S. Pratt*1,2, R. Zeng1, Z. Yuan1 and J. Keller1 1 Advanced Wastewater Management Centre (AWMC), The University of Queensland, St Lucia, QLD 4072, Australia 2
Centre for Environmental Technology and Engineering (CETE), Massey University, Palmerston North, New Zealand (*corresponding author:
[email protected]) Abstract The mass transfer coefficient for oxygen in water (KLaO2) is an important parameter for respirometric studies. But determination of KLaO2 in process conditions is not straightforward. In this paper, two distinct procedures for determining KLaO2 in process conditions are outlined and tested. The off-gas method relies on a gas mass balance over a bioreactor while the non-steady state methods rely on analysing DO recovery after perturbation. Various means for inducing perturbation are tested and compared. KLaO2 values for a bioreactor are determined by the listed methods. It was found that the off-gas method resulted in the highest KLaO2 for the given reactor, while the non-steady state method, whereby perturbation is caused by exogenous activity on acetate, resulted in the lowest KLaO2. It is shown that the gas mass balancing technique is robust to unexpected exogenous activity (caused by for example, the oxidation of storage polymers formed or nitrite accumulated), while the non-steady state methods that involve inducing perturbations by exogenous activity appear susceptible to such continued exogenous activity in the DO recovery period. Keywords In-process; off-gas analysis; oxygen transfer coefficient; respirometry
Water Science and Technology Vol 50 No 11 pp 153–161 ª IWA Publishing 2004
Comparison of methods for the determination of KLaO2 for respirometric measurements
Introduction
Respirometric techniques have long been used as a tool for monitoring biological activity in activated sludge systems (Spanjers et al., 1998). Furthermore, respirometric experiments are useful for the calibration of some extended biokinetic models (Dochain et al., 1995; Vanrolleghem et al., 1995). Henze et al. (1987), in presenting the well known Activated Sludge Model No. 1 (generally accepted as state-of-the-art in modelling activated sludge systems), suggested that many of the parameters listed in their model be calibrated from respirometric data. This paper focuses on the methods available for the identification of KLaO2, a parameter that can be used for the preparation of respirometric data. Several techniques have been developed for identifying the oxygen transfer coefficient (ASCE, 1992; ASCE, 1996; Capela et al., 1999). In this work, non-steady state and off-gas methods are considered. For non-steady state methods the oxygen transfer coefficient is determined by monitoring the dissolved oxygen concentration over time after perturbation from steady-state conditions. The widely used ASCE Standard Measurement of Oxygen Transfer in Clean Water (ASCE, 1992) is a non-steady state method. Dissolved oxygen (DO) is removed by the addition of sodium sulfite in the presence of a cobalt catalyst (ASCE, 1992). The transfer coefficient is then calculated from data obtained during DO recovery. However, it should be noted that the transfer coefficient determined in clean water is not suitable for subsequent respirometric studies (Blok, 1974; ASCE, 1996). The presence of surfactants, oil, salts and particles have significant effect on the coefficient. As such, researchers have developed non-steady state procedures for providing perturbations for
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determining KLaO2 in process conditions. The first practical approach was reported by Nogaj and Hurwitz (1963) (Kayser, 1979). The authors presented a method whereby perturbation of DO concentration was caused by stopping aeration. The DO concentration decrease resulted from exogenous activity in the reactor. The decrease can be accelerated by bubbling a non-oxygen gas (e.g. nitrogen, helium) through the reactor. This is known as the desorption method. Sperandio and Paul (1997) made use of a desorption method whereby nitrogen gas was used to “strip” oxygen from the system. An alternative method to cause a decrease in DO concentration has been proposed and used by, among others, Vanrolleghem (1994). The basis of the technique involves temporarily increasing OUR by means of an addition of a readily biodegradable substrate pulse. The aim was to measure the exogenous oxygen uptake rate and the oxygen transfer coefficient in a batch reactor. Methods also exist in which perturbation is achieved by increasing the oxygen concentration, via the addition of hydrogen peroxide (Kayser, 1979; Mueller and Boyle, 1988) or pure oxygen (Wagner, 1998). Off-gas analysis may also be used to determine the transfer coefficient (Redmon et al., 1983; Fujie et al., 1994; Capela et al., 1999). This method is typically a steady state method. KLaO2 can be determined based on measurement of oxygen consumption rate, which can be determined by an oxygen