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Inhibition effect of linear alkylbenzene sulphonates on the biodegradation mechanisms of activated sludge
Bioresource Technology 101 (2010) 92–97
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Inhibition effect of linear alkylbenzene sulphonates on the biodegradation mechanisms of activated sludge Özlem Karahan * Istanbul Technical University, Environmental Engineering Department, Maslak, Istanbul, TR-34469, Turkey
a r t i c l e
i n f o
Article history: Received 8 May 2009 Received in revised form 27 July 2009 Accepted 31 July 2009 Available online 31 August 2009 Keywords: Activated sludge Inhibition Linear alkylbenzene sulphonates (LAS) Respirometry Modeling
a b s t r a c t The study presents a conceptual approach for the identification of the inhibition mechanisms of biodegradable inhibitors. Synthetic sewage was selected as the model degradable substrate to simulate domestic wastewaters. LAS, known to be a biodegradable but inhibitory compound, was selected as a model substrate for the determination of the inhibition mechanisms. Biodegradation of synthetic sewage and LAS were monitored through oxygen uptake rate (OUR) tests conducted to observe the dynamic response of the system when fed with synthetic sewage and synthetic sewage–LAS mixtures. The approach uses respirometry to calibrate the kinetic and stoichiometric coefficients of the proposed biochemical model. Model simulation results confirmed that presence of LAS has inhibitory effects on the biodegradation mechanisms of synthetic sewage. LAS imposed non-competitive inhibition on the hydrolysis process with an inhibition coefficient of 500 mg COD/L and effected heterotrophic growth through a competitive inhibition mechanism with an inhibition coefficient of 150 mg COD/L. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The wastewaters of detergent manufacturing, textile and food industries contain high levels of inhibitory surfactants that pose appreciable impacts on the environment (Mensah and Forster, 2003). In addition to industrial wastewaters, domestic wastewaters also include synthetic organic compounds like surfactants, personal-care products, plasticizers and industrial additives which have severe effects on wildlife and human health due to their toxicological properties (Birkett and Lester, 2003). Therefore there is a growing research need on the inhibitory effects and removal of such compounds. Linear alkylbenzene sulphonates (LAS) play an important role in our everyday life, since they have been announced as biodegradable and environmentally friendly surfactants compared to branched benzene sulphonates like Dodecyl Benzene Sulphonates (Børglum et al., 1994). The use of LAS in many different areas like textile and food industries in addition to their household use has resulted in the presence of LAS at considerable concentrations in wastewaters (Fauser et al., 2003; Statistics Denmark, 1999). LAS is known as biodegradable due to the linear structure of the alkyl group present in its molecular structure. However, it has also been reported that surfactants, specifically linear alkyl benzene sulphonates decrease the affinity of substrate to biomass and lower
* Tel.: +90 212 2856579; fax: +90 212 2851344. E-mail address: [email protected] 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.07.088
maximum growth rates are observed in the presence of surfactants (Liwarska-Bizukojc et al., 2008). Many studies have been conducted to understand the effect of surfactants including LAS on the biodegradation processes. The surfactants containing benzene ring deteriorate activated sludge systems (Liwarska-Bizukojc et al., 2008). LAS itself and the intermediates produced by advanced oxidation techniques have been reported to be biodegradable, however the intermediates are less biodegradable (Mehrvar and Tabrizi, 2006). The negative effects imposed on degradation by the advanced oxidation intermediates have also been stated for other xenobiotics (Ubay Cokgor et al., 2004). In a risk assessment study it has been pointed out that the risk imposed on the aquatic environment by the anionic surfactants depends upon the applied treatment scheme prior to discharge of wastewaters. However activated sludge systems were found to pose the minimum risk (Mungray and Kumar, 2008). Therefore treatment of surfactants by activated sludge plays an important role in minimizing the reported environmental impacts. Increasing concentrations of LAS in activated sludge systems increase inhibition on the degradation capacity of the system, resulting in elevated residual COD levels in the effluent (Oviedo et al., 2004). Activated sludge modeling is a very useful tool to design and predict the behavior of biological treatment systems. Therefore it is important to understand the mechanisms taking place in an activated sludge system under different conditions, such as when an inhibitory compound like LAS is introduced to the system. The effect of inhibitory substances like LAS on the biochemical
Ö. Karahan / Bioresource Technology 101 (2010) 92–97
mechanisms should be determined in terms of process kinetics for incorporating these effects on the response of activated sludge systems. Respirometry has been used very efficiently for identifying and understanding the biochemical mechanisms involved in the activated sludge process (Spanjers and Vanrolleghem, 1995). On-line oxygen uptake rate (OUR) measurements have been realized as more reliable methods compared to instant determination of the OUR response of the activated sludge systems for toxicity and inhibition studies (Ubay Cokgor et al., 2007). A number of successful applications of respirometry have been reported which have been used to understand and predict different biochemical mechanisms of activated sludge systems (Karahan et al., 2006a,b). Respirometric tests are widely used for biodegradability assessment of various xenobiotic compounds (Stasinakis et al., 2008). Despite the research efforts to understand the extent of the inhibition caused by LAS, the inhibition effects on activated sludge systems have not been determined using respirometry. The assessment of the inhibition by the use of respirometry enables implementation of inhibition effects in modeling, and thus achieved results can be applied globally on similar system disturbances. Determination of toxicity effects have been performed successfully for several inhibitors by the use of respirometric methods (Meriç et al., 2003). This study aims to investigate the inhibition effects imposed by LAS on the biodegradation mechanisms of an aerobic activated sludge system using respirometry. The study identifies the different inhibition effects on the process stoichiometry and kinetics of the different biodegradation mechanisms involved, such as hydrolysis and direct growth for a substrate mixture (synthetic sewage) accepted to represent domestic wastewaters. The proposed model structure also includes the mechanisms for the biodegradation of LAS. The study provides a systematic approach for the respirometric assessment and modeling of inhibition caused by compounds like LAS, which are partially biodegradable. Specific industries like textile and food industries have activated sludge systems that suffer from low growth rates and thus low sludge production. This problem can only be solved if the inhibitory effects imposed on microorganisms are taken into account while setting the operational parameters of the activated sludge systems. Incorrect operation of wastewater treatment plants which receive substances like cleaning agents, disinfectants and solvents has been reported as one of the main causes of bad removal performance (Bodík et al., 2008). In this context, this study will provide a better understanding of the inhibition mechanisms and the results of the study will be useful in the solution of operational problems of activated sludge systems. 2. Conceptual approach The synthetic sewage representing domestic wastewaters contains carbohydrates, peptides and amino acids, organic acids, minerals, vitamins and urea together with different compounds for the nutritional requirements of biomass. Since synthetic sewage mixture has a complex structure it is only possible for synthetic sewage to be used after an hydrolysis step. Therefore the complex structures present in the synthetic sewage mixture should be hydrolyzed prior to their utilization. LAS also has a complex structure. LAS is composed of a sulphonated benzene group attached to a linear alkyl which can be composed of 10–14 C-atoms. Although LAS is known to be biodegradable this compound cannot be directly used by the microorganisms. LAS has to be hydrolyzed and a linear alkyl group and a phenolic group are formed as a result of hydrolysis. Adsorption of LAS on biomass prior to its degradation is also a well established phenomenon (Mehrvar and Tabrizi, 2006). The alkyl group generated through hydrolysis can be utilized by activated sludge while
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the phenolic group exerts inhibitory effects on the system. The degradation of the alkyl group is confirmed with the reported results stating that higher degradation rates were observed for LAS homologues having longer alkyl chains (Oviedo et al., 2004). Inhibition of the metabolic processes in an activated sludge system can occur by different mechanisms depending on the structure and properties of the inhibitor. Competitive inhibition is observed when the inhibitor has a similar structure with the substrate and competes for the same enzymes used for growth. This type of inhibition is expected for the hydrolyzed biodegradable fractions of LAS that would compete with the hydrolysis products of synthetic sewage for growth enzymes. In the case of non-competitive inhibition the inhibitor also reacts with the enzymes used for the degradation of the substrate without competition. Non-competitive inhibition would be the type of inhibition exerted when the LAS molecules also consume the hydrolysis enzymes that are necessary for the hydrolysis of synthetic sewage.
