The inhibition of activated sludge respiration is one of the most important parameters for monitoring wastewater toxicity. The main objective of this study was to improve respiration inhibition testing in order to protect the biological degradation within the aerobic process in a wastewater treatment plant more efficiently. In order to influence the sensitivity of the testing bacteria, two different nutrient solutions were selected for biological testing, synthetic wastewater according to ISO 8192 and NaAc (sodium acetate). The effects of the nutrient solutions on heavy metal speciation and their respiration inhibition were investigated. The toxicants Zn(II), Cu(II), Cr(VI) and 3,5 DCP (3,5-dichlorophenol) were used as standards to determine toxicities and to study the influence of nutrient solutions. Results have shown that NaAc as a nutrient solution sensitized the respiration inhibition test up to a factor of 7.7. Furthermore, an oxidation reduction potential electrode can be used as an alternative tool to verify the results obtained with an oxygen sensor.

INTRODUCTION

The activated sludge process is considered as a common method for the biological treatment of wastewaters. To assess this process, it is necessary to monitor the influent of the wastewater treatment plant (WWTP). The high importance of wastewater monitoring relating to the inhibition of activated sludge due to toxic wastewater was described by Rudolph et al. (2012, 2015) using a specific example in Vietnam. This statement is proven by the incidence that 45 out of 109 tested municipal WWTPs, received inhibiting wastewater (Jönsson et al. 2000). Another example was reported by Grau & Da-Rin (1997). The authors stated that the operation of a WWTP in Brazil was affected for six months due to receiving wastewater with a high concentration of phenol. There are numerous methods for measuring toxicity. Dalzell et al. (2002) compared five methods for determining the inhibition of activated sludge by wastewater. In the most recent research, the transcriptional response of activated sludge bacteria was examined to study the inhibiting effect of heavy metals (Kapoor et al. 2015, 2016). The activated sludge respiration inhibition test is an appropriate method for determining the inhibition by pollutants, as this method uses microorganisms directly from the WWTP. The procedure of activated sludge inhibition tests is described in International Organization for Standardization (ISO) (ISO 8192 2007) and the Organisation for Economic Co-operation and Development (OECD) (OECD 209 1993) due to its necessity in determining the inhibition of pollutants to activated sludge bacteria.

The most important variable for improving monitoring with biological test organisms is the sensitivity of the test organisms (Gu et al. 2002). Improving the sensitivity of activated sludge respiration inhibition tests ensures the detection of toxic pollutants and contaminants in lower concentrations, and this allows protecting microorganisms within the activated sludge process at the WWTP and hence makes the biological degradation of contaminants more efficient.

In order to enhance the sensitivity of the activated sludge respiration inhibition test the procedure of this biological test according to ISO (ISO 8192 2007) was modified for this work by altering the nutrient solutions. This was carried out by replacing the nutrient solution according to ISO (ISO 8192 2007) with quickly biologically degradable NaAc as a nutrient to stimulate bacterial growth of the activated sludge bacteria. The second purpose of selecting NaAc was to avoid the formation of less toxic complexes with the chemical compounds of synthetic wastewater, which is used as a nutrient as per ISO (ISO 8192 2007). The influence of heavy metal speciation on nitrification was described by (Çeçen et al. 2010a, 2010b) and needs to be considered when investigating the inhibition effect of heavy metals on activated sludge bacteria (Li et al. 2016). The novelty of this work is that NaAc was specifically chosen as a nutrient to sensitize the biological test organisms and to avoid the formation of complexes with a lower bioavailability.

The second objective of this study was to evaluate an alternative detector to an oxygen sensor in order to verify the results of activated sludge respiration inhibition testing and to find a more cost efficient sensor compared to an optical oxygen sensor, which is commonly used for online respiration measurements. In this work, it was possible to obtain similar results with an oxidation-reduction potential (ORP) electrode, as compared to an oxygen sensor which is commonly used for the inhibition test of oxygen consumption by activated sludge.

