CPE Services, Inc.
Enfield, NH

OPTIMIZING CLARIFIER DESIGN and PERFORMANCE
Prepared for the University of Wisconsin BNR Short-Course
March 2007
By: John K. Esler, P.E.¹

INTRODUCTION:

The most important point to remember when evaluating any wastewater treatment facility …. whether small, medium or large ….. is that it’s performance is impacted by a combination of factors. These factors have been categorized by Bob Hegg in his EPA manual “Retrofitting POTW’s” into the areas of operations, maintenance, design and administration. So, when evaluating and looking for opportunities to improve clarifier performance, remember to investigate each of these functional areas for factors that may be limiting its performance.

How can you tell what’s best for your clarifier? The answer is that often you can’t tell without trying it out first at your plant. We’ve seen too many theories presented …. and nifty modifications proposed ….. that simply don’t work in many wastewater plants. There are often small differences …. and sometimes major differences ….. between clarifiers that appear to look alike. This is a lesson to remember. Because of these differences, we have to learn to apply some caution when dealing with clarifiers. We have to try to find out first what makes your particular clarifier “tick”. This is the real challenge in the optimizing process.

The following outline is a summary of ideas that are intended to help you avoid some of the problems that have been identified in clarifiers, and to point you in the right direction for change. Most of what I've written comes directly from our field experience, but some is from the shared experience of others. I think you’ll be able to find something useful here that will make your life at the plant a bit easier.

1. REMEMBER THAT THE CLARIFIER IS A PART OF A SYSTEM
   
   
   1. Preliminary Treatment: effects rag, grit and grease removal.
   2. Equalization: what every operator dreams of; should be sized for production excesses and storm water.
   3. Raw Sewage Pumping: should always be variable speed.
   4. Cooling systems are major contributors, esp. for paper industry
      1. President, CPE Services, Inc., Enfield, NH
         (www.clarifiers.com or John.Esler @ Clarifiers.com)
   5. Aeration Units
      1. Mechanical aerators:
         • May have an adverse impact on floc formation …. but it can be remedied.
           
         • Effect on heat transfer? Definitely, esp. in northern climates.
           
         • Effect on Nocardia scum production? You bet!!!
           
      2. High-purity oxygen systems can increase a temperature problem.
         
      3. Take advantage of the benefits of step aeration and contact
         stabilization for storing solids during high flow events.
         
   6. Flow Distribution to clarifiers dramatically affects performance !!
      
      1. Plan for future expansion ..... without compromising the present.
         
      2. The lack of hydraulic balance generally leads to poor distribution.
         
      3. Design to be able to control flows during the "operational imbalance” caused by taking a clarifier out of service.
         
      4. Automated flow-balancing systems work best when paced by open- channel effluent flow meters for each clarifier.
         
      5. An upflow, overflow distribution box of adequate size ….. with adjustable overflow weirs ….. works best.
         
      6. Always provide a flow measurement device for each clarifier!
         
   7. Stormwater and Stormwater Storage Flows
      
      1. Provide for bypass or storage to protect the biological process.
         
      2. If no bypass ??
         
         1. Go to step-feed or contact stabilization mode.
            
         2. Reduce aeration (i.e. mixing) to retain solids in aeration.
            
         3. Try forcing excess flow to just one basin..... to save the solids in the other basins.
            
         4. Be aware of the temperature effects of the colder stormwater on the biological system and its settling characteristics.
            
   8. Return Sludge Flows
      
      1. Provide lots of pump capacity as well as good turn-down capability.
         
      2. Avoid RAS pump headers or wet wells that serve multiple clarifiers.
         
      3. always provide separate RAS pumps for each clarifier ….. with individual flow meters …. that work!
         
         
2. IS SHAPE A FACTOR IN PERFORMANCE ?
   
   
   1. . “Yes” and “No”; rectangular and circular clarifiers each have their advantages and disadvantages due to their shapes.
      
   2. However….. the best-performing clarifier we’ve seen ….. as tested by the ASCE-CRTC protocol, and proven by time!….. is rectangular ….. 150’ long by 20’ wide ….. and only 9.5’ deep! ….. and with co-current sludge removal! (L.A. County San. Dist. – San Jose Creek)
      
   3. The poorest shape is always the "squircle" (square) ….. followed closely by the “double squircle” …. and, yes, the “triple squircle” (Cincinnati)!
      
      
3. IS DEPTH A FACTOR ?
   
   In unmodified clarifiers, extra depth has a beneficial effect, primarily for additional sludge storage during thickening overload. It also may be effective in reducing the solids loss from the upwelling of the density current at the wall ...... but is it really cost-effective in your situation?
   
