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Reverse Osmosis – Unglamorous but Vital to Water Purification!

Posted on September 29th, 2016 by in New Materials & Applications

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Admittedly basic chemical processes are not among the most glamorous topics of discussion. To most these processes have been around for a long time and are considered roads much traveled. In contrast nanotechnology, bio-based materials, solar planes, 3D printing and others are new and exciting. Yet, some chemical processes like reverse osmosis (RO), used for water purification among its other applications, are vital to mankind. RO allows installation of relatively inexpensive water treatment systems in remote places thus providing clean drinking water to people.

Chemical processes including reverse osmosis are researched continuously to improve productivity, to decrease cost, to reduce environmental emissions and to minimize their carbon footprints.  Here is a report about progress in the management of the biggest operating problem of RO which is biofouling caused by microorganisms.

What is Reverse Osmosis?

The phenomenon of osmosis takes place when pure water flows from a dilute salt solution through a membrane into a higher concentration salt solution (Figure 1).  To illustrate the concept a semi-permeable membrane is placed between two legs of a U-shape tube. “Semi-permeable” is defined as the membrane being permeable to some material but impermeable to others.  Let’s assume the membrane is permeable to water, but not to salt.  If salt solution is poured into one leg of the U-shape tube and tap water into the other, osmosis will take place. The membrane will allow water to permeate through it to either side. But salt cannot pass through the membrane thus resulting in the dilution of the salt solution[1].

Reverse Osmosis

 

Figure 1 Schematic of Osmosis and Reverse Osmosis Mechanisms (Source: www.kandrwaterservice.com)

If a force is applied to a column of seawater (Figure 1), the direction of water flow through the membrane can be reversed.  Water actually flows from the seawater leg into the freshwater leg of the U-shape tube. This is the basis of the term reverse osmosis. This reversed flow produces pure water from the salty seawater, since the membrane is not permeable to salt.  One important application of RO is indeed in the desalination of seawater.

Commercial membranes usually consist of three main layers (Figure 2).  The top layer (permeable) in contact with the feed water is usually a thin (0.2 µm) aromatic polyamide film.  Two layers of materials support the thin film layer.  Immediately beneath the thin film is a polysulfon layer (45 µm thickness) followed by a base non-woven fabric (100 µm thickness) that imparts strength to the entire laminate.

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Figure 2 Structure of a typical RO membrane (Source: Umar Farooq, NOMAC, umarfarooq@NOMAC.com)

RO can remove dissolved solids, color, organic contaminants, and nitrate from water.  Other applications include treatment of effluents from beverage industry; distillery spent wash, ground water treatment, recovery of phenol compounds, and reclamation of wastewater (e.g., car wash).

Biofouling

In a September 2016 paper on Chemical Engineering Progress, Arza and Kucera state: “The bane of existence for users of reverse osmosis (RO) membrane systems (Figure 1) is controlling membrane fouling from microorganisms”[2].  They analyzed 150 membranes after they had been in use for some time.  They found every one of the membranes had some degree of biofouling. Forty-nine membranes contained microbial colonies with densities greater than 105 CFU/cm2 (CFU stands for colony-forming unit).

The authors attributed the direct cause of the decline, in membranes’ performance, to the colonies. Biofouling was determined to be a contributing factor to the decline of the performance of the other 101 membranes.  In nearly every application the RO process is required to operate consistently and smoothly.  Of special importance are water treatment processes that operate in remote locations in the third world and elsewhere.  Any disruption because of biofouling could easily cut off some or an entire community from safe drinking water supply.

Figures 3 and 4 show a biofouled membrane and a scanning electron micrograph of that membrane’s surface.  Arza and Kucera defined biofouling as irreversible adhesion of microorganisms on a membrane and the extracellular polymers (biofilm) that they produce. The process of adhesion entails three steps:

  1. Bacterial adhesion, which can become irreversible in just hours, even without nutrients present
  2. Micro-colony formation
  3. Biofilm maturation, which protects the bacteria from biocides, flow shear, and predators.

Certain properties of membranes favor adhesion and biofilm formation including:

  1. Surface roughness (Figure 4) — the rougher the surface, the more adhesion
  2. Surface charge — the more neutral the charge, the more adhesion of bacteria

(negatively charged)

  1. Hydrophobicity – the more hydrophobic membrane surface, the more adhesion.

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Figure 3 Example of a membrane covered with a black biofoulant[2]

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Figure 4 The rough surface and hydrophobic properties of polyamide RO membrane favor bacterial adhesion[2]

Biofilm adhesion is also promoted by dissolved nutrients in the laminar boundary layer next to the membrane, which is called the concentration polarization layer. Because no convection occurs at the membrane surface, dissolved and suspended solids, including nutrients, build up.

Management and Prevention of Biofouling

There are different approaches to address biofouling[3-4].  The objectives of biofouling treatment methods include killing the microbes; removing microbes and dead microbial bodies (which can become food for new growth); preventing adhesion, propagation, and biofilm maturation; and removing nutrients that foster microbial growth.  These approaches are:

  1. Modification of membrane surface (from sandpaper roughness to paper smoothness).
  2. Modification of the bacterium and/or organic nutrient source (biological means of microorganism management).
  3. Disinfection, removal, or sterilization of the microbes (direct approach to exterminate the microorganism[2].

Techniques to modify membrane properties, such as roughness, charge, or hydrophilicity, include coating the membrane and using a membrane made of a different polymer to minimize bacterial adhesion. Antimicrobial nanoparticles, such as silver, titanium dioxide, and carbon nanotubes, incorporated into membranes can help to limit adhesion. [2]

Bacterium modification and disinfection reduce the concentration of viable microorganisms in the feed water flowing to a membrane. The three basic methods of bacterium modification and disinfection are physical, thermal, and chemical.

Physical disinfection techniques include ultraviolet (UV) radiation, membrane filtration (microfiltration [MF] and ultrafiltration [UF]), and sand filtration. These techniques either modify the bacterium itself to hinder reproduction (UV) or remove bacteria according to particle size (MF, UF, and sand filtration). These methods, however, can be capital-intensive and do little to address biofilm once it has formed.

When lifting the next glass of drinking water consider this: Not withstanding the significant progress made in the installation of water purification systems around the world, there are still nearly one billion people without access to clean drinking water.  Alas, 85% of the world population lives in the driest part of the climate and sadly annually 6-8 million people die from the consequences of disasters and water-related diseases (Source: www.unwater.org).  More work and installations are needed.

* This post has made extensive use of the excellent review article by Anne Arza and Jane Kucera published in the September 2016 issue of CEP Magazine, Minimize Biofouling of RO Membranes, www.aiche.org/resources/publications/cep/2016/september/minimize-biofouling-ro-membranes.

References:

  1. FILMTEC™ Membranes, How Reverse Osmosis Works, Form No. 609-02003-1004, Dow Chem Co, filmtec.com.
  2. aiche.org/resources/publications/cep/2016/september/minimize-biofouling-ro-membranes.
  3. Nguyen, F. A. Roddick and L. Fan, Biofouling of Water Treatment Membranes: A Review of the Underlying Causes, Monitoring Techniques and Control Measures, Membranes 2, pp804-840, 2012 and
  4. Walker, M. Cardenas, and G. Richinik, Debugging the Plant: Managing Reverse Osmosis Biofouling at a Groundwater Treatment Plant, Journal of American Water Association, March 2016)

 

All opinions shared in this post are the author’s own.

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