How to Obtain Better Performance from an SST by Exploiting the Sludge Blanket Momentum Preservation ^†

Abstract

In static conditions, the only mechanism available for sludge/water separation is sludge sedimentation by gravity. In dynamic conditions an additional mechanism is available: sludge momentum preservation. In order to achieve a better understanding of the operation of a secondary sedimentation tank (SST), the authors analyzed the behavior of the sludge blanket (taking note of the concentration in vertical and horizontal directions) and how it relates to the hydrodynamic fields within the SST. These findings have been interpreted based on hydrodynamic principles: momentum preservation, in case of any energy loss; motion of fluids from an area with higher potential energy to an area with lower potential energy; and the ratio between inertia and gravity forces. The results indicated that the sludge blanket momentum is a parameter of great importance for understanding the behavior of an SST. According to these principles, a longitudinal flow rectangular clarifier has been converted into a transverse flow clarifier, obtaining considerable improvement in operating performance. Moreover, it should be noted that there are already design strategies based on the optimization of water/sludge different momentum as a mechanism to improve the performances of a secondary clarifier. Peripheral feeding in the circular decanter; perforated baffles installed on a rectangular decanter; and the distance to be maintained between the bottom wall of a rectangular SST and the clarified water collection channel are all design strategies explained on the basis of the different sludge/water momentum rather than solid flux theory.

1. Introduction

In order to obtain more information about the behavior of the sludge blanket in an SST the author carried out investigations on many circular SSTs with the following procedure:

• The scraper bridge was stopped;
• Sludge samples were taken at different heights from the bottom on several vertical lines at different distances from the tank center;
• The sludge samples were sent in a laboratory for analysis.
• The results can be summarized as follows:
• The sludge blanket’s upper surface is horizontal throughout the decanter;
• The potential energy of the sludge blanket is at the maximum at the back of the external vertical wall and at the minimum at the center of the tank; where the suction for RAS is located;
• The sludge on the back of the external wall is in a thickened state.

Similar results were obtained also for rectangular SSTs. The results obtained are summarized in Figure 1 and superimposed on top of the hydrodynamic field inside an SST.

In examining this picture, the following points appear reasonable:

• The bottom flow, having centripetal direction towards the RAS suction point, is made up of thickened sludge;
• The upper flow immediately above, having a centrifugal direction (which, on arriving close to the peripheral vertical wall of the tank, divides into two streams: one which turns upwards towards the outlet weir; and another which turns downwards towards the thickened sludge,) is made up of MLSS (mixed liquor suspended solids) still not separated into water and sludge. In this layer, the significant change of the sludge concentration at different depths is mitigated by the turbulent diffusion phenomenon, which increases as the rate of the two layers increases: the thickened sludge moves towards the central bottom of the tank; the upper layer towards the vertical peripheral wall. Thus, in the central part of the tank where the velocity of the two layers is very high, no sedimentation can occur at all;
• The uppermost layer is made of clean water, and close to the outlet weir, it divides into two streams: the first goes towards the outlet weir, the second generates a superficial stream which tends to return towards the center of the tank.

The prime issue to achieve in the correct performance of an SST concerns the division of the intermediate stream into two streams going to feed both the bottom flow of thickened sludge and the upper stream of clean water, in other words: the sludge/water separation. This division can be explained as follows:

As long as the peripheral vertical wall (on which the outlet weir is located) is far enough away and the stream speed is high enough, the stream proceeds with a centrifugal direction without any difficulty and without any, or very little, sedimentation hampered by turbulent diffusion. However, when the mixed liquor stream is close to the peripheral wall, the clear water is diverted upstream towards the drainage channel, but the sludge flocks, for the stronger inertia, to go straight on, until it impacts in the peripheral vertical wall. The separation occurs when the ratio between drag and gravity forces affecting the sludge flocks is low enough.

When synthesizing, the behavior of the sludge blanket is affected much more by the density currents than by the scraper bridge. “… it means that the chain-and flight sludge collectors may simply move at the bottom of the sludge blanket and have very little real effect on the gross movement of the majority of the sludge solids”: (see reference [1], included in the Clarifier Design Book, edited by Water Environment Federation, second edition). The impact of the solid particles in the vertical wall generates a stockpile of thickened sludge in the so-called “dead zone” of the decanter. If there is no loss of energy, this impact produces an increase in the potential energy of the sludge blanket which, as a first approximation, we can hypothesize to be directly proportional to the variation of the momentum and inversely proportional to the quantity of mass in the system, that is, to the recirculation ratio.

Ep = K* C/r * (Q + rQ) * (Q + rQ) = K* C/r * (Q + rQ)²

(1)

where:

• C is the MLSS concentration;
• Q and r the incoming flow and the recicle ratio;
• K is a constant in the system;
• C*(Q + rQ) is the sludge mass entering the SST in unit time, that we can define as “horizontal solid flux”;
• (Q + rQ) for a given SST (that is for an equal peripheral cylindrical surface) is proportionate to the horizontal solid flux velocity noted above which is that pertaining to the middle layer with centrifugal direction.

The flow of thickened sludge which heads towards the RAS suction point comes from the stockpile of thickened sludge found at the back of the peripheral walls.

This expression has been confirmed by the measures taken by the authors on circular SSTs, as represented in Figure 2 (See Appendix A).

2. The Modifications Carried out on the Rectangular SST of the “Grandis Line” of Lido di Camaiore WWTP

If the interpretation of an SST operation given in the previous paragraph is correct, increasing the length of the outlet weir would be of no or little use; rather, the Q/S ratio should be decreased; where Q is the clear water flow and S the vertical surface that must be crossed by the liquid current Q near the outlet. These concepts were applied to improve the functioning of a rectangular SST of Lido di Camaiore WWTP.

