Ensuring an adequate supply of "food" to microorganisms helps maintain the health of biological treatment systems. This is typically achieved using the "food-to-microorganism ratio" or "F:M ratio." When the F:M ratio is too low, there is insufficient "food," and the microorganisms will "starve." If the F:M ratio is too high, the high organic matter content in the wastewater will quickly overwhelm the microorganisms, leading to inadequate removal of pollutants. Both situations result in low biological treatment efficiency; therefore, it is necessary to find and maintain an optimal F:M balance to ensure sufficient pollutant removal to meet regulatory discharge requirements.
The F:M ratio is typically determined by two common assays. In the F:M ratio parameter, the F (food) component represents the organic pollutant content, generally measured using biochemical oxygen demand (BOD5). A 5-day test is used to detect the oxygen consumed when bacteria decompose organic matter, indirectly inferring the carbon content in the water. In the F:M ratio, the M (microorganisms) component is generally measured using mixed liquor suspended solids (MLSS). These assays have several limitations that make F:M unsuitable for effective process control. MLSS assays, used to quantify microbial levels, cannot distinguish between live and dead biomass, making it very difficult to maintain an optimal F:M ratio and unreliable for understanding the overall health of the biological system. A 5-day BOD assay is too slow for process decision-making. By the time a wastewater treatment plant detects a carbon load imbalance, the unhealthy state of the biomass has often persisted for several days. This is particularly problematic for treatment plants with variable wastewater loads. Furthermore, because BOD5 depends on bacterial usage, it lacks acceptable accuracy and precision, and the presence of toxic compounds in the sample can severely interfere with the test results.
A more accurate and effective method for monitoring the "food" of biomass is to directly determine the carbon content in wastewater using Total Organic Carbon (TOC) analysis. Unlike indirect BOD measurement, TOC analyzers directly detect the carbon content in samples. This results in more accurate detection and eliminates the interference problems common in BOD testing. TOC testing can be completed within minutes, making it a more effective tool for process control and treatment optimization. By using TOC analysis to detect the organic load in biological treatment, the treatment unit can determine a "more accurate F:M ratio."
Case 1
A large U.S. refinery conducted a 12-month study analyzing the advantages of using TOC analysis to determine the "true F:M ratio" in a conventional activated sludge biological treatment unit. By using the rapid and accurate results obtained through TOC analysis, the treatment unit was able to quickly identify changes in organic load and determine the ideal F:M balance. The plant concluded that when the treatment unit operated within acceptable limits, removal efficiency was very stable, and TOC analysis was a more effective tool for maintaining F:M balance compared to typical aerobic demand (ADO) testing. Furthermore, the continuous online data provided by TOC analysis allowed the treatment unit to quickly adjust flow rates and achieve more effective process control by ensuring an adequate supply of "food" for using the F:M ratio. This reduced process turbulence inherent in biological treatment and ultimately saved time and costs associated with poor microbial health.
Case 2
Besides the F:M ratio, TOC analysis has become a useful tool for optimizing the nutrient balance in wastewater biological treatment. Many treatment plants require a proper balance between carbon content and nutrients (typically nitrogen and phosphorus) in wastewater. A large beverage plant in the United States decided to upgrade its traditional biological treatment system to a high-flow-rate membrane bioreactor (MBR) system. While this helped reduce the plant's footprint and improve the removal of organic matter from wastewater, the high flow rate of the new MBR system and the fluctuating sugar load in the plant's effluent meant that oxygen demand (OD) testing was too slow to ensure the nutrient balance of the biological treatment system. The plant required a C:N:P nutrient balance ratio of 100:5:1. After adding TOC analysis, the plant was able to track changes in the organic matter content in the wastewater and make rapid process adjustments. Plant operators were able to determine the carbon content and adjust the nitrogen added to the wastewater to maintain optimal nutrient balance. The new process can continuously remove organic matter, significantly reducing process turbulence and saving the plant hundreds of thousands of dollars annually.
Traditionally, biochemical oxygen demand (BOD) has been used to determine the effectiveness of wastewater treatment.
Total Organic Carbon (TOC) analysis, which directly monitors carbon content, can be a powerful tool for wastewater treatment plants attempting to optimize biological treatment processes. Unlike traditional oxygen demand (OCD) testing, TOC analysis provides accurate data within minutes, enabling operators to make rapid control decisions. Maintaining an effective F:M ratio or C:N:P nutrient balance using TOC data ensures the optimization of biological treatment processes. Monitoring the health of bioreactors through TOC analysis helps plants minimize process turbulence, effectively remove contaminants, and achieve compliant effluent discharge.
