Sludge Blanket Considerations In Final Clarifiers, by William H. Boyle, P.E.

In the activated sludge process, the final clarifier follows the aeration unit and its primary purposes are to separate, by gravity, the solids from the mixed liquor flow from the aeration system, to collect the settled solids at the bottom, and to return some of these solids to the same aeration basins (return activated sludge) as well as waste some to the solids handling system (waste activated sludge).

It is important to know and evaluate how activated sludge physically acts at the bottom of the final clarifier because sludge deposition and removal are critical to the mass loading (lb/day/ft²) design factor as well as to the concentrations of return and waste sludge. Examination of the logic and data from field installations provides an insight into the sludge blanket level and its solids concentration. Such an examination may show reasons for discrepancies in certain solids flux models and other clarifier design assumptions.

Logic:

    Water seeks its own level.

    Water is a fluid (liquid).

    Activated sludge acts as a fluid (liquid).

    Therefore: Activated sludge seeks its own level.

According to technical literature mixed liquor is denser than the free water in a final clarifier. This fact has been substantiated by studies showing that the heavier mixed liquor, when entering the final clarifier basin, drops to the floor or sludge blanket. It then travels along the floor until it hits an obstruction; normally the peripheral wall for the center feed clarifier. This wall effect is a major process concern when designing final clarifiers. There are few reported instances where the solids in a final clarifier do not reach the wall. It is evident that activated sludge does not settle as a discreet particle (such as grit or primary solids). Rather, the activated sludge settles in a hindered zone of influence and is not governed solely by Stokes law of settling. Activated sludge has no choice but to seek its own liquid mass level in a clarifier if no mechanical or hydraulic limitations are imparted on it.

Sludge Concentration Variance in the Basin 

The sludge, most likely, will not be a homogeneous solids mixture in the sludge blanket, nor will the sludge blanket have a constant concentration of solids per unit area of the tank floor. At two Wisconsin wastewater plants, for example, data and measurements of solids in final clarifiers show random solids contour lines of where the sludge settles. 

Since sludge in each plant is different, the relative concentration will vary, but the ‘random’ pattern of the contours probably will not. Obviously, these contour lines will not be a perfect circle about the center line of the basin. A theoretical removal point will pass through as many as eight different solids contours for half the basin. It would be most difficult to design a sludge collector for rapid adjustment to each different concentration point. 

Even if there were well defined radial concentration points, it would be impossible to say where the heavy versus the light sludge would be at any given time. These contour lines and concentration points would be very ‘flow’ dependent and could vary throughout the day and hour.  

Design of Basin Collection Troughs (Part 2), by William H. Boyle, P.E.

If the outlet channel has a water depth within it that will be above the critical depth of the collection trough, then a “submerged discharge” condition exists. The forces that would govern this water level in the outlet channel would be based on the condition downstream. The point where the collection trough enters the outlet channel would then have a submerged depth (HL) greater than the critical depth (Hc). To compute the upstream water depth (Ho), the following formula would be used:

Ho= [HL^2+2Q^2/gb^2HL]^1/2   HL = submerged depth in ft.

By the use of the submerged discharge formula, the headloss would be minimized and the additional head required in the project scheme would be eliminated.

The above formulas are based on the momentum theorem and the use of simplifying assumptions. These assumptions are: the kinetic energy of the water falling into the trough does not contribute to longitudinal velocity, level inverts, flow is substantially horizontal in direction, and the water surface curve is approximated by a parabola.

To design the trough, a sufficient freefall should be available at the upstream part of the trough so flooding of the weirs does not occur. Total headloss from basin water surface to the water surface in the outlet channel is therefore computed by adding the difference between Ho and HL + freefall into the basin trough + head on weir. On short runs of troughs, friction could be neglected, but friction should be incorporated when the headloss is extremely critical and the length of run is extensive. 

When a specific headloss (H0 – HL) is required for the basin collection trough, the trial and error method must be used.  This headloss requirement is a result of the outfall condition mentioned above. In many cases this headloss requirement will be the governing factor for the basin trough design. And remember, by increasing the basin trough cross section, the loss of head becomes smaller.

In summary, it is important for the design engineer to determine if the collection trough will have free or submerged discharge, and size it accordingly.

Design of Basin Collection Troughs, by William H. Boyle, P.E.

An important item to be investigated when designing a water or wastewater treatment plant is the design of the collection troughs required by the many different basins in the treatment facility. These collection troughs have a direct bearing on both the process and the economic aspects of the facility.

