Financial and Process Performance of a Wastewater Reuse System in the Upper Midwest

A new 100 million gallon ethanol plant was recently constructed approximately 25 miles west of the City of Fargo in eastern North Dakota. Finding a suitable water supply for the ethanol plant was a challenging task that involved unsuccessful exploration of private wells, regional water systems, and municipal drinking water. However, with persistence and open minds, stakeholders turned their attention to recycled wastewater from the City of Fargo. After fast-tracking a water supply/diversion permit for the water supply system, the City, Ethanol Plant, and Cass Rural Water Users District (CRWUD) developed a thorough Memorandum of Understanding (MOU) that defined the project requirements and responsibilities for each party. An Effluent Reuse Facility (ERF) capable of producing 1.4 million gallons per day (MGD) of reverse osmosis (RO) quality water was designed and the construction was completed in the fall 2008. The ERF started delivering RO water to the ethanol plant since December 2008 and more than 700 million gallons RO water has been provided so far. Due to the seasonal fluctuation of influent water quality, such as temperature and TSS, the ultrafiltration (UF) process in the ERF has experienced rapid fouling in the last two and half years and the original designed chemical cleaning procedure cannot remove the biological fouling of the UF membranes. The City conducted an intensive study of cleaning solutions and processes for the UF membrane fouling issues and concluded that an enzyme cleaner and a detergent are most effective. The RO process has been running stably at the design operational conditions over the last two and half years. The City estimated operations and maintenance (O&M) costs of 1.02 per 1,000 gallons during development of the MOU. Over the first two and a half years, the actual average O&M cost per 1,000 gallons has been 1.50. The costs of chemicals and energy contribute more than 70 percent of total O&M. As the ethanol plant is quickly expanding its designed production capacity and increasing its water demand, the City is generating increasing revenue. Encouraged by the overall success of this wastewater reclamation project, the City of Fargo is continuing to consider potential options for expanding its wastewater reuse system.