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Chattanooga, Tennessee

Power savings and ease of operation are hard to…

Chattanooga, Tennessee has been operating a 145 TPD cryogenic oxygen generator for about 40 years.  The cryogenic plant has been well maintained and upgraded periodically to simplify operation and/or improve maintenance.  But newer oxygen generation technologies are now so much more energy efficient and easy to operate that they are hard to ignore.  So Chattanooga’s plant management decided to take a look at alternate methods of oxygen supply, just to see.  Working in conjunction with HDR Engineering (Omaha, NB), DWG ASSOCIATES developed detailed, site specific costs for numerous methods of oxygen supply.  The alternates considered for the Chattanooga plant were (1) refurbishing the existing cryogenic plant, (2) buying and owning a new, appropriately sized Vacuum Pressure Swing Adsorption (VPSA) oxygen generator, (3) contracting for a third party owned and operated VPSA oxygen generator, and (4) contracting for a third party owned and operated used but considerably newer cryogenic oxygen generator.  Although all cost estimates developed were site specific to the Chattanooga plant, the results of the analysis are consistent with what has been indicated by other similar analyses for most high purity oxyen wastewater treatment plants.  The VPSA’s ease of operation and power consumption make this newer, ambient temperature oxygen generation technology THE most cost effective method of oxygen supply, including for Chattanooga.  Implementation of the recommendation to install a new VPSA at Chattanooga awaits a detailed plant loading study, which will establish the exact VPSA size.

Western Lake Superior Sanitary District, Duluth, Minnesota

Equipment upgrades can help you sleep at night…

Thirty or forty years is really an eternity in the life of a wastewater treatment plant.  The changes in service area, customer base, wastewater characteristics, and plant personnel during that time can significantly influence plant efficiency.  In the case of personnel, changes can also cause angst for plant management.  The Western Lake Superior Sanitary District (WLSSD) in Duluth, MN looked recently at the potential future changes they might expect for many of these parameters in the coming years.  They saw changes galore!  The U.S Pulp & Paper industry (a large contributor to the plant) has been waning.  Some critical senior plant personnel were nearing retirement age.  And most of the secondary plant equipment was nearing 40 years of age.  What to do?  DWG ASSOCIATES, in conjunction with Donohue & Associates (Donohue, Sheboygan, WI), was asked to evaluate the current condition of the UNOX System equipment at the WLSSD plant and recommend a cost effective path forward.  The existing UNOX System consisted of 4 aeration trains of 4 stages each with an oxygen supply system of two 40 TPD cryogenic (cryo) oxygen generators.  We quickly determined that both the aeration system and oxygen generators were in good shape due to exceptional maintenance.  Furthermore, the aeration system was appropriately sized for current conditions, had considerable remaining life, and could easily be modified to handle changed future conditions if and when they occur.  The oxygen supply system was a different story.  What would it cost to upgrade the existing system to last another 20 years?  Could it be configured to operate efficiently should future plant loads decrease significantly?  How does the existing cryo system compare from an operations and maintenance standpoint with newer technologies?  To address these questions, DWG ASSOCIATES and Donohue developed a financial model to account for WLSSD's identified potential future scenarios.  During scenario analysis it became obvious that 3rd party cryo plant operator assistance and training availability in the future was of critical importance to WLSSD because of impending personnel retirements.  Our financial model allowed us to analyze all potential future operating scenarios, including operator assistance and training costs.  Given the scenarios considered, our analysis indicated that the most cost effective path forward was to replace the existing cryo units with new state-of-the-art, fully automatic, less energy intensive Vacuum Swing Adsorption (VSA) oxygen generators (note:  VSA and VPSA are the same technology).  The VSA's automatic operation and remote monitoring capabilities virtually eliminates future 3rd party operator assistance & training worries and costs.  And the production turndown capabilities and overall power efficiencies of the VSA are outstanding.  That means sound sleeping for management.
The new VSA equipment is up and running as of the end of November, 2019.  Duluth’s operations staff report that running the VSAs is much simplier than the old cryogenic plants.  Startup, shutdown, and capacity control of both VSA units is a simple "point and click" procedure through the plant’s DCS.  Power comsumption is about 35% less than with the old cryogenic equipment at comparable production rates.  Management’s general attitude is "why didn't we do this sooner?"

Rocky Mount, North Carolina

Old oxygen generators might last forever but…

Thirty years ago, when the Tar River Regional Wastewater Treatment Plant in Rocky Mount, NC was new, it’s OASES secondary system had a state-of-the-art 50 ton per day (TPD) cryogenic oxygen generator to supply oxygen for its BNR secondary and ozone disinfection systems.  That cryogenic plant was still functional in 2011; but at a significant cost to Rocky Mount because of decreasing plant loading and the elimination of ozone disinfection.  DWG ASSOCIATES and The Wooten Company (Raleigh, NC) looked at the efficiency of the cryogenic plant relative to current state-of-the-art oxygen generators.  That analysis indicated that there was considerable operating dollars to be saved by replacing the old oxygen generator with a newer technology.  DWG ASSOCIATES and Wooten subsequently prepared specifications for a new, state-of-the-art AirSep Corporation Vacuum Pressure Swing Adsorption (VPSA) oxygen generator, which has since been installed.  During the engineering phase of the project, estimated energy savings just for oxygen production with the new VPSA were projected to be $200,000 annually.  The new VPSA was started in January, 2013, and after almost 10 years of actual operation the energy reduction results are more than confirmed.  "The new VPSA system is costing us $345/day for electricity, compared to more than $1,000/day for the old cryo plant," says now retired superintentent of wastewater treatment Kirk Bass.  The actual operating saving therefore is approximately $50,000/year more than initially projected.  Bass went on to say, "This [investment] has not only paid off, it has done so as quickly as the engineers promised."

