Marshalltown, IA

Influent Pump Station and Grit Removal (28 MGD)

  • Client City of Marshalltown
  • Type Wastewater
  • Services Evaluation, Design, Construction
  • Project Manager Keith Hobson, P.E., BCEE
  • Project Engineer Lance Aldrich, P.E., BCEE


The Influent Pumping Station and Pretreatment Building at the WPCP were constructed in 1987 to transport wastewater flows to the treatment plant and excess flow to the holding lagoon. The vertical pumps had experienced excess wear from grit and high speed operation so pump replacement was desired. The city needed additional raw wastewater pumping capacity as well as the ability to remove grit from their wastewater system.



The pump station and pretreatment building were constructed to provide for transport of wastewater flows to the treatment plant and the excess flow holding lagoon. This project incorporated fine screening with Aquaguard moving screens and Eutek Teacup free vortex grit removal units.  Automatic blowdown valves periodically deliver the grit slurry to the Grit Snail for grit separation and dewatering.  The dewatered grit is deposited onto a belt conveyor, which also transports screenings, for deposit into a dumpster located in an adjacent room. A lime system also provides stabilization of the material.


The pump station includes six pumps with a firm capacity of 28 MGD. Maintaining operation and providing higher efficiency, lower maintenance pumps were the project goals. In 2002, FOX Engineering investigated several options to replace the existing raw wastewater pumps. Newer pumps were identified that could produce higher efficiency results with lower speed operation and hardened materials for better abrasion resistance.

The wetwell top slab required additional structural support for the new pumps; the construction was managed with the divided wetwell to provide continuous operation during replacement.

The project addressed additional important needs: UV disinfection, dome covers for final clarifiers, jet aeration, and RAS (return-activated sludge) and WAS (waste-activated sludge) pumping.



At the time, the grit units (14+ MGD) were the largest forced vortex grit units in Iowa/US. Built in 1986, this system is still in good working order. Pumps were replaced in 2004 as part of a large biosolids project; and in 2010 screw pumps were added and flow equalization was expanded.

Four of the six pumps have been replaced and are operating continuously with adjustable frequency drives to pump all influent flows to the plant. The pumps are providing improved performance with less power and maintenance requirements. These design improvements should provide additional benefits throughout the extended service of the pump station.