METHODS

Activated sludge

The activated sludge for testing was taken directly from the recycle line of the industrial WWTP at ‘Mekong’ seafood factory in Tra Noc industrial zone, Can Tho City, Vietnam. The biomass was stored in a 20 L laboratory-size batch activated sludge reactor while being aerated and additionally fed with 50 mL synthetic wastewater every day. After two weeks, the sludge was discarded and new activated sludge was taken from the same plant.

Before toxicity measurements, the activated sludge was analyzed by measuring pH and mixed liquor suspended solids (MLSS). Afterwards, the pH value of the activated sludge was adjusted to 7.3–7.8 and the MLSS concentration was adjusted to 3 g L−1. For further quality assurance, the standards 3,5 DCP (3,5-dichlorophenol) 21 mg L−1 and Cu(II) 25 mg L−1 were analyzed in order to prove that the activated sludge respiration inhibition test provides reproducible results. These concentrations were selected to achieve the EC 50 values of the appropriate standard. If the coefficient of variation was higher than 10% the activated sludge was discarded and new activated sludge was obtained from the ‘Mekong’ seafood factory.

Toxic standard solutions

Standards used for the inhibition experiment were 3,5 DCP, ZnSO4, CuSO4 and K2Cr2O7. All applied chemicals were analytical grade. The solutions were prepared with deionized water. Zn(II), Cu(II) and Cr(VI) were selected as exemplary heavy metal toxicants commonly found in industrial wastewater (Karvelas et al. 2003; Friedrichs et al. 2016) and hence these toxicants are widely used to investigate the effect of heavy metals on activated sludge bacteria (Çeçen et al. 2010b; Vaiopoulou & Gikas 2012; Ouyang et al. 2016). 3,5 DCP is a commonly used organic reference toxicant (ISO 8192 2007).

Nutrient solutions

The synthetic wastewater according to ISO (ISO 8192 2007) was used as a nutrient solution to feed the biomass while it was stored in the laboratory-size batch activated sludge reactor and also as a nutrient solution for the activated sludge respiration inhibition test. The synthetic wastewater was prepared with the following compounds: peptone 16 g L−1, meat extract 11 g L−1, urea 3 g L−1, NaCl 0.7 g L−1, CaCl·3H2O 0.4 g L−1, MgSO4·7H2O 0.2 g L−1, K2HPO4 2.8 g L−1.

The NaAc solution used as a nutrient for the activated sludge respiration inhibition test was prepared with the same chemical oxygen demand (COD) concentration as the synthetic wastewater nutrient solution, which was 30 g L−1. The pH values of the NaAc solution and of the synthetic wastewater were adjusted to a pH value of 7 with HCl or NaOH (0.1 M).

Analytical methods

The COD was measured with a Quick CODLab device by LAR Process Analysers AG. The oxygen concentration was determined using the optical oxygen sensor Hamilton – VISIFERM DO 120 and the ORP was measured with the electrode WTW – SENTIX ORP 100. Whatman Glass Microfiber Filters were used for MLSS measurements.

Activated sludge respiration inhibition test

The activated sludge respiration inhibition test was performed according to ISO (ISO 8192 2007) but while varying the nutrient solution. In order to prepare the test mixture 250 mL activated sludge with a MLSS concentration of 3 g L−1 was poured into a 500 mL Erlenmeyer flask, a defined volume of the stock toxicant solution and 16 mL of the nutrient solution (synthetic wastewater or NaAc) were then added. This solution was topped up to 500 mL in the Erlenmeyer flask and was incubated under aeration for 30 min, afterwards the respiration was measured at a constant temperature of 20–22 °C with an optical oxygen sensor. The oxygen concentration was measured online every 20 seconds while stirring until a concentration of 1 mg L−1 was reached. The respiration rate was calculated based on oxygen consumption. Additionally, an ORP electrode was placed into the Erlenmeyer flask and the ORP values were recorded every 20 seconds. For each analysis a fivefold determination was done and the mean values and coefficients of variation were calculated.

