Elsevier

Water Research

Volume 44, Issue 11, June 2010, Pages 3434-3444
Water Research

Monitoring off-gas O2/CO2 to predict nitrification performance in activated sludge processes

https://doi.org/10.1016/j.watres.2010.03.022Get rights and content

Abstract

Nitrification/denitrification (NDN) processes are the most widely used technique to remove nitrogenous pollutants from municipal wastewater. The performance of nitrogen removal in the NDN process depends on the metabolism of nitrifying bacteria, and is dependent on adequate oxygen supply. Off-gas testing is a convenient and popular method for measuring oxygen transfer efficiency (OTE) under process conditions and can be performed in real-time. Since carbon dioxide is produced by carbonaceous oxidizing organism and not by nitrifiers, it should be possible to use the off-gas carbon dioxide mole fraction to estimate nitrification performance independently of the oxygen uptake rate (OUR) or OTE. This paper used off-gas data with a dynamic model to estimate nitrifying efficiency for various activated sludge process conditions. The relationship among nitrification, oxygen transfer, carbon dioxide production, and pH change was investigated. Experimental results of an online off-gas monitoring for a full-scale treatment plant were used to validate the model. The results showed measurable differences in OUR and carbon dioxide transfer rate (CTR) and the simulations successfully predicted the effluent ammonia by using the measured CO2 and O2 contents in off-gas as input signal. Carbon dioxide in the off-gas could be a useful technique to control aeration and to monitor nitrification rate.

Introduction

Nitrification/denitrification (NDN) in the activated sludge process (ASP) is the state of the art technique to remove nitrogenous pollutants in municipal wastewater. The activated sludge process consists of many different types of active bacteria, often called “active mass” in structured ASP models (Clifft and Andrews, 1986), which can have a wide range of biodegradation mechanisms for different types of pollutants. Consumption of carbonaceous compounds and denitrification are generally performed by heterotrophic bacteria, and nitrification is performed by autotrophic bacteria. The performance of an ASP is highly dependent on the operating conditions. In aeration basins, many criteria must be carefully maintained to provide a suitable habitat for microorganisms, especially for nitrifiers, such as proper pH, temperature, (Painter, 1970, Painter and Loveless, 1983), adequate solids retention time (SRT, Poduska and Andrews, 1975), and sufficient dissolved oxygen (DO) concentration (Stenstrom and Poduska, 1980).

Nitrification failure can easily occur under low DO conditions, which are controlled by a number of factors, such as oxygen transfer efficiency (OTE) and the overall oxygen uptake rate (OUR, Stenstrom and Song, 1991). The off-gas test described by Redmon et al. (1983) can be used to estimate process water oxygen transfer status for aeration systems (Rosso et al., 2005), and has been widely used to test aeration basins under process conditions (American Society of Civil Engineering, ASCE, 1997). The off-gas analyzer and testing procedure can also be used to measure other gas fractions (nitrogen, carbon dioxide, water vapor, volatile organic chemicals), but it is commonly used to measure only oxygen mole fraction. The CO2 mole fraction can be easily measured if an additional analyzer, such as a CO2 absorption tube or infrared sensor is used.

Because the reaction by-products of carbonaceous and nitrogenous compounds are different, the relative amounts of CO2 produced and oxygen consumed can become the basis for a new method of analyzing nitrification rate: the molar fraction of carbon dioxide in the off-gas should be greater if nitrification is limited, or the fraction of nitrogenous compounds of the total oxygen demand is less.

