Upgrade of the Big Gulch WWTP Oxidation  Ditches for Energy Efficiency and Improved 

Nitrogen Removal

Authors: Thomas E. Coleman, dTEC Systems

Thomas G. Bridges, Mukilteo Water & Wastewater District

Mukilteo Water & Wastewater District

Bend, Oregon October 24-27, 2010

Introduction

• In 2008 and 2009 the Mukilteo Water and  Wastewater District undertook a two phased  project to upgrade the Big Gulch WWTP  oxidation ditches.

• The project goals included:Improve oxidation ditch loading capacity

Reduce aeration energy costs

Increase nitrogen removal efficiency

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Mukilteo Location Map

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Big Gulch WWTF

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Big Gulch WWTF Location

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Olympus Terrace/Big Gulch WWTF HistoryThe original plant, built in 1970, included• coarse bar screening of influent wastewater• one 639,800‐gallon oxidation ditch with two 40  HP brush rotor aerators

• one 58‐foot diameter secondary clarifier, • two 36‐inch diameter screw pumps for RAS  pumping 

• chlorine injection equipment with no chlorine  contact tanks 

There were no provisions for handling waste  activated sludge. 

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Olympus Terrace/Big Gulch WWTF  Upgrades

The first upgrade was completed in 1984 with the addition  of a 54‐foot diameter secondary clarifier

A major upgrade completed in 1989 included:•A new headworks with grit removal•A 1.07 million gallon oxidation ditch with four 30 HP brush  rotor aerators•One 54‐foot diameter secondary clarifier, •Aerobic Digesters, sludge pumping facilities, and  dewatering equipment•Chlorine contact tanks

In 1993 a submersible mixer was added to each ditchIn 2002 the plant converted from chlorine to UV disinfection

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Big Gulch WWTF Layout

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Big Gulch WWTF Aerial view

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Big Gulch WWTF Neighbors

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Design Criteria in the NPDES permit issued in November 2004

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Average flow for the max. month: 2.61 MGD

BOD5 loading for max. month: 4492 lbs/day

TSS loading for max. month: 3605 lbs/day

This permit expired in November 2009 and has not yet been reissued.

Influent Loading Data 2005 ‐ 2006

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Capacity Study and Engineering Report (Gray & Osborne, March 2008)

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Influent Loading Trends at MWWD

0

2,000

4,000

6,000

8,000

10,000

1997 1999 2001 2003 2005 2007 2009

Load

ing

Rat

e (lb

s/da

y)

Average Annual BOD5 Average Annual TSSMaximum Monthly BOD5 Maximum Monthly TSS

Capacity Study and Engineering Report

• Intermittent soluble BOD loading spikes in  2006 *

• New sampler revealed TSS loadings much  higher than previously measured

• Long‐term trend of increasing loading rates

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Findings:

Intermittent soluble BOD and TSS loading spikes have  continued to the present time (October 2010). 

*

Capacity Study and Engineering Report

Report Recommendations:

• Loading source investigations in collection  system

• Grit removal improvements

• Activated sludge aeration capacity  expansion

• Aerobic digester aeration capacity expansion

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Activated sludge aeration capacity expansion

1. Aeration system capacity determined to be a  limiting factor in establishing the BOD design  capacity.

2. Evaluation of aeration system capacity upgrade  alternatives:

• Upgrade brush rotor aeration system

• Replace rotors with diffusers and PD blowers

• Replace rotors with diffusers and turbo blowers

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Evaluation of aeration system capacity  upgrade alternatives

The evaluation of aeration system alternatives  considered capital and O&M costs.

Incentives payments available from the utility  (Snohomish PUD) for improvements which  reduce electrical energy consumption and  demand.

Other operational factors also considered. 

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Energy Costs at a WWTPAeration accounts for 50-75% of the total energy cost at a municipal wastewater treatment plant.

AerationLightingPumpingHVACMisc.

