Calculators
Process Control
The daily activated sludge numbers in one place: F/M, MCRT, SVI and the WAS rate that holds a target sludge age. Your numbers are worked through underneath.
Your numbers, step by step
- Food: pounds of BOD entering aeration140 mg/L × 2.5 MGD × 8.34 = 2,919 lb/day
- Microorganisms: pounds of MLVSS2,100 mg/L × 0.75 MG × 8.34 = 13,136 lb
- F/M ratio2,919 ÷ 13,136 = 0.22
Why it works
Every number here except SVI comes from the same pounds formula used in the Davidson pie chart: concentration in mg/L × volume or flow in millions of gallons × 8.34. A tank volume gives pounds sitting in the tank; a flow gives pounds moving per day. The process control numbers are ratios of those pounds.
- F/M compares the food arriving each day with the bugs available to eat it. More food per pound of bugs means a younger, faster growing, poorer settling biomass.
- MCRT divides the solids you are holding by the solids leaving each day, through the WAS line and over the weirs. The answer is the average number of days a floc particle stays in the system.
- WAS rate runs MCRT backwards. Pick the sludge age you want, work out how many pounds must leave each day to hold it, subtract what the effluent already carries out, and waste the rest.
- SVI is a settling test, not a pounds problem. It asks how much space one gram of sludge takes up after 30 minutes. The × 1,000 turns milligrams into grams.
The MCRT formula matches the one on the formula sheet, including clarifier solids. If you leave those blank you get the simpler aeration-only version many plants use for daily control.
Example: sludge age is drifting short
A 2.5 MGD plant has 0.75 MG of aeration at 2,800 mg/L MLSS. The crew has been wasting 30,000 gal/day of 8,000 mg/L return sludge, and effluent TSS is running 12 mg/L. Ammonia has started creeping up, and nitrifiers need age, so the goal is 9 days.
- 2,800 × 0.75 × 8.34 = 17,514 lb under aeration
- 8,000 × 0.03 × 8.34 = 2,002 lb/day wasted
- 12 × 2.5 × 8.34 = 250.2 lb/day lost over the weirs
- 17,514 ÷ (2,002 + 250.2) = 7.8 days MCRT today
- 17,514 ÷ 9 = 1,946 lb/day may leave at a 9-day MCRT
- 1,946 − 250.2 = 1,696 lb/day to waste
- 1,696 ÷ (8,000 × 8.34) = 0.0254 MGD, about 25,400 gal/day or 17.7 gpm
That is a cut of about 4,600 gal/day. Make it in steps over several days rather than all at once, and watch the blanket depth as the inventory builds.
Do the numbers look reasonable?
Your plant's normal range is the one that matters: the values it runs at when the effluent is good. Use these broad ranges to catch a typo or a units slip, not to set targets.
| Number | Conventional | Extended aeration | If it is far off |
|---|---|---|---|
| F/M | About 0.2–0.5 | About 0.05–0.15 | Check that BOD and MLVSS are mg/L and volume is MG, not gallons. |
| MCRT | About 5–15 days | About 20–30 days or more | A WAS flow entered in gpm but labeled gal/day will be off by 1,440×. |
| SVI | About 80–150 mL/g settles well | Above ~150 suggests bulking. If SSV30 reads above ~800 mL/L, dilute the sample and retest. | |
| MLVSS ÷ MLSS | About 0.65–0.85 | Low values point to grit or inert solids building up. | |
Before you change the wasting
- Trend it. One day's MCRT bounces with every lab result. Steer by a 3- to 7-day rolling average.
- Move slowly. Change the WAS rate by no more than about 10 to 15% a day, then give the system a sludge age or so to respond.
- Same-day samples. The WAS concentration, MLSS and effluent TSS should come from the same day. A stale RAS number is the most common reason the math and the tanks disagree.
- Know your BOD sample point. F/M uses the load reaching aeration. Recycle streams from digesters and belt presses add load that a raw influent sample won't show.
- Watch the clarifier too. Holding more solids to raise MCRT only works if the clarifier can store them. Check blanket depth before and after.
This is a study and planning aid. Your plant's SOPs, permit and process engineer take priority over any number on this page.