June 2013, Vol. 25, No.6

Operator Essentials

What every operator should know about secondary clarification

Tim Miller and John Esler



Practical considerations  

Secondary clarification 

The clarifier enables the activated sludge particles to come in contact through flocculation and to clump together and settle. They are later returned to the aeration tank. The treated effluent is discharged over the effluent weirs for further treatment. 


Typically, clarifiers are rectangular or circular. 

Clarifiers are not miracle workers; you need to grow good quality activated sludge in the aeration tank first. 


If you have a small or undersized clarifier, a faster-settling sludge with a low sludge volume index may work best. 


Big, oversized clarifiers mean you may be able to carry a higher mixed liquor suspended solids (MLSS) level and/or SVI in the aeration tank. 


If high flows from wet weather events are a regular concern, prepare the system to handle this condition rather than operating for normal flows. 

Effluent weirs 

A weir is a plate, usually with V-shaped notches, used to ensure a uniform flow leaving the clarifier. 

Weirs must be level and clean to avoid short-circuiting within the clarifier. Weirs may be located along the end or the perimeter of a clarifier. They also may be set inboard from the end wall or perimeter.  


Weirs located along the outside clarifier wall may undergo increased solids loss due to currents rising along the wall. 

Solids collectors 

These collectors are any devices used to remove the settled biomass from the bottom of the clarifier. 


Several devices are used in circular clarifiers, including scrapers to a hopper, rapid withdrawal tubes, and suction headers. 


Rectangular clarifiers generally use a set of continuous chains with flights. Less common is a traveling-bridge setup for solids and scum removal. 

Circular solids collectors operate beneath the water surface. The operator must rely on regular dewatering and inspection to assess equipment condition. 


Condition of the rectangular flights and chains can be observed as the flights bring the scum back to the scum trough. 


Daily observations — such as solids quality from samples taken at the pump, clarifier-drive torque readings, observation of the solids-withdrawal tubes, and watching the clarifier for unusual floating solids or unexplained high blanket readings — may help identify potential problems with solids withdrawal. 

Scum collection 

The scum-collection system usually consists of a scum baffle, skimmer arm, and scum trough located at the perimeter of a circular clarifier or the end of a rectangular clarifier. The system is designed to contain and remove floating material from the clarifier surface. 


A simple flushing mechanism often is used to ensure the removal of the scum from the scum hopper or trough. 

On circular clarifiers, properly adjusted and maintained skimmer-arm squeegees ensure that floating material is removed during every revolution. Scum baffles may be deepened near the scum hopper to avoid the loss of solids. Older clarifiers often have small scum troughs that can be overwhelmed by filamentous foaming episodes. Good operation in the aeration tank can minimize this issue. 


In rectangular clarifiers, the flights skim the surface scum on their return path to a scum trough at the end of the clarifier. 

(also called centerwell)

The feedwell is a cylinder surrounding the center feed column in circular clarifiers. Its purpose is to promote flocculation and assist in flow distribution. 

The feedwell must be 

large enough to avoid concentrating the downward flow, 

shallow enough to avoid scouring the sludge blanket, and 

small enough to effectively contain the influent flow. 

Energy-dissipating inlet 

This device is an “insert” or “tub” within the feedwell that is designed to dissipate the energy of the incoming influent and promote flocculation. 

Biological flocs are light and can be carried easily far into the clarifier by the incoming flow. These inlets slow down the incoming liquid and provide a better environment for flocculation and settling. 

Hydraulic detention time 

In theory, this is the time one drop of water would stay in the clarifier before discharging into the effluent.  


The formula is 

clarifier volume (gal) ÷ effluent flow (mgd). 

Typical values are 3 to 6 hours. 

In reality, the actual operating detention time (DT) is much shorter than the calculated time. Various factors, such as uneven weirs, poor inlet conditions, and density currents, act to shorten the actual DT. These factors all can contribute to reduced clarifier efficiency. 

