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Sample calculation for citrus processing plant estimating quantity and size of aerators required to meet the oxygen demand and provide complete mix conditions.
Design Flow = 2.54 MGD
Temperature = 25 oC (given)
Influent BOD = 4750 mg/l
Basin Dimensions
Water Depth = unknown ft
Volume = 38.36 MG
Elevation = 100 ft
Hydraulic Retention Time
HRT = 38.36 MG / 2.536 MGD x 24 hr / 1 day = 363 hrs
Actual Oxygen Requirement
The oxygen demand is based on 1.25 lb O2 / lb BOD applied.
AOR (BOD) = 1.25 lb/lb x 4750 mg/l x 2.536 MGD x 8.34 / 24 hr = 5232.48 lb O2 / hr
Field Oxygen Transfer Efficiency
FTE = SOTE x [(Cs x ) - Cr] x 1.024(T-20) x 9.09
where:
SOTE = 2.8 lb O2 / BHP-hr T = 25oC
Cs = 8.23 mg/l (at 25oC and 100 ft) = 0.95 (typical, assumed) = 0.85 (typical, assumed)
Cr = 2.0 mg/l
FTE = 1.72 lb O2 / BHP-hr
Power Requirement
Power (aeration) = 5232.48 lb/hr
1.72 lb/BHP-hr x 0.92 = 3306.7 HP
A mixing level of approximately 80 HP/MG is recommended to provide complete mix conditions.
Power (mixing) = 80 HP/MG x 38.36 MG = 3068.8 HP
This leads to a recommendation of forty four (44) - 75 HP surface aerators.
Need to have basin geometry and dimensions to verify this and to make recommendation for mooring and electrical arrangement.
This is simply quite straight-forward number crunching to illustrate oxygen and mixing requirements. One would probably consider prior anaerobic pretreatment to deal with this so precariously characterized stream. For the sake of sample calc it's OK.
Sample calculation for paper mill, id est aeration required to meet the oxygen demand/mix conditions.
Excerpt from project files (a trickling filter was originally requested):
The four lagoons would have an organic loading rate of 0.055 kg/cu m/d and a hydraulic retention time of 7.27 d. Because paper mill wastewaters generally have a low rate of biodegradation and little suspended solids, I don't think a trickling filter would work well. Also, there is more risk of odors with a trickling filter. The aerated lagoon approach should work well if the lagoons are arranged in stages so that the bulk of the biodegradation takes place in the first stage (one or two lagoons) and the other lagoons are used for polishing. DAF units work well to removed solids from lagoons, but will require the use of an coagulant aid (usually ferric chloride or alum) and a polymer (usually a cationic polyelectrolyte).
The COD/BOD ratio for paper mill wastewaters often if very high so that the measured BOD often underestimates the organic loading significantly. If the COD/BOD ratio is less than 3, then there should be no problems using measured BOD values for sizing aeration equipment. But if the COD/BOD ratio is greater than 3 ( I have seen ratios as high as 10:1), then the aerators based on measured BOD values may be too small. The same problem would occur when sizing TFs, that is, they would be too small if sized on measured BOD values alone.
Sample Assignment: Select suitable aeration equipment for a 50m * 50m * 2.5m basin to be used for a slurry-phase bioremediation treatment.
Consulting engineer provided a basic "mini" SARA (Saturates, Aromatics, Resins and Asphaltenes) analysis, a chemical characterization method used in the petroleum industry to fractionate heavy crude oilsIt separates the sample into four main groups based on solubility and polarity, essential for optimizing refining, assessing crude quality, and predicting colloidal stability.
It's not that easy to settle on a given MLSS typical of wastewater projects, as well as most other parameters. Annotations from archives:
Power densities range from 20 to 50 kW per 1,000 m3 for moderately thick suspensions and 100 to 250 kW per 1,000 m3 for thick suspensions. Operation is typically batch. Solids content of between 5 and 20 percent is typical (US EPA, 1993) - still solids content can range between 10 and 40 percent. Dissolved oyxgen 2 mg/L desirable.
Preliminary setting and selection was as follows:
Basin dimensions: 50m x 50m x 2.5m water level
8-10% slurry
four (4) 150 HP high speed surface aerators
four (4) 15 HP submersible mixers
System was to operate for about twelve (12) months.
Feel free to work out the proposed power density and compare with regular wwtp values. Rationale behind not that oxygen requirement were that large but the need to sustain mixing throughout said period.
Given Actual Oxygen Requirement (AOR) determine the size and number of low speed aerators for a to-be-built oxidation ditch.
AOR = 300 kg O2/hr = 300 * 2.2046 lb O2/hr = 661 lb O2/hr
Using Alpha = 0.55, Beta = 0.95 and 12 °C and residual D.O. = 1.5 mg/L Standard Oxygen Requirement turns out to be about 1,515 lb O2/hr.
Each aerator is designed to transfer 3.5 lb oxygen pero HP hour.
Therefore, required HP is 15153/5 = 432 HP. Preliminary recommendation would then be three (3) 150 HP low speed surface aerators in a single oxidation ditch with a volume of approximately 1.5 million gallons enough to provide a 24 hour retention time at design flow.
