The Miller-Holder Interceptor is a long, slow flowing, high sulfide-producing gravity sewer that discharges into Treatment Plant No. 2 of the Orange County Sanitation District's (OCSD) system. There, the wastewater undergoes chemically-enhanced primary treatment and 50% secondary treatment, and is then discharged to the ocean. Except for contributing force main discharges just prior to the midway point, and a siphon just after the midway point, the interceptor is a free-flowing gravity RCP with average velocity 2.5 fps and diameters increasing from 48” at the upper reaches to 78" at the treatment plant. The wastewater is typical of mixed residential to commercial origins, with no major industrial inputs. The total interceptor flow terminating at the treatment plant is 23 mgd with a transit time of 14 hours. Sulfide loading rates at the plant are 1,900 lbs per day.  

Since the early 1990's, OCSD used routine periodic shock dosing of caustic soda (sodium hydroxide) to mitigate odors and corrosion along the interceptor associated with hydrogen sulfide. Biofilm recovery and thus H2S reappearance, however, was complete within a few days of each shock treatment. The result was inconsistent sulfide control and inefficient, difficult to manage chemical additions at the treatment plant. The final straw was when infrequent episodes of plant effluent quality deterioration were linked to upstream caustic shocking events. Finding an alternative program became a top priority, particularly one that was affordable, could achieve consistent low level control of sulfides throughout the duration of the interceptor and was highly compatible with treatment plant operations.

To remedy the increasing sulfide related odor and corrosion problems along the Miller-Holder Interceptor, OCSD embarked on a search for a comprehensive and cost effective treatment program that could consistently meet their control objectives. These were 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. It was also important that the overall strategy provided synergy between the collection system and treatment plant. After determining that conventional sulfide control alternatives would not cost effectively satisfy the program objectives, OCSD chose to implement a proprietary technology developed by US Peroxide, Inc. called PRI-SC^TM (Peroxide Regenerated Iron-Sulfide Control^TM, patent pending). PRI-SC^TM is a "master plan" strategy that 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 case of OCSD, an additional regeneration step at the treatment plant enabled the use of much of the ferrous salt added in the collection system to be converted to hydrous ferric oxide for enhanced primary clarification purposes. Once implemented, the full-service PRI-SC^TM program cost effectively met the OCSD treatment objectives, including the need for synergy between the collection system and plant.

PRI-SC^TM in Practice

PRI-SC^TM was implemented as a full-service program that included characterization of the Miller-Holder Interceptor, application development, chemicals, storage and dosing systems and ongoing operations management.  Based on results of initial 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 top of the interceptor, and hydrogen peroxide solution (H2O2) would be added first at the midway point and again at the treatment plant. This meant spacing the chemical injection facilities at transit time intervals of 6-7 hours. The feed rates were programmed to vary hourly, being matched to diurnal sulfide loading rates. The optimal feed rates to achieve the specified sulfide control objectives along the 25-mile interceptor were within 20% of theoretical rates (i.e., 1,000 gpd FeCl2-36% and 325 gpd H2O2-50%).

By adding a second H2O2 regeneration step at the treatment plant, PRI-SC^TM was not only able to meet the treatment objectives in the collection system, but also provided significant odor control and operational benefits at the plant level. Influent dissolved sulfide levels were reduced to < 0.5 mg/L and vapor H2S levels at the influent sewer were lowered from 150-250 ppm to about 10 ppm. This reduction in overall sulfide loading resulted in more efficient use of chemicals at the headworks and primary clarifiers, as well as generally reduced fugitive emissions. Additionally, the regenerated iron became a useful source of hydrous ferric oxide for enhanced primary clarification. The resulting full-service PRI-SC^TM program cost was only $1.02 per lb-Sulfide, which was lower than the other conventional treatment methods previously evaluated by OCSD.

 

Conclusion

The results achieved within the Miller-Holder Interceptor support PRI-SC^TM being a cost effective alternative to conventional methods for controlling sulfides in long, slow-moving, high sulfide producing interceptors. The results also demonstrate that PRI-SC^TM is highly synergistic with treatment plant operations and provides measurable benefits. Since implementing a full-service PRI-SC^TM program along the Miller-Holder Interceptor, OCSD is able to consistently meet their treatment objectives at an affordable cost of $1.02 per lb-Sulfide. In addition, operational benefits at the treatment plant have been realized.

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

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