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AIT, Bangkok, Thailand Bauhaus-Universität Weimar University of Leeds, UK

Physico-Chemical Processes

2004 1. Edition

Chapter 4 SEDIMENTATION

Prof.C.Visvanathan

Copyright: The contents of the following chapters are prepared under the direct supervision of Professor C. Visvanathan, AIT (herein referred to as the Author) for the Asia-link Project, Development of an International Long Distance Internet-Based Master Course on Environmental Technology and Management, Contract # ASI / B7 -301 / 98 / 679-019. The ownership of the following document(s) belongs to the Author and the Asialink Project. Any electronic or physical copying/distribution of the material(s) must be informed either to the Author or to the Project Management in advance.

Physico-Chemical Processes 1. Edition

Prof.C.Visvanathan: SEDIMENTATION

Chapter 4 - Table of Contents

4.1 4.2 4.3 4.4 4.5 4.6

Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Size Determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Efficiency of an Ideal Settling Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of Settling Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process Description of Sedimentation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.1 4.6.2 4.6.3 4.6.4

1 1 1 2 4 7

Conventional Sedimentation Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Plate and Tube Settlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Clariflocculators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Sludge Blanket Clarifiers (SBC) (Figure 4.9) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.7

Type of Sedimentation Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.7.1 Static Settling Tanks without Sludge Scrapers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.7.2 Static Settling Tanks with Mechanical Sludge Scrapers . . . . . . . . . . . . . . . . . . . . . . . . 16 4.7.3 Sludge Contact Settling Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.8

Factors Affecting the Sedimentation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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Chapter 4 - List of Figures

Fig. 4.1: Fig. 4.2: Fig. 4.3: Fig. 4.4: Fig. 4.5: Fig. 4.6: Fig. 4.7: Fig. 4.8: Fig. 4.9: Fig. 4.10: Fig. 4.11: Fig. 4.12: Fig. 4.13: Fig. 4.14: Fig. 4.15: Zones of Sedimentation (a) Horizontal Flow Clarifier (b) Upflow Clarifier . . . . . . . . . Settling Paths of Discrete Particles in a Horizontal Flow Tank (Idealized) . . . . . . . . . . Settling in an Upflow Clarifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Set-up for Settling Studios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Particle Trajectory in Tube Settler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Different Types of Tube Settlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lamella Separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Treatment Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Treatment Flow Diagram with SBC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process Description of Zone Settling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cylindrical Settling Tanks with Conical Bottom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plate Type Static Settling Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flocculator Clarifier with Scraper Bridge (peripheral drive) . . . . . . . . . . . . . . . . . . . . . . . Rectangular Tank with Longitudinal Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sludge Blanket Clarifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 4 6 8 9 10 12 13 14 15 16 17 17 18

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Chapter 4 - List of Tables

Table 4.1: Table 4.2: Table 4.3: Table 4.4:

Removal Efficiency (%) as Function of Sedimentation Depth . . . . . . . . . . . . . . . . . . . . . 6 Basic Design Criteria for Horizontal Flow Sedimentation Tanks . . . . . . . . . . . . . . . . . . . 7 Tube Setters ­ Overflow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Advantages and Disadvantages of SBC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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4

4.1

SEDIMENTATION

Objective

Removal of suspended particles whether such particles already exist in wastewater (primary settling) or are produced by the action of coagulation-flocculation or result from a biological treatment (secondary clarification).

4.2

Principle

Solid liquid separation by gravitational force.

Fig. 4.1:

Zones of Sedimentation (a) Horizontal Flow Clarifier (b) Upflow Clarifier

4.3

1. 2. 3. 4.

Size Determination

Surface loading (m3/m2.h) Detention time (h) Weir loading rate (m3/m.h) Solids loading (kg of SS/m2.h) - not an important deciding factor.

Note: Sedimentation rate of SS depend on their size.