   Remember, the best clarifier that we’ve seen (ref. II B) has only a 9.5-foot SWD! So don’t be fooled by rating curves that try to relate performance solely to depth.
   
   
4. INTERNAL DETAILS HAVE A GREAT EFFECT ON PERFORMANCE
   
   
   1. Influent flow balance: must provide a means to measure flow !
   2. Inlet Design:
      
      1. Avoid jetting.
         
      2. Provide for flocculation: see L.A. City/County inlet nozzle design for rectangular clarifiers; consider an energy-dissipating inlet for circular clarifiers. (see the new L.A. City-Hyperion plant design / ”LA-EDI”)
         
      3. Distribute flow horizontally ? Yes; see L.A. City/County nozzle design.
         
      4. Distribute flow vertically ?
         
         • Only necessary in primaries ….. or in secondaries following fixed-film reactors.
           
         • Avoid blowing out the blanket with some tangential port inlets.
           
         • Be wary of the ”waterfall" effect from straight influent weirs in rectangular clarifiers.
           
   3. Sludge Withdrawal Mechanisms in Circular Clarifiers:
      
      1. RAS draft tubes should be aligned horizontally, not vertically …. and sized properly!. Beware of inlet “jetting”.
         
      2. Manifold-type suction mechanisms have a full floor suction pattern ….. if they don’t plug!
         
      3. Slow the collectors down?? Definitely should in “squircles”.
         
      4. Speed them up?? Faster than conventional rotation (10 fpm tip speed) can have a detrimental effect on sludge blanket compaction.
         
      5. Avoid (like the plague!) collector arms with pantographic corner
         sweeps ...... used in squircles.
   4. Hydraulic Sludge Withdrawal (RAS) Tube Flow Control:
      
   1. Submerged gates are difficult to control individually.
      
   2. Telescopic valves theoretically provide better control, but they are difficult to adjust ….. and usually collect rags. Avoid them.
      
   3. Use “twist-turn” control tubes for easier control and maintenance.
      
   4. Avoid plugging problems by reducing the number of tubes in service.
      
   5. Sludge Scrapers:
      
      1. Standard segmented plows work well (may be even better than draft-tube suction-type?).
         
      2. Consider adding some extra depth to scraper collectors.
         
      3. Spiral scrapers? An expensive retrofit ….. may not be necessary ….. and definitely not a “silver bullet”!
         
      4. Spiral scrapers alone didn’t improve performance at Passaic Valley …. or L.A. ….. or Phoenix ….but did cost a lot of money!!!
   6. Circular Centerwells:
      
      1. Problems with scum? Design relief ports for it; use a skimmer blade, or, extend ducking scum trough into the centerwell.
         
      2. Optimum depth? approx. 0.5 x SWD; avoid DEEP centerwells.
         
      3. Optimum diameter? 0.2 D+/-; avoid LARGE centerwells.
         
      4. Designing to enhance flocculation? Beware of problems with the standard tangential port energy-dissipating inlet. (A much better proven device is the new “L.A.-Hyperion inlet”).
         
      5. Avoid using a return shelf ("lip") on the bottom of the centerwell.
         
      6. Provide flocculators? They are effective in some chemical plants, esp. when using polymers, but generally not useful in POTW’s.
         
   7. Scum Collection w/ Circular Clarifiers:
      
      1. Avoid a perpendicular alignment of the skimmers
         
      2. Use large scum hopper and drain pipe (8” min.).
         
      3. Multiple scum hoppers are an unnecessary option.
         
      4. Provide underwater flushing port for hoppers (Hinsdale, IL)
         
      5. Best idea: Install a simple “anti-rotation scum baffle".
         
      6. Plan for getting lots of scum ...... like when there’s Nocardia !!
         
      7. Always provide for safe access to scum hoppers (Elizabeth, NJ).
         
      8. Ducking skimmers? NG! Cost lots of $$$ …. return lots of water …. and preclude ever having the very useful option of providing an algae sweep mechanism.
         
