مبادئ تصميم خزانات المعالجة البيولوجية بنظام الرياكتور/the principal biological reactor design

Bioselector

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General

Bioselectors (also referred to as selective reactors) are biological reactor processes that are placed just ahead of the principal biological reactor (activated sludge, etc.).

The selector process involves reacting the influent wastewater with return activated sludge from the secondary clarifiers.

Selectors are of three types depending upon the degree of oxidation of the biological sludge: aerobic, anoxic, and anaerobic.

The most prevalent application of selectors involves the anoxic process. For biological phosphorous removal, the aerobic selector is not used; the anaerobic mode is used.

Only the anoxic selector is briefly addressed in this manual.

most extensively applied to the treatment of both municipal and industrial wastewaters.
Anoxic selectors are a means of controlling SVI in the biological treatment of wastewater.

In particular, selectors may be used in the treatment train of wastewater treatment plants using a suspended growth process as the principal biological treatment method.

Anoxic selectors can be used in an industrial wastewater treatment plant in which foaming or bulking problems may be expected.

Industrial wastewaters, which are expected to produce a severe foaming problem during the main aeration step, may employ selectors just ahead of the aeration.

Many industrial and some municipal treatment processes with short to long sludge ages, including extended aeration, experience bulking (nonsettling sludge) problems. Again, application of an anoxic selector just ahead of the main aeration step may be applied for the attenuation of potential bulking problems.

Foaming and bulking conditions can be expected to exist for industrial wastewaters that consist of relatively simple sugars and other soluble substrates.

These kinds of wastewaters are produced by pulp and paper mills, food processing facilities (fruit processing in particular), breweries with high alcohol content in the wastewater, and so on.

Wastewater with elevated temperatures will exacerbate the problem of bulking and foaming.

Temperatures to the bioreactor should not exceed 104° F, with temperatures below 100° F being more desirable

.
The third application for anoxic selectors is for nutrient removal.

Municipal wastewater treatment plants that employ a selector reactor system typically experience nitrogen compound and phosphorus reduction.

Reactor designs that promote selective growth of certain microorganisms and which have enhanced nitrogen and/or phosphorus removal have been developed.

In some cases, these proprietary processes are configured with a two (or more) stage biological reactor.

The design criteria may be different depending on the primary objective for the application.

Selector design for bulking and foam control may use a somewhat different set of criteria than a selector with the principal objective of nutrient removal.
2.
Foaming and Bulking Control

The purpose of including a selector in the treatment train for the reduction of foaming or bulking potential is to change the competitive environment among the various types of microorganisms that are present in the wastewater.

In particular, the objective is to selectively remove the BOD5 through absorption under conditions that are the least advantageous to filamentous types of microorganisms.

Two phenomena have been reported as having an impact. The first is reduction in available BOD for the growth of filamentous microorganisms;

the second is reduction in residual soluble BOD that remains towards the end of the aeration step.

Both of these actions reduce the concentration of filamentous microbes in the activated sludge.

In turn, these microbes, which are more likely to partition into the foam or float in the activated sludge, are reduced in concentration.

Criteria for Sewage Works Design

Design for this type of condition typically involves return of a portion of the RAS to the influent to the selector.

Hydraulic detention times for this type of selector may be as short as 10 minutes and as long as 45 minutes.

Typical sizing of a selector for this application involves hydraulic sizing for 30 minutes at the design flow, with detention times to be no less than 10 minutes under peak flow conditions.

In addition, the selector should be compartmentalized into three or more equal volume tanks, each with a mixer capable of maintaining complete mix conditions.

A high food-to-micro-organism ratio (F/M) ratio should be designed for the first stage selector tank.

F/M values of 6 to over 30 have been reported as being successful designs.The designer should make provision for returning only a portion of the RAS to the influent of the selector process.

The return flow to the selector should be selected by the operator from about 30 percent to 100 percent of the total RAS flow.

In the absence of any pilot plant data, a design F/M value of 10 to 15 should be used initially.

It should be anticipated that the operator will need to make adjustments to this value once the treatment plant is in operation.

3.
Nutrient Control

The anoxic/oxic (A/OTM) process for removal of phosphorus uses a selector reactor quite different from that described for bulking or foaming control.

This process uses an anaerobic reactor followed by an aerobic reactor, with both tanks being about equal in volume.

RAS full flow is returned to the influent to the anaerobic reactor.

The mixed liquor is then piped into the aeration chamber. Nitrogen reduction typically does not occur with this process.

For this process, Metcalf & Eddy recommend an F/M ratio of 0.2 to 0.7 and an anaerobic reactor detention time of 0.5 to 1.5 hours followed by an aerobic reactor detention time of 1 to 3 hours

Nitrogen reduction in a municipal wastewater can be accomplished with the inclusion of an anoxic selector just before the aeration process.

Reductions of 50 to 80 percent of the TKN may be accomplished depending upon unit sizing, MLVSS and TKN concentration, etc.

The design of an anoxic selector for denitrification is not straightforward.

Both the anoxic reactor and the aerobic reactor are sized based on the desired effluent.

When the treatment plant is required to produce very low residual TKN the designer should consider an alternative process, such as the Bardenpho™ process.

When reductions of TKN are required to be on the order of 50 percent, an anoxic selector can be used ahead of the aeration reactor.

With this type of process, the anoxic selector has a longer detention time and the full flow RAS is returned to the influent to the selector.

Selector detention times for this type of application can exceed 2 hours, although this is rare.

Metcalf & Eddy report a range of 0.2 to 2 hours as a typical detention time for the anoxic selector, with a detention time of 6 to 15 hours for the aeration chamber.

Since much of the denitrification will occur from the sludge, all of the RAS is returned to
Biological Treatment
the selector.

Metcalf & Eddy present a rational approach to the design of this type of system.
Regardless of the objective for including an anoxic selector in the treatment train, some reduction in nutrients will occur.

The rational approach cited above may be used to predict the amount of reduction. However, a number of assumptions are required to use the approach, or pilot study data must be developed for a more accurate prediction.

4.
Discussion

Bioselectors control bulking, and can reduce capacity requirements by 30 to 50 percent.
Application of bioselectors in the treatment train should be used by the designer, either:
•
To reduce the potential for bulking and/or foaming in the aeration chamber of an industrial or municipal wastewater treatment system, or
•
For partial nutrient removal from a municipal or industrial wastewater treatment system.

» Batch Treatment (Sequencing Batch Reactor design/مبادئ تصميم وحدات المعالجة البيولوجية بنظام الاس.بى.ار
» مبادئ تصميم خزانات الترسيب للمعالجة البيولوجية الثانوية بطريقة الحماه النشطة/General design guidelines for secondary sedimentation as a part of the activated sludge process
» General Design Considerations FOR Continuous Flow ofActivated Sludge procesess/lfمبادئ تصميم خزانات المعالجة البيولوجية باسلوب الحماة النشطة
» Equipment Design Features Required for SBR Systems/اسس تصميم واختيار معدات المعالجة بنظام الاس بى ار
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