Membrane and Desalination Technologies - TCE Moodle Website

540 J.P. Chen et al.
construction so that the waste heat from the facilities can be properly handled and the water
pollution is minimized. For the newly built distillation systems, the thermal discharge is not a
major concern because brine water with high temperatures is cooled by the raw feed water.
2.7.2. Materials for Construction
The corrosive nature of high-temperature brines, acid pretreatments, and chemical scaling
can cause plant failure. Presently, the only acceptable construction materials for wetted
surfaces in high-temperature systems are an austenitic stainless steel. Anodized aluminum
and many thermoplastic materials are acceptable for use in low-temperature systems.
2.7.3. Energy for Operation
Operation of distillation is mainly energy driven. The crude oil prices have increased by
almost 100% in the last 2 years. It is, therefore, important to consider the energy cost for
operations and a sound forecast of the same is essential.
2.7.4. Characteristics of Raw Water
The important parameters of raw water are TDS, salt contents, organic contents (TOC),
and temperature. The availability of raw water is also important.
2.7.5. Final Disposal of Waste Brine
The waste brine has high temperatures and TDS. Thus, it must be properly treated before it
is discharged to nearby waters. Cooling and IX are recommended for its treatment.
3. ELECTRODIALYSIS
3.1. Introduction
ED was commercially introduced in the early 1960s, about 10 years before the introduction
of RO. The development of ED provides a cost-effective technology to remove salt from
seawater. ED is also used in water and wastewater treatment, food processing, and chemical
and pharmaceutical manufacturing.
In ED, ionic components are removed from aqueous solutions through the IX membranes
using the driving force of an electric field. The membrane is selective and semipermeable and
it only allows the passage of either the anions or the cations. The separation is because of the
nature of charged substances rather than the difference in their sizes. In seawater desalination,
the dissolution of salt leads to the formation of positively charged cations such as Na + ,Ca 2+ ,
and Mg 2+ and negatively charged anions such as Cl - . The cations and the anions are attracted
to the cathode and the anode, respectively, which are subsequently removed. As a result, the
water after treatment carries less ionic substances. A typical electrodialysis plant operates at
50–90% conversion and this conversion value is determined by the brine recirculation rate.
An ED system can operate over a wide pH range (pH 1.0–13.0) as well as a wide temperature
range (up to 43 C). The cost for O&M of ED (EDR) is slightly higher than that for RO.
The drawback of this process is that the surfaces of membranes can be easily fouled. Thus,
a modified process of EDR was developed. By intermittently reversing the polarity, the