      9. Full-radius scum beach? Maybe OK on smaller clarifiers.
         
   8. Rectangular Sludge Collection (w/ scrapers):
      
      1. MUST provide for scum removal !!!
         
      2. Scraper speed? Normal rate is 2 fpm; 4+ fpm OK, esp. to avoid denite.
         
      3. How slow can they go? 1 fpm OK w/ fixed film; maybe OK w/A.S. too.
         
      4. What kind of chain? Non-metallic! It’s easiest and long-lasting!!
         
      5. Traveling Bridges?
         
         1. They really complicate the collection process, and
            
         2. force the currents and flow to launders at the far end, and
            
         3. are difficult to correct for short-circuiting conditions, and
            
         4. can be maintenance night-mares! 'Nuff said??
   9. Inlets for Rectangular Clarifiers:
      
      1. L.A. City/County opposing jet nozzle design is best .
         
      2. Submerged gates w/ head differential are good, but plan for lots of scum removal from the distribution channel.
         
      3. Deep inlet baffles are NG; they increase the density current effect.
         
      4. Overflow inlet weirs increase the density current and nocardia foam generation; they foul too easily.
   10. Location of RAS Hoppers in Rectangular Clarifiers:
       
       1. At inlet end is the standard condition. It's OK, but be prepared to deal with the typical density current.
          
       2. At middle (Gould type II) is best in long (200'+/-) clarifiers; will have a useful density current in first half …. and variable currents in second half!
          
       3. RAS hopper at effluent end ?? This configuration works great at all L.A. County plants!
          
          
5. WEIR PLACEMENT IS CRITICAL!!
   
   
   1. In standard Rectangular Clarifiers (w/o baffles):
      
      1. Worst Conditions:
         
         1. at or near the end wall, or close together
            
         2. with launders deeper than necessary.
            
         3. with short (finger) weirs perpendicular to end wall.
            
         4. with submerged pipe launders at end.
            
      2. Better Conditions:
         
         1. covering at least 20% of surface.
            
      3. Best Conditions:
         
         1. covering at least 30% of surface.
            
         2. having the ability to measure the flow.
            
         3. w/ adjustable weirs that are able to be taken out of service.
            
         4. no deeper than necessary !!
            
         5. or ….. having launders parallel to the flow (see L.A. County/City)
   2. In Circular Clarifiers (w/o baffles):
      
      1. Worst Conditions:
         
         1. with a single perimeter weir that’s flush w/ face of wall, or
            even cantilevered inward.
            
         2. an inboard launder that's deeper than necessary.
            
         3. an inboard launder that’s too close to the wall.
            
         4. a single perimeter weir with a close inboard launder.
            
      2. Better Conditions:
         
      1. an inboard cantilevered launder ...... that’s not too deep!
         
      2. Spiral flow (peripheral feed) w/ central launder (for lower OFR’s).
         
      3. Best Conditions:
         
      1. a “reasonably sized” (say, 20% diameter) centerwell w/ inboard launders
      2. with the new “L.A.-EDI” flocculating/ energy-dissipating inlet
         
         
6. THE EFFECT OF MAINTENANCE ON PERFORMANCE
   
   
   • Periodic Maintenance: should dewater and inspect at least once/yr!
     
   • Torque Overload Protection should be checked frequently.
     
   • Rotating top and bottom seals (on suction manifolds) must be checked; must dewater the clarifier in order to inspect properly.
     
   • The RAS sight well center column seal needs periodic replacement.
     
   • Weir leveling is very important.
     
   • Algae growth can actually shut off flow over portions of the weir. Plan for periodic cleaning ….. or else!
     
   • For effective algae control …. consider that:
     
     1. a hypochlorite solution piping under water near the weir works well.
        
     2. the automatic brushing systems are slick!(AKA the "algae sweep"
        
        

   For safety's sake, provide …..

   1. safety screens/bars at the outlet of the effluent launders.
      
   2. safe access to the launders and the scum hopper.
      
   3. for easy access for maintenance of gear driv

7. THE EFFECT OF OPERATIONS ON PERFORMANCE
   
   
   1. Sludge Blanket Level Control: a critical activity.
      • Keeping blankets low is the best way to accommodate high flows.
        
      • Increase the blanket monitoring activity during high flows.
        
      • Manual core samplers are useful, but very subjective.
        
      • Some automatic blanket detectors are very reliable and useful.
        
      • Hand-held electronic blanket detectors are very useful; they provide for uniform measurements by staff, even w/ typically poor lighting conditions at the clarifiers.
        
   2. SVI Control:
      
      • This is the most important process control activity.
        
      • Every operator should know how to identify and control filaments. (or have access to someone who does!).
        
      • Larger plants should have a good phase-contrast microscope.
        
   3. MLSS Control:
      
      • It’s your call on what’s the proper concentration; i.e. use whatever works best for your process and sludge dewatering system.
        