3. Methods Two 4 L lab-scale fill and draw reactors were seeded with activated sludge taken from a domestic wastewater treatment _ plant in Istanbul. The reactors were operated for the acclimation of biomass. The reactors had hydraulic retention times of 1 day and both of them were fed with synthetic sewage representing domestic wastewater (ISO 8192, 2007), while LAS was also added to one of the reactors. LAS sample was a surfactant mainly composed of LAS used in a glass manufacturing plant. The first reactor (R1) was only fed with synthetic sewage with an initial COD of 535 mg/L and the second reactor (R2) was fed with a mixture of synthetic sewage and LAS where the total COD was constant with gradual increases on the amount of COD coming from LAS during the acclimation period. R2 was fed with 855 mg/L COD that was eventually composed of 40% synthetic sewage and 60% LAS. The acclimation period was monitored with TSS, VSS and effluent COD measurements. The reactor fed with synthetic sewage and LAS mixture reached steady state after 5 months of acclimation. The stock synthetic sewage solution used had a COD of 13,360 mg/L and the LAS stock solution had a COD content of 23,650 mg/L. R1 was fed with 40 mL of synthetic sewage stock solution and R2 was fed with 20 mL synthetic sewage and 25 mL of LAS stock solutions. The synthetic sewage stock solution was composed of 16 g Peptone G, 11 g meat extract, 3 g urea, 0.7 g NaCl, 0.4 g CaCl22H2O, 0.2 g MgSO47H2O, and 2.8 g K2HPO4, dissolved in 1 L of distilled water. The synthetic sewage used consists of all the nutritional requirements of the bacterial population. Pancreatic digest of gelatin (Peptone G) provides nitrogen, amino acids, vitamins, and carbon. Pancreatic digest of gelatin (Peptone G) is deficient in carbohydrates. Beef extract is a mixture of peptides and amino acids, nucleotide fractions, organic acids, minerals and vitamins. Beef extract products are not exposed to the harsh treatment used for protein hydrolysis, so they can provide some of the nutrients lost during peptone manufacture (Flickinger and Drew, 1999). The function of beef extract is complementing the nutritive properties of peptone by contributing minerals, phosphates, carbon sources providing energy and those essential factors missing from peptone (Bridson and Brecker, 1970). Urea Agar Base consists of urea, enzymatic digest of gelatin, dextrose, phenol red, sodium chloride and potassium phosphate. NaCl, CaCl22H2O, MgSO47H2O, K2HPO4 provide the necessary micro-nutrients and the buffer capacity required for cell growth. Respirometric tests were conducted with biomass acclimated to synthetic sewage–LAS mixture in R2. The tests were conducted with
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an Applitek Ra-COMBOÒ respirometer equipped with a 2 L reactor vessel. The respirometer was operated at a temperature controlled room at 20 °C. The system was aerated so that the dissolved oxygen levels were above 5 mg/L in the reactor vessel; pH was checked periodically and was always between 6.5 and 7.5 during the experiments. Biomass taken from acclimation reactor (R2) was washed to remove residual organic matter, settled and re-suspended in the 2 L aerated reactor vessel. Hach nitrification inhibitor, Formula 2533(TM) which includes 2-chloro-6(trichloromethyl) pyridine and sodium sulfate, was also added to the reactor sustaining 0.16 g inhibitor in 300-mL sample. The OUR measurements were started with only biomass present in the reactor vessel in order to determine the endogenous respiration rate. Substrate mixtures were added after the endogenous level was determined for 1 h OUR measurement. OUR response of the system was monitored and COD samples were taken at predetermined time intervals. The COD measurements were conducted according to ISO 6060 method (1986). All other analyses were done according to Standard Methods (2005). 4. Experimental results The acclimation reactors fed with synthetic sewage (R1) and synthetic sewage–LAS mixture (R2) were monitored for VSS and effluent COD at steady state in order to determine the degradation achieved for synthetic sewage and LAS. The synthetic sewage reactor (R1) fed with 535 mg COD/L reached steady state at a sludge age of 15 days and the 855 mg COD/L synthetic sewage–LAS fed reactor (R2) had a sludge age of 25 days. COD monitoring results have shown that 93% COD removal was achieved in R1 while only 82% COD was removed in R2. In addition to the reduced COD removal, the effluent filtered COD results are also misleading for the reactor fed with synthetic sewage–LAS mixture (R2) since biomass growth corresponding to the COD reduction was not detected as indicated by the longer sludge age. Respirometric studies involved two sets of OUR measurements with the synthetic sewage–LAS acclimated biomass. The first set was run with synthetic sewage as the feed and the second set was fed with synthetic sewage–LAS mixture, similar to the acclimation studies. Filtered COD was also monitored during the OUR measurements with frequent sampling. The synthetic sewage feed solution used in the OUR measurements had a COD concentration of 15,127 mg/L and the LAS mixture prepared had 14,158 mg/ L COD concentration. Respirometric tests were started with the detection of endogenous respiration OUR level with only aerated biomass for 60 min.