Furthermore, the specific oxygen uptake rate (SOUR) were determined with an oxygen sensor and an OPR electrode for reference water using the activated sludge respiration inhibition test after adding synthetic wastewater or NaAc as a nutrient solution. For each analysis 15 measurements were carried out and the mean values, standard deviations and the coefficients of variation were calculated. The oxygen respiration rate was determined after adding synthetic wastewater or NaAc as a nutrient solution and after an incubation time of 30 min.

Theoretical heavy metal speciation under test conditions

The initial theoretical heavy metal speciation was calculated with the chemical equilibrium software MINTEQ 3.1. The calculations for the heavy metal speciation were carried out for NaAc (960 mg L−1) and for synthetic wastewater as nutrients with the consideration of following chemical compounds and concentrations: NaCl 22.4 mg L−1, CaCl·3H2O 12.8 mg L−1, MgSO4·7H2O 6.4 mg L−1 and K2HPO4 89 mg L−1. The program MINTEQ 3.1 does not provide a model for peptone and meat extract since both bacterial growth media consist of undefined amino acids and peptides. Instead of using peptone and meat extract, the amino acid glycine was selected as a model for the chemical speciation calculation with a concentration of 864 mg L−1. These values are the concentrations of the total test mixtures for the activated sludge respiration inhibition test; glycine is the sum of the concentrations of peptone and meat extract. Urea was disregarded in chemical speciation calculations since MINTEQ 3.1 does not give information about this model. The chosen heavy metal concentrations were the determined EC 50 values of Zn(II), Cu(II) and Cr(VI). The calculated heavy metal speciation in terms of free ions, inorganic and organic complexes and log K (stability constant of the metal species) are summarized in Tables 13.

Table 1

Theoretical Cu(II) speciation under initial conditions

Synthetic wastewater
NaAc
Cu species% of total concentrationLog KCu species% of total concentrationLog K
 0.423 8,56  37.03 – 
 99.577 15.7  0.26 3.94 
    8.46 −7.50 
    0.14 −16.23 
    2.93 −10.49 
    0.28 −20.79 
    44.53 2.21 
    6.36 3.4 
Synthetic wastewater
NaAc
Cu species% of total concentrationLog KCu species% of total concentrationLog K
 0.423 8,56  37.03 – 
 99.577 15.7  0.26 3.94 
    8.46 −7.50 
    0.14 −16.23 
    2.93 −10.49 
    0.28 −20.79 
    44.53 2.21 
    6.36 3.4 
Table 2

Theoretical Zn(II) speciation under initial conditions

Synthetic wastewater
NaAc
Zn species% of total concentrationLog KZn species% of total concentrationLog K
 23.95 –  77.594 – 
 0.201 −8.997  0.431 1.91 
 0.024 −16.894  0.56 −8.997 
 0.08 2.34  0.063 −16.894 
 12.245 15.69  21.351 1.57 
 50.153 5.38    
 13.296 9.81    
Synthetic wastewater
NaAc
Zn species% of total concentrationLog KZn species% of total concentrationLog K
 23.95 –  77.594 – 
 0.201 −8.997  0.431 1.91 
 0.024 −16.894  0.56 −8.997 
 0.08 2.34  0.063 −16.894 
 12.245 15.69  21.351 1.57 
 50.153 5.38    
 13.296 9.81    
Table 3

Theoretical Cr speciation under initial conditions

Synthetic wastewater
NaAc
Cr species% of total concentrationLog KCr species% of total concentrationLog K
 77.9 –  78.81 – 
 0.19 0.70  18.18 6.51 
 0.21 0.57  0.11 14.56 
 19.89 6.51  2.90 0.70 
 0.13 15.32    
 1.63 14.56    
 0.16 26.68    
Synthetic wastewater
NaAc
Cr species% of total concentrationLog KCr species% of total concentrationLog K
 77.9 –  78.81 – 
 0.19 0.70  18.18 6.51 
 0.21 0.57  0.11 14.56 
 19.89 6.51  2.90 0.70 
 0.13 15.32    
 1.63 14.56    
 0.16 26.68    