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4.4

Efficiency of an Ideal Settling Basin

Four zones of a sedimentation basin (Figure 4.1): 1. 2. 3. Inlet zone: evenly distribute the flow and suspended solids across the cross-section of the settling zones approximately 25% of the tank length. Settling zone: where the actual settling of particles take place. Sludge zone: collection of sludge. · · · · · 4. · · Configuration and depth of the sludge zone depends on the method of cleaning and quantity of sludge deposited. Well flocculated solids, 75% settle in the 1/5th of the tank length. Ex: 2m near the inlet, 0.3m at the outlet, with a sludge hopper at the tank. Manual Cleaning: 1 for 3 - 6 months (Slope 5 - 10 %) Mechanical Cleaning: 1 % Removal of settled water without carrying away any of the flocs. Should be designed to avoid scouring by having either weirs or trough.

Outlet zone:

Fig. 4.2:

Settling Paths of Discrete Particles in a Horizontal Flow Tank (Idealized)

vo is the velocity of the particle falling through the full depth h0 of the settling zone in the detention time to (Figure 4.2)

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Formula 4.1: h0 v 0 = ---t0 Legend: vd vs Formula 4.2: l × w × h0 V t 0 = --- = ----------------------Q Q Legend: V w Formula 4.3: Q Qv 0 = ----------- = ----l×w As = Surface Loading or Overflow Velocity As = surface area Removal is independent of the depth. All particles with vs v0 are removed For Design : Calculate/estimate vs : vo = 0.8 vs Particles of vs < v0 can be removed from horizontal flow basins if they are within vs vertical striking distance of ---- × l 0 from the sludge zone (Figure 4.3). vd

(4.3)

(4.1)

Displacement velocity Settling velocity

(4.2)

Volume Width of the channel

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Fig. 4.3:

Settling in an Upflow Clarifier

4.5

Determination of Settling Velocity

Four types of Sedimentation: 1. Class - 1 clarification: Settling of dilute suspensions which have little or no tendency to flocculate. 2. 3. Class - 2 clarification: Settling of dilute suspensions with flocculation taking place during the settling process. Zone settling: This occurs when particles settle as a mass and not as discrete particle. Inter-particle forces hold the particle (which are sufficiently close) in fixed position, so that the settlement takes place in a zone. High concentration > 1000 mg/L; and Compression settling: Settlement taking place over the resistance provided by the compacting mass resulting from particles that are in contact with each other.

4.

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Type I Sedimentation (Discrete Particle Settling)

For spherical particles, Formula 4.4: ( ­ ) 2 g s L v c = ------------------------- d 18µ Legend: vc s L g d µ Terminal velocity Density of the particle Density of the liquid Acceleration due to gravity Diameter of particle Dynamic viscosity

(4.4)

In the design of sedimentation basins, select a particle with terminal velocity vc and design the basin so that all particles that have settling velocity equal to or greater than vc will be removed (Figure 4.4)

Type II Sedimentation (Flocculated Settling) (Figure 4.4)

Here due to continuous flocculation, size and settling velocity change as a function of time. The overall removal `R' of discrete particles of different sizes and densities is expressed as: Formula 4.5:

P0

1 R = ( 1 ­ P 0 ) + ---- v s dp v0

0

(4.5)

Po is the portion of the particles with a settling velocity vo (Surface overflow rate). The settling characteristics for the flocculant particles are obtained from a settling column. Condition for Settling Analysis: · Column Diameter 300 mm. (avoid piston effect) · · · · Depth Sedimentation Tank Depth. Sampling Points: 0.6 m interval. Physical Condition: Constant temperature and non-turbulence. Measurement: % removal as a function of time and depth.

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Fig. 4.4:

Experimental Set-up for Settling Studios

R at time t Formula 4.6: Rn ­ R n + 1 h n = ---------------------------- -------------------------- 2 Total height

h 1 R 1 + R 2 h 2 R 2 + R 3 h 3 R 3 + R 4 h 4 R 4 + R 5 = -------- ------------------ + -------- ------------------ + -------- ------------------ + -------- ------------------ (4.6) 2 h5 2 h5 2 h5 h5 2 In practice settling velocity or overflow rate obtained from column study is often multiplied by factor 0.65 - 0.85, and the detention times are multiplied by a facor 1.25 - 1.5. Question A batch settling test using 2.0 m column and coagulated water yield the following data (Table 4.1). Design a tank to remove 65% of the influent suspended solids from their design flow of 0.5 m3/s and calculate the total removal efficiency at 60 minutes.

