and the actual volatilization measurement of AS and PAC under different temperatures to calculate the MLBSS and MLCSS. Most organic components of biomass can be carbonized and volatilized at 400 ◦ C, while active carbon powder or granular activated carbon can be volatilized at 600 ◦ C for 1 h. The following equations were used: X + Y + Z = W,
(1)
X = W − W 1,
(2)
Y = W 1 − Z,
(3)
where W is the MLSS (mg/L), X is the MLBSS (mg/L), Y is the MLCSS (mg/L), Z is the inorganic matters (mg/L), and W1 and Z are the residual weights (mg/L) after drying at 400 and 600 ◦ C, respectively. To determine W, a certain volume of powdered activated carbon activated sludge mixed liquor is filtered by a burned glass fiber filter, which is constant weight, then dried at 105 ◦ C, and then weighted. The remaining sample is dried at 400 ◦ C for 0.5 h and weighted to obtain W1 and then dried at 600 ◦ C for 1 h and weighted to obtain Z. 2.3.2. Specific oxygen uptake rate analysis Specific oxygen uptake rate (SOUR) referred to the rate of oxygen utilization per unit mass of sludge per unit of time
Q.-Y. Hu et al. / Journal of Hazardous Materials 295 (2015) 1–8
2.4. Biolog-ECO microplate analysis In this study, the Biolog-ECO microplates were used for the rapid identification of bacterial metabolism characteristics by multicarbon source utilization [23,24]. The detailed experimental procedures were described by Kong et al. [25]. The reading for each well of the Biolog-ECO microplate data was corrected by subtracting the value of the water blank for that replicate and standardized by dividing by the average well color development (AWCD) for the replicate [26]: AWCD=
1 31 (R − R0t ) , 3 i=1 it
(4)
where Rit and R0t are the absorbencies of the sole carbon source i and the water blank at time t. To analyze the metabolic characteristics of microorganisms on different carbon sources, the carbon source of the BiologECO microplate is divided into seven categories, namely, alcohols, amines, amino acids, carbohydrates, carboxylic acids, esters, and polymers. The microbial metabolic activity of the carbon source is represented by the absorbance ratio: Ri
fi =
31
,
(5)
Ri
i=1
1 nj f , nj i=1 i
Fj =
(6)
where fi is the absorbance of the carbon source i, Fj is the average absorbance of the category of carbon source j, and nj is the amount of category of the carbon source j. Microbial diversity (DQ) indicates the microbial diversity in a system, and dissimilarity index (D) evaluates the differences between the microbial communities in different systems:
DQ=
2 31
,
(7)
1 31 |f1 − f2 |, 2 i=1
(8)
i=1
D=
fi
where f1 is the absorbance of the carbon source i in the PAC–AS system, and f2 is the absorbance of the carbon source i in the AS system. 3. Results and discussion 3.1. Performance comparison of the PAC–AS and AS systems Because the compositions of the refractory comprehensive industrial wastewater were mainly some refractory biodegradable organic compounds, less carbon sources were available for microbes in the wastewater. Therefore, the degradation of COD was difficult and it became the major pollution indicator of the effluent. Therefore, removal efficiencies of COD were selected to present the performance of systems. The COD removal efficiencies of the three systems are shown in Fig. 2. The PAC system showed removal efficiencies of 45% in the first 4 days mainly because of PAC
50 Removal efficiency of COD %
(SOUR = DO/t × MLSS) and indirectly indicates microbial activity. SOUR was determined using the standard method of OECD209. Blank control trials of two groups were conducted. PAC and some mixed liquor of AS, which were taken from the PAC–AS system and inactivated at 120 ◦ C were, respectively, added to water. The oxygen consumption rates of the two trials were almost zero, which excluded the effect of PAC on dissolved oxygen in the PAC–AS system.
3
40 30
PAC-AS AS PAC
20 10 0 0
10
20
30
40
Time d
50
60
70
80
Fig. 2. Comparison of COD removal efficiency.
adsorption. With the operation of the system, the COD removal efficiencies decreased to 12% until the 15th day. During this period of operation, the COD removal efficiencies improved slightly every time after adding PAC. After 15 days, the COD removal efficiencies of the PAC system were maintained in the range of 6–7% and the active carbon supplement no longer had a significant influence on COD removal efficiency, thus indicating that PAC adsorption played a major role in COD removal in the early stage of operation and had a lesser effect in the later stage. The removal efficiencies of the AS system fluctuated at approximately 20%, whereas the average COD removal efficiencies of the PAC–AS system nearly doubled, reaching 39%. The results showed that PAC adsorption contributed to 7% of the COD removal efficiencies, whereas AS biodegradation contributed to 33% of the COD removal efficiencies in the PAC–AS system. These findings indicated that the COD removal efficiencies of biodegradation in the PAC–AS system were remarkably higher than that in the AS system. These results indicated that the increases in the COD removal efficiencies were caused by the improvement of biodegradation by PAC addition and not by adsorption. Therefore, the influence of PAC addition was examined in detail in the following analyses. 3.2. Effects of PAC addition on water quality The hydrophilicity and hydrophobicity of the influent and effluent of the three systems are shown in Fig. 3. The total removal efficiencies of HPI and TPI-N in the PAC, AS, and PAC–AS systems were 30%, 65%, and 72%, respectively. The PAC system treated wastewater only by activated carbon adsorption. Therefore, in the biological process, HPI and TPI-N were easily removed [21]. Hydrophobic organic compounds (HPO-A and HPO-N) are not easily degraded by AS. The total removal efficiencies of HPO-A and HPO-N in the PAC, AS, and PAC–AS systems were 65%, 50%, and 75%, respectively. The hydrophobic substance removal efficiencies of the PAC system were higher than those of the AS system, thus indicating that when the disposable PAC dosage was large and active carbon adsorption was unsaturated, the adsorption of PAC was stronger than the biodegradation of AS on removing hydrophobic substances. The hydrophobic substance removal efficiency of the PAC–AS system increased by approximately 25% compared with the AS system. These results indicated that PAC preferentially adsorbed and facilitated the removal of hydrophobic matter. The removal efficiency of acidic components was further analyzed. HPO-A and TPI-A are the acidic substances of the five components. Fig. 3 shows that the acid content (HPO-A and TPI-A) of wastewater was relatively high (greater than 30%). The removal efficiencies of HPO-A and TPI-A in the PAC, AS, and PAC–AS systems were 55%, 30%, and 60%, respectively. The removal efficiencies of
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10 8
SOUR
6 4 PAC-AS
2
AS
0 0
20
40
60
80
Time d Fig. 6. SOUR of PAC–AS and AS system.
