Energy Savings for Marshalltown

In 2011, FOX Engineering began a detailed evaluation of the Marshalltown Water Pollution Control Plant (WPCP) in cooperation with Alliant Energy.  The mission: identify improvements that could be made to the facility that would reduce the plant’s energy consumption.  The Marshalltown WPCP processes include conventional activated sludge with primary clarifiers for the main municipal treatment system, a sequencing batch reactor dedicated for a packing plant waste stream, dissolved air flotation for waste activated sludge (WAS) thickening, anaerobic digestion using a temperature phased anaerobic digestion (TPAD) process, and liquid sludge storage.

The first step in that process was data collection.  For the entire year of 2010, monthly operating reports provided daily wastewater flows and organic load information.  SCADA records were used for equipment operation and runtimes, utility records were examined for electrical and natural gas usage, and interviews with plant staff were conducted to fill in missing information.

Field measurements of equipment voltage and current draw were also conducted to accurately determine actual equipment power use.  Combined with daily runtime data from the SCADA system, this allowed for accurate calculations of power consumption by various processes.

The plant used 7.2 million kilowatt hours (KWHrs) of electricity and 26.4 billion BTUs of gas for electricity and heat generation.  Of the BTUs utilized, 44% came from biogas from the anaerobic digesters; the rest were from purchased natural gas.

Of the electricity used, 17% was from the co-generation engines (a combination of biogas and natural gas produce heat and electricity in these engines).

The main municipal aeration system process was the single largest power user at 2.35 million KWhrs and was found to be the process with the greatest potential energy savings.

The goal of the study was to identify a project with a 5 to 7 year payback.  The study revealed that most of the facility was already highly efficient.

Previous projects had upgraded the anaerobic digestion process, raw wastewater pumping, and WAS sludge thickening; little improvement could be found in the industrial sequencing batch reactor plant.

The main municipal aeration system could be upgraded with a project yielding about a 10-year payback.

The interior lighting system could be upgraded with a 2-3 year payback.

The improvements to the main municipal aeration system included:

  • Automatic DO Control – new DO sensors and automatic control valve on the air to each of the three (3) treatment trains
  • 9-inch membrane disc diffusers in a full-floor grid pattern to replace the jet aeration system
  • A high efficiency “turbo” blower on a VFD to replace an older multi-stage centrifugal blower
  • Integrated controls to operate the whole system

The interior lighting system upgrades included:

  • Replacing the high pressure sodium (HPS) fixtures with multi-lamp T8 fluorescent fixtures
  • Installing occupancy sensors in most spaces to control the lights

While the original high pressure sodium lights were reasonably efficient even for today, they can’t practically be turned on and off due to the long warm up time required to come to full brightness.  Therefore, they remained on 24 hours a day, 365 days a year whether anyone was in that part of the plant or not.  Most of the overall energy savings is from the ability to have the lights off in most areas of the facility most of the time, and the occupancy sensors in conjunction with the new T8 fixtures allow the lights to only be on when a person is in the room.  The T8 fixtures are also slightly more efficient than the HPS lamps and give off a more natural white light.

There is only 6 months of operating data since the construction project was completed, and direct comparisons between 2014/2015 and 2010 can be difficult due to varying flow rates and organic loadings.  However, so far it appears that the WPCP is saving approximately 25 to 30 percent in electrical use.

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.