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Challenging Source Water Leads to Unique Membrane Pilot Study

The lessons learned from a unique pilot study at the Fargo, ND Water Treatment Plant (WTP) were shared recently with water professionals across the country. Brian Bergantine, PE, Advanced Engineering and Environmental Services Inc. (AE2S) Project Manager, presented “Membrane Technology Application in the Red River Valley,” alongside Mark Peterson, City of Fargo Water Treatment Facility Operations Supervisor, at the 2013 Membrane Technology Conference & Exposition in San Antonio, Texas. The presentation focused on the City of Fargo’s Facility Plan for the WTP that was completed in 2011. The City retained AE2S, in association with Black & Veatch (B&V), to evaluate the potential improvements to the WTP.

The City of Fargo relies on the Red River as its primary water source, and augments its water supply with the Sheyenne River and Lake Ashtabula. The WTP Facility Plan was prepared to address increased capacity needs resulting from the area’s growth as well as the ability to treat high dissolved solids, primarily sulfates, coming from the Sheyenne River. The high sulfate concentrations in the Sheyenne River are due to flooding in the Devils Lake basin, and the subsequent operation of two emergency outlets on the lake that pump up to 600 cubic feet per second (cfs) of high sulfate water into the Sheyenne River. 

Sulfates can have a negative impact on the taste of water and can create gastric distress in certain populations. The U.S. Environmental Protection Agency (USEPA) has set a secondary standard for sulfates of 250 milligrams per liter (mg/l), and the North Dakota Health Department recommends drinking water include no more than 250 mg/l. With the emergency outlet impact, the sulfate levels in the Sheyenne River are expected to be in the range of three times that limit - 750 mg/l. Fargo’s WTP currently does not have the capability to treat for sulfates without the additional treatment technology. 

AE2S and B&V recommended reverse osmosis (RO) as the most appropriate technology for sulfate reduction. “We knew from experience that the performance of different reverse osmosis membranes can vary, particularly with respect to fouling, the unique characteristics of the water being treated (high total organic carbon, temperature, seasonal), and membrane chemistry,” says Bergantine. In addition, the team knew that optimization of pretreatment requirements and the RO system design parameters - such as optimal permeate flux and recovery - is paramount prior to the design of a full-scale project. To address these issues, the City initiated a Pilot Study in July 2011. Bergantine explains, “The study was conducted over a full year, in order to allow for the dramatic seasonal variations in water quality in North Dakota’s source waters. We wanted to study cold and warm water seasonal impacts, as well as capture the water quality differences within the Sheyenne River - particularly the spring runoff.”


WTP

Two pilot study scenarios were used to test the membrane application on Fargo's water. RO membranes experienced rapid fouling in the Polishing Scenario, which used RO to further treat filtered water after pretreatment, lime softening, ozone, and granular filtration at the existing WTP. The RO membranes exhibited superior performance in the Parallel Scenario, which was a separate treatment process that included pretreatment plus microfiltration/ultrafiltration (MF/UF) and RO, parallel to the existing WTP. RO membrane autopsies conducted at the conclusion of the piloting efforts indicated that membrane fouling was generally organic and biological in the Polishing Scenario, while organic and scaling in the Parallel Scenario. 

Fargo WTP

Optimization studies were performed in the Parallel Scenario to determine optimal coagulation conditions for pretreatment; as well as establish stable flux, recovery, and membrane cleaning regimes for both the MF/UF and the RO systems. Uniquely, an RO membrane selection pilot was conducted for both scenarios to evaluate RO membranes from four different manufacturers. One RO membrane could not be cleaned adequately, although it has many successful applications elsewhere. 

The one year pilot study proved that RO technology is feasible to reduce sulfate concentrations to acceptable levels in the City’s finished water and to meet the City's other finished water quality goals. The Fargo Membrane WTP is currently in the design phase. 

If you have questions about RO technology or sulfate treatment options, contact Brian Bergantine at Brian.Bergantine@ae2s.com, or 218-299-5610.  

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If you have any questions concerning the content of this newsletter,
please contact Heather Syverson at 701-364-9111 or Heather.Syverson@ae2s.com.
 
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