Fig. 3. Hydrophilicity and hydrophobicity analysis (a) and the ratio of hydrophilic and hydrophobic substances (b).
the PAC–AS and PAC system were similar, but significantly higher than that of the AS system, thus indicating that the addition of PAC facilitated the removal of acidic substances and the biodegradation of microorganism on acidic substances was inefficient. Acidic components were produced in the microbial metabolic process and caused microorganism inhibition [27]. Thus, we observed that the acid content of the effluent in the AS system was relatively higher than that of the influent. PAC can eliminate this inhibition effect. Therefore, in the PAC–AS system, acidic organic compounds were removed by PAC adsorption. 3.3. Effects of PAC addition on sludge properties
300 PAC-AS
SVI mL/g
250
3.3.1. Sludge settleability Sludge volume index (SVI) is an important indicator of the sedimentation performance of AS. Fig. 4shows the changes in SVI values with the operation of the PAC–AS and AS systems. The initial SVI of the two systems is 180 mL/g. After two weeks of operation, the SVI of the AS system increased. The sludge floc dispersed, and the sedimentation performance became worse. After 21 days, the SVI of the AS system remained stable at approximately 250 mL/g and the sludge bulking phenomenon occurred mainly because of the comprehensive water quality of industrial wastewater and the high concentration of acid contents. By contrast, the SVI of the PAC–AS
AS
200 150 100 50 0 0
7
14
21
28
35
42
49
56
63
70
2.5
Time d
PAC-AS
Fig. 4. Sludge volume index of PAC–AS and AS system.
AS
2 8000
MLSS
6000
AWCD(cm-1)
MLBSS˖PAC-AS MLBSS˖AS MLSS˖PAC-AS
4000
1.5 1
0.5
2000
0 0
7
14
21
28
35
42
49
Time d
56
63
70
0 0
30
60
90
120
Time h
Fig. 5. Biomass of PAC–AS and AS system. Fig. 7. AWCD of PAC–AS and AS system (day 45).
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5
1 Polymers
Absorbance ratio
0.8
Amines Esters
0.6 Carboxylic acids Amino acids
0.4
Carbohydrates
0.2 Alcohols
0 5
20
45 Time
70
d
(a) 1 Polymers 0.8
Absorbance ratio
Amines Esters
0.6
Carboxylic acids 0.4
Amino acids Carboh ydrates
0.2 Alcoho ls 0 5
20
Time
d
45
70
(b) Fig. 8. The average absorbance ratio of carbon source in ECO flat in PAC–AS system(a) and in AS system(b).
system decreased continuously with the addition of PAC and then remained stable at approximately 80 mL/g after 28 days, which was close to the normal value of 100 mL/g. These results indicated that PAC addition could improve sludge settleability. 3.3.2. Biomass Before the addition of PAC, the initial MLBSS of the PAC–AS and AS systems was 4000 mg/L. Given that the wastewater lacked a biodegradable carbon source, the biomass in the AS system
declined significantly to 3000 mg/L (day 0–day 21). Thereafter, MLBSS was maintained at approximately 3600–3800 mg/L after 35 days. The comprehensive water quality and adverse environment resulted in sludge bulking, which negatively influenced microorganism growth. The biomass of the PAC–AS system remained stable in the first four weeks and then increased slowly to approximately 5000 mg/L (day 28–day 49). During the operation, the biomass in the PAC–AS system was always higher than that of the AS system (Fig. 5). The addition of PAC improved the adaptability of
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Powdered activated carbon (PAC)
Adsorption
Bioaugmentation
Hydrophobic organic compounds
Enhance
Improve
Strengthen
biomass
metabolic activity
sludge settleability
Improve
System environment
Activated sludge
Fig. 9. Influence of powdered activated carbon on biological treatment.
