Using Process Optimization and Energy Audits to Reduce

Using Process Optimization and Energy Audits to Reduce Energy Costs

PNCWA Annual Conference

23 October 2012

Ron Moeller Operations Services Leader

Presentation Outline

• What is a process energy-audit?

• Energy use in WWTPs

• Process interrelationships

• What questions to ask yourself

• Potential obstacles

• Case Studies

What Is a “Process Energy Audit”?

• Different from an Energy Audit

• Looks primarily at O&M

• Savings found in process adjustments

• Process understanding is key

• Understand interrelationships

Collaborative Workshops

• Auditor/facilitator with WWTP

experience

• Engage management,

operations, maintenance,

and engineering staff

• Create understanding and

buy-in

WWTP Energy Use

Typical Energy Use Profile for 10‐mgd Secondary Treatment Processes.

Source: WEF 2009, Figure 7.1

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Energy Costs

1 HP running 24/7/365 costs the following:

Utility Cost per kWh Annual Cost

Tacoma, WA $0.042 $274.32

Western WA $0.065 $424.54

California $0.17 $1,110.33

Hawaii $0.33 $2,155.36

Savings Opportunities

• Headworks/Influent Pumping

• Primary Clarification

• Secondary Treatment

• Filtration (media and membrane)

• Disinfection

• Solids Handling

Be Careful – Everything is Related!

A change in one area may have unintended

consequences in other areas!

Inf.

Pump

Headworks

Primary

Clarifier

Aeration

Basin

Secondary

Clarifier Filter Disinfection

Thickener Digestion Dewatering

Ask Yourself…

• Have we tried to modify process operation to

gain energy savings?

• When was the last time we asked “why are we

doing it this way?”

• Have we adopted SOPs or does each shift

change operation based on their preference?

• Do we collect data and understand how our

process behaves?

• Can we turn this off?

• Can we improve power quality?

Obstacles to Energy Efficiency

• Resistance to change

• Perception that saving energy = reduced

effluent quality

• Skepticism with new technology

• Already optimized

• Cost to implement

• I’m too busy

EPA Evaluation of ECMs

…does not discuss process-related ECMs

Example ECMs

1 Source: EPA 832-R-10-005

Case Study #1

• City of Greeley, Colorado

• Average flow 7.4 mgd

• Selector activated sludge

– Ammonia limits

– Anoxic selector for alkalinity gain

– MLR pump constant speed @ 1Q

– Average DO 2.4 mg/L

– F/M of 0.23

– MCRT of 10-22 days

• Operate 3 of 4 clarifiers

• RAS rates near 100%

• Avg. electric cost $0.066 kWh

Case Study #1

Case Study #1 – Project Methodology

1. Information Collection

Data review, request for additional information, and

synthesis of this information followed.

2. Site Visit

The site visit and facility tour were conducted with workshop

participants.

3. Workshop

The workshop focused on energy utilization and wastewater

process control, and allowed an exchange of information

that increased the number and applicability of

recommendations.

4. Technical Memo

Provided a summary of findings and recommendations

CS#1: Primaries

• Condition

– Septic primary clarifiers identified

– Liquid stream pH drops from 7.6 to 6.5

– 5% cBOD removal and increased ammonia

• Huge impact on AB DO requirement

– Primary sludge pH 5

– Condition for greater than 10 years

• Actions and Expected Results

– Increase primary pumping

– Improve cBOD removal and reduce ammonia

– Lower cBOD air demand 25-35%

CS#1: Re-Aeration

• Condition

– 1,000,000 gallon tank aerating already well-oxidized

RAS only

– Totally unnecessary in current set-up

– Increases DO to anoxic selector


– Remove tank from service

– Reduces DO to selector

– Lowers overall blower requirement

CS#1: Anoxic Selector

•Condition

–Not effective for filament control

–Nitrate reduction not consistent

–Too much DO in MLR and Re-Aeration

–Inconsistent ALK gain

•Actions and Expected Results

–Lower mixed liquor DO

–Take Re-Aeration off line

–Reduced cBOD to aerobic by approx.15 mg/L

–Improved ALK and pH

–Improved filament control

CS#1: RAS

• Condition

– RAS rate too high / VFDs manually controlled

– Thins WAS concentration / increases WAS costs

– Increases RAS pumping costs

– Impacts selector, aerobic, and clarifier performance

• Action and Expected Results

– Pace RAS off raw sewage meter

– RAS rates should be 30-60%

– Reduces RAS and WAS pumping costs

– Reduces WAS volume 25%

– Improves selector, aerobic, and clarifier

performance

CS#1: DO & MLSS

• Condition

– AB DO at 2.4 mg/L

– MLSS at 2600 mg/L

– 3 250 HP blowers in service


– Reduce AB DO to 1.5 mg/L

– Increase MLSS to 3200 mg/L

– Reduce sludge yield

– Reduce DO recycle to selector

CS#1: Filaments

• Condition

– Current SVI good but historically variable

– Reoccuring problem with Microthrix parvicella and

Nostocoida limicola


– Improved selector performance from DO recycle

control should reduce filaments (esp. N. Limicola)

