The Knott-Interplant Interceptor is a long gravity main system with a major contribution from a low-velocity force main just prior to the midway point. Downstream of this juncture, the line size increases from 66" to 96" as it picks up flow and travels 10 miles further to where 55 mgd of by-pass wastewater from Orange County Sanitation District’s (OCSD) Treatment Plant No. 1 is added. The line continues about three miles further and then discharges into Plant No. 2 where the wastewater undergoes chemically-enhanced primary treatment and 50% secondary treatment, and is then discharged to the ocean. The wastewater composition is typical of mixed residential to commercial origins, with no major industrial inputs. The total system flow averages 80 mgd with an overall transit time of about 13 hours. The resulting sulfide loading at the treatment plant is around 2,000 lbs per day.  

Controlling odors within the Knott-Interplant Interceptor is a difficult challenge due to high sulfide loading rates and emissions at the force main discharge point. 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 effluent quality deterioration were linked to upstream caustic shocking events.

To remedy these problems, OCSD sought an alternative "master plan" strategy that could consistently control liquid and vapor sulfide levels at all points along the interceptor to less than 0.5mg/L dissolved sulfide and 25 ppm vapor H2S.  It was also essential that the strategy be compatible with plant operations.  After determining through trial that conventional approaches were not cost effective, OCSD chose to implement US Peroxide Inc.'s proprietary technology called PRI-SC^TM (Peroxide Regenerated Iron - Sulfide Control^TM, patent pending), which had already proven successful in meeting the program objectives at other major interceptors within their system. 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 (oxidize) the spent iron (FeS) in-situ, yielding ferric / ferrous iron and colloidal sulfur.The "regenerated" ferric iron affords subsequent sulfide control further downstream. Two key attributes of PRI-SC^TM made it ideally suited for the Knott-Interplant Interceptor: (1) by being able to add less iron to the low-velocity force main (and subsequently regenerating the spent iron for downstream sulfide control), pH depression was minimized and thus vapor H2S levels at the high-turbulence force main discharge were reduced; and (2) an additional regeneration step at the treatment plant enabled much of the ferrous salt added in the collection system to be converted to hydrous ferric oxide for enhanced primary clarification purposes. Since implementing US Peroxide’s full-service PRI-SC^TM program in 2000, OCSD's Knott-Interceptor has experienced an 80% reduction of vapor sulfide levels, eliminating odor complaints downstream of the force main discharge. Further optimization steps will be implemented in the near future.

PRI-SC^TM in Practice

PRI-SC^TM was implemented as a full-service program that included characterization of the Knott-Interplant 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 upstream force main, and hydrogen peroxide solution (H2O2) would be added first at the midway point and again at the treatment plant. 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 1,400 gpd FeCl2-36% and 350 gpd H2O2-50%. These feed rates resulted in vapor H2S levels at the force main discharge being reduced from 250-300 ppm to < 5 ppm and provided for continuous downstream control as well.

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 discharge point into the plant were lowered from 100-150 ppm to < 5 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 $1.25 per lb-Sulfide and was less expensive than the conventional alternatives that OCSD had previously evaluated.  

 

Conclusion

The results achieved within the Knott-Interplant Interceptor support PRI-SC^TM being a cost-effective approach for controlling sulfides in mixed force main and gravity main systems where sulfide levels range from a few mg/L to 10 mg/L. 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 within the Knott-Interplant Interceptor, OCSD has been able to consistently meet their sulfide control treatment objectives at an affordable cost ($1.25 per lb-Sulfide). In addition, operational benefits at the treatment plant have been realized without additional expense, including: reduced sulfide loading in the influent; improved chemical usage efficiency at the headworks; reduced H2S emissions; and, the ability to utilize the regenerated (ferrous) iron for enhanced clarification purposes

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

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