عدد المساهمات : 3492
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: General Design Considerations FOR Continuous Flow ofActivated Sludge procesess/lfمبادئ تصميم خزانات المعالجة البيولوجية باسلوب الحماة النشطة الجمعة مارس 23, 2012 2:36 am|| |
TECHNOLAB EL-BAHAA GROUP
General Design Considerations
Specific Process Selection
The activated sludge process and its many modifications may be used to accomplish various degrees of removal of suspended solids and reduction of carbonaceous and/or nitrogenous oxygen demand.
Choosing the most applicable process will be influenced by the degree and consistency of treatment required, type of waste to be treated, proposed plant size, anticipated degree of operation and maintenance, and operating and capital costs.
All designs shall provide for flexibility in operation and should provide for operation in various modes, if feasible.
Submittal of Calculations
Calculations shall be submitted, upon request, to justify the basis of design for the activated sludge process.
The calculations shall show the basis for sizing the aeration tanks, aeration equipment, secondary clarifiers, return sludge equipment, and waste sludge equipment.
Where primary settling tanks are not used, effective removal or exclusion of grit, debris, excessive oil or grease (greater than 100 mg/l), and screening of solids shall be accomplished prior to the activated sludge process.
Fine screens (6 mm or less) should always be used if primary clarifiers are not provided.
In severe climates, consideration should be given to minimizing heat loss and protecting against freezing.
Table T3-1 is a sample worksheet showing the data requirements typically necessary for designing biological systems processes.
Table T3-1. Sample Worksheet Showing Input Data Requirements for Biological Systems
(1) If COD:BOD5 ratio is not 1.9-2.2:1.0, the conventional design equation can be in error.
(2) If nutrient removal is required, TKN and/or TP will be needed.
Volume of Aeration Tanks
The volume of the aeration tanks for any adaptation of the activated sludge process shall be determined based on full scale experience, pilot plant studies, or rational calculations.
Design equations based on mean-cell residence time (sludge age) can be found in WEF .
When aeration tanks are sized for carbonaceous BOD removal using rational calculations, the ability to maintain a flocculent, well settling mixed liquor must be considered.
The use of selectors, as described in this chapter, may be desirable or necessary.
For carbonaceous BOD removal, sludge age values in the range of 5 to 15 days are typical, with the lower values used for high temperatures and the higher values used for low temperatures. Significant levels of nitrification will generally occur at 5-day SRT and temperatures of 61° F or greater.
Mixed liquor suspended solids (MLSS) concentrations in the range of 1,500 to 3,500 mg/L are often used. Because the mixed liquor concentration affects the solids loading on the secondary clarifiers, selection of the MLSS concentration must be coordinated with the secondary clarifier design.
Oxygen requirements for carbonaceous BOD removal include oxygen to satisfy the BOD of the wastewater plus the endogenous
respiration of the microorganisms. Additional oxygen is required if nitrification occurs.
Oxygen requirements depend on the influent loading to the aeration tank as well as the process design and should be determined using rational calculations.
Calculations should be based on the peak hourly BOD loading to the aeration tanks.
Recycle flows from solids processing operations must be considered since these streams often have high BOD concentrations.
Oxygen requirements for carbonaceous BOD removal are dependent on the SRT and are typically 0.9 to 1.3 pounds of O2 per pound of BOD removed. Provisions for nitrogenous oxygen demand should be considered separately and are typically 4.6 pounds of O2 per pound of TKN applied.
Sludge Recycling Requirements
Sludge recycle rates can be calculated using the rational equations referenced above.
The recycle rate deserves careful consideration since it affects the size of the secondary clarifiers without influencing the size of the aeration tanks.
Because the recycle requirements also depend on the sludge settling and thickening characteristics, which may change, the rate of sludge recycle should be variable.
The range is typically from 25 to 100 percent of the average design flow, though peak hourly flow needs must be accommodated.
Sludge Production and Wasting
When full scale or pilot plant data is not available, net sludge production can be estimated using the rational calculation procedures referenced above.
In order to obtain a reasonable estimate of the total sludge production, it is important to include solids present in the influent to the plant.
Net sludge production increases with decreasing temperature and sludge age. In plants with primary sedimentation and operating at a sludge age of 15 days, net sludge production can be expected to be approximately 0.60 pounds of TSS per pound of BOD removed (0.48 lb VSS/lb BOD) at temperatures near 68° F.
If the sludge age is decreased to 5 days, the net sludge production can be expected to increase slightly, to about 0.75 lbs/lb BOD removed (0.60 lb VSS/lb BOD).
In plants without primary sedimentation, net sludge production can be expected to range from 1.2 lbs TSS/lb BOD removed (0.92 lb VSS/lb BOD)to 1.0 lbs TSS/lb BOD removed (0.75 lb VSS/lb BOD) at sludge ages from 5 to 15 days at 68° F.
