ox is affected by oxygen, nitrate and phosphate. Nitrate inhibits PAOs. Sampling the Internal MLSS Recycle to the anaerobic zone is preferable in which case a target < 1 mgN/L and a critical limit of 3 mgN/L applies. P release occurs in the anaerobic zone. Anoxic Zone Control Point 130 Parameter Target Critical Limit DO (mg/L) Redox potential 0 As negative as possible. > 0.5 Nitrate at the end of zone pH As close to zero as possible. 7 - 7.5 <6 >8 Comment DO inhibits anoxic bacteria. Redox potential can vary significantly however regular measurement will allow an operator to know the range of values that work best for that plant. The measurement of redox is affected by oxygen and nitrate. Depends on the plant configuration and may be as high as several mgN/L or as low as zero. Low pH inhibits denitrification. High pH inhibits denitrification.9b:T6d2,Chapter 11 - Monitoring the Activated Sludge Process Aerobic Zone Control Point Parameter Target Critical Limit DO (mg/L) 1-2 < 0.7 > 2.5 7.0 - 8.0 < 6.5 > 8.5 MLSS (mg/L) 2,300 - 4,500 < 1,200 DSVI (mg/L) 100 150 SVI 120 180 MLVSS % 70 - 80 Ammonia N (mgN/L) Nitrate N (mgN/L) Soluble phosphate P (mgP/L) Total phosphate P (mgP/L) Iron (soluble) (mg/L) Turbidity (NTU) Alkalinity (mg/L) <2 5 Sufficient oxygen is necessary to ensure adequate nitrification. DO above 2 mg/L wastes power. Low pH inhibits nitrification. High pH inhibits nitrification. A sufficient and stable biomass is necessary to ensure effective treatment. Too low inhibits settling. Bulking sludge makes operation difficult. Too low leads to pin floc carryover. Bulking sludge makes operation difficult. Too low leads to pin floc carryover. Depends on SRT to some extent. Chemical dosing decreases % volatiles. Depends mainly on licence limits. < 10 < 0.25 - 0.5 > 15 1 Depends mainly on licence limits. Depends mainly on licence limits. < 0.5 3 Depends mainly on licence limits. < 0.5 3 < 10 > 75 20 < 60 Relevant for plants using iron salts for chemical P removal. Ref