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O W E A B i o s o l i d s W o r k s h o p
D e c e m b e r 8 , 2 0 1 1
Liquid Solids
Handling Economics
General Ideas
• “Pumping Systems Account for Nearly 50-percent of
Electricity Used by Pumping-Intense Fields Such as
Municipal Water and Wastewater Industries.” –
Excerpt From Life Cycle Savings in Water Treatment
Pump Systems by Robert Lax, ITT Gould Pumps
• Based on a US Department of Energy Study, the
Optimization of Pumping Systems can Reduce a
Facilities Energy Costs by 20-Percent
System Curves
• Typical System Curve -
Ask Engineer
• Pumps in Parallel - Add
Flows at Same Head
• Pumps in Series - Add
Heads at Same Flows
Factors Affecting System Hydraulics
• Age of Pipes
• Pipe Sizing
• Sludge % Solids
• Rate of Flow
• Number of Valves &
Fittings
Losses - Fittings
• k factors from Darcy-Weisback
• k 𝑉2
2𝑔 = Headloss
• Pipe Exit into a Tank
k = 1.0
Losses - Fittings
• Velocity Matters!
•𝑉2
2𝑔 @ 200 gpm
• 4” – 5.11 ft/sec, 𝑉2
2𝑔 = 0.406 ft
• 6” – 2.27 ft/sec, 𝑉2
2𝑔 = 0.08 ft
Losses - Fittings
• 0.406 ft times 1.0 = 0.406 ft
• 0.08 ft times 1.0 = 0.08 ft
• Recall that 1.0 is the k factor for an exit loss
Losses - Pipe
• 4” @ 100 ft = 2.73’
• 6” @ 100 ft – 0.346’
• Reduced Diameter in any Fashion Creates Additional
Headloss
• Pipe Size
• Age
• Walls Coated with Material
• Air
Percent Solids
Primary 1% - 5%
Waste 0.5% - 1.5%
Return 0.5% - 1.5%
Thickened 2% - 12%
Digested 2% - 6%
Why is % Solids Critical?
Pumping Solids
• Increased Solids Harder
to Pump
• Increases Horse Power
Required
• Extremely Thick Solids
Start-up Head Increases
Must Get Product Moving
Pumps
• Static Head – You Can
Check
• Minimum Pump Must
overcome
• Wetwell Fluctuations
Piping Systems
• Watch for High Points –
Pressure
• Air Release Valves
• Check Valves Horizontal
if Possible
• Maintain 2 ft/sec
• Once Grit Settles Out,
Takes 7-10 ft/sec to scour
System Curves
• What will a VFD do?
Variable Frequency Drives
• Increased Operating Control and Flexibility
• Reduced Energy Consumption at Non-Peak Times
• Complete Motor Protection and Diagnostics
• User Interface and System Integration
• Soft Starting and Stopping
• Reduced Equipment Wear
• Energy Savings
Variable Frequency Drives
• Energy Savings:
• Energy Savings as Flow Requirements Change
• Non-Peak Loads Reduce Speed/Horsepower
• Operation/Start-up Cost less Because of Slow Starts
• Will Not Allow Motor to Exceed Full Load Amp Rating
Sludge - Types
• Primary
• Waste
• Return/Waste
• Thickened
• Digested – Aerobic or Anaerobic
}Co-settled
Primary Sludge
• Often Times Co-settled With Waste
• Low Volume, Medium Head
• Pumping Difficulty
• Scale 1-10
• 5-9 Why
Primary Sludge
• Why a 9?
• Headworks Dependent
• Often Times Grinding Required
• Gritty
• Pump Wear
• Live with Pumping Inefficiencies
Primary Sludge
• Pumping Operation
• Intermittent Times
• Constant Small Volumes
• Instrumentation?