Let's calculate an estimate of the solids retention time for a number of aerobic installations using the iterative formula - all data self-explanatory. Please be aware that these calculations are a simplification of the actual project data. For example, actual solids inventories could be smaller or larger and all other derived magnitudes adjusted accordingly. SRT is thus about ballpark.
| SRT Sludge Age Sizer | Dairy Biological Nutrient Removal | ||||||||||||
| T | 20 | °C | T | 20 | °C | T | 20 | °C | |||||
| Yoc | 0.5 | Yon | 0.31 | Ydn | 2.28 | ||||||||
| Kd | 0.200 | Kd | 0.200 | Kd | 0.200 | ||||||||
| flow in | 2000 | m3/d | 0.528 | mgd | |||||||||
| CODin | 483 | mg/L | 2126 | lbCOD/day | 964 | kgCOD/day | TKNin | 44 | mg/L | ||||
| TKNin | 44 | mg/L | (for nitrif!) | 194 | lbTKN/day | 88 | kgTKN/day | ||||||
| Nitrates | 44 | mg/L | 194 | lbCOD/day | 88 | kgNO3/day | 0 | used up for COD removal/growth | |||||
| nVSS | 0 | mg/L | 0 | lbSS/day | 0 | kgSS/day | TKNin used for growth | 0 | mg/L | ||||
| Percent | 100 | % Treated | 964 | kgCOD/day | TKNin avail for nitrification | 44 | mg/L | ||||||
| Percent | 95 | % COD removed | 916 | kgCOD/day | |||||||||
| Vreactor | 899300 | gallon | 3404 | m3 | (from order of magnitude sizing) | ||||||||
| 0.8993 | mg | 0.28 | kgCOD/day per m3 | ||||||||||
| Target MLVSS | ballpark bug mix | ||||||||||||
| 1086 | mg/L | heterotrophs | 3690 | kg heterotrophs | 1086 | mg/L | 68 | % of Target MLVSS | |||||
| 61.5 | mg/L | nitrifiers | 209 | kg nitrifiers | 62 | mg/L | 4 | % of Target MLVSS | |||||
| 450 | mg/L | denitrifiers | 1529 | kg denitrifiers | 450 | mg/L | 28 | % of Target MLVSS | |||||
| 0 | mg/L | other solids | 0 | kg other | 0 | mg/L | 0 | % other | (this entry's not used) | ||||
| 1598 | mg/L | ||||||||||||
| 1598 | mg/L | 11967 | lb bugs | 5428 | kg bugs | ||||||||
| Heterotrophic Calcs | |||||||||||||
| SRT | 30.04 | days | (iterates automatically) | ||||||||||
| Yn = | 0.134 | ||||||||||||
| biomass yield = | 123 | kg/day bugs | |||||||||||
| Global Sludge Age | |||||||||||||
| Nitrifiers | 30.01414 | 181 | kg/day | ||||||||||
| SRT | 30.16 | days | (iterates automatically) | synthesis | |||||||||
| Yn = | 0.083 | ||||||||||||
| biomass yield = | 7 | kg/day bugs | |||||||||||
| Denitrifiers | |||||||||||||
| SRT | 29.93 | days | (iterates automatically) | ||||||||||
| Yn = | 0.612 | ||||||||||||
| biomass yield = | 51 | kg/day bugs | |||||||||||
| HRT = V / Q = | 1.7 | days | |||||||||||
| 41 | hours | ||||||||||||
Let's calculate an estimate of the required solids retention time for a small petroleum refinery stream containing benzene (1850 ppm -> COD = 1850* 3.08 = 5700 mg/L). Please be aware that these calculations are a simplification of the actual project data. For example, actual solids inventories could be smaller or larger and all other derived magnitudes adjusted accordingly. SRT is thus about ballpark.
| SRT Sludge Age Sizer | ||||||||||||
| T | 25 | °C | ||||||||||
| Yoc | 0.5 | Petroleum Refinery Stream | ||||||||||
| Kd | 0.250 | (also called b) | Stream containing about 1850 ppm benzene | |||||||||
| Carbonaceous Removal Sludge Age | ||||||||||||
| flow in | 240 | m3/d | 0.063 | mgd | ||||||||
| CODin | 5700 | mg/L | 3011 | lbCOD/day | 1366 | kgCOD/day | applied | |||||
| 0 | kgCOD/day | via denitrif | ||||||||||
| 1366 | ||||||||||||
| Percent | 100 | % Treated | 1366 | kgCOD/day | ||||||||
| Percent | 98 | % COD removed | ||||||||||
| Vreactor | 190440 | gallon | 721 | m3 | (from order of magnitude sizing) | |||||||
| 0.19044 | mg | 1.89 | kg applied COD/day per m3 | (Residual OK) | ||||||||
| Target MLVSS | 1.89 | kg applied + from denitri COD d/m3 | ||||||||||
| 5320 | mg/L | 8439.463 | lb bugs | 3828 | kg bugs | total bugs in tank but say only | 100.0 | % heterotrophs | ||||
| 5320 | mg/L | 8439.463 | lb bugs | 3828 | kg bugs | (heterotrophic biomass) | ||||||
| SRT | 20.04 | days | (iterates automatically) | |||||||||
| Ync | 0.143 | ok | f/m | 0.357 | kg AppliedCOD d per total kg bugs (Residual OK) | |||||||
| f/m | 0.357 | kg Applied + From Dentir / total kg bugs | ||||||||||
| biomass yield = | 191 | kg/day bugs | due to appliedCOD | |||||||||
| HRT = V / Q = | 3.0 | days | ||||||||||
| 72 | hours | ok | ||||||||||
Sample keywords: hydrogen sulfide EGSB tannery (no commas!)
