Ammonia Breakthrough After a Cold Snap at a Small Activated Sludge Plant
A 0.75 MGD extended-aeration plant serves a community with a year-round ammonia limit of 4.0 mg/L monthly average. Current flow is 0.55 MGD. The aeration basin volume is 0.50 MG, MLSS 2,500 mg/L, D.O. 2.0 mg/L. WAS is 20,000 gpd at 8,000 mg/L; effluent TSS averages 8 mg/L. Influent TKN is 45 mg/L with ammonia at 30 mg/L, alkalinity 180 mg/L as CaCO₃. After the first cold week (basin temperature 11°C), effluent ammonia climbs to 6–9 mg/L and stays there even after D.O. is raised to 3.0 mg/L. The operator has been increasing wasting to “clear the turbidity” caused by a rising sludge blanket.
1. Compute the current solids retention time and explain why the plant is failing in cold weather.
Read the worked answer
Solids inventory:
0.50 MG × 2,500 mg/L × 8.34 = 10,425 lb MLSS
Solids leaving the system:
- WAS: 0.020 MG × 8,000 × 8.34 = 1,334 lb/d
- Effluent: 0.55 MGD × 8 × 8.34 = 37 lb/d
- Total ≈ 1,371 lb/d
SRT = 10,425 / 1,371 ≈ 7.6 days
Why it fails: nitrifier growth rate collapses with temperature, while the solids inventory does not. At ~11°C the minimum SRT for reliable nitrification is roughly 8–10 days for a marginal system and designers typically hold 15–20 days to carry a safety factor against cold snaps and peak loads. At 7.6 days the plant has no margin and is at or below the washout threshold in cold weather, so nitrifiers are progressively lost in the WAS.
Raising D.O. to 3.0 mg/L does not help because oxygen is not the limiting factor - the population is being wasted out faster than it can grow. Increasing wasting is exactly the wrong move: the rising blanket is a symptom of solids overloading the clarifier or of a low-density sludge, not of too much biomass in the aeration basin, and every extra pound wasted removes nitrifiers.
2. What SRT should be targeted and what wasting rate achieves it? Then check whether the plant's chemistry and oxygen supply can support that target.
Read the worked answer
Target SRT = 15 days.
- Required solids out: 10,425 / 15 = 695 lb/d
- Less effluent solids (37 lb/d): WAS = 658 lb/d
- At 8,000 mg/L: 658 / (8.34 × 8,000) = 0.00986 MG ≈ 9,900 gpd
That is roughly half of the current 20,000 gpd - the wasting setpoint must be cut, not raised. Expect MLSS to rise to roughly 3,500–4,000 mg/L at the same inventory-plus-growth; confirm with weekly MLSS/30-min settle checks rather than setting and forgetting.
Alkalinity check - this is the second, hidden failure mode:
Nitrification consumes 7.14 mg alkalinity (as CaCO₃) per mg N oxidized. With effluent ammonia at 4 mg/L, about 26 mg/L is oxidized:
26 × 7.14 ≈ 186 mg/L as CaCO₃ required against 180 mg/L available.
The plant is right at the point where alkalinity, not biomass, limits nitrification; pH will sag toward 6.5 or below and stall the process. Provide supplemental alkalinity (soda ash, caustic, or lime) to hold 70–80 mg/L residual effluent alkalinity, or use the denitrification credit (~3.57 mg alkalinity per mg N denitrified) if an anoxic zone exists.
Oxygen check: carbonaceous plus nitrogenous demand at 4.57 mg O₂/mg N oxidized on 26 mg/L N alone adds ~119 mg/L of oxygen demand - verify blower capacity and diffuser condition before assuming D.O. control is available at the higher MLSS.
3. Raising SRT loads the secondary clarifier. What would you check, and what failure would tell you the SRT increase is not sustainable without capital work?
Read the worked answer
What to check:
- Solids loading rate (SLR): SLR = (Q + Qr) × MLSS × 8.34 / clarifier area. At MLSS 3,500 and R = 1.0 on 0.55 MGD, the mass roughly doubles versus today; compare against ~25 lb/ft²·d for a well-settling sludge and derate heavily for high SVI.
- Hydraulic overflow rate at peak hourly flow, not average.
- SVI / 30-minute settle and blanket depth profile - the rising blanket the operator reported may be a density problem (SVI > 200) that no SRT change fixes.
- Return sludge capacity and RAS concentration: you must be able to return the solids you keep.
- Filamentous organism screen - low F/M at 15-day SRT can select for filaments and for foam-forming organisms.
Failure signals that mean capital work: persistent blanket above the clarifier mid-depth at average flow with SVI > 200 after the SRT change, SLR above the derated limit at peak flow, or MLSS that cannot be held because RAS capacity is exhausted. In that case the choices are clarifier rehabilitation, flow equalization, or converting part of the aeration volume - not abandoning the SRT target, which the permit requires.
Practical note: raise the SRT gradually (about 10–15% per week), watch nitrite as the leading indicator, and hold D.O. at 2.0 mg/L in the last pass rather than 3.0 everywhere.