Elimination of Odor and Hydrogen Sulfide Gas by Superoxygenation of the Bluebird Force Main in Laguna Beach, California

ISSN： 1938-6478

Source： Proceedings of the Water Environment Federation, Vol.2008, Iss.4, 2008-01, pp. : 815-825

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Abstract

Like many cities, the City of Laguna Beach has a transmission main that was sized twenty years ago for a projected level of development that was never realized. Consequently, the transmission main is oversized with daily service velocities running less than optimum. The mountainous terrain of the City requires a complex system of twenty-eight lift stations to serve approximately 18,000 users. The wastewater collection travel time is lengthened due to the lift station trips necessary to reach the transmission pipeline. Travel time is further exacerbated at night when flows are essentially stopped until enough flow is collected to trigger a pumping cycle in some of the smaller residential lift stations. Collectively this typical diurnal cycle is the cause for significant anaerobic conditions to develop in the collection system. The generation of malodorous hydrogen sulfide gas has plagued the City for many years. Over the last ten years, the City has tried many methods and spent a great deal of money to reduce the odor production. Three years ago the City conducted a test to determine the level of hydrogen sulfides released at blow-off vaults. Some of the vaults were located near heavily traveled pedestrian crosswalks in the City. The odorous gas levels were extraordinary and proved that antiquated conventional methods to treat odors were not effective. Costly chemical treatment was discontinued. The City sought new methods for preventative, non-chemical in pipe treatment.The root cause of most odor problems associated with wastewater collection and treatment is the presence of naturally created malodorous gases. The gases are formed by Sulfate Reducing Bacterial, (SRB's) under anaerobic conditions resulting from the imbalance of high BOD loading and the limited solubility of oxygen into wastewater. This problem is exacerbated in long force mains and slow-cycled residential lift stations due to high microbial oxygen uptake rates, long detention times, and low dissolved oxygen (D.O.) levels. These anaerobic conditions produce significant amounts of hydrogen sulfide (H₂S) which is both odorous and highly corrosive. Since anaerobic conditions are precursors to H₂S formation, a logical solution to H₂S control is to induce aerobic or oxic conditions. SuperOxygenation is a process to sustain aerobic conditions by using an ECO2 “Speece” Cone (Downflow Bubble Contactor) to dissolve pure oxygen into wastewater to prevent the formation of H₂S.An ECO₂ SuperOxygenation system was designed and installed on the Bluebird force main in August of 2006. The system has successfully reduced untreated hydrogen sulfide gas in amounts exceeding 300 ppm to an average of under 2 ppm at the force main discharge ∼ 1.5 miles down stream. This is being done at a capital and annual operating cost resulting in a 3.6-year payback versus previously used chemical treatment.The system operates by redirecting a side-stream flow of 1,000 gpm of raw, unscreened wastewater from the primary lift station discharge manifold and pumping it through an ECO2 Cone. Gaseous oxygen is generated onsite by a Pulse Swing Absorption (PSA) oxygen generator. The oxygen is fed at a rate of up to 1,000 lbs per day into the ECO2 Cone which has an exceptionally large oxygen/wastewater interface generated by an intense bubble swarm to achieve rapid oxygen dissolution. The ECO₂ SuperOxygenation system completely dissolves the gaseous oxygen into the wastewater side-stream before it is blended back into the force main. Enough oxygen is dissolved into the force main to sustain aerobic conditions reducing the amount of H₂S generated to near non-detectable levels.Generation of malodorous hydrogen sulfide gas has plagued the City of Laguna Beach for many years. In 2006, the City sought new methods for preventative, non-chemical, in pipe treatment and found unprecedented success in odor reduction by installing ECO₂ SuperOxygenation technology to sustain aerobic conditions in the force main preventing the formation of H₂S.