The main purpose of adding nutrients (such as nitrogen, phosphorus, etc.) in sewage treatment plants is to provide sufficient nutrition for microorganisms to maintain their normal metabolism and treatment efficiency. The following are the calculation basis, formula, and precautions for the dosage:
1, Calculation basis
Microbial nutritional requirements
In traditional activated sludge process, the demand ratio of microorganisms for carbon (C), nitrogen (N), and phosphorus (P) is usually BOD ₅: N:P = 100: 5: 1 (by quality).
-Carbon source: generally based on the BOD ₅ concentration in wastewater.
-Nitrogen source: commonly used ammonia nitrogen (NH3-- N) or organic nitrogen (such as urea).
-Phosphorus source: commonly used phosphates (such as potassium dihydrogen phosphate).
Actual water quality analysis
It is necessary to determine whether it is necessary to supplement nutrients by detecting the concentrations of BOD ₅, N, and P in the sewage.
-If the actual N or P concentration is lower than the theoretical requirement, it needs to be added; If the carbon source is insufficient (such as industrial wastewater), it is necessary to supplement the carbon source (such as methanol, sodium acetate).
2, Calculation formula
1. Calculation of nitrogen dosage
Formula:
Text {Nitrogen dosing rate (kg/d)}= frac { text {design flow rate (m ³/d)} times ( text {target N concentration} - text {actual N concentration})} { text {nitrogen source purity} times 1000}
Example:
-Design flow rate=10000 m ³/d
-Target N concentration=5 mg/L (calculated proportionally)
-Actual N concentration=2 mg/L
-Urea purity=46% (nitrogen content)
Text {dosage}= frac {10000 times (5-2)} {0.46 times 1000} estimate 65.22 , text {kg/d (urea)}
2. Calculation of phosphorus dosage
Formula:
Text {Phosphorus dosage (kg/d)}= frac { text {design flow rate (m ³/d)} times ( text {target P concentration} - text {actual P concentration})} { text {phosphorus source purity} times 1000}
Example:
-Target P concentration=1 mg/L
-Actual P concentration=0.5 mg/L
-Potassium dihydrogen phosphate (KH ₂ PO ₄) purity=90% (with a P content of approximately 22.8%)
Text {dosage}= frac {10000 times (1-0.5)} {0.228 times 1000} estimate 21.93 , text {kg/d (potassium dihydrogen phosphate)}
3, Precautions
Dynamic adjustment:
-Regularly check the quality of the incoming water and adjust the dosage according to the actual load.
-If the carbon source in the sewage is insufficient (such as BOD ₅/N<100), it is necessary to supplement carbon sources (such as methanol, glucose).
Process differences:
-The biofilm process (such as MBR) may require a higher N/P ratio, which needs to be adjusted according to the specific process.
Safety and Cost:
-Excessive addition may lead to excessive N/P in the effluent or sludge expansion.
-Priority should be given to low-cost and easily storable agents such as urea and industrial phosphoric acid.
4, Summary
The core of nutritional dosage is based on BOD ₅: N:P = 100: 5: Calculate the difference based on the actual water quality using a ratio of 1. In practical operation, flexible adjustments are required, and the dosing strategy should be optimized through small-scale trials or online monitoring.