RESULTS AND DISCUSSION

SOUR obtained by oxygen respiration and ORP measurements

The average SOUR obtained by measuring the oxygen concentrations was lower for synthetic wastewater in comparison to NaAc as a nutrient solution as shown in Table 4. The SOUR ratio of synthetic wastewater to NaAc was 1/1.6. The activating effect of acetate compared to municipal wastewater on the respiration of activated sludge was described by Strotmann et al. (1999). The authors determined the maximum respiration rate for municipal wastewater and NaAc with a ratio of 1/1.5. In comparison, the results obtained by the SOUR with an ORP electrode were slightly lower when NaAc was used as a nutrient as shown in Table 5, the ratio of SOUR with an ORP of synthetic wastewater to NaAc was 0.9/1.

Table 4

SOUR of reference water obtained by oxygen respiration measurements

 Synthetic wastewaterNaAc
SOUR average (mg g−1 h−110.4 16.4 
n 15 15 
StDev (mg g−1 h−12.3 3.9 
CV (%) 21.8 23.9 
 Synthetic wastewaterNaAc
SOUR average (mg g−1 h−110.4 16.4 
n 15 15 
StDev (mg g−1 h−12.3 3.9 
CV (%) 21.8 23.9 
Table 5

SOUR of reference water obtained by ORP measurements

 Synthetic wastewaterNaAc
SOUR average (mg g−1 h−1120.5 109.3 
n 15 15 
StDev (mV/g*h) 28.0 21.0 
CV (%) 23.2 19.3 
 Synthetic wastewaterNaAc
SOUR average (mg g−1 h−1120.5 109.3 
n 15 15 
StDev (mV/g*h) 28.0 21.0 
CV (%) 23.2 19.3 

Sensitization of activated sludge respiration inhibition testing by varying nutrient solutions

In order to enhance the sensitivity of activated sludge respiration inhibition testing, toxicities were determined after using synthetic wastewater and NaAc as nutrient solutions. The results were obtained by measuring oxygen respiration. In this study, the EC 50 values were 37.1, 25.5, 20.8 and 39.2 mg L−1 using synthetic wastewater and 20.2, 3.3, 6.2 and 6.5 mg L−1 using NaAc as nutrients solution for Cr(VI), Cu(II), 3,5 DCP and Zn(II), the results are shown in Figure 1. The results show that compared to synthetic wastewater NaAc increased the sensitivity of activated sludge inhibition testing.
Figure 1

Comparison of EC 50 values for the toxicants: Cr(VI), Cu(II), 3,5 DCP and Zn(II) measuring the respiration with an oxygen sensor and an ORP electrode using synthetic wastewater or NaAc as a nutrient solution. The results are presented with standard deviation.

Figure 1

Comparison of EC 50 values for the toxicants: Cr(VI), Cu(II), 3,5 DCP and Zn(II) measuring the respiration with an oxygen sensor and an ORP electrode using synthetic wastewater or NaAc as a nutrient solution. The results are presented with standard deviation.

One of the most common standards to evaluate activated sludge respiration inhibition testing is 3,5 DCP. In this study, an EC 50 value for 3,5 DCP of 20.8 mg L−1 was obtained under standard conditions using synthetic wastewater as a nutrient solution and it was decreased by using NaAc with a concentration of 6.2 mg L−1. This corresponds to a calculated sensitization factor of 3.4. The result using synthetic wastewater was very good when compared with the range of validity according to ISO (ISO 8192 2007), where 20 laboratories participated in a round robin test and reported an EC 50 value of 20.3 mg L−1 (σ = 8.6 mg L−1). In the work of Gendig et al. (2003), the sensitization was realized by increasing the incubation time from 30 min to 180 min, the factor of sensitization was only 1.64.

