4 November 2015

Purifying water with nano technology

By Dr Md Bazlul Mobin Siddique

Water is undoubtedly a necessary and important resource. However, not many of us are bothered about the source of our water as most of us may have filters installed. Our concern for clean and safe water supply may be raised if we are forced to consume unsafe water when the filter breaks down.

The World Health Organisation has in fact set clear guidelines for the safe management of drinking water. The guidelines, with the latest edition released in 2011, emphasises aspects such as microbial, chemical, radiological and aesthetic.

It also covers ensuring the safety of the entire water system, including activities in catchment areas, transmission infrastructure, treatment plants, storage reservoirs and distribution systems. This would be much appreciated in a place where water is abundant, but in many places where abundance of water is an issue, recycling can be a solution. The process of recycling water goes through many steps, and purification is one of them. A number of processes and materials have been used for water purification till now. Among them, membrane filtration technique is one of the more advanced and effective processes. Membrane filtration uses a physical barrier with extremely tiny pores to filter impurities or particles in liquids. The membrane used for this separation process depends on the size and shape of the particles. The advancement in this technology effectively helps with cleanup and desalination of brackish water, recycled water, and seawater. It also offers a solution for removing heavy metals, such as lead, arsenic and mercury as well as large organic or inorganic molecules, which pose a threat to human health.

However, the two main drawbacks that are withholding the implementation of this technology in large scale in developing countries are energy consumption, and membrane fouling – when the particles are deposited on the membrane, blocking the pores and thus reducing the efficiency of the membrane. Over the past few years, researchers were able to reduce the energy consumption to as low as about 3 kW/m3 of water by using membranes with better permanence, and effective energy management. But the energy consumption is still considered high, incurring huge costs. Membrane fouling in any of its forms, such as crystalline, organic, and colloid fouling, results in low filtration performance. This eventually leads to high operating and maintenance costs. Nanoparticles, the marvel of recent developments in the technological arena, have been adopted and modified to fight this problem.

There are different types of challenges involved in this scenario and various approaches have been taken to overcome these problems. One of the solutions is to use the right material to produce a membrane. Nevertheless, the modification of the membrane surface could also produce a promising result.

When a conventional polymer-based membrane can be useful and cheaper, modification of its surface by nano materials can yield a much better life of this membrane and increase efficiency. A range of materials are being used in nano form for this purpose. From carbon nanotube to gold nano particle, from silver to iron nano composites, all are offering interesting results in water purification. The usability of these particles depends on the materials they are made of. Carbon nano tubes allow water to pass through the hollow pipe at a faster rate due to certain chemical properties, yet it restricts the impurity sizes to as low as 10 nanometers. With slight modification and alignments of these tubes, many ions can be separated from water without a significant reduction of purification rate. On the other hand, as silver nano particles possess anti-microbial property it can penetrate the bacterial cell membrane and thus cause structural damage resulting in its death. Nano particle has a huge surface area due to its tiny size and this facilitates to establish contact with more particles as well as microbes. So, the efficiency increases.

Titanium oxide based nano particles, after being excited by UV light, show good capability in removing organic impurities from water which are otherwise problematic to remove by conventional methods. Therefore, they can help against organic fouling. Similarly, a recent investigation on iron oxide nano particles has shown a promising result of faster purification rate and better filtration of impurities, thus enhancing the anti-fouling property of the membrane.

Continual research will find many more effective, state of the art processes for water purification in future and thereby provide us with safe water. However, having said that, research and technical implementation are expensive. Despite all this, it is worthwhile to ensure that common water body are kept clean for our safety and to reduce the burden of purification.

Dr Md Bazlul Mobin Siddique is a lecturer with the Faculty of Engineering, Computing and Science at Swinburne University of Technology Sarawak Campus. He is contactable at msiddique@swinburne.edu.my