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A System-Wide Sulfide Control Plan for the Lee County, Florida Wastewater Collection System

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Lee County Utilities (LCU) undertook a comprehensive study of sulfide generation, corrosion, and odor in the 80 square mile collection system serving Fort Myers and Lee County. Due to long collection system detention times, numerous pump stations, and elevated wastewater temperatures, high concentrations of dissolved sulfide form in the force mains. Dissolved sulfide concentrations as high as 40 mg/l and hydrogen sulfide concentrations as high as 260 ppm were measured. Much of the system consists of long force mains with numerous contributing pump stations forming a ‘manifold rsquo;, with few ‘master stations ’ carrying large flows. Hydrogen peroxide, calcium nitrate, and iron salts are currently used for sulfide control, at a total of 18 locations, for an annual cost of 1.2 million. Additionally, 24 pump stations have odor control systems.

The system-wide sulfide control plan is intended to meet the following objectives:

Meet a zero-tolerance requirement for odor complaints

Protect vulnerable assets, such as ductile iron and concrete pipelines and pump stations

Optimize the use of sulfide control chemicals to maintain a cost-effective program

The field activities included grab samples for dissolved sulfides, pH, dissolved oxygen, wastewater temperature, and headspace hydrogen sulfide concentrations in addition to corrosion assessments at 320 pump stations in six of the county's collection systems. Odalogs were installed at selected stations to record diurnal trends. At ten of the pump stations, Odalogs were deployed simultaneously with ISCO 24-hr sequential samplers for analysis of dissolved sulfide. The 24 bottles were analyzed in the field as soon as the sampler was turned off. The intention was to produce a useful regression relationship between sulfide and headspace hydrogen sulfide.

In order to determine optimal chemical dosage, mass flow of sulfide from major pump stations had to be determined. Pump station flow rates were not readily available so they were calculated by measuring the rate of water level drawdown in the wet well. Additionally, at eight pump stations, In-Situ Troll level logger probes were deployed. These probes logged wastewater levels of the pump stations at 5-second intervals. The level data was processed to generate profiles of wastewater pumping rates and calculate the volumes of wastewater handled by the pump stations over the course of a day. The results showed significant variation in pumping rate from one pump cycle to another. This was because of varying back-pressure in the force main due to the activities of other pumps on the manifold. These results provided an insight into probable reasons for air-locks in the collection system and potential bottle necks.

System maps were produced for each of the six collection systems. Each system was divided in to sub-drainage areas for which sulfide mass flows and detention times were calculated. During two workshops with LCU, each sub-area was scored for prioritization, with points assigned for observed corrosion in pumping stations, presence of DI and concrete pipelines, odor complaints, and odor risk due to high hydrogen sulfide and proximity to sensitive receptors. A full range of liquid-phase sulfide controls and air-phase ventilation and odor controls were evaluated. The final result was a program specifically designed for each sub-area consisting of an optimal combination of liquid and air-phase solutions, with capital and O&M costs and prioritization based on cost/benefit ratio. Through a series of workshops, a program was developed to protect all the vulnerable assets in the six collection systems and prevent odor complaints, at a 30 percent increase over current annual expenditures for liquid-phase sulfide control.
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Keywords: Wastewater collection systems; collection system odor; sulfide control

Document Type: Research Article

Publication date: 2010-01-01

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