Table 4.1: Removal Efficiency (%) as Function of Sedimentation Depth Time (min) 5 Depth (m) 0.5 1.0 2.0 41 19 15 50 33 31 60 45 38 67 58 54 72 62 59 73 70 63 76 74 71 10 20 40 60 90 120

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Type III Sedimentation (Hindered Settling)

If you increase the concentration of particles/flocs in suspension, a point will be reached, above which particles are so close to each other that they no longer settle independently. The interaction between the particles become more important and velocity fields of the fluid displaced by adjacent particles overlap. Thus, there is a net upward flow of liquid displaced by the settling particles. This results in a reduced particle settling velocity and the effect is known as HINDERED SETTLING. New Settling velocity of the Suspension (Sludge) has to be equal to the upflow velocity of the water, designed to maintain high concentration of solids in suspension and take advantage of the increased opportunity for particles to colloids and aggloromate - assist removing very fine particles. Often such type of settling is known as ZONE SETTLING, because when the particle concentration is so high (> 1000 mg/L), the whole suspension tends to settle as a ,,Blanket". Type IV Sedimentation (Compression Settling) Normally used for sludge dewatering. This section will be discussed in detail in the sludge dewatering chapter.

4.6

4.6.1

· ·

Process Description of Sedimentation Methods

Conventional Sedimentation Tanks

Flow is usually horizontal. Circular or Rectangular sedimentation tanks.

Basic Design Criteria for Horizontal Flow Sedimentation Tanks

Table 4.2:

Parameter Surface loading rate (m /m .d) Mean horizontal velocity (m/min) Water depth (m) Detention time (h) Weir loading rate (m /m.d) Solid loading rate (kg/m2.d) (primary sedimentation) (secondary clarifier)

3 3 2

Design value 20 - 60 0.15 - 0.90 2-3 2-4 100 - 200 1.5 - 34 49 - 98

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4.6.2

Plate and Tube Settlers

1. As mentioned earlier, the removal efficiency is directly related to the settling velocity and not the depth of the basin. · Shallow as possible to optimize the efficiency.

2. Plate and tube settlers are shallow settling devices consisting of stacked off-set trays or bundles of small plastic tubes of various geometries. Formula 4.7:Q v 0 = ----As when Q = constant 3. When As increase vo decrease thus will increase the removal efficiency 4. The principle of plate settlers is to increase As by tubes or plates. Practical limitations of this theory (Figure 4.5): a) Needs laminar flow through the tube; b) c) When the velocity through plates reaches a certain level - deposited solids could be lifted; Difficulties related to sludge removal which leads to the development of Inclined Plate and Tube settlers.

(4.7)

Fig. 4.5:

Particle Trajectory in Tube Settler

Horizontal tube distance to be traveled = d Formula 4.8: d cos = ---' d When = 0 d = d'

(4.8)

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d When cos60 = ---- = 0.5 ' d d' = 2d

' d d = ----------cos

Formula 4.9: L V 1max = v s ---' d L cos V 1max = v s --------------d

(4.9)

In tube settlers, tubes can be rectangular or circular. Angle of inclination: 45-60° 1. If angle > 60° efficiency decreases. 2. If angle < 45° sludge tends to accumulate within the plates and tubes.

Advantages: · Cost effective method of upgrading; · · · Do not require any mechanical device; Low cost options; Less land area.

Tube Inclination

Fig. 4.6:

Different Types of Tube Settlers

Horizontal Tube Settler (Figure 4.6) · Settling angle is slightly inclined (50) in the direction of the flow. · Horizontal Tube Settlers theoretically requires less tube volume, because the depth of particle fall varies inversely with the Cosine of , is smallest. At = 60° would effectively double the maximum fall distance for particle entering the tube. Requires frequent cleaning to wash down the accumulated sludge.

