Bioresource Technology 153 (2014) 160–164
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
New insight into the biological treatment by activated sludge: The role of adsorption process Xiaochun Zhang a,⇑, Xinrun Li a, Qingrui Zhang a,⇑, Qiuming Peng b, Wen Zhang a, Faming Gao a a b
Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The growth of microorganism is not
directly related to substrate concentration. Wastewater treatment by activated sludge is actually the process of adsorption. The microorganism could grow by using the loaded organic matters without substrate.
a r t i c l e
i n f o
Article history: Received 19 September 2013 Received in revised form 25 November 2013 Accepted 30 November 2013 Available online 7 December 2013 Keywords: Adsorption Activated sludge Microorganism Specific growth rate
a b s t r a c t The objective of this study was to evaluate the effect of adsorption on the biological treatment process of wastewater. In the absence of substrate in the water, activated sludge developed well in the first hour, indicating that the growth of microorganism was not directly related to substrate concentration and the dissolved organic matter in the water assays were performed, no organic matter was detected out, revealing that there was no desorption in the activated sludge adsorption process. Activated sludge batch growth experiments in the presence of different adsorption capacities indicated that specific growth rate increased as specific adsorption capacity increased. The experiment on the relationship of adsorption capacity and substrate concentration or sludge concentration was also carried out. Specific adsorption capacity increased as sludge load increased, presenting linear correlation. The experiment results showed that adsorption should be taken into account in the study of the biological treatment process of wastewater. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, water pollution by organic compounds is a worldwide issue and has posed serious threaten towards ecology and human health. Up to now, various methods including flocculation (Gasperi et al., 2012; Ismail et al., 2012), adsorption (Kong et al., 2013; Zhang et al., 2012), biological methods (Di Trapani et al., 2013; Falås et al., 2012) and electrolysis (Wang et al., 2013) were ⇑ Corresponding authors. Tel.: +86 335 8387 741; fax: +86 335 8061 549. E-mail addresses: [email protected] (X. Zhang), [email protected] (Q. Zhang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.11.084
exploited to achieve the efficient removal of organic pollutants. Among the available technologies, biological methods might be one of the most promising approaches due to its low costs, technically simple and high retention efficiencies towards target pollutants. Particularly, the activated sludge methods for biological treatments have been regarded as the core treatment units for wastewater purification (Jones and Schuler, 2010; Krzeminski et al., 2012; Tizghadam et al., 2008). Activated sludge, a suspended microbial aggregate, is frequently applied to wastewater to remove organic compounds through the metabolic reactions of microorganisms and the growing of microorganisms is closely interrelated with the degradation of
X. Zhang et al. / Bioresource Technology 153 (2014) 160–164
organic pollutants in wastewater. As well known, wastewater treatment by activated sludge is actually the process of adsorption (Fujita et al., 2005; Liu et al., 2011; Ren et al., 2007; Soda et al., 1999), metabolism and utilization of organic matter by microorganism communities in activated sludge. This process could be divided into two stages. Firstly, the organic matter is adsorbed by activated sludge; Then, organic subtracts cell surfaces are subjected to degradation by the microorganism within activated sludge. According to the conventional Monod equation (Orhon et al., 2009; Petersen et al., 2003), the microorganism specific growth rate is considered to be a function of the substrate concentration. However, in a non-steady state system, the microorganism concentration and substrate concentration are constantly changing; besides, the growing rates are relative to the substrate contents and the concentration of microorganisms. Reviewing about the process, it can be found that organic matter removal in wastewater is the direct results of adsorption; but, durative removal is dependent on micro-organism metabolism; in other words, the effect of metabolism is not immediate. Thus, the adsorption might be a key factor in order to suitably describe the organic pollutants treatments by activated sludge. Unfortunately, reviewing the published literatures, most of the research works focused on the relationship of microorganism growth and the corresponding substrate concentration (Brandt et al., 2004; Daigger and Grady, 1982; Khoyi and Yaghmaei, 2005; Lee et al., 2001; Scruggs and Randall, 1998), and the process of adsorption was neglected. As for the growing process, microorganisms could utilize the organic matters adsorbed onto activated sludge for growth and the concentration of organic matter in wastewater doesn’t directly affect the microorganism growth rate. In other words, the removal efficiency of organic components in wastewater might mainly be determined by the adsorption process, while the growth of microorganism depends on the organic degradation rates, i.e. utility of the organic matters adsorbed onto the activated sludge. Therefore, it is important to elucidate the relationship between microorganism growth and the adsorption capacity to accurately represent the removal process of organic matter in the water. In the present study, fresh domestic wastewater was selected as the model water effluents and series tests were performed to determine and verify the potential relationship of microorganism specific growth, adsorption capacity and removal efficiencies. This work shed light on the novel information of the role of adsorption process in biological treatment process. 2. Methods 2.1. Materials The microorganisms used in the experiments were from laboratory-scale aerobic activated sludge batch reactor (20 L). The pure culture was taken from the aerobic stage of the oxidation ditch in the second municipal sewage treatment plant, Qinhuangdao. The reactor was fed with domestic wastewater with the quality parameters: COD = 800 ± 50 mg L1, pH was 7.0 ± 0.4 and such composition was relatively stable. The water in the reactor was replaced with fresh domestic wastewater once a day. Aeration was stopped at first, after the mixed liquor was left to stand for 1 h, the supernatant was removed, and then fresh domestic wastewater was added. After culturing for 24 h, the microorganisms were in endogenous respiration and the activated sludge microorganisms were used in the following experiments. During the experiments, the temperature of the reactor was 20 ± 2 °C. Dissolved oxygen (DO) concentration was above 2–4 mg L1, pH = 7.0 ± 0.4.