Surface overflow rate (SOR) 

This is a measure of the hydraulic loading on a clarifier. 


The formula is: 

SOR =   effluent flow (gal/d) 

         clarifier surface area (ft2). 

This is an arbitrary value used by designers to size clarifiers. This value varies by state standards. 


An SOR of 800 gal/ft2•d (33,000 L/m2•d) often is a criterion for an average flow condition. An SOR of 1200 gal/ft2•d (49,000 L/m2•d) often is the peak acceptable loading condition. 


Target SOR often determines how many clarifiers will be kept in operation. 

Solids loading rate 

The clarifier solids loading rate is a key operating parameter, especially during high-flow conditions. If/when flows double or triple, the solids loading rate also increases significantly. A lower solids loading is always better for clarifier operation.  


Solids loading rate is defined as the mass of solids applied to the clarifier surface area. 


The formula is 

MLSS × (influent flow + return sludge flow) × 8.34  

clarifier surface area (ft2)  


Typical maximum range is 20 to 30 lb/ft2•d
(100 to 150 kg/m2•d).

To lower the solids loading rate, consider the elements of the formula: 

You can increase the clarifier surface area by putting more clarifiers on-line. 

You can increase wasting to decrease the MLSS concentration only if the lower MLSS still enables you to meet effluent limits. 

Recognize that increasing return sludge flow is not always the answer, because this increases the solids loading rate — much depends on how well the sludge is settling. 

If you have the capability, move the influent point farther along the aeration tank’s length (step feed). This change stores thicker solids at the head end of the aeration tank and reduces the MLSS concentration entering the clarifier. 

Sludge blanket levels
(expressed as blanket thickness [BLT] or depth of blanket [DOB])

The sludge blanket level is the amount of sludge held in the clarifier at any particular moment. The level usually is determined by inserting a “sludge judge” into the clarifier or using a suspended-solids meter. More and more, in-line meters are being used to monitor blanket thickness. 


Results of these tests are expressed as BLT, which is the feet or meters of sludge in the clarifier, or as DOB, which is the clear water depth to the top of the sludge blanket. 

Critical to providing good information are consistency in sampling location (this should be marked on the clarifier), measurement technique, and reading. These skills require hands-on training for new operators. 


The typical maximum target value is 0.3 to 0.6 m (1 to 2 ft). 


The balancing act is twofold. Operators should maintain a thick sludge to minimize the water returned to the aeration tanks or wasted to the solids handling unit, as well as avoid storing too much solids in the clarifier. An excessively high blanket can stress bacteria, result in solids loss into the clarifier effluent, or promote denitrification, which can result in floating solids on the clarifier surface or in the effluent. 

Density currents 

Ideally, the entire clarifier volume would be used to settle the activated sludge. However, the simplified diagram below shows a typical density current observed in clarifiers. 







Obviously, these density currents result in a large portion of the clarifier not being used for settling. In addition, the currents typically travel at velocities ranging from 1.2 to 3.0 m/min (4 to 10 ft/min). With these velocities, lighter floc material is carried in the current to the effluent, rather than settling out. 


Density currents are a major contributing factor leading to the loss of solids and clarifier failure. 


A baffle is a structure within the clarifier that is designed to interrupt or redirect the short-circuiting currents within the clarifier. 

Baffles have been used successfully in many clarifiers to slow density currents.  


In circular clarifiers with suction solids collectors, midtank baffles attached to the collector interrupt the current. These are referred to as Crosby cylindrical baffles. 


Angled baffles, called Stamford or Crosby peripheral baffles, are attached to the outer wall or bottom (knee) of the launder to redirect the current coming up the wall. 


In rectangular clarifiers, solid or open baffles (depending on the situation, one to four baffles have been used) are installed to slow the density current. 

Tim Miller is an operations specialist, and John Esler is president of Clarifier Performance Evaluations Inc. (Enfield, N.H.).