مبادئ تصميم خزانات الترسيب للمعالجة البيولوجية الثانوية بطريقة الحماه النشطة/General design guidelines for secondary sedimentation as a part of the activated sludge process

Sedimentation
by

technolab el-bahaa group

general.dr

bahaa badr

Overview

General
design guidelines for secondary sedimentation as a part of the activated sludge process.

Applicability

The activated sludge process requires separation of treatment organisms from the treated mixed liquor.

In almost all activated sludge processes currently in use, this separation takes place in a gravity sedimentation tank or in a gravity sedimentation phase of a cyclic feed process.

Since the effluent from the sedimentation process is the final step, sedimentation determines effluent quality for every activated sludge process.

Process Design Considerations

Design of sedimentation for activated sludge processes requires consideration of the overall process.

Process loading parameters that determine the efficiency of the activated sludge sedimentation include overflow rate, solids loading rate, sludge settleability, underflow or return sludge pumping rate, and tank hydraulic characteristics. Design values should be identified for each of these process parameters.

Overflow Rate

The overflow rate is the rate of effluent flow from the sedimentation tank divided by the tank surface area.

The overflow rate is the average upward velocity of process effluent from the sedimentation tank.

Early researchers in sedimentation identified overflow rate as the critical factor in sedimentation tank design.

By this early theory, a given size particle will be captured in the sedimentation tank if its settling velocity is more than the average tank overflow rate.

Current design practice recognizes the hindering effect of high influent solids concentrations on settling in the activated sludge clarifier and includes overflow rate as only one of the factors used to determine sedimentation tank size.

If, in overall activated sludge process design, the aeration tank size is determined to maintain MLSS concentration and settleability less than critical values for performance of the sedimentation tank, then the overflow rate may be the primary design parameter for
the sedimentation tank.

Table gives values for design tank overflow rate during the peak sustained flow period that have proven effective under three different process configurations for the activated sludge process.

Typical values for process variables⎯MLSS, sludge volume index (SVI), and RAS rate⎯are shown with corresponding values for design peak overflow rate.

Overflow rate is given in units of gallons per day of effluent flow per square foot of total clarifier area.

Some engineers subtract the influent area of the feed zone of the clarifier from the total sedimentation area.

This practice may be considered as an additional safety factor in design and is not necessary as long as adequate safety factors are provided in the overall process design.

Table T3-2. Typical Process Design Values for Sedimentation Overflow Rate
Process Configuration

Typical MLSS, mg/L(1)

Typical SVI, mL/g

RAS rate, %

Peak Overflow Rate, gpd/sf(2)

Conventional Activated Sludge

1,500-3,500

150

50-75

1,200

Extended Aeration

2,500-3,500

200

100

500

Oxidation Ditch

2,500-3,500

150

100

700

(1) Not true if bioselectors are used.

(2) Depends on process parameters and tank design.

Solids Loading Rate

The solids loading rate is as important as overflow rate in determining the capacity of an activated sludge clarifier.

The solids loading rate is the total mass rate of suspended solids into the clarifier divided by the tank cross-sectional area.

The total mass rate to the clarifier is the sum of the tank effluent flow rate and the tank underflow or RAS pumping rate times the MLSS concentration.

The limiting solids loading rate to an activated sludge clarifier should be no greater than the limiting solids flux in the clarifier.

A factor of safety should also be applied that takes into consideration reasonably foreseen variations in design loading, settleability, and other variables.

SF = GL/SLR, where

SF = Safety factor

GL = Limiting solids flux, ppd

SLR = Solids loading rate, ppd

The limiting solids flux to an activated sludge clarifier is the limiting rate of solids loading
to the clarifier that will reach the tank bottom.

The limiting solids flux is a function of MLSS concentration, RAS rate, and sludge settleability.

It can be calculated for given design conditions in a number of ways. Riddell, et al., in “Method for Estimating the Capacity of an Activated Sludge Plant” (1983), provides a procedure for direct
calculation of limiting solids flux. Graphical procedures are provided in numerous references

Rational designs should demonstrate that design assumptions for MLSS concentration, RAS rate, and sludge settleability have been taken into account in determining the size of activated sludge aeration tanks and clarifiers.

The overflow rate values in Table each yield a safety factor of approximately 1.5 when applied at the indicated values for MLSS, SVI, and RAS rate using the method of Riddell, et al.

For circular clarifiers, the SLR should not exceed 80 percent of the loading as a function of SVI (or DSVI) and return sludge concentration.

See Daigger, “Development of Refined Clarifier Operating Diagrams Using an Updated Settling Characteristics Database” (1995).

Sludge Settleability

Sludge settleability determines the everyday capacity of an activated sludge clarifier since it partly determines the sludge settling rate against which the effluent overflow rate acts.

The common measure of settleability in the activated sludge process is the SVI. Several models have been developed to relate SVI to sludge settling velocity.

However, SVI is a poor procedure for MLSS of 3,000-4,000 mg/l and DSVI and SSVI tests should be used.

Where possible, designs for activated sludge clarifiers should be based on field measurement of sludge settling velocity using batch settling tests at varying initial suspended solids concentration.

In order to eliminate high SVI conditions, bioselectors should be used in activated sludge plants.

Return Sludge Pumping Rate

Return sludge pumping is required to maintain a mass balance of solids in the secondary clarifier.

The rate of sludge pumping as a ratio of the effluent flow from the clarifier is called the return sludge ratio.

Values for this ratio have an inversely proportional effect on RAS concentration.

» General Design Considerations FOR Continuous Flow ofActivated Sludge procesess/lfمبادئ تصميم خزانات المعالجة البيولوجية باسلوب الحماة النشطة
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» مبادئ تصميم وحدة التهوية الطويلة للمعالجة البيولوجية/Extended Aeration
» طرق المعالجة البيولوحية بالتهوية المطولة/برك الاكسدة/الحماة النشطة التقليدية