The most significant decrease of the EC 50 value was obtained with Cu(II) resulting in a sensitization factor of 7.7. The EC 50 value under standard conditions was 25.5 mg L−1 and was determined using NaAc at the low concentration of 3.3 mg L−1. In the literature there are several reports about the effect of Cu(II) on activated sludge bacteria. For example, Ochoa-Herrera et al. (2011) published an EC 50 value of 4.6 mg L−1 using glucose as a nutrient. For Zn(II) as a toxicant, the high factor of 6.6 was determined. The EC 50 value for using synthetic wastewater as a nutrient solution was 39.2 mg L−1 and 6.5 mg L−1 for NaAc.

In comparison to the other tested toxicants Cr(VI) showed the lowest factor of sensitization with a value of only 1.8. The EC 50 value with synthetic wastewater as a nutrient solution was 37 mg L−1 and 20 mg L−1 with NaAc. In addition, the effect of Cr(VI) on activated sludge bacteria is described in numerous literature reports. The EC 50 values for Cr(VI) are ranging from 40 to 90 mg L−1 in literature (Vaňková et al. 1999; Cokgor et al. 2007).

One reason for the sensitization of the activated sludge bacteria to toxins is that the maximum growth rate of the activated sludge bacteria is enhanced by NaAc as a nutrient solution. The effect of the nutrient solution on the maximum growth rate was reported by Blok (1974). The literature also suggests that NaAc is an easily degradable nutrient solution when compared to synthetic wastewater, which results in a higher growth rate. In the research of Curless et al. (1990) and Ryan et al. (1996), it was reported that cloned gene expression is higher at high growth rates. Due to the increased growth rates, the cells focus on the production of genes, which is necessary for cell division. The cell division of slowly growing cells is much more proficient as compared to fast growing cells. Therefore, the higher growth rate induced by NaAc as a nutrient solution could be one reason for the sensitization of activated sludge respiration inhibition testing.

Another impact on activated sludge is a decreased sensitivity after storing and feeding it with synthetic wastewater as reported by Gendig et al. (2003). This effect can be excluded for the results of this work, since the toxicities for 3,5 DCP 21 mg L−1 and Cu(II) 25 mg L−1 were measured every day. In addition, the adaptation of the microorganisms to the nutrient solution is an important issue affecting their resistance to toxicants. In comparison to the results of this work, Cokgor et al. (2007) reported a decrease in the sensitivity of activated sludge inhibition testing for Ni(II) and Cr(VI) with a sludge age of 10 days and feeding the activated sludge with glucose and a starch/acetic acid mixture.

Influence of the nutrient solution on heavy metal speciation

As shown in Table 1, the theoretical calculated speciation of Cu(II) are highly dependent on the nutrient solution. For the chemical speciation calculations using synthetic wastewater as nutrient solution, the amino acid glycine was selected as a model for peptone and meat extract. On the assumption that the different amino acids of both bacterial growth media have a similar behavior to form complexes, it is most likely that 100% of the total Cu(II) forms species with the amino acids of peptone and meat extract. The formation of Cu(II) complexes with amino acids has been reported by Li & Doody (1954) and the diminishing inhibition of the heavy metal Ag+ due to the formation of strong complexes with peptone by Çeçen & Kılıç (2016). The author supposed that the diminishing effect can be explained by the hindered sorption or/and diffusion of the Ag-peptone complex in cells.

In contrast, for NaAc as a nutrient solution, 37% is present as the free Cu(II) ion and 51.2% in form of weak Cu-(Acetate) complexes (Log K = 2.21–3.94). Hence, the high factor of sensitization of 7.7 for Cu(II) using NaAc as a nutrient can be explained, because Cu(II) in the presence of synthetic wastewater forms complexes with the amino acids of peptone and meat extract up to 100%. While using NaAc as nutrient solution, 37% Cu(II) is present as the free form which is more toxic than the Cu(II) complexes of the amino acids derived from peptone and meat extract.

