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oxic system
Journal of Environmental Management 184 (2016) 281e288
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Degradation behavior of dimethyl phthalate in an anaerobic/anoxic/ oxic system Tao Zhang b, Zehua Huang c, Xiaohong Chen a, Mingzhi Huang a, *, Jujun Ruan b, ** a
Department of Water Resources and Environment, Guangdong Provincial Key Laboratory of Urbanization and Geo-simulation, Sun Yat-sen University, Guangzhou 510275, PR China b School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-Sen University, Guangzhou 510275, PR China c Fujian Quanzhou Foreign Language Middle School, Quanzhou 362002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2016 Received in revised form 2 October 2016 Accepted 4 October 2016 Available online 8 October 2016
Dimethyl phthalate (DMP) as one of the most important and extensively used Phthalic acid esters (PAEs) is known to likely cause dysfunctions of the endocrine systems, liver, and nervous systems of animals. In this paper, the degradation and behavior of DMP were investigated in a laboratory scale anaerobic/ anoxic/oxic (AAO) treatment system. In addition, a degradation model including biodegradation and sorption was formulated so as to evaluate the fate of DMP in the treatment system, and a mass balance model was designed to determine kinetic parameters of the removal model. The study indicated that the optimal operation condition of HRT and SRT for DMP and nutrients removal were 18 h and 15 d respectively, and the degradation rates of anaerobic, anoxic and aerobic zones for DMP were 13.4%, 13.0% and 67.7%, respectively. Under the optimal conditions, the degraded DMP was 73.8%, the released DMP in the effluent was 5.8%, the accumulated DMP was 19.3%, and the remained DMP in the waste sludge was 1.1%. Moreover, the degradation process of DMP by acclimated activated sludge was in accordance with the first-order kinetics equation. The model can be used for accurately modeling the degradation and behavior of DMP in the AAO system. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Dimethyl phthalate (DMP) Anaerobic/anoxic/oxic system Degradation Behavior Kinetic
1. Introduction In recent years, phthalic acid esters (PAEs) have caught extensive concerns because they are widely used as plastic plasticizers, and additives in more than hundred varieties of products, such as toy, packing material and cosmetics production. Therefore, these persistent and toxic organic compounds, which could harm the health of organisms and human by transmission of food chain and bioaccumulation, commonly exists in various environments (Latini, 2005; Magdouli et al., 2013). Dimethyl phthalate (DMP) known as one of the most important and extensively used PAEs, has been already measured in various environment, such as various surface water, groundwater, sediments of water, atmosphere, aerosol particle, soil (Wu et al., 2011; Huang et al., 2015; Montuori et al., 2008;
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Huang), ruanjujun@mail. sysu.edu.cn (J. Ruan). http://dx.doi.org/10.1016/j.jenvman.2016.10.008 0301-4797/© 2016 Elsevier Ltd. All rights reserved.
Ogunfowokan et al., 2006; Zeng et al., 2009), and is known to likely cause dysfunctions of the endocrine systems, liver, and nervous systems of humans and animals (Wang et al., 2008; Yuan et al., 2008; Calafat and McKee, 2006; Wu et al., 2011). Therefore, DMP has been listed as a priority control pollutant by American Environmental Protection Agency (1991), Ministry of Environmental Protection of the People's Republic of China (Mohan et al., 2007) and European Union (1993). The plasticizers in the water environment is mainly from discharge of industrial effluent without treatment. However, due to its persistence and refractory, the wastewater treated though the wastewater treatment plants (WWTP) is also an important source of The plasticizers in the water environment (Gao et al., 2014; Loraine and Pettigrove, 2006). When these toxic chemicals pass through the domesticated biological treatment process, which can overcome the shortcoming of traditional-activated sludge process, they are quite recalcitrant to be degraded and may direct discharge into the receiving water bodies. If PAEs are not removed at WWTP, there may be toxic or endocrine disrupting influences on aquatic organisms in the receiving water bodies (Knudsen and Pottinger,
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Abbreviation PAEs DMP WWTP AAO COD BOD TN NHþ 4 -N TP DO HRT SRT MLSS HPLC MAPE RMSE SOM
Phthalic acid esters Dimethyl phthalate Wastewater treatment plants Anaerobic/anoxic/oxic Chemical organic demand Biochemical oxygen demand Total nitrogen Ammonium nitrogen Total phosphorus Dissolved oxygen Hydraulic retention time Sludge retention time Mixed liquor suspended solid High performance liquid chromatography Mean absolute percentage error Root mean squared error Suspended organic matter
1999; Meghdad et al., 2009). As such compounds are degraded slowly by photodegradation and hydrolyzation under natural conditions, biodegradation is thought of as one of the major degradation pathways for PAEs (Staples et al., 1997; Huang et al., 2011). Previous studies have demonstrated that several PAEs can take the biodegradation under aerobic conditions in activated sludge (Sanna et al., 2004; Cendrine et al., 2009), in soil and sediments (Peng and Li, 2012), in river, lake or sea water (Chen et al., 2009; Fang et al., 2009), and under anaerobic conditions (Liang et al., 2007; Wang et al., 2000), and the degradation rate of PAEs can reach from 60% to 100% though WWTP, which includes aerobic and anaerobic treatment zones (Gao et al., 2014; Oliver et al., 2005). From these studies it can be concluded PAEs could be effectively removed under aerobic conditions, but for anaerobic conditions, the degradation efficiency of PAEs becomes much slower (Huang et al., 2010; Mahmoud et al., 2012; Ruan and Xu, 2011). However, these studies have tended to pay attention to the biodegradability and degradation pathway of different PAEs. Hence, the effective degradation of DMP in WWTP is essential to be understood so that the effluent concentration of DMP can be minimized to discharge into the receiving water bodies. Previous researches demonstrated that various PAEs have high biodegradation efficiency in the present of activated sludge (Kumar et al., 2014; Brar et al., 2009), and various PAEs could be removed by adsorption onto activated sludge. As a consequence, it is believable that the main removal mechanisms for DMP removal could be sorption on activated sludge and biodegradation. However, it is also very controversial and very challenging how these removal processes interacts with in WWTP. Moreover, in order to describe the removal process of pollutants in WWTP and understand the degradation of pollutants in WWTP, many researchers focus on the kinetics of pollutants in WWTP (Meghdad et al., 2009; Patrik et al., 2003; Jarungwit et al., 2016). But in the previous study, the kinetic model is only used for modeling the biodegradation of PAEs, not take into account the fate of PAEs in WWTP. Thereby, it still needs to clearly investigate the degradation mechanism and behavior of DMP in the treatment system. And on this basis a model describing the degradation behavior of DMP should be built. In this work, the degradation behavior of DMP were investigated
in an anaerobic/anoxic/oxic (AAO) treatment system under various operating conditions. In order to evaluate the fate of DMP in the treatment system, three removal models including biodegradation and sorption process for anaerobic, anoxic and aerobic reaction were developed, respectively. In the end, a mass balance model based on degradation and behavior of DMP was designed to determine the kinetic parameters of the removal models. 2. Materials and methods 2.1. Reactor system As shown in Fig. 1, the AAO treatment system made of polyethylene includes mainly four parts: one anaerobic zone with volume of 40 L, one anoxic zone with volume of 40 L, three aerobic zone with total volume of 160 L and one settling zone. there were two motor-driven stirrers employed in anaerobic and anoxic zones. An air blower was used to supply oxygen to the microorganisms of aerobic zone. The mixed liquor passing through the aerobic zones was recycled to the anoxic zone, and the sludge was returned from the bottom of the settling zone to the anaerobic zone. The reflux ratios of the mixed liquor and sludge were same, and set to 1. The pH of anaerobic digester is controlled at 6.5e8.0. 2.2. Feed The sludge from a sewage treatment plant in Guangzhou was cultivated in a laboratory scale AAO treatment system with synthetic wastewater as feed. The synthetic wastewater with five different concentrations of DMP, which included 30, 40, 50, 60, and 80 mg L-1, was used. Chemical organic demand (COD) was supplied from glucose. Ammonium nitrate and potassium dihydrogen phosphate were added to maintain the nitrogen and phosphorous sources in the system. The ratio of COD:N:P was kept at 100:7:1. 2.3. Operation conditions of the experiment In order to maintaining at a constant temperature of 25 C, the work environment reactor system was controlled by the temperature control system. Dissolved oxygen (DO) was measured by the online dissolved oxygen meter (D53, HACH), and the concentrations of DO in anaerobic, anoxic and aerobic zones were within the scope of 0e0.30 mg L1, 0e0.60 mg L1 and 2.54e5.72 mg L1, respectively. The mixed liquor suspended solid (MLSS) concentration of about 3000 mg L1 was controlled. On the basis of changing the influent pump flow, hydraulic retention time (HRT) would be adjusted. Just as well sludge retention time (SRT) would be adjusted through altering the amount of the discharged excess sludge in the bottom of the settling zone. In order to investigate the influence of HRT and SRT on the degradation and behavior of DMP, different HRTs with 12, 18, 24, and 30 h and different SRT with 10, 15, 20, and 25 d were used. The continuous period of the operated system was one year. 2.4. Sampling and extraction Water phase: in this work, the samples from the inlet and outlet of the reactor system and mixed liquors in anaerobic, anoxic and aerobic zones were filled into 1 L glass bottles once per day. Firstly, after all samples were treated by centrifugation at 3000 rpm for 15 min, 3% sulfuric acid was added to adjust pH of the supernatant at 3. And then a solid phase extraction column (Waters Oasis HLB, 200 mg/6 mL) was used to extract the centrifuged samples. When the flow rate of the supernatant flowing through the columns was up to 1e2 mL min1, the columns were washed with 3 mL
T. Zhang et al. / Journal of Environmental Management 184 (2016) 281e288
283
Fig. 1. Diagram of the AAO treatment system.
