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