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Comparison of Aeration Alternatives

Oxidation Ditch “A” peak energy consumption

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Parameter Aeration MethodBrush Rotors

Diffused Aeration with

PD Blower

Diffused Aeration with Turbo Blower

Actual Oxygen Demand (lb/day) 4,527 4,527 4,527 Standard Oxygen Demand (lb/day) 6,658 7,611 7,611 Diffuser Air Flow Rate (scfm) N/A 1,300 1,300 Max. Motor Power (hp) 115 60 50 Max. Motor Power (kW) 86 45 37 Power Savings at Max. Load vs. Brush Rotor (%) 48% 57%

Power Savings at Max. Load vs. PD Blower (%) 17%

Comparison of Aeration AlternativesOxidation Ditch “A” annual avg. energy consumption

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Parameter Aeration MethodBrush Rotors

Diffused Aeration with

PD Blower

Diffused Aeration with Turbo

Blower

Actual Oxygen Demand (lb/day) 2,440 2,440 2,440 Diffuser Air Flow Rate (scfm) N/A 665 665 Average motor power req'd (hp) 58 28 21 Avg. Annual Power Consumption (kWH) 380,000 183,000 137,000 Annual Power Savings (kWh) 197,000 243,000 Annual Power Cost Savings ($) $13,800 $17,000 Annual Power Savings vs. brush rotor (%) 52% 64%Annual Power Savings vs. PD blower (%) 25%

Comparison of Aeration AlternativesOxidation Ditch “A” annual O&M costs

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Item Quantity Unit Price AmountA. Brush Rotor Alternative

Power 380,000 kWH $0.07 $26,600 Repair and Maintenance 1 LS $2,300 $2,300 Total $28,900

B. Diffused Air with PD Blower AlternativePower 183,000 kWH $0.07 $12,800 Repair and Maintenance 1 LS $2,100 $2,100 Total $14,900

C. Diffused Air with Turbo Blower AlternativePower 137,000 kWH $0.07 $9,600 Repair and Maintenance 1 LS $1,800 $1,800 Total $11,400

Comparison of Aeration Alternatives

Oxidation Ditch “A” lifecycle cost comparison

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Alternative Total Construction

Cost

Annual O&M Cost Estimate

20-year Net Present Value

Brush Rotor Alternative $261,000 $28,900 $808,000

Diffused Air with PD Blower Alternative $279,000 $14,900 $507,000

Diffused Air with Turbo Blower Alternative $321,000 $11,400 $490,000

Operational Considerations   Brush Aerators

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ProsSimple “In-House” Repair

Submersible Mixer Not Needed

ConsFixed Hydraulic Level

Damages Floc Structure

Lubrication Required

Create Aerosols

Energy inefficient

Difficult to Control D.O.

Noise

Operational Considerations Brush Rotor Aeration Pros and Cons

Operational Considerations   Turbo Blower With Fine Bubble 

Diffusers

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ProsHydraulic Level can vary

No Shearing of Floc

No Lubrication Required

Minimal Maintenance

Minimal Aerosols

Energy Efficient

Good D.O. Control

Low Noise

ConsNeeds Submersible Mixer

Proprietary Blower Technology*

Diffuser Inspection/Repair Requires Dewatering of Basin

*(in the case of Turbo Blowers)

Diffused Aeration Pros and Cons

Comparison of Aeration Alternatives• Selected alternative: Diffused air with turbo  blower

• Despite higher capital cost, energy efficiency  resulted in a payback period of 4 years

• Lowest 20 year lifecycle costs (3% lower than  PD blower)

• 65% energy savings compared to brush rotors

• 25% energy savings compared to PD blower

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Turbo Blower TechnologySingle‐stage centrifugal blower with integrated  high‐frequency VFD

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Turbo Blower Cutaway View

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Image courtesy of K-Turbo Inc.

Impeller directly mounted on motor shaftHigh-speed permanent magnet motor (up to 24,000 rpm)

Turbo Blower – Air Foil Bearing

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Image from:http://www.grc.nasa.gov/WWW/Oilfree/bearings.htm

Turbo Blower Features

• Reduced maintenance: no maintenance  required for seals, lubrication, etc.

• Low power at motor start

• Can run unloaded at 1% of rated power

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Phase I Project

Convert Oxidation Ditch “A” from brush rotor aeration to diffused aeration with a Turbo Blower

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Installation of Blower & Diffusers

Demolition/Removal of Old Equipment

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Installation of Blower & Diffusers

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Install Turbo Blower

Front View of Turbo Blower

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New diffusers in place in Ditch A

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Oxidation Ditch A after upgrade

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Electric utility rebate

• Snohomish County PUD No. 1 monitored  electricity consumption before and after  Ditch “A” project construction

• Rebate for construction costs equal of  $0.17/kWH of annual electricity savings

• District received SNOPUD Incentive Rebate of  $39,171 for Phase 1

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Phase II ‐ Conversion of Ditch “B”

• Phase II of the project converted Oxidation  Ditch “B” to diffused air with turbo blowers

• 35% reduction in annual energy consumption  were estimated for the oxidation ditch  system

