The Milan Wastewater Treatment Plant, located just south of Moline, Illinois, is a 1.7 MGD (6,434 m3/day) activated sludge facility with an average wastewater flow of 700,000 gpd to 800,000 gpd.
This facility dewaters primary sludge using a belt filter press (Figure 1). The press operates an average of three times per month, 8 hours per day at a pump rate of 100 gpm. The sludge is dewatered from 3-4% solids to about 23% solids.
On days when the bio-solids from the primary settling tank are dewatered, nuisance odors were a common problem. Not only did the plant experience bad odors, but the neighbors living in the general area also complained. On days when the plant dewatered sludge, City Hall employees did not welcome their odorous co-workers. The plant used deodorizers in the past, with little to no success.
A major contributor to the bad odor was hydrogen sulfide (H2S). Hydrogen sulfide is a colorless gas that is moderately soluble in water. In addition to the rotten egg odor, H2S is very corrosive to metal pipes, equipment, and general metal surfaces (Figure 2). More important, H2S is a dangerous gas with grave consequences to persons exposured to over 10 ppm over an extended period of time.
A preliminary evaluation of the odor problem found atmospheric H2S levels above the sludge conveyor belt as high as 57 ppm. The atmospheric H2S level in the press-room was 5 ppm with some measurements as high as 9.8 ppm close to the press. Without proper ventilation, levels in the plant could potentially exceeded the TWA (time weighted average) of 10 ppm during dewatering.
Permanganate (MnO4-) is an easy compound to evaluate in odor control applications because the reaction with H2S is selective and very fast. Its characteristic pink or purple color, when rapidly reacting with H2S, turns to the yellow or brown color manganese dioxide (MnO2). Simple laboratory jar tests will indicate very quickly what dosages would provide the degree of odor control desired.
For this application, ECONOX® agent was selected for its ease of use. The material was supplied in a 55-gallon (208.2 L) drum and is fed directly into the sludge stream by means of a simple feed pump.
ECONOX agent dosed at a rate of 2.5 gph (9.5 L/hr) effectively reduced the atmospheric H2S levels above the sludge conveyor belt from 57 ppm to 16 ppm and reduced the atmospheric H2S levels in the dewatering room from 5 ppm to below the detection limit (Figure 4). In addition, the dissolved sulfide (S2-) in the filtrate was lowered from 20 mg/L to 5 mg/L.
Since implementing odor control with ECONOX agent, the operators have noticed immediate odor control effectiveness during the dewatering operation. Complaints by coworkers have been significantly reduced and the neighborhood complaints have been eliminated.
ECONOX agent dosed at 2.5 gph (9.5 L/hr) reduced the H2S emissions during the dewatering process. The following additional benefits resulted:
Reduced operator exposure to dangerous H2S gas
Eliminated odor complaints
Reduced atmospheric corrosion
Reduced sulfide return to process
Reduced odors associated with cake storage and transportation
Improvements in sludge dewatering and reduction in polymer usage will be monitored as the process is optimized.
Carus Value Added
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As an integral part of our technical support, Carus provides extensive on-site treatment assistance. We offer full application services, including technical expertise, supervision, testing, and feed equipment design and installation in order to accomplish a successful evaluation and/or application.
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Various options and accessories are available to meet a wide range of applications. Custom-Engineered Feed Systems are complete, preengineered and prepackaged systems. System designs are customized to meet specific applications and customer needs.
During its 98-year history, Carus’ ongoing emphasis on research and development, technical support, and customer service has enabled the company to become the world leader in permanganate, manganese, oxidation, and base-metal catalyst technologies.