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Inclined Plate Clarifier Design and Sizing Procedure

Reviewing and filling out the HydroFlo Tech, "Application Questionnaire" will also help you gather pertinent information required for the proper design and application of inclined plate clarifiers and clarification systems.

Inclined Plate Clarifier (parallel plate, lamella) Theory of Operation

Basic concepts and theories of clarification

If you are unfamiliar with basic inclined plate clarifier (plate settler, parallel plate clarifier, lamella plate settler, etc.) design principles, a review of the "Inclined Plate Clarifier (parallel plate, lamella) Theory of Operation" might prove helpful.

This basic document covers Stokes' law and other basic separation concepts.

General Considerations

Inclined plate clarifiers are typically considered very simple devices. However, several factors that could potentially affect safety, efficiency, and proper management must be given careful consideration prior to the installation or modification of any clarifier or clarification system:

  1. Flow Rate
    In general, clarifiers are sized by the flow rate verses the separation chambers "effective" surface area (or, as it is more commonly called. the "projected" surface area). Therefore, the effectiveness of any clarifier is affected by the flow rate. The slower the flow, the better the results.

  2. Design Capacity
    A clarifier has upper limits to the amounts of suspended solids that can effectively accumulate while it is in operation. If too much "sludge" or settled suspended solids accumulate on the plates, it may carry over into the wastewater outlet chamber and end up being discharged to the environment. Proper clarifier design will take the plate spacing into consideration and also allow for the removal and storage of accumulated sludge from the clarifier to ensure that the accumulated products do not effect the operation of the clarifier.

  3. Maintenance Practices
    The ability of any clarifier to function properly depends upon the timely performance of required service and maintenance. Clarifiers must be monitored and maintained by competent personnel who understand how the systems operate. Clarifiers should be given the same close attention shown to any other important piece of equipment. The operators, users, and maintainers of the clarifier must be clear on who will be responsible for monitoring, inspecting, maintaining, and servicing the system. Frequent inspections should be made of the system and all associated piping, valves, etc. to prevent operational and mechanical failures or inefficiencies. Sludge needs to be regularly removed from the clarifier to keep it operating properly. Additionally, leaks from clarifiers can result in environmental pollution, which can trigger costly investigative studies and cleanups. Rigorous implementation of a clarifier inspection and maintenance plan can prevent discharges from the clarifier that may contaminate the environment.

  4. Suitability of Inclined Plate Clarifier System to Process
    A clarifier designed and installed to meet a past process requirement may no longer be suitable when process requirements change and/or the original maintenance plan is no longer followed. A clarifier that is put to a use for which it was not originally designed may be damaged or may not function properly, and could become an environmental liability. For example, a clarifier designed to receive the wastewater discharge from a small heavy metal precipitation system will not be able to properly treat larger wastewater volumes for removing chrome from a tannery effluent. Process changes can also result in changes to the physical/chemical makeup of the wastewater being treated by a clarifier.

  5. Contaminants Contained in the Wastewater Stream
    Metallic particles in the wastewater will settle into the sludge at the bottom of the clarifier. Solvents or fuel compounds may also be entrained in the clarifier's sludge. This sludge could require management under the Resource Conservation and Recovery Act (RCRA) as a hazardous waste, if it exhibits certain toxicity characteristics. Therefore, it is important to prohibit the discharge of certain types of potential contaminants into a clarifier, and to regularly analyze sludge samples to determine toxicity prior to disposal. General improvements in spill/drip control and containment of hazardous materials and solvents will also reduce the amount of contamination in clarifier discharges.

Inclined Plate Clarifier Decision - Flow Diagram

Determining if your process require a clarifier or clarification system

STEP 1:

Identify Wastewater Source
Buildings and areas, as well as ALL activities and processes within the buildings and areas, that generate wastewater

 

STEP 2:

Institute Pollution Prevention and Source Elimination / Reduction Procedures
Can the processes that generate the wastewater be eliminated?  Can the process be converted to a dry process?

 

               

 

STEP 2-A:

Process altered or eliminated, no further discharge
STOP
No Further Action Required

STEP 3:

Source Diversion
Can the process be moved to an area that has existing clarification equipment in place?  Can the existing equipment handle the increased flow?  Is moving the process, diverting the flow, economically feasible?

 

               

 

STEP 3-A:

Process relocated or discharge diverted
STOP
No Further Action Required

STEP 4:

Wastewater Compliance Evaluation

Identify permit limits on ALL pollutants generated at the site.  Characterize raw wastewater prior to any treatment, if appropriate.  Characterize treated wastewater if existing treatment equipment is in place.

 

STEP 5:

Discharge & Pretreatment Requirements

Does the raw wastewater meet permit limits and environmental requirements?  Does the treated wastewater meet permit limits and environmental requirements?