After that, feed was added. Forty five millilitre of synthetic sewage solution was added as feed for the first test and 45 mL stock synthetic sewage and 50 mL of stock LAS solutions were added in the second test. The feed addition was done so that the tests are conducted with 50–50% synthetic sewage–LAS mixture on COD basis. The initial feed COD of the synthetic sewage fed system was 340 mg/L and the second test was run with an initial COD of 694 mg/L, composed of 340 mg COD/L from synthetic sewage and 354 mg COD/L from LAS solutions. The OUR profile obtained in the two respirometric tests are given in Fig. 1. The endogenous OUR level in both tests were measured as 13 mg/L/h, showing that the biomass was at the same activity level in both tests and that the OUR results were directly comparable. The OUR level has increased to 130 mg/L/h in the first 3 min and decreased to the initial endogenous OUR level in 200 min in the first test where only synthetic sewage was fed on the biomass. The amount of dissolved oxygen consumed for the degradation of 340 mg/L synthetic sewage was calculated as 103 mg/L from the area below the OUR curve (Fig. 1), corresponding to a growth yield of 0.70 g COD/g COD. This result is in accordance with the yield values stated in literature (Movahedyan et al., 2008). In the OUR test conducted with synthetic sewage–LAS mixture, the maximum OUR level of 90 mg/L/h could only be attained 13 min after the addition of feed mixture and the initial endogenous OUR level was reached after 300 min. This observation shows that synthetic sewage present in this system is degraded at a slower rate, indicating that the presence of LAS induces inhibitory effects on synthetic sewage degradation. Dissolved oxygen utilized in the experiment was calculated as 144 mg/L. Considering that all synthetic sewage was degraded during the test and 103 mg/L dissolved oxygen was consumed for synthetic sewage, it can be concluded that 41 mg/L of the dissolved oxygen is utilized for the degradation of LAS. The COD profiles obtained in the tests showed that degradation of synthetic sewage resulted in 32 mg/L soluble metabolic product generation and 264 mg/L soluble COD was left in the reactor fed with synthetic sewage–LAS mixture. The effluent filtered COD levels show that 135 mg COD/L of LAS was removed from the mixed liquor (considering similar amount of soluble metabolic product generation as synthetic sewage), resulting in a heterotrophic yield value of 0.70 g COD/g COD for the degradation of LAS. Although the heterotrophic yield value of 0.70 g COD/g COD has been reported in similar studies (Liwarska-Bizukojc et al., 2008), this yield is considerably high for LAS, when compared with the degradation results obtained for LAS in the acclimation studies and thus it is concluded
140 Synthetic sewage (45ml)+LAS(50ml) Synthetic sewage (45ml)
120
OUR (mg/l/h)
100 80 60 40 20 0 -100
-50
0
50
100
150
200
Time (min) Fig. 1. OUR profiles obtained for peptone and peptone – LAS mixture.
250
300
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Ö. Karahan / Bioresource Technology 101 (2010) 92–97 Table 1 Synthetic sewage–LAS System Model Matrix. Process
SO2
Hydrolysis of synthetic sewage (with non-competitive inhibition) Growth on synthetic sewage (with competitive inhibition)
INC
H 1Y YH
Endogenous decay
(1fES)
I
1þKLAS
;
SP
IC ¼ 1 þ
ILAS K I;C
;
ILAS ¼ X S;LAS ;
t=tau
trans ¼ 1 e
XS,SUB
XS,LAS
XH
1
fES
=X
H INC kh;SUB K X;SUBS;SUB þX S;SUB =X H X H
SS;SUB H;SUB K S;SUB IC þSS;SUB
trans l
X
1
Y1H
Rate X
1
Y1H 1
Growth on hydrolyzed LAS
1
SS,LAS
1 H 1Y YH
Hydrolysis of LAS
¼
SS,SUB
XH
=X
H kh;LAS K X;LASS;LAS þX S;LAS =X H X H
1
lH;LAS K S;LASSS;LAS þSS;LAS X H
1
bH X H
.
I;NC
Table 2 Values of model coefficients obtained by simulations. Coefficient
Unit
fES YH bH kh,SUB KX,SUB
g COD/g COD g COD/g COD L/d L/d g COD/g COD L/d mg COD/L L/d g COD/g COD L/d mg COD/L d mg COD/L mg COD/L
lH,SUB KS,SUB kh,LAS KX,LAS
lH,LAS KS,LAS tau KI,NC KI,C
Value Synthetic sewage
Synthetic sewage + LAS
0.2 0.70 0.2 3.8 0.08 4.8 5 – – – – 0.001 – –
0.2 0.70 0.2 3.8 0.08 4.8 5 0.8 0.1 1.5 150 0.0028 500 150
that LAS is primarily adsorbed on the biomass and is hydrolyzed prior to its degradation. 5. Modeling Modeling studies were conducted in order to understand the degradation mechanism and the inhibition effect of LAS on the biochemical mechanisms present in the activated sludge system. Modeling efforts were directed towards the identification of the stoichiometric relationships and kinetic processes involved in the degradation mechanisms of synthetic sewage and LAS. Simulation studies were done for the determination of the coefficients of the identified biodegradation and inhibition mechanisms. The model suggested for the system fed with synthetic sewage–LAS mixture is given in a matrix format in Table 1. The general structure of the model matrix has been adapted from Activated Sludge Model No. 1 (Henze et al., 1987). The matrix presented in Table 1 shows that synthetic sewage (XS,SUB) fed to the system is first hydrolyzed according to surface reaction kinetics. The hydrolysis produces readily biodegradable substrates (SS,SUB) utilized for the growth of heterotrophic biomass. The rate of growth process on synthetic sewage hydrolyzates has been formulated according to Monod kinetics as generally accepted in activated sludge models and the trans function has been used as suggested by Vanrolleghem et al. (2004) for the simulation of the transient response observed when readily biodegradable substrate is fed on endogenous biomass. The transient response is observed for the first few minutes when biomass adapts its internal metabolic functions for shifting from endogenous respiration to growth. The mechanism suggested for the degradation of LAS starts with the adsorption of LAS on the biomass and hydrolysis of LAS (XS,LAS). Since the adsorption process is very fast, it has been incorporated in the hydrolysis process for simplicity in the model. The readily
biodegradable substrate (SS,LAS) generated after the hydrolysis of LAS is also used for biomass growth. The model suggests that there is only one heterotrophic biomass culture (XH) responsible for all biochemical processes. Since all the degradation mechanisms for synthetic sewage and LAS are carried out by the acclimated mixed culture, only one endogenous decay process is defined in the model. Similarly the heterotrophic growth yield (YH) of the biomass for synthetic sewage and LAS was selected to be identical. The metabolic products generated in the system due to the biochemical transformations are defined collectively as soluble metabolic products (SP), to prevent complexity in the model. Addition of LAS on the system growing on synthetic sewage causes inhibition on the hydrolysis and growth processes. Hydrolysis of synthetic sewage slows down by non-competitive inhibition since the hydrolysis enzymes are also used by LAS. The non-competitive inhibition of hydrolysis is defined by INC function in the model, which effectively decreases maximum rate of hydrolysis with increasing inhibitor concentration and 50% reduction is observed at the inhibitor concentration of KNC. Inhibition of growth on synthetic sewage is defined by competitive inhibition since a second biodegradable substrate generated by the hydrolysis of LAS is present and this second substrate is in competition with synthetic sewage hydrolyzates for the enzymes used for growth. As a result of this competition the substrate affinity coefficient (KS,SUB) increases and the rate of growth on synthetic sewage decreases. In accordance with competitive inhibition the substrate affinity coefficient, KS,SUB is doubled when the inhibitor reaches KI,C concentration. The model matrix given in Table 1 has been calibrated with the kinetic and stoichiometric coefficients given in Table 2. The maximum growth rate on LAS was found 1.5 1/d and that of synthetic substrate was 4.8 1/d as given in Table 2. The maximum growth rate on LAS is 1/3 of the synthetic sewage, which is in total agreement with the work of Liwarska-Bizukojc et al. (2008), where the maximum growth rate for control reactor fed with synthetic wastewater was reported as 21.4 1/d and the maximum growth rate for LAS was found as 14 1/d. The maximum growth rate on LAS (1.5 1/d) is in the same range as the value of 2.8 1/d stated in literature (Mehrvar and Tabrizi, 2006). Liwarska-Bizukojc et al. (2008) reported substrate affinity constants KS as 245 mg COD/L for LAS, which is comparable to the KS value of 150 mg COD/L estimated in this study. The experimentally determined endogenous decay coefficient, bH was 0.25 1/d, similar to the bH value of 0.20 1/d given in Table 2. The proposed model was dynamically simulated for the initial conditions and substrate additions applied in the respirometric tests by Aquasim 2.0 software (Reichert et al., 1998). Model simulations were conducted for the OUR response of the synthetic sewage and synthetic sewage–LAS mixture fed systems with the coefficients presented in Table 2 and their comparison with the experimentally obtained OUR data are given in Figs. 2 and 3, respectively. Fig. 2 shows the OUR simulation results for the
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Ö. Karahan / Bioresource Technology 101 (2010) 92–97
160 OUR model
140
OUR data
OUR (mg/l/h)
120 100 80 60 40 20 0 -50
0
50
100
150
200
250
300
Time (min) Fig. 2. Model simulation results for synthetic sewage fed system.
