Carbon source calculation formula
Carbon source selection
Generally, available carbon sources for denitrification are divided into fast carbon sources (such as methanol, acetic acid, sodium acetate, etc.), slow carbon sources (such as starch, protein, glucose, etc.) and cell substances. Different added carbon sources have different effects on the denitrification of the system, and even if the same amount of carbon is added, the denitrification effect is different.
Compared with slow carbon sources and cell substances, methanol, ethanol, acetic acid, and sodium acetate have the fastest denitrification rate among fast carbon sources, so they are more widely used. Table 1 compares the performance of four fast carbon sources.
Carbon source dosage calculation
Both influent total nitrogen and effluent total nitrogen include various forms of nitrogen. Influent total nitrogen is mainly ammonia nitrogen and organic nitrogen, while effluent total nitrogen is mainly nitrate nitrogen and organic nitrogen.
Influent total nitrogen enters the biological reactor, part of which is discharged into the atmosphere through denitrification, part of which enters the activated sludge through assimilation, and the remaining effluent total nitrogen needs to meet the relevant water quality discharge requirements.
Carbon source dosage calculation
The nitrogen assimilated into the sludge is calculated as 5% of the BOD5 removal, i.e., 0.05(Si-Se), where Si and Se are the influent and effluent BOD5 concentrations, respectively.
The nitrogen removed by denitrification is related to the size of the anoxic tank in the denitrification process and the influent BOD5 concentration. The concept of denitrification design parameters is defined as the ratio of denitrification nitrate nitrogen concentration to influent BOD5 concentration, expressed as Kde (kgNO3--N/kgBOD5).
Therefore, the denitrification-removed nitrate nitrogen can be calculated as
Theoretically, denitrification of 1kg of nitrate nitrogen consumes 2.86kg BOD5, i.e.:
Kde=1/2.86(kg NO3--N/kgBOD5)
The formula for calculating the amount of external carbon source required by the wastewater treatment plant to correspond to the amount of nitrogen is:
N=Ne计-NsNe计=Ni-KdeSi-0.05(Si-Se)
N—Amount of nitrogen corresponding to the external carbon source required, mg/L;
Ne计—Effluent total nitrogen that can be achieved based on the designed wastewater quality and process parameters, mg/L;
Ns—Effluent total nitrogen discharge standard of the secondary sedimentation tank, mg/L;
Si—Influent BOD5 concentration, mg/L;
Se—Effluent BOD5 concentration, mg/L;
Ne计 needs to be calculated by establishing a nitrogen balance equation. The nitrogen balance of the biochemical reaction system is shown in Figure 1.
The calculated amount of nitrogen is converted into the amount of carbon required.
Phosphorus removal calculation formula
Calculation of phosphorus removal agent dosage
Iron salts or aluminum salts are commonly used in China, and their chemical reactions with phosphorus are as shown in formulas (1) and (2).
The reaction competing with the precipitation reaction is the reaction of metal ions with OH-, as shown in formulas (3) and (4).
Formulas (1) and (2) show that removing 1 mol of phosphate requires 1 mol of iron or aluminum ions.
Because the reaction is not 100% effective in actual engineering, and OH- will compete and react with metal ions to generate corresponding hydroxides, such as formulas (3) and (4), the actual chemical precipitation agent generally needs to be added in excess to ensure that the required effluent P concentration is reached.
For the sake of convenience in calculation, moles are converted into mass units in actual calculations. For example:
1molFe=56gFe，1 molAl=27gAl，1molP=31gP。
That is to say, when iron salt is used to remove 1kg of phosphorus,
1.5×(56/31)=2.7 kgFe/kgP;
When aluminum salt is used, 1.5×(27/31)= 1.3kgAl/kgP needs to be added.
Calculation of phosphorus amount to be removed by auxiliary chemical phosphorus removal
In a simultaneous precipitation chemical phosphorus removal system, the key to calculating the dosage of phosphorus removal agent is to first determine the amount of phosphorus that needs to be removed by auxiliary chemical phosphorus removal. The algorithm is different for wastewater treatment plants that have been in operation and those under design.
For wastewater treatment plants that have been in operation, PPrec=PEST-PER
PPrec——Amount of phosphorus to be removed by auxiliary chemical phosphorus removal, mg/L;
PEST——Measured concentration of total phosphorus in the effluent of the secondary sedimentation tank, mg/L;
PER——Permitted total phosphorus concentration in the effluent of the wastewater treatment plant, mg/L.