 

               

 

STEP 5-A:

Discharge meets requirements,
STOP
No Further Action Required

STEP 6:

New Clarifier or System Upgrades Required.  Proceed to Inclined Plate Clarifier Design - Flow Diagram

 

Inclined Plate Clarifier Design - Flow Diagram

Basic process in the design of a clarifier or clarification system

STEP 1:

Conduct a Wastewater Characterization Study
The engineer / designer may conduct a wastewater characterization study to establish clarifier or clarification system design parameters.

The first step in a wastewater characterization study is to conduct the "Clarifier Settling Velocity" test.  This bench test will help determine the clarifier's "TARGET" effluent quality at a specific particle settling rate.  If possible, you should conduct this test numerous times to allow for variances in your overall process.

                   

STEP 2:

Determine the Type of Clarifier or Clarification System Necessary for Your Process
Evaluate the results of the characterization study and determine the category of suspended solids to be removed.  Decide what type of clarifier or clarification system to use.

NOTE:
HydroFlo ClariMax clarifiers are an excellent separation solution for complex, evolving processes.  The HydroFlo ClariMax clarifiers are designed to evolve with your growing, expanding process requirements.  These robust clarifiers can be retrofit with a variety of plate packs, increasing the efficiency of the separator as your process or discharge requirements change.  Surface drag skimmers for processes, vapor tight lids, etc. are simple bolt on modifications.

                   

STEP 3:

Do You Have an Existing Clarifier at Your Facility
If there is an existing clarifier on site, evaluate if it may be upgraded to accept the total flow or partial flow from the proposed source to reduce loadings.

 

                   

STEP 3-A:

Will Upgrading the Existing Clarifier Bring the Discharge into Compliance
Can the existing system be upgraded (by the installation of alternate plate packs, etc.) or adding additional treatment capabilities, etc.

                

STEP 4:

Review Concerns and Requirements of the New Clarifier or Clarification System with the Engineer / Designer
List all provisions that need to be considered to ensure the new system will be accessible for maintenance, will meet site specific area classifications (i.e.: seismic and explosion proof area classifications, etc.) and will meet all regulatory and effluent discharge requirements.

STEP 3-B:

Can the Existing Clarifier be Used to Pre-treat the Wastewater Prior to the New Clarifier
Even if the existing clarifier is not capable of handling the proposed load, it may be useable as a form of pre-treatment or used in conjunction with the proposed upgrades.

 

                

STEP 5:

Write the Specifications for the New or Upgraded Clarifier or Clarification Systems
Write specifications for the new clarifier, the clarifier upgrade or the new clarification system required to meet all the above listed concerns and requirements.

STEP 3-C:

Incorporate existing clarifier into design

STEP 3-D:

If determined that the existing clarifier is no longer suitable for its current use or is unusable in the new system, incorporate the closure of the existing clarifier into the specifications

STEP 3-E:

Design the upgrade of the existing clarifier

The Impact of Settling Rate on Inclined Plate Clarifier Design

Determining the hydraulic loading ratio required to meet your effluent requirements (ppm, mg/l) is an important step in the design and sizing of any clarification system. A small change in the hydraulic loading ratio can result in a large change in the size of the specified clarifier. For example look at the following spreadsheets.

  Input            
What is the Process Flow Rate 100.00            
What is the Desired Hydraulic Loading Ratio 0.25            
What is the Desired Plate Spacing 2.00            
                     
  Output            
Required PSA 400            
  Available Models Cross Sectional Velocity
In Ft/Min 
Total Effective PSA Actual Hydraulic
Loading Ratio
  139 - 10 - 1 - ST OR PB 1.02 559 0.18
  139 - 10 - 2 - ST OR PB 1.02 559 0.18
  129 - 8 - 1 - ST OR PB 0.87 512 0.20
  129 - 8 - 2 - ST OR PB 1.02 439 0.23
  127 - 6 - 1 - ST OR PB 0.68 479 0.21
  139 - 6 - 2 - ST OR PB 0.61 532 0.19
MODEL NOT AVAILABLE 0 - 4 - 1 - ST OR PB #DIV/0! 0 #DIV/0!
  120 - 4 - 2 - ST OR PB 0.51 399 0.25
  Required Cubic Feet of Media   Overall Plate Length   Number of Influent Troughs   Sludge Thickening
or
Pyramid Hopper
     

EXAMPLE ONE

 

  Input            
What is the Process Flow Rate 100.00            
What is the Desired Hydraulic Loading Ratio 0.40            
What is the Desired Plate Spacing 2.00            
                     
  Output            
Required PSA 250            
  Available Models Cross Sectional Velocity
In Ft/Min 
Total Effective PSA Actual Hydraulic
Loading Ratio
  104 - 10 - 1 - ST OR PB 1.53 373 0.27
  139 - 10 - 2 - ST OR PB 1.02 559 0.18
  86 - 8 - 1 - ST OR PB 1.22 366 0.27
  86 - 8 - 2 - ST OR PB 1.53 293 0.34
  81 - 6 - 1 - ST OR PB 1.02 319 0.31
  92 - 6 - 2 - ST OR PB 1.02 319 0.31
  74 - 4 - 1 - ST OR PB 0.76 266 0.38
  74 - 4 - 2 - ST OR PB 0.76 266 0.38
  Required Cubic Feet of Media   Overall Plate Length   Number of Influent Troughs   Sludge Thickening
or
Pyramid Hopper
     

EXAMPLE TWO

The only change in the above spreadsheets was the "Hydraulic Loading Ratio". All other variables remain the same. 

