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?"