The past few months have been difficult for some, if not most of us. The slow quite time of winter that we experience in our little town has now stretched into June. Our shops and retailers find themselves behind where we feel we should be for our up and coming summer shopping season.
With the lack of our normal numbers of spring shoppers, also comes the lack of the normal spring flow in gallons that we see here at the wastewater treatment plant. This may sound like a good thing, but it comes with its own challenges for me.
One of the tools that an operator uses is something called and F to M ratio (See Below). I try to key into a very specific balance of “bugs” to gallons of wastewater that I receive daily. Too many “bugs” equals problems, and not enough “bugs” lead to other problems. So, trying to run the treatment plant like years past just will not work. But like everyone else, we are doing what we need to do to get through this while staying in compliance with our permit, and the state.
You may have seen us along the roads throughout town pulling pumps at our lift stations. The toilet paper shortage that America experienced did not go unnoticed here at Shipshewana. Some people have resorted to using “flushable wipes”. Please know that even though the package may say flushable, they are not. They do not break down like toilet paper. However, what they do is collect in masses and clog our lift station pumps. This leads to pump failures, sewer backups, and flooded basements. Please only flush poop, pee, and toilet paper. Flushable wipes and disinfectant wipe belong in the trash can, not in our sewers.
With the slow ease of restrictions, we need to keep in mind that summer is heading our way quickly, and with-it come’s longer days, warmer weather, and summer shoppers. I know that the people of our town will be ready for it. I know that we here at the treatment plant will be too.
Food-to-Microorganism Ratio(F/M Ratio)
Food-to-Microorganism ratio (F/M ratio)
The food-to-microorganism ratio (F/M ratio) is a process control method/calculation based upon maintaining a specified balance between available food materials (BOD or COD) in the aeration tank influent and the aeration tank mixed liquor volatile suspended solids (MLVSS) concentration The chemical oxygen demand test is sometimes used, because the results are available in a relatively short period of time. To calculate the F/M ratio, the following information is required:
- Aeration tank influent flow rate (MGD)
- Aeration tank influent BOD or COD (mg/L)
- Aeration tank MLVSS (mg/L)
- Aeration tank volume (MG)
F/M ratio= primary effluent COD/BOD (mg/L) x Flow (MGD) x 8.34 lb/mg/L/MG /MLVSS (mg/L) x aerator volume (MG) x 8.34 lb/mg/L/MG
The aeration tank influent BOD is 145 mg/L, and the aeration tank influent flow rate is 1.6 MGD. What is the F/M ratio if the MLVSS is 2300 mg/L and the aeration tank volume is 1.8 MG?
F / M ratio= 145mg/L x1.6 MGD x 8.34 lb/mg/L /MG
2300 mg /L x1.8 MG x 8.34 lb/mg/L/MG
= 0.06 BOD (lb) / MLVSS (lb)
- Key Point:
If the MLVSS concentration is not available, it can be calculated if the percent volatile matter (%VM) of the mixed liquor suspended solids (MLSS) is known.
MLVSS = MLSS x % (decimal) volatile matter (VM)
- Key Point:
The food (F) value in the F/M ratio for computing loading to an activated biosolids process can be either BOD or COD. Remember, the reason for biosolids production in the activated biosolids process is to convert BOD to bacteria. One advantage of using COD over BOD for analysis of organic load is that COD is more accurate.