The process aspect of collection troughs involves the requirement to handle a maximum specific flow rate. All basin collection troughs have one thing in common: they must be sized properly so the hydraulics of the troughs will not adversely affect the intended unit process function and design. The trough must be designed properly so that flooding of the weirs does not occur. If flooding of any portion of the weirs did occur, the basin hydraulics would lose their continuity; thus the individual unit process would suffer. Each basin requires a different arrangement for the collection troughs, depending upon its designated application. 

Another important aspect of the design of the collection trough would be that of economics. A trough should not be over-sized because this would increase the cost of the total project. On the other hand, a trough should not be undersized, because that would adversely affect the process aspect of the project and the headloss required for it. A proper economic sizing of the trough should be a compromise between an economic determination of the cost of the trough versus the cost of handling additional flow and headloss.

Many of the treatment facilities now being planned are extensions of existing installations. Most existing treatment facilities are located on or near a receiving body of water and therefore the outfalls are set in accordance with the existing water level of the body of water. This outfall condition would affect all the unit process basins upstream of the outfall structure and set and/or limit the head available to them.

If head is no problem in the treatment scheme, a collection trough that discharges into a collection/outlet channel will be at a free discharge condition. The water level, as it flows into the outlet channel, will be flowing at approximately critical depth. To compute the upstream water depth (Ho), 1.73 x the critical depth (Hc) can be used.

           Hc = [Q^2/gb^2]^1/3      Q in cfs;  b in ft.;  g=32.2

The use of free discharge arrangement would give the most economical design, because it yields the smallest basin trough cross section possible. However this design utilizes the maximum head.

In my next post, I will examine a scenario with a “submerged discharge" condition.

Side Water Depth Considerations (Part 2) by William H. Boyle, P.E.

Solids Can Hinder Performance

If there is an appreciable amount of solids in the clarification zone, the performance of the clarifier will be inadequate.

The zones beneath the clarification zone are affected by the amount and makeup of the solids coming in to the clarifier. As the mixed liquor comes into the clarifier and settles, the associated water flows from the mixed liquor mass towards the effluent, thereby depositing the solids towards the bottom zones. The incoming mixed liquor will flow over the settled sludge if there is a noticeable difference between their densities. With low MLSS, a noticeable difference would occur at the bottom of the clarifier where the ultimate concentration appears below the compression zone. With higher MLSS, the hindered zone and transition zone become more of a governing factor on the flow pattern of the mixed liquor.

Let’s look at five cases of possible theoretical bench test results for further understanding. 

The dimension-less parameter Sludge Volume Index (SVI) will be used for the development of this rationale. Sludge Volume Index is the volume in milliliters occupied by one gram of activated sludge after settling for 30 minutes, expressed as:

SVI = ml of settled mixed liquor after 30 min / ppm SS in mixed liquor x 1000

SVI = ml @ 30 min / MLSS x 1000

It is readily agreed upon that as the SVI increases over 100 poorer settling characteristics of the mixed liquor will be realized. Looking at a typical SVI of 100 and relating this to various MLSS concentrations, the following can be developed: 

For SVI = 100	MLSS	ml @ 30 min.	% of depth
Case 1	2000	200	20
Case 2	3000	300	30
Case 3	4000	400	40
Case 4	5000	500	50
Case 5	6000	600	60

For every 1,000 ppm of MLSS increase, the ml settled solids @ 30 min. increased by 100. Percentage of depth is based on the milliliters of solids settling in a 1,000 ml graduated cylinder.

It is obvious by looking at the five cases the clarification zone in Case 1 is larger than in Case 5.  In an actual plant, if a given clarifier with a given depth was designed and operated at the condition found in Case 5, it could be anticipated that the effluent quality would suffer due to the lack of available clarification zone. If this was the case, then the high MLSS would require a greater basin depth to maintain an adequate clarified water zone because the incoming mixed liquor solids contact more solids in the hindered and transition zones.

The design engineer normally does not have the luxury of running actual bench tests for the plant that is about to be designed. Since this is often the case, then the following might be a useful procedure to use when determining the side water depth for the final clarifier, keeping in mind the above discussion. An investigation into an existing clarifier’s performance could also use this tool as to evaluate performance.

Depth Percentage

Based on the premise that a clarifier handling 2,000 MLSS with an SVI of 100 performs adequately at a 10-foot side water depth, a percentage of depth to MLSS versus mL ratio can be determined. The above shows the percentage of depth at the various MLSS. Note that for every 1000 ppm MLSS increase, the percentage of depth, based on the milliliters of solids settling in a 1,000 ml graduated cylinder, increases by 10%. Thus, a rational conclusion would be: as the MLSS increases by one thousand ppm, the side water depth should increase by one foot. This is to maintain the same clear water depth above the sludge blanket.