balance in the gas phase, and the dissolved oxygen concentration. The method is ideal for the study of in-process conditions as the transfer coefficient can be derived under actual operating conditions (relatively constant DO and gas flow rate) (Fujie et al., 1994). The objective of this paper is to compare the oxygen transfer coefficients under process conditions obtained by non-steady state and off-gas methods. The non-steady state methods trialled include those commonly used (perturbation caused by addition of H2O2, stopping aeration and exogenous activity on acetate) along with the less used ones (perturbation caused by exogenous activity on ethanol, ammonia and nitrite). The off-gas method is based on methods reported in the literature, but modified to make use of the equipment available. The focus of the work is on determination of KLaO2 for subsequent use for the preparation of respirometric data for laboratory studies. The advantages and potential limitations of each of the methods are discussed in this context. Theoretical framework
Both the non-steady state and the off-gas analysis methods rely on an oxygen mass balance around a completely mixed tank (1). dCL =KL aO2 (C*1 xCL )xrO dt
(1)
where: CL is the dissolved oxygen (DO) concentration [mg/L]; KLaO2 is the volumetric mass transfer coefficient [hrx1]; rO is the respiration rate [mg/hr]; C*1 is the oxygen saturation concentration [mg/L] and is defined as the value in equilibrium with the concentration in the bulk gas phase (expressed as the partial pressure of oxygen PO). C*1 =
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PO r32r1;000 H
(2)
H is the Henry’s law constant [atm/(mole/L)]; PO is the partial pressure of oxygen [atm] In process conditions, a pseudo saturation concentration may be used (C*1 p) [mg/L]. If it is assumed that the rate of endogenous activity (rOendog) is constant throughout the
experiment then the saturation concentration may simply be reported as: dCL =KL aO2 (C*1 pxCL )xrOexog dt
(3)
where C*1 p=CLendog =C*1 xrOendog =KL aO2 and rOexog is the exogenous oxygen uptake rate.
CL =C*1 px(C*1 pxCL )exKLaO2 :t
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Non-steady state methods. The non-steady state method relies on DO perturbation. This can be induced by exogenous activity, changes in the inlet gas flow rates/composition or addition of hydrogen peroxide. The data collected over the period during which the DO returns to the saturation level is used for the determination of KLaO2. During this period, it is assumed that there is no change in exogenous biological activity (generally rOexog is non-existent over the period concerned). As such, integration of (3) leads to: (4)
The DO data, along with a nonlinear regression analysis of (4) may be used to identify KLaO2. Gas mass balancing method. A gas mass balancing technique may be used to determine KLaO2 (Kayser, 1979). The method relies on recording DO over a period during which the rate of biological activity is constant. A period of exogenous or endogenous activity is suitable. It is a steady state method as the DO concentration in the system is unchanging. As such, the oxygen transfer rate (OTR) is constant and can be written as: OTR=KL aO2 (C*1 xCL )
(5)
Gapes et al. (2003) showed that a gas mass balance can be used to quantify OTR. In this work, a gas mass balancing technique is used to determine OTR (see below), which, along with DO measurements and knowledge of C*1 , is used to identify KLaO2. In biological systems, the true saturation concentration (C*1 ) is difficult to quantify. This problem can be overcome with measurement of C*1 p and the exogenous oxygen uptake rate (calculated by subtracting the OTR caused by endogenous activity from the total OTR). OTRexog =KL aO2 (C*1 pxCL )
(6)
Materials and methods Experimental design
Activated sludge from a local domestic wastewater treatment plant was used for the batch experiments. The sludge was aerated prior to the tests to ensure the absence of readily biodegradable material. For each of the non-steady state experiments, the objective was to cause a change in the DO and to then record the DO concentration as the system returned to steady state. For the off-gas experiments, the objective was to record off-gas data and DO during a period of constant biological activity. The experiments conducted were for KLaO2 calculation from data obtained from: a gas mass balance during constant exogenous activity due to ammonia oxidation, and DO data recorded after a perturbation caused by (i) exogenous activity on a readily biodegradable compound (acetate, ammonia, nitrite, and ethanol), (ii) H2O2 addition, and (iii) stopping aeration and bubbling with helium. For the ammonia experiment samples were taken prior to and during the DO recovery period for analysis of the concentration of readily biodegradable substrate. This data was used to investigate if the soluble substrate was exhausted at the time suspected.