Pump Types
• Chopper
• Two Functions – Chop &
Pump
• Low Efficiency
Chopper Pump
Performance Curves
Pump Types
• Solids Handling
• Vortex & Semi-Vortex
• Headworks - Critical
• More Efficient
Vortex Pump
Vortex Pump
Vortex Pump
Pump Types
• Progressive Cavity
• Rotor & Stator or Rotary Lobes – “Squeeze Types”
• Plunger or Diaphragm Types – “Push Types”
Pump Types
• Progressive Cavity Styles
• Grit – Can be an Issue with “Squeeze” Style
• Lower Speed Less Wear
• Easy to Control
• Consider Grinding
Plunger Pump
Plunger Pump
Primary Sludge - Economics
• Grinding and Costs
• Breakdowns - Manpower
• Pumps – Repairs or Parts Replacement
Return Sludge
• Normally Constant 24/7
• Need Control – Typically VFD
• Typically Low Head High Volume
• % Solids – 0.5% - 1.5%
Return Sludge
• Pump Efficiency Important
• Range 25% to 100-150% of Design Flow
• Higher Range Normally not Necessary but Required
by Ten States
• Pump Type – Solids Handling – Non-Clog
Non-Clog
Non-Clog
Return Sludge - Economics
• 85% Efficiency vs. 45% Efficiency
• Conditions
• 500 gpm @ 20 feet
• 24/7 Operation
• Brake Hp = gpm x H x SG
3960 x eff
• BHp85% = 3 Hp
• BHp45% = 5.6 Hp
Power Cost
• Motor Efficiency (Assume 90%)
• Bhp/Motor Eff. = Electrical Hp
• 3 Hp/0.9 – 3.33 Hp
• 5.6 Hp/0.9 – 6.2 Hp
Power Cost
• Electrical Hp x 0.746 = kw
• 2.5kw x 24 x 365 = 21,900 kw Hrs
• 4.6 kw x 24 x 365 = 40,296 kw Hrs
• 21,900 kw Hrs x 0.08¢/kw Hrs = $1,752
• 40,296 kw Hrs x 0.08¢/kw Hrs = $3,223
• Savings = $1,500/year
Return - Control
• Telescoping Valves Used to Keep a Consistent
Product
• Can Pull Direct or to a Wet Well
• May Rat Hole at Higher Flows in Direct Applications
Return - Control
• Variable Frequency Drives
• Flow Paced to Maintain a Constant % of Plant Flow
• Reduced MLSS Concentration Variations
• Keeps MLSS in Clarifier for Shorter Periods – Less Chance
to Denitrify
Return - Control
• Constant RAS Flow
• Simple for Smaller Plant
• Less Operation Attention
• At Start of Peak Flows, High Solids Loadings to Clarifiers
Return - Control
• Check with Sludge Judge
• Check via Lab Results
• Every Plant Unique as to Rate
Waste Sludge
• Typically Same as Return Consistency
• Try Again for High Efficiency Pumping
• Motor Operated Valve off Return
• Saves Pumping Again
Waste Sludge
• Often Returned to Primaries to be Co-Settled
• Why?
• Thicker is Normally Better
• Less Initial Volume Utilized in Digester
• Less Supernating Return from Digester
• Less Water to Heat
Thickening
• Why
• Reduces the Flow & Volume Requirement for Down Steam
Processes
• Example: 4 to 6% solids to an Anaerobic Digester Reduces
Heating and Volume Requirements compared to 2-4%
• Increase Detention Times
• Eliminate Digester Expansion
Gravity Thickeners
• Most Often Used to Thicken Primary Solids due to
Their Ease of Settling
• Have Been Used for Waste Activated, Mixed Results
Gravity Thickener
Gravity Thickeners
• Advantages
• Increases Solids – Reduces Down Stream Process Sizes
• Potential to Generate Volatile Fatty Acids for BNR
• Provides Some Flow EQ & Storage
• Easier to Enable a More Consistent Loading on Digesters
• Disadvantages
• Increased Odor Potential
• Covered Tanks – Air Returned to Aeration Tanks
• Additional Process Step
• More Land Required
Waste Sludge Thickening
• Typically a Mechanical Means – 3-5% TS
• Many Technologies to Achieve
• Most Widely Used DAF, Gravity Belts, Rotary Drum
and Centrifuges
DAF Thickening
• Dissolved Air Floatation
• Utilizes Microscopic Air
Bubbles to Float the
Suspended Solids
DAF Thickening
• Typical Results 3-4% & 90% Capture
• With and Without Polymer
• Sized Based on Solids Loading Rates
DAF Thickening
• Advantages
• Simple Operation
• Little Attention
• Low Polymer Usage
• Disadvantages
• Relative High Energy Use
• Possible Odor
• Lower Concentrations
• Large Footprints
Gravity Belt Thickeners
• Separation of Solids on Horizontal Belts
• Polymer Required
• 4-8% Solids Typically Achieved
Gravity Belt Thickener
Gravity Belt Thickeners
• Advantages
• Simple Operation
• Little Attention
• Low Power Consumption
• Disadvantages
• Odor Potential
• High Wash Water Flows
Rotary Drum Thickeners
• Utilize a Perforated
Rotating Drum to Free
Water
• Polymer Utilized
• 4-6% Solids
Rotary Drum Thickeners
• Advantages
• Simple Operation
• Little Attention
• Low Power
• Low Odor
• Disadvantages
• High Polymer Consumption Potential
• Wash Water
Rotary Drum
Liquid Disposal
• Who liquid land applies?
