The Baker-Main Interceptor is a 10-mile mixed force main and gravity main system that begins with a solids transfer line from the Irvine Ranch Water District (which conveys 1 mgd of primary solids of 1% concentration for one mile through a 12" force main with a 3.5 hr transit time).  The IRWD force main discharges to a gravity interceptor where it picks up additional domestic and commercial flows, travels three miles further and discharges into the intermediate Main St. pump station.  It leaves the pump station in a 24" force main that transitions to gravity one mile later, whereupon it flows another 4.5 miles, under the Santa Ana River siphon and into Plant No. 1 of the Orange County Sanitation District's (OCSD) system. The total flow is 25 mgd with the transit time from IRWD to Plant No. 1 being about 9 hours. The sulfide loading rate at the treatment plant is approximately 1,500 lbs per day.

Due to the primary solids contribution in its upper reaches, the Baker-Main Interceptor has long experienced high vapor H2S levels and has through the years been the cause of persistent nuisance odor complaints. H2S levels at the IRWD force main discharge were typically 300-400 ppm while those at the force main discharge downstream of the Main St. pump station were 150-200 ppm. These levels were suspected to have been made worse by inadequate sewer ventilation. Initial attempts to control sulfides involved the addition of nitrate at the influent of the IRWD solids force main. While this provided localized control within the force main and at the discharge point, OCSD determined that the dosage required for nitrate to provide adequate control downstream and throughout the entire interceptor was cost prohibitive.

Based upon their need to satisfy the objectives of their odor and corrosion control "master plan", OCSD sought a more cost effective treatment approach that would consistently control liquid and vapor sulfide levels at all points along the interceptor to less than 0.5 mg/L dissolved sulfide and 25 ppm vapor H2S. To meet these objectives, OCSD chose to utilize a proprietary technology from US Peroxide, Inc. called PRI-SC^TM (Peroxide Regenerated Iron-Sulfide Control^TM, patent pending), which was already successfully controlling sulfides within two of their other major interceptors. PRI-SC^TM integrates the use of iron salts and hydrogen peroxide in a synergistic fashion.  The combination treatment involves adding an iron salt in the upper reaches of the collection system and hydrogen peroxide at specific points downstream. In this way, iron is used for primary sulfide control and hydrogen peroxide is used to regenerate the spent iron (FeS) in-situ, yielding ferric / ferrous iron and colloidal sulfur. The "regenerated" ferric iron affords subsequent sulfide control further downstream in the system. Two key advantages made PRI-SC^TM an ideal choice for use within the Baker-Main Interceptor: (1) the iron component is very efficient at treating the solids transfer line since it is highly selective toward sulfide and largely unaffected by the high oxygen uptake rates; and (2) the downstream regeneration of the iron using H2O2 is less inhibited by the presence of high BOD and TSS contributed from the solids waste line. Since implementing US Peroxide's full-service PRI-SC^TM program in 2001, OCSD's Baker-Main Interceptor has experienced a 75% reduction of vapor sulfide levels and has significantly reduced the number of odor complaints along the line.  Further optimization steps will occur.

PRI-SC^TM in Practice

PRI-SC^TM was implemented as a full-service program that included characterization of the Baker-Main Interceptor, application development, chemicals, storage and dosing systems and ongoing operations management.  Based on results of baseline sampling and the availability of suitable sites for chemical storage and dosing, it was determined that ferrous chloride solution (FeCl2) would be added to the influent of the IRWD force main, and hydrogen peroxide solution (H2O2) would be added at the Main St. pump station. The feed rates were programmed to vary hourly, being matched to diurnal sulfide loading rates.  The optimal feed rates to achieve the sulfide control objectives along the interceptor were 900 gpd FeCl2-36% and 250 gpd H2O2-50%.  As a result, vapor H2S levels at the IRWD force main discharge were reduced to < 25 ppm, while those at the Main St. force main discharge were reduced to < 40 ppm. Continued optimization is underway to lower these levels even further.

The resulting PRI-SC^TM program cost to control to < 0.5 mg/L dissolved sulfide and a 75% reduction in vapor sulfide levels for the entire length of the 10 mile interceptor is $0.74 per lb-Sulfide.  By comparison, it cost roughly the same amount to treat with nitrate only the IRWD solids transfer line portion of the interceptor, which represents only a fraction of the required control objective.  

 

Conclusion

The results achieved within the Baker-Main Interceptor support PRI-SC^TM being a cost effective alternative to conventional methods for providing long duration sulfide control within large interceptor systems impacted by the negative effects of upstream solids transfer force main systems. Since implementing a full-service PRI-SC^TM program within the Baker-Main Interceptor, OCSD is able to consistently meet their treatment objectives at cost of only $0.74 per lb-Sulfide.

PRI-SC^TM is a proprietary technology from US Peroxide, Inc., patent pending

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