methanol/water (5:95 v/v). Finally, the purified product was obtained through vacuum concentration and drying with a gentle nitrogen flow for 5 min. Sludge phase: the sludge treated by centrifugation was extracted by the soxhlet extractor with 90 mL petroleum ether for 48 h after drying at room temperature. The extracts were reduced to approximately 1.0 mL under gentle nitrogen flow through concentrating with anhydrous sodium sulfate and solventexchanging to n-hexane. The concentrated extracts were separated and isolated on a silica-alumina gel glass column. The prepared process was in accordance with the process procedure of the water phase when the extracts were adjusted to 100 mL. During the extraction process, it need to add the activated copper granules into the extraction flasks for removing elemental sulfur.
2.5. Analytical methods High performance liquid chromatography (HPLC) (Agilent 1100, USA) with a reverse phase Hypersil C-18 column was used to for determination and identification of the concentration of DMP. The mobile phase consisted of methanol and water (90:10, v/v) was used as gradient elution for separation with HPLC, and the flow rate and the injection volume of the mobile phase were 0.8 mL min-1 and 20 mL, respectively. COD, biochemical oxygen demand (BOD), MLSS, total nitrogen (TN), ammonium nitrogen (NHþ 4 -N) and total phosphorus (TP) were measured according to Standard Methods (China's State Environmental Protection Administration, 2002).
2.6. Calculation processes For investigate the fate of DMP in the system, the calculating equations for the concentrations of DMP in anaerobic, anoxic, and aerobic zones are designed as following:
Pi1 ¼
Pi þ R Pe1 1þR
(1)
Pi2 ¼
ð1 þ RÞ P1 þ r P3 1þRþr
(2)
Pi3 ¼ P2
(3)
And the calculating Equation (4) is the total removal rate of DMP through the AAO treatment system:
Total removal rate of DMP ð%Þ ¼
Pi Q ðQ Qs Þ Pε Qs P3 Pi Q (4)
The total amount of DMP removed in the system includes the removal mount of DMP in anaerobic zone, the removal mount of DMP in anoxic zone, the removal mount of DMP in aerobic zones and the removal mount of DMP in settling zone. The calculating equations for the removal rates of DMP in anaerobic, anoxic, aerobic and settling zones are shown as the following:
RRanaerobic ¼
ðPi1 P1 Þ Q ð1 þ RÞ 100% ðPi1 P1 Þ Q þ ðPε P3 Þ Qs
(5)
RRanoxic ¼
ðPi2 P2 Þ Q ð1 þ R þ rÞ 100% ðPi Pε Þ Q þ ðPε P3 Þ Qs
(6)
RRaerobic ¼
ðPi3 P3 Þ Q ð1 þ R þ rÞ 100% ðPi Pε Þ Q þ ðPε P3 Þ Qs
(7)
RRsettling ¼
P3 ðQ þ R Q Qs Þ Pε ðQ Qs Þ R Q Pε1 ðPi Pε Þ Q þ ðPε P3 Þ Qs 100% (8)
where RRanaerobic , RRanoxic , RRaerobic and RRsettling are the removal rates of DMP in anaerobic, anoxic, aerobic and settling zones, respectively; Pi is the concentration of the influent DMP (mg L1); Pε1 is the concentration of DMP for returning activated sludge (mg L1); P1 ,P2 ,P3 ,Pε are the concentrations of the effluent DMP in anaerobic, anoxic, aerobic and settling zones (mg L1), respectively; Pi1 ,Pi2 ,Pi3 are the concentrations of DMP in mixed liquors of the anaerobic, anoxic, aerobic zones (mg L1), respectively; Q And Qs are the flow rate of the influent and the flow rate of the waste sludge (L h1), respectively; R and r are the reflux ratios of the returning activated sludge and the recycling mixed liquor, respectively. On the basis of the mass balance, the fate and behavior of DMP in the AAO treatment process, which consisted of the amount of the degraded DMP by microorganisms in the system, the amount of the effluent DMP, the amount of accumulating DMP in the system, the amount of the discharging DMP with waste sludge, was calculated as the following equation:
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Min Meff ¼ Msludge þ Mbio þ Macc
(9)
where Min ,Mout , Msludge , Mbio and Macc are the amount of the influent DMP, the effluent DMP, the degraded DMP by microorganisms in the system, and the discharging DMP with waste sludge, respectively (mg).
PAEs can be also adsorbed on the suspended organic matter (SOM) in the wastewater, which could cause that PAEs can be not degraded in the near future (Patrik et al., 2003). Moreover, SOM could go through structural changes during the biological treatment process, such as degradation and hydrolysis, so eventually the adsorbed PAEs maybe be released in the effluents. Therefore, it is very important to explore the adsorbent influence of PAEs in the further.
