Hydrometrics, Inc.

An innovative, non-fouling reverse osmosis process termed High Efficiency Reverse Osmosis (HERO™) was recently installed and tested at a metals smelter to treat process wastewater for thallium, lead and antimony removal. Installation of this treatment system has allowed the site to meet its new National Pollutant Discharge Elimination System (NPDES) permit limits as well as become a zero-liquid discharging facility. Zero-liquid discharge is achieved by routing the permeate to the cooling tower for makeup instead of adding fresh water.

Prior to installing the HERO™ process, a number of alternative technologies were investigated, including evaporation, ion exchange and several proprietary processes. The specialized HERO™ process described in this paper was found to be the most cost-effective technology and was subsequently pilot-tested. Conventional RO systems were not considered due to the extremely fouling nature of the wastewater.

The HERO™ process pretreats water with a special ion exchange resin to remove hardness and metals. The water is then degassed to remove the alkalinity. After the hardness and alkalinity are removed, the pH of the water is increased above 9 and fed to a reverse osmosis unit. In conventional reverse osmosis systems, membranes foul with scale (divalent cation precipitates such as hardness, especially at high pH), organics (oil and hydrocarbons) and microbiological growth. The HERO™ process eliminates these problems by removing the divalent cations, and then operating the RO at a high pH where oils are emulsified and biological growth does not occur.

A performance test of the full-scale HERO™ process at the smelter was recently completed. This test was conducted at conditions that met or exceeded the original design specifications. The average HERO™ feed rate was 90 gpm, the First Pass RO recovery was 93.5 percent and the Second Pass RO recovery was 95 percent. The total reject flow was 10 gpm (5.8 gpm for the First Pass RO and 4.2 gpm for the Second Pass RO) and the final product water flow was 80 gpm. The feed water during the test contained a total dissolved solids concentration of 4,400 mg/L, which is typical for this facility.

Table 1 shows a comparison between the design and performance test conditions, as well as a comparison between the HERO™ product water (permeate) quality and the new NPDES permit limits. Also shown for comparison is the water quality that is currently used for cooling tower makeup. The HERO™ product water quality met all NPDES limits, including those for thallium, lead and antimony, and was found to be significantly better than water used for cooling tower makeup.

Hydrometrics, Inc., 2727 Airport Road, Helena, MT 59601. Tel: 406-443-4150; Fax: 406-443-0760.