Membrane Separation Technologies – Techniques for Minimising Wastes & Effluents

Many industrial processes generate effluent streams that contain a mixture of substances. To reduce the cost of processing these streams, operators can remove the substances that they contain using a range of membrane separation technologies.Using separation technologies as part of the production process – for instance as a method of cleaning continuously re-circulating process water – can prove more cost-effective than using a larger effluent treatment plant. It is an approach that usually requires less energy and: is less capital-intensive in terms of plant; allows the recovery and reuse of individual substances, many of which may be valuable; retains water of an appropriate purity within the process so that it can be recycled.This guide describes proven technologies and techniques for the cost-effective separation of four types of dissolved substance from liquid streams:

heavy metals;
anions (metals and non-metals);
organics;
water in organics.

It also considers the separation of gases and liquids.

Environmental best practice

This guide is intended to help companies decide whether separation technology can be used to minimise their waste streams and, if so, to indicate the most appropriate technologies available.

For each of the technologies examined, this guide first discusses the general principles of the technology, its typical applications, main advantages and disadvantages and approximate costs.

It then considers issues such as the technology’s efficiency and limitations, scale of operation, the main outputs and any ancillary plant required.

The choice of technology in any situation will depend on the specific nature of the streams involved. However, this guide offers guidelines as to the likely suitability of each technology for certain tasks, basing its conclusions on that technology’s general characteristics.

In some cases, a technology is applicable to the removal of a whole range of substances. Reverse osmosis and nanofiltration, two membrane separation technologies, can, for instance, be used to remove dissolved heavy metals, anions, organics and water in organics. The same is true for evaporation.

In other instances, a technology has a much narrower application. Adsorption and air stripping are only used in the separation of dissolved organics, for example. Some technologies fall between these two extremes.

The main application of the various electrical technologies, for instance, is in the removal of dissolved heavy metals and anions, and the same is true for ion exchange and precipitation techniques.

In the separation of gases from liquids, demisters and electrostatic precipitation techniques are used to remove mists in gas streams, while defoaming technologies and separation vessels are used where bubbles of gas occur in a liquid and also for treating two-phase mixtures.