100 90
OUR model
80
OUR data
OUR (mg/l/h)
70 60 50 40 30 20 10 0 -50
0
50
100
150
200
250
300
Time (min) Fig. 3. Model simulation results for synthetic sewage–LAS mixture fed system.
synthetic sewage fed activated sludge system achieved in the absence of the inhibitory effects of LAS and thus the inhibition functions were discarded in the model simulations. The coefficients for the simulation of the OUR response of synthetic sewage fed system were determined without inhibition. Upon the addition of LAS with synthetic sewage the model was simulated with the inhibition functions using the inhibition coefficients given in Table 2 and the OUR response obtained is presented in Fig. 3. The model coefficients of the biochemical mechanisms involved in the utilization of LAS and inhibition coefficients were determined with the calibration of the model for experimental OUR results obtained for synthetic sewage–LAS mixture fed system. As shown in Figs. 2 and 3, the model simulation results are in good agreement with the experimental OUR data, showing that the biochemical mechanisms defined in the model and the coefficients used were able to predict the behavior of the activated sludge system fed with two different substrate mixtures. Model results have revealed that 173.6 mg/L of total oxygen utilization occurred in the synthetic sewage fed system. The amount of dissolved oxygen consumed for endogenous respiration is estimated as 71.6 mg/L and the remaining 102 mg/L dissolved oxygen is utilized for growth on synthetic sewage. Similarly, 213.5 mg/L dissolved oxygen is utilized in the synthetic sewage–LAS mixture fed system. The total dissolved oxygen consumption is composed
of 70.6 mg/L consumption for endogenous respiration, 100.9 mg/L utilization for growth on synthetic sewage and 42 mg/L dissolved oxygen consumption for growth on LAS hydrolyzates. The model simulations have revealed that the complete degradation of synthetic sewage took 200 min in the first system, however 300 min were required for synthetic sewage degradation when LAS was added in the second system. It could be estimated from 42 mg/L dissolved oxygen consumption that 139 mg COD/L of LAS was degraded which was fed at a concentration of 354 mg COD/L. Two hundred and fifteen milligram of COD per litre LAS remained undegraded in the experimental period of 300 min. This result is also in agreement with the filtered COD measurements. The inhibition effects imposed on the hydrolysis and growth on synthetic sewage by LAS is clearly shown in the model simulation results obtained for the OUR response of the system for complete degradation of synthetic sewage as given in Fig. 4. The level differences observed in the two OUR curves for synthetic sewage is due to the non-competitive inhibition of LAS which reduces the rate of hydrolysis and therefore the rate of oxygen consumption for growth. The decreased slope observed for the OUR response transition from the higher first plateau to the second lower OUR level in Fig. 4 is due to competitive inhibition imposed by LAS degradation. As shown in the figure, although the same amount
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Ö. Karahan / Bioresource Technology 101 (2010) 92–97
OUR for Growth on Synthetic Sewage (mg/l/h)
140 Synthetic sewage(45 ml) 120 Synthetic sewage(45 ml)+LAS(50ml) 100 80 60 40 20 0 -50
0
50
100
150
200
250
300
Time (min) Fig. 4. Inhibition effects of LAS on OUR response for synthetic sewage degradation.
of dissolved oxygen was utilized for the same amount of synthetic sewage fed, the time for complete degradation has increased 50% and reached 300 min. These results are in agreement with the study of Oviedo et al. (2004) since they have also reported that LAS influenced the degradation capacity of, both, organic matter and itself in aerobic processes.