The calculation of the Zn(II) species presence in synthetic wastewater showed that only 63.45% formed Zn-Glycine complexes. In contrast to Cu(II), the concentration of the free ion for Zn(II) ion using synthetic wastewater as nutrient increased to 23.95% and the percentage of the strong complex increased to 12.45%. In this relation, it can be explained that the factor of sensiti-zation decreased to a value of 6.6, since 23.95% of free Zn(II) is present already, while using synthetic wastewater as nutrient, which has a higher toxicity than the strong ZnHPO4 complex.

The lowest factor of sensitization was achieved for Cr(VI). After calculation of the theoretical Cr(VI) speciation, the reason for the relatively weak effect was understood. In contrast to Cu(II) and Zn(II), Cr(VI) does not form any cationic complexes with chemical compounds of the synthetic wastewater in significant concentrations, as shown in Table 3. For both synthetic wastewater and NaAc, Cr(VI) was present as the species or anionic complexes with a percentage of the total Cr(VI) concentration >99%. The difference in behavior between Cr(VI) and the other tested heavy metals can be explained due to the negative charge of and its anionic complexes, since the surface of activated sludge is negatively charged the might not be absorbed by the usually negative charged surface of activated sludge (Çeçen et al. 2010a). The authors could prove that that Cr(VI) in a mixed liquor with activated sludge is not absorbed onto the biomass and remained in the soluble phase.

In conclusion, higher toxicities for Cu(II) and Zn(II) were found because NaAc hardly forms complexes with these heavy metals. This effect can be explained by the free ion concentration being higher using NaAc as nutrient solution because the free form of a heavy metal exerts the highest toxicity. While using synthetic wastewater as nutrient solution, the mixed organic matter and the inorganic salts formed heavy metal complexes and the concentration of the free ion was reduced. But also for 3,5 DCP, a sensitization of the activated sludge respiration inhibition test was achieved. This organic reference material was selected, because it most likely does not form complexes with compounds of the nutrient solution. However, a sensitization factor of 3.4 was obtained for 3,5 DCP while using NaAc as nutrient solution. This might be evidence that the sensitization is not only influenced by the speciation of the toxicant but also by the increased growth rate of the activated sludge bacteria due to NaAc as nutrient solution.

Verifying the results of activated sludge respiration inhibition testing using an ORP electrode

In order to find an alternative sensor and to verify the results for the inhibition of activated sludge respiration, experiments were carried out using an additional ORP electrode. A measurement curve of the activated sludge respiration inhibition test is shown in Figure 2 for reference water and 3,5 DCP (20 mg L−1) using an oxygen sensor and an ORP electrode simultaneously. Notably, the ORP is still increasing at oxygen depletion. The relationship of ORP and oxygen concentrations and the effect that the oxygen concentration stabilizes quickly at low values while the ORP is decreasing slowly in an activated sludge reactor was reported by Heduit & Thevenot (1989).
Figure 2

Comparison of respiration for reference water and 3,5 DCP (20 mg L−1) with an oxygen sensor and an ORP electrode using synthetic wastewater as a nutrient solution.

Figure 2

Comparison of respiration for reference water and 3,5 DCP (20 mg L−1) with an oxygen sensor and an ORP electrode using synthetic wastewater as a nutrient solution.

The comparison of the resulting EC 50 values obtained with the activated sludge respiration inhibition test using an oxygen sensor and an ORP electrode is shown in Figure 1. Due to a high conformity of the results, it was possible to prove that the ORP electrode can be used as an alternative tool instead of the oxygen sensor to verify the effect of the nutrient on the activated sludge respiration inhibition test. The standard deviation of the EC 50 values of the oxygen sensor and ORP electrode for both tested nutrient solutions is between 0.6 mg L−1 and 1.5 mg L−1. This variation of the results can be considered to be a deviation of the activated sludge inhibition test. No significant difference of the two different sensors and the tested nutrient solutions was observed.