3. Results and discussions 3.2. Influence of SRT on the degradation and behavior of DMP 3.1. Influence of HRT on the degradation and behavior of DMP As shown in Table 1, the concentrations of DMP in the influent, anaerobic, anoxic, aerobic and settling zones, final effluent and returning activated sludge of the AAO treatment system under different HRT condition were obtained. From Table 1, it can be seen that, DMP concentrations in the water and sludge phases were separately measured to determine the ratio between the aqueous and solid phases. The total removal rate of DMP in reactor system and the percentage of removed DMP for anaerobic, anoxic, and aerobic zones are shown in Fig. 2, which indicates that, for different HRTs with 12, 18, 24, the total removal rate of DMP in reactor system was all above 91%. With the change of HRT, there was no significant influence on the removal rate of DMP in the system. For each reactor, the percentage of DMP in the sludge phase was a value between 81 and 95%, and the quantities of DMP in the sludge phase for anaerobic, anoxic, and aerobic zones were within the scope of 14.80e17.00 mg g1, 12.46e14.33 mg g1, and 10.04e12.35 mg g1, respectively. From the measured experimental data, it implies that absorption on sludge was the dominant status. And the degradation efficiency of DMP in anaerobic zone was under the degradation efficiency of DMP in anoxic zone, while the degradation efficiency of DMP in aerobic zone was greater than anoxic zone. The degradation efficiencies of DMP for anaerobic, anoxic and aerobic zones were within the scope of 10e18%, 13e21%, and 55e68%, respectively. Hence, comparing with the anaerobic and anoxic microorganisms, the aerobic microorganisms made a greater contribution to DMP removal. In addition it needed to maintain the activity of sludge in the reactor system. As shown in Fig. 3 and S1 Table, The calculation results demonstrated that the percentage of the degraded DMP by the microorganisms was a value between 67.3 and 73.8%, the percentage of the released DMP in the effluent was in the range of 5.8e8.7%, the percentage of the accumulated DMP was 19.1e26.2%, and the percentage of the remained DMP in the waste sludge was only 1.1e1.4%. Therefore, we may reasonably conclude that, as HRT became longer, DMP was easier to be absorbed onto the waste sludge and the amount of the accumulated DMP in the system became more. It would be helpful to grasp the removal process of DMP. However, due to the low solubility and the high logKow values, Table 1 The concentrations of DMP in different zones of the AAO treatment system under different HRT condition. HRT(h) 1
Influent (mg L ) Aqueous phase in anaerobic reactor (mg L1) Solid phase in anaerobic reactor (mg g1) Aqueous phase in anoxic reactor (mg L1) Solid phase in anoxic reactor (mg g1) Aqueous phase in oxic reactor (mg L1) Solid phase in oxic reactor (mg g1) Return sludge (mg g1) Final effluent (mg L1)
12
18
24
30
49.98 9.38 17.00 5.17 14.09 3.02 10.04 9.52 4.34
53.08 7.40 14.80 3.61 14.33 1.39 10.13 10.25 3.07
64.29 8.82 16.13 4.62 13.76 2.26 12.16 12.81 3.73
47.78 9.11 16.27 4.74 12.48 2.21 12.35 12.52 3.12
As shown in Table 2 and Fig. 4, the concentrations of DMP in the influent, anaerobic, anoxic, aerobic and settling zones, final effluent and returning activated sludge under different SRT condition were obtained. From Fig. 4, it can be seen that, with the variation of SRT from 10 to 15 d, the total removal rate of DMP through the AAO treatment system increased from 90.9% to 94.2%. However, with the change of SRT from 15 to 25 d, there was no significant influence on the removal rate of DMP. As shown in Table 2, we can see that the quantities of DMP in the sludge phase for anaerobic, anoxic, and aerobic zones were within the scope of 14.05e16.93 mg g1, 12.61e14.33 mg g1 and 10.13e13.13 g g1, respectively. The degradation efficiencies of DMP for anaerobic, anoxic and aerobic zones to the total DMP removal in the system were within the scope of from 13.4 to 21.4%, 13.0e17.9% and 56.6e67.7%, respectively. It can be concluded that the results from the influence of SRT on the degradation and behavior of DMP were in accord with the results obtained from the variation of HRT. As shown in Fig. 5 and S2 Table, the percentage of the degraded DMP by the microorganisms was a value between 64.9 and 75.5%, the percentage of the released DMP in the effluent was in the range of 5.8e9.1%, the percentage of the accumulated DMP was 17.1e26.4%, and the percentage of the remained DMP in the waste sludge was only 1.1e1.4%. And it can been seen that, the percentage of the remained DMP in the releasing waste sludge decreased from 1.4% to 1.1% with the change of SRT. On the contrary, the percentage of the degraded DMP by the microorganisms increased from 64.9% to 75.5%. These results indicated it was rather slow in the growth of the microorganisms for degrading DMP, and It is beneficial to increase the retention time of the microorganisms with the increase of SRT. 3.3. Removal efficiencies of nutrients under the optimal operation condition In generally, AAO treatment process is designed to treat the domestic sewage, so the removal efficiencies of conventional nutrients parameters were investigated under the optimal operation conditions in AAO treatment system. The measured data indicated that with the increase of HRT from 12 to 30 h, there was no significant influence on the removal efficiencies of COD, BOD5, NHþ 4 -N, TN and TP. Therefore, considering the degradation and behavior of DMP with different HRT, the most optimal HRT for treating DMP and nutrients in AAO system was determined to be 18 h. The removal efficiencies of nutrients parameters under different SRT condition is shown in Fig. 6. From Fig. 6, it can be seen, with the increase of SRT, there was only little influence on the removal efficiencies of COD, BOD5, NHþ 4 -N and TN. However, the removal rate of TP decreased with the increase of SRT, especially during the change of SRT from 15 to 20 d. Because the eliminating sludge in AAO system determines biological phosphorus removal, a large amount of the produced sludge will generally become more and more at shorter SRT, and the removal rate of phosphorus can become higher. Thus, the removal rate of TP decreased with the change of SRT from 15d to 20d, while there was only little influence
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285
Fig. 2. The total removal rate of DMP in reactor system and the percentage of removed DMP for anaerobic, anoxic, and aerobic zones at different HRT.
Fig. 3. The degradation and behavior of DMP with different HRT in AAO treatment process.
Table 2 The concentrations of DMP in different zones of the AAO treatment system under different SRT condition. SRT(d)
10
15
20
25
Influent (mg L1) Aqueous phase in anaerobic reactor (mg L1) Solid phase in anaerobic reactor (mg g1) Aqueous phase in anoxic reactor (mg L1) Solid phase in anoxic reactor (mg g1) Aqueous phase in oxic reactor (mg L1) Solid phase in oxic reactor (mg g1) Return sludge (mg g1) Final effluent (mg L1)
50.88 11.21 15.55 6.37 12.61 4.19 12.83 12.56 4.61
53.08 7.40 14.80 3.61 14.33 1.39 10.13 10.25 3.07
59.78 8.72 14.05 3.94 13.33 2.08 10.41 10.23 3.72
49.79 8.88 16.93 3.84 12.81 1.89 13.02 13.13 3.16
on the removal efficiencies of COD, BOD5, NHþ 4 -N and TN. Therefore, considering the degradation and behavior of DMP with different
SRT, the most optimal SRT for treating DMP and nutrients in AAO system was determined to be 15 d. On the basis of the optimal operation condition with HRT of 18 h and SRT of 15 d, the degradation and behavior of DMP was shown in Tables 1 and 2 The fate of DMP in AAO treatment process could be shown in details as the following: the percentage of the degraded DMP by the microorganisms was 73.8%, the percentage of the released DMP in the effluent was 5.8%, the percentage of the accumulated DMP was 19.3% was accumulated in the system, and the percentage of the remained DMP in the waste sludge was only 1.1%. Similar results were obtained by some researchers. A range of 73%e87% as removal efficiencies for DMP, diethyl phthalate, bis(2ethylhexyl) phthalate, di-n-butyl phthalate, and butyl-benzyl phthalate in the aerobically activated sludge WWTP were reported by Balabanic and Klemencic (2011). In this work, AAO
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Fig. 4. The total removal rate of DMP in reactor system and the percentage of removed DMP for anaerobic, anoxic, and aerobic zones at different SRT.
Fig. 5. The degradation and behavior of DMP with different SRT in AAO treatment process.
treatment system, which is a modified form of the activated sludge process, added anaerobic and anoxic units, can provide high removal efficiencies for PAEs and have advantages of excellent nitrogen and phosphorus removal. The research results indicated that, the high removal efficiency of DMP in AAO treatment process could be obtained under the optimal operation condition with HRT of 18 h and SRT of 15 d, provided that the nutrient can meet the requirements of the AAO system satisfied. Meanwhile nutrient removals were consistent with DMP removal, which were high in the system. 3.4. Kinetics of DMP degradation In order to describing exactly the degradation behavior of DMP in AAO treatment system, the degradation models including biodegradation and sorption according to the Activated Sludge
Model was developed based on the fate of DMP. The anaerobic, anoxic and aerobic hydrolysis rate can be designed as following:
rh ðanaerobicÞ ¼ hFe K
rh ðanoxicÞ ¼ hNO3 K
rh ðaerobicÞ ¼ K
KO2 KNO3 S X KO2 þ SO2 KNO3 þ SNO3 KS þ S H
KO2 KNO3 S X KO2 þ SO2 KNO3 þ SNO3 KS þ S H
KO2 S X KO2 þ SO2 KS þ S H
(10)
(11)
(12)
where rh (anaereobic), rh (anobic), and rh (aerobic) are the anaerobic hydrolysis process rate, anoxic hydrolysis process rate and aerobic
T. Zhang et al. / Journal of Environmental Management 184 (2016) 281e288
parameters of DBP for the anaerobic degradation, anoxic degradation and aerobic degradation shown as in S3eS5 Tables were determined. Thus according to the linear formula (Equation (15)), the kinetic parameters of the models (K, Ks and h) shown in Table 3 were calculated. From Table 3, it can be seen that, the parameter h of DMP for the anaerobic degradation, anoxic degradation and aerobic degradation, which were above the guided values by International Water Association, were 0.68, 0.80 and 1.00, respectively. That is because the mixed liquor passing through the aerobic zones was recycled to the anoxic zone, and the sludge in the settling zone was returned back to the anaerobic zone, which caused anaerobic sludge, anoxic sludge and aerobic sludge to have the similar characteristic. Thus, the removal efficiency of DMP in AAO treatment process was higher. In order to assess the performance of the proposed models, the models were utilize for forecasting the removal efficiency of DMP. Table 4 lists the predicted values for the concentrations of 78.97 and 59.33 mg l1. It is very clear from Table 4 that the modeling approach gave good predictions. Due to the degradation model including biodegradation and sorption, the models can exactly describe the fate of DMP in the treatment system. Therefore, the forecasting errors were very small, RMSE and MAPE of the model were 5.1892 and 11.0%, respectively, and the average value of relative errors were below 15.0%. The results clearly indicated the proposed model can describe exactly the degradation behavior of DMP in AAO treatment system.
