Current News

Tampa, Florida/Daly City, California

New Aerators can buy you…

    The high purity oxygen system at the Howard F. Curren Advanced Wastewater Treatment 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 Curren 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 (known originally as the UNOX Aerator) are constructed of fiberglass with multiple enclosed, mixed liquor pumping channels per turbine.  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, again in conjunction with Hazen and Sawyer 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 as well as technical support from a major U.S. surface aerator manufacturer.

Philadelphia, Pennsylvania

Will a new type of oxygen generator effect…

    Like many HPO plants, the Philadelphia SW wastewater plant has on-going programs to reduce power consumption.  The Philly SW plant’s original oxygen generators were cryogenic systems.  The power savings and simplicity of operation of vacuum pressure swing adsorption (VPSA) systems appear very attractive as the Philly plant staff looks to the next 20 years of operation.  But how would the lower feed oxygen purity effect the aeration basins?  Of paramount importance is what happens to the dissolved oxygen concentrations if lower purity oxygen is fed to the basins?  That’s something that must be investigated when changing from the 98% pure cryogenic oxygen gas to the 90% purity of a VPSA system.  DWG ASSOCIATES’ HPODesign© simulation software answers those exact questions.  Working with Hazen and Sawyer, DWG ASSOCIATES simulated numerous current and anticipated future influent conditions to confirm that a switch to VPSA oxygen supply would not adversely effect the operation of the existing aeration system.

South Essex Sewerage District, Salem, Massachusetts

Upgrading, where to start…

    A few years ago, South Essex put together a plan for sequentially upgrading the components of their UNOX HPO secondary system.  DWG ASSOCIATES assisted with the development of that plan.  The plan was comprehensive and included recommendations ranging from immediate minor fixes that would improve operating efficiency to a sequential equipment replacement schedule driven by increasing maintenance expenditures on the existing equipment.  The sequential equipment replacement schedule was rearranged almost immediately because of the energy efficiency improvements that would be realized by switching from PSA to VPSA oxygen generation.  Economic analyses indicated that oxygen production energy consumption would decrease by 50% with a VPSA oxygen generator.  The power savings over the life of the new unit totally offsets the capital needed for purchase of the new generator.  Preliminary engineering also indicates that the VPSA equipment footprint is actually smaller than that of the existing PSA.  Therefore, existing infrastructure can house the new VPSA oxygen generator.

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, and wastewater characteristics during that time can significantly influence plant efficiency.  And when it comes to trained operating personnel, changes can often cause paralytic 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!  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.  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.  So was minimum energy consumption.  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 simpler 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 consumption 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 superintendent 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."

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

Energy audits can identify…

    Working with HDR Engineering (Omaha, NB), DWG ASSOCIATES conducted comprehensive energy audits of East Bay’s Main Wastewater Treatment Plant (MWWTP) in Oakland, CA, Louisville’s Morris Forman Water Quality Treatment Center, and Harrisburg’s Advanced Wastewater Treatment Facility 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 operation of the entire plant.  Similarly, approximately $300,000 in annual electrical 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.  For the Advanced Wastewater Treatment Facility in Harrisburg, which is an HPO BNR facility removing both nitrogen and phosphorus, $186,000 in annual electrical power savings were identified for the HPO portion of the plant.  The total, entire plant energy savings identified approached approximately $450,000.

Oxygen Supply System Upgrades

Justifying new oxygen generation is…

    The energy efficiencies of all types of equipment are improving constantly.  That is especially the case for some of the equipment associated with high purity oxygen wastewater treatment.  Oxygen supply system improvements have been significant in recent years.  Vacuum Pressure Swing Adsorption (VPSA) or Vacuum Swing Adsorption (VSA) oxygen generators (no matter what you call them, they’re the same technology) are far more energy efficient than either cryogenic or PSA oxygen generators.  VPSA is such a "killer" technology that units are currently up and running at numerous HPO plants including Rocky Mount (NC), Camden (NJ), Duluth (MN), New Orleans (LA), Miami (FL), Louisville (KY), Seattle (WA), and Ft. Lauderdale (FL).  And that's a condensed list!  Both efficiency of production and ease of operation have been proven at all of these locations.  Many other HPO locales have already completed economic analyses that have shown them the compelling economics for replacing their original oxygen generator(s) with VPSA systems.  To produce the same amount of oxygen, a VPSA plant consumes half the power of the old PSA technology and about 35% to 40% less power than a cryogenic plant.  Those are savings that are truly noticeable on your power bill.  DWG ASSOCIATES has been involved with virtually all of the VPSA conversions.  If you’d like to talk about what’s involved to make the switch, contact us.
    And if you decide that a new VPSA is in your future, don't automatically assume your old equipment is worthless.  There is a small but active market for used oxygen generators.  We can’t guarantee anything but if we know your old equipment will be scrapped soon, we may be able to find someone that can utilize it.  To find out, all we need is your "go ahead."