161
2.2. Biomass growth tests in distilled water and wastewater loaded with organic matters The experiment was performed in 2000 mL beakers, the beakers were aerated. Fresh wastewater and activated sludge in endogenous respiration were added into one beaker; the concentration of activated sludge was 3000 mg L1. After 30 min (adsorption of organic matter on the activated sludge attained saturated) the mixture was filtered with a Buchner funnel, the sludge filtered out was placed in another two beakers, and then was added with 1000 mL distilled water and 1000 mL wastewater respectively, and were aerated immediately. At the beginning of aeration and afterward, samples were taken to measure the mixed liquor suspended solid (MLSS) concentration and chemical oxygen demand (COD) of the water every 10 min.
2.3. Adsorption capacity effects on microorganism growth rate In order to study the effect of adsorption capacity on the biomass growth rate, batch assays were carried out in six 2000 mL aerated beakers. The fresh wastewater and activated sludge used in the batch tests was similar to that described in the previous section; in all beakers the initial chemical oxygen demand (COD) of the water was 700 mg L1, the biomass concentration (MLSS) was 3000 mg MLSS L1. The residence time in six beakers was designed for 5, 10, 15, 20, 25, 30 min respectively; At the end of the reaction, samples were taken to measure chemical oxygen demand (COD) of the water in each beaker, activated sludge specific adsorption capacity (qC, mg g1) was calculated as follows (Kong et al., 2013):
qC ¼
COD0 COD MLSS
ð1Þ
Then the mixture in each beaker was filtered with a Buchner funnel, the sludge filtered out was placed in another six beakers, added with 1000 mL distilled water, and aeration was carried out immediately. The mixed liquor suspended solid concentration (MLSS0) at the beginning of aeration and the mixed liquor suspended solid concentration (MLSS20) after 20 min were determined, the specific growth rate (l, h1) was calculated as follows:
l¼
ðMLSS20 MLSS0 Þ=MLSS0 0:333h
ð2Þ
Note that 20 min was selected because the generation of most microorganisms is within 20 min.
2.4. The effects of substrate concentration and sludge concentration on adsorption In order to determine the maximum adsorption capacity of organic matter onto activated sludge, series of experiments were performed in the same 2000 mL beakers with constant temperature (20 ± 2 °C) and pH (7.00 ± 0.05).
2.4.1. The relationship between adsorption capacity and substrate concentration Five beakers were filled with different amounts of fresh domestic wastewater with the COD values of 400, 500, 600, 700, 800 mg COD L1 respectively. Subsequently, equal amount of activated sludge in endogenous respiration was added to the seven beakers to ensure the same sludge concentration (MLSS) in each beaker (3000 mg L1). Then the mixed liquor was aerated. The COD values of the filtrate were assailed.