      • Generally, operating with the lowest MLSS is better (i.e. leads to a lower solids loading)
        
   4. Nutrient Control: be aware of your minimum N and P requirements.
      
   5. Be diligent about scum removal, esp. to control odors and to
      reduce freezing problems.
      
   6. Flow Balancing:
      
      • You must be able to measure and control the flow to (or from) each clarifier; that means ….. install flow measurement weirs, etc.
        
      • You must be able to measure and control RAS from each clarifier.
        
   7. RAS Control:
      
      • RAS rate can effect hydraulic performance, esp. in circular centerfeed clarifiers.
        
      • The RAS rate effects treatment time in aeration reactors.
        
      • RAS tube selection: you should control or take tubes out of service in order to optimize RAS concentration and blanket control.
        
      • “Solids Flux / State Point" concept: can be useful in predicting the typical clarifier blanket thickening failure, or “what if?” situations.
        
      • Remember the basic concept that “pounds out" (as RAS) must balance the "pounds in" (as MLSS).
        
      • The diurnal fluctuation of blanket levels yields a helpful diurnal change (increase) in the RAS concentrations. But it can also lead to diurnal solids washouts!
        
   8. Effluent TSS monitoring
      
   • Monitor the ETSS periodically from each individual clarifier for better process control.
     
   • Compare clarifier ETSS with settleometer supernate TSS for better clarifier process control.
     
   • Use DSS and FSS tests for diagnostic observations.
     
   • Occasionally, check the diurnal ETSS pattern.
     
   • Use a low-level TSS meter or turbidimeter for on-line control.
     
     
8. USE OF POLYMERS (especially important for industrial plants)
   
   
   1. Consider CEPT (chemically enhanced primary treatment) to reduce organic loadings on secondary systems.
      
   2. Consider polymer use to enhance flocculation for improving settling rate, reducing sludge blanket thickness, and reducing ETSS.
      
   3. Consider polymer use for the control of certain types of filaments.
      
   4. May be a cost-effective temporary solution (esp. during wet weather events), and even a cost-effective long-term solution (i.e. instead of adding more clarifiers).
      
   5. Laboratory jar testing should mimic the actual plant conditions with respect to mixing time and mixing intensity.
      
   6. Always use the "stirred" settlometer test for determining SSV’s for SVI when using polymers for treatment.
      
   7. Provide for multiple feed points, effective initial mixing and distribution.
      
   8. Be wary of the effect of a more compact blanket on collector torque.
      
   9. Be wary of too much polymer >>>> flotation problems, “whale” problems.
   
   
9. STEPS IN ANALYZING CLARIFIER PERFORMANCE
   
   

   First ………..

   1. Determine and equalize the flow distribution to each individual clarifier.
      
   2. Determine and equalize the individual clarifier return sludge flows; confirm RAS flow meter accuracy.
      
   3. Monitor the biological or chemical treatment performance with respect to flocculation (use microscope; jar tests; settleometer).
      
   4. Determine individual clarifier ETSS performance at various
      overflow rates and blanket conditions.
      
   5. Monitor changes in blanket profiles at selected locations; monitor the diurnal blanket changes. (do Vertical Solids Profiles, or,VSP’s)
      
   6. Monitor effluent turbidity during stress tests.
      
   7. Look for diurnal ETSS variations (esp. the time of the peak ETSS).
      
   8. Optimize the activated sludge quality for your plant conditions.

Then ……..
Determine the Actual Clarifier Hydraulic Characteristics.

1. Determine the actual detention times.
   
2. Observe overall flow patterns for different conditions.
   
3. Look for reverse currents; unusual currents.
   
4. Examine the currents at different depths and locations.
   
5. Determine the full depth vertical solids profiles (VSP’s)
   
6. Determine the effects of the individual launders and weirs.
   
7. Determine the location and intensity of short-circuiting currents.
   
8. Look for temperature effects if following fixed-film reactors or with warm industrial wastes (esp. in primary clarifiers), or with steel tanks.
   
9. Determine the impact of higher and lower RAS rates.
   
   
1. IMPROVING INTERNAL CLARIFIER HYDRAULICS
   
   
   1. This activity is …. by far …. the most cost-effective means of improving clarifier performance and increasing plant capacity!!
      
   2. Refer to the work of Bob Crosby in improving circular clarifiers:
      
      1. the Crosby sloped-peripheral baffle (@ Stamford, CT, it improved average ETSS by > 30%). NOTE: It does not reduce short-circuiting; it does increase density currents; it must have at least a 45¬o slope.
         
