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NEFCO Dual Launder/ Baffle Study

Many clarifiers feature a launder positioned away from the tank wall with weirs on both sides. The intent is to maximize the weir length in the clarifier. However, this configuration often suffers the effects of density currents that carry lighter solids upward and deposit them in the space between the tank wall and the trough or drive the solids to the trough itself.

This study was designed to determine how effective the SB2.0 Density Current Baffle was in reducing solids when used with this clarifier configuration, and shows that the baffle reduced solids by more than 30% in all cases.

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NEFCO Dual Launder Modeling

1. Setup One (120.0 ft diameter clarifier, dual weir set 5.0 ft from wall)

2. Setup Two (120.0 ft diameter clarifier, dual weir set 10.0 ft from wall)

3. Setup Three (100.0 ft diameter clarifier, dual weir set 12.0 ft from wall)

Three different configurations were studied

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Setup One

Not to Scale

120 ft

20 ft

5 ft 3 ft

2 ft

Sludge Blanket3 ft

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Setup One – Without Baffle

Treatment Flow Rate = 8.5 mgd

Flow Pattern (colored by speed, red is fast)

In this case, the density current proceeds unimpeded across the top of the blanket, carrying solids up the tank wall and depositing them in the space between the tank wall and the outer weir of the launder.

These views depict a 2D cross section representation of the flow within the clarifier, from the center to the outer tank wall.

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Setup One – Density Current Baffle

Density Current Baffle Design

Vertical Position of the Baffle

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Setup One – With Baffle

Flow Pattern (colored by speed, red is fast)

Treatment Flow Rate = 8.5 mgd

Short-Circuiting current is deflected

Density Current gets Underneath of Baffle

In this case, the baffle intercepts the short-circuiting current and redirects the current and solids away from the launder and into the main volume of the clarifier

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Setup One – With and Without Baffle

Higher Solids Concentration

Greater Approach Flow Speeds

Flow Pattern (colored by density, red is high)

Just as in the velocity plot, solids fill the space between the tank and outer weir.

In this case the baffle has diverted the current and reduced the solids concentration at the launder.

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Setup Two

120 ft

20 ft

10 ft 3 ft

2 ft

Sludge Blanket3 ft

Not to Scale

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Setup Two – Without Baffle

Flow Pattern (colored by speed, red is fast)

Treatment Flow Rate = 8.5 mgd

Again, the current rises unimpeded into the space around the launder. The speed of the current in the vicinity of the launder is still high. Note the width of the current as it rises up the tank wall. This explains why NEFCO’s SB 2.0 Baffle is designed with a larger horizontal projection than the original Stamford Baffle.

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Setup Two – Density Current Baffle

Density Current Baffle Configuration

Vertical Position of the Baffle

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Setup Two – With Baffle

Flow Pattern (colored by speed, red is fast)

Treatment Flow Rate = 8.5 mgd

Flow speeds around launders are less

Density Current gets Underneath of Baffle

Here again, the baffle has intercepted and redirected the current away from the launder.

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Setup Two – With and Without Baffle

Higher solids concentration

Greater approach flow speeds

Flow is directed towards launder, but its speed is less near launders

Flow Pattern (colored by density, red is high)

Note the difference in solids concentration at the launder in the two images

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Setup Three

100 ft

18 ft

12 ft 3 ft

2 ft

Sludge Blanket3 ft

Not to Scale

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Setup Three – Without Baffle

Treatment Flow Rate = 6.0 mgd

Flow Pattern (colored by speed, red is fast)

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Setup Three – Density Current Baffle

Density Current Baffle ConfigurationVertical Position of the Baffle

Flow Pattern (colored by speed, red is fast)

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Setup Three – With Baffle

Treatment Flow Rate = 6.0 mgd

Speed of short-circuiting current is less with baffle in place

Density Current gets underneath of baffle

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Setup Three – With and Without Baffle

Higher solids concentration

Greater approach flow speeds

Flow is directed towards launder, but its speed is comparatively low

Flow Pattern (colored by density, red is high)

The difference in the solids concentration at the the launder is striking.

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CONCLUSIONS

1. A “classic” density current forms in each of the cases and rides along the top of the sludge blanket.

2. Without the baffle, the density current rises unimpeded and the solids concentration increases in the area between the tank wall and the launder

3. The SB 2.0 Density Current Baffle effectively disrupts the short-circuiting current and redirects it.

4. Depending on the position of the launder, the baffle may cause the current to flow toward the launder, but speed is greatly reduced.

5. For the operating conditions studied, TSS is reduced by 30% or more.