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Ditch B Conversion Energy Savings

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Conversion of Ditch “B”

• Removal of four 30‐hp brush rotors (120 hp  total)

• Installation of two additional 50‐hp turbo  blowers (one duty, one standby)

• Installation of two new 6‐hp submersible  mixers

• Equipment was pre‐purchased

• Construction completed in March 2010

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Brush Rotors in Ditch B before upgrade

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Conversion of Ditch “B”

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Phase 2 Baseline Metering

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Phase 2 Post Metering Results

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Combined Results from Phase 1 and Phase 2

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Before

After

dNOx® System for Improved  Denitrification

Originally developed under an EPA Small  Business Innovation Research (SBIR) Project  at the Grand Coulee, WA WWTP.

Patent # 5,582,734 process for oxidation  ditch control

Inventors: Coleman, Stensel, Denham, and  Fleischman

WAS

Process Flow Schematic for Oxidation Ditch with Cyclic Aerobic/Anoxic Operation

WSEO funded mixer installations in the early 1990s.

The automated anoxic cycle control system was developed during the mid 1990s under an EPA SBIR research contract.

The GC/EC WWTP has consistently achieved energy savings, good total nitrogen removal, and SVI control for over 15 years.

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dNOx® System Description

• Detects ORP inflection point = reliable detection of nitrate/nitrite depletion in activated sludge processes

• Provides automated aeration control for oxidation ditches and other wastewater treatment processes such as SBRs and Aerobic Digesters

2NH4+ + 3O2

→ 2NO2− +4H+ + 2H2

O (Eq. 1)

Ammonia Oxidizing Bacteria (AOB)

2NO2− + O2

→ 2NO3− (Equation 2)

Nitrite Oxidizing Bacteria (NOB)

NH4+ + 2O2

→ NO3− + 2H+ + H2

OOverall Reaction

Nitrification Fundamentals

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~ 4.6 lb of O2 required per lb of NH3

-N oxidized

Denitrification

NO3 ̄ → NO2 ̄ → NO → N2 O → N2

C6 H12

O6 + 4.8NO3

- + 4.8H+ → 6CO2 + 2.4N2

+ 8.4H2 O

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~ 2.9 lb of O2 equivalent per lb of NO3

-N reduced

OR

P m

V

DO

, NO

3- , N

H3+ m

g/L

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dNOx Anoxic Control Panel

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_

dNOx® Process Control Algorithm

dNOx control system algorithms utilizes

IEC 61131‐3 Compliant Structured Text and  Function block diagrams

The Structured Text uses a syntax similar to  the Pascal programming language

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dNOx® System Benefits• Energy savings

• Reliable effluent nitrogen  as low as 5 mg/L

• Quick installation, very  little maintenance

• Fully automatic  nitrification/denitrification  cycles control

• Compatible with  multiple  wastewater  processes

• Improved sludge  settling

• Selective pressure  against filamentous  bacteria growth

• Alkalinity recovery

Nitrogen removal at Big Gulch

During the first 6 months of operation  using the dNOx system to control  anoxic/aerobic cycling in the oxidation  ditches, a number of operational  strategies have been evaluated.

Currently the process has been  optimized to consistently achieve less  than 5 mg/L total inorganic nitrogen.

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Next Phase of Improvements at the  Big Gulch WWTF

The next phase of improvements at the Big  Gulch WWTF will include the upgrade of  screening and grit removal at the Headworks  and increasing the aeration capacity of the  Aerobic Digester.A Selector Basin is also being designed into  the Headworks upgrade.  Depending on the  amount of nitrate being returned to the  selector it could be anoxic or anaerobic. If  anaerobic, we can expect EBPR to occur.

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Process Flow Schematic of Heyburn, ID WWTP(Recently upgraded to meet new P limits for discharge to the

Snake River)

WAS

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Heyburn,ID WWTP (Bio-P removal)

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ConclusionsBlowers and diffused air may be advantageous  for oxidation ditch operation:•Significant energy savings•Operational flexibility Turbo blowers may offer further energy savings  in comparison with positive displacement  blowers.

An ORP based control system optimizing  aerobic/anoxic cycling in an oxidation ditch can  achieve additional energy savings and  improved total nitrogen removal.

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http://www.dsireusa.org/

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Electric utility rebate

Pre‐Metering Results: Ditch “A” Week 2 Rotor 1

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Electric utility rebate

Pre‐Metering Results: Ditch “A” Week 2 Rotor 2

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Electric utility rebate

Post‐Metering Results: Ditch “A” Blower