Shell-media technology controls lift station odor in Georgia community
Oconee County lies in the center of Georgia. Its lift station on Mars Hill Road, a heavily traveled low-speed limit area with a retirement community and church nearby, generated a number of odor complaints. The county hired an odor consultant who determined that the odor coming from the lift station was complex in that it contained both mercaptans and hydrogen sulfide (H2S).
Control of odor-causing compounds emitted from wastewater and industrial treatment processes has become a growing area of concern. As populations increase and housing and commercial developments encroach on once remote treatment facilities and systems, the importance of effective yet simple odor control technology continues to grow as well. In addition to being a nuisance, odorous compounds can be corrosive to equipment, requiring ventilation to extend equipment life and reduce capital replacement expenditure.
Oconee County tried a number of technologies, including aeration systems, misting systems, hydrogen peroxide and chlorine—all with limited success. Ultimately, the county decided it wanted a long-term solution that did not require chemicals due to their expenses and the manpower required to monitor and continually add and replenish them.
After receiving competitive bids for ozone and biofiltration systems, Mónashell was selected as the most economical solution. Design parameters include a ventilation rate of 465 cu ft per minute and H2S of 40 to 60 ppm.
Utility Department Director Chris Thomas recognized the many benefits of Mónashell relative to other technologies. In comparison to biofilters, it:
• Treats high and variable concentrations of H2S and a broader range of organic sulfides;
• Offers up to 67% footprint reduction compared to conventional biofilters due to higher efficiencies;
• Reduces contact time and deeper filter bed;
• Does not short-circuit or channel through media;
• Features shells with high porosity, which results in low pressure drop;
• Includes no nutrient addition; and
• Has lower water consumption due to water recirculation.
Versus carbon and wet scrubbers, it:
• Features low pressure drop and low energy costs;
• Reduces media replacement costs;
• Eliminates the need for chemicals;
• Operates in a sustainable fashion using naturally occurring media;
• Achieves high effectiveness on highly variable inlet conditions; and
• Offers lower operating and maintenance costs.
Laboratory and field trials demonstrated treatment effectiveness, and the Mónashell process was patented in 1994 (Patent No. 2300824). There currently are more than 600 systems installed worldwide for applications with H2S exceeding 500 ppm for:
• Wastewater pumping stations;
• Wastewater treatment plants;
• Sludge-handling and treatment processes;
• Municipal solid waste and composting facilities; and
• Industrial facilities.
Seashells were identified as an optimal media because of the built-in buffering from the calcium carbonate (CaCO3) makeup of shells. Other qualities of the shell media include:
• High porosity;
• The ability to sustain high irrigation rates;
• Capacity to retain large quantities of water;
• Size and shape in the correct range for good mass transfer;
• Support for biological activity; and
• High affinity for sulfur compounds.
Shell-media technology can be viewed as a combination of biofilter and biotrickling filter technologies. High irrigation rates are required in conventional biotrickling filters to control pH, but the seashells slowly release calcareous compounds that neutralize the acid, so water usage is lower.
The shell media eventually is consumed, but lifespans are longer than with conventional biofilter media. Additionally, a more neutral pH in the system improves the removal of other odorous compounds.
Within two days of installation, the significant H2S odor was gone; the more complex odor was removed effectively within a week. This was accomplished by containing the odorous gases that are ventilated from the source for treatment. Ventilation is achieved via ducting and extraction fans under low-negative-pressure conditions. The airstream is pulled through the system, rather than pushed, decreasing the risk of escaping odor.
The waste airstream is directed through the shell bed of the odor removal unit, allowing contact between the shell media, water, selected microorganisms and odorous compounds. Microorganisms reside on the shell media, which contain high levels of CaCO3 to neutralize the acid byproducts of sulfide oxidation.
The physical, structural and chemical properties of the media allow for smaller filters and higher efficiencies than conventional biofilters, and the end products of the biofiltration process are harmless and free of odor.
Thomas has not received any odor complaints since the unit was installed. He now is looking at Anua’s new Compact Mónashell—the same technology as the standard unit, but with an even smaller footprint for above- and below-ground use—for other lift and pump stations.