Calculating the Food to Microorganism Ratio

The food to microorganism ratio (F/M) is a process control tool that is used to help optimize the wastewater treatment plant. It is a ratio of pounds of biochemical oxygen demand (BOD) entering the plant (F) to the pounds of mixed liquor volatile suspended solids in the aeration section of the plant (M). It is a baseline established to determine how much food a single pound of organisms will eat every day.

We use this number to help determine something about the bacteria’s growth rate, condition and what phase of respiration they are in. By using the chart below we can determine what phase of their life cycle they are in and how they can be optimized.

Lag-phase: During this phase bacteria becomes acclimated to their new surroundings. The bacteria that can adapt to the new variables (temperature, ph, BOD concentration etc.) survive. They are digesting food, developing enzymes and other things required for growth. This phase is near the influent portion of the plant.

Accelerated Growth-phase: The bacteria are dispersed and growing as fast as they can because of an excess of food. Normally this phase has a high F/M which indicates an increased usage of dissolved oxygen by the bacteria. As they use the BOD for reproduction and energy production, they use more dissolved oxygen.

Declining Growth-phase: Reproduction slows down because there is not an excess of food. A lot of food has been eaten and there are now a large number of bacteria to compete for remaining food. As a result, the bacteria do not have enough remaining food to keep the growth rate at a maximum.

The number of bacteria is the highest possible, but not much food is left, so the bacteria cannot increase in number. There is some reproduction, but some cells are also dying, so the number of bacteria remains relatively constant. The bacteria have now lost their flagella and have a sticky substance covering the outside of the cell, allowing them to agglomerate into floc. In fact, the floc gets big enough that if aeration and mixing were stopped, the floc could settle to the bottom. At this point the bacteria, (sludge) is allowed to settle in a clarifier and is either returned to the aeration section of the plant as return activated sludge or wasted to the digester or thickner for further treatment.

During this phase we normally maintain the recommended F/M of .05 to .15 pounds of influent BOD per pound of MLVSS in the aeration section of the treatment plant. The recommended range normally produces a well stabilized sludge, an acceptable effluent and the least amount of sludge production.

Death-phase: The death rate increases with very little if any growth occurring. Therefore, the total number of living bacteria keeps reducing. The bacteria are just trying to keep alive. This is the endogenous respiration phase where the bacteria are reducing the volume of sludge to be disposed of. This phase should be carried out in a thickner or a digester.

The food to microorganism ratio is only one tool that is used to help optimize the treatment plant. It should be used in combination with other process control tools (settlometer, sludge judge, organic loading rate, SVI) to determine the optimum amount of sludge to waste and the amount to return in order to produce the best effluent with the least amount of sludge to be disposed of.

If you would like to practice calculating Food to Microorganism ratio, check out Test Your Knowledge-Food to Microorganism Ratio.

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