the PAC–AS system to the adverse environment, thus enabling the PAC–AS system to always maintain a high biomass. 3.3.3. Specific oxygen uptake rate The changes in SOUR with the operation of the PAC–AS and AS systems are shown in Fig. 6. The SOUR in the two systems showed a similar trend: the SOUR first decreased and then increased because the poor water quality affected the normal metabolism of cells. Thereafter, the activity recovered gradually after the microorganisms adapted to the environment. The initial SOUR of the PAC–AS and AS systems was 7.9 and 6.1 mg/(g h), respectively. At the 14th day, the SOUR of the PAC–AS system decreased to the minimum value of 5.7 mg/(g h). By contrast, at the 21st day, the SOUR of the AS system decreased to the minimum value of 3.4 mg/(g h). The SOUR value then increased gradually and became stable. The average SOUR in the entire PAC–AS experiment was 7.5 mg/(g h), whereas that of the AS system was 4.3 mg/(g h), thus indicating that PAC could improve treatment efficiency by maintaining high microbial activity. 3.4. Effects of PAC addition on microbial characteristics 3.4.1. Average metabolic activity The AWCD curves under different conditions of the PAC–AS and AS systems could indicate the differences in internal biological activity. The typical AWCD curve of the two systems at the 45th day is shown in Fig. 7. The AWCD curves consisted of three phases, namely, early period with color variation, index change period with color variation, and final stable period (the cell death phase was not observed because of limited observation time). The AWCD curves of the two systems showed an apparent lag phase in the first 30 h, then increased rapidly, and finally were almost constant after 100 h. The value of the PAC–AS system was stable at approximately 2.0, whereas that of the AS system was 1.5. This result indicated that the microbial metabolic activity of the PAC–AS system was higher. The rate of AWCD change was fastest between 40 h and 60 h. Thus, the rate of this stage could characterized the average activity of aerobic microbial metabolism. The average metabolic activity of the microbial community on the Biolog-ECO microplate during the operation in the PAC–AS and AS systems is shown in Table 1. The microbial activity in the same stage and the change trend of
the two systems were different. In the first 20 days, the microbial metabolic activity of the PAC–AS system was basically unchanged, whereas the metabolic activity of the AS system decreased because of refractory and toxic wastewater quality. During the entire operation process, the average microbial metabolic activity of the PAC–AS system was higher than that of the AS system; thus, indicating that PAC enhanced the microbial ability of resistance to the adverse environment and increased the stability of the microbial metabolic activity. 3.4.2. Substrate utilization and microbial community The substrates can be divided into groups of alcohols, amines, amino acids, carbohydrates, carboxylic acids, esters, and polymers [28]. Qualitative measures of substrate utilization by each group can be calculated using the average absorbency fraction in the same manner as for the entire plate [23,29]. Fig. 8 shows the average absorbency fraction of the microbial metabolic carbon source in the Biolog-ECO microplate with time, from which the similarity of substrate utilization in the two systems can be observed. The average absorbency fraction of microbial metabolic alcohols was the highest, i.e., greater than 0.2 in the two systems. The result indicated that alcohol could be easily utilized by the microbial community in the two systems. The metabolism of carbohydrate decreased first (day 5–day 20) and then gradually increased (day 20–day 45). This result was contrary to the metabolism of lipids. After 45 days of operation, the microbial metabolism of different carbon sources recovered similarly to the initial situation of the experiment. During the entire test, the microbial utilization of various carbon sources was basically unchanged. The metabolism of alcohol increased slightly, whereas the metabolism of amino acids and carboxylic acids decreased slightly. The similarity of the two systems could also be observed from the analyses of DQ and D [30]. The initial values of DQ in the two systems were approximately 30.5, and the value of D was only 0.036. Table. 1 The average microbial metabolism in ECO plate of PAC–AS and AS systems(h−1 ). Days
5
20
45
70
PAC–AS AS
0.0352 0.0276
0.0354 0.0240
0.0369 0.0291
0.0336 0.0238
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With the operation of the system, the value of DQ in the PAC–AS system significantly decreased and reached 26.5 at the 20th day, whereas the value of DQ in the AS system was 29.0. The result showed that the addition of PAC reduced the microbial diversity of the system. This might be explained as follows. Since microbes are special organics, there existed adsorption of PAC for microbes, which contributed to the decrease of the microbial diversity in the initial operation. The value of D increased to 0.12, which was still far less than one. After 45 days, the values of DQ in the two systems recovered to 30.5 and were almost constant. This was mainly because the microbes adapted to the environment and the adsorption of PAC for microbes reduced. At the same time, the value of D decreased to 0.045–0.048. We observed from the entire experiment that the characteristics of the microbial community in the PAC–AS and AS systems did not differ. 4. Influence pathway of PAC addition on biological treatment The water used in the experiment was refractory comprehensive industrial wastewater from a chemical industrial park in Tianjin, China, which had the characteristic of poor biodegradability. The average removal efficiencies of COD in the AS system was 20%. By contrast, the average removal efficiencies of COD increased to nearly double after PAC addition. Through further analysis, we observed that the removal efficiency increased mainly because of the improvement of the metabolic rate of slowly biodegradable organic matter and the removal efficiency of refractory organic matter. The average removal efficiencies of COD in the PAC–AS system was 39%, with 7% of the removal efficiency attributed to the adsorption effect of PAC. The removal of COD by biodegradation in the PAC–AS system was 13% higher than that in the AS system. This finding indicated that the addition of PAC not only improved the adsorption effect of activated carbon for hydrophobic organic compounds, but also strengthened the biodegradation of AS in the PAC–AS system by enhancing biomass, improving metabolic activity and strengthening sludge settleability. The influence mechanisms of PAC addition on biological treatment are shown in Fig. 9. 5. Conclusion Analyzing the impact of PAC on water quality characteristics, the properties of AS, and the microbial community characteristics during the long-term operation, we observed that the mechanism in which PAC enhances the effect of the AS system mainly included the following several aspects: 1. PAC has a strong adsorption capacity for hydrophobic substances. The removal of hydrophobic refractory organic compounds is enhanced after the addition of PAC to the AS system. Acidic substances will accumulate in the system during microbial metabolism. The adsorption of PAC on acid can remove this part of organic matter and eliminate the adverse effects of metabolites on the system environment. 2. In the PAC–AS system, in addition to the adsorption effect, PAC also plays a role in the reinforcement of biodegradation. The addition of PAC does not change the structure of the microbial community and the metabolic pathway of microbial degradation, but can improve the biomass, the metabolic activity, and the settleability performance of AS. In summary, a synergistic effect is observed between the adsorption of PAC and biodegradation. The interaction enables a more effective treatment of comprehensive chemical wastewater in the PAC–AS system compared with the AS system.
