As with all mechanical assemblies, circular clarifiers require annual maintenance to maintain and extend service life and to prevent possible future operational issues that could lead to a shutdown.  Many of these systems were installed in the 1970s and 1980s and have thus surpassed their design life of 20 years. 

Plant personnel should not only investigate these clarifiers’ mechanical/structural welfare but also check their original design capacities against the plant’s current flow patterns.  Some types of clarifiers are flow-sensitive.  If the current flow exceeds the clarifier’s design range, the clarifier’s efficiency could be severely reduced.  Many innovations and refinements to the design of clarifiers have been made over the years, to increase flow capacities and to improve efficiency in the settling and collection of settled solids. 

Circular clarifiers fulfill a variety of functions in the scheme of a treatment plant, from grit collection, primary collection, secondary fixed film or activated sludge collection, thickeners, etc.  Because of the complexity of all of the different mechanisms, this article will focus on the secondary clarifier in an activated sludge process.  The article will provide plant operators with a guide of structural items to check at the annual inspection and some insight into the innovations and upgrades available to improve an existing clarifier’s performance.

Annual Inspection Checklist

To ensure a circular clarifier’s continued proper maintenance, drain and hose off the clarifier mechanism at least once a year for inspection.  Examine each of the major components within a secondary clarifier, paying particular attention to the drive unit and structural components at the water line. 

Drive Unit

Drive unit configurations vary greatly between different clarifier manufacturers.  This discussion is based on a cast iron, split gear, pier-supported drive unit that is furnished by several OEMs.  First, review the OEM manual for items unique to the drive unit on-hand.  Before the clarifier mechanism is shut down and the basin is drained and cleaned, investigate the unit for any unusual noises or vibrations while it is still in operation.  Noises most commonly originate from the motor fan or from a poorly lubricated/loose chain and sprocket arrangement.  Also check sprockets for signs of “barreling” at the root of the sprocket tooth.  Inspect all exposed shaft seals for signs of leakage and the shaft itself for grooving.  Clean the gear reducer vents so they remain operational.  

To use a simple, old-time method of checking for vibration, stand, or actually sit, on the drive housing while it runs for a full revolution.  If vibration occurs, note the position of the main tank skimmer.  While the interaction of the rotating skimmer assembly with the fixed scum beach/trough might be the cause, it should not be a point of concern.  However, the skimmer assembly may have fallen out of proper adjustment.  A severe vibration, accompanied by a “snap, crackle and pop” noise, might be caused by a broken ball in the main bearing, in which case the OEM should be called for repair options.

Once the mechanism is properly shut down and locked out, drain the oil and visually inspect the oil for condensate/water content.  Water can occur in the oil or grease for a myriad of reasons, ranging from a simple collection of condensate to corroded dust shields.  As contaminated oil and grease is the main cause for deterioration of service life, study periodic oil samples to determine a proper maintenance schedule.

Center Pier and Cage

The center pier and cage usually have areas of corrosion at the water line due to the fluctuation of water level and exposure to the atmosphere.  Often, a significant reduction in the material occurs at the upper influent port areas of the center pier.  When assembling the clarifier mechanism, the pier is the first component to be erected, which in turn means that just about the entire mechanism must be disassembled for the pier to be replaced.  It is sometimes more cost effective to simply replace the entire unit than it is to fully disassemble the existing one.  For instance, if the pier is corroded beyond repair then the high cost of disassembly and re-assembly labor would warrant investing in a whole new mechanism. 

Inspect the individual angle members of the center cage structure for signs of corrosion at welded joints or deformed members.  Examine bolted connections to the drive, manifold and truss arms for both corrosion and tightness of bolts.

Settled Sludge Collection Device 

There are many methods of sludge collection in a clarifier, including using a hydraulic sludge header with manifold, riser pipe clarifier with pipes, V-plows and sludge box, spiral scraper and others.

With hydraulic removal types, one critical component is the condition of the seals that allow transfer of the collected sludge in the rotating mechanism to a fixed withdrawal point.  When these seals fail, the suction head is significantly reduced from the collection arm, thereby prohibiting uniform pickup along the entire length of the arm.  It is important to regularly examine the condition of these seals, per recommendations in the OEM O&M manuals. 

Skimmer Arrangement 

Inspect the existing skimmer assembly and scum blade arrangement for deformed members, broken connections and corrosion.  Also check and replace skimmer wipers.  Evaluate the operation of the skimmer 

traveling up the scum beach and over the trough and, if needed, adjust the mounting for smooth operation.

Access Bridge 

Review the access bridge support structure as well as the walkway material, checkered floor plate or grating, and handrail.  Additionally, give special attention to the structure’s underside as this is where moisture collects and corrosion is common.  As part of the inspection, scrutinize the expansion/contraction slip type connection located either at the tank wall or at the drive unit, to verify that movement from fluctuations in ambient temperatures can occur.  Otherwise, thermal movement in the bridge can pull or push the mechanism’s center pier out of plumb, causing skimmer-to-scum trough issues, interference with the mechanism and basin floor, and related stress on the structure and drive unit. 

Plant Flows 

Review the original O&M manual for the original design flows of the clarifier, which should be compared to current plant flows.  Over the equipment’s life, the incoming flows, changes in pumps or the plant’s process sometimes significantly alter the loading to the clarifier.  These changes, especially higher or lower RAS rates, can negatively impact the effectiveness and efficiency of the hydraulic sludge removal system. 

