1. Desalination Becomes A Reality In Tampa Bay Florida
Interaction of Clarifiers with Other Facilities
Clarifier Design MOP FD-8
Jim Jensen
Senior Project Manager PB Water Area Manager
Parsons Brinckerhoff Quade & Douglas, Inc. San Diego
NBWA Water Conference
April 2, 2004
Nikolay Voutchkov, PE, DEE
Senior Vice President
Poseidon Resources Corporation

2. Clarifier Interactions
Clarifier Performance Is Affected by:
Wastewater Collection System;
Screening Facilities;
Grit Removal System;
Plant Side-streams.
Clarifier Performance Impacts:
Biological Treatment;
Solids Handling.

3. Effect of WW Collection System on Clarifier Design
Combined Sewer Systems – Subject to Wide Flow Variations;
CSO Control Measures Necessitate Clarifier Design for Peak Wet-Weather Conditions;
Transient Flows Impact Clarifier Effluent Quality and Sludge Density;
Cooler Storm Water Deteriorates Clarifier Settling and Overall Hydraulic Performance;
Prolonged Wet-Weather Events Result in Grit Washout to Primary Clarifiers;
I&I Reduction Programs Result in 5 to 25 % of Influent Flow Decrease and Wastewater Strength Increase:
Increase in Sludge Production and Blanket Depth;
Elevated Potential for More Frequent Sludge Bulking.

4. Impact of Sewer System on Clarifier Design
Average and Peak Daily Flows Used to:
Calculate Hydraulic and Solids Loading Rates;
Select Type, Size and Configuration of Sludge Collection and Withdrawal Systems.
Peak Hourly Flow Used to:
Estimate Maximum Clarifier and Sludge Blanket Depths;
Identify the Need for Flow Equalization;
Determine Sludge Inventory Control Strategy During Transient Flow Conditions.
Peak Instantaneous Flow Used to:
Determine Influent Pump Capacity;
Select Sludge Blanket Depth Control Strategy.

5. Transient Flow Impact Mitigation – Sewer System
Complete More Frequent Sewer Line Cleaning Restores System Storage Capacity;
Reduce Peak Industrial Discharge Flows by On-Site Equalization;
Enlarge Bottlenecking Sections of the Sewer System;
Construct Sewer System Retention Tanks;
Implement Comprehensive I&I Reduction Program.

6. Transient Flow Impact Mitigation – Treatment Plant
Equalize Influent Flows/Loads;
Use Deep Clarifiers;
Reduce Sludge Inventory;
Increase RAS and WAS Rate;
Use Sludge Contact Stabilization;
Implement Step-Feed Aeration;
Construct Adjustable Aeration Basin Effluent Weirs;
Shut Down Aeration For a Brief Period of Time.

7. Flow and Load Equalization
Typically Cost-Effective if Plant Peak Hourly Factor > 2.5;
Allows Decreasing Size and Depth of Primary & Secondary Clarifiers;
Provides Opportunity to Run at Higher Sludge Inventories (MLSS & SRT);
May Cause Odor Problems;
Equalization of Primary Effluent More Desirable.

8. Use of Deeper Clarifiers

9. Clarifier Depth vs. Performance

10. Clarifier Depth – Rules of Thumb
Plants with Wet Weather Peaking Factors > 2.5:
SWD of 14 to 16 ft;
Transient Sludge Blanket Allowance – 6 ft.
Conventional Activated Sludge Tanks – Sludge Blanket of 1 to 2 ft (avg. conditions);
BNR Plants – Keep Minimal Sludge Blanket Depth (< 1.5 ft);
Maintain a Minimum Buffer Distance of 3 feet Between Sludge Blanket Level & Clarifier Surface.

11. Depth - Surface Overflow Rate Trade-Off

12. When Shallow Clarifiers Work? – Reduced Sludge Inventory

13. Lower Solids Inventory = Reduced Sludge Blanket
An Alternative to Deeper Clarifiers or Lower SORs;
Only Suitable When SRT can Be Reduced Significantly (20 to 40 %) w/o Effluent Quality Penalty;
Limited Application for BNR Systems;
Main Reason Why Shallow and Deep Clarifiers May Show Similar Performance.

14. RAS and WAS Rate Increase
RAS Rate Increase Has Only Temporary Relief Effect and is Limited by Critical Flux;
WAS Increase After Critical Flux is Reached;
Gradual Increase in Essential – “Rat-holing”;
Design RAS Pump Capacity % of Avg. Dry-Weather Flow or 50% of Peak Wet Weather Flow.
WAS Pump Capacity – Determined by Min SRT Acceptable at Wet Weather Conditions.