The relationship of ORP and oxygen concentration in activated sludge respiration inhibition testing originates from the oxidation of carbon sources by the activated sludge heterotrophs, which consumes oxygen. Heduit & Thevenot (1989) conducted tests in a full scale WWTP to define an empiric equation for the dissolved oxygen concentration and the ORP, the authors reported that the relationship obeys an Nernst equation. The ORP in activated sludge inhibition testing might also be influenced by heavy metals or the ORP ammonia relationship but these factors have a minor significance. Thereby, the high dependence and consistency between the results when using an oxygen sensor and an ORP electrode can be explained. Therefore, an oxygen sensor can be replaced by an ORP electrode while obtaining similar results. The advantage of the ORP electrode over an optical oxygen sensor is higher cost efficiency. For online respiration measurements, the inexpensive membrane oxygen sensor cannot be used, since the activated sludge bacteria form a biofilm on the membrane, which would increase the maintenance effort. Therefore, the information that an optical oxygen sensor can be replaced by an ORP electrode is very valuable for the manufacture of online respirometers to reduce the production costs.

CONCLUSION

The obtained experimental results and their evaluation proved that activated sludge respiration inhibition testing can be sensitized by varying the nutrient solution. The EC 50 value using NaAc as a nutrient solution was up to 7.7 times lower compared to synthetic wastewater. This effect can be explained by the sensitization of the activated sludge bacteria by NaAc as a nutrient solution. Furthermore, the heavy metal speciation due to the compounds of the nutrient solutions have great significance for respiration inhibition. It is therefore possible to detect toxic pollutants in lower concentrations, which have an inhibiting effect on activated sludge bacteria. Furthermore, the ORP electrode can be used as an alternative tool to verify the results obtained with the oxygen sensor and to reduce the production costs of online respirometer.

ACKNOWLEDGEMENTS

This research project was supported by the German Ministry of Education and Research – BMBF in the framework of the AKIZ Project, project no. 02WA1067–02WA1068 and the Vietnamese Ministry of Science and Technology – MOST. The authors gratefully acknowledge the support of the BMBF and the MOST.