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2.4.2. The relationship between adsorption capacity and sludge concentration Equal amount of fresh domestic wastewater with the same organic contents (COD = 600 mg L1) was added to five beakers; then different amounts of activated sludge in the endogenous respiration were added to each beaker, the activated sludge concentration (MLSS) in five beakers were 2250, 2570, 3000, 3600, 4500 mg L1 respectively. Then the mixed liquor was aerated. The COD values of the filtrate were determined with 5 min interval until 30 min. 2.5. Analytical methods To determine the MLSS, one piece of quantitative filter paper was heated in an electrical heating desiccator at 105 °C for 1 h, and then the exact weights was recorded as w0. 100 mL of Sample was filtered with the above filter paper, subsequently as-obtained filter paper were heated at 105 °C for around 1 h, and then weighed, the weight was recorded as w1. MLSS (mg L1) of sample was calculated as the following equation:
MLSS ¼
w1 w0 0:1
ð3Þ
The concentration of organic matter (COD) in wastewater was measured with standard potassium dichromate method. 3. Results and discussion 3.1. The effect of adsorption on growth of microorganism In the process of microorganism growth, organic matters were necessary nutrients; as well known, organic components were ubiquitous in wastewater and the surface of activated sludge. In this section, the microorganism growth of activated sludge in wastewater and distilled water loaded with organic matter was evaluated respectively. The results were shown in Fig. 1, it could be seen that in distilled water (absence of nutritional substrate), the activated sludge microorganisms still showed a favorable growth trends, which suggested microorganisms could normally develop by utilizing the organic matter adsorbed onto the sludge surfaces. While in the domestic wastewater with nutritional substrate, the microorganism exhibited similar growing behaviors in the beginning stage (10 min). Such phenomenon implied that the amount of substrate in the water could hardly affect the growth of microorganism and the microorganism growth was not directly related to the concentration of organic matter in water at the beginning stage. It was also noted that, in contrast with the blank
Fig. 1. Activated sludge growth in distilled water and waste water.
water, sludge growth increase was detected in the wastewater after 10 min, which might ascribed that microorganisms could continuously adsorb the organic matters from wastewater after the organic matters within sludge were degraded. However, microorganism growth in distilled water was inhibited due to the lack of nutrition supplement. Moreover, the released COD values were measured in distilled water and the results (Fig. 1) show the COD varied with time in the distilled water. It can be seen that COD concentration did not change in the process of microorganism growth, and negligible organic matter in water was detected, indicating that the adsorption of organic subtracts onto activated sludge was stable and the reversed desorption onto sludge did not occurred in distilled water. In this case, the adsorbent was activated sludge, which was different from the conventional non-biological adsorbents; The organic matter adsorbed did not always attached to the microorganism surface of sludge, and the loading organic compounds could diffuse into the inner structure of microorganism for degradation, thus, it might be difficult for the matters to desorb from the sludge.
3.2. Effect of substrate concentration and sludge concentration on specific adsorption capacity As well known, the adsorption capacity of organic matter onto activated sludge was relative to substrate concentration and sludge concentration. In this section, series of experiments were performed to determine the relationship between adsorption capacity and substrate concentration and sludge concentration. Fig. 2 shows the specific adsorption capacity changes as the function of substrate concentration. In this experiment, the activated sludge concentration in reactors was kept constant. It could be found that the sludge presented preferential sorption performance with the substrate concentration increase. Particularly, the maximum adsorption capacities were obtained with substrate concentration above 700 mg L1, which indicated that the adsorption reached saturation state. Next, the similar tests with different sludge doses were also performed and the results were depicted in Fig. 3, the specific adsorption capacity increased as the activated sludge concentration decreased until reaching the maximum adsorption, similarly indicating that the adsorption reached saturation state. Synthetically considering the effects of substrate concentration and sludge concentration, it was that the specific adsorption capacity was relative to the ratio of substrate concentration to sludge
Fig. 2. The relationship between specific adsorption capacity and substrate concentration.
X. Zhang et al. / Bioresource Technology 153 (2014) 160–164
163
organic matter adsorption on activated sludge were the same. Thus, the specific adsorption capacity was the function of sludge loadings (Fig. 5) The function of specific adsorption (q) and sludge load (x) was linear correlation, can be expressed as:
q ¼ ax
ð4Þ
Coefficient a was determined by experiment. In the presence of different water quality and sludge status, the coefficient a was different. According to the above Fig. 5, similar adsorption appearances could also be found that organic matter adsorption on activated sludge would reach saturation state; specific adsorption capacity increased as sludge load increased; when reached a certain value, the specific adsorption capacity no longer increased, and stayed steady.
Fig. 3. The effects of activated sludge concentration on adsorption capacities.
Fig. 4. The effects of sorption capacities of activated sludge on the microorganisms specific grow rate.