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No temperature correction was applied to KLaO2 as the comparisons were made between consecutive tests at the same temperature. Experimental set-up
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The tests to determine KLaO2 were carried out in a custom made perspex cylindrical reactor. The base diameter of the reactor was 115 mm. The working volume was 3 L. Gas was fed to the vessel through an aluminium porous plate (Metapor, Portec LTD, Switzerland). The inlet gas stream was the resultant of a helium stream (controlled using a mass flow controller) and an oxygen/argon stream (controlled using a mass flow controller). The total flow into the reactor was calculated as the sum of the two stream flowrates. Dissolved oxygen (DO) was measured using a polarographic electrode (YSI model 5739, Yellow Springs, USA). The DO readings were automatically logged every eight seconds. An off-gas arrangement (see Pratt et al. (2003) for full description) was used to determine the oxygen transfer rate (OTR), which was required for determination of KLaO2 by the gas mass balancing technique. The item of equipment central to the off-gas arrangement was a quadrupole mass spectrometer (OmniStar, Balzers AG, Liechtenstein). Data analysis
The non-steady state method is based on a nonlinear regression of the model (equation (4)) through the DO versus time data. The best estimates of the parameters KLaO2, C*1 p, and C0 are selected as the values that drive the model equation through the prepared DO concentration-versus-time data points with a minimum residual sum of squares. A residual refers to the difference in concentration between the DO value at a given time and the DO value predicted by the model at the same time. The ‘solver’ tool in MS-Excel was used to determine the parameter values, and model fits in AQUASIM were used to estimate the standard errors. For the gas mass balancing method, KLaO2 was found using CL and OTR data, measured over a 15 minute steady state period, and C*1 p data, determined during endogenous respiration. Eqn. (6) and the “data analysis” tool in MS-Excel were used to determine KLaO2 and estimate standard error. Results and discussion Measured and modelled data
Figure 1 shows the measured DO profile for a typical non-steady state experiment. Shortly after time 0, readily biodegradable substrate was added to the system. The biomass then fed on that substrate causing a sharp drop in DO. Once the substrate had been exhausted, the DO was able to recover. The data recorded during the recovery period was used to determine the model (equation (4)) parameters. Figure 1 also shows simulation of the model, using the identified parameter values. The DO values recorded immediately after apparent substrate exhaustion were not used for the parameter estimation exercise. Figure 2 shows the typical DO and oxygen uptake rate (OUR) profiles required for the gas mass balance method. The figure shows that the concentration of ammonia in the system approaches zero prior to the period of DO recovery. Comparison of methods for determination of KLaO2
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Figure 3 shows the KLaO2 values found using the various methods. It can be seen that the coefficient determined using the gas mass balancing method (0.25 minx1) was significantly higher than that found using the non-steady state methods. In a similar study reported by Capela et al. (1999), it was also found that oxygen transfer coefficients obtained by
8 7 6 DO (mg/L)
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Figure 1 DO recovery after exogenous activity on ethanol. [Measured data (- -) and regression fit ()]
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Figure 2 Measured OUR, DO data for calculation of KLaO2 by both a non-steady state and gas mass balancing method. Ammonia data (m) measured off-line shows its removal
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Figure 3 Comparison of KLaO2 determined by various methods (standard errors included)
