North Liberty, IA

Membrane Bioreactor Wastewater Treatment Plant: Phase II

  • Client City of North Liberty
  • Type Wastewater
  • Services Treatment
  • Construction Cost Estimated $16 Million
  • Project Manager Steve Troyer, P.E., BCEE
  • Project Engineer Jenny Ruddy, P.E.


In May 2006, the FOX team began design of Phase I WWTP Improvements. Design was completed on schedule in January 2007; bids were received, and the contract awarded in March 2007.  Construction was completed, and the plant was placed in operation in August 2008.  The $8.5 million project was completed within budget and two months ahead of schedule. As the first plant of its kind in Iowa, the plant and design have won several awards, including the 2010 Iowa ACEC Engineering Excellence Grand Conceptor Award (1st Place Overall) and the Grand Place Award in the Water/Wastewater Category (1st Place). The project also received the Governor’s Award for Environmental Excellence in 2009.

Continued rapid growth of 2 to 5 percent per year required implementation of the Phase II plant expansion. Major goals of the expansion project included increasing the design capacity of the state-of-the-art membrane bioreactor (MBR) treatment plant to accommodate a population of 28,000, adding biological nutrient removal (BNR), and adding biosolids dewatering and storage facilities.


FOX completed design and bidding in 2015 for the $15.6 million Phase II plant expansion. Biological nutrient removal (nitrogen and phosphorous) was incorporated to meet the Iowa Nutrient Reduction Strategy (INRS) and contribute to reducing the hypoxic region in the Gulf of Mexico. Chemical and biological removal of nutrients were evaluated. The design effort included using BioWin modeling software (a wastewater treatment process simulator) to evaluate and select the best configuration and most economical sizing for the biological nutrient removal process.

Based on this effort, the Modified University of Cape Town (MUCT) process was selected for biological nutrient removal to meet the INRS treated effluent discharge goals of 10 mg/L total nitrogen and 1 mg/L total phosphorus.

Existing MBR treatment was expanded with the Phase II project by adding a fourth membrane bioreactor (MBR) train. General Electric (GE), the equipment supplier for the Phase I improvement project, also supplied the MBR equipment for the Phase II expansion. The newest membrane technology, LEAPprimary, was installed which promises a net energy savings of 25 percent for biological treatment; this is due to the treatment efficiency of the MBR technology once all MBR trains are converted to LEAPprimary. This is another example of how the North Liberty facility continues to take exemplary measures to be good environmental stewards.

Phase II includes the addition of a biosolids dewatering system and dewatered biosolids storage facility; this increases operational flexibility and reduces the cost of hauling. Facility planning in 2012 evaluated liquid storage versus dewatering. To reduce solids handling costs and storage requirements, biosolids dewatering was selected as the preferred method. FOX Engineering worked closely with plant staff to determine the most beneficial, operator-friendly, and cost-effective system.

Membrane Trains

Permeate Pumps

On-site pilot testing was conducted during the summer of 2015. The goals were to evaluate dewaterability; confirm operational parameters such as solids loading, flow, speed, and pressures; determine optimum polymer dose and full-scale design conditions; and evaluate operation and maintenance requirements. The pilot studies also provided an ideal opportunity for plant staff to get first-hand operational and maintenance experience with the equipment. Based on a life-cycle-cost evaluation completed by FOX, the plant operator’s preference for operational and maintenance features, and redundancy feature by having more than one dewatering channel, the rotary press technology was selected for full-scale design.

The design included construction of a new building to house one rotary press with six dewatering channels and a new biosolids storage facility.  The dewatering building includes space to add one more rotary fan press unit in the future. The rotary press is designed to handle up to 108,000 gallons (18,000 lbs) per week.


The new dewatering and storage facilities started operation in December 2016, and plant staff have been pleased with performance and results of the new dewatering facilities. During the first hauling event spring 2017, biosolids disposal costs were significantly reduced compared to previous liquid land application methods, resulting in a savings of more than $50,000.

Construction of Phase II was substantially complete in May 2018.