·

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· ·

Lower construction cost. Due to this complex cleaning process, this type of installations are not advocated for large water treatment plants - limited only for small water treatment plants.(1-2 MGD)

Inclined Tube Settler (Figure 4.6) · Inclined to an angle of 450 - 500 · · Higher slope facilitates gravity drainage of sludge. Suitable for high capacity installations.

Lamella Separator (Figure 4.7) · Used in most of the upflow clarifiers. · Equipped with sludge movement deflecting blades.

Fig. 4.7:

Lamella Separator

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Table 4.3:

Tube Setters ­ Overflow Rate Horizontal Flow Basins Loading Rates

Overflow Rate, Based on Total Raw Water Turbidity Clarifier Area (GPM/ft2) For water temperatures 40°F or less : 2.0 2.0 2.0 2.0 For water temperatures 50°F or above: (> 10°C) 2.0 2.0 2.0 3.0 2.0 2.0 1 GPM = 0.06308 L/sec; 1 C = (°F - 32) x 0.556 ft2 = 0.0929 0-100 0-100 0-100 0-100 100-1,000 100-1,000 m2; 0-100 0-100 100-1,000 100-1,000

Overflow Rate, Portion of Basin Covered by Tubes (GPM/ft2)

L/sec/m2

2.5 3.0 2.5 3.0

1.72 2.04 1.72 2.04

2.5 3.0 4.0 3.5 2.5 3.0

1.72 2.04 2.75 2.40 1.72 2.04

Usually in a Tube Settler, an overflow rate of 2.04 L/sec/m2 (3 GPM) is acceptable for the basin area covered by the tube settlers, when next unit is either dual or mixed media filters. Problem: Upgrading existing treatment plant using tube settlers. Present Set-up: Rectangular sedimentation basin: Design overflow rate (Table 4.3): T: Rapid sand filters: Flocculation mechanical:

(9m × 40.5m × 4.5 m) (40.7 m3/day/m2) 2.5 h. 0.18 m3/sec. (T = 40 min)

Proposed Modification: 1. Doubling the plant capacity 0.35 m3/sec (8 MGD) Sedimentation basin overflow rate = 8.15 m3/day/m2 T = 1.25 h. If you install a tube settler, an overflow rate of 2.04 L/sec/m2 is acceptable.

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The area covered by tubes: Capacity = -------------------------------------------------Allowable Tube Rate

2 0.35m / sec = ----------------------------------------------------------- = 172m 3 2m 1 2.04L / sec m ------ ------- L 10 3 3

2.

Assume the tubes are extended over whole width, the tube length will be: A = L × 9 = 172m2 L = 19.1 m

Limitations of Plate and Tube Settlers: 1. Tropical climatic conditions found in most of the Asian countries, cause Algae growth in tubes and plates, causing maintenance and odor problems. Easy to clean in Lamella but not in Tubular Module. 2. 3. 4. 5. Careful attention is necessary for the design of inlet and outlet structures or else the tube performance could be affected by turbulence and uneven flow. In wastewater treatment, buildup of oil and grease. Sometimes high pressure hose water is injected to flush out the solids. However, this flushing is a problem. Sometimes the sludge deposition rate >> removal rate.

4.6.3

Objective: ·

Clariflocculators

Flocculation and Sedimentation in a single unit reducing plant size (Figure 4.8).

Fig. 4.8:

Water Treatment Flow Diagram

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1. 2. 3.

Practically it is difficult to maintain the velocity of 0.6 m/s To avoid this problem combine clariflocculator flocculation + sedimentation usually circular sedimentation tanks. Simple modification of conventional sedimentation unit. This technique will not use Sludge Blanket Formation.

Applications: 1. Water Treatment a) b) 2. 3. 4. High Turbid waters; Chemically formed flocs and impurities.

Final Sedimentation of Activated Sludge. Chemical Treatment of Sewage. Iron and Manganese Removal.

