Faced with the challenge of expanding its treatment plant to meet growing demands, Osceola County Rural Water System (OCRWS) worked with DGR Engineering (DGR) to find an innovative solution. As a first step, the chemical feed operations were revamped, maximizing the capability of the existing facility. Yet, the plant still couldn’t keep up with the rising needs.

OCRWS considered a traditional WTP expansion to increase the capacity from 5.3 to 8.6 MGD, but a $10 million estimated cost prompted a search for a more cost-effective solution. Due to high levels of iron and manganese in the raw water, the existing filters experienced short filter run times at loading rates below the DNR’s allowed limits. DGR designed a pilot study using dual media (anthracite and silica sand) which increased filter loading rates by 25%. This would boost plant capacity to 7.0 MGD for a more manageable cost of $1.8 million.

Anthracite expands significantly more than silica sand during the backwash process. Consequently, the existing backwash troughs needed to be replaced to prevent the loss of the lighter anthracite media during the combined air and water backwash procedure. The OCRWS WTP comprises three distinct filtration units that have been constructed at different times over the course of 45 years. The replacement of the backwash troughs presented a considerable challenge in retrofitting new troughs into a multi-phase WTP. Thanks to a seamless collaboration among the contractor, OCRWS, and DGR, the construction proceeded efficiently, addressing one filtration unit at a time, and was successfully completed during a single off-peak season.

Project Approach

• Custom Alternatives: Worked with OCRWS to develop tailored solutions to meet system demands.
• Pilot Study: Developed and administered a pilot study to verify the effectiveness of the proposed solution.
• Funding and Permitting: Secured necessary funding and permits to commence construction.
• Construction Collaboration: Worked closely with OCRWS and contractor during construction to minimize operational impacts.

Challenges

• Hydraulic Capacity: Verified the hydraulic capacity of existing piping and treatment equipment to support increased treatment capacity.
• Equipment Installation: Overcame difficulties in installing new equipment within an existing building.
• Operational Continuity: Maintained water treatment plant operations during construction.
• Mudballing Issues: Addressed “mudballing” issues in existing filters by replacing clogged underdrain nozzles.

Project Solutions

• Media: Replaced 27″ of silica sand filter media with 12″ of anthracite and 18″ of silica sand.
• Media Coating: A manganese dioxide coating system was installed on the new filter media to improve manganese removal performance.
• Backwash Troughs: Replaced existing backwash troughs to allow the transition to dual media.
• Underdrain nozzles: Replaced existing media retaining underdrain nozzles in the older filters.
• Pre-procurement: Pre-procured equipment to save time during construction.
• Baffling: Replaced baffling in detention tanks to enhance iron oxidation with air

Ultimately, a capacity increase of 1.7 MGD was achieved by completing a project that cost $1.8 million.