3.3. Effect of adsorption on microbial specific growth rate Microbial growth rate was related with the amount of nutrients used. The nutrients which microorganisms were able to use, were the dissoluble organic matter adsorbed onto microorganism surface; it seems that there is apparent relationship between the microorganism growth and adsorption capacities onto sludge rather the organic matter in waters; thus, some experiments were carried out to determine the relationship between the microbial specific growth rate and specific adsorption capacity. In this section, the activated sludge loaded with different amounts of organic matters was placed in distilled water, stirred and aerated, then the increased amount of sludge was measured at 20 min; the specific growth rate in the presence of different adsorption capacity can be calculated from experiment data, as shown in Fig. 6. Fig. 6 shows the two series of experimental results, the difference between these two series is that activated sludge used in the experiments came from different culture vessels. As was shown, a certain relationship existed between the specific growth rate (l) and the specific adsorption capacity (q), l increased as q increased, basically fitting the linear relation. And, in Fig. 6, a maximum value of specific growth rate could be seen. When specific adsorption capacity reached a certain value, the specific growth rate will no longer increase, but stayed at a certain value; which agreed with the results of previous studies.
Fig. 5. The relationship between specific adsorption capacity and sludge load.
concentration (sludge load, F/M) by analyzing the experimental data (Fig. 4). In the case of different substrate concentration and sludge concentration with the same sludge loaded, the values of
Fig. 6. The effects of activated sludge loads on its adsorption capacities.
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X. Zhang et al. / Bioresource Technology 153 (2014) 160–164
4. Conclusion The organic matter degradation by microorganisms played a leading role in the process of organic matter removal in the water; this effect was indirect, emerging through the adsorption process. The continuous removal of organic matters depended on the microbial metabolic activity. The nutrients that can be used by activated sludge microorganisms in their growth process were the organic matters adsorbed to the microorganisms’ surface; and not directly related to the organic matters in the water; thus, the organic matters in the water must be first adsorbed onto the surface of microorganisms, and then utilized in the microbial metabolism. Acknowledgements We greatly acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21207112), New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0690), the Natural Science Foundation of Hebei Province (No. B2012203060), the China Postdoctoral Science Foundation (No. 2012M510770, 2013T60265) and the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (No. Y2011113).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech. 2013.11.084. References Brandt, B.W., Kelpin, F.D.L., van Leeuwen, I.M.M., Kooijman, S.A.L.M., 2004. Modelling microbial adaptation to changing availability of substrates. Water Res. 38 (4), 1003–1013. Daigger, G.T., Grady Jr, C.P.L., 1982. The dynamics of microbial growth on soluble substrates: a unifying theory. Water Res. 16 (4), 365–382. Di Trapani, D., Christensson, M., Torregrossa, M., Viviani, G., Ødegaard, H., 2013. Performance of a hybrid activated sludge/biofilm process for wastewater treatment in a cold climate region: influence of operating conditions. Biochem. Eng. J. 77, 214–219.
Falås, P., Baillon-Dhumez, A., Andersen, H.R., Ledin, A., la Cour Jansen, J., 2012. Suspended biofilm carrier and activated sludge removal of acidic pharmaceuticals. Water Res. 46 (4), 1167–1175. Fujita, Y., Hata, T., Nakamaru, M., Iyo, T., Yoshino, T., Shimamura, T., 2005. A study of boron adsorption onto activated sludge. Bioresour. Technol. 96 (12), 1350– 1356. Gasperi, J., Laborie, B., Rocher, V., 2012. Treatment of combined sewer overflows by ballasted flocculation: removal study of a large broad spectrum of pollutants. Chem. Eng. J. 211–212, 293–301. Ismail, I.M., Fawzy, A.S., Abdel-Monem, N.M., Mahmoud, M.H., El-Halwany, M.A., 2012. Combined coagulation flocculation pre treatment unit for municipal wastewater. J. Adv. Res. 3 (4), 331–336. Jones, P.A., Schuler, A.J., 2010. Seasonal variability of biomass density and activated sludge settleability in full-scale wastewater treatment systems. Chem. Eng. J. 164 (1), 16–22. Khoyi, M.R., Yaghmaei, S., 2005. Simulation of competition between two microorganisms in a biofilm reactor based on different growth models. Biochem. Eng. J. 23 (1), 63–72. Kong, L., Xiong, Y., Tian, S., Luo, R., He, C., Huang, H., 2013. Preparation and characterization of a hierarchical porous char from sewage sludge with superior adsorption capacity for toluene by a new two-step pore-fabricating process. Bioresour. Technol. 