Zero Liquid Discharge of Brackish Water

Water treatment by desalination is increasing worldwide to meet growing water demands, preserve the value of reclaimed water, provide drought proof supplies, and protect public health and aquatic ecosystems from emerging contaminants. Implementing desalination, however, can be constrained by the challenge of managing the concentrate byproduct generated when water is desalinated by membrane processes.

The options for managing concentrate are:

• Direct discharge

• Deep well injection

• Discharge to a wastewater treatment facility

• Zero liquid discharge

Discharge options that fail to remove salts and contaminants from the water cycle increasingly are considered unsustainable. In zero liquid discharge (ZLD) desalination, concentrate is treated to produce product water and there is no discharge of liquid waste from the process. Currently ZLD desalination is applied primarily to industrial waste streams or power plant cooling water. The established technologies for ZLD are thermal desalination and evaporation ponds. Each has disadvantages that can make its use prohibitively expensive for drinking water applications. Research was conducted to evaluate ZLD desalination of brackish water using a new electrodialysis technology referred to as Electro Dialysis Metathesis (EDM). The goal of this work was to reduce the costs and energy requirements for ZLD desalination.

The ZLD treatment approach is illustrated in the figure. Concentrate from reverse osmosis (RO) is desalinated with EDM to generate a product stream and two EDM concentrate streams. Concentrate from EDM is treated with thermal desalination, in this case a mechanical vapor compression crystallizer. The water sources evaluated in this project are in Florida, where the wet climate precludes use of evaporation ponds. In arid climates, evaporation ponds could be an economical alternative to thermal desalination as the final ZLD step. The final product water is a blend of RO permeate, EDM diluate, and crystallizer distillate.

Electrodialysis has been used for decades to remove ions from water. In drinking water applications, the objective is to produce desalinated water for potable use. In the chemical and food industries, electrodialysis has been used to concentrate solutions to recover valuable salts or brine products and to produce chemical products.

A conventional electrodialysis stack comprises alternating cation and anion selective membranes between a cathode and anode. The driving force is the electric potential gradient between the anode and cathode. Anions are drawn toward the positively charged anode, and cations are drawn toward the negatively charged cathode. Cations pass through the negatively charged cation exchange membrane and are rejected by the positively charged anion exchange membrane. Similarly, anions pass through the anion exchange membrane and are rejected by the cation exchange membrane. As a result, water flowing through alternate compartments is depleted of ions and concentrated with ions.

In terminology typically used in the electrodialysis industry, the solution being depleted of ions is referred to as diluate, and the solution receiving ions is referred to as concentrate. The basic unit of electrodialysis is a cell pair comprising a diluate compartment, a concentrate compartment, an anion exchange membrane, and a cation exchange membrane. A typical electrodialysis stack contains hundreds of cell pairs.

EDM was evaluated for treatment of RO concentrate in this research. The primary difference between EDM and electrodialysis is the use of four solution compartments and four membranes, rather than two of each in the repeating unit. The EDM membrane configuration is shown in. The repeating unit in EDM comprises one diluate compartment, two concentrate compartments, one NaCl solution compartment, one ordinary anion exchange (A), one ordinary cation exchange (C), one monovalent selective anion exchange (SA), and one monovalent selective cation (SA). This unique configuration is designed to separate EDM concentrate into two streams of highly soluble salts: one containing sodium with anions and the other containing chloride with cations.

This characteristic of EDMprovides a significant advantage in treating RO concentrate because the membrane-fouling potentials of typical scalants such as CaSO4 and CaCO3 do not increase with recovery, as is the case with RO, nanofiltration, and other forms of electrodialysis, such as electrodialysis reversal (EDR).

Results & Discussion

EDM was pilot tested with concentrate samples from existing desalination plants. The total dissolved solids range of the concentrate samples was 3,000 to 16,000 milligrams per liter, and the samples were supersaturated with salts that would foul RO or electrodialysis membrane systems if either were used for further treatment.

Water quality and EDM performance parameters were monitored to evaluate: 1) the effectiveness of EDM in separating the concentrate into two streams of highly soluble salts, 2) the rate of product water recovery by EDM, and 3) energy requirements for desalination with EDM.