A wastewater treatment plant in Moundsville, West Virginia, uses variable frequency drives to improve aeration control and lower costs.
In 2003 the Moundsville Wastewater Treatment plant encountered a blower failure. They were about to repair the damaged unit when their lab manager, Duane, started discussions that led to substantial savings in power costs and improvement in plant performance.
The plant was equipped with three 75 HP centrifugal blowers, two of which fed air to four aeration tanks via a main header. The third blower served as a spare.
After studying tank conditions, the plant determined that dissolved oxygen levels varied from about 0.5 PPM to around 6.0 PPM throughout the day.
The plant personnel and the Danfoss sales representative back-calculated the air requirements for the individual tanks and came up with the idea to put a new valve in the line to the secondary aeration tanks with a new 50 HP rotary positive displacement blower. To operate the blower at optimum efficiency, a Danfoss drive was installed along with a dissolved oxygen meter, allowing the system to respond to actual conditions in the aeration tanks.
With the new arrangement, the drive was programmed to maintain a dissolved oxygen level of 2.2 PPM and a minimum of 40% blower speed.
After four days, the plant’s pinfloc problem ceased. The plant believes this resulted from better control of the dissolved oxygen levels.
The 50 HP blower was generally observed to be operating at 40–50% of full speed or 20 to 25 HP. After a year of operation, it was apparent that with one 75 HP blower operating full time and one blower operating at a rough average of 22 HP, they had saved about $24,000 in power costs.
The plant then performed further studies with a dissolved oxygen meter in the two remaining tanks and found that excess air was being introduced.
The plant ordered a 100 HP blower with a drive and DO meters to provide optimized airflow for the two first-stage aeration tanks.
The original centrifugal blowers are now being maintained as backups for the new blowers, which also run much more quietly. After two years of service, the new blower arrangement is saving approximately $50,000 to $60,000 per year in energy costs.