GRIT HAPPENS: KNOW HOW TO DEAL WITH IT

Transcription

1 GRIT HAPPENS: KNOW HOW TO DEAL WITH IT A CASE STUDY ON SELECTION OF GRIT REMOVAL SYSTEMS FOR WWTPs Murali Erat, P.E. November 18, 2014

2 Outline What is Grit? Characteristics of Grit Grit Removal Technologies Case Study: Grit Removal System Evaluation for City of Houston s Sims Bayou South WWTP Conclusion Key Factors for Design

3 What is Grit? EPA s Wastewater Technology Fact Sheet defines grit as: Particles > 210- micron (.0084 in or 65 mesh) SpeciFic gravity > 2.65

4 Grit Characteristics SpeciFic Gravity (SG) Settling Velocity Specific Gravity of Various Materials in WW Quartz Sand 1.20 Sand, wet 1.92 Limestone 1.55 Gravel 2.00 Granite 1.65 Asphalt 2.20 Clay 1.80 Concrete 2.40 Red Brick 1.90 Grit 2.65 ParNcle Size (microns) Aggregate Class Time Required to SeTle 1 SG = 2.65 Time Required to setle 1 SG = Fine Sand 38 Seconds 2 min. 48 sec.

5 Grit Characteristics Modeling Grit as Sand (Se6ling Characteris:cs): Clean Sand Particle Settling Velocity (Fast) CLEAN SAND Same Size Particles Grit Particle Settling Velocity (Slow) GRIT Design Criteria Sand Particle With Same Settling Velocity Sand Equivalent Size (SES) Target grit cut point: μ SpeciHic gravity:

6 Types of Grit Removal System Mechanical Vortex Aerated Grit Chambers Gravity Vortex (HeadCellTM)

7 Gravity Vortex (HeadCell TM ) Screened Influent Degritted Effluent Grit to dewatering

8 Case Study Sims Bayou South WWTP (City of Houston) Sims Bayou South WWTP

9 Sims Bayou South WWTP InHluent Lift Station Chlorine Contact Basins Headworks Aeration Basins ClariHiers

10 Sims Bayou South WWTP Plant built in 1987 Current Permit: Annual Average Daily Flow (AADF) 36 MGD 2- hour Peak Flow 90 MGD Future Ultimate 2- hour Peak Flow 190 MGD Actual Flow Conditions: Current average Flow 16 MGD Max. peak Flows during wet weather MGD

11 Existing Aerated Grit Chambers Total Capacity of Aerated Grit Chambers MGD Out of Service for ~ 10 years

12 Existing Aerated Grit Chambers Air Li] Pumps

13 Existing Grit Dewatering System: Vibra- Screens

14 Existing Aerated Grit Chambers DeHiciencies Vibra- Screen dewatering system failed was hard to operate and maintain Air lift pumps failed to remove settled grit Air lift pumps were manually operated Existing Basin Geometry: Width to Depth ratio 2:1 (Recommended :1) Length to Width 1.6:1 (Recommended 3-8:1) Floor slope 22 deg (Recommended 30 deg)

15 Grit Removal System Options 1 2 Refurbish Exis:ng Aerated Grit Chambers Retrofit with Mechanical Vortex System 3 Retrofit with HeadCell TM

16 1 Refurbish Exis:ng Aerated Grit Chambers Replace air lift pumps with self- priming grit pumps Replace vibra- screen unit with traditional grit washer/classihier

17 2 Retrofit with Mechanical Vortex System Entrance Channel: MGD 100 MGD

18 3 Retrofit with HeadCell TM 31.7 MGD 31.7 MGD 31.7 MGD Three (3) 12 dia tray units, Capacity per basin 95 MGD 18

19 3 Retrofit Exis:ng Aerated Grit Chambers with HeadCell TM Relocate efhluent troughs Add partition walls BafHle Wall 31.7 MGD 31.7 MGD 31.7 MGD EfHluent Trough Three (3) 12 dia tray units, Capacity per basin 95 MGD Existing Aerated Grit Chamber Add isolation gates

20 Option 3: RetroHit with HeadCell TM HeadCell Grit Pump

21 Option 3: RetroHit with HeadCell TM Grit Washing/ Dewatering Station #1 Basin #1 Basin #2 Grit Washing/ Dewatering Station #2

22 Grit Removal EfHiciency 85% HeadCell/ Vortex 50% Aerated Grit Chambers

23 Grit Removal System Options - Cost Capital Annual O&M Life Cycle Cost (25 years) 1 Refurbish Existing Aerated Grit Chambers $1.44 M $114 K $12.6 M 2 Mechanical Vortex System $4.5 M $45 K $7.5 M 3 RetroFit with HeadCell TM $4.0 M $64 K $7.3 M

24 Grit Removal System Options - Cost More frequent cleaning of aeration basins Increase in Blower HP $$$ $$$ $$ Damage to diffusers during grit cleanout NO GRIT REMOVAL Wear & tear of sludge piping $ Wear & tear of mechanical equip. $$

25 Grit Removal System Options - Cost Capital Annual O&M Life Cycle Cost (25 years) 0 Status Quo No Grit Removal $211 K $90 K $12.4 M 1 Refurbish Existing Aerated Grit Chambers $1.44 M $114 K $12.6 M 2 Mechanical Vortex System $4.5 M $45 K $7.5 M 3 RetroFit with HeadCell TM $4.0 M $64 K $7.3 M

26 Option 3: HeadCell Option 2: Mechanical Vortex Easily retrofitted in existing grit basins Flexible operation No mechanical parts within the basin Very efficient in removing Fine grit Pilot HeadCell system for use at other plants SOLE SOURCED!! Not a sole- sourced technology EfFicient in removing Fine grit Cannot be retrofitted into existing grit basins Larger footprint No turn down

27 Grit Removal Options - Recommendation 3 Retrofit Exis:ng Aerated Grit Chambers with HeadCell TM Includes: Six (6) HeadCell Units Six (6) Tea- Cup girt washes and four (4) Grit Snail grit dewatering equipment Grit pumps Structural repair to grit basins New isolation gates Contingency and overhead factors Capital Cost Annual O&M Life Cycle Cost (25 years) $4.0 M $64 k $7.3 M

28 Grit Removal System Design Key Points 70% Grit Separa:on Aerated grit chambers Mechanical Vortex Gravity Vortex - HeadCell Cyclone Type (Tea- Cup), Conical Type (Pista Turbo, Huber etc.) Grit Washing 80% 90% Grit Dewatering Inclined Screw Type (Pista Turbo) Escalator Type (Grit- Snail) Overall EfHiciency =50%

29 Grit Removal System Design Key Points Know your grit! Fine grit? There are treatment options! Keep grit dewatering system close Grit Slurry Pipe ü Velocity 4-7 ft/sec ü Avoid 90 deg bends ü Provide cleanouts

30 GRIT HAPPENS DEAL WITH IT

31 Questions Murali Erat, P.E