On the first spreadsheet the "Hydraulic Loading Ratio" was input at .25. On the second spreadsheet the "Hydraulic Loading Ratio" was input at .40. You will note that the model #'s (which represent the total cubic feet of media required) changed from a high of 139 cubic feet to a low of 74 cubic feet. That is a 46% change in the total size of the separator.

Both examples are suitable for 100 gallons per minute. Both examples will remove the targeted suspended solid. The real world difference between all the listed clarifiers is overall clarifier efficiency.

This example should reinforce the importance of accurately conducting the "Test for Determination of Clarifier Hydraulic Loading Ratio" and the "Clarifier Settling Rate" studies. If you size a clarifier without knowing the target "Hydraulic Loading Ratio", you will have to greatly over size the clarifier, because erring on the safe side, you will have to size the clarifier for maximum efficiency.

Sizing the Clarifier or Clarification System

Inputting the Accumulated Data to the HydroFlo Clarifier Sizing Spreadsheet to Accurately Size a Clarifier or Clarification System for Your Application

The final step is to insert the data into the "clarifier sizing spreadsheet".

NOTE:
ALWAYS SPECIFY THE "SHORTEST" PLATE LENGTH FEASIBLE FOR YOUR INSTALLATION.

This is VERY important.

All things being equal (projected surface area and plate spacing), a clarifier with 4' long plates will generate better results than a clarifier with 8' long plates.

A clarifier with 4' long plates (as opposed to a clarifier with 8' long plates) will accumulate 1/2 the amount of sludge on the surface of the plates. Lower accumulation of sludge per plate helps prevent sludge re-entrainment and clarifier fouling.

Also, shorter plates will lower cross sectional velocities and improve overall laminar flow conditions.

The trade off is that clarifiers with short plate lengths take up more floor space. A clarifier with 8' long plates will take up half the floor space that a comparable clarifier with 4' long plates.

Comparing and Evaluating Clarifiers and Clarification Systems from Different Suppliers

If you go to ten different clarifier manufacturers and ask them to size a clarifier for a specific application (even when giving them a written specification), you will get ten different answers. Everyone bases the efficiency of their clarifier on their clarifier's projected surface area. But, other design variables are all over the map. Plate spacing will vary from 3/4" to 2", and more. The angles and configurations of the plates are vastly different from manufacturer to manufacturer. Overall clarifier volume, from the largest to the smallest designs, can fluctuate as much as 150%, or more.

When comparing one manufacturer to another, it is best to compare the volume of the separation chamber (or size of the plate pack) in cubic feet verses the sell price. This ratio should give you the clarifiers cost per cubic foot of plate pack. This is important because the physical size of the clarifier is the greatest factor in the cost, as well as the efficiency, of the device. The published projected surface area has a much smaller impact on the clarifier's overall cost and performance.

NOTE:
DO NOT COMPARE CLARIFIERS BASED SOLELY ON THE AMOUNT OF PROJECTED SURFACE AREA!

This is VERY important.

Many clarifier manufacturers use the smallest plate spacing possible in their clarifiers. This allows them to post the largest projected surface area numbers possible, giving the customer the impression that they are purchasing the most efficient separator available.

Unfortunately, using the smallest possible plate spacing is not the answer. Care must be taken when specifying plate spacing. If the plate spacing is too narrow, the results will be excessively high cross sectional velocities and unacceptably high Reynolds numbers, improper plate pack distribution and short circuiting issues, as well as sludge re-entrainment and high volumes of floc carry over.

There are many clarifiers in the field built with 8 foot long plates and 3/4" plate spacing that are only capable of operating at 30% of the designed flow rate! There are also many clarifier manufacturers who specify 1" plate spacing, but in fact, they are actually less than that because they do not account for the plate thickness in their calculations.

Make sure that, when you are talking about plate spacing, you are talking about the distance between the plates - not the distance from plate center to center. On smaller applications, the difference does not amount to much, but on large projects, the difference can be substantial.

Also, many manufacturers claim that their clarifiers are 100% efficient. Current fluid dynamic studies have shown that even the best designs are 85% efficient at best and that some are even as low as 50% efficient. We rate our clarifiers at a conservative 80% efficiency rating. Make sure that you receive confirmation of the clarifier's total projected surface area as well as the clarifier's "effective" surface area.