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[27] P.F. Storm, P. Hu, Effect of pH on fungal growth and bulking in laboratory – activated sludge, Res. J. WPCF 63 (1991) 276–277. [28] C. Wang, J.Y. Xi, H.Y. Hu, Y. Yao, Effect of UV pretreatment on microbial community structure and metabolic characteristics in a subsequent biofilter treating gaseous chlorobenzene, Bioresour. Technol. 100 (2009) 5581–5587.
[29] J.C. Zak, M.R. Willig, D.L. Moorhead, H.G. Wildman, Functional diversity of microbial communities: a quantitative approach, Soil Biol. Biochem. 26 (1994) 1101–1108. [30] M.J. Preston, L. Boddy, P.F. Randerson, Analysis of microbial community functional diversity using sole-carbon-source utilization profiles – a critique, FEMS Microbiol. Ecol. 42 (2002) 1–14.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Influence of powdered activated carbon addition on water quality, sludge properties, and microbial characteristics in the biological treatment of commingled industrial wastewater Qing-Yuan Hu, Meng Li, Can Wang ∗ , Min Ji School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
h i g h l i g h t s • • • •
PAC–AS system shows better COD removal efficiencies in the long-term operation. PAC facilitates the removal of hydrophobic matter and metabolic acidic products. PAC shows bioaugmentation through enhancing biomass, sludge settleability and SOUR. PAC enhances microbial ability but does not change microbial community structures.
a r t i c l e
i n f o
Article history: Received 3 November 2014 Received in revised form 28 February 2015 Accepted 31 March 2015 Available online 1 April 2015 Keywords: Powdered activated carbon Commingled industrial wastewater Water quality Microbial characteristics
a b s t r a c t A powdered activated carbon–activated sludge (PAC–AS) system, a traditional activated sludge (AS) system, and a powdered activated carbon (PAC) system were operated to examine the insights into the influence of PAC addition on biological treatment. The average COD removal efficiencies of the PAC–AS system (39%) were nearly double that of the AS system (20%). Compared with the average efficiencies of the PAC system (7%), COD removal by biodegradation in the PAC–AS system was remarkably higher than that in the AS system. The analysis of the influence of PAC on water quality and sludge properties showed that PAC facilitated the removal of hydrophobic matter and metabolic acidic products, and also enhanced the biomass accumulation, sludge settleability, and specific oxygen uptake rate inside the biological system. The microbial community structures in the PAC–AS and AS systems were monitored. The results showed that the average well color development in the PAC–AS system was higher than that in the AS system. The utilization of various substrates by microorganisms in the two systems did not differ. The dissimilarity index was far less than one; thus, showing that the microbial community structures of the two systems were the same. © 2015 Published by Elsevier B.V.
1. Introduction The powdered activated carbon–activated sludge (PAC–AS) is a wastewater treatment method that combines powdered activated carbon (PAC) and biological treatment, which is also called PACT (powdered activated carbon treatment) process [1,2]. PAC–AS technology integrates adsorption and biodegradation of organic matter with activated sludge (AS) in one unit [3,4]. Activated carbon adsorption has the disadvantages of high price and the biological treatment is not effective for refractory contaminants [5–7]. The PAC–AS process overcomes the shortcomings of the stand-alone
∗ Corresponding author. Tel.: +86 22 27406057; fax: +86 22 27406057. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.jhazmat.2015.03.070 0304-3894/© 2015 Published by Elsevier B.V.
technology. The addition of PAC enhances the biological stability and improves the effectiveness of the AS system with respect to the removal of the refractory compounds with simple operation and low price [4,8]. That was mainly because PAC has large surface area, acting as a supporting medium and supplying bacteria with suitable living micro-environment [8,9]. PAC–AS has been extensively used in wastewater treatment and has also been the subject of an in-depth study around the world [5]. Aziz et al. [10] compared the treatment performance for landfill leachate using conventional sequencing batch reactor (SBR) and PAC–SBR processes. The result showed that the PAC–SBR displayed superior performance in term of removal efficiencies when compared to SBR and also improved the sludge settleability. A municipal sewage treatment plant in Nebraska, USA, obtained high BOD removal efficiencies and improved nitrification performance with
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Fig. 1. Schematic diagram of the experimental apparatus.
