Activated Sludge SRT, Wasting, and Nitrification Alkalinity
A municipal activated-sludge plant receives 15,000 m3/d with influent BOD5 250 mg/L, TSS 220 mg/L, TKN 38 mg/L, and alkalinity 160 mg/L as CaCO3. Four aeration tanks provide 8,000 m3 total volume, and MLSS is 3,500 mg/L. Current waste activated sludge (WAS) flow is 450 m3/d at 7,500 mg/L; effluent TSS is 15 mg/L. The ammonia limit is 3 mg/L, but effluent ammonia has risen to 7 mg/L at 12 C. The operator proposes reducing wasting to raise SRT to 12 d. The utility wants to know whether that will fix nitrification and whether alkalinity or aeration will become limiting.
1. Compute the current SRT and the WAS flow needed for an SRT of 12 d, assuming MLSS stays at 3,500 mg/L.
Read the worked answer
Solids inventory=8,000 m3(3.5 kg/m3)=28,000 kg MLSS. Effluent solids=15,000 m3/d(0.015 kg/m3)=225 kg/d. Current WAS solids=450(7.5)=3,375 kg/d. Total loss=3,600 kg/d; SRT=28,000/3,600=7.8 d. For SRT=12 d, total loss=28,000/12=2,333 kg/d; WAS solids=2,333-225=2,108 kg/d; WAS flow=2,108/7.5=281 m3/d. So wasting should be reduced from 450 to about 280 m3/d, provided MLSS is held at 3,500 mg/L and the WAS concentration stays at 7,500 mg/L.
2. Will SRT 12 d fix nitrification? Evaluate alkalinity, pH, and aeration constraints.
Read the worked answer
SRT 12 d gives more nitrifier inventory than 7.8 d and is likely adequate at 12 C if DO and pH are suitable, but it is not a guarantee. Alkalinity is the bigger constraint: nitrification consumes about 7.14 mg alkalinity as CaCO3 per mg NH4-N oxidized. With TKN 38 mg/L and effluent NH3-N 3 mg/L, about 35 mg/L is nitrified, requiring 35(7.14)=250 mg/L as CaCO3. Influent alkalinity is only 160 mg/L, so complete nitrification would drive pH down and inhibit nitrifiers. Add alkalinity, such as lime or soda ash, or provide denitrification, which recovers about half the alkalinity. Also check aeration capacity and confirm MLSS does not rise above the assumed 3,500 mg/L.
3. After the wasting reduction, MLSS rises to 4,200 mg/L. What WAS flow is now needed to hold SRT at 12 d, and what operational risk does this create?
Read the worked answer
New inventory=8,000(4.2)=33,600 kg MLSS. At SRT=12 d, total solids loss=33,600/12=2,800 kg/d. Subtract effluent solids, 225 kg/d, leaving WAS solids=2,575 kg/d. At 7.5 kg/m3, WAS flow=2,575/7.5=343 m3/d. Thus the operator must waste more, not less, than the original 280 m3/d to hold SRT once MLSS rises. The risk is a higher solids load on the clarifiers, possible solids washout, lower F/M, and increased oxygen demand. Monitor sludge blanket, return sludge, DO, and effluent TSS, and adjust wasting gradually rather than chasing a single SRT number.