Determination of activated sludge biological activity using model corrected CO2 off-gas data

Area of Science:

• Environmental Engineering
• Environmental Microbiology
• Water Treatment Technology

Background:

• Online carbon dioxide (CO2) off-gas monitoring is valuable for assessing biological activity in activated sludge systems, particularly under oxygen-limited conditions such as simultaneous nitrification-denitrification (SND).
• Previous limitations in using off-gas CO2 monitoring were due to the complex influence of the bicarbonate system on CO2 liquid-gas transfer.
• Respirometric measurements are often inapplicable in systems like SND, highlighting the need for alternative monitoring methods.

Purpose of the Study:

• To present a practical method for correcting measured off-gas CO2 concentrations to accurately reflect biological activity in wastewater treatment.
• To account for pH variations and influent alkalinity changes that affect the bicarbonate system and CO2 transfer.
• To enable wider application of off-gas CO2 monitoring as a reliable indicator of biological processes.

Main Methods:

• Development of a simple model based on the physicochemical equilibrium of the bicarbonate/CO2 system and liquid-gas mass transfer in aerated systems.
• Utilizing standard online measurements (pH, temperature, flow rates) and periodic alkalinity titrations as input data.
• Estimating CO2 mass transfer coefficients specific to the plant and calculating inorganic CO2 transfer rates to correct measured CO2 concentrations.

Main Results:

• The developed model effectively corrects measured off-gas CO2 concentrations by considering pH shifts and alkalinity variations.
• The method allows for the estimation of the biological carbon dioxide production rate (CPR), indicative of heterotrophic activity.
• The model differentiates between biological CO2 production and inorganic CO2 transfer, providing a more accurate assessment of microbial activity.

Conclusions:

• The proposed method provides a practical and reliable approach to using off-gas CO2 monitoring for assessing biological activity in activated sludge systems.
• Accurate correction for the bicarbonate system's influence overcomes previous limitations, expanding the utility of CO2 off-gas analysis.
• This technique is particularly beneficial for monitoring under challenging conditions like simultaneous nitrification-denitrification (SND) where traditional methods are insufficient.