Energy Conservation by Optimizing Aeration Systems By Tom Jenkins, P.E. Dresser Roots, Inc. Vikram M. Pattarkine, Ph.D. Brinjac Engineering, Inc Michael K. Stenstrom, Ph.D., P.E. C & EE Dept, UCLA

Outline Overview Types of Aeration Systems, Terminology and Relative Efficiency Operational Issues – Cleaning and Avoiding Power Loss through Fouling and Scaling Blower Overview and Optimization Conclusion

Types of Aeration Systems Mechanical or surface aerators –High speed – 900 to 1200 RPM, no gear boxes, easy to install, high heat loss, spray issues, low efficiency –Low speed – gear boxes to reduce RPM to 30 to 60, long lead time to install, high heat loss, spray issues, medium efficiency

Types of Aeration Systems Diffused or subsurface aerators –Coarse bubble – ¼ to ½ inch orifices, low efficiency, low maintenance, low to ultra-low efficiency –Fine bubble or fine pore – millimeter to sub- millimeter orifices or porous media, highest efficiency, significant cleaning and maintenance issues

Types of Aeration Systems Combined systems –Jets, turbines, and aspirating devices – generally two prime movers such as a blower and a motor-gearbox, low efficiency, generally not used for new applications unless there are special concerns or needs

Terminology Efficiency –Standard oxygen transfer efficiency (SOTE) (percent oxygen transferred) –Standard oxygen transfer rate (SOTR) (mass transferred per unit time) –Standard aeration efficiency (SAE) (mass transferred per unit time per unit power) All “standard” terminologies defined for clean water such as tap water (secondary process effluent is never suitable for clean water testing)

Terminology Process Conditions (OTE, OTR, AE) –Adjustment formulas based upon driving force, temperature, barometric pressure, water quality, saturation concentration, etc. –Driving force and water quality the most significant –Driving force = (DO S – DO)/DO S –Water quality – alpha factor, 0 to 1 ! –Total correction can result in process water transfer of only 30 to 80% of clean water transfer

ASCE/EWRI Standards Clean Water Oxygen Transfer Standard –1984, 1991 and Process Water Testing Guidelines –1996 These two documents are quite useful in defining aeration performance, and create a “level playing” field to evaluate aeration systems and facilitate low bid or life-cycle purchase evaluations – use them!

Energy Approximations (wire power) Aerator Type SAE lbO 2 /hp-h (kgO 2 /kW-h) Low SRT AE at 2 mg/L DO High SRT AE At 2 mg/L DO High Speed 1.5–2.2 (0.9–1.3)0.7–1.4( ) Low Speed 2.5–3.5 (1.5–2.1) (0.7–1.5) Turbine 2-3 ( ) ( ) ( ) Coarse Bubble (0.6 –1.5)0.5 – 1.2 ( ) 0.6–1.6 ( ) Fine Pore 6–8 (3.6–4.8) (0.7–1.0) (2–2.6) Approximations – use only as a guideline – transfer efficiency will depend on site specific conditions

Most Common Systems Today Municipal Treatment Plants – fine pore systems: –Discs, ceramic, plastic and membranes –Tubes, membranes –Panels and strips Municipal HPO-AS Systems –Low speed mechanical –Some new impeller designs to improve efficiency

Most Common Systems Today Lagoons, ditches, industrial systems sometimes are best designed with alternative aeration systems due to extremely high oxygen uptake rates, odd geometries, heat loss considerations, requirements for wet installation or wet maintenance

Fine Pore Aeration Systems Why fine “pore” and not fine “bubble” ??? –Fine bubbles can be created by turbines and other mechanical devices. –Fine pore systems create bubbles by passing air through pores or orifices Generally the best design choice for energy conservation, but there are issues and problems to avoid

Some Example Systems Ceramic domes – legacy system Ceramic discs – popular today Membrane discs – maybe most popular at present Membrane tubes – popular today Membrane panels and strips – popular today, and among the most energy efficient

Ceramic Domes

Ceramic Discs

Membrane and Plastic Discs

EPDM PVC Ceramic EPDM Plastic Tubes

Panels and Strips

Efficiency Varies Key to overall transfer efficiency is the air flow per unit area of diffuser surface and the number of diffusers used More diffusers and more area creates efficiencies that are at the upper part of the fine pore range Few diffusers and high flow per diffusers will provide only low efficiency, at the low end of the range or even approximating lower efficiency devices

Fouling and Scaling Fine pore diffusers invariably undergo fouling, scaling and material changes that reduce efficiency Some type of routine maintenance program is always required: otherwise, efficiency declines to values that may be so low that they don’t justify the capital investment Also, back pressure may built which may prevent plant operation