– SVI control results in increased realized clarifier

capacity and possible increase in RAS/WAS

concentration

CS#1: Sludge Yield/WAS

• Condition

– Current sludge yield is 0.76


– Data trending indicated that decreasing F/M from

0.24 to 0.19 produces a sludge yield of 0.6

– This correlates to a 22% reduction in WAS mass

– Lower WAS production saves money throughout

solids train

CS#1: Clarifiers

• Condition

– Three clarifiers on-line

– Three RAS pumps on-line


– Evaluation showed that only 2 clarifiers and 2 RAS

pumps needed

– Reduced energy related to clarifier and RAS pump

operation

– Less maintenance on equipment

CS#1: Pieces of a Puzzle

CS#1: Projected Results

Equipment/Process Change Description Horse Power Volts Current Phase (1 or 3) Hours Monthly

Reduction (kWh)

Annual Reduction

(kWh)

Monthly

Reduction ($)

Annual

Reduction ($)

Percentage

Reduction ($)

WAS Thickening

Centrifuge

Reduced runtime from increased

WAS concentration.

150 3.4 11,566 138,792 $751.79 $9,021.47 2.0%

Blower cBOD and ammonia reduction,

coupled with other passive DO

reduction measures, allows 2nd

blower to be turned off at night (9

PM to 9 AM.)

250 12 68,035 816,422 $4,422.29 $53,067.46 11.6%

Blower Taking the 1,000,000 gallon RAS Re-

aeration Tank offline will reduce the

need for a third blower.

250 24 136,070 1,632,845 $8,844.58 $106,134.91 23.2%

RAS Pump RAS pump associated with secondary

clarifer taken offline.

15 24 8,164 97,971 $530.67 $6,368.09 1.4%

Secondary Clarifier Remove one of three from service.

State-point analysis shows one can

be safely taken offline.

1 24 544 6,531 $35.38 $424.54 0.1%

Monthly Totals 224,380 kWh $14,584.71

Annual Totals 2,692,561 kWh $175,016.47 38.2%

Proposed Energy Control Measures Table


Up to 2,697,000 kWh/year reduction in power

• $175,000/year, or $875,000 every 5 years!

• Potential 38% reduction in total plant electrical

use

Improved sludge quality

20-40% reduction in WAS sludge production

More stable and consistent secondary operation

Lower effluent nitrate

Less odors

Reduced O&M labor

Staff has improved process control

procedures/knowledge

Environmental Impact

The reduced greenhouse gasses (measured by carbon dioxide [CO2]

equivalent) that could be realized from the project are significant. The annual

environmental equivalents of reducing the electricity are shown below:

Equivalent to avoiding emissions of 1,937 metric tons of CO2 per year

CO2 emissions from 217,873 gallons of gasoline consumed

CO2 equivalent of removing 370 cars from the road per year

CO2 emissions from 4,504 barrels of oil consumed

CO2 emissions from electricity use of 235 homes per year

CO2 emissions from 80,704 propane cylinders used for home barbecues

Carbon sequestered by 49,664 tree seedlings grown for 10 years

Carbon sequestered annually by 413 acres of pine or fir forests

Case Study #2

• City of Tacoma, Washington

• Average flow 22.7 mgd

• Pure oxygen activated sludge

– Peak wet weather facility creates high peak demand

charge for remainder of the year

– Primaries hydraulically taxed in the winter

– SVI highly variable (150 to 280 ml/g)

– UNOX first stage DO is 7.0 mg/l

– Mixed liquor channel is aerated

• Solids handling includes DAFT, ATAD, anaerobic

digestion, and belt presses for sludge dewatering

• Avg. electric cost $0.042 kWh

Case Study #2 – Project Methodology

1. Information Collection

Data review, request for additional information, and

synthesis of this information followed.

2. Site Visit

A site visit and facility tour were conducted with senior staff.

3. Workshop

The workshop focused on energy utilization, and allowed an

exchange of information that increased the number and

applicability of recommendations.

