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

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.

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.

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.

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.

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.

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.

Use of Deeper Clarifiers

Clarifier Depth vs. Performance

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.

Depth - Surface Overflow Rate Trade-Off

When Shallow Clarifiers Work? – Reduced Sludge Inventory

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.

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 - 120 % of Avg. Dry-Weather Flow or 50% of Peak Wet Weather Flow. WAS Pump Capacity – Determined by Min SRT Acceptable at Wet Weather Conditions.

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

Step-Feed Aeration Lower MLSS Higher MLSS

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.

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.

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.

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

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;

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!

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.

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.

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.

Clarifier Design MOP FD-8 Questions?