15. Contact Stabilization
Sludge Moved Out of the Anaerobic Clarifier Blanket
And Shifted to Aerobic Conditions.
Contact Zone – 1 to 2 Hrs of HRT
Stabilization Zone – 4 to 6 Hrs of HRT

16. Step-Feed Aeration
Lower
MLSS
Higher MLSS

17. Clarifiers & Pretreatment
Influent Pumps (Hydraulic Surges):
VFDs;
Screw Pumps – Dampen Flow Variations.
Screens – Type and Size Affects Sludge:
Grinders – 5 to 10 % Sludge Increase.
Clarifier Sludge Collection & Withdrawal Affected;
Suction Sludge Withdrawal Not Suitable for Grinders;
Use of Grinders May Plug Lamella Openings.

18. Grit Chamber Performance Impacts Sludge Quantity and Quality
Effective Grit Removal Is Essential;
Grit Accumulates in Primary Sludge and Digesters;
Excessive Grit in Primary Sludge May Cause Collector Mechanism Damage;
De-gritting of Primary Sludge Recommended if Grit Chambers are Overloaded/Ineffective;
Aerated Grit Chambers Help Address Sludge Septicity/Excessive H2S;
Good Point of Chemical Addition for Mixing & Flocculation.

19. Primary Clarifiers & Nutrient Removal
Sedimentation Affects BOD : N : P Ratio;
Chemical Phosphorus Removal:
Over 90 % of Particulate Phosphorus Could Be Removed;
Use of Iron Salts Also Controls Odors/H2S;
Use of Aluminum Salts Minimizes Phosphate Release in Anaerobic Digesters;
Increased Sludge Amount and Elevated Metal Content;
Reduction in Influent Alkalinity (5.8 mg/mg Al & 2.5 mg/mg Fe);
Excessive Removal Could Hinder Denitrification Due to Nutrient Deficiency.

20. Use of Primary Clarifiers for Solids Pre-fermentation
Volatile Fatty Acids
Volatile Fatty Acids
Sludge Recirculation
Regular Removal of Methane Organisms

21. Secondary Clarifier Design for Enhanced Nutrient Removal
Apply Conservative SOR – 300 to 600 gpd/sf;
Use Deep Clarifiers – 12 to 16 ft SWD;
Maintain Shallow Sludge Blankets < 1.5 ft/ Leave 0.6 ft of Blanket for Compaction;
Keep Sludge Blanket Retention Time Below 3 Hrs;
Design for High RAS Rates (50 to 75 %);
Provide Anaerobic or Anoxic Selectors to Control Filaments;
Operate at High DO Levels – 2.5 to 3 mg/L;

22. Interaction With Thickeners
Thickening in Deep Primary Clarifiers is OK - 3 to 6 % Solids;
If Influent Prone to Septicity – Use Shallow Clarifiers & Thicken Separately;
Thickening in Secondary Clarifiers Usually Causes More Harm than Good;
Co-thickening in Primary Clarifiers:
Suitable for Trickling Filter Sludge;
Usually Detrimental for Primary Clarifiers in WAS Used.
When in Dough – Thicken Separately!

23. Interaction With Anaerobic Digesters
Homogenous Sludge Feed is of the Essence;
Thicken if Feed Sludge is Below 1 % Solids;
Optimum Primary Sludge Concentration – 4 to 6 %;
Minimize Activated Sludge Production;
Ferric Salt Addition is Effective in Controlling H2S (Ferrous Sulfate – the Best);
Avoid Addition of Chlorine and FeCl3 at the Same Point – Iron Sulfide is Difficult to Settle;
Chemically Enhanced Sedimentation Increases Sludge Quantity.

24. Clarifiers & Aerobic Digestion
Aerobic Digestion of Primary Sludge is Several Times More Costly than WAS (More Energy);
Thickening is Essential – Aeration Costs Driven by Mixing;
Higher Target Thickening Goals – 4 to 6 % vs. 3 to 4 % for Primary Digesters;
Achilles Heal – Foaming – Control Using Activated Sludge System Selectors.

25. Summary & Conclusions
Clarifiers are the “Cross-Roads” of the Treatment Plant;
Clarifier Performance and Design are Strongly Influenced by:
Wastewater Collection System Type and Performance;
Screening and Grit Removal;
Effluent Treatment Goals.
Influent Septicity Most Critical for Primary Clarifier Design;
Solids Inventory Most Critical for Secondary Clarifier Performance;
Optimized Clarifier Operation Is Essential For Cost Effective Solids Handling.

26. Clarifier Design MOP FD-8
Questions?