      2. the Crosby mid-radius/cylindrical baffle: CPE projects @ Franklin, (NH), improved ETSS by >35%; worked well at Atlanta (GA), etc.; it does reduce short-circuiting and provides for additional flocculation.
         
      3. "Distributive Centerwell" (tried at Stamford CT, and an industrial site),
         didn’t reduce short-circuiting and didn't improve ETSS. Don’t repeat it!
         
   3. A combination of Crosby peripheral baffle and Crosby mid-radius baffle can be more effective than either one individual baffle.
      See CPE projects at Augusta (ME), a 1997 EPA award-winner; and also at New Haven (CT))
      
   4. The peripheral horizontal shelf baffles may not improve performance (Atlantic City, Orlando, Hyperion - L.A.); and they always, always, collect solids on the surface!
      
   5. Use interior baffles in rectangular clarifiers (Esler-Miller baffles)
      
   1. The right baffle will improve most clarifiers
      
   2. Beware! The wrong baffle(s) can make them worse !
      
   3. Two baffles are better than one.
      
   4. Three baffles can be better than two (Branford, CT).
      
   5. Four baffles can be even better. (Waterford, NY)
      
   6. Avoid horizontal end-wall baffles.
   6. Focus on improving the center feedwell to give better flow distribution and minimize turbulence; look for the new “L.A.-Hyperion Opposing-Jet EDI” inlet.
      
      
2. SOME INNOVATIONS (Some are good ...... but some are not so good !!)
   
   
1. Stacked rectangular units in parallel:
   
   1. This is a difficult clarifier to maintain and operate: consider especially the confined space entry requirements …. and monitoring the blanket in the bottom unit!
      
   2. They will have the same (poor) hydraulic characteristics as most counter-current sludge withdrawal rectangular clarifiers.
      
   3. Haven’t worked well at the Boston project (MWRA-Deer Island) or at Mamaroneck, NY.
      
2. Multiple compartments in series in rectangular clarifiers:
   
1. This system is used in Japan's best-performing clarifiers!
   
2. Can be an excellent retro-fit for existing clarifiers.
   
3. Can be approximated by using multiple interior baffles.
   
3. End-around / Folded-flow / "Boomerang" configurations:
   Worked well at Toronto and at NYC's Hunt's Point plant; may be the best configuration for rectangular clarifiers.
   
4. Speeding up the sludge collector in circular clarifiers didn’t work well in Washington, DC
   
5. Lamellas / tube / trays are not recommended for biological systems.
   
6. DEEEEEP clarifiers: Are they really worth the extra $$$$$ ?
   
7. “Standard” tangential port energy-dissipating inlets: this type of EDI is supposed to enhance flocculation and distribution, but it is certainly not living up to it’s claims! i.e. The clarifier will still have strong density currents (Orlando; Cedar Rapids; Phoenix), maybe even worse currents (Atlantic City; Santa Rosa, L.A.-Hyperion). This standard type of EDI has also even caused premature clarifier failure!!!!
   
8. The new LA-EDI inlet ….. yielded 50% more capacity than any other modification tested with the new 150-ft. diam. LA-Hyperion clarifiers.
   
9. Beware of novel inlet designs without good supporting field data !

	CLOSING THOUGHTS:
	Use the “Comprehensive Process Evaluation” approach to evaluating and improving clarifier performance. i.e. Identify all the performance-limiting factors in the Design / Operation / Maintenance / and Management of the biological system as well as in the clarifier system. There are usually several of these factors present that are limiting the clarifier's performance. Your challenge is to find them and optimize them!
	Making your existing clarifiers (typically with a 20% to 30% hydraulic efficiency) more efficient is much more cost-effective than simply adding more of the same inefficient clarifiers! Always start by considering that your existing clarifiers are units with a money-saving potential !!
	Don't forget your primary clarifiers! These units are also affected by many of the same conditions that cause problems in secondary clarifiers. Remember ….. this is where you can remove BOD most economically ….. and where you often have real opportunities to capture more materials for recycle and re-use. Improvements in primary clarifier performance will pay multiple dividends!!
	Wherever possible, provide “spill tanks” for unexpected dumps, and equalization of influent flows and sidestream loadings!

Remember ….. There are a lot of good ideas ….. here and elsewhere ….. for your clarifiers. However, you’ll never know if they’re right for you unless you try them! Just go for it!!!

 

 

Note: This outline is presented with gratitude and appreciation for the good work begun in the 1970’s and 1980’s by the late Bob Crosby. His insight and ingenuity and principles have always been an inspiration for our work.