Challenge

When the city of Knoxville, Iowa, built a new water treatment plant in the fall of 1994, they required a treatment scheme that would remove iron and radium from the ground water supply in a single process.  They also wanted a system that would need only minimal operator attention and maintenance.

Solution

We installed a 2.4-million-gallons-per-day (MGD) water treatment system consisting of two General Filter horizontal manganese greensand pressure filters with pre-formed hydrous manganese oxide (HMO) chemical treatment.

The raw water to the plant is provided by three wells that draw from the Jordan aquifer.  As raw water enters the plant, all chemicals are injected in-line prior to a static mixer.  Reaction time is provided in roughly 20 linear feet of pipe before entering the filters.  Potassium permanganate and manganous sulfate are added to form HMO; sodium hypochlorite is added for iron oxidation and disinfection; and sodium hydroxide is added for distribution system corrosion control.

Radium is reduced by sorption to the produced manganous oxides.  Additional solids produced as a result of HMO formation have a negligible effect on filter run length.  The filters are backwashed on time every two weeks before terminal headloss of 3.5 psi differential is reached.  The HMO’s containing radium are washed out of the filter, and the waste is sent to the sanitary sewer.  The Knoxville plant uses a backwash waste holding tank to equalize flow and discharge it at a minimum rate into the sanitary sewer.  The waste treatment plant treats the combined filter backwash waste and municipal water, requiring no changes in their process.

Results

The water treatment equipment, chemical treatment scheme and overall process have been operating successfully for over nine years.

• The HMO process, combined with the manganese greensand filters, produce a finished water with iron and manganese close to non-detectable levels
• Total radium is reduced to a level safely below the maximum contaminant level of 5 pCi/l
• According to Steve Inskeep, general manager of the Knoxville Water Works, the highly automated water treatment system requires minimal operator attention and maintenance
• A telemetry system is in place should an event occur that requires operator attention