6. Conclusions The study revealed that LAS exhibits non-competitive inhibition on the hydrolysis mechanism of synthetic sewage and imposes competitive inhibition on the growth of biomass. Model simulation results were in good agreement with experimental results indicating that proposed model structure successfully defines the biochemical mechanisms. The results obtained will help the engineers and the plant operators to improve their understanding about detailed design and operation of WWTPs which receive biodegradable inhibitory compound discharges. The results of the study present that the applied methodology is successful for the interpretation of the response of activated sludge systems to inhibitory biodegradable compounds like LAS. Acknowledgement The author deeply acknowledges FEYZI_ AKKAYA RESEARCH FUND FOR SCIENTIFIC ACTIVITIES (FABED) for the financial support provided by Research Support for Outstanding Young Scientists Award for this study. References ˇ an, M., Drtil, M., 2008. Bodík, I., Gašpariková, E., Dancˇová, L., Kalina, A., Hutn Influence of disinfectants on domestic wastewater treatment plant performance. Bioresour. Technol. 99 (3), 532–539. Børglum, B., Hansen, A.M., Kortlgning af Vaske- Rengøringsmidler, 1994. A Survey of Washing and Cleaning Agents, Arbejdsmiljøinstituttet, København, AMI rapport nr. 44. Birkett, J.W., Lester, J.N., 2003. Endocrine Disrupters in Wastewater and Sludge Treatment Processes. CRC Press LLC, Florida. Bridson, E.Y., Brecker, A., 1970. Design and formulation of microbial culture media. In: Norris, Ribbons (Eds.), Methods in Microbiology, vol. 3A. Academic Press, New York, pp. 229–292. Fauser, P., Vikelsøe, J., Sørensen, P.B., Carlsen, L., 2003. Phthalates, nonylphenols and LAS in an alternately operated wastewater treatment plant-fate modeling based on measured concentrations in wastewater and sludge. Water Res. 37, 1288– 1295.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Inhibition effect of linear alkylbenzene sulphonates on the biodegradation mechanisms of activated sludge Özlem Karahan * Istanbul Technical University, Environmental Engineering Department, Maslak, Istanbul, TR-34469, Turkey
a r t i c l e
i n f o
Article history: Received 8 May 2009 Received in revised form 27 July 2009 Accepted 31 July 2009 Available online 31 August 2009 Keywords: Activated sludge Inhibition Linear alkylbenzene sulphonates (LAS) Respirometry Modeling
a b s t r a c t The study presents a conceptual approach for the identification of the inhibition mechanisms of biodegradable inhibitors. Synthetic sewage was selected as the model degradable substrate to simulate domestic wastewaters. LAS, known to be a biodegradable but inhibitory compound, was selected as a model substrate for the determination of the inhibition mechanisms. Biodegradation of synthetic sewage and LAS were monitored through oxygen uptake rate (OUR) tests conducted to observe the dynamic response of the system when fed with synthetic sewage and synthetic sewage–LAS mixtures. The approach uses respirometry to calibrate the kinetic and stoichiometric coefficients of the proposed biochemical model. Model simulation results confirmed that presence of LAS has inhibitory effects on the biodegradation mechanisms of synthetic sewage. LAS imposed non-competitive inhibition on the hydrolysis process with an inhibition coefficient of 500 mg COD/L and effected heterotrophic growth through a competitive inhibition mechanism with an inhibition coefficient of 150 mg COD/L. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The wastewaters of detergent manufacturing, textile and food industries contain high levels of inhibitory surfactants that pose appreciable impacts on the environment (Mensah and Forster, 2003). In addition to industrial wastewaters, domestic wastewaters also include synthetic organic compounds like surfactants, personal-care products, plasticizers and industrial additives which have severe effects on wildlife and human health due to their toxicological properties (Birkett and Lester, 2003). Therefore there is a growing research need on the inhibitory effects and removal of such compounds. Linear alkylbenzene sulphonates (LAS) play an important role in our everyday life, since they have been announced as biodegradable and environmentally friendly surfactants compared to branched benzene sulphonates like Dodecyl Benzene Sulphonates (Børglum et al., 1994). The use of LAS in many different areas like textile and food industries in addition to their household use has resulted in the presence of LAS at considerable concentrations in wastewaters (Fauser et al., 2003; Statistics Denmark, 1999). LAS is known as biodegradable due to the linear structure of the alkyl group present in its molecular structure. However, it has also been reported that surfactants, specifically linear alkyl benzene sulphonates decrease the affinity of substrate to biomass and lower
* Tel.: +90 212 2856579; fax: +90 212 2851344. E-mail address: [email protected] 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.07.088
maximum growth rates are observed in the presence of surfactants (Liwarska-Bizukojc et al., 2008). Many studies have been conducted to understand the effect of surfactants including LAS on the biodegradation processes. The surfactants containing benzene ring deteriorate activated sludge systems (Liwarska-Bizukojc et al., 2008). LAS itself and the intermediates produced by advanced oxidation techniques have been reported to be biodegradable, however the intermediates are less biodegradable (Mehrvar and Tabrizi, 2006). The negative effects imposed on degradation by the advanced oxidation intermediates have also been stated for other xenobiotics (Ubay Cokgor et al., 2004). In a risk assessment study it has been pointed out that the risk imposed on the aquatic environment by the anionic surfactants depends upon the applied treatment scheme prior to discharge of wastewaters. However activated sludge systems were found to pose the minimum risk (Mungray and Kumar, 2008). Therefore treatment of surfactants by activated sludge plays an important role in minimizing the reported environmental impacts. Increasing concentrations of LAS in activated sludge systems increase inhibition on the degradation capacity of the system, resulting in elevated residual COD levels in the effluent (Oviedo et al., 2004). Activated sludge modeling is a very useful tool to design and predict the behavior of biological treatment systems. Therefore it is important to understand the mechanisms taking place in an activated sludge system under different conditions, such as when an inhibitory compound like LAS is introduced to the system. The effect of inhibitory substances like LAS on the biochemical
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mechanisms should be determined in terms of process kinetics for incorporating these effects on the response of activated sludge systems. Respirometry has been used very efficiently for identifying and understanding the biochemical mechanisms involved in the activated sludge process (Spanjers and Vanrolleghem, 1995). On-line oxygen uptake rate (OUR) measurements have been realized as more reliable methods compared to instant determination of the OUR response of the activated sludge systems for toxicity and inhibition studies (Ubay Cokgor et al., 2007). A number of successful applications of respirometry have been reported which have been used to understand and predict different biochemical mechanisms of activated sludge systems (Karahan et al., 2006a,b). Respirometric tests are widely used for biodegradability assessment of various xenobiotic compounds (Stasinakis et al., 2008). Despite the research efforts to understand the extent of the inhibition caused by LAS, the inhibition effects on activated sludge systems have not been determined using respirometry. The assessment of the inhibition by the use of respirometry enables implementation of inhibition effects in modeling, and thus achieved results can be applied globally on similar system disturbances. Determination of toxicity effects have been performed successfully for several inhibitors by the use of respirometric methods (Meriç et al., 2003). This study aims to investigate the inhibition effects imposed by LAS on the biodegradation mechanisms of an aerobic activated sludge system using respirometry. The study identifies the different inhibition effects on the process stoichiometry and kinetics of the different biodegradation mechanisms involved, such as hydrolysis and direct growth for a substrate mixture (synthetic sewage) accepted to represent domestic wastewaters. The proposed model structure also includes the mechanisms for the biodegradation of LAS. The study provides a systematic approach for the respirometric assessment and modeling of inhibition caused by compounds like LAS, which are partially biodegradable. Specific industries like textile and food industries have activated sludge systems that suffer from low growth rates and thus low sludge production. This problem can only be solved if the inhibitory effects imposed on microorganisms are taken into account while setting the operational parameters of the activated sludge systems. Incorrect operation of wastewater treatment plants which receive substances like cleaning agents, disinfectants and solvents has been reported as one of the main causes of bad removal performance (Bodík et al., 2008). In this context, this study will provide a better understanding of the inhibition mechanisms and the results of the study will be useful in the solution of operational problems of activated sludge systems. 2. Conceptual approach The synthetic sewage representing domestic wastewaters contains carbohydrates, peptides and amino acids, organic acids, minerals, vitamins and urea together with different compounds for the nutritional requirements of biomass. Since synthetic sewage mixture has a complex structure it is only possible for synthetic sewage to be used after an hydrolysis step. Therefore the complex structures present in the synthetic sewage mixture should be hydrolyzed prior to their utilization. LAS also has a complex structure. LAS is composed of a sulphonated benzene group attached to a linear alkyl which can be composed of 10–14 C-atoms. Although LAS is known to be biodegradable this compound cannot be directly used by the microorganisms. LAS has to be hydrolyzed and a linear alkyl group and a phenolic group are formed as a result of hydrolysis. Adsorption of LAS on biomass prior to its degradation is also a well established phenomenon (Mehrvar and Tabrizi, 2006). The alkyl group generated through hydrolysis can be utilized by activated sludge while
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the phenolic group exerts inhibitory effects on the system. The degradation of the alkyl group is confirmed with the reported results stating that higher degradation rates were observed for LAS homologues having longer alkyl chains (Oviedo et al., 2004). Inhibition of the metabolic processes in an activated sludge system can occur by different mechanisms depending on the structure and properties of the inhibitor. Competitive inhibition is observed when the inhibitor has a similar structure with the substrate and competes for the same enzymes used for growth. This type of inhibition is expected for the hydrolyzed biodegradable fractions of LAS that would compete with the hydrolysis products of synthetic sewage for growth enzymes. In the case of non-competitive inhibition the inhibitor also reacts with the enzymes used for the degradation of the substrate without competition. Non-competitive inhibition would be the type of inhibition exerted when the LAS molecules also consume the hydrolysis enzymes that are necessary for the hydrolysis of synthetic sewage.