Fig. 6. The removal efficiencies of nutrients parameters under different SRT condition.
Table 3 The kinetics parameters of DMP in the removal models. Parameters
Fitting Eq.
R2
Ks
K
h
Anaerobic reaction Anoxic reaction Aerobic reaction
Y ¼ 15.320xaþ0.1033 Y ¼ 15.911xþ0.0880 Y ¼ 6.720xþ0.0701
0.9884 0.9812 0.9973
148.31 180.81 95.89
9.68 11.36 14.27
0.68 0.80 1
a
287
x ¼ 1S ,S is the biodegradable DMP.
Table 4 Predictions and the performance of the developed models. Indexes
Real values (mg L1)
Predicted values (mg L1)
Relative errors (%)
Mean relative errors (%)
Anaerobic
68.40 51.39 58.11 43.65 4.67 3.51
60.88 44.89 52.08 38.56 5.34 3.73
11.0 12.6 10.4 11.7 14.4 6.2
11.8
Anoxic Oxic
hydrolysis process rate, respectively; hFe andhNO3 are anaerobic hydrolysis reduction factor and anoxic hydrolysis reduction factor; KNO3 , K and KO2 are saturation/inhibition coefficient for nitrate, hydrolysis rate constant and saturation/inhibition coefficient for oxygen, respectively; SO2 and S are dissolved oxygen and biodegradable substrate; XH is heterotrophic biomass. In addition, due to the uniformity of the mixed liquors in each reactor, the metabolic rate of DMP by microorganism in AAO system is uniform. Hence, the degradation rate of DMP in AAO treatment process could be described as the following equation:
dS S S0 ¼ dt t
(13)
From what had been mentioned above, the model for describing the degradation rate of DMP could be simplified:
Xt K 1 1 ¼ S$ þ S0 S K S K
(14)
where 1S , SoXtS, KKS and K1 were represent as the variables of x, y, k and a, respectively. Therefore, Equation (14) could be derived to a linear formula for relating the transformed values as following:
y ¼ kx þ a
(15)
Base on the kinetic model of DMP degradation, kinetic
10.5
RMSE
MAPE (%)
5.1892
11.0
10.3
4. Conclusions The degradation and behavior of DMP under different operation condition were investigated in a laboratory scale AAO treatment system. Under optimal operation condition with HRT of 18 h and SRT of 15 d, the percentage of the degraded DMP by the microorganisms was 73.8%, the percentage of the released DMP in the effluent was 5.8%, the percentage of the accumulated DMP was 19.3% was accumulated in the system, and the percentage of the remained DMP in the waste sludge was only 1.1%. In addition, it was proved that the developed degradation model including biodegradation and sorption can be used for accurately modeling the degradation and behavior of DMP in the AAO system. Acknowledgements This research was supported by National Natural Science Foundation of China (No. 51208206 and 51210013), Guangdong Provincial Natural Science Foundation (No. 2016A030306033), Guangdong Provincial Science and Technology Plan Project Foundation (No. 2014A020216007), Technological Innovation Young Talents of Guangdong Special Support Plan (No. 2014TQ01Z530), Pearl River S&T Nova Program of Guangzhou (No. 201506010058), and the Fundamental Research Funds for the Central Universities (No. 15lgpy11). The authors are thankful to the anonymous reviewers for their insightful comments and suggestions.
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Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2016.10.008. References Balabanic, D., Klemencic, A.K., 2011. Presence of phthalates, bisphenol a, and nonylphenol in paper mill wastewaters in Slovenia and efficiency of aerobic and combined aerobic-anaerobic biological wastewater treatment plants for their removal. Fresenius Environ. Bull. 20, 86e92. ro, J.R., Surampalli, R.Y., 2009. Concurrent Brar, S.K., Verma, M., Tyagi, R.D., Vale degradation of dimethyl phthalate (DMP) during production of Bacillus thuringiensis based biopesticides. J. Hazard. Mater 171, 1016e1023. Calafat, A.M., McKee, R.H., 2006. Integrating biomonitoring exposure data into the risk assessment process: phthalates [diethyl phthalate and di(2-ethylhexyl) phthalate] as a case study. Environ. Health Persp. 114, 1783e1789. Cendrine, D., Marie-Jeanne, T., Marc, C., Martine, B., 2009. Phthalate removal throughout wastewater treatment plant: case study of Marne Aval station (France). Sci. Total. Environ. 407, 1235e1244. Chen, C.Y., Wu, P.S., Chung, Y.C., 2009. Coupled biological and photo-Fenton pretreatment system for the removal of di-(2-ethylhexyl) phthalate (DEHP) from water. Bioresour. Technol. 100, 4531e4534. China's State Environmental Protection Administration, 2002. Standard Methods for the Examination of Water Wastewater. China Environmental Science Press, Beijing. European Union, 1993. Council Regulation (EEC), No. 793/93 of 23 March 1993 on the Evaluation and Control of the Risks of Existing Substances (OJ L 84, 5 April 1993). European Union, Brussels. Fang, C.R., Long, Y.Y., Shen, D.S., 2009. Comparison on the removal of phthalic acid diesters in a bioreactor landfill and a conventional landfill. Bioresour. Technol. 100, 5664e5670. Gao, D., Li, Z., Wen, Z., Ren, N., 2014. Occurrence and fate of phthalate esters in fullscale domestic wastewater treatment plants and their impact on receiving waters along the Songhua River in China. Chemosphere 95, 24e32. Huang, M.Z., Ma, Y.W., Wang, Y., Wan, J.Q., Zhang, H.P., 2010. The fate of dinbutylphthalate in a laboratory-scale anaerobic/anoxic/oxic wastewater treatment process. Bioresour. Technol. 101, 7767e7772. Huang, M.Z., Ma, Y.W., Wan, J.Q., Zhang, H.P., Wang, Y., Chen, Y.M., Yoo, C.K., Guo, W.J., 2011. A hybrid genetic - neural algorithm for modeling the biodegradation process of DnBP in AAO system. Bioresour. Technol. 102, 8907e8913. Huang, K.Y., Chou, W.L., Wang, C.T., Chang, Y.C., Shu, C.M., 2015. Electrochemically assisted coagulation for the adsorptive removal of dimethyl phthalate from aqueous solutions using iron hydroxides. J. Taiwan Inst. Chem. E 50, 236e241. Jarungwit, B., Chart, C., Wilai, C., Kazuo, Y., 2016. Kinetics of phenolic and phthalic acid esters biodegradation in membrane bioreactor (MBR) treating municipal landfill leachate. Chemosphere 150, 639e649. Knudsen, F.R., Pottinger, T.G., 1999. Interaction of endocrine disrupting chemicals, singly and in combination, with estrogen-, androgen-, and corticosteroidbinding sites in rainbow trout. Aquat. Toxicol. 44, 159e170. Kumar, P., Surender, S., Debjani, D., Surabhi, C., Sudershan, G., Lata, N., 2014. Biodegradation of dimethyl phthalate by an entomopathogenic nematode symbiont Xenorhabdus indica strain KB-3. Int. Biodeterior. Biodegr. 89, 23e28.