The main problem of the proposed method is the “super-saturatation” of dissolved carbon dioxide due to change of pH. At normal pH, carbon dioxide concentration in gas phase is a function of dissolved CO2 concentration, influent alkalinity, and pH. When pH changes, dissolved carbon dioxide acts as a buffer and shifts the fraction between carbonic acid (H2CO3) and bicarbonate (HCO3) to consume or release hydrogen ions. Since CO2 transfer relates to the concentration of carbonic acid, increasing the fraction of bicarbonate creates a supersaturated condition until the dissolved CO2 can be stripped. Stripping of CO2 also leads to an increase in pH, which shifts the equilibrium towards bicarbonate and similar to the response of receiving high alkalinity influents. Hellinga et al. (1996) calculated the ratio of carbon dioxide production rate (CPR) and oxygen uptake rate (OUR) for different wastewater compositions. The authors demonstrated that off-gas measurements are useful for evaluating the reactivity of carbonaceous substrate (i.e. COD/TOC ratio), because the changes of CPR are small due the buffering capacity of bicarbonate equilibrium. Similar discussions were also presented by others (Nogita et al., 1982, Minkevich and Neubert, 1985, Spérandio and Paul, 1997).

Experimental and modeling studies have evaluated or simulated the supersaturated conditions of dissolved CO2 in bioreactors. Pratt et al., 2003, Pratt et al., 2004 used a titrimetric technique with off-gas analysis, called on-line titrimetric and off-gas analysis (TOGA), to calculate the effect of changing pH in batch systems. Hydrogen ion production rate (HPR) was measured by monitoring the shift of pH due to in-situ titration, and CPR was monitored in the off-gas using a mass spectrometer. By knowing HPR and CPR, the transfer rates of oxygen, nitrogen and carbon dioxide were calculated from stoichiometry. Weissenbacher et al. (2006) proposed a simplified model to take account the effect of pH shift or change in alkalinity to off-gas CO2 mole fraction. Instead of simulating alkalinity and pH shift, the authors used on-line pH measurements as input signals to experimentally evaluate CPR. This modeling approach is similar to what is used in this paper.

Goals of this paper are to understand the relationships of CO2 and O2 in off-gas emissions and to develop a method for estimating nitrification efficiency of a full-scale ASP bioreactor based upon off-gas O2/CO2 monitoring. A mathematical model was built to simulate the temporal concentrations of the major components in wastewater and off-gas, and results of field measurements were applied to validate the model. Online off-gas tests were performed in a full-scale wastewater treatment plant, and the relationships between oxygen uptake/transfer, carbon dioxide production/transfer, change of pH, and nitrification performance were investigated. The model was used to create a profile between off-gas O2/CO2 and reaction status of carbonaceous substrate/ammonia, in order to study the effects of changing pH to CO2 stripping, and to simulate/predict ammonia discharge.

Section snippets

Off-gas test

A floating hood on the surface of the aeration basinas was used to collect off-gas and the mole fractions of oxygen and carbon dioxide were measured. In the ASCE (1997) Standard Guidelines for Process Water Testing, oxygen transfer efficiency (OTE, %) is calculated by comparing the oxygen content in the supplied air and the off-gas. If clean water transfer efficiency is known, the alpha factor (α) can be calculated as the ratio of the OTE in process water, adjusted to standard conditions, to

Primary analyses of off-gas data

Table 3 shows the operating conditions and the average measurement data during the four testing periods. The average DO and off-gas data are relatively constant during the year, but the solids retention time (SRT) was changed in response to water temperature changes. SRT increased from approximately 8 days in summer to 15.5 days in winter to provide better nitrification. Fig. 2 shows the results of online off-gas monitoring and nitrification conditions. Examples of two testing periods are

Conclusions

  • -

    This paper presented a strategy for using online off-gas O2 and CO2 monitoring to predict nitrification performance. The ratio of carbon dioxide transfer rate (CTR) to oxygen transfer rate (OTR) can be used to assess the relative rates of the heterotrophic and nitrifier communities. The ratio of the CTR to the OTR decreased during ammonia break through periods.

  • -

    The simulation results showed that rapid change of pH may significantly affect the accuracy of CTR estimation; but in a well controlled

Acknowledgements

This research was partially supported by the California Energy Commission and Southern California Edison Inc., a power company in California, U.S.A. The authors thank Anne Schuchardt and Chris Sahlmann for the data collection and water quality analysis.

Shao-Yuan Leu was a Ph.D. student at the time of the research, and is now a post-doctroal scholar and Michael K. Stenstrom is a professor in the Civil Engineering Department at the University of California, Los Angeles. Judy A. Libra is a

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