146, 457–462. Krzeminski, P., Iglesias-Obelleiro, A., Madebo, G., Garrido, J.M., van der Graaf, J.H.J.M., van Lier, J.B., 2012. Impact of temperature on raw wastewater composition and activated sludge filterability in full-scale MBR systems for municipal sewage treatment. J. Membr. Sci. 423–424, 348–361. Lee, J., Ahn, W.-Y., Lee, C.-H., 2001. Comparison of the filtration characteristics between attached and suspended growth microorganisms in submerged membrane bioreactor. Water Res. 35 (10), 2435–2445. Liu, J.J., Wang, X.C., Fan, B., 2011. Characteristics of PAHs adsorption on inorganic particles and activated sludge in domestic wastewater treatment. Bioresour. Technol. 102 (9), 5305–5311. Orhon, D., Cokgor, E.U., Insel, G., Karahan, O., Katipoglu, T., 2009. Validity of Monod kinetics at different sludge ages – peptone biodegradation under aerobic conditions. Bioresour. Technol. 100 (23), 5678–5686. Petersen, B., Gernaey, K., Devisscher, M., Dochain, D., Vanrolleghem, P.A., 2003. A simplified method to assess structurally identifiable parameters in Monodbased activated sludge models. Water Res. 37 (12), 2893–2904. Ren, Y.-X., Nakano, K., Nomura, M., Chiba, N., Nishimura, O., 2007. A thermodynamic analysis on adsorption of estrogens in activated sludge process. Water Res. 41 (11), 2341–2348. Scruggs, C.E., Randall, C.W., 1998. Evaluation of filamentous microorganism growth factors in an industrial wastewater activated sludge system. Water Sci. Technol. 37 (4–5), 263–270. Soda, S., Ike, M., Fujita, M., 1999. Adsorption of bacterial cells onto activated sludge flocs. J. Biosci. Bioeng. 87 (4), 513–518. Tizghadam, M., Dagot, C., Baudu, M., 2008. Wastewater treatment in a hybrid activated sludge baffled reactor. J. Hazard. Mater. 154 (1–3), 550–557. Wang, Y.-Z., Wang, A.-J., Liu, W.-Z., Sun, Q., 2013. Enhanced azo dye removal through anode biofilm acclimation to toxicity in single-chamber biocatalyzed electrolysis system. Bioresour. Technol. 142, 688–692. Zhang, W., Li, H., Kan, X., Dong, L., Yan, H., Jiang, Z., Yang, H., Li, A., Cheng, R., 2012. Adsorption of anionic dyes from aqueous solutions using chemically modified straw. Bioresour. Technol. 117, 40–47.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
New insight into the biological treatment by activated sludge: The role of adsorption process Xiaochun Zhang a,⇑, Xinrun Li a, Qingrui Zhang a,⇑, Qiuming Peng b, Wen Zhang a, Faming Gao a a b
Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The growth of microorganism is not
directly related to substrate concentration. Wastewater treatment by activated sludge is actually the process of adsorption. The microorganism could grow by using the loaded organic matters without substrate.
a r t i c l e
i n f o
Article history: Received 19 September 2013 Received in revised form 25 November 2013 Accepted 30 November 2013 Available online 7 December 2013 Keywords: Adsorption Activated sludge Microorganism Specific growth rate
a b s t r a c t The objective of this study was to evaluate the effect of adsorption on the biological treatment process of wastewater. In the absence of substrate in the water, activated sludge developed well in the first hour, indicating that the growth of microorganism was not directly related to substrate concentration and the dissolved organic matter in the water assays were performed, no organic matter was detected out, revealing that there was no desorption in the activated sludge adsorption process. Activated sludge batch growth experiments in the presence of different adsorption capacities indicated that specific growth rate increased as specific adsorption capacity increased. The experiment on the relationship of adsorption capacity and substrate concentration or sludge concentration was also carried out. Specific adsorption capacity increased as sludge load increased, presenting linear correlation. The experiment results showed that adsorption should be taken into account in the study of the biological treatment process of wastewater. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, water pollution by organic compounds is a worldwide issue and has posed serious threaten towards ecology and human health. Up to now, various methods including flocculation (Gasperi et al., 2012; Ismail et al., 2012), adsorption (Kong et al., 2013; Zhang et al., 2012), biological methods (Di Trapani et al., 2013; Falås et al., 2012) and electrolysis (Wang et al., 2013) were ⇑ Corresponding authors. Tel.: +86 335 8387 741; fax: +86 335 8061 549. E-mail addresses: [email protected] (X. Zhang), [email protected] (Q. Zhang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.11.084