• How many land apply cake?
Example - Liquid - Dry Tons
1,000,000 gallons at 2% solids
Lmggallons
gallonpounds /000,20000,000,1
000,000,1/34.8
tonsdrytonslbs
lbs4.83
/000,2
800,166
Disposal Costs - Liquid - Land Application
1,000,000 gal x $0.04/gal = $40,000
What if 4% solids in lieu of 2%
8.34 x ? x 40,000 = 166,800 dry lbs
=
0.5 million gals or 500,000 gallons
500,000 x $0.04/gal = $20,000
$239.50/dry ton
Thickening Saves!
tonsdrytonsdry
/479$4.83
000,40$
Liquid Storage
• Requirements
• 120-180 Days
• Seems to be Getting Longer
• Can Include Your Digester Capacity Outside of the
Digester Operating Parameters
• Requires 3/8” or Equivalent Screenings
Liquid Storage
Liquid Storage
• Access for Cleaning
• Mixing
• Side Entry Mixers
• Air
• Fixed Nozzles System
• Others – Linear Motion
Fixed Nozzle System
Liquid Storage
• Supernating Tree
• Floating Devices
Supernating Tree
Example - Cake - Wet Tons
Remember the 1,000,000 gallons of liquid @ 2%
equated to 83.4 dry/tons
83.4 dry tons @ 16% cake solids
tonswettonsdry
25.52116.0
4.83
tonswettonsdry
cakeif 41720.0
4.83%20
Disposal Costs - 16% Cake - Landfill
Cost to Landfill $35/wet ton
521.25 wet tons x $45/ton = $23,456
What if 20% cake
417 wet tons x $45/ton = $18,765
$225/dry ton
tondrytonsdry
/25.281$4.83
456,23$
Disposal Costs - 16% Cake -
Land Application
Cost to land apply - $25/yard
How many yards de we have?
How much does 16% cake weigh/ft3?
Water fraction 0.84 x 62.4 lbs/ft3 = 52.4 lbs/ft3
Solids fraction 0.16 x 90/lbs/ft3 = 14.4 lbs/ft3
Total = 66.8 lbs/ft3
If 20% - 67.9 lbs/ft3
Disposal Costs - 16% Cake -
Land Application
66.8 lbs/ft3 x 27 ft3/yd = 1,803.6 lbs/yd
ydsydlbs
tonlbsxtonswet578
/6.803,1
/000,225.521
578 yds x $25/yard = $14,450
tondrytondry
/26.173$4.83
450,14$
If 20% = $136.40/dry ton
Cost to Make Cake?
• Let’s assume a belt press or centrifuge
• Other ways
• Rotary fan press
• Wedgewire beds
• Old fashion way - sand beds
Costs
• Polymer Cost - generally $35/dry ton (can be higher or lower)
• Electrical @ $0.05/kwh • Belt press
• Drive
• Booster water pump
• Feed pump
• Conveyor
• Hydraulic pump
• Lights & HVAC
Total $10/dry ton
Costs
• Assuming labor is a wash since no employees are
added or subtracted
• Total $60/dry ton
• $281.25 + $60/dry ton = $341.25/dry ton
• $285/dry ton – 20%
Washwater & Filtrate Treatment = $5/dry ton
Equipment maintenance will vary can use $5-
$10/dry ton
Comparison
• Liquid @ 2% - $479/dry ton
• Liquid @ 4% - $239.50/dry ton
• Cake Landfill @16% - $341.25/dry ton
• Cake Land Apply @ 16% - $235/dry ton
• Cake Landfill @ 20% = $285/dry ton
• Cake Land Apply @ 20% = $196/dry ton