Latini, G., 2005. Monitoring phthalate exposure in humans. Clin. Chim. Acta 361, 20e29. Liang, D.W., Zhang, T., Herbert, H.P.F., 2007. Anaerobic degradation of dimethyl phthalate in wastewater in a UASB reactor. Water Res. 41, 2879e2884. Loraine, G.A., Pettigrove, M.E., 2006. Seasonal variations in concentrations of pharmaceuticals and personal care products in drinking water and reclaimed wastewater in Southern California. Environ. Sci. Technol. 40, 687e695. Magdouli, S., Daghrir, R., Brar, S.K., Drogui, P., Tyagi, R.D., 2013. Di 2-ethylhexylphtalate in the aquatic and terrestrial environment: a critical review. J. Environ. Manag. 127, 36e49. , R.U., Raúl, O.P., Jose , D.M.D., Manuel, S.P., 2012. Environmental Mahmoud, M.A., Jose impact of phthalic acid esters and their removal from water and sediments by different technologies-A review. J. Environ. Manag. 109, 164e178. Meghdad, P., Ali-Reza, M., Seyed, J.S., Ali, A.Z., 2009. Kinetic evaluation and process performance of a fixed film bioreactor removing phthalic acid and dimethyl phthalate. J. Hazard. Mater 167, 500e506. Mohan, S.V., Shailaja, S., Krishna, M.R., Sarma, P.N., 2007. Adsorptive removal of phthalate ester (Di-ethyl phthalate) from aqueous phase by activated carbon: a kinetic study. J. Hazard. Mater 146, 278e282. Montuori, P., Jover, E., Morgantini, M., Bayona, J.M., Triassi, M., 2008. Assessing human exposure to phthalic acid and phthalate esters from mineral water stored in polyethylene terephthalate and glass bottles. Food Addit. Contam. A 25, 511e518. Ogunfowokan, A.O., Torto, N., Adenuga, A.A., Okoh, E.K., 2006. Survey of levels of phthalate ester plasticizers in a sewage lagoon effluent and a receiving stream. Environ. Monit. Assess. 118, 457e480. Oliver, R., May, E., Williams, J., 2005. The occurrence and removal of phthalates in a trickle filter STW. Water Res. 39, 4436e4444. Patrik, F., Jorgen, V., Peter, B.S., Lars, C., 2003. Phthalates, nonylphenols and LAS in an alternately operated wastewater treatment plantdfate modeling based on measured concentrations in wastewater and sludge. Water Res. 37, 1288e1295. Peng, X., Li, X., 2012. Compound-specific isotope analysis for aerobic biodegradation of phthalate acid esters. Talanta 97, 445e449. Ruan, J., Xu, Z., 2011. Environmental friendly automated line for recovering the cabinet of waste refrigerator. Waste Manag. 31, 2319e2326. Sanna, K.M., Kari, H., Jukka, A.R., 2004. Removal of DEHP in composting and aeration of sewage sludge. Chemosphere 54, 265e272. Staples, C.A., Peterson, D.R., Parkerton, T.F., Adams, W.J., 1997. The environmental fate of phthalate esters: a literature review. Chemosphere 35, 667e749. US EPA, 1991. National Primary Drinking Water Regulations, Federal Register; 40 CFR Chapter I, Part 141. US Environmental Protection Agency, Washington, DC. Wang, J., Chen, L., Shi, H., Qian, Y., 2000. Microbial degradation of phthalic acid esters under anaerobic digestion of sludge. Chemosphere 41, 1245e1248. Wang, Y.L., Yin, B., Hong, Y.G., Yan, Y., Gu, J.D., 2008. Degradation of dimethyl carboxylic phthalate ester by Burkholderia cepacia DA2 isolated from marine sediment of South China Sea. Ecotoxicology 17, 845e852. Wu, M.H., Liu, N., Xu, G., Ma, J., Tang, L., Wang, L., Fu, H.Y., 2011. Kinetics and mechanisms studies on dimethyl phthalate degradation in aqueous solutions by pulse radiolysis and electron beam radiolysis. Radiat. Phys. Chem. 80, 420e425. Yuan, B.L., Li, X.Z., Graham, N., 2008. Aqueous oxidation of dimethyl phthalate in a Fe(VI)-TiO2-UV reaction system. Water Res. 42, 1413e1420. Zeng, F., Wen, J.X., Cui, K.Y., Wu, L.N., Liu, M., Li, Y.J., Zhu, F., Ma, Z., Zeng, Z., 2009. Seasonal distribution of phthalate esters in surface water of the urban lakes in the subtropical city, Guangzhou, China. J. Hazard. Mater. 169, 719e725.
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Research article
Degradation behavior of dimethyl phthalate in an anaerobic/anoxic/ oxic system Tao Zhang b, Zehua Huang c, Xiaohong Chen a, Mingzhi Huang a, *, Jujun Ruan b, ** a
Department of Water Resources and Environment, Guangdong Provincial Key Laboratory of Urbanization and Geo-simulation, Sun Yat-sen University, Guangzhou 510275, PR China b School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-Sen University, Guangzhou 510275, PR China c Fujian Quanzhou Foreign Language Middle School, Quanzhou 362002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2016 Received in revised form 2 October 2016 Accepted 4 October 2016 Available online 8 October 2016
Dimethyl phthalate (DMP) as one of the most important and extensively used Phthalic acid esters (PAEs) is known to likely cause dysfunctions of the endocrine systems, liver, and nervous systems of animals. In this paper, the degradation and behavior of DMP were investigated in a laboratory scale anaerobic/ anoxic/oxic (AAO) treatment system. In addition, a degradation model including biodegradation and sorption was formulated so as to evaluate the fate of DMP in the treatment system, and a mass balance model was designed to determine kinetic parameters of the removal model. The study indicated that the optimal operation condition of HRT and SRT for DMP and nutrients removal were 18 h and 15 d respectively, and the degradation rates of anaerobic, anoxic and aerobic zones for DMP were 13.4%, 13.0% and 67.7%, respectively. Under the optimal conditions, the degraded DMP was 73.8%, the released DMP in the effluent was 5.8%, the accumulated DMP was 19.3%, and the remained DMP in the waste sludge was 1.1%. Moreover, the degradation process of DMP by acclimated activated sludge was in accordance with the first-order kinetics equation. The model can be used for accurately modeling the degradation and behavior of DMP in the AAO system. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Dimethyl phthalate (DMP) Anaerobic/anoxic/oxic system Degradation Behavior Kinetic
1. Introduction In recent years, phthalic acid esters (PAEs) have caught extensive concerns because they are widely used as plastic plasticizers, and additives in more than hundred varieties of products, such as toy, packing material and cosmetics production. Therefore, these persistent and toxic organic compounds, which could harm the health of organisms and human by transmission of food chain and bioaccumulation, commonly exists in various environments (Latini, 2005; Magdouli et al., 2013). Dimethyl phthalate (DMP) known as one of the most important and extensively used PAEs, has been already measured in various environment, such as various surface water, groundwater, sediments of water, atmosphere, aerosol particle, soil (Wu et al., 2011; Huang et al., 2015; Montuori et al., 2008;
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Huang), ruanjujun@mail. sysu.edu.cn (J. Ruan). http://dx.doi.org/10.1016/j.jenvman.2016.10.008 0301-4797/© 2016 Elsevier Ltd. All rights reserved.