exploited to achieve the efficient removal of organic pollutants. Among the available technologies, biological methods might be one of the most promising approaches due to its low costs, technically simple and high retention efficiencies towards target pollutants. Particularly, the activated sludge methods for biological treatments have been regarded as the core treatment units for wastewater purification (Jones and Schuler, 2010; Krzeminski et al., 2012; Tizghadam et al., 2008). Activated sludge, a suspended microbial aggregate, is frequently applied to wastewater to remove organic compounds through the metabolic reactions of microorganisms and the growing of microorganisms is closely interrelated with the degradation of
X. Zhang et al. / Bioresource Technology 153 (2014) 160–164
organic pollutants in wastewater. As well known, wastewater treatment by activated sludge is actually the process of adsorption (Fujita et al., 2005; Liu et al., 2011; Ren et al., 2007; Soda et al., 1999), metabolism and utilization of organic matter by microorganism communities in activated sludge. This process could be divided into two stages. Firstly, the organic matter is adsorbed by activated sludge; Then, organic subtracts cell surfaces are subjected to degradation by the microorganism within activated sludge. According to the conventional Monod equation (Orhon et al., 2009; Petersen et al., 2003), the microorganism specific growth rate is considered to be a function of the substrate concentration. However, in a non-steady state system, the microorganism concentration and substrate concentration are constantly changing; besides, the growing rates are relative to the substrate contents and the concentration of microorganisms. Reviewing about the process, it can be found that organic matter removal in wastewater is the direct results of adsorption; but, durative removal is dependent on micro-organism metabolism; in other words, the effect of metabolism is not immediate. Thus, the adsorption might be a key factor in order to suitably describe the organic pollutants treatments by activated sludge. Unfortunately, reviewing the published literatures, most of the research works focused on the relationship of microorganism growth and the corresponding substrate concentration (Brandt et al., 2004; Daigger and Grady, 1982; Khoyi and Yaghmaei, 2005; Lee et al., 2001; Scruggs and Randall, 1998), and the process of adsorption was neglected. As for the growing process, microorganisms could utilize the organic matters adsorbed onto activated sludge for growth and the concentration of organic matter in wastewater doesn’t directly affect the microorganism growth rate. In other words, the removal efficiency of organic components in wastewater might mainly be determined by the adsorption process, while the growth of microorganism depends on the organic degradation rates, i.e. utility of the organic matters adsorbed onto the activated sludge. Therefore, it is important to elucidate the relationship between microorganism growth and the adsorption capacity to accurately represent the removal process of organic matter in the water. In the present study, fresh domestic wastewater was selected as the model water effluents and series tests were performed to determine and verify the potential relationship of microorganism specific growth, adsorption capacity and removal efficiencies. This work shed light on the novel information of the role of adsorption process in biological treatment process. 2. Methods 2.1. Materials The microorganisms used in the experiments were from laboratory-scale aerobic activated sludge batch reactor (20 L). The pure culture was taken from the aerobic stage of the oxidation ditch in the second municipal sewage treatment plant, Qinhuangdao. The reactor was fed with domestic wastewater with the quality parameters: COD = 800 ± 50 mg L1, pH was 7.0 ± 0.4 and such composition was relatively stable. The water in the reactor was replaced with fresh domestic wastewater once a day. Aeration was stopped at first, after the mixed liquor was left to stand for 1 h, the supernatant was removed, and then fresh domestic wastewater was added. After culturing for 24 h, the microorganisms were in endogenous respiration and the activated sludge microorganisms were used in the following experiments. During the experiments, the temperature of the reactor was 20 ± 2 °C. Dissolved oxygen (DO) concentration was above 2–4 mg L1, pH = 7.0 ± 0.4.
161
2.2. Biomass growth tests in distilled water and wastewater loaded with organic matters The experiment was performed in 2000 mL beakers, the beakers were aerated. Fresh wastewater and activated sludge in endogenous respiration were added into one beaker; the concentration of activated sludge was 3000 mg L1. After 30 min (adsorption of organic matter on the activated sludge attained saturated) the mixture was filtered with a Buchner funnel, the sludge filtered out was placed in another two beakers, and then was added with 1000 mL distilled water and 1000 mL wastewater respectively, and were aerated immediately. At the beginning of aeration and afterward, samples were taken to measure the mixed liquor suspended solid (MLSS) concentration and chemical oxygen demand (COD) of the water every 10 min.