Tampa, Florida/Daly City, California

New Aerators can buy you…

The high purity oxygen system at the Howard F. Curren wastewater plant in Tampa, Florida needed new aerators.  The old units had exceeded their useful life and many were from a manufacturer that is no longer in business.  There was also excess aeration tank capacity because of process modifications that have been made to the Current plant since it was originally designed.  And, finally, surface aeration equipment has become much more efficient since the original aerators were first installed.  DWG ASSOCIATES, in conjunction with Hazen and Sawyer (New York, NY) evaluated myriad aeration tank scenarios to determine the least number of aeration trains with appropriate aerator sizes to wring the most value out of new, more efficient aeration equipment.  Going forward Tampa will operate with fewer aeration trains on line while maintaining higher D. O. concentrations than the plant has in the past.  This will all result in improved performance at less operating cost.
The high purity oxygen system at the Daly City, CA wastewater treatment plant also needed new aerators.  Daly City has been operating for the last 40 years with surface aeration equipment that was considered obsolete 30 years ago.  These now defunct surface aerator turbines are constructed of fiberglass with multiple closed, mixed liquor pumping channels per turbine (known as the UNOX Aerator).  Needless to say, channel clogging with the commensurate loss of mass transfer and mixing has caused plant operating problems for a number of years.  DWG ASSOCIATES, in conjunction with Hazen and Sawyer (New York, NY) designed new, high efficiency surface aeration equipment that will allow the plant to operate with much improved mass transfer efficiency (and higher D.O. concentrations) and no increase in power consumption.  Added benefits will be no clogging and technical support from an existing major U.S. surface aerator manufacturer.

South Essex Sewerage District, Salem, Massachusetts

How to sequentially upgrade to get ready for the coming…

The South Essex plant's UNOX System equipment is running as close to flawlessly as the staff can expect.  But the plant is approaching 25 years of age.  Plant management contracted with DWG ASSOCIATES to evaluate and quantify equipment replacement or refurbishment approaches that would (1) correct longstanding original design flaws and/or (2) provide cost effective methods to upgrade plant equipment to yield another 25 years of plant life.  DWG ASSOCIATES completed the entire plant evaluation.  Recommendations ranged from immediate fixes that would improve and ensure long term operating efficiency to a non-budget busting sequential equipment replacement/refurbishment schedule that would upgrade overall plant operating efficiency while positioning the plant for 25 more years of operation with minimal maintenance expenditures.  The sequential schedule will ensure that upgrade expenditures will only be made when justified by (1) energy efficiency improvements or (2) escalating maintenance expenditures associated with impending equipment failure.  The South Essex plant can now move forward knowing what areas of their UNOX System need replacing or refurbishiment each year.

East Bay MUD (Oakland, California)/Louisville MSD (Kentucky)

Energy audits are a lot of work but…

Working with HDR Engineering (Omaha, NB), DWG ASSOCIATES conducted comprehensive energy audits of both East Bay’s Main Wastewater Treatment Plant (MWWTP) in Oakland, CA and Louisville’s Morris Forman Water Quality Treatment Center UNOX Systems.  Connected load and current operating philosophy were determined through detailed discussions with the operations staffs.  Using the prevailing power cost schedules for each plant, plant specific power saving and load shedding modes of operation were defined.  The capital costs of energy saving upgrades were established and a net present value-type analysis was used to identify the most attractive energy saving and/or load shedding modes of operation for each plant.  Approximately $500,000 in potential annual electric energy savings for just the East Bay UNOX System have been identified to date.  For the entire plant, total identified MWWTP energy reductions are currently resulting in annual operating cost savings approaching $3,000,000; which, in 2012, helped the MWWTP become the first wastewater treatment plant in North America to produce more renewable energy onsite than is consumed during normal operatation of the entire plant.  Similarly, approximately $300,000 in annual electical savings were identified (and are currently being realized) for just the UNOX secondary system at Morris Forman.  Total, entire plant Morris Forman energy savings were approximately $1,100,000 annually.  And most of the Morris Forman savings required little or no capital investment to capture.

Oxygen Supply System Upgrades

If you’re wondering…

The energy efficiencies of all technologies change with time.  For HPO systems, oxygen supply technologies have changed considerably over the last 20 years.  VPSA/VSA and LOX supply systems have proven to be cost effective alternatives to the PSA and cryo oxygen generators originally installed at most high-purity oxygen plants.  VPSA/VSA systems, in particular, are seeing significant efficiency improvements as vendors’ R & D money continues to pour into this state-of-the-art, ambient operating temperature technology.  Detailed economic evaluations have recently shown it’s cost justified to replace old, inefficient oxygen generation equipment at Orange County (CA), Rocky Mount (NC), Camden (NJ), Yuba City (CA), Duluth (MN), New Orleans (LA), Chattanooga (TN), Houston (TX), Miami (FL), Ft. Lauderdale (FL), and Louisville (KY); all of which have or will replace their original oxygen supply systems.  DWG ASSOCIATES’ familiarity with the vendors of these newer technologies and experience with the economics and efficiencies of these upgrades will ensure you of the most appropriate and cost effective upgrade for your future needs.  Remember, these technologies are changing (improving) fast. You need someone that understands oxygen generation or . . .
And if your future does include new oxygen supply equipment, don’t just assume that your old equipment is worthless.  Even though you might consider your equipment old and worthless, there is a small but existing market for used oxygen generators.  We can't guarantee anything but if you let us know your equipment is coming available as soon as you can; we may successfully find someone that can utilize it.