This value must be divided by the percent VSS/TSS in
the mixed liquor to generate net yields of lb TSS/lb BOD.
Aeration equipment must be selected to satisfy the maximum oxygen requirements and provide adequate mixing.
In processes designed for carbonaceous BOD removal, oxygen requirements normally control aeration equipment design and selection.
Consideration for aeration and mixing requirements should always be reviewed independently.
Aeration equipment should be designed to maintain a minimum dissolved oxygen concentration of 2 mg/L at maximum monthly design loadings and 0.5 mg/L at peak hourly loadings.
Because aeration consumes significant energy, careful consideration should be given to maximizing oxygen utilization and matching the output of the aeration system to the diurnal oxygen requirements.
Diffused Air Systems
Air requirements for diffused air systems should be determined based on the oxygen requirements and the following factors, using industry-accepted equations:
Beta value of waste.
Aeration-device standard oxygen-transfer efficiency.
Minimum aeration tank dissolved oxygen concentration.
Critical wastewater temperature.
Altitude of plant.
Values for alpha and the transfer efficiency of the diffusers should be selected carefully to ensure an adequate air supply.
For all the various modifications of the activated sludge process, except extended aeration, the aeration system should be able to supply 1,500 cf of air (at standard conditions) per pound of BOD applied to the aeration tank.
This aeration rate assumes the use of equipment capable of transferring at least 1.0 pound of oxygen per pound of BOD loading to the mixed liquor.
Air required for other purposes, such as aerobic digestion, channel mixing, or pumping, must be added to the air quantities calculated for the aeration tanks.
Multiple blowers must be provided. The number of blowers and their capacities must be such that the maximum air requirements
can be met with the largest blower out of service.
Because blowers consume considerable energy, the design should provide for varying the volume of air delivered in proportion to the demand.
Flow meters and throttling valves, where applicable, should be provided for air flow distribution and process control.
Mechanical Aeration Systems
In the absence of specific performance data, mechanical aeration equipment should be sized based on a transfer efficiency of 2.0 lbs of oxygen per hp/per hr in clean water under standard conditions.
Mechanical aeration devices must be capable of maintaining biological solids in suspension. In a horizontally mixed aeration tank, an average velocity of not less than 1 fps must be maintained.
Provisions to vary the oxygen transferred in proportion to the demand should be considered in order to conserve energy.
Protection from sprays and provisions for ease of maintenance should be included with any mechanical aeration system.
Where extended cold weather conditions occur, the aeration device and associated structure should be protected from freezing due to splashing.
Freezing in subsequent treatment units must also be considered due to the high heat loss resulting from mechanical aeration equipment agitation, i.e., splash and wave action.
Sludge Recycle Equipment
The sludge recycle rate should be variable over the range recommended ,When establishing the flow range, initial operating conditions should be considered.
Sludge is normally recycled using pumps, and the most common method of controlling the sludge recycle rate is with variable speed pump motors. When pumps are used, the maximum sludge recycle flow shall be obtained with the largest pump out of service.
Sludge return pumps should operate with positive suction head and should have suction and discharge connections at least 3 inches in diameter. One pump should not be connected to two clarifiers for continuous withdrawal.
Air-lift pumps may also be used to return sludge. When air-lift pumps are used to pump sludge from the hopper in each clarifier, it is not practical to install standby units.
Therefore, the design should provide for rapid and easy cleaning. Air-lift pumps should be at least 3 inches in diameter.
Flow meters should be provided for process control.
Waste Sludge Equipment
The sludge wasting rate will depend on the quantity of sludge produced and the process which receives the waste sludge.
Sludge is most commonly wasted using pumps.
Waste sludge pumps could have capacity of up to 25 percent of the average daily flow.
Minimum capacities in most smaller plants are governed by the practical turndown capabilities of the pumps.
Variable speed drives and/or timers should be considered to control the wasting rate. Careful pump selection is also key in small flow-wasting applications (such as positive displacement vs. centrifugal)
Means should be provided for observing and sampling waste activated sludge.
Flow meters with totalizers and recorders should be provided for process control and mass balance determinations.
عدد المساهمات : 3492
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: رد: General Design Considerations FOR Continuous Flow ofActivated Sludge procesess/lfمبادئ تصميم خزانات المعالجة البيولوجية باسلوب الحماة النشطة الجمعة مارس 23, 2012 2:41 am|| |
A complete detailed listing of design criteria shall be provided for the entire plant during wet-weather and dry-weather flow conditions, including the following:
Flows (peak hour, maximum month, average daily).
Anticipated effluent quality.
Treatment units, size, depth, detention, overflow, etc.
Equipment HP, rated capacity, size, RPM, etc.
Outfall length, material, diameter.
Diffuser ports, depth, minimum dilution.
Solids handling process units, equipment, metering, etc.
Standby power type, capacity, fuel consumption and storage, etc.