Paper Number: 1375

Application of Electrotechnology for Removal and Prevention of Reverse Osmosis Biofouling

Rodrigo F.V. Romo, M. Michael Pitts.
Zeta Corporation, Tucson, AZ.

Prepared for presentation at the 1998 AIChE Spring Meeting