4.6.4

Sludge Blanket Clarifiers (SBC) (Figure 4.9)

Fig. 4.9:

Water Treatment Flow Diagram with SBC

Process description (Figure 4.10): · Rapid mixing and flocculation in the central conical zone - here high floc concentration is maintained. · The flocculated water is directed through the sludge blanket at the bottom of the tank - to promote growth of larger clusters of flocs where heavier particles have already settled. Precipitated flocs helps capturing micro-particles flocs (Sweep Flocculation).

·

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Fig. 4.10:

Process Description of Zone Settling

Dimension of a SBC: · Using a Jar Test method find the optimum flocculation; · · · · Prepare fresh suspension using data in #1; Pour into 1 liter cylinder; Make a homogenous mixer using manual mixing. Set time = 0; Observe the height of the solid/liquid interface and note for 30 minutes.

Overflow rate of clarification: Such that average rise velocity of the liquid < Zone Settling velocity of the suspension.

Table 4.4: Advantages and Disadvantages of SBC Advantages Working of the unit is very simple Better economy because of the combination of clarifier and flocculation and at times the filter as well. Suitable for application in rural areas due to ease of maintenance and operation. Disadvantages Floc blanket formation after starting up takes time. Artificial feeding of clay suspension. Proper upflow velocity must be provided to prevent sludge settling and possible clogging of inlet. Low turbid waters require addition of artificial turbidity at start-up for rapid blanket formation.

Can be applied to varying initial turbidi- Care must be taken to prevent algae growth. ties, particularly for high turbidities. Head loss is low compared to the other conventional techniques. Hydraulic overloading or turbidity may lead to floc carryover and plugging of the filters. SBC requires the attention of a skilled operator, especially if raw water quality and flow rates are variable.

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4.7

4.7.1

Type of Sedimentation Processes

Static Settling Tanks without Sludge Scrapers

a) Ordinary Cylindrical Settling Tank with Conical Bottom (Figure 4.11)

Application: · Primary Settling Characteristics · Flow rate: < 20 m3/h · · Upward flow rate: 1 - 2 m/h Slope of conical part: 45 to 650

Fig. 4.11:

Cylindrical Settling Tanks with Conical Bottom

b) Plate Type Settling Tanks (Figure 4.12)

Characteristics: · Combines in one unit · · · A mixing zone for sewage and added reagents An accelerated flocculator A plate settling zone

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Fig. 4.12:

Plate Type Static Settling Tank

4.7.2

Static Settling Tanks with Mechanical Sludge Scrapers

a) Circular Settling Tanks (Figure 4.13)

Application: · Primary settling and secondary clarification Characteristics: · Slope of the floor : 4 - 10% · · · · · Sludge is scraped into a central hopper. Scraper system: radial bridge with peripheral drive. These types of settling tanks can be equipped with a flocculator. Sweep Area: Flocculator chamber diameter x depth; Paddle Area: 10% sweep area of the flocculator.

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Fig. 4.13:

Flocculator Clarifier with Scraper Bridge (peripheral drive)

b) Rectangular Settling Tanks (Figure 4.14)

Application: · Primary settling and secondary · Clarification

Characteristics · length/width : 2.5 - 4 m · slope of the bottom : 1%

Fig. 4.14:

Rectangular Tank with Longitudinal Flow

4.7.3

Sludge Contact Settling Tanks

Application: · Physico Chemical Treatment. Principle: · Slow stirring of suspension in order to enable colloidal matter and added flocculating reagents to coalesce.

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Characteristics: · Sludge is separated form water in calm zones (sludge hoppers or ,,concentrators") from where it is automatically extracted by valves or siphons.

Fig. 4.15:

Sludge Blanket Clarifier

4.8

Factors Affecting the Sedimentation Process

Currents: · Eddy currents: setup by the inertia of the incoming liquid · · · · Surface currents: wind induced in open basins Vertical convection currents: thermal in origin Density currents: causing cold or heavy water to underun a basin and warm or light water to flow across its surface Current induced by outlet structures:

Short circuiting and basin stability: · Variation in detention time. · Can be detected by adding dye, electrolyte or other tracer substance.

Scour of bottom deposits: · Lifting of settled sludge from the sludge zone · Can be avoided by the geometry of the structure.

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