Using Process Optimization and Energy Audits to Reduce Energy Costs

Transcription

1 Using Process Optimization and Energy Audits to Reduce Energy Costs PNCWA Annual Conference 23 October 2012 Ron Moeller Operations Services Leader

2 Presentation Outline What is a process energy-audit? Energy use in WWTPs Process interrelationships What questions to ask yourself Potential obstacles Case Studies

3 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

4 Collaborative Workshops Auditor/facilitator with WWTP experience Engage management, operations, maintenance, and engineering staff Create understanding and buy-in

5 Power use Million kwh/year 30 ft TDH Preliminary Treatment Primary Settling 60 ft TDH Trickling Filter - Rock Trickling Filter - Plastic RBC Activated Sludge Coarse Bubble Activated Sludge Fine Bubble Gravity Thickening DAF Thickening Anaerobic Digestion Vacuum Filters Centrifuge Belt Filters Aerobic Digestion WWTP Energy Use Typical Energy Use Profile for 10 mgd Secondary Treatment Processes. Source: WEF 2009, Figure 7.1

6 Energy Costs 1 HP running 24/7/365 costs the following: Utility Cost per kwh Annual Cost Tacoma, WA $0.042 $ Western WA $0.065 $ California $0.17 $1, Hawaii $0.33 $2,155.36

7 Savings Opportunities Headworks/Influent Pumping Primary Clarification Secondary Treatment Filtration (media and membrane) Disinfection Solids Handling

8 Be Careful Everything is Related! Headworks Primary Clarifier Aeration Basin Secondary Clarifier Filter Disinfection Inf. Pump Thickener Digestion Dewatering A change in one area may have unintended consequences in other areas!

9 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?

10 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

11 EPA Evaluation of ECMs does not discuss process-related ECMs

12 1 Source: EPA 832-R Example ECMs

13 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 1Q Average DO 2.4 mg/l F/M of 0.23 MCRT of days Operate 3 of 4 clarifiers RAS rates near 100% Avg. electric cost $0.066 kwh

14 Case Study #1

15 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

16 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%

17 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 Actions and Expected Results Remove tank from service Reduces DO to selector Lowers overall blower requirement

18 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

19 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

20 CS#1: DO & MLSS Condition AB DO at 2.4 mg/l MLSS at 2600 mg/l HP blowers in service Action and Expected Results Reduce AB DO to 1.5 mg/l Increase MLSS to 3200 mg/l Reduce sludge yield Reduce DO recycle to selector

21 CS#1: Filaments Condition Current SVI good but historically variable Reoccuring problem with Microthrix parvicella and Nostocoida limicola Action and Expected Results 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

22 CS#1: Sludge Yield/WAS Condition Current sludge yield is 0.76 Action and Expected Results 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

23 CS#1: Clarifiers Condition Three clarifiers on-line Three RAS pumps on-line Action and Expected Results Evaluation showed that only 2 clarifiers and 2 RAS pumps needed Reduced energy related to clarifier and RAS pump operation Less maintenance on equipment

24 CS#1: Pieces of a Puzzle

25 CS#1: Projected Results Proposed Energy Control Measures Table 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 Blower Blower RAS Pump Secondary Clarifier Reduced runtime from increased WAS concentration. 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.) Taking the 1,000,000 gallon RAS Reaeration Tank offline will reduce the need for a third blower. RAS pump associated with secondary clarifer taken offline. Remove one of three from service. State-point analysis shows one can be safely taken offline , ,792 $ $9, % , ,422 $4, $53, % ,070 1,632,845 $8, $106, % ,164 97,971 $ $6, % ,531 $35.38 $ % Monthly Totals 224,380 kwh $14, Annual Totals 2,692,561 kwh $175, %

26 CS#1: Projected Results 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

27 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

28 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

29 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

30 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 Actions and Expected Results 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

31 CS#2: Primaries Condition Hydraulically taxed in winter Reduced BOD and TSS removal Actions and Expected Results 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

32 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 Actions and Expected Results 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

33 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?) Actions and Expected Results 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

34 CS#2: DAFT Condition No control of recycle water pump Actions and Expected Results 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

35 CS#2: Projected Results Equipment/Process Change Description HP Saved Hours Annual Savings Influent Wet Well Run with higher wet well level to increase pump suction head $7,021 Headworks Odor Scrubber Shut off in winter. 6 months/year is equivalent to shutting off 12 hours/day $3,511 Grit Odor Scrubber Shut off in winter. 6 months/year is equivalent to shutting off 12 hours/day $2,106 Grit Tank Air Reduce air to tanks. Change pulley or install VFDs. Assume 20% reduction $1,404 Peak Wet Weather Fans. There are 5 ea. 10HP fans that could be turned off for 9 months/year, equivalent to shutting off 18 hours/day $10,532 Transformers. There are 5 ea. 900KVA transformers that could Peak Wet Weather be turned off for 9 months/year, equivalent to shutting off 18 hours/day. $25,040 Winter operation of chemically enhanced primary treatment (CEPT). 15% reduction of BOD to UNOX system equates to Primary Clarifiers lower O 2 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 Install VFDs and monitoring equipment for DO control on 2nd, UNOX 3rd, and 4th stage mixers. Assume 10% reduction on one train in service year-round = 25HP $7,021 Install VFDs and monitoring equipment for DO control on 2nd, UNOX 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 $3,511 UNOX Repair air leakage on tanks. HP equivalent is an estimate $5,617 Secondary Clarifiers Reduce air in the MLSS channel to the clarifiers. Assume 1/3 runtime $7,021 DAFT Install VFDs. Allow 50% turndown. Assume 80% reduction of current 80% efficiency = 64HP savings $17,974 DAFT (under CEPT) Turn off one recycle pump during winter. 6 months/year is equivalent to shutting off 12 hours/day $14,042 #6 Digester Turn off gas mixing compressor or operate in ON/OFF mode. Assume 50% reduction in runtime $3,511 Blend Tank Mixer pump is oversized. Reduce runtime $7,489 Total $133,547

36 CS#2: Projected Results 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

37 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)

38 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

39 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!

40 Questions and Answers