Our Database of Full-Scale Results More than 20 years of observations of ~ 35 plants Ceramic discs, ceramic domes, membrane discs, membrane and plastic tubes, panels and strips New ( 24 months) and cleaned Cleaning – tank top hosing, brushing, acid washing

Our Database of Full-Scale Results Process operation matters Conventional, low MCRT or sludge age – lowest efficiency Long MCRT, nitrifying, good efficiency Long MCRT, nitrifying, denitrifying, best efficiency Flow per unit area of diffuser and tank surface is more important to defining performance that the generic diffuser type or material

Efficiency per process type CONVENTIONALN-ONLYNDN 3.75%/m 4.30%/m 4.60%/m  SOTE/Z (%/ft)  SOTE/Z (%/m) NEW & CLEANED <24 mo. >24 mo.

Transfer Efficiency Here-to-fore, transfer efficiency measurements required an expert using an off-gas analyzer, a few days of time, and thousands of dollars in fees A real-time off-gas oxygen transfer efficiency analyzer has been developed by the UCLA- Southern California Edison Team, with California Energy Commission Funding, and the design is in the public domain It is described in detail during the technical sessions

Economics of Fouling

Summary Fine pore systems generally, but not always offer the best energy conservation Fine pore systems require a dedication to maintenance; otherwise, select different alternatives Reputable manufactures have valuable experience with piping and assembly – Listen to them! The consultant or process engineer must define the efficiency – Require this information from them!

Blowers All fine pore diffuser systems, coarse bubble systems and most combined aeration systems require blowers – they are an indispensable part of the system The next section describes blower types and guides for selection

Positive Displacement (PD) Constant flow at constant speed Pressure varies with load Most common <200 hp Energy Conservation: Most Common Blower Types

Multistage Centrifugal Variable flow Approx. Constant Pressure Most common

Single Stage Centrifugal Variable flow Pressure varies with load High efficiency Most common > 500 hp Energy Conservation: Most Common Blower Types

Energy Conservation: Uncommon Blower Types Regenerative Very High Speed Centrifugal New proprietary technology High efficiency Limited size range Characteristics similar to PD Limited to small flows and low pressures

Energy Conservation: Blower System Design Considerations Provide lots of turndown capability Use multiple smaller blowers Select blowers for current requirements Evaluate energy over range of actual near term operating condition

Energy Conservation: Blower System Design Considerations Minimize system pressure Most Open Valve Control for automatic controls Keep diffuser drop leg valves open for manual control Minimize diffuser pressure drop – orifice size, diffuser configuration, clean diffusers

Energy Conservation: Blower System Design Considerations Use automatic DO control 20% to 50% energy reduction Newer technology IS reliable

Energy Conservation: Blower System Design Considerations Use efficient blower control Use technology appropriate to blower system Integrate with basin controls

Energy Conservation: Blower Upgrades / Revamps Collect Actual Operating Data on Process: Dissolved Oxygen (DO) Concentration Air Flow Rates Dissolved Oxygen (DO) Concentration System Pressure Minimum, Maximum, and Average for Typical Operation Compare to Design Conditions

Energy Conservation: Blower Upgrades / Revamps Collect Actual Operating Data on Blowers: Number of Units Operating Blower Power (kW) or Amps Minimum, Maximum, and Average for Typical Operation Frequency of Manual Adjustments Compare to Design Conditions

Energy Conservation: Blower Upgrades / Revamps All Blower Types Provide proper maintenance – filters, seals, diffuser cleaning Change to energy efficient motors Add smaller blowers to achieve turndown Combine air use for other functions (Post-Aeration, Channel Aeration, etc.) Update Controls

Energy Conservation: Blower Upgrades / Revamps PD Blowers Change sheaves to optimize capacity Multistage Centrifugal Blowers Change impellers to match actual conditions Single Stage Centrifugal Blowers Change impellers to match actual conditions Add Inlet Guide Vanes and/or Variable Discharge Diffuser Vanes

Energy Conservation: Control System Techniques All Blower Types Automatic DO Control to match air rates to process demand Use MOV Control to minimize pressure Automatic starting and stopping of blowers Parallel control instead of cascade control Design Control System for Reasonable Payback – 2 to 5 years Include Process Improvement in Evaluation

Energy Conservation: Control System Techniques PD Blowers Use VFDs (Variable Frequency Drives) to modulate air flow Multistage Centrifugal Blowers VFDs to modulate air flow (with appropriate curves) Automatically controlled inlet throttling to modulate flow and improve turndown Single Stage Centrifugal Blowers Inlet Guide Vanes and Variable Discharge Diffusers to modulate flow and improve turndown