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the off-gas methods were higher than those obtained by non-steady state, liquid phase methods. Also, it can be seen that the non-steady state methods did not reveal a single KLaO2 value, rather values ranging from 0.22 minx1 (using the H2O2 addition method) down to only 0.05 minx1 (using the exogenous activity on acetate method). This finding was similar to that reported by Capela et al. (1999) where it was shown that the KLaO2 values obtained by non-steady state methods (stopping aeration and H2O2 addition) varied by up to 43%. The likely reasons for the large variation found in this study are presented later in the paper. As was found in this work, Capela et al. (1999) reported that the H2O2 addition method gave the closest match to the off-gas method. The figure clearly shows that for the system tested, the acetate method resulted in the lowest KLaO2 value. The possible reasons for this are discussed later in the paper. Table 1 shows the KLaO2 values found using the non-steady state methods, along with both the observed maximum perturbation in the DO signal and the estimated saturation DO concentration. The table shows that there is no correlation between the magnitude of the perturbation and the value of the resulting transfer coefficient. KLaO2 determination by non-steady state methods
Each of the non-steady state methods trialled in this study relies on the assumption that biological activity in the DO recovery period remains constant (i.e. the rate of endogenous activity). It is suspected that in reality, this may not be the case, especially for the methods whereby perturbation is caused by exogenous activity on readily biodegradable substrate. Figure 4 confirms significant exogenous activity during DO recovery for the acetate experiment. While acetate is invariably quickly exhausted in heterotrophic sludges there is potential for some substrate to be converted to carbon storage polymers and not immediately oxidised. The formation of storage polymers from acetate has often been observed (Beun et al., 2000). These storage polymers may then act as a substrate during the DO recovery period. It is likely that similar, albeit less pronounced, storage phenomena affected the DO recovery period for the ethanol experiment. Of the experiments conducted using readily biodegradable substrates, ethanol gave the highest KLaO2 value, indicating that for the ethanol experiments, the rate of biological activity in the DO recovery period was close to the rate of endogenous activity. There is also potential for significant exogenous activity during DO recovery for the ammonia experiment. Nitrite accumulation has often been observed in activated sludge Table 1 KLaO2 calculated after perturbation Method
Perturbation caused by exogenous activity on Acetate a Ethanol a Acetate* Ethanol* Ammonia Nitrite Perturbation caused by stopping aeration Perturbation caused by H2O2 addition
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KLaO2 (min–1)
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C*¥ p (mg/L)
0.05 0.19 0.10 0.21 0.16 0.15 0.16 0.22
3.4 3.4 3.9 4.0 2.8 0.8 2.5 3.5
3.5 3.4 6.0 6.6 3.6 3.6 3.8 3.4
For both Figure 3 and Table 1: Gas flow: 680 ml/min * Gas flow: 660 ml/min with a higher oxygen fraction causing a higher C*1 p a DO reached 0 mg/L during exogenous activity. Gas mass balance performed during a period of constant biological activity on ammonia
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exogenous activity during DO recovery
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Figure 4 Exogenous activity during DO recovery. Acetate added at time 0
systems (Gapes et al., 2003). As such, it is possible that oxidation of nitrite will continue well after ammonia removal. The DO recovery curve after ammonia removal shown in Figure 5, which was recorded in an experiment with another sludge, displays an obvious inflection point. It is likely caused by nitrite accumulation. Unfortunately, NO2 was not measured during this experiment. It is suggested that even in cases where such inflection points are not obvious, some exogenous activity may continue. Vanrolleghem (1994) suggested a data screening method to avoid the use of inappropriate DO data points (or the entire data set) for KLaO2 estimation. The method involves adjusting the starting point of the period over which the DO data is used for KLaO2 estimation, with the aim to produce a satisfactory fit between the model-predicted DO profile and the measured one over this period. If such a starting point is not found, which would likely happen with the data sets presented in Figure 4 and Figure 5, the data set would be considered inappropriate for KLaO2 estimation. The non-steady state experiments not considered to this point are the stopping aeration and the H2O2 addition experiments. The reasons for the reduced KLaO2 obtained for these experiments are not so evident. However, Gogate and Pandit (1999) reported that bringing the system to a new steady state by changing the oxygen concentration (in this case turning