REFERENCES

REFERENCES
Çeçen
F.
Kılıç
B.
2016
Inhibitory effect of silver on activated sludge: effect of organic substrate and the carbon to nitrogen ratio
.
Journal of Chemical Technology & Biotechnology
91
(
4
),
1190
1198
.
Çeçen
F.
Semerci
N.
Geyik
A. G.
2010a
Inhibition of respiration and distribution of Cd, Pb, Hg, Ag and Cr species in a nitrifying sludge
.
Journal of Hazardous Materials
178
(
1–3
),
619
627
.
Çeçen
F.
Semerci
N.
Geyik
A. G.
2010b
Inhibitory effects of Cu, Zn, Ni and Co on nitrification and relevance of speciation
.
Journal of Chemical Technology & Biotechnology
85
(
4
),
520
528
.
Cokgor
E. U.
Ozdemir
S.
Karahan
O.
Insel
G.
Orhon
D.
2007
Critical appraisal of respirometric methods for metal inhibition on activated sludge
.
Journal of Hazardous Materials
139
(
2
),
332
339
.
Dalzell
D. J. B.
Alte
S.
Aspichueta
E.
de la Sota
A.
Etxebarria
J.
Gutierrez
M.
Hoffmann
C. C.
Sales
D.
Obst
U.
Christofi
N.
2002
A comparison of five rapid direct toxicity assessment methods to determine toxicity of pollutants to activated sludge
.
Chemosphere
47
(
5
),
535
545
.
Friedrichs
F.
Rudolph
K. U.
Panning
F.
Huyen
P. T.
Genthe
W.
Trung
D. Q.
2016
Occurrence of nitrification inhibition in Vietnam's industrial zones
.
VNU Journal of Science
32
(
1S
),
159
168
.
Gendig
C.
Domogala
G.
Agnoli
F.
Pagga
U.
Strotmann
U. J.
2003
Evaluation and further development of the activated sludge respiration inhibition test
.
Chemosphere
52
(
1
),
143
149
.
Grau
P.
Da-Rin
B. P.
1997
Management of toxicity effects in a large wastewater treatment plant
.
Water Science and Technology
36
(
2–3
),
1
18
.
Heduit
A.
Thevenot
D. R.
1989
Relation between redox potential and -sludge oxygen levels in activated reactors
.
Water Science and Technology
21
(
8–9
),
947
956
.
ISO 8192
2007
Test for inhibition of oxygen consumption by activated sludge for carbonaceous and ammonium oxidation
.
Jönsson
K.
Grunditz
C.
Dalhammar
G.
La Cour Jansen
J.
2000
Occurrence of nitrification inhibition in Swedish municipal wastewaters
.
Water Research
34
(
9
),
2455
2462
.
Kapoor
V.
Li
X.
Elk
M.
Chandran
K.
Impellitteri
C. A.
Santo Domingo
J. W.
2015
Impact of heavy metals on transcriptional and physiological activity of nitrifying bacteria
.
Environmental Science & Technology
49
(
22
),
13454
13462
.
Kapoor
V.
Li
X.
Chandran
K.
Impellitteri
C. A.
Domingo
J. W. S.
2016
Use of functional gene expression and respirometry to study wastewater nitrification activity after exposure to low doses of copper
.
Environmental Science and Pollution Research
23
(
7
),
6443
6450
.
Karvelas
M.
Katsoyiannis
A.
Samara
C.
2003
Occurrence and fate of heavy metals in the wastewater treatment process
.
Chemosphere
53
(
10
),
1201
1210
.
Li
N. C.
Doody
E.
1954
Copper(II) and zinc complexes of some amino acids and glycylglycine
.
Journal of the American Chemical Society
76
(
1
),
221
225
.
Li
X.
Kapoor
V.
Impelliteri
C.
Chandran
K.
Domingo
J. W. S.
2016
Measuring nitrification inhibition by metals in wastewater treatment systems: current state of science and fundamental research needs
.
Critical Reviews in Environmental Science and Technology
46
(
3
),
249
289
.
Ochoa-Herrera
V.
León
G.
Banihani
Q.
Field
J. A.
Sierra-Alvarez
R.
2011
Toxicity of copper(II) ions to microorganisms in biological wastewater treatment systems
.
Science of the Total Environment
412–413
(
0
),
380
385
.
OECD 209
1993
Test No. 209: Activated Sludge, Respiration Inhibition Test (Carbon and Ammonium Oxidation)
.
OECD Publishing, Paris, France
.
Rudolph
K.
Kreuter
S.
Genthe
W.
Friedrichs
F.
Dong
P. H.
Heinrich
R.
Long
N. V.
The
N. M.
2012
Monitoring of indirect industrial discharges – development of a monitoring strategy and first results of a monitoring survey in the drainage system of Tra Noc industrial zone in Vietnam
.
Vietnam Journal of Chemistry
51
(
2
),
224
232
.
Rudolph
K.
Dong
P. H.
Dung
H. V.
Friedrichs
F.
Genthe
W.
Long
N. V.
Meinardi
D.
Trung
D. Q.
2015
Abwasser-Monitoring in Echtzeit mit Toxizitäts-Screening zur technischen und wirtschaftlichen Optimierung von Abwassersystemen
,
KA - Korrespondenz Abwasser, Abfall
6
,
520
528
.
Vaňková
S.
Kupec
J.
Hoffmann
J.
1999
Toxicity of chromium to activated sludge
.
Ecotoxicology and Environmental Safety
42
(
1
),
16
21
.