4. Technical Memo

Provided a summary of findings and recommendations

CS#2: PWWF

• Condition

– PWWF runs for a few hours each year, but

establishes the peak electrical demand charge for

the next 11 months

– Five 900 HP pumps

– Five 10 HP fans run continuously

– Transformers are on year round


– Turn off fans and transformers 6 to 9 months out of

the year

– Establish SOP for consistent operation of PWWF

– Annual savings of $35,000

CS#2: Primaries

• Condition

– Hydraulically taxed in winter

– Reduced BOD and TSS removal


– Encouraged winter operation of CEPT

– Increase BOD and TSS removal

– Reduced oxygen demand in UNOX equals savings of

$17,700

– Possibility of greater oxygen demand in ATAD

– 15% more primary sludge production increases gas

production

– Potential energy value of $77,900

CS#2: UNOX

• Condition

– Well operated and in good working order

– No D.O. control in stages 2-4

– Highly variable SVIs; good sludge yield (0.60)

– ATAD off gas is routed back to the UNOX first stage


– Lower first stage D.O. from 7 to 6 mg/l

– Route ATAD off gas to other stages

– Install VFDs and D.O. monitoring/control on mixers in

stages 2-4

CS#2: Mixed Liquor Channels

• Condition

– Air in the channel is provided by a 75 HP blower and

appears to be over-aerated

– There is foaming in the channel that feeds the secondary

clarifiers (damage to floc?)


– Reduce air to the channel (lower the blower VFD speed,

use a smaller blower, etc.)

– One-third reduction results in annual savings of $7,000

– Potential to reduce/eliminate foaming and floc shear

CS#2: DAFT

• Condition

– No control of recycle water pump


– Install VFDs on recycle water pump

– A 50% turndown results in annual savings of $18,000

– Under CEPT, shut down one DAFT in winter, resulting in

annual savings of $14,000


Equipment/Process Change Description HP Saved Hours Annual Savings

Influent Wet Well Run with higher wet well level to increase pump suction head. 25 24 $7,021

Headworks Odor ScrubberShut off in winter. 6 months/year is equivalent to shutting off 12

hours/day. 25 12 $3,511

Grit Odor ScrubberShut off in winter. 6 months/year is equivalent to shutting off 12

hours/day. 15 12 $2,106

Grit Tank AirReduce air to tanks. Change pulley or install VFDs. Assume

20% reduction. 25 4.8 $1,404

Peak Wet WeatherFans. There are 5 ea. 10HP fans that could be turned off for 9

months/year, equivalent to shutting off 18 hours/day. 50 18 $10,532

Peak Wet Weather

Transformers. There are 5 ea. 900KVA transformers that could

be turned off for 9 months/year, equivalent to shutting off 18

hours/day. $25,040

Primary Clarifiers

Winter operation of chemically enhanced primary treatment

(CEPT). 15% reduction of BOD to UNOX system equates to

lower O2 production. Assume 40% net reduction of oxygen

related to the additional BOD removal in the primary clarifiers.

$44,382 x 40% = $17,747. $17,747

UNOX

Install VFDs and monitoring equipment for DO control on 2nd,

3rd, and 4th stage mixers. Assume 10% reduction on one train

in service year-round = 25HP. 25 24 $7,021

UNOX

Install VFDs and monitoring equipment for DO control on 2nd,

3rd, and 4th stage mixers. Assume 10% reduction on a second

train in service during higher winter flows = 25HP running only 6

months/year = 12.5HP. 12.5 24 $3,511

UNOX Repair air leakage on tanks. HP equivalent is an estimate. 20 24 $5,617

Secondary ClarifiersReduce air in the MLSS channel to the clarifiers. Assume 1/3

runtime. 25 24 $7,021

DAFTInstall VFDs. Allow 50% turndown. Assume 80% reduction of

current 80% efficiency = 64HP savings. 64 24 $17,974

DAFT (under CEPT)Turn off one recycle pump during winter. 6 months/year is

equivalent to shutting off 12 hours/day. 100 12 $14,042

#6 DigesterTurn off gas mixing compressor or operate in ON/OFF mode.

Assume 50% reduction in runtime. 25 12 $3,511

Blend Tank Mixer pump is oversized. Reduce runtime. 40 16 $7,489

Total $133,547


Up to 2,697,000 kWh/year reduction in power

• $133,500/year, or $667,500 every 5 years ---

Hey, this is at $0.042 kWh too!!

• Potential 14% reduction in total plant electrical

use

Improved UNOX process control

Improved primary clarifier performance

More stable and consistent secondary operation

Reduced O&M labor

Staff has improved process control

procedures/knowledge

Recommendations

• Savings from some recommendations are not

enumerated (labor, reduced maintenance,

chemical, etc.)

• No capital costs…process changes only!

• The recommendations presented are viable.

However, Utilities may choose not to

implement the recommendations for the

following reasons:

– Political (Odors)

– Socio-Economic (TAGRO)

– Regulatory (Energy, Rerate)

Obstacles to Energy Efficiency

• Resistance to change

• Perception that saving energy = reduced

effluent quality

• Skepticism with new technology

• Already optimized

• Cost to implement

• I’m too busy

Summary

• Many savings opportunities can be

identified using a process energy audit.

• Operator involvement is paramount!

• Look for opportunities in design as well.

• Energy Performance Contracting (EPC)

is a possible contracting vehicle that

reduces financial risk.

• There are resources out there to help

you get started!

Questions and Answers

Documents

Using Process Optimization and Energy Audits to Reduce