DO (mg/L)
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Time (minutes) Figure 5 DO recovery profiles resulting from an ammonia addition, showing the result of exogenous activity during DO recovery. Point (A) shows a point of inflection
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on the O2 supply) can considerably lower the measured/observed value of the mass transfer coefficient because the change in gas flow rate will affect the gas hold-up. And for the H2O2 addition experiment the significant increase in DO could have caused an increase in the rate of endogenous respiration, which would result in a violation of the assumption that biological activity in the DO recovery period remains constant. S. Pratt et al.
Which method is best?
It is difficult to identify the best method as the true KLaO2 for the systems was not known. However, it is reasonable to believe that the off-gas method produces KLaO2 values that are closer to the true one. Also, the method can be carried out while biological activity takes place, making the method ideal for in-process conditions. The standard errors associated with all of the methods were found to be very low. However, as the OTR signal used for KLaO2 determination (Figure 2) is quite noisy, the standard error on the KLaO2 estimated by the gas mass balancing method (0.005 on a KLaO2 of 0.25) is significantly higher than the standard errors produced by the non steady state methods (typically 0.0005 on a KLaO2 of 0.02). Standard error is a function of the number of measured data points, so can be reduced by increasing measurements. For the gas mass balancing method, this would be achieved by extending the measurement period over which the exogenous activity remains constant. With the off-gas value as a reference, it is apparent that the methods that relied on DO perturbation all led to relatively low KLaO2 values. It has been argued that, in most cases, this could have been caused by the inaccurate assumption that biological activity in the DO recovery period remains constant. However, it must be recognised that these methods are relatively simple to implement. The H2O2 addition method has been found to produce a KLaO2 value that is close to that of the off-gas method. For this reason, along with the fact that the method can be used in a low DO environment, the H2O2 addition method is apparently an attractive option (TusseauVuillemin et al., 2002). However, while Tusseau-Vuillemin et al. (2002) found H2O2 to be harmless to heterotrophic organisms, H2O2 should be used with caution as its presence has been shown to be potentially detrimental to autotrophic biomass (Cole et al., 2002). The substrate addition methods may also be used when the requirement on accuracy is not high. However, the substrate needs to be selected carefully. The most suitable substrate is one that causes the least exogenous oxygen uptake during DO recovery. In this regard, acetate is typically not a suitable substrate due to storage product formation. In this study, ethanol was found to yield a KLaO2 value that is similar to that of the H2O2 method. However, caution is required to draw a general conclusion that ethanol is suitable substrate as storage polymer may well be produced with a different sludge (Beccari et al., 2002). Ammonia may be used when no nitrite accumulation occurs, which requires substantiation. Nitrite is typically unsuitable due to the fact that it often causes a small variation in DO, leading to unreliable estimates. Conclusion
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Non-steady state methods that involve inducing perturbations by exogenous activity appear susceptible to continued exogenous activity in the DO recovery period. However, it was observed that DO recovery after exogenous activity on ethanol was relatively unaffected. Non-steady state methods that involve inducing perturbations by addition of H2O2 result in KLaO2 values that are close to those obtained using the off-gas method. A gas mass balancing technique is robust to unexpected exogenous activity, likely yielding more accurate KLaO2 values.
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