2.3. Adsorption capacity effects on microorganism growth rate In order to study the effect of adsorption capacity on the biomass growth rate, batch assays were carried out in six 2000 mL aerated beakers. The fresh wastewater and activated sludge used in the batch tests was similar to that described in the previous section; in all beakers the initial chemical oxygen demand (COD) of the water was 700 mg L1, the biomass concentration (MLSS) was 3000 mg MLSS L1. The residence time in six beakers was designed for 5, 10, 15, 20, 25, 30 min respectively; At the end of the reaction, samples were taken to measure chemical oxygen demand (COD) of the water in each beaker, activated sludge specific adsorption capacity (qC, mg g1) was calculated as follows (Kong et al., 2013):
qC ¼
COD0 COD MLSS
ð1Þ
Then the mixture in each beaker was filtered with a Buchner funnel, the sludge filtered out was placed in another six beakers, added with 1000 mL distilled water, and aeration was carried out immediately. The mixed liquor suspended solid concentration (MLSS0) at the beginning of aeration and the mixed liquor suspended solid concentration (MLSS20) after 20 min were determined, the specific growth rate (l, h1) was calculated as follows:
l¼
ðMLSS20 MLSS0 Þ=MLSS0 0:333h
ð2Þ
Note that 20 min was selected because the generation of most microorganisms is within 20 min.
2.4. The effects of substrate concentration and sludge concentration on adsorption In order to determine the maximum adsorption capacity of organic matter onto activated sludge, series of experiments were performed in the same 2000 mL beakers with constant temperature (20 ± 2 °C) and pH (7.00 ± 0.05).
2.4.1. The relationship between adsorption capacity and substrate concentration Five beakers were filled with different amounts of fresh domestic wastewater with the COD values of 400, 500, 600, 700, 800 mg COD L1 respectively. Subsequently, equal amount of activated sludge in endogenous respiration was added to the seven beakers to ensure the same sludge concentration (MLSS) in each beaker (3000 mg L1). Then the mixed liquor was aerated. The COD values of the filtrate were assailed.
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2.4.2. The relationship between adsorption capacity and sludge concentration Equal amount of fresh domestic wastewater with the same organic contents (COD = 600 mg L1) was added to five beakers; then different amounts of activated sludge in the endogenous respiration were added to each beaker, the activated sludge concentration (MLSS) in five beakers were 2250, 2570, 3000, 3600, 4500 mg L1 respectively. Then the mixed liquor was aerated. The COD values of the filtrate were determined with 5 min interval until 30 min. 2.5. Analytical methods To determine the MLSS, one piece of quantitative filter paper was heated in an electrical heating desiccator at 105 °C for 1 h, and then the exact weights was recorded as w0. 100 mL of Sample was filtered with the above filter paper, subsequently as-obtained filter paper were heated at 105 °C for around 1 h, and then weighed, the weight was recorded as w1. MLSS (mg L1) of sample was calculated as the following equation:
MLSS ¼
w1 w0 0:1
ð3Þ
The concentration of organic matter (COD) in wastewater was measured with standard potassium dichromate method. 3. Results and discussion 3.1. The effect of adsorption on growth of microorganism In the process of microorganism growth, organic matters were necessary nutrients; as well known, organic components were ubiquitous in wastewater and the surface of activated sludge. In this section, the microorganism growth of activated sludge in wastewater and distilled water loaded with organic matter was evaluated respectively. The results were shown in Fig. 1, it could be seen that in distilled water (absence of nutritional substrate), the activated sludge microorganisms still showed a favorable growth trends, which suggested microorganisms could normally develop by utilizing the organic matter adsorbed onto the sludge surfaces. While in the domestic wastewater with nutritional substrate, the microorganism exhibited similar growing behaviors in the beginning stage (10 min). Such phenomenon implied that the amount of substrate in the water could hardly affect the growth of microorganism and the microorganism growth was not directly related to the concentration of organic matter in water at the beginning stage. It was also noted that, in contrast with the blank
Fig. 1. Activated sludge growth in distilled water and waste water.
water, sludge growth increase was detected in the wastewater after 10 min, which might ascribed that microorganisms could continuously adsorb the organic matters from wastewater after the organic matters within sludge were degraded. However, microorganism growth in distilled water was inhibited due to the lack of nutrition supplement. Moreover, the released COD values were measured in distilled water and the results (Fig. 1) show the COD varied with time in the distilled water. It can be seen that COD concentration did not change in the process of microorganism growth, and negligible organic matter in water was detected, indicating that the adsorption of organic subtracts onto activated sludge was stable and the reversed desorption onto sludge did not occurred in distilled water. In this case, the adsorbent was activated sludge, which was different from the conventional non-biological adsorbents; The organic matter adsorbed did not always attached to the microorganism surface of sludge, and the loading organic compounds could diffuse into the inner structure of microorganism for degradation, thus, it might be difficult for the matters to desorb from the sludge.
