Water entering the plant is called raw water. Upon entry, potassium permanganate is injected to convert soluble ferrous iron to insoluble ferric iron so it can be filtered out. Another benefit to potassium permanganate injection is the oxidation of hydrogen sulfide (which gives the rotten egg odor). This oxidation process is vital to the plant operation in that it filters out iron and sulfur bacteria which would otherwise deposit on the membrane surface and cause fouling. It also enables the City to "BLEND" filtered water back with the reverse osmosis (R/O) product water to naturally stabilize the water. This process will be discussed further during the stabilization process.
The water then enters the greensand pressure filters utilizing the water pressure provided by the supply wells. The filters are constructed as horizontal cylinders with layers of filtering material. The top layer is anthracite. It filters out the precipitated insoluble iron produced by the injection of the potassium permanganate. Manganese greensand is the middle layer. This layer absorbs any residual potassium permanganate left over in the water. It also filters out and converts any iron still left in the water at this point. The final layers are multi-sized gravel to support the upper layers and provides filtering for any particles which may have passed through the upper layers. All of the water entering the plant passes through this iron removal process.
Sulfuric acid is added to lower the pH of the water to prevent calcium carbonate scaling on the R/O membranes. Antiscalant (polyacrylic acid) is added to prevent calcium sulfate and barium sulfate scaling on the membrane. Any scaling on the membrane surface would prevent the passage of the water through the membrane. As scaling occurs, the pressure it takes to force the water through the membrane increases, which in turn increases the power requirement.
After the injection of chemicals to prevent scaling on the membranes, the water passes through a series of
5 micron polypropylene cartridge filters. These remove any silt, turbidity, or debris larger than 5 micron
which could cause irreparable damage to the membrane surface. Bacteria or other organic matter can also
be filtered out during this process. The micron filters cannot be cleaned and/or reused. They are changed
out as the pressure loss across them dictates or the water quality from them deteriorates to a point that
possible damage to the R/O membranes could occur. This step is the last and most important in the
pre-treatment process the membranes. The water is then termed pre-treated feedwater. All treatment
done to this point is to protect the RO membranes from any scaling or fouling from natural constituents in
the raw water.