Wastewater Treatment Review 10 Sludge Thickening nw ate rc oll e ge 20 Gravity Thickening ca Principle of Operation 1. Uses gravitational forces to separate water from sludge 2. Solids settle to bottom and become compacted as more solids settle 3. Thickened sludge is sent to stabilization process 4. Secondary sludge is not well suited for gravity thickening Am eri Main Components 1. Inlet distribution assembly 2. Sludge rake 3. Steel pickets mounted on the sludge rake 4. Effluent overflow weir 5. Scum removal equipment Factors Affecting Gravity Thickeners 1. Type of sludge 2. Age of feed sludge 3. Sludge temperature 4. Sludge blanket depth 5. Solids and hydraulic detention time 6. Solids and hydraulic loading - 66 -
Sludge Type Gravity Thickener Performance Solids Loading lbs/day/ft2 % TS of Thickened Sludge Primary 20-30 8-10 Activated Sludge 5-8 2-4 Trickling Filter 8-10 7-9 Primary and Activated Sludge 6-12 4-9 Primary and Trickling filter 10-20 7-9 Gravity Thickener Trouble Shooting Guide Parameter Cause Check Solution Supernatant clear but Gasification Sludge detention time Increase sludge pumping sludge is rising and Septic feed sludge Sludge characteristics Increase sludge pumping carrying over Blanket disturbances Sludge collector speed Reduce speed Polymer dose too low Chemical system Raise polymer dose Loading too high Flow rate Reduce flow Thin sludge with clear supernatant Thin sludge with solids in supernatant and carrying over Thin blanket Blanket level Reduce sludge pumping Removal rate too high Hydraulic overload Loading rate Reduce flow Polymer dose too low Chemical system Increase polymer dose - 67 -
Wastewater Treatment Review nw ate rc oll e ge 20 10 Dissolved Air Floatation (DAF) Am eri ca Principles of Operation 1. Pressurized air-saturated liquid injected into incoming sludge stream (45 70 psi) 2. Drop in pressure causes air bubbles to come out of solution 3. Bubble make contact with sludge and attach 4. Bubbles and sludge rise to surface 5. Heavier sludge settles to bottom 6. Concentrated solids are removed and sent to stabilization process (3-5% TS) 7. Polymer may be used to enhance the process Main Components 1. Air injection equipment 2. Pressure tank 3. Recycle pump 4. Inlet distribution assembly 5. Sludge scrapers 6. Effluent baffle - 68 -
Factors Affecting DAF Units 1. Type of sludge 2. Solids and hydraulic loading 3. Air to Solids ratio (A/S) 4. Recycle rate 5. Sludge blanket depth DAF Unit Typical Performance Parameter With Polymer Without Polymer Solids Loading (lbs/day/ft 2 ) 24-48 10-24 Hydraulic Loading (gpm/ ft 2) 0.5-2.0 0.5-1.5 Recycle % 100-200 100-200 Air/Solids Ratio (lbs/lbs) 0.01-0.1 0.01-0.1 Minimum Feed TS (mg/l) 5,000 5,000 Floating Sludge TS (%) 3-5 2-4 Solids Recovery (%) 90-98 50-90 DAF Troubleshooting Guidance Problem Cause Check Solution Solids carry over Floating sludge Flight speed Increase flight speed with good thickening blanket too thick Solids loading Reduce flow Thin sludge blanket with quality effluent Poor effluent quality with thin floating sludge blanket Floating sludge Flight speed Reduce flight speed blanket too thin Solids loading Increase flow A/S too low Air rate/compressor Restore compressor Incorrect pressure Pressure gauge Adjust valve Recycle pump off Pump Turn on pump Reaeration pump off Pump Turn on pump Insufficient polymer Chemical system Increase dose Excessive loading Loading rates Lower flow rate - 69 -
Gravity Belt Thickening Principles of Operation 1. Sludge is pre-conditioned with polymer 2. Conditioned sludge is applied to the belt traveling 2-10 feet per minute 3. Water drains through pores in belt 4. Sludge removed from belt and sent to stabilization process Main Components 1. Belt 2. Belt wash sprayers 3. Filtrate trough 4. Sludge collection hopper Factors Affecting Gravity Belt Thickeners 1. Chemical conditioning of sludge (too much polymer can blind belt and cause washout) 2. Belt speed (ideal speed is the slowest speed that can be maintained without washing out the belt) 3. Belt size and type 4. Hydraulic and solids loading Possible Causes of Gravity Belt Problems 1. Polymer dose too low 2. Polymer dose too high 3. Belt speed too high or too low 4. Hydraulic load too high 5. Belt Blinded (need to increase belt wash pressure after securing sludge feed) - 70 -
Wastewater Treatment Review nw ate rc oll e ge 20 10 Centrifuge Thickening Principles of Operation 1. Sludge is fed through inner scroll for even distribution 2. Inner screw accelerates the sludge 3. Centrifugal force is used to force sludge to outer part of bowl (800-2,800 rpm which produce 500-3,000 g s) 4. Outer screw moves thickened sludge to discharge point and liquid is discharged continuously over an adjustable weir eri ca Operational Controls 1. Bowl speed Ranges from 800-2,800 rpm Higher speeds produce higher solids Am 2. Differential Scroll Speed Controls how fast sludge moves through the system Increasing differential speed produces lower solids 3. Pool Depth Controlled by adjustable weir Usually factory set for maximum depth which produces maximum solids recovery but a wetter cake - 71 -
4. Polymer Dose Improves solids recovery Low dose results in poor solids capture High dose results in wetter cake and possible foam in centrate Factors Affecting Operation of Scroll Centrifuge 1. Type of sludge a. Digested Primary Sludge = 35%-40% TS b. Digested Primary + WAS = 25%-30% TS c. Extended Aeration WAS = 18%-22% TS 2. Solids loading (typically 4,000 7,000 mg/l) 3. Hydraulic Loading (typically 75 100 gpm of WAS) Scroll Type Centrifuge Troubleshooting guide Parameter Cause Check Solution Centrate quality good Scroll speed too high Scroll RPM Reduce scroll speed but discharge solids diluted Pool depth too high Pool depth Reduce pool depth Centrate quality poor but discharge sludge solids are fine Centrate quality poor and discharge sludge solids are low Scroll speed too low Scroll speed Raise scroll speed Hydraulic load too high Flow rate Reduce flow Pool depth too low Pool depth Raise pool depth Polymer dose too low Chemical system Raise polymer dose Bowl speed too low Bowl speed Raise bowl speed Loading too high Flow rate Reduce flow Polymer dose too low Polymer system Raise polymer dose Scroll speed/pool depth not optimized Scroll speed and pool depth Adjust scroll speed and pool depth - 72 -