Wastewater treatment plants under design
According to the material balance of phosphorus, we can get: PPrec = PIAT - PER - PBM - PBioP
PIAT——Design concentration of total phosphorus in the influent of the biochemical system, mg/L;
PBM——Amount of phosphorus removed through biosynthesis, PBM = 0.01CBOD, IAT, mg/L;
CBOD，IAT——Measured concentration of BOD5 in the influent of the biochemical system, mg/L;
PBioP——Amount of phosphorus removed through excessive biological adsorption, mg/L.
The value of PBioP is related to many factors. The German ATV-A131 standard recommends that the value of PBioP can be estimated according to the following situations:
(1) When there is a pre-anaerobic tank in the biochemical system, PBioP can be estimated as (0.01~0.015)CBOD, IAT.
(2) When the water temperature is low and the concentration of nitrate nitrogen in the effluent is ≥15mg/L, even if there is a pre-anaerobic tank, the effect of biological phosphorus removal will be affected to a certain extent, and PBioP can be estimated as (0.005~0.01)CBOD, IAT.
(3) When there is a pre-denitrification or multi-stage denitrification tank in the biochemical system, but no anaerobic tank, PBioP can be estimated as ≤0.005CBOD, IAT.
(4) When the water temperature is low and part of the internal reflux mixture refluxed to the denitrification zone is refluxed to the anaerobic tank (in this case, the anaerobic tank is used as an anoxic tank to improve the denitrification effect), PBioP can be estimated as ≤0.005CBOD, IAT.
Water pump calculation formula
The calculation of pump head is an important basis for pump selection, which is determined by the installation and operating conditions of the pipe network system. Before calculation, a process sketch, plan and elevation layout should be drawn first, and the length, diameter, type and quantity of pipelines and pipe fittings should be calculated.
The general pipe network is shown in the figure below (more examples can be found in the Chemical Process Design Handbook).
D——Discharge geometric height, m;
Value: Above the pump inlet centerline: positive; Below the pump inlet centerline: negative;
S——Suction geometric height, m;
Value: Above the pump inlet centerline: negative; Below the pump inlet centerline: positive;
Pd, Ps——Operating pressure in the container, m liquid column (gauge pressure);
Value: Based on the positive and negative gauge pressure
Hf1——Straight pipe resistance loss, m liquid column;
Hf2——Pipe fitting resistance loss, m liquid column;
Hf3——Inlet and outlet local resistance loss, m liquid column;
h——Pump head, m liquid column.
h = D + S + hf1 + hf2 + h3 + Pd - Ps
h = D - S + hf1 + hf2 + hf3 + Pd - Ps
h = D + S + hf1 + hf2 + hf3 + Pd - Ps
The meaning of each parameter symbol in the calculation formula ↓
Approximate values of ε for some industrial pipe materials are shown in the table below ↓
Local resistance calculation of pipe network ↓
Local resistance coefficient ζ of common pipe fittings and valves ↓
Oil-water separator calculation formula
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Minimum particle size of oil that can be separated: d ≥ 15μm;
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Oil density: ρ = 0.92～0.95g/cm3;
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Horizontal flow velocity of oil-water separator: v ≤ 0.9m/min, and not more than 15 times the oil droplet rising velocity;
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Size range of the pool: depth 0.9～2.4m; width 1.8～6.1m; depth/width 0.3～0.5; safety factor k = 1.6.
Water flow cross-sectional area A: A = Q/v, m2 (1) In the formula:
Q——Water treatment volume, m3/min;
v——Horizontal flow velocity, m/min;
G——Acceleration of gravity, 980cm/s2
ρwater——Water density, g/cm3
d——Oil droplet particle size, generally 0.015cm
μ——Dynamic viscosity coefficient, (g·s)/cm2, when the water temperature is 20℃, μ = 0.0102
u——Oil droplet rising velocity, m/min
The pool width B and effective water depth h1 are taken as the lower limit according to the design basis, and then Bh1 ≥ A is checked. Otherwise, the values of B and h1 are reset.
Total length of the pool L = L1 + L2 + L3 + L4
L1——Width of the distribution tank, generally 0.5～0.8m;
L2——Effective length of oil-water separation zone, m;
t——Sedimentation time, min
Other symbols are the same as before
L3 - Collection tank width, generally 0.8m;
L4 - Water intake well width, m.
The effective volume of the water intake well is greater than the drainage pump's drainage volume for 5 minutes.
Floating oil is collected via a skimming pipe and flows out of the water. When the amount of floating oil is small and the inflow is relatively stable, it can be collected in buckets outside the pool. Otherwise, an oil storage pit needs to be set up, with its top surface level with the top of the oil-water separator. For floating oil with high viscosity at low temperatures, steam heating can be installed in the oil storage pit.
1 - Hopper; 2 - Quantitative feeder; 3 - Dissolution solution tank;
4 - Mixer; 5 - Metering pump; 6 - Y-type filter