Clarifier Upgrades and Conversions

Significant improvements have been made to the performance of circular clarifiers over the last 20 years.  Many early designs with shallow side water depths of 8’ - 10’, along with outboard effluent channel designs, led to elevated levels of solids in the effluent, especially at high flows.  Alternate methods that introduce the flow into the basin and collect the settled solids in an efficient and uniform manner have also helped improve the clarifiers’ overall performance. 

Energy Dissipating Inlets 

Oversized center pier ports and increased influent well diameters alone do not sufficiently reduce inlet kinetic energy and related flow vectors within the body of the clarifier.  At higher flows, these currents can re-suspend the settled solids and deteriorate effluent quality.  This led to the introduction of energy dissipating inlet (EDI) baffle systems which, as the name indicates, dissipate influent energy.  Some systems also promote particle contact.  The combination of both promotes lower effluent suspended solids and higher RAS solids concentrations.  Numerous designs are available, many of which can be added to existing clarifier mechanisms.  Some important design factors to be considered when evaluating a system include:  

• The number of times the flow changes direction before entering the influent well, as kinetic energy is consumed with each redirection. 
• The individual ports’ effect on reducing the incoming velocity and just as importantly, possible clogging issues.
• Areas of flow impingement on itself create mixing and passive flocculation, which promotes better settling.
• Possible “dead” areas in the basic arrangement that could lead to solid deposition and septic conditions.
• Within the basic arrangement, the degree of field modifications to the existing clarifier structure and ease of assembly which impacts the overall costs.
• Before purchasing, design calculations furnished by the manufacturer, verifying overall reduction of velocities and required head losses.

The addition of these baffle systems can have a positive effect on the performance of all existing clarifiers including those with shallow side water depths and even primary clarifiers.

Peripheral Feed, Peripheral Overflow Clarifiers

Some treatment plants have large diameter existing pier-supported side-fed clarifiers that were designed and installed when skimming was not a priority.  The inlet pipe is supported from the access bridge, making it almost impossible to add a skimming arrangement.  An alternative to excavating outside the tank and the basin floor, burying the inlet pipe and then converting the unit to a siphon center feed is to retrofit the clarifier to a peripheral feed, peripheral overflow (PFPO) design clarifier. 

With a PFPO clarifier, the influent is introduced into the clarifier through a hydraulically designed channel that extends around the entire circumference of the basin.  This channel must be designed to include orifices in the floor, of specific diameter and spacing to accommodate the plant’s current flow requirements.  In addition, proper orifice and skirt baffles are necessary to obtain optimum clarifier efficiencies.  The clarifier effluent is collected by an additional peripheral channel located adjacent to the influent channel. 

First introduced in the late 1960s by Siemens Water Technologies, this concept can provide greater hydraulic efficiency – 50 to 80% more than a center feed clarifier.  The combination of influent/effluent channels can be fabricated steel or poured-in-place concrete, doweled to the existing basin wall.  The existing clarifier mechanism requires few modifications if it was recently replaced or is in good shape with remaining service life.  With this concept, conventional main tank skimming is available along with numerous influent channel skimming options.

Riser Pipe Clarifiers 

In the 1970s and 1980s, many secondary clarifiers were furnished with a “riser pipe” design clarifier consisting of two truss arms located at the basin floor.  The design also had a series of V-plows and sludge suction pipes that terminated in a sludge box, located beneath the clarifier drive unit.  Each pipe had a flow control device within the sludge box that allowed the operator to fine-tune the withdrawal process depending on flow rates.  In reality, due to the number of devices and frequency of flow fluctuations, the control devices were seldom adjusted after the initial setting at startup.  Other issues included the clogging of pipes, breakage of elbows (caused by rodding out the plugged pipes) and, at higher withdrawal rates, “rat holing” of clearer liquid through the sludge blanket at the sludge pipe inlets.

A tapered tube design is an alternative for sludge collection.  It utilizes various sized orifices, is spaced not more than 30” on center, and is hydraulically designed to uniformly withdraw settled sludge along the entire basin radius.  With this design, the available suction is spread evenly along the tube to prevent “hot spots” of suction.  Internal velocities must also be uniform and sufficient, to prevent solids from settling within the tube itself. 

Additionally, there are several other methods of retrofitting riser pipe clarifiers with a sludge collection header, with most requiring the replacement of the mechanism.  In some cases, the drive unit and/or the access bridge can be reused to keep costs to a minimum.  If the RAS rate is controlled outside of the basin proper, the basin can be easily modified at the basin’s center by core drilling a new RAS sump or cutting and replacing some concrete.  If the only available RAS control is in the clarifier, then an alternate mechanism can be furnished, complete with a sludge collection header, a single vertical riser duct with one control device, and a sludge well.  Due to variations of clarifier basin/piping construction, plant personnel should request that the selected manufacturer evaluate the most practical and cost-efficient method of upgrading the clarifier. 

Conclusion

Many different models of circular clarifier mechanisms are available for water and wastewater treatment.  Regardless of the specific model or which application it is used for, basic knowledge of the major components within a clarifier is essential.  Routine maintenance and general troubleshooting can be performed through a combination of visual/audio observation, reference to the O&M manual, and consultations with the equipment manufacturer.  Certain manufacturers also offer short seminars on available options and onsite evaluations to help plants select the most appropriate and cost-effective replacement or upgrade.  Many innovations and upgrades are available for plants wishing to ameliorate their existing circular clarifier’s performance.  Improved drive units and enhanced skimming arrangements are just some of several upgrade options that are available. 

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