Monitoring off-gas O2/CO2 to predict nitrification performance in activated sludge processes
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
References (28)
- et al.
Activated sludge model no. 3
Water Sci. Technol.
(1999) - et al.
The potential of off-gas analyses for monitoring wastewater treatment plants
Water Sci. Technol.
(1996) - et al.
Substrate flux into storage and growth in relation to activated sludge modelling
Water Res.
(1999) - et al.
Basic studies on carbon dioxide/air control system for activated sludge processes
Water Res.
(1982) A review of literature on inorganic nitrogen metabolism in microorganisms
Water Res.
(1970)- et al.
The effect of dissolved oxygen concentration on nitrification
Water Res.
(1980) ASCE Standard Measurement of Oxygen Transfer in Clean Water
(2006)Standard Guidelines for In-Process Oxygen Transfer Testing
(1997)- et al.
Gas-liquid interactions in oxygen activated sludge
J. Environ. Eng.
(1986) - et al.
Relationship between oxygen transfer rate and airflow for fine-pore aeration under process conditions
Water Environ. Res.
(2001)
Real-time aeration efficiency monitoring in the activated sludge process and methods to reduce energy consumption and operating costs
Water Environ. Res.
Principles of gas absorption
Ind. Eng. Chem.
Influence of carbon dioxide solubility on the accuracy of measurements of carbon dioxide production rate by gas balance technique
Acta Biotechnol.
Cited by (18)
Nutrient and carbon fluxes in terrestrial agroecosystems
2023, Marschner's Mineral Nutrition of PlantsNitrogen recovery from wastewater as nitrate by coupling mainstream ammonium separation with side stream cyclic up-concentration and targeted conversion
2023, Chemical Engineering JournalCitation Excerpt :GHG produced from WWTPs including direct emissions (organic oxidation - CO2, nitrogen removal - CO2 and N2O) in biological processes and indirect emissions during electricity consumption [54]. Alkalinity in the influent CAIRM process (majorly Na2CO3, mainstream MBR and SNR) and oxic tank of AAO process (majorly NaHCO3) were both consumed in nitrification [55], emitting 7433 and 14143 kg/d CO2, respectively. According to the model proposed by Mannina, et al. [54], 1.74 % of the TN load in the influent of WWTP with AAO leads to the N2O emission (11440 kgCO2eq/d) in nitrification (0.12 %) and denitrification (1.62 %) process.
Potential of off-gas analyses for sequentially operated reactors demonstrated on full-scale aerobic granular sludge technology
2021, Science of the Total EnvironmentCitation Excerpt :To get an idea of the sensitivity of the carbon dioxide emission to variations in the influent organics composition, the theoretically expected values for oxidation of oleic acid (C18H34O2) and formate (CH2O2) were included in Fig. 5. This shows that off-gas analysis could give an indication of the carbon content of wastewater organics without calibration of an extensive bioconversion model (Leu et al., 2010): a batch with strongly deviating TOC/COD ratio could be noticed for example as a blue dot which lies far away from the bold line. By producing this graph with a long-term dataset, deviations from typical cycles could be detected for a specific plant.
Online measurement of CO<inf>2</inf> and total gas production in parallel anaerobic shake flask cultivations
2020, Biochemical Engineering JournalCitation Excerpt :The necessity of strictly oxygen-free conditions causes difficulties in the application of several online measurement techniques in anaerobic bioconversion processes [1–4], especially on the small-scale level [5].