3. Methods Two 4 L lab-scale fill and draw reactors were seeded with activated sludge taken from a domestic wastewater treatment _ plant in Istanbul. The reactors were operated for the acclimation of biomass. The reactors had hydraulic retention times of 1 day and both of them were fed with synthetic sewage representing domestic wastewater (ISO 8192, 2007), while LAS was also added to one of the reactors. LAS sample was a surfactant mainly composed of LAS used in a glass manufacturing plant. The first reactor (R1) was only fed with synthetic sewage with an initial COD of 535 mg/L and the second reactor (R2) was fed with a mixture of synthetic sewage and LAS where the total COD was constant with gradual increases on the amount of COD coming from LAS during the acclimation period. R2 was fed with 855 mg/L COD that was eventually composed of 40% synthetic sewage and 60% LAS. The acclimation period was monitored with TSS, VSS and effluent COD measurements. The reactor fed with synthetic sewage and LAS mixture reached steady state after 5 months of acclimation. The stock synthetic sewage solution used had a COD of 13,360 mg/L and the LAS stock solution had a COD content of 23,650 mg/L. R1 was fed with 40 mL of synthetic sewage stock solution and R2 was fed with 20 mL synthetic sewage and 25 mL of LAS stock solutions. The synthetic sewage stock solution was composed of 16 g Peptone G, 11 g meat extract, 3 g urea, 0.7 g NaCl, 0.4 g CaCl22H2O, 0.2 g MgSO47H2O, and 2.8 g K2HPO4, dissolved in 1 L of distilled water. The synthetic sewage used consists of all the nutritional requirements of the bacterial population. Pancreatic digest of gelatin (Peptone G) provides nitrogen, amino acids, vitamins, and carbon. Pancreatic digest of gelatin (Peptone G) is deficient in carbohydrates. Beef extract is a mixture of peptides and amino acids, nucleotide fractions, organic acids, minerals and vitamins. Beef extract products are not exposed to the harsh treatment used for protein hydrolysis, so they can provide some of the nutrients lost during peptone manufacture (Flickinger and Drew, 1999). The function of beef extract is complementing the nutritive properties of peptone by contributing minerals, phosphates, carbon sources providing energy and those essential factors missing from peptone (Bridson and Brecker, 1970). Urea Agar Base consists of urea, enzymatic digest of gelatin, dextrose, phenol red, sodium chloride and potassium phosphate. NaCl, CaCl22H2O, MgSO47H2O, K2HPO4 provide the necessary micro-nutrients and the buffer capacity required for cell growth. Respirometric tests were conducted with biomass acclimated to synthetic sewage–LAS mixture in R2. The tests were conducted with
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an Applitek Ra-COMBOÒ respirometer equipped with a 2 L reactor vessel. The respirometer was operated at a temperature controlled room at 20 °C. The system was aerated so that the dissolved oxygen levels were above 5 mg/L in the reactor vessel; pH was checked periodically and was always between 6.5 and 7.5 during the experiments. Biomass taken from acclimation reactor (R2) was washed to remove residual organic matter, settled and re-suspended in the 2 L aerated reactor vessel. Hach nitrification inhibitor, Formula 2533(TM) which includes 2-chloro-6(trichloromethyl) pyridine and sodium sulfate, was also added to the reactor sustaining 0.16 g inhibitor in 300-mL sample. The OUR measurements were started with only biomass present in the reactor vessel in order to determine the endogenous respiration rate. Substrate mixtures were added after the endogenous level was determined for 1 h OUR measurement. OUR response of the system was monitored and COD samples were taken at predetermined time intervals. The COD measurements were conducted according to ISO 6060 method (1986). All other analyses were done according to Standard Methods (2005). 4. Experimental results The acclimation reactors fed with synthetic sewage (R1) and synthetic sewage–LAS mixture (R2) were monitored for VSS and effluent COD at steady state in order to determine the degradation achieved for synthetic sewage and LAS. The synthetic sewage reactor (R1) fed with 535 mg COD/L reached steady state at a sludge age of 15 days and the 855 mg COD/L synthetic sewage–LAS fed reactor (R2) had a sludge age of 25 days. COD monitoring results have shown that 93% COD removal was achieved in R1 while only 82% COD was removed in R2. In addition to the reduced COD removal, the effluent filtered COD results are also misleading for the reactor fed with synthetic sewage–LAS mixture (R2) since biomass growth corresponding to the COD reduction was not detected as indicated by the longer sludge age. Respirometric studies involved two sets of OUR measurements with the synthetic sewage–LAS acclimated biomass. The first set was run with synthetic sewage as the feed and the second set was fed with synthetic sewage–LAS mixture, similar to the acclimation studies. Filtered COD was also monitored during the OUR measurements with frequent sampling. The synthetic sewage feed solution used in the OUR measurements had a COD concentration of 15,127 mg/L and the LAS mixture prepared had 14,158 mg/ L COD concentration. Respirometric tests were started with the detection of endogenous respiration OUR level with only aerated biomass for 60 min.
