Hydrogen sulfide issues have become increasingly problematic for the City of Moorhead, MN. As a result of these concerns, Apex Engineering Group was contracted to develop a strategy for effectively and economically controlling odors and corrosion within the sanitary collection system and the City’s Wastewater Treatment Facility (WWTF).

Odor in wastewater collection systems and treatment facilities, particularly related to excessive hydrogen sulfide (H2S) concentrations, can result in severe corrosion and shortened life-span of system components, public complaints, and worker health and safety issues due to toxicity.

The initial task was to establish a sampling plan to evaluate the existing odors and measure the parameters influencing odor and corrosion within the sanitary collection system and WWTF. Sampling revealed high odor conditions in the downstream portions of the collection system and WWTF headworks. Based on these results, Apex provided recommendations and alternatives to the City of Moorhead.

A pilot study was conducted to evaluate the effectiveness of FeCl2 for the City of Moorhead. FeCl2 was injected into the sanitary collection system, and sampling results verified this was an effective strategy for controlling odors in the collection system and at the WWTF.

As a result of the pilot study, detailed design was initiated for three FeCl2 chemical injection stations to control odors and corrosion. Budgetary and rate impacts associated with the necessary capital improvements as well as ongoing operation, maintenance and chemical costs were also evaluated during the design. The chemical injection stations were constructed in 2014, and startup occurred in the spring of 2015.

Besides reduced odor and corrosion, City staff have reported several operational benefits including reduced frequency of carbon media replacement in odor control scrubbers, lower sludge volume index values and final clarifier sludge blanket levels, higher return activated sludge suspended solids concentrations, high solids concentration in gravity belt thickener feed