Veolia Water Technologies & Solutions

How to Improve Process Control of Secondary Biological Treatment using Total Organic Carbon (TOC) Analysis

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In the wastewater treatment process, secondary biological treatment is used to remove dissolved organic matter using microorganisms. A common practice for determining the performance of this treatment is the Food to Microorganism ratio (F:M). To determine the food (F) portion of the ratio, total organic carbon (TOC) analysis can provide more accurate and reliable data than other methods to reduce risk of regulatory non-compliance and help optimize secondary biological treatment.

Secondary biological treatment is a critical step in wastewater treatment that uses microorganisms to remove organic and other nutrients (nitrogen and phosphorus). Optimal operation requires balance between microorganisms and the contamination (organic food) from the wastewater. Biological treatment can vary in implementation from one facility to another. Municipal wastewater facilities tend to have more consistent organic loading in their wastewater, whereas industrial wastewater tends to fluctuate more frequently and rapidly based on production activities. These fluctuations can be problematic for biological treatment because the microorganisms used in this process react slowly to loading changes and are easily overwhelmed. This can lead to a disruption in the treatment efficiency and removal of contaminants. To prevent treatment disruption and optimize process performance, operators can monitor their organic loading rates and microorganism population using the Food to Microorganism ratio (F:M). The F:M ratio is often used to control and manage aeration, equalization, return activated sludge and any microorganism maintenance.

The F:M ratio can be a beneficial parameter for wastewater operators because it provides a fundamental understanding of the balance between the organic loading in the wastewater and the population of the microorganisms available to use it as a fuel source. However, determining the appropriate ratio depends on adequate, reliable, and timely measurements for the F and M that achieves optimal contaminant removal and efficient operation.

The food portion (F) from wastewater loading of the F:M is typically determined by using Biological Oxygen Demand (BOD) or Chemical Oxygen Demand (COD). BOD and COD are analytical procedures that measure biological or chemical consumption of oxygen from a sample which correlates to the amount of organic matter present in the wastewater. The speed of these methods and their susceptibility to interference are not ideal for process optimization, whereas TOC offers many advantages. These will be discussed further in the next section.

The microorganism (M) portion or the biomass in the reactor is measured by mixed liquor suspended solids (MLSS) total solids or mixed liquor volatile suspended solids (MLVSS) volatile fraction of total solids also considered microbiological suspension. Operators calculate the MLSS by multiplying the concentration of MLSS (mg/L) to the aeration tank volume (MG) and water density. To calculate the MLVSS, the MLSS is multiplied by the % of volatile matter. While these methods may seem simple and straightforward, there are some drawbacks to these measurements that can make it difficult to rely on the F:M ratio for process optimization.

How TOC Improves Process Control

Operators cannot solely rely on using the F:M ratio with these traditional methods because of several disadvantages and interferences. The F portion can include compounds that will not be degraded easily by the microorganisms while the M portion assumes any solids are microorganisms (even the dead ones).  BOD takes up to 5 days to obtain results and are susceptible to interferences such as sanitizers, chlorine, and salts which can lead to inaccurate results. While COD takes 2-3 hours to obtain results, it is also susceptible to interferences including nitrite, ferric compounds, sulfide, and chlorides and uses toxic chemicals during analysis. In addition, oxygen demand by regulated methods is lab based which increases the time to data results.

These limitations make it difficult for operators to obtain accurate results which is especially problematic for facilities that frequently vary in organic loading. Using total organic carbon (TOC) to monitor organic content in wastewater is more efficient than BOD and COD because it directly quantifies the organic contamination (food) in wastewater loading. Additional advantages of using TOC over oxygen demand include capturing all organics completely, obtaining results in minutes rather than hours or days, achieving accuracy of 2-5% versus 10-20%, and avoiding interferences from possible co-contaminants in the wastewater feed. To determine the organic loading of a wastewater stream in real time, an online analyzer can be installed to observe changes in wastewater organic loading and make rapid decisions to optimize treatment efficiency. More accurate and reliable data from TOC monitoring can reduce risk of regulatory non-compliance and help optimize aeration. Similarly ATP offers significant advantages over MLSS or MLVSS for the microorganism (M) portion of the ratio by measuring the actual active biomass population. This can reduce site to site variability associated with suspended solids. Using TOC and ATP offer true, reliable, and adaptable F:M ratios that can offer clear optimum operating ranges. 

Biological treatment plays a critical step in removing organic compounds from wastewater and monitoring the F:M ratio helps ensure treatment efficiency. By monitoring food from wastewater loading using TOC and microorganisms in the bioreactor using ATP, operators can take action when there is a surge in organic loading and adjust parameters to optimize treatment using real-time data.


Author: Sara Speak

Sara Speak is a Product Application Specialist with Veolia Water Technologies & Solutions, providing support and application expertise to Sievers Analytical Instruments customers in industries such as chemical, petrochemical, food & beverage, and municipal wastewater. 

Sara works with customers to provide training, support product installations, optimize equipment use, and demo feasibility across different testing applications. Prior to her current role, she was a Factory Service Technician, responsible for repairing and troubleshooting Sievers instruments. Sara previously worked in the food and beverage industry as a QA Laboratory Technician at MillerCoors and Leprino Foods. She holds a Bachelor of Science (B.S.) in Chemistry and a Bachelor of Music (B.M.) in Violin Performance from Metropolitan State University of Denver.

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  5. MECC. Lesson 26: Activated Sludge Calculations. Accessed on 02/21/2024.


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