The City of McCook, Nebraska has a population of 8,000 and a drinking water supply from nine shallow wells located in an alluvial aquifer adjacent to the Republican River. High nitrate levels and concentrations of arsenic and uranium in the wells exceeded the EPA Minimum Contaminant Levels. W Design Associates and Jacobson Satchell Consultants provided design criteria and invited potential vendors to submit proposed treatment options. In addition to nitrate, arsenic and uranium removal, TOC reduction and hardness reduction were also required.
The selected design process recommended by Tonka Water uses ion exchange in a two-step arrangement to first reduce the hardness of the raw water using Tonka Water’s RidION™, follow by Nion-IX™ to reduce the nitrate, arsenic, uranium and TOC levels in a single vessel train. Design consisted of six trains of RidION™ ion exchange vessels, each with a matched pair of strong acid cation and strong base anion resin vessels.
Treated water from the cation vessels and the anion vessels blends with raw water to achieve the Nebraska EPA requirements. Blending the raw and treated water allows flow to be bypassed, thus reducing treatment cost.
A Tonka Water plant PLC control system operates the cation and anion resin vessels. As flow demand increases or decreases, pairs of vessels are placed on line or taken off line as needed. The units are also regenerated as needed with salt from on-site salt brine pits located in a separate building.
The raw water analysis summarized below reflects arsenic, nitrate and uranium existing in the water. The treatment process has been highly effective in delivering quality water as indicated below.
Blended Finished Water
|Arsenic||15 ppb||8 ppb|
|Nitrate||15 ppm as N||8 ppm as N|
|Uranium||50 ppb||25 ppb|
|TOC||> 2.0 to 4.0 ppm||<1.7 to 3.4 ppm|