Ogunfowokan et al., 2006; Zeng et al., 2009), and is known to likely cause dysfunctions of the endocrine systems, liver, and nervous systems of humans and animals (Wang et al., 2008; Yuan et al., 2008; Calafat and McKee, 2006; Wu et al., 2011). Therefore, DMP has been listed as a priority control pollutant by American Environmental Protection Agency (1991), Ministry of Environmental Protection of the People's Republic of China (Mohan et al., 2007) and European Union (1993). The plasticizers in the water environment is mainly from discharge of industrial effluent without treatment. However, due to its persistence and refractory, the wastewater treated though the wastewater treatment plants (WWTP) is also an important source of The plasticizers in the water environment (Gao et al., 2014; Loraine and Pettigrove, 2006). When these toxic chemicals pass through the domesticated biological treatment process, which can overcome the shortcoming of traditional-activated sludge process, they are quite recalcitrant to be degraded and may direct discharge into the receiving water bodies. If PAEs are not removed at WWTP, there may be toxic or endocrine disrupting influences on aquatic organisms in the receiving water bodies (Knudsen and Pottinger,
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Abbreviation PAEs DMP WWTP AAO COD BOD TN NHþ 4 -N TP DO HRT SRT MLSS HPLC MAPE RMSE SOM
Phthalic acid esters Dimethyl phthalate Wastewater treatment plants Anaerobic/anoxic/oxic Chemical organic demand Biochemical oxygen demand Total nitrogen Ammonium nitrogen Total phosphorus Dissolved oxygen Hydraulic retention time Sludge retention time Mixed liquor suspended solid High performance liquid chromatography Mean absolute percentage error Root mean squared error Suspended organic matter
1999; Meghdad et al., 2009). As such compounds are degraded slowly by photodegradation and hydrolyzation under natural conditions, biodegradation is thought of as one of the major degradation pathways for PAEs (Staples et al., 1997; Huang et al., 2011). Previous studies have demonstrated that several PAEs can take the biodegradation under aerobic conditions in activated sludge (Sanna et al., 2004; Cendrine et al., 2009), in soil and sediments (Peng and Li, 2012), in river, lake or sea water (Chen et al., 2009; Fang et al., 2009), and under anaerobic conditions (Liang et al., 2007; Wang et al., 2000), and the degradation rate of PAEs can reach from 60% to 100% though WWTP, which includes aerobic and anaerobic treatment zones (Gao et al., 2014; Oliver et al., 2005). From these studies it can be concluded PAEs could be effectively removed under aerobic conditions, but for anaerobic conditions, the degradation efficiency of PAEs becomes much slower (Huang et al., 2010; Mahmoud et al., 2012; Ruan and Xu, 2011). However, these studies have tended to pay attention to the biodegradability and degradation pathway of different PAEs. Hence, the effective degradation of DMP in WWTP is essential to be understood so that the effluent concentration of DMP can be minimized to discharge into the receiving water bodies. Previous researches demonstrated that various PAEs have high biodegradation efficiency in the present of activated sludge (Kumar et al., 2014; Brar et al., 2009), and various PAEs could be removed by adsorption onto activated sludge. As a consequence, it is believable that the main removal mechanisms for DMP removal could be sorption on activated sludge and biodegradation. However, it is also very controversial and very challenging how these removal processes interacts with in WWTP. Moreover, in order to describe the removal process of pollutants in WWTP and understand the degradation of pollutants in WWTP, many researchers focus on the kinetics of pollutants in WWTP (Meghdad et al., 2009; Patrik et al., 2003; Jarungwit et al., 2016). But in the previous study, the kinetic model is only used for modeling the biodegradation of PAEs, not take into account the fate of PAEs in WWTP. Thereby, it still needs to clearly investigate the degradation mechanism and behavior of DMP in the treatment system. And on this basis a model describing the degradation behavior of DMP should be built. In this work, the degradation behavior of DMP were investigated
in an anaerobic/anoxic/oxic (AAO) treatment system under various operating conditions. In order to evaluate the fate of DMP in the treatment system, three removal models including biodegradation and sorption process for anaerobic, anoxic and aerobic reaction were developed, respectively. In the end, a mass balance model based on degradation and behavior of DMP was designed to determine the kinetic parameters of the removal models. 2. Materials and methods 2.1. Reactor system As shown in Fig. 1, the AAO treatment system made of polyethylene includes mainly four parts: one anaerobic zone with volume of 40 L, one anoxic zone with volume of 40 L, three aerobic zone with total volume of 160 L and one settling zone. there were two motor-driven stirrers employed in anaerobic and anoxic zones. An air blower was used to supply oxygen to the microorganisms of aerobic zone. The mixed liquor passing through the aerobic zones was recycled to the anoxic zone, and the sludge was returned from the bottom of the settling zone to the anaerobic zone. The reflux ratios of the mixed liquor and sludge were same, and set to 1. The pH of anaerobic digester is controlled at 6.5e8.0. 2.2. Feed The sludge from a sewage treatment plant in Guangzhou was cultivated in a laboratory scale AAO treatment system with synthetic wastewater as feed. The synthetic wastewater with five different concentrations of DMP, which included 30, 40, 50, 60, and 80 mg L-1, was used. Chemical organic demand (COD) was supplied from glucose. Ammonium nitrate and potassium dihydrogen phosphate were added to maintain the nitrogen and phosphorous sources in the system. The ratio of COD:N:P was kept at 100:7:1. 2.3. Operation conditions of the experiment In order to maintaining at a constant temperature of 25 C, the work environment reactor system was controlled by the temperature control system. Dissolved oxygen (DO) was measured by the online dissolved oxygen meter (D53, HACH), and the concentrations of DO in anaerobic, anoxic and aerobic zones were within the scope of 0e0.30 mg L1, 0e0.60 mg L1 and 2.54e5.72 mg L1, respectively. The mixed liquor suspended solid (MLSS) concentration of about 3000 mg L1 was controlled. On the basis of changing the influent pump flow, hydraulic retention time (HRT) would be adjusted. Just as well sludge retention time (SRT) would be adjusted through altering the amount of the discharged excess sludge in the bottom of the settling zone. In order to investigate the influence of HRT and SRT on the degradation and behavior of DMP, different HRTs with 12, 18, 24, and 30 h and different SRT with 10, 15, 20, and 25 d were used. The continuous period of the operated system was one year. 2.4. Sampling and extraction Water phase: in this work, the samples from the inlet and outlet of the reactor system and mixed liquors in anaerobic, anoxic and aerobic zones were filled into 1 L glass bottles once per day. Firstly, after all samples were treated by centrifugation at 3000 rpm for 15 min, 3% sulfuric acid was added to adjust pH of the supernatant at 3. And then a solid phase extraction column (Waters Oasis HLB, 200 mg/6 mL) was used to extract the centrifuged samples. When the flow rate of the supernatant flowing through the columns was up to 1e2 mL min1, the columns were washed with 3 mL
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283
Fig. 1. Diagram of the AAO treatment system.
methanol/water (5:95 v/v). Finally, the purified product was obtained through vacuum concentration and drying with a gentle nitrogen flow for 5 min. Sludge phase: the sludge treated by centrifugation was extracted by the soxhlet extractor with 90 mL petroleum ether for 48 h after drying at room temperature. The extracts were reduced to approximately 1.0 mL under gentle nitrogen flow through concentrating with anhydrous sodium sulfate and solventexchanging to n-hexane. The concentrated extracts were separated and isolated on a silica-alumina gel glass column. The prepared process was in accordance with the process procedure of the water phase when the extracts were adjusted to 100 mL. During the extraction process, it need to add the activated copper granules into the extraction flasks for removing elemental sulfur.
2.5. Analytical methods High performance liquid chromatography (HPLC) (Agilent 1100, USA) with a reverse phase Hypersil C-18 column was used to for determination and identification of the concentration of DMP. The mobile phase consisted of methanol and water (90:10, v/v) was used as gradient elution for separation with HPLC, and the flow rate and the injection volume of the mobile phase were 0.8 mL min-1 and 20 mL, respectively. COD, biochemical oxygen demand (BOD), MLSS, total nitrogen (TN), ammonium nitrogen (NHþ 4 -N) and total phosphorus (TP) were measured according to Standard Methods (China's State Environmental Protection Administration, 2002).