Approximately, 10 g of PAC was added to the first system and the system was replenished with 1 g of PAC every 3 days (PAC–AS system). The second system was operated without PAC addition (AS system). In the third system, 10 g of PAC was added and the AS was inactivated by 120 ◦ C autoclave to prevent biodegradation (PAC system). During the operation, 1 mg/L HgCl was added as a biological inhibitor every 3 days to inhibit the growth of active microorganisms in the PAC system. The MLSS of the AS in the three systems was 4000 mg/L. The three systems were operated for 70 days. The hydraulic retention time was 40 h, and the sludge reflux ratio was 200%. Testing water was collected from a chemical industrial park in Tianjin, China. The COD of the influent reached 800–1000 mg/L with a BOD5 /COD ratio of less than 0.2. The AS was obtained from an SBR treating municipal wastewater and was then cultivated with wastewater. The PAC was produced by a company in Zhejiang and dried for 2 h before use. 2.2. Wastewater fraction analysis
the addition of 95–470 mg/L PAC [11]. The technology of powdered activated carbon-membrane bioreactor (PAC–MBR) has also been widely applied in the treatment of polluted water. The addition of PAC not only improves the removal performance and stability of MBR, but also helps to control membrane fouling [12–14]. These studies showed that the addition of PAC significantly improved the treatment performance of wastewater. However, only a few studies have focused on the influence of PAC on subsequent biological processes. A few studies indicated that PAC eliminated the inhibition effects of toxic contaminants on microorganisms. Lee et al. [15] investigated the effect of PAC on the removal of Cr(VI) and COD. The result showed that the average COD and Cr(VI) removal efficiencies with PAC addition were 96% and 41%, respectively, while those without PAC addition were 85% and 9%, respectively. Lim et al. [16] treated wastewater with Cu(II) and Cd(II) using the PAC–SBR process, and the removal efficiencies were improved from 60% to 85%. Martin et al. [17] obtained higher removal efficiencies of phenol and organic compounds using the PAC–AS process to treat highly concentrated organic wastewater containing phenol compared with the traditional AS process. The removal of aqueous Hg(II) and microcystin toxins by PAC is also proven to be effective [18–20]. In this study, a PAC system, a traditional AS system, and a PAC–AS system were established to treat refractory comprehensive industrial wastewater from a chemical industrial park in Tianjin, China. The main pollutants in the water were refractory biodegradable organic compounds, i.e., aniline compounds and benzene serial materials, which was analyzed using GC–MS technology (Aglient 7890A–5975C). The effects of PAC on water quality characteristics, sludge properties, and metabolic characteristics were analyzed to examine the insights into the effects and influence of PAC addition on the biological treatment of commingled industrial wastewater.
2. Materials and methods 2.1. Experimental setup and operation Three sets of parallel operational biological systems were established in this study (Fig. 1). The reactor was made of organic glass with an effective volume of 10 L and a size of 250 mm × 100 mm × 600 mm. A mixer and a pump were used to maintain the concentration of dissolved oxygen at less than 0.5 mg/L in the anaerobic tank and at approximately 2 mg/L in the aerobic tank. The temperature was maintained relatively constant at 20 ◦ C by a heating rod.
The hydrophilicity and hydrophobicity of the influent and effluent were analyzed by the resin fractionation method [21]. Jar tests of three groups were carried out to make resin fractionation of the stand-alone AS, PAC, and PAC–AS systems. The content of each component was characterized by TOC. Amberlite XAD-8 and XAD-4 resin adsorption columns were used in series to adsorb and elute different organic matters in wastewater. On the basis of the adsorption characteristics of components with different resin contents, organic compounds in the water samples were divided into strong hydrophobic acid compounds (HPO-A), strong hydrophobic neutral compounds (HPO-N), transition of hydrophilic acidic compounds (TPI-A), transition of hydrophilic neutral compounds (TPI-N), and hydrophilic compounds (HPI). 2.3. Sludge fraction analysis 2.3.1. Biomass analysis MLSS in the PAC–AS system consists of mixed liquid biomass suspended solids (MLBSS), mixed liquid carbon suspended solids (MLCSS), and inorganic matters. Differential heating method [22] was used to measure biomass, which took advantage of the material balance equation and the actual volatilization measurement of AS and PAC under different temperatures to calculate the MLBSS and MLCSS. Most organic components of biomass can be carbonized and volatilized at 400 ◦ C, while active carbon powder or granular activated carbon can be volatilized at 600 ◦ C for 1 h. The following equations were used: X + Y + Z = W,
(1)
X = W − W 1,
(2)
Y = W 1 − Z,
(3)
where W is the MLSS (mg/L), X is the MLBSS (mg/L), Y is the MLCSS (mg/L), Z is the inorganic matters (mg/L), and W1 and Z are the residual weights (mg/L) after drying at 400 and 600 ◦ C, respectively. To determine W, a certain volume of powdered activated carbon activated sludge mixed liquor is filtered by a burned glass fiber filter, which is constant weight, then dried at 105 ◦ C, and then weighted. The remaining sample is dried at 400 ◦ C for 0.5 h and weighted to obtain W1 and then dried at 600 ◦ C for 1 h and weighted to obtain Z. 2.3.2. Specific oxygen uptake rate analysis Specific oxygen uptake rate (SOUR) referred to the rate of oxygen utilization per unit mass of sludge per unit of time
Q.-Y. Hu et al. / Journal of Hazardous Materials 295 (2015) 1–8