After that, feed was added. Forty five millilitre of synthetic sewage solution was added as feed for the first test and 45 mL stock synthetic sewage and 50 mL of stock LAS solutions were added in the second test. The feed addition was done so that the tests are conducted with 50–50% synthetic sewage–LAS mixture on COD basis. The initial feed COD of the synthetic sewage fed system was 340 mg/L and the second test was run with an initial COD of 694 mg/L, composed of 340 mg COD/L from synthetic sewage and 354 mg COD/L from LAS solutions. The OUR profile obtained in the two respirometric tests are given in Fig. 1. The endogenous OUR level in both tests were measured as 13 mg/L/h, showing that the biomass was at the same activity level in both tests and that the OUR results were directly comparable. The OUR level has increased to 130 mg/L/h in the first 3 min and decreased to the initial endogenous OUR level in 200 min in the first test where only synthetic sewage was fed on the biomass. The amount of dissolved oxygen consumed for the degradation of 340 mg/L synthetic sewage was calculated as 103 mg/L from the area below the OUR curve (Fig. 1), corresponding to a growth yield of 0.70 g COD/g COD. This result is in accordance with the yield values stated in literature (Movahedyan et al., 2008). In the OUR test conducted with synthetic sewage–LAS mixture, the maximum OUR level of 90 mg/L/h could only be attained 13 min after the addition of feed mixture and the initial endogenous OUR level was reached after 300 min. This observation shows that synthetic sewage present in this system is degraded at a slower rate, indicating that the presence of LAS induces inhibitory effects on synthetic sewage degradation. Dissolved oxygen utilized in the experiment was calculated as 144 mg/L. Considering that all synthetic sewage was degraded during the test and 103 mg/L dissolved oxygen was consumed for synthetic sewage, it can be concluded that 41 mg/L of the dissolved oxygen is utilized for the degradation of LAS. The COD profiles obtained in the tests showed that degradation of synthetic sewage resulted in 32 mg/L soluble metabolic product generation and 264 mg/L soluble COD was left in the reactor fed with synthetic sewage–LAS mixture. The effluent filtered COD levels show that 135 mg COD/L of LAS was removed from the mixed liquor (considering similar amount of soluble metabolic product generation as synthetic sewage), resulting in a heterotrophic yield value of 0.70 g COD/g COD for the degradation of LAS. Although the heterotrophic yield value of 0.70 g COD/g COD has been reported in similar studies (Liwarska-Bizukojc et al., 2008), this yield is considerably high for LAS, when compared with the degradation results obtained for LAS in the acclimation studies and thus it is concluded
140 Synthetic sewage (45ml)+LAS(50ml) Synthetic sewage (45ml)
120
OUR (mg/l/h)
100 80 60 40 20 0 -100
-50
0
50
100
150
200
Time (min) Fig. 1. OUR profiles obtained for peptone and peptone – LAS mixture.
250
300
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Ö. Karahan / Bioresource Technology 101 (2010) 92–97 Table 1 Synthetic sewage–LAS System Model Matrix. Process
SO2
Hydrolysis of synthetic sewage (with non-competitive inhibition) Growth on synthetic sewage (with competitive inhibition)
INC
H 1Y YH
Endogenous decay
(1fES)
I
1þKLAS
;
SP
IC ¼ 1 þ
ILAS K I;C
;
ILAS ¼ X S;LAS ;
t=tau
trans ¼ 1 e
XS,SUB
XS,LAS
XH
1
fES
=X
H INC kh;SUB K X;SUBS;SUB þX S;SUB =X H X H
SS;SUB H;SUB K S;SUB IC þSS;SUB
trans l
X
1
Y1H
Rate X
1
Y1H 1
Growth on hydrolyzed LAS
1
SS,LAS
1 H 1Y YH
Hydrolysis of LAS
¼
SS,SUB
XH
=X
H kh;LAS K X;LASS;LAS þX S;LAS =X H X H
1
lH;LAS K S;LASSS;LAS þSS;LAS X H
1
bH X H
.
I;NC
Table 2 Values of model coefficients obtained by simulations. Coefficient
Unit
fES YH bH kh,SUB KX,SUB
g COD/g COD g COD/g COD L/d L/d g COD/g COD L/d mg COD/L L/d g COD/g COD L/d mg COD/L d mg COD/L mg COD/L
lH,SUB KS,SUB kh,LAS KX,LAS
lH,LAS KS,LAS tau KI,NC KI,C
Value Synthetic sewage
Synthetic sewage + LAS
0.2 0.70 0.2 3.8 0.08 4.8 5 – – – – 0.001 – –
0.2 0.70 0.2 3.8 0.08 4.8 5 0.8 0.1 1.5 150 0.0028 500 150
that LAS is primarily adsorbed on the biomass and is hydrolyzed prior to its degradation. 5. Modeling Modeling studies were conducted in order to understand the degradation mechanism and the inhibition effect of LAS on the biochemical mechanisms present in the activated sludge system. Modeling efforts were directed towards the identification of the stoichiometric relationships and kinetic processes involved in the degradation mechanisms of synthetic sewage and LAS. Simulation studies were done for the determination of the coefficients of the identified biodegradation and inhibition mechanisms. The model suggested for the system fed with synthetic sewage–LAS mixture is given in a matrix format in Table 1. The general structure of the model matrix has been adapted from Activated Sludge Model No. 1 (Henze et al., 1987). The matrix presented in Table 1 shows that synthetic sewage (XS,SUB) fed to the system is first hydrolyzed according to surface reaction kinetics. The hydrolysis produces readily biodegradable substrates (SS,SUB) utilized for the growth of heterotrophic biomass. The rate of growth process on synthetic sewage hydrolyzates has been formulated according to Monod kinetics as generally accepted in activated sludge models and the trans function has been used as suggested by Vanrolleghem et al. (2004) for the simulation of the transient response observed when readily biodegradable substrate is fed on endogenous biomass. The transient response is observed for the first few minutes when biomass adapts its internal metabolic functions for shifting from endogenous respiration to growth. The mechanism suggested for the degradation of LAS starts with the adsorption of LAS on the biomass and hydrolysis of LAS (XS,LAS). Since the adsorption process is very fast, it has been incorporated in the hydrolysis process for simplicity in the model. The readily
biodegradable substrate (SS,LAS) generated after the hydrolysis of LAS is also used for biomass growth. The model suggests that there is only one heterotrophic biomass culture (XH) responsible for all biochemical processes. Since all the degradation mechanisms for synthetic sewage and LAS are carried out by the acclimated mixed culture, only one endogenous decay process is defined in the model. Similarly the heterotrophic growth yield (YH) of the biomass for synthetic sewage and LAS was selected to be identical. The metabolic products generated in the system due to the biochemical transformations are defined collectively as soluble metabolic products (SP), to prevent complexity in the model. Addition of LAS on the system growing on synthetic sewage causes inhibition on the hydrolysis and growth processes. Hydrolysis of synthetic sewage slows down by non-competitive inhibition since the hydrolysis enzymes are also used by LAS. The non-competitive inhibition of hydrolysis is defined by INC function in the model, which effectively decreases maximum rate of hydrolysis with increasing inhibitor concentration and 50% reduction is observed at the inhibitor concentration of KNC. Inhibition of growth on synthetic sewage is defined by competitive inhibition since a second biodegradable substrate generated by the hydrolysis of LAS is present and this second substrate is in competition with synthetic sewage hydrolyzates for the enzymes used for growth. As a result of this competition the substrate affinity coefficient (KS,SUB) increases and the rate of growth on synthetic sewage decreases. In accordance with competitive inhibition the substrate affinity coefficient, KS,SUB is doubled when the inhibitor reaches KI,C concentration. The model matrix given in Table 1 has been calibrated with the kinetic and stoichiometric coefficients given in Table 2. The maximum growth rate on LAS was found 1.5 1/d and that of synthetic substrate was 4.8 1/d as given in Table 2. The maximum growth rate on LAS is 1/3 of the synthetic sewage, which is in total agreement with the work of Liwarska-Bizukojc et al. (2008), where the maximum growth rate for control reactor fed with synthetic wastewater was reported as 21.4 1/d and the maximum growth rate for LAS was found as 14 1/d. The maximum growth rate on LAS (1.5 1/d) is in the same range as the value of 2.8 1/d stated in literature (Mehrvar and Tabrizi, 2006). Liwarska-Bizukojc et al. (2008) reported substrate affinity constants KS as 245 mg COD/L for LAS, which is comparable to the KS value of 150 mg COD/L estimated in this study. The experimentally determined endogenous decay coefficient, bH was 0.25 1/d, similar to the bH value of 0.20 1/d given in Table 2. The proposed model was dynamically simulated for the initial conditions and substrate additions applied in the respirometric tests by Aquasim 2.0 software (Reichert et al., 1998). Model simulations were conducted for the OUR response of the synthetic sewage and synthetic sewage–LAS mixture fed systems with the coefficients presented in Table 2 and their comparison with the experimentally obtained OUR data are given in Figs. 2 and 3, respectively. Fig. 2 shows the OUR simulation results for the
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160 OUR model
140
OUR data
OUR (mg/l/h)
120 100 80 60 40 20 0 -50
0
50
100
150
200
250
300
Time (min) Fig. 2. Model simulation results for synthetic sewage fed system.