2.6. Calculation processes For investigate the fate of DMP in the system, the calculating equations for the concentrations of DMP in anaerobic, anoxic, and aerobic zones are designed as following:
Pi1 ¼
Pi þ R Pe1 1þR
(1)
Pi2 ¼
ð1 þ RÞ P1 þ r P3 1þRþr
(2)
Pi3 ¼ P2
(3)
And the calculating Equation (4) is the total removal rate of DMP through the AAO treatment system:
Total removal rate of DMP ð%Þ ¼
Pi Q ðQ Qs Þ Pε Qs P3 Pi Q (4)
The total amount of DMP removed in the system includes the removal mount of DMP in anaerobic zone, the removal mount of DMP in anoxic zone, the removal mount of DMP in aerobic zones and the removal mount of DMP in settling zone. The calculating equations for the removal rates of DMP in anaerobic, anoxic, aerobic and settling zones are shown as the following:
RRanaerobic ¼
ðPi1 P1 Þ Q ð1 þ RÞ 100% ðPi1 P1 Þ Q þ ðPε P3 Þ Qs
(5)
RRanoxic ¼
ðPi2 P2 Þ Q ð1 þ R þ rÞ 100% ðPi Pε Þ Q þ ðPε P3 Þ Qs
(6)
RRaerobic ¼
ðPi3 P3 Þ Q ð1 þ R þ rÞ 100% ðPi Pε Þ Q þ ðPε P3 Þ Qs
(7)
RRsettling ¼
P3 ðQ þ R Q Qs Þ Pε ðQ Qs Þ R Q Pε1 ðPi Pε Þ Q þ ðPε P3 Þ Qs 100% (8)
where RRanaerobic , RRanoxic , RRaerobic and RRsettling are the removal rates of DMP in anaerobic, anoxic, aerobic and settling zones, respectively; Pi is the concentration of the influent DMP (mg L1); Pε1 is the concentration of DMP for returning activated sludge (mg L1); P1 ,P2 ,P3 ,Pε are the concentrations of the effluent DMP in anaerobic, anoxic, aerobic and settling zones (mg L1), respectively; Pi1 ,Pi2 ,Pi3 are the concentrations of DMP in mixed liquors of the anaerobic, anoxic, aerobic zones (mg L1), respectively; Q And Qs are the flow rate of the influent and the flow rate of the waste sludge (L h1), respectively; R and r are the reflux ratios of the returning activated sludge and the recycling mixed liquor, respectively. On the basis of the mass balance, the fate and behavior of DMP in the AAO treatment process, which consisted of the amount of the degraded DMP by microorganisms in the system, the amount of the effluent DMP, the amount of accumulating DMP in the system, the amount of the discharging DMP with waste sludge, was calculated as the following equation:
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Min Meff ¼ Msludge þ Mbio þ Macc
(9)
where Min ,Mout , Msludge , Mbio and Macc are the amount of the influent DMP, the effluent DMP, the degraded DMP by microorganisms in the system, and the discharging DMP with waste sludge, respectively (mg).
PAEs can be also adsorbed on the suspended organic matter (SOM) in the wastewater, which could cause that PAEs can be not degraded in the near future (Patrik et al., 2003). Moreover, SOM could go through structural changes during the biological treatment process, such as degradation and hydrolysis, so eventually the adsorbed PAEs maybe be released in the effluents. Therefore, it is very important to explore the adsorbent influence of PAEs in the further.
3. Results and discussions 3.2. Influence of SRT on the degradation and behavior of DMP 3.1. Influence of HRT on the degradation and behavior of DMP As shown in Table 1, the concentrations of DMP in the influent, anaerobic, anoxic, aerobic and settling zones, final effluent and returning activated sludge of the AAO treatment system under different HRT condition were obtained. From Table 1, it can be seen that, DMP concentrations in the water and sludge phases were separately measured to determine the ratio between the aqueous and solid phases. The total removal rate of DMP in reactor system and the percentage of removed DMP for anaerobic, anoxic, and aerobic zones are shown in Fig. 2, which indicates that, for different HRTs with 12, 18, 24, the total removal rate of DMP in reactor system was all above 91%. With the change of HRT, there was no significant influence on the removal rate of DMP in the system. For each reactor, the percentage of DMP in the sludge phase was a value between 81 and 95%, and the quantities of DMP in the sludge phase for anaerobic, anoxic, and aerobic zones were within the scope of 14.80e17.00 mg g1, 12.46e14.33 mg g1, and 10.04e12.35 mg g1, respectively. From the measured experimental data, it implies that absorption on sludge was the dominant status. And the degradation efficiency of DMP in anaerobic zone was under the degradation efficiency of DMP in anoxic zone, while the degradation efficiency of DMP in aerobic zone was greater than anoxic zone. The degradation efficiencies of DMP for anaerobic, anoxic and aerobic zones were within the scope of 10e18%, 13e21%, and 55e68%, respectively. Hence, comparing with the anaerobic and anoxic microorganisms, the aerobic microorganisms made a greater contribution to DMP removal. In addition it needed to maintain the activity of sludge in the reactor system. As shown in Fig. 3 and S1 Table, The calculation results demonstrated that the percentage of the degraded DMP by the microorganisms was a value between 67.3 and 73.8%, the percentage of the released DMP in the effluent was in the range of 5.8e8.7%, the percentage of the accumulated DMP was 19.1e26.2%, and the percentage of the remained DMP in the waste sludge was only 1.1e1.4%. Therefore, we may reasonably conclude that, as HRT became longer, DMP was easier to be absorbed onto the waste sludge and the amount of the accumulated DMP in the system became more. It would be helpful to grasp the removal process of DMP. However, due to the low solubility and the high logKow values, Table 1 The concentrations of DMP in different zones of the AAO treatment system under different HRT condition. HRT(h) 1
Influent (mg L ) Aqueous phase in anaerobic reactor (mg L1) Solid phase in anaerobic reactor (mg g1) Aqueous phase in anoxic reactor (mg L1) Solid phase in anoxic reactor (mg g1) Aqueous phase in oxic reactor (mg L1) Solid phase in oxic reactor (mg g1) Return sludge (mg g1) Final effluent (mg L1)
12
18
24
30
49.98 9.38 17.00 5.17 14.09 3.02 10.04 9.52 4.34
53.08 7.40 14.80 3.61 14.33 1.39 10.13 10.25 3.07
64.29 8.82 16.13 4.62 13.76 2.26 12.16 12.81 3.73
47.78 9.11 16.27 4.74 12.48 2.21 12.35 12.52 3.12
As shown in Table 2 and Fig. 4, the concentrations of DMP in the influent, anaerobic, anoxic, aerobic and settling zones, final effluent and returning activated sludge under different SRT condition were obtained. From Fig. 4, it can be seen that, with the variation of SRT from 10 to 15 d, the total removal rate of DMP through the AAO treatment system increased from 90.9% to 94.2%. However, with the change of SRT from 15 to 25 d, there was no significant influence on the removal rate of DMP. As shown in Table 2, we can see that the quantities of DMP in the sludge phase for anaerobic, anoxic, and aerobic zones were within the scope of 14.05e16.93 mg g1, 12.61e14.33 mg g1 and 10.13e13.13 g g1, respectively. The degradation efficiencies of DMP for anaerobic, anoxic and aerobic zones to the total DMP removal in the system were within the scope of from 13.4 to 21.4%, 13.0e17.9% and 56.6e67.7%, respectively. It can be concluded that the results from the influence of SRT on the degradation and behavior of DMP were in accord with the results obtained from the variation of HRT. As shown in Fig. 5 and S2 Table, the percentage of the degraded DMP by the microorganisms was a value between 64.9 and 75.5%, the percentage of the released DMP in the effluent was in the range of 5.8e9.1%, the percentage of the accumulated DMP was 17.1e26.4%, and the percentage of the remained DMP in the waste sludge was only 1.1e1.4%. And it can been seen that, the percentage of the remained DMP in the releasing waste sludge decreased from 1.4% to 1.1% with the change of SRT. On the contrary, the percentage of the degraded DMP by the microorganisms increased from 64.9% to 75.5%. These results indicated it was rather slow in the growth of the microorganisms for degrading DMP, and It is beneficial to increase the retention time of the microorganisms with the increase of SRT. 3.3. Removal efficiencies of nutrients under the optimal operation condition In generally, AAO treatment process is designed to treat the domestic sewage, so the removal efficiencies of conventional nutrients parameters were investigated under the optimal operation conditions in AAO treatment system. The measured data indicated that with the increase of HRT from 12 to 30 h, there was no significant influence on the removal efficiencies of COD, BOD5, NHþ 4 -N, TN and TP. Therefore, considering the degradation and behavior of DMP with different HRT, the most optimal HRT for treating DMP and nutrients in AAO system was determined to be 18 h. The removal efficiencies of nutrients parameters under different SRT condition is shown in Fig. 6. From Fig. 6, it can be seen, with the increase of SRT, there was only little influence on the removal efficiencies of COD, BOD5, NHþ 4 -N and TN. However, the removal rate of TP decreased with the increase of SRT, especially during the change of SRT from 15 to 20 d. Because the eliminating sludge in AAO system determines biological phosphorus removal, a large amount of the produced sludge will generally become more and more at shorter SRT, and the removal rate of phosphorus can become higher. Thus, the removal rate of TP decreased with the change of SRT from 15d to 20d, while there was only little influence
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Fig. 2. The total removal rate of DMP in reactor system and the percentage of removed DMP for anaerobic, anoxic, and aerobic zones at different HRT.