2.4. Biolog-ECO microplate analysis In this study, the Biolog-ECO microplates were used for the rapid identification of bacterial metabolism characteristics by multicarbon source utilization [23,24]. The detailed experimental procedures were described by Kong et al. [25]. The reading for each well of the Biolog-ECO microplate data was corrected by subtracting the value of the water blank for that replicate and standardized by dividing by the average well color development (AWCD) for the replicate [26]: AWCD=
1 31 (R − R0t ) , 3 i=1 it
(4)
where Rit and R0t are the absorbencies of the sole carbon source i and the water blank at time t. To analyze the metabolic characteristics of microorganisms on different carbon sources, the carbon source of the BiologECO microplate is divided into seven categories, namely, alcohols, amines, amino acids, carbohydrates, carboxylic acids, esters, and polymers. The microbial metabolic activity of the carbon source is represented by the absorbance ratio: Ri
fi =
31
,
(5)
Ri
i=1
1 nj f , nj i=1 i
Fj =
(6)
where fi is the absorbance of the carbon source i, Fj is the average absorbance of the category of carbon source j, and nj is the amount of category of the carbon source j. Microbial diversity (DQ) indicates the microbial diversity in a system, and dissimilarity index (D) evaluates the differences between the microbial communities in different systems:
DQ=
2 31
,
(7)
1 31 |f1 − f2 |, 2 i=1
(8)
i=1
D=
fi
where f1 is the absorbance of the carbon source i in the PAC–AS system, and f2 is the absorbance of the carbon source i in the AS system. 3. Results and discussion 3.1. Performance comparison of the PAC–AS and AS systems Because the compositions of the refractory comprehensive industrial wastewater were mainly some refractory biodegradable organic compounds, less carbon sources were available for microbes in the wastewater. Therefore, the degradation of COD was difficult and it became the major pollution indicator of the effluent. Therefore, removal efficiencies of COD were selected to present the performance of systems. The COD removal efficiencies of the three systems are shown in Fig. 2. The PAC system showed removal efficiencies of 45% in the first 4 days mainly because of PAC
50 Removal efficiency of COD %
(SOUR = DO/t × MLSS) and indirectly indicates microbial activity. SOUR was determined using the standard method of OECD209. Blank control trials of two groups were conducted. PAC and some mixed liquor of AS, which were taken from the PAC–AS system and inactivated at 120 ◦ C were, respectively, added to water. The oxygen consumption rates of the two trials were almost zero, which excluded the effect of PAC on dissolved oxygen in the PAC–AS system.
3
40 30
PAC-AS AS PAC
20 10 0 0
10
20
30
40
Time d
50
60
70
80
Fig. 2. Comparison of COD removal efficiency.
adsorption. With the operation of the system, the COD removal efficiencies decreased to 12% until the 15th day. During this period of operation, the COD removal efficiencies improved slightly every time after adding PAC. After 15 days, the COD removal efficiencies of the PAC system were maintained in the range of 6–7% and the active carbon supplement no longer had a significant influence on COD removal efficiency, thus indicating that PAC adsorption played a major role in COD removal in the early stage of operation and had a lesser effect in the later stage. The removal efficiencies of the AS system fluctuated at approximately 20%, whereas the average COD removal efficiencies of the PAC–AS system nearly doubled, reaching 39%. The results showed that PAC adsorption contributed to 7% of the COD removal efficiencies, whereas AS biodegradation contributed to 33% of the COD removal efficiencies in the PAC–AS system. These findings indicated that the COD removal efficiencies of biodegradation in the PAC–AS system were remarkably higher than that in the AS system. These results indicated that the increases in the COD removal efficiencies were caused by the improvement of biodegradation by PAC addition and not by adsorption. Therefore, the influence of PAC addition was examined in detail in the following analyses. 3.2. Effects of PAC addition on water quality The hydrophilicity and hydrophobicity of the influent and effluent of the three systems are shown in Fig. 3. The total removal efficiencies of HPI and TPI-N in the PAC, AS, and PAC–AS systems were 30%, 65%, and 72%, respectively. The PAC system treated wastewater only by activated carbon adsorption. Therefore, in the biological process, HPI and TPI-N were easily removed [21]. Hydrophobic organic compounds (HPO-A and HPO-N) are not easily degraded by AS. The total removal efficiencies of HPO-A and HPO-N in the PAC, AS, and PAC–AS systems were 65%, 50%, and 75%, respectively. The hydrophobic substance removal efficiencies of the PAC system were higher than those of the AS system, thus indicating that when the disposable PAC dosage was large and active carbon adsorption was unsaturated, the adsorption of PAC was stronger than the biodegradation of AS on removing hydrophobic substances. The hydrophobic substance removal efficiency of the PAC–AS system increased by approximately 25% compared with the AS system. These results indicated that PAC preferentially adsorbed and facilitated the removal of hydrophobic matter. The removal efficiency of acidic components was further analyzed. HPO-A and TPI-A are the acidic substances of the five components. Fig. 3 shows that the acid content (HPO-A and TPI-A) of wastewater was relatively high (greater than 30%). The removal efficiencies of HPO-A and TPI-A in the PAC, AS, and PAC–AS systems were 55%, 30%, and 60%, respectively. The removal efficiencies of
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10 8
SOUR
6 4 PAC-AS
2
AS
0 0
20
40
60
80
Time d Fig. 6. SOUR of PAC–AS and AS system.