By using the data, the five cases depict the change in MLSS and indicate that although the initial settling rate might be somewhat the same; the zones below the clarification zone become deeper, reducing the depth of the clarification zone. An extreme case would occur if the clarifier was operating with the sludge blanket level at or near the basin water surface causing gross carryover of solids.

If the one foot increase in side water depth for every 1,000 increase of MLSS is used as a guideline, then any mixed liquor below 100 SVI should be acceptable.

High SVIs?

There is certainly a limitation with the use of the above SVI approach in evaluating side water depth in relation to the effluent quality. As stated above, an SVI over 100 results in poorer settling characteristics of the mixed liquor. Looking at sludge with an SVI of 200 and an MLSS of 5,000 the percentage of depth would be 100% (200 SVI * 5000 MLSS/1000 = 1000 ml @ 30 min = 100% depth). Since this is somewhat unrealistic, the guidelines of using SVI for determining side water depth would be in question for high SVIs. However, it might be an indication of why there is a poor effluent quality with high SVI sludges. In this instance, the high SVI relationship to effluent quality from the final clarifier should be looked at relative to the overflow rate, inlet structure, sludge removal mechanism, and/or mass loading.

The above are parameters that need field data and/or field observations to verify. The state point analysis is another useful tool to get a “feel” for what is happening inside of a circular clarifier.

Side Water Depth Considerations by William H. Boyle, P.E.

Side Water Depth (SWD) is critical when the design engineer is planning new clarification equipment or evaluating existing units. Side water depth/sidewall depth is the depth of the water at the wall of the basin.

The application of the clarifier/settling tank has a lot to do with the selection of the side water depth. For a grit collector where the material acts according to Stokes Law, the side water depth can be fairly shallow. With waste treatment primary units, the side water depth is determined by holding solids inventory. In chemical clarifiers, solids inventory and detention time for flocculation and separation might be the basis of determining the side water depth. For water treatment units, the same parameters as chemical clarifiers/settling tanks are used. All of these parameters are site specific as well.

The following is a discussion based on waste activated sludge final clarifiers. There appears to be more of an art to the design than just following a set of scientific guidelines with respect to SWD.

Waste Activated Sludge Final Clarifiers/Settling Basins 

Much has been written on the design parameters to be used for final clarifiers, such as overflow rate, detention time, mass loading, settling velocity, etc. Unfortunately, not much has been written about SWD and its effect on clarifier performance.

It is common practice to design and evaluate a clarifier based on surface overflow rate and mass loading. In the past, detention time was used in selecting the depth of the clarifier. Current practices involve selecting the side water depth independent of detention time. However, little or no hard data has been generated on the effects that side water depth has on the performance of final clarifiers. This is probably due to the theoretical nature of the items to be considered.

A clarifier should have sufficient depth to handle the transportation and solids inventory required for efficient operation. If the solids-holding depth is not adequate, gross carry-over of the solids into the effluent can result, thus reducing the effluent quality. The question then arises: Will the clarifier need additional depth to handle additional mixed liquor suspended solids (MLSS)? With this consideration, it would appear the side water depth should be selected on the basis of both required sludge detention time and the need to handle the above additional mixed liquor suspended solids.

The need for the correct design depth of the clarifier is made clear by examining the different settling zones (which have different settling characteristics) created by waste activated mixed liquor. These various zones as stated by Eckenfelder and Ford (starting at the top of the basin) are:

• Clarified water zone
• Individual particle zone
• Hindered zone
• Transition zone
• Compression zone
• Alternate concentration zone

The top and bottom zones are the most critical for final clarifiers. The bottom concentration zone is affected by the removal device and the mass loading. The top clarified zone is affected by the type of inlet and overflow rate. Since the main purpose of a final clarifier is to have ultimate clarification of the treated sewage, these zones should be looked at most critically.

Clarifier Drive Torque: Operating Use of Torque by William H. Boyle, P.E.

Normally a clarifier runs at one torque value for 90 to 98 percent of the time. This torque value could be termed the "running torque" of the clarifier. Sometimes the running torque is called the "design torque". The drive unit should be designed for this running torque, which will give the most economical mechanism based on the intended use of the clarifier.