Fig. 3. The degradation and behavior of DMP with different HRT in AAO treatment process.
Table 2 The concentrations of DMP in different zones of the AAO treatment system under different SRT condition. SRT(d)
10
15
20
25
Influent (mg L1) Aqueous phase in anaerobic reactor (mg L1) Solid phase in anaerobic reactor (mg g1) Aqueous phase in anoxic reactor (mg L1) Solid phase in anoxic reactor (mg g1) Aqueous phase in oxic reactor (mg L1) Solid phase in oxic reactor (mg g1) Return sludge (mg g1) Final effluent (mg L1)
50.88 11.21 15.55 6.37 12.61 4.19 12.83 12.56 4.61
53.08 7.40 14.80 3.61 14.33 1.39 10.13 10.25 3.07
59.78 8.72 14.05 3.94 13.33 2.08 10.41 10.23 3.72
49.79 8.88 16.93 3.84 12.81 1.89 13.02 13.13 3.16
on the removal efficiencies of COD, BOD5, NHþ 4 -N and TN. Therefore, considering the degradation and behavior of DMP with different
SRT, the most optimal SRT for treating DMP and nutrients in AAO system was determined to be 15 d. On the basis of the optimal operation condition with HRT of 18 h and SRT of 15 d, the degradation and behavior of DMP was shown in Tables 1 and 2 The fate of DMP in AAO treatment process could be shown in details as the following: the percentage of the degraded DMP by the microorganisms was 73.8%, the percentage of the released DMP in the effluent was 5.8%, the percentage of the accumulated DMP was 19.3% was accumulated in the system, and the percentage of the remained DMP in the waste sludge was only 1.1%. Similar results were obtained by some researchers. A range of 73%e87% as removal efficiencies for DMP, diethyl phthalate, bis(2ethylhexyl) phthalate, di-n-butyl phthalate, and butyl-benzyl phthalate in the aerobically activated sludge WWTP were reported by Balabanic and Klemencic (2011). In this work, AAO
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Fig. 4. The total removal rate of DMP in reactor system and the percentage of removed DMP for anaerobic, anoxic, and aerobic zones at different SRT.
Fig. 5. The degradation and behavior of DMP with different SRT in AAO treatment process.
treatment system, which is a modified form of the activated sludge process, added anaerobic and anoxic units, can provide high removal efficiencies for PAEs and have advantages of excellent nitrogen and phosphorus removal. The research results indicated that, the high removal efficiency of DMP in AAO treatment process could be obtained under the optimal operation condition with HRT of 18 h and SRT of 15 d, provided that the nutrient can meet the requirements of the AAO system satisfied. Meanwhile nutrient removals were consistent with DMP removal, which were high in the system. 3.4. Kinetics of DMP degradation In order to describing exactly the degradation behavior of DMP in AAO treatment system, the degradation models including biodegradation and sorption according to the Activated Sludge
Model was developed based on the fate of DMP. The anaerobic, anoxic and aerobic hydrolysis rate can be designed as following:
rh ðanaerobicÞ ¼ hFe K
rh ðanoxicÞ ¼ hNO3 K
rh ðaerobicÞ ¼ K
KO2 KNO3 S X KO2 þ SO2 KNO3 þ SNO3 KS þ S H
KO2 KNO3 S X KO2 þ SO2 KNO3 þ SNO3 KS þ S H
KO2 S X KO2 þ SO2 KS þ S H
(10)
(11)
(12)
where rh (anaereobic), rh (anobic), and rh (aerobic) are the anaerobic hydrolysis process rate, anoxic hydrolysis process rate and aerobic
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parameters of DBP for the anaerobic degradation, anoxic degradation and aerobic degradation shown as in S3eS5 Tables were determined. Thus according to the linear formula (Equation (15)), the kinetic parameters of the models (K, Ks and h) shown in Table 3 were calculated. From Table 3, it can be seen that, the parameter h of DMP for the anaerobic degradation, anoxic degradation and aerobic degradation, which were above the guided values by International Water Association, were 0.68, 0.80 and 1.00, respectively. That is because the mixed liquor passing through the aerobic zones was recycled to the anoxic zone, and the sludge in the settling zone was returned back to the anaerobic zone, which caused anaerobic sludge, anoxic sludge and aerobic sludge to have the similar characteristic. Thus, the removal efficiency of DMP in AAO treatment process was higher. In order to assess the performance of the proposed models, the models were utilize for forecasting the removal efficiency of DMP. Table 4 lists the predicted values for the concentrations of 78.97 and 59.33 mg l1. It is very clear from Table 4 that the modeling approach gave good predictions. Due to the degradation model including biodegradation and sorption, the models can exactly describe the fate of DMP in the treatment system. Therefore, the forecasting errors were very small, RMSE and MAPE of the model were 5.1892 and 11.0%, respectively, and the average value of relative errors were below 15.0%. The results clearly indicated the proposed model can describe exactly the degradation behavior of DMP in AAO treatment system.
Fig. 6. The removal efficiencies of nutrients parameters under different SRT condition.
Table 3 The kinetics parameters of DMP in the removal models. Parameters
Fitting Eq.
R2
Ks
K
h
Anaerobic reaction Anoxic reaction Aerobic reaction
Y ¼ 15.320xaþ0.1033 Y ¼ 15.911xþ0.0880 Y ¼ 6.720xþ0.0701
0.9884 0.9812 0.9973
148.31 180.81 95.89
9.68 11.36 14.27
0.68 0.80 1
a
287
x ¼ 1S ,S is the biodegradable DMP.
Table 4 Predictions and the performance of the developed models. Indexes
Real values (mg L1)
Predicted values (mg L1)
Relative errors (%)
Mean relative errors (%)
Anaerobic
68.40 51.39 58.11 43.65 4.67 3.51
60.88 44.89 52.08 38.56 5.34 3.73
11.0 12.6 10.4 11.7 14.4 6.2
11.8
Anoxic Oxic
hydrolysis process rate, respectively; hFe andhNO3 are anaerobic hydrolysis reduction factor and anoxic hydrolysis reduction factor; KNO3 , K and KO2 are saturation/inhibition coefficient for nitrate, hydrolysis rate constant and saturation/inhibition coefficient for oxygen, respectively; SO2 and S are dissolved oxygen and biodegradable substrate; XH is heterotrophic biomass. In addition, due to the uniformity of the mixed liquors in each reactor, the metabolic rate of DMP by microorganism in AAO system is uniform. Hence, the degradation rate of DMP in AAO treatment process could be described as the following equation:
dS S S0 ¼ dt t
(13)
From what had been mentioned above, the model for describing the degradation rate of DMP could be simplified:
Xt K 1 1 ¼ S$ þ S0 S K S K
(14)
where 1S , SoXtS, KKS and K1 were represent as the variables of x, y, k and a, respectively. Therefore, Equation (14) could be derived to a linear formula for relating the transformed values as following:
y ¼ kx þ a
(15)
Base on the kinetic model of DMP degradation, kinetic
10.5
RMSE
MAPE (%)
5.1892
11.0
10.3
4. Conclusions The degradation and behavior of DMP under different operation condition were investigated in a laboratory scale AAO treatment system. Under optimal operation condition with HRT of 18 h and SRT of 15 d, the percentage of the degraded DMP by the microorganisms was 73.8%, the percentage of the released DMP in the effluent was 5.8%, the percentage of the accumulated DMP was 19.3% was accumulated in the system, and the percentage of the remained DMP in the waste sludge was only 1.1%. In addition, it was proved that the developed degradation model including biodegradation and sorption can be used for accurately modeling the degradation and behavior of DMP in the AAO system. Acknowledgements This research was supported by National Natural Science Foundation of China (No. 51208206 and 51210013), Guangdong Provincial Natural Science Foundation (No. 2016A030306033), Guangdong Provincial Science and Technology Plan Project Foundation (No. 2014A020216007), Technological Innovation Young Talents of Guangdong Special Support Plan (No. 2014TQ01Z530), Pearl River S&T Nova Program of Guangzhou (No. 201506010058), and the Fundamental Research Funds for the Central Universities (No. 15lgpy11). The authors are thankful to the anonymous reviewers for their insightful comments and suggestions.
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