In Lido di Camaiore WWTP, four lines were available for biological wastewater treatment (oxidation followed by secondary sedimentation tank), with roughly the same size and operating parameters as MLSS concentration and SVI; as is shown in the following

Table 1. Main characteristics of the four biological process lines of Lido di Camaiore WWTP.

Process Line Name	Oxidation Tank Volume	SST Number and Shape	SST Size	Overall SST Volume W and Surface S	Overall Overflow Lenght
Italba 3	900 mc	N°1 circular	D = 13.4 m Hu = 2.25 m	W = 315 mc S = 140 mq	42.1 m
Tecnitalia 1 + 2	800 mc	N°1 circular	D = 14.9 m H u = 2.60 m	W = 455 mc S = 175 mq	46.8 m
SIAF	850 mc	N°1 circular	D = 14.9 m Hu = 2.25 m	W = 392 mc S = 175 mq	46.8 m
Grandis	800 mc	N°2 rectangular	Lungh. 15 m Largh: 5.0 m Hu = 2.45 m	W = 367 mc S = 150 mq	36.0 m

:

The overflow weirs of the two rectangular line Grandis SSTs were located on the short side (5 m length), as well as on the two longitudinal sides for 7 m from the corners. The settled sludge removal was operated by aspirated scraper bridge and a longitudinal concrete channel. The design flow for all the process lines was 150 mc/h; but while for the first three it was possible to increase the incoming flow even above 200 mc/h without problems, it was impossible to supply the Grandis line with more than 75 mc/h because of massive-activated sludge overflow in the drainage channel. It is to be noted that the design SST loads, as overflow rate and solid flux, were roughly the same for all the process lines. The only little difference concerned the overflow specific speed: for the first three it was roughly 1.0 L/s per m (but it has been increased up to 1.35 L/s per m without any problem) for the Grandis line, instead, at design flow it should be 1.16 L/s per ml, but it was impossible to feed the SSTs with more than 75 mc/h, corresponding to 0.58 L/s per ml.

By applying the principles set out above, the problem has been solved by transforming only one of the two rectangular SSTs available for Linea Grandis from longitudinal flow to transversal flow (see Figure 3), that is: The MLSS incoming flow, that following the original design was fed on a short side with clear water collected on the opposite side, was instead fed on a longitudinal side, and the clear water collected on the same side. Originally it was planned to modify both rectangular SSTs, but since after the first was modified there were no more problems for the whole plant, the second SST was not modified. It has not been possible (to this date) to carry on with the collection of more data of the performance of the modified rectangular SST, but it can be said that it was possible to feed to the modified SST up to the original design flow of 75 mc/h without any problems. The obtained results cannot be explained at all by solid flux theory, but only on the basis of the behaviour of an SST, as illustrated in the previous paragraph.

3. Existing Design Strategies That Comply with the Given Explanation of the SST Operation

Moreover, it should be noted that design strategies for improving the SST performance that cannot be explained on the basis of flux solid theory already exist. However, the obtained results can be understood on the basis of the behavior of an SST, such as the one above illustrated.

3.1. Peripheral Feed Decanter

In a secondary sedimentation tank with the flow of liquid aerated from the periphery and collected in the sludge from the center, the hydrodynamic flows are those represented in Figure 4.

The water in these tanks is forced to undergo a reversal in direction, while the sludge enters from the periphery and exits from the center without undergoing any reversal in direction. This performance is exactly the opposite of that of a traditional sedimentation decanter (Figure 1) in which the sludge is forced to undergo a reversal in direction while the water enters from the center and exits from the periphery. With the interpretation of a secondary sedimentation tank as indicated above, it is obvious why the peripheral feed tank, with equal dimensions, guarantees a much better performance than the traditional sedimentation decanter. In fact, it requires much less energy to invert the flow of the clarified water than that required to invert the flow of thickened sludge.

3.2. Vertical Baffles, Perforated or Not, Installed on Rectangular SSTs

One of the design strategies investigated by several authors for a rectangular SST is the installation of vertical baffles, perforated or not, along the tank at different distances from the inlet side. This suggestion has been included also in the WEF manual for settling tank design (see reference [2]). The impact of the sludge blanket current on a vertical wall has already been described above, it is not necessary for the vertical wall to be the external tank wall.

Figure 5a represents the hydrodynamic field (top) and sludge blanket (bottom) in a rectangular decanter with an intermediate vertical baffle. Figure 5b represents the sludge blanket in a rectangular decanter with several perforated vertical baffles.

3.3. Recommended Placement of Transverse Weir Away from Effluent End

The previously mentioned WEF manual [2] recommends that the terminal drainage channel of a rectangular and longitudinal flow sedimentation tank be arranged at a distance from the back wall that is larger than the depth of the tank, and that in any case, is not less than 5 m. (See Figure 6). This recommendation derives from the interpretation of the functioning of a secondary decanter indicated above. Although the trials were run on circular decanters, it is clear that the fundamental physical principles are the same whether for a circular decanter or for a rectangular one.

This suggestion becomes clear with the interpretation of the sludge blanket behavior described above, for which the thickened sludge is piled at the back of the peripheral wall where the potential energy of the sludge blanket reaches the maximum value.

3.4. Recommended Placement of Sludge Hopper

As for the previous point 3.3, the suggestion represented in Figure 7, becomes clear with the interpretation of the sludge blanket behaviour described above.

4. Conclusions

A new approach was developed for studying secondary sedimentation tank operations. This study covered the basic physical principles applied to suspended sludge in an SST preservation of momentum, the ratio of drag to gravity, and the potential energy variations of the sludge blanket under changing hydrodynamic conditions. Tests carried out demonstrated that the approach is correct, and that it is possible to define new design and verification criteria for SSTs.