Clean Water Agencies. From Dewatering to Disposal/Use: Lynne H. Moss, P.E., BCEE

Transcription

1 Texas Association of Clean Water Agencies From Dewatering to Disposal/Use: Sludge Cake Handling Considerations Lynne H. Moss, P.E., BCEE March 26, 2010

2 Overview Approaches General considerations Technologies Applicability Design Considerations Pros/Cons Case studies

3 Low-Tech Approach Limited or no conveyance Often seen at small plants Goals are low cost, simplicity

4 More Complex Approaches Conveyors to multiple bays Multiple conveyors Some storage

5 High-Tech Approaches May include both sophisticated conveyance plus storage Cake pumps Vertical screws s Loading silos Rare in Texas

6 General Considerations What is it (solids content/grit)? Conveyor type What s next? Subsequent processing Landfill Storage Required loading time Storage Redundancy needs Bypass Downstream process reliability/use Ease of repair Capital vs. O&M Dual trains

7 Belt Conveyors Flat Trough Side-wall Basic Conveyor Types Cleated Articulating Pivoting Reversing More

8 Belt Conveyor Design Considerations Good for long conveyance Slope 16-20% max, without cleats Concern is roll-back 25-30% with cleats Keep cleats close together (6-in) Scrapers For non-cleated belts Not as effective as most would like Covers Completely enclosed can help with odor control, but access an issue

9 Belt Conveyor Advantages & Disadvantages Advantages Cost Durable Easily repaired, maintained Disadvantages Messy Scrapers not always effective Odor (enclosures required)

10 Screw Conveyors Types Shaftless (most common) Shafted Applicability Can go steep/vertical Good for small footprints

11 Screw Conveyor General Design Considerations Consider shaftless for >15 ft length Hangers/bearings in shafted screws can cause blockage Allow room for screw removal Will have liner wear if rpms too high for design load (cfh) Remember dead space beyond outlet Consider carry-over after a discharge with an open gate

12 Shafted Screw Conveyor Design Considerations Avoid using shafted screws with more than 12% trough loading Intermediate bearing blockage Remember loss of capacity compared to shaftless Shafted spiral surface area: trough surface = 1.3:1 Shaft spiral surface area: trough surface = 0.3:1

13 Special Considerations for Inclined Screw Conveyors Over 25 degrees, recommend push-in design, not gravity load. Recognize extra liner wear Account for roll-back on shaftless inclines over degrees Accounting for loss of capacity because of incline

14 Screw Conveyor Sizing

15 Screw Conveyor Advantages & Disadvantages Shaftless Advantages Low friction No hanger bearings Clean Can go steep Disadvantages Thixotropic transformation Shafted Advantage Lower capital cost than shaftless Clean Can go steep Disadvantages Thixotropic transformation Blockage at hangar bearings

16 Cake Storage Generally for larger facilities (> 20 mgd) Does not have to be complex Can provide valuable wide spot in line if not available upstream of dewatering Cake pumping can be considered, but may change thixotropic characteristics Typically used in conjunction with additional processing

17 Case Study: San Antonio River Authority (SARA) Martinez System Issues Old equipment Sludge through pugmill Double-handling Messy Salatrillo WWTP Issues Stop dewatering to load trucks Solids back-up in plant Inefficient

18 Case Study: San Antonio River Authority (SARA) Approach Martinez System new operation at Martinez II WWTP Salatrillo WWTP modify existing facilities Improvements Transition to roll-offs Winches to help with even roll-off loading Reversing conveyors to allow continuous loading Concrete pads for roll-off storage Conveyance improvements about $ per Conveyance improvements about $50,000 per WWTP

19 Martinez II WWTP Dewatering and Conveyance Improvements

20 Salaltrillo WWTP Conveyance Improvements

21 Nuese River WWTP (Raleigh, NC): Solids Handling Issues Aged conveyance and stabilization system Unreliable All dewatered cake had to run through pugmill mixer Time consuming truck loading operation (60-90 min) Lack of control on alkaline operations Extremely high chemical use to create Raleigh Plus

22 New Dewatered Cake Storage and Handling System New Conveyance System Horizontal and vertical shaftless screw conveyors Increased flexibility in truck loading operation Minimized odor emissions Full redundancy by means of dual conveyors

23 Belt Conveyor Collecting Dewatered Cake

24 Discharge of Dewatered Cake into Screw Conveyors

25 Alkaline Chemical Feed Screw Replaced

26 New Alkaline/Sludge Blending Unit New blend proportions, mixes and blends lime and sludge Discharges product onto belt conveyors Capacity: 67,000 lbs/hour Wet sludge cake: 15-25% solids content

27 New Cake Storage Silos and Load Out Facility Cake Storage Live Bottom Hoppers 2 Units, welded steel construction Inverted, truncated, four-sided pyramid Capacity: 100 cu. Yd. (each) Direct loading of trucks trailers

28 New Dewatered Cake Storage and Handling System Truck weigh scales 2 units, 11-ft x 70 ft Capacity: 100 tons Electronic, digital indicators

29 Original vs. New Storage and Handling System Performance Reduced use of lime/dry ton of Raleigh Plus 3.50 Original System: 3.00 $3,266 per day ($ per dry ton Raleigh Plus) Average )/Raleigh Plus (dry tons) New System: $2,000 per day ($86 per dry ton Raleigh Plus) LKD (dry tons) January-05 May-05 August-05 December-05 April-06 August-06 December-06 April-07 August-07 December-07 April-08 August-08 December-08 April-09 August-09 December-09

30 Nuese River WWTP Solids Handling Improvement Summary New blender contributed to a reduction of 30% in use of lime New conveyance system and hoppers reduced the loading time from min to 5 minutes The new weigh scales allow for better control and monitoring

31 QUESTIONS?