AWWA M38:1995 pdf download.Electrodialysis and Electrodialysis Reversal
Spacers create flow paths within the membrane stack for demineralized and concentrated water. Different models and sizes of spacers satisfy specific design applications. The main difference in spacer models is the number of flow paths, which determines water velocity across the membrane stack and contact time of the source water with the membrane. Since water velocity is responsible for the degree of mixing and the amount of desalting that occurs across membranes, velocity is an important design parameter for spacer choice. Because the same spacers are used for both demineralized and concentrated water in EDR systems, the flow rates of both these streams should be equalized to prevent high differential pressures across the membranes (Siwak 1993).
Spacer design and flow velocity are also important parameters in determining the maximum amount of desalting or the current density (discussed later) at which a given system can operate.
A metal electrode at each end of the membrane stack conducts direct current (DC) electric current into the stack. The electrode is usually made of titanium and plated with platinum. Its life span is dependent on the ionic composition of the source water and the amperage applied to the electrode. Large amounts of chlorides in the source water and high amperages reduce electrode life. Polarity reversal (as in EDR) also results in significantly shorter electrode lifetimes than for nonreversing systems.
Normally located at each end of the membrane stack, electrode compartments consist of an electrode, an electrode water-flow spacer, and a heavy cation membrane. The electrode spacer is thicker than a normal spacer, which increases water velocity to prevent scaling. This spacer also prevents the electrode waste from entering the main flow paths of the stack.
Effluent from these compartments may contain oxygen, hydrogen, and chlorine gas. Concentrate from the electrode stream is sent to a degasifier to remove and safely dispose of any reaction gases.AWWA M38 pdf download.