Typical activated sludge operating ranges
Conventional activated sludge with nitrification. Your plant's best values come from its own data and design.
| Parameter | Typical range | Notes |
|---|
| MLSS | 1,500 to 4,000 mg/L | Higher for extended aeration |
| MLVSS/MLSS | 70 to 85% | Lower with chemical P removal or high inert solids |
| SRT, conventional (no nitrification) | 3 to 6 days | |
| SRT, nitrifying | 8 to 20 days | Longer in cold weather |
| F/M | 0.05 to 0.4 lb BOD/lb MLVSS/d | Lower values for nitrifying and extended aeration |
| DO in aerobic zones | 1.5 to 2.5 mg/L | 2.0 mg/L is a common nitrification setpoint |
| SVI | 80 to 150 mL/g | Above 150 often indicates bulking |
| RAS rate | 50 to 100% of Q | Higher during peak flow if clarifiers need it |
| Internal recycle (MLE) | 200 to 400% of Q | |
| Secondary clarifier SOR | 400 to 800 gpd/ft² average | 1,000 to 1,200 peak hour |
| Secondary clarifier SLR | 20 to 30 lb/d/ft² average | 40 to 50 peak |
| Sludge blanket | 1 to 3 ft | |
| pH | 6.8 to 7.5 | Nitrification slows below about 6.8 |
| Residual alkalinity | 50 to 100 mg/L as CaCO3 | Keep at least 50 for stable nitrification |
Dissolved oxygen saturation in clean water at sea level
| Temperature | DO saturation |
|---|
| 0 °C (32 °F) | 14.6 mg/L |
| 5 °C (41 °F) | 12.8 mg/L |
| 10 °C (50 °F) | 11.3 mg/L |
| 15 °C (59 °F) | 10.1 mg/L |
| 20 °C (68 °F) | 9.1 mg/L |
| 25 °C (77 °F) | 8.3 mg/L |
| 30 °C (86 °F) | 7.6 mg/L |
| 35 °C (95 °F) | 7.0 mg/L |
Saturation drops about 3% per 1,000 ft of elevation.
Stoichiometric factors
| Item | Factor |
|---|
| Alkalinity consumed by nitrification | 7.14 mg/L as CaCO3 per mg/L NH3-N oxidized |
| Alkalinity recovered by denitrification | 3.57 mg/L as CaCO3 per mg/L NO3-N reduced |
| Oxygen for nitrification | 4.57 lb O2 per lb NH3-N oxidized |
| Oxygen credit from denitrification | 2.86 lb O2 per lb NO3-N reduced |
| Oxygen for BOD removal | about 1.1 lb O2 per lb BOD removed (varies 0.9 to 1.3 with SRT) |
| Alkalinity used by alum | about 0.5 mg/L per mg/L dry alum |
| Alkalinity used by ferric chloride | about 0.9 mg/L per mg/L FeCl3 |
| Lime (as CaCO3 equivalent) | 1 mg/L Ca(OH)2 adds about 1.35 mg/L alkalinity |
| Sodium hydroxide | 1 mg/L NaOH adds about 1.25 mg/L alkalinity |
| Sodium bicarbonate | 1 mg/L NaHCO3 adds about 0.6 mg/L alkalinity |
Conversion factors
| From | To |
|---|
| 1 MGD | 694.4 gpm, 1.547 cfs, 3,785 m³/d |
| 1 ft³ | 7.48 gal |
| 1 gal water | 8.34 lb |
| 1 psi | 2.31 ft of water |
| 1 hp | 0.746 kW |
| 1 mg/L | 8.34 lb/MG |
| 1 lb | 0.4536 kg |
| 1 ft | 0.3048 m |
| 1 acre | 43,560 ft² |
Circular clarifier surface areas
| Diameter | Area (US) | Area (metric) |
|---|
| 30 ft | 707 ft² | 65.7 m² |
| 40 ft | 1,257 ft² | 116.7 m² |
| 50 ft | 1,963 ft² | 182.4 m² |
| 60 ft | 2,827 ft² | 262.7 m² |
| 65 ft | 3,318 ft² | 308.3 m² |
| 70 ft | 3,848 ft² | 357.5 m² |
| 80 ft | 5,027 ft² | 467.0 m² |
| 90 ft | 6,362 ft² | 591.0 m² |
| 100 ft | 7,854 ft² | 729.7 m² |
| 110 ft | 9,503 ft² | 882.9 m² |
| 120 ft | 11,310 ft² | 1,050.7 m² |
| 130 ft | 13,273 ft² | 1,233.1 m² |
| 150 ft | 17,671 ft² | 1,641.7 m² |
Filament indicators (common associations)
| Condition | Filaments often associated |
|---|
| Low DO | Sphaerotilus natans, type 1701, Haliscomenobacter hydrossis |
| Low F/M, long SRT | Microthrix parvicella, type 0041, type 0675, type 1851, type 0092 |
| Septicity, sulfides | Thiothrix, Beggiatoa, type 021N |
| Nutrient deficiency | Thiothrix, type 021N, Sphaerotilus |
| Fats, oils and grease | Nocardioforms, Microthrix parvicella |
| Low pH (below 6.5) | Fungi |
These are tendencies, not rules. Confirm the cause with process data.
| Dominant organisms | Usually suggests |
|---|
| Amoebae, flagellates | Young sludge, startup, high F/M, or recovery from upset |
| Free swimming ciliates | Moderate sludge age, improving |
| Stalked ciliates, crawling ciliates | Stable, well operated process |
| Rotifers | Older, stable sludge |
| Nematodes, water bears, worms | Very old sludge, long SRT |
