Padre Dam Municipal Water District, Helix Water District, City of El Cajon and the County of San Diego were named “Recycled Water Agency of the...
Orange County Sanitation District (OCSD), a large utility composed of 27 municipalities in southern California, contains more than 400 miles of interceptor sewers. With an annual chemical budget for odor prevention of $6.5 million, Orange County is proactive in keeping abreast of developing technologies that can provide more efficient and economical odor control.
Recently, the district completed a full-scale demonstration of a new green technology that prevented H2S formation in a 14,000-ft long, 42-in. diameter force main at the Seal Beach Lift Station without adding any liquid chemicals to the wastewater. Instead of remediating odor and corrosion problems, the new process, “superoxygenation,” designed by ECO2, helps prevent odor production by keeping conditions aerobic throughout the force main, resulting in non-detectable levels of H2S at the force main discharge.
Foul odors caused by sulfide generation in wastewater can only occur in the absence of dissolved oxygen (DO). Commonly, the DO in the wastewater is 0 to 2 mg/L while the BOD is 200 to 300 mg/L. This critical disparity between available DO and the abundance of oxygen-demanding organic pollution is the cause of anaerobic conditions in the wastewater, which generate H2S.
H2S is a more favorable food source than carbonaceous BOD and is therefore quite rapidly oxidized by the bacteria in wastewater within about 15 minutes if DO is added and the oxic conditions preclude further generation of H2S. No energy input to heterotrophic bacteria other than sulfide approaches such high levels, so sulfide oxidation dominates. Warm temperatures and long travel time combine to exacerbate odor generation in collection systems that are DO deficient.
Odor control or prevention?
There are three strategies now available for the control of odors in wastewater collection systems and treatment plants:
Injection of gaseous pure oxygen into force mains has been practiced in the past using venturi aspirators and pump impellers as well as direct injection into the pipe, with varying absorption efficiencies. Because even pure oxygen is exceptionally insoluble, the undissolved O2 bubbles have tended to collect at high points in the pipes, increasing the head on the pumps as well as “burping” out the end of rising mains, thus losing any value of the O2 in the system.
The best results to date of direct injection of O2 into a force main have been from a pure oxygen gas injection system operated less than 440 ft of head at the beginning of a 2-mile long force main with a 2-hour hydraulic detention time. This installation yielded only 50-70% oxygen absorption efficiency, however, even under such considerable pressure and long detention time.
In contrast, a superoxygenation system has been achieving 98% oxygen absorption efficiency for more than five years in a California lake while dissolving 18,000 lb of DO per day by adding 100 mg/L DO to a side stream. The oxygen transfer reactor, known in the industry as the Speece Cone, maintains the stratified lake in an aerobic condition year round and thereby prevents iron, manganese and H2S formation.
The same Speece Cone technology has now also been applied by ECO2 to the municipal facility at the OCSD Seal Beach demonstration, dissolving 50 mg/L DO in a side stream of raw unscreened sewage before it re-enters the force main. Biological purification of the wastewater within the collection system begins immediately due to the high concentrations of DO present in the earliest phase.
The ECO2 system maximizes all three critical imperatives for maintaining oxic conditions in sewer infrastructure by providing:
These provisions negate many problems associated with undissolved gases present in pipe flows and successfully inhibit H2S formation in the entire sanitary system.
Without liquid chemical treatment in the manhole at the transition of the Seal Beach Force Main into a gravity line, H2S levels were more than 100 ppm before superoxygenation. After superoxygenation was introduced, the H2S concentration dropped to non-detectable levels measured by an instrument capable of detecting H2S as low as 1 ppm in the air. In the early morning hours after lower nighttime usage caused longer retention times, the H2S rose for a short time to 3 to 5 ppm and then returned to non-detectable levels of H2S by mid-morning.
Pure oxygen sources and cost
On-site generated high purity oxygen (HPO) costs approximately $0.03/lb or about 1/10 to 1/20 of the cost for nitrate or hydrogen peroxide on an oxygen equivalent basis.
HPO at 93% O2 purity can be generated on-site using vacuum or pulsed swing adsorption technology. This technology utilizes a blower or air compressor to move air through a molecular sieve, which selectively retains the N2 and yields the O2 product. HPO can be generated at the same rate it is used, thereby precluding any need for liquid oxygen storage on site.
On-site HPO generators have a capital cost of approximately $60,000 for each ton/day of HPO capacity and electrical operating cost of approximately 500 kwhr/ton of HPO generated. Thus the amortization cost is only about $30/ton HPO, and the operating cost is also about $30/ton HPO if electricity is $0.06/kwhr.
“I’m impressed with this technology, and I think it has a bright future in this business,” said Ed Ratledge, project manager and senior environmental specialist for OCSD. “My background is in plant operations, so the idea of starting plant processes in the collection system while doing useful odor control is very attractive to me.”