3.2. Effect of substrate concentration and sludge concentration on specific adsorption capacity As well known, the adsorption capacity of organic matter onto activated sludge was relative to substrate concentration and sludge concentration. In this section, series of experiments were performed to determine the relationship between adsorption capacity and substrate concentration and sludge concentration. Fig. 2 shows the specific adsorption capacity changes as the function of substrate concentration. In this experiment, the activated sludge concentration in reactors was kept constant. It could be found that the sludge presented preferential sorption performance with the substrate concentration increase. Particularly, the maximum adsorption capacities were obtained with substrate concentration above 700 mg L1, which indicated that the adsorption reached saturation state. Next, the similar tests with different sludge doses were also performed and the results were depicted in Fig. 3, the specific adsorption capacity increased as the activated sludge concentration decreased until reaching the maximum adsorption, similarly indicating that the adsorption reached saturation state. Synthetically considering the effects of substrate concentration and sludge concentration, it was that the specific adsorption capacity was relative to the ratio of substrate concentration to sludge
Fig. 2. The relationship between specific adsorption capacity and substrate concentration.
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organic matter adsorption on activated sludge were the same. Thus, the specific adsorption capacity was the function of sludge loadings (Fig. 5) The function of specific adsorption (q) and sludge load (x) was linear correlation, can be expressed as:
q ¼ ax
ð4Þ
Coefficient a was determined by experiment. In the presence of different water quality and sludge status, the coefficient a was different. According to the above Fig. 5, similar adsorption appearances could also be found that organic matter adsorption on activated sludge would reach saturation state; specific adsorption capacity increased as sludge load increased; when reached a certain value, the specific adsorption capacity no longer increased, and stayed steady.
Fig. 3. The effects of activated sludge concentration on adsorption capacities.
Fig. 4. The effects of sorption capacities of activated sludge on the microorganisms specific grow rate.
3.3. Effect of adsorption on microbial specific growth rate Microbial growth rate was related with the amount of nutrients used. The nutrients which microorganisms were able to use, were the dissoluble organic matter adsorbed onto microorganism surface; it seems that there is apparent relationship between the microorganism growth and adsorption capacities onto sludge rather the organic matter in waters; thus, some experiments were carried out to determine the relationship between the microbial specific growth rate and specific adsorption capacity. In this section, the activated sludge loaded with different amounts of organic matters was placed in distilled water, stirred and aerated, then the increased amount of sludge was measured at 20 min; the specific growth rate in the presence of different adsorption capacity can be calculated from experiment data, as shown in Fig. 6. Fig. 6 shows the two series of experimental results, the difference between these two series is that activated sludge used in the experiments came from different culture vessels. As was shown, a certain relationship existed between the specific growth rate (l) and the specific adsorption capacity (q), l increased as q increased, basically fitting the linear relation. And, in Fig. 6, a maximum value of specific growth rate could be seen. When specific adsorption capacity reached a certain value, the specific growth rate will no longer increase, but stayed at a certain value; which agreed with the results of previous studies.
Fig. 5. The relationship between specific adsorption capacity and sludge load.
concentration (sludge load, F/M) by analyzing the experimental data (Fig. 4). In the case of different substrate concentration and sludge concentration with the same sludge loaded, the values of
Fig. 6. The effects of activated sludge loads on its adsorption capacities.
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4. Conclusion The organic matter degradation by microorganisms played a leading role in the process of organic matter removal in the water; this effect was indirect, emerging through the adsorption process. The continuous removal of organic matters depended on the microbial metabolic activity. The nutrients that can be used by activated sludge microorganisms in their growth process were the organic matters adsorbed to the microorganisms’ surface; and not directly related to the organic matters in the water; thus, the organic matters in the water must be first adsorbed onto the surface of microorganisms, and then utilized in the microbial metabolism. Acknowledgements We greatly acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21207112), New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0690), the Natural Science Foundation of Hebei Province (No. B2012203060), the China Postdoctoral Science Foundation (No. 2012M510770, 2013T60265) and the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (No. Y2011113).
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