The City of Fargo utilizes an intermediate ozone process in their lime softening water treatment plant (LSWTP) as a primary disinfectant and to reduce taste and odor compounds prior to filtration. The LSWTP went online in 1997 and has had minimal improvements since that time. Apex and Carollo were hired to evaluate the existing ozone process and propose improvements to the ozone process.

A number of improvements ware proposed for the LSWTP ozone process; these improvements are broken into two efforts: near-term improvements and long-term improvements. Near-term improvements are fast-tracked improvements that are intended to address immediate safety concerns. Long-terms improvements are larger project or projects focused on increases in capacity and performance. Long-term improvements are still being developed, while near-term improvements are currently under construction.

Near-term improvements include the following:

• Segregation of supplemental air and ozone gas prior to the ozone contractors, as moisture in the air has been identified as the driving agent for corrosion inside the piping.
• Replacement of corroded supplemental air and ozone gas piping and diffusers inside the ozone contractor. New diffusers and piping will be provided for the segregated gas systems.
• Modifications to ancillary systems serving the contractor as needed to accommodate the segregation of piping.
• Replacement of gas and supplemental air diffusers in both ozone contractors.
• Access improvements to each of the two ozone contractors to improve operator safety.
• Relocation of ozone gas monitors.

Existing ozone piping has experienced corrosion in locations where supplemental air  has been mixed with ozone. Nitrous oxides are formed when ozone, nitrogen, and moisture from ambient air are combined. This combination of gases and moisture then condenses in the piping. This has likely impacted all the piping downstream of the supplemental air injection location, and may have had a localized impact upstream of the supplemental air. By segregating the supplemental air from the
ozone product gas, internal pipe corrosion will be dramatically reduced.

The existing diffusers will be replaced with state-of-the-art diffusers for fine-bubble diffusion that have no gaskets requiring periodic replacement. Since gas transfer efficiency is not a concern with supplemental air, a higher flow per diffuser can be anticipated.

Access to the ozone contractors is provided through manways located on top of the contractor which is over 25 feet deep. Operational staff members are lowered into the contractors and must remove the diffusers, which are mounted several feet above the contractor floor, to get to the floor for maintenance activities. New hatches will be provided to each contractor along with bottom, improving access and safety for operational staff. A fall protection cable configuration is included for use with each ladder, as well as davit crane anchorage points at each new hatch to enable
extraction of personnel from each contractor, as the contractors are confined spaces.

Construction of the near-term improvements was completed in March of 2020. PKG Contracting of Fargo, ND was the contractor completing the improvements. Long-term improvements are moving forward and include the following:

• New low-demand ozone generator.
• New vacuum swing absorption unit for oxygen production in the ozone generation process.
• New oxygen receiver tank to be used in conjunction with both the new USA system and the existing liquid oxygen (LOX) system.
• New electric LOX vaporizers to be used in parallel with the existing water circulating vaporizer.
• Modifications to existing ozone generators to facilitate turn-down.

The design phase for the long-term improvements were completed in early 2021 with construction to follow.