The drive unit should be protected from excessive loads that would require it to run above its design requirements. An alarm should be set that provides adequate protection, and tells the operator that there is something affecting the clarifier mechanism. This alarm setting should be above the running torque; 120% of the running torque is generally sufficient.

Use of an alarm torque value is beneficial not only in protecting the drive unit, but also in giving an added range of running torque in case the loading values were incorrectly selected.  The drive unit should not be designed to run continuously at or near the alarm setting. This would defeat the reasoning behind choosing the running torque requirement.

For the protection of the drive unit, a shut-off or cut-off torque rating should be used. If there is some abnormality in the clarifier causing a rapid increase in torque, the unit would shut itself off without adversely affecting the mechanism. Shut-off torque should be used only as a protective rating, however, and the unit should not be running continuously at this value; 140% of the running torque appears to be adequate. If the drive was sized and designed to run continuously at this value, it would be over-sized and therefore an uneconomical choice consideration.   

Peak torque is the value determined by the supplier of the drive unit indicating what the unit can handle at a momentary or instantaneous load. This ultimate load should be of very short duration, e.g. in the three (3) second range, in case the torque is increased so rapidly that response time to the shut-off mode is not enough. Peak torque is normally two (2) times the running torque. 

As described above, an overload device is essential for the drive unit so that the alarm and shut-off torques can be sensed and the unit protected. This is particularly important in the larger units because a good deal of money is invested in the drive and adequate protection is critical. Overload protections can be electronic and/or mechanical. On the smaller drive units, such as those that rest on the bridges of small clarifiers, the complete range of protection is not as critical. A simple shear pin device that turns the unit off when an excessive torque is sensed is generally adequate.

As engineers and designers progress in their determination of the drive units to be furnished, they should remember that reputable equipment suppliers stand behind their drives. It is apparent by the operation of the clarifier if the drive is working properly.  However, if project engineers state a torque value above the necessary range, they want to be assured of the results. Therefore, the specification should clearly state desired torque ratings: running, alarm, and shut-off.

As an additional requirement for a project, a torque testing procedure could be specified if required. This is a costly procedure for most projects, but if the engineer feels he or she is not getting, or will not get the drive that was specified, he or she could call for torque testing of the drive unit in the field.    

Clarifier Drive Torque: Selection and Clarity by William H. Boyle, P.E.

In order to provide an accurate determination of torque for specific applications, specifying engineers should be familiar with the application, selection, and terminology related to drive torque requirements. Also, required design specifications should be clearly communicated to suppliers.

The dictionary definition of torque is “a force or combination of forces that produces a twisting or rotating monitor.”

When engineers specify a particular torque value for a clarifier, they are limiting the suppliers to one particular design requirement. By selecting and specifying a torque, the designers are trying to assure the drive will function as intended. To truly receive what they are specifying, specifications should be made clear and simple.

In the circular clarifier field, the types of drives vary with each equipment supplier. The basic arrangements are: (1) the half bridge design where the drive rests on the center column; (2) the full bridge design where the drive rests on the bridge spanning the tank. While other arrangements do exist, the two described above are most commonly seen in the circular clarifier field. 

Torque Calculation

Calculation of torque for a circular drive unit is based on the simple cantilevered beam type of equation, with a uniform load L (#/ft. – pounds per foot) applied to the rotating sludge removal arm. Torque required to turn a rake arm would equal the resultant force of the uniform load (L*R) [R, Radius of the basin] multiplied by the moment arm (R/2). Since most circular clarifiers have two arms, the resulting equation will be T = L*R^2 (expressed in ft•lbs [foot pounds]).

Loading varies with the type of waste to be handled, material density, depth of sludge, and angle of repose of the solids, as well as other intangible parameters. Loadings for various industrial sludges are sometimes available based on pilot work or field data. The following table shows suggested uniform loadings (L) in lbs/ft. for various sewage treatment applications.

Application	Loading L - #/ft.
Grit	40
Primary w/grit treatment	8
Primary w/o grit treatment	10
Secondary (scraper)	6
Secondary (suction)	4
Gravity Thickener primary only	30
Gravity Thickener primary w/ secondary	20 - 30
Torque = L*R^2 = foot pounds    Radius R in ft.

There are times the skimmer could be hung up on the scum trough and possibly impart a torque to the center mechanism and subsequently to the drive unit. This is not a major factor since the skimmer and the supports would act as a torsion-bar (in most cases) and dissipate the torque energy, causing a mechanical failure before it got to the drive unit. There are some designs that use a fail-safe release to minimize this concern.