Current News

South Essex Sewerage District, Salem, Massachusetts

Upgrading, where to start…

    A few years ago, the South Essex Sewerage District Plant (Salem, Massachusetts) put together a plan for sequentially upgrading the components of their UNOX HPO secondary system.  With plant staff, DWG ASSOCIATES assisted with the development of the plan.  The plan was comprehensive and included recommendations ranging from immediate minor fixes that would rapidly 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% (or more) 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.  Potential plant permit changes have temporarily delayed the PSA replacement.

Southtowns (Erie County), New York/Daly City, California

New Aerators can buy you…

    Over the past few years, the high purity oxygen system at the Southtowns Advanced Wastewater Treatment Facility in Erie County, New York has been upgrading equipment as dictated by needs.  The old (original) aeration equipment had exceeded its useful life years ago and finally reached the top of the "replace now" list.  There was also a need to add aeration tank capacity because of flow and loading increases to the Southtowns plant since it was originally designed.  Because surface aeration equipment has become much more efficient since the original aerators were first installed, it was cost effective to purchase all new aerators for the entire, upgraded plant.  DWG ASSOCIATES, in conjunction with Arcadis (Buffalo, NY), determined new aerator sizing for each stage of the system to minimize power draw for each influent condition anticipated.  The aerator sizing effort included determining the least number of operating aeration trains needed at each influent condition to wring the most value out of new, more efficient aeration equipment.  Going forward the Southtowns facility will operate much more efficiently yet will maintain higher D. O. concentrations than the plant has in the past.  This will all result in improved performance at less operating cost; which is exactly Southtowns long term objective.
    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.  The original, 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 and the commensurate loss of mass transfer and mixing has caused significant 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 as well as continuing technical support from a major U.S. surface aerator manufacturer, SPX Flow.  The new aerators were commissioned and lined-out in late 2024 and are performing exactly as predicted.

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, staff changes can often cause paralytic angst for plant management.  The Western Lake Superior Sanitary District (WLSSD) in Duluth, MN looked recently at all these potential changes for the coming years and they saw issues galore!  What to do?  DWG ASSOCIATES, in conjunction with Donohue & Associates (Sheboygan, Wisconsin), 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 new future conditions if and when they occur.  The oxygen supply system was a different story.  So the questions became, what would it cost to upgrade the existing cryogenic (cryo) system to last another 20 years?  Could the cryo be configured to operate efficiently should future plant loads decrease significantly?  And 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 3^rd 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 3^rd 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 (two 40 TPD Air Products VSAs) 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 40% less than with the old cryogenic equipment at comparable production rates.  Management’s general attitude is "why didn't we do this sooner?"

Cedar Rapids, Iowa

Do you really need a new oxygen generator…

    Like many large HPO plants, the Cedar Rapids Water Pollution Control Facility (CRWTF) has a cryogenic (cryo) oxygen generator to produce oxygen for their aeration basins.  The CRWTF operations staff has been operating this oxygen generator since it was originally installed in 1979.  Like so many well maintained cryogenic oxygen generators, the 100 ton per day (TPD) unit at the CRWTF is in exceptional physical condition due to proper, timely preventative maintenance.  The coldbox is in such good condition that approximately 10 years ago DWG ASSOCIATES and HDR (Omaha, Nebraska) determined via a detailed economic analysis that it was most cost effective to upgrade the mechanical equipment associated with the cryo plant than to replace the entire generator with either a new cryo plant or with a new, more energy efficient oxygen generator like a Vacuum Pressure Swing Adsorption (VPSA) plant.  Consequently, within the last 10 years, the existing cryo plant has been totally refurbished and is now in essentially "as new" condition.  The final refurbishment step was just recently completed with the addition of a Pre-Purifier Unit (PPU) between the main air compressors and the coldbox.  The PPU, which was recommended by DWG ASSOCIATES, essentially replaces the cryo plant’s reversing heat exchanger.  The PPU will reduce plant startup times, essentially eliminate 1 year and 5 year turnarounds, and positively prevent water vapor, CO₂, and hydrocarbons from entering the coldbox.  The PPU will also provide many other economic benefits too numerous to detail here.  These benefits will significantly reduce O & M costs and improve plant operability and longevity.
    If your current and future operations’ staffs are comfortable operating and maintaining your cryogenic plant, it might be worth looking at the cost effectiveness of installing a PPU on your facility.

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

Energy audits can identify…

    Working with HDR Engineering (Omaha, Nebraska), 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.  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.  Of note, 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 systems 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), Louisville (KY), and Seattle (WA).  And that's a condensed list!  Both energy efficiency 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 and consequently now have VPSA construction projects that are ongoing.  Why is this the case?  Primarily it is ENERGY saving.  To produce the same amount of oxygen, a VPSA plant consumes less than 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 these VPSA conversions.  If you’d like to see the economics or 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 promise anything but if we know your old oxygen supply 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."

Existing Cryogenic Plants & PPUs

Old Cryo Plants can last forever if…

    As any cryo plant operator knows, frozen material in cryogenic distillation columns can and does plug internal piping and interferes with the distillation process that’s going on in the coldbox.  In a reversing heat exchanger plant, the reversing heat exchanger is supposed to prevent freezable material from entering the coldbox.  But reversing heat exchangers don't work very well.  Some water vapor and CO₂, as well as all hydrocarbons, do get through a reversing heat exchanger, necessitating the dreaded and costly annual turnaround.  A Pre-Purifier Unit (PPU) replaces the reversing heat exchanger.  A PPU (sometimes called a molecular sieve front end) is an easy and cost effective replacement for a reversing heat exchanger.  Unlike a reversing heat exchanger, a PPU positively and effectively removes water vapor, CO₂, and even most hydrocarbons from the feed air to the coldbox.  The industrial gas industry introduced PPUs on cryogenic plants in the 1970s, so there is a lot of operational history with these things.  This long historical record proves that PPUs are dependable, low maintenance, simple to operate, and cost effective add-ons to simplify and improve the operation of cryogenic gas separation plants.  The advantages of PPUs are too numerous to list here but two of the more significant ones are the essential elimination of annual and five year turn-arounds.  If you'd like to know what all the advantages of PPUs are or what the economics of these units are, DWG ASSOCIATES knows them.  DWG ASSOCIATES can also assist with the details associated with retrofitting an existing cryo plant with a PPU.  To be sure, PPU additions are not always cost effective but it doesn't hurt to "take a look."  If you like your cryo plant and want to save it, give us a call.