100 90
OUR model
80
OUR data
OUR (mg/l/h)
70 60 50 40 30 20 10 0 -50
0
50
100
150
200
250
300
Time (min) Fig. 3. Model simulation results for synthetic sewage–LAS mixture fed system.
synthetic sewage fed activated sludge system achieved in the absence of the inhibitory effects of LAS and thus the inhibition functions were discarded in the model simulations. The coefficients for the simulation of the OUR response of synthetic sewage fed system were determined without inhibition. Upon the addition of LAS with synthetic sewage the model was simulated with the inhibition functions using the inhibition coefficients given in Table 2 and the OUR response obtained is presented in Fig. 3. The model coefficients of the biochemical mechanisms involved in the utilization of LAS and inhibition coefficients were determined with the calibration of the model for experimental OUR results obtained for synthetic sewage–LAS mixture fed system. As shown in Figs. 2 and 3, the model simulation results are in good agreement with the experimental OUR data, showing that the biochemical mechanisms defined in the model and the coefficients used were able to predict the behavior of the activated sludge system fed with two different substrate mixtures. Model results have revealed that 173.6 mg/L of total oxygen utilization occurred in the synthetic sewage fed system. The amount of dissolved oxygen consumed for endogenous respiration is estimated as 71.6 mg/L and the remaining 102 mg/L dissolved oxygen is utilized for growth on synthetic sewage. Similarly, 213.5 mg/L dissolved oxygen is utilized in the synthetic sewage–LAS mixture fed system. The total dissolved oxygen consumption is composed
of 70.6 mg/L consumption for endogenous respiration, 100.9 mg/L utilization for growth on synthetic sewage and 42 mg/L dissolved oxygen consumption for growth on LAS hydrolyzates. The model simulations have revealed that the complete degradation of synthetic sewage took 200 min in the first system, however 300 min were required for synthetic sewage degradation when LAS was added in the second system. It could be estimated from 42 mg/L dissolved oxygen consumption that 139 mg COD/L of LAS was degraded which was fed at a concentration of 354 mg COD/L. Two hundred and fifteen milligram of COD per litre LAS remained undegraded in the experimental period of 300 min. This result is also in agreement with the filtered COD measurements. The inhibition effects imposed on the hydrolysis and growth on synthetic sewage by LAS is clearly shown in the model simulation results obtained for the OUR response of the system for complete degradation of synthetic sewage as given in Fig. 4. The level differences observed in the two OUR curves for synthetic sewage is due to the non-competitive inhibition of LAS which reduces the rate of hydrolysis and therefore the rate of oxygen consumption for growth. The decreased slope observed for the OUR response transition from the higher first plateau to the second lower OUR level in Fig. 4 is due to competitive inhibition imposed by LAS degradation. As shown in the figure, although the same amount
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Ö. Karahan / Bioresource Technology 101 (2010) 92–97
OUR for Growth on Synthetic Sewage (mg/l/h)
140 Synthetic sewage(45 ml) 120 Synthetic sewage(45 ml)+LAS(50ml) 100 80 60 40 20 0 -50
0
50
100
150
200
250
300
Time (min) Fig. 4. Inhibition effects of LAS on OUR response for synthetic sewage degradation.
of dissolved oxygen was utilized for the same amount of synthetic sewage fed, the time for complete degradation has increased 50% and reached 300 min. These results are in agreement with the study of Oviedo et al. (2004) since they have also reported that LAS influenced the degradation capacity of, both, organic matter and itself in aerobic processes.
6. Conclusions The study revealed that LAS exhibits non-competitive inhibition on the hydrolysis mechanism of synthetic sewage and imposes competitive inhibition on the growth of biomass. Model simulation results were in good agreement with experimental results indicating that proposed model structure successfully defines the biochemical mechanisms. The results obtained will help the engineers and the plant operators to improve their understanding about detailed design and operation of WWTPs which receive biodegradable inhibitory compound discharges. The results of the study present that the applied methodology is successful for the interpretation of the response of activated sludge systems to inhibitory biodegradable compounds like LAS. Acknowledgement The author deeply acknowledges FEYZI_ AKKAYA RESEARCH FUND FOR SCIENTIFIC ACTIVITIES (FABED) for the financial support provided by Research Support for Outstanding Young Scientists Award for this study. References ˇ an, M., Drtil, M., 2008. Bodík, I., Gašpariková, E., Dancˇová, L., Kalina, A., Hutn Influence of disinfectants on domestic wastewater treatment plant performance. Bioresour. Technol. 99 (3), 532–539. Børglum, B., Hansen, A.M., Kortlgning af Vaske- Rengøringsmidler, 1994. A Survey of Washing and Cleaning Agents, Arbejdsmiljøinstituttet, København, AMI rapport nr. 44. Birkett, J.W., Lester, J.N., 2003. Endocrine Disrupters in Wastewater and Sludge Treatment Processes. CRC Press LLC, Florida. Bridson, E.Y., Brecker, A., 1970. Design and formulation of microbial culture media. In: Norris, Ribbons (Eds.), Methods in Microbiology, vol. 3A. Academic Press, New York, pp. 229–292. Fauser, P., Vikelsøe, J., Sørensen, P.B., Carlsen, L., 2003. Phthalates, nonylphenols and LAS in an alternately operated wastewater treatment plant-fate modeling based on measured concentrations in wastewater and sludge. Water Res. 37, 1288– 1295.
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