Fig. 3. Hydrophilicity and hydrophobicity analysis (a) and the ratio of hydrophilic and hydrophobic substances (b).
the PAC–AS and PAC system were similar, but significantly higher than that of the AS system, thus indicating that the addition of PAC facilitated the removal of acidic substances and the biodegradation of microorganism on acidic substances was inefficient. Acidic components were produced in the microbial metabolic process and caused microorganism inhibition [27]. Thus, we observed that the acid content of the effluent in the AS system was relatively higher than that of the influent. PAC can eliminate this inhibition effect. Therefore, in the PAC–AS system, acidic organic compounds were removed by PAC adsorption. 3.3. Effects of PAC addition on sludge properties
300 PAC-AS
SVI mL/g
250
3.3.1. Sludge settleability Sludge volume index (SVI) is an important indicator of the sedimentation performance of AS. Fig. 4shows the changes in SVI values with the operation of the PAC–AS and AS systems. The initial SVI of the two systems is 180 mL/g. After two weeks of operation, the SVI of the AS system increased. The sludge floc dispersed, and the sedimentation performance became worse. After 21 days, the SVI of the AS system remained stable at approximately 250 mL/g and the sludge bulking phenomenon occurred mainly because of the comprehensive water quality of industrial wastewater and the high concentration of acid contents. By contrast, the SVI of the PAC–AS
AS
200 150 100 50 0 0
7
14
21
28
35
42
49
56
63
70
2.5
Time d
PAC-AS
Fig. 4. Sludge volume index of PAC–AS and AS system.
AS
2 8000
MLSS
6000
AWCD(cm-1)
MLBSS˖PAC-AS MLBSS˖AS MLSS˖PAC-AS
4000
1.5 1
0.5
2000
0 0
7
14
21
28
35
42
49
Time d
56
63
70
0 0
30
60
90
120
Time h
Fig. 5. Biomass of PAC–AS and AS system. Fig. 7. AWCD of PAC–AS and AS system (day 45).
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5
1 Polymers
Absorbance ratio
0.8
Amines Esters
0.6 Carboxylic acids Amino acids
0.4
Carbohydrates
0.2 Alcohols
0 5
20
45 Time
70
d
(a) 1 Polymers 0.8
Absorbance ratio
Amines Esters
0.6
Carboxylic acids 0.4
Amino acids Carboh ydrates
0.2 Alcoho ls 0 5
20
Time
d
45
70
(b) Fig. 8. The average absorbance ratio of carbon source in ECO flat in PAC–AS system(a) and in AS system(b).
system decreased continuously with the addition of PAC and then remained stable at approximately 80 mL/g after 28 days, which was close to the normal value of 100 mL/g. These results indicated that PAC addition could improve sludge settleability. 3.3.2. Biomass Before the addition of PAC, the initial MLBSS of the PAC–AS and AS systems was 4000 mg/L. Given that the wastewater lacked a biodegradable carbon source, the biomass in the AS system
declined significantly to 3000 mg/L (day 0–day 21). Thereafter, MLBSS was maintained at approximately 3600–3800 mg/L after 35 days. The comprehensive water quality and adverse environment resulted in sludge bulking, which negatively influenced microorganism growth. The biomass of the PAC–AS system remained stable in the first four weeks and then increased slowly to approximately 5000 mg/L (day 28–day 49). During the operation, the biomass in the PAC–AS system was always higher than that of the AS system (Fig. 5). The addition of PAC improved the adaptability of
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Powdered activated carbon (PAC)
Adsorption
Bioaugmentation
Hydrophobic organic compounds
Enhance
Improve
Strengthen
biomass
metabolic activity
sludge settleability
Improve
System environment
Activated sludge
Fig. 9. Influence of powdered activated carbon on biological treatment.
the PAC–AS system to the adverse environment, thus enabling the PAC–AS system to always maintain a high biomass. 3.3.3. Specific oxygen uptake rate The changes in SOUR with the operation of the PAC–AS and AS systems are shown in Fig. 6. The SOUR in the two systems showed a similar trend: the SOUR first decreased and then increased because the poor water quality affected the normal metabolism of cells. Thereafter, the activity recovered gradually after the microorganisms adapted to the environment. The initial SOUR of the PAC–AS and AS systems was 7.9 and 6.1 mg/(g h), respectively. At the 14th day, the SOUR of the PAC–AS system decreased to the minimum value of 5.7 mg/(g h). By contrast, at the 21st day, the SOUR of the AS system decreased to the minimum value of 3.4 mg/(g h). The SOUR value then increased gradually and became stable. The average SOUR in the entire PAC–AS experiment was 7.5 mg/(g h), whereas that of the AS system was 4.3 mg/(g h), thus indicating that PAC could improve treatment efficiency by maintaining high microbial activity. 3.4. Effects of PAC addition on microbial characteristics 3.4.1. Average metabolic activity The AWCD curves under different conditions of the PAC–AS and AS systems could indicate the differences in internal biological activity. The typical AWCD curve of the two systems at the 45th day is shown in Fig. 7. The AWCD curves consisted of three phases, namely, early period with color variation, index change period with color variation, and final stable period (the cell death phase was not observed because of limited observation time). The AWCD curves of the two systems showed an apparent lag phase in the first 30 h, then increased rapidly, and finally were almost constant after 100 h. The value of the PAC–AS system was stable at approximately 2.0, whereas that of the AS system was 1.5. This result indicated that the microbial metabolic activity of the PAC–AS system was higher. The rate of AWCD change was fastest between 40 h and 60 h. Thus, the rate of this stage could characterized the average activity of aerobic microbial metabolism. The average metabolic activity of the microbial community on the Biolog-ECO microplate during the operation in the PAC–AS and AS systems is shown in Table 1. The microbial activity in the same stage and the change trend of
the two systems were different. In the first 20 days, the microbial metabolic activity of the PAC–AS system was basically unchanged, whereas the metabolic activity of the AS system decreased because of refractory and toxic wastewater quality. During the entire operation process, the average microbial metabolic activity of the PAC–AS system was higher than that of the AS system; thus, indicating that PAC enhanced the microbial ability of resistance to the adverse environment and increased the stability of the microbial metabolic activity. 3.4.2. Substrate utilization and microbial community The substrates can be divided into groups of alcohols, amines, amino acids, carbohydrates, carboxylic acids, esters, and polymers [28]. Qualitative measures of substrate utilization by each group can be calculated using the average absorbency fraction in the same manner as for the entire plate [23,29]. Fig. 8 shows the average absorbency fraction of the microbial metabolic carbon source in the Biolog-ECO microplate with time, from which the similarity of substrate utilization in the two systems can be observed. The average absorbency fraction of microbial metabolic alcohols was the highest, i.e., greater than 0.2 in the two systems. The result indicated that alcohol could be easily utilized by the microbial community in the two systems. The metabolism of carbohydrate decreased first (day 5–day 20) and then gradually increased (day 20–day 45). This result was contrary to the metabolism of lipids. After 45 days of operation, the microbial metabolism of different carbon sources recovered similarly to the initial situation of the experiment. During the entire test, the microbial utilization of various carbon sources was basically unchanged. The metabolism of alcohol increased slightly, whereas the metabolism of amino acids and carboxylic acids decreased slightly. The similarity of the two systems could also be observed from the analyses of DQ and D [30]. The initial values of DQ in the two systems were approximately 30.5, and the value of D was only 0.036. Table. 1 The average microbial metabolism in ECO plate of PAC–AS and AS systems(h−1 ). Days
5
20
45
70
PAC–AS AS
0.0352 0.0276
0.0354 0.0240
0.0369 0.0291
0.0336 0.0238
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With the operation of the system, the value of DQ in the PAC–AS system significantly decreased and reached 26.5 at the 20th day, whereas the value of DQ in the AS system was 29.0. The result showed that the addition of PAC reduced the microbial diversity of the system. This might be explained as follows. Since microbes are special organics, there existed adsorption of PAC for microbes, which contributed to the decrease of the microbial diversity in the initial operation. The value of D increased to 0.12, which was still far less than one. After 45 days, the values of DQ in the two systems recovered to 30.5 and were almost constant. This was mainly because the microbes adapted to the environment and the adsorption of PAC for microbes reduced. At the same time, the value of D decreased to 0.045–0.048. We observed from the entire experiment that the characteristics of the microbial community in the PAC–AS and AS systems did not differ. 4. Influence pathway of PAC addition on biological treatment The water used in the experiment was refractory comprehensive industrial wastewater from a chemical industrial park in Tianjin, China, which had the characteristic of poor biodegradability. The average removal efficiencies of COD in the AS system was 20%. By contrast, the average removal efficiencies of COD increased to nearly double after PAC addition. Through further analysis, we observed that the removal efficiency increased mainly because of the improvement of the metabolic rate of slowly biodegradable organic matter and the removal efficiency of refractory organic matter. The average removal efficiencies of COD in the PAC–AS system was 39%, with 7% of the removal efficiency attributed to the adsorption effect of PAC. The removal of COD by biodegradation in the PAC–AS system was 13% higher than that in the AS system. This finding indicated that the addition of PAC not only improved the adsorption effect of activated carbon for hydrophobic organic compounds, but also strengthened the biodegradation of AS in the PAC–AS system by enhancing biomass, improving metabolic activity and strengthening sludge settleability. The influence mechanisms of PAC addition on biological treatment are shown in Fig. 9. 5. Conclusion Analyzing the impact of PAC on water quality characteristics, the properties of AS, and the microbial community characteristics during the long-term operation, we observed that the mechanism in which PAC enhances the effect of the AS system mainly included the following several aspects: 1. PAC has a strong adsorption capacity for hydrophobic substances. The removal of hydrophobic refractory organic compounds is enhanced after the addition of PAC to the AS system. Acidic substances will accumulate in the system during microbial metabolism. The adsorption of PAC on acid can remove this part of organic matter and eliminate the adverse effects of metabolites on the system environment. 2. In the PAC–AS system, in addition to the adsorption effect, PAC also plays a role in the reinforcement of biodegradation. The addition of PAC does not change the structure of the microbial community and the metabolic pathway of microbial degradation, but can improve the biomass, the metabolic activity, and the settleability performance of AS. In summary, a synergistic effect is observed between the adsorption of PAC and biodegradation. The interaction enables a more effective treatment of comprehensive chemical wastewater in the PAC–AS system compared with the AS system.
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