Alternative strategy controls sulphides in slow-moving interceptors

The new patented PRI-SC™ technology from US Peroxide controls sulphides in a gravity sewer interceptor and improves treatment plant operations in Southern California, USA.

A comprehensive treatment strategy that integrates the use of iron salts and hydrogen peroxide proved to be a cost-effective alternative for controlling sulphides in a long, slow-moving, high sulphide-producing interceptor in the Orange County Sanitation District (OCSD) in the US state of California. Most importantly, the overall treatment programme provided synergy between the collection system and treatment plant, achieving operational benefits at the treatment plant without additional expense.

The Miller-Holder Interceptor gravity sewer discharges into Treatment Plant No. 2 of the OCSD treatment system, which derives its wastewater from mixed residential and commercial origins, with no major industrial inputs. The total interceptor flow terminating at the treatment plant is 23 million gallons per day (mgd) with a transit time of 14 hours. Sulphide loading rates at the plant are 1,900 lbs per day.

Since the 1990s, OCSD used periodic shock dosing of caustic soda to mitigate odours and corrosion associated with hydrogen sulphide (H2S) along the interceptor. Biofilm recovery and thus H2S reappearance, however, occurred within a few days of each shock treatment, resulting in inconsistent sulphide control and inefficient chemical additions at the treatment plant. In addition, infrequent episodes of plant effluent quality deterioration were linked to upstream caustic shocking events. Finding an affordable, alternative programme that could achieve consistent low-level control of sulphides and was compatible with treatment plant operations became a top priority.

OCSD soon embarked on a search for a treatment programme that could consistently meet their control objectives. These objectives included liquid and vapour sulphide levels at all points along the Miller-Holder Interceptor of less than 0.5 mg/L dissolved sulphide and 25-ppm vapour H2S. In addition, the overall strategy needed to provide synergy between the collection system and treatment plant. OCSD chose to implement a patented proprietary technology developed by US Peroxide called Peroxide Regenerated Iron-Sulphide Control™ (PRI-SC™).

PRI-SC is a “master plan” strategy that integrates the use of iron salts and hydrogen peroxide. This 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 sulphide control and hydrogen peroxide is used to regenerate the spent iron (FeS) in-situ, yielding ferric / ferrous iron and colloidal sulphur. The “regenerated” ferric iron affords subsequent sulphide 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.

OCSD implemented PRI-SC as a full-service programme that included characterisation of the Miller-Holder Interceptor, application development, chemicals, storage and dosing systems and ongoing operations management. Based on results of initial baseline sampling, 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. Feed rates were programmed to vary hourly, being matched to diurnal sulphide loading rates. The optimal feed rates to achieve the specified sulphide control objectives along the 25-mile interceptor were within 20% of theory (i.e., 1,000 gallons per day [gpd] FeCl2-36% and 325 gpd H2O2-50%).

By adding a second H2O2 regeneration step at the treatment plant, PRI-SC met treatment objectives in the collection system and provided significant odour control and operational benefits in the plant. Influent dissolved sulphide levels decreased to < 0.1 mg/L and vapour H2S levels at the influent sewer decreased from 150 to 250 ppm to < 10 ppm. This reduction in overall sulphide loading improved the efficiency of chemical use at the headworks and primary clarifiers, and reduced fugitive emissions. Additionally, the regenerated iron became a useful source of ferric hydroxide for enhanced primary clarification. The full-service PRI-SC programme cost only US$ 1.02 per lb for sulphide, lower than the other conventional treatment methods previously evaluated by OCSD.

Author’s Note

John Walton is the vice president of Technology & Program Development at US Peroxide, located in Laguna Niguel, California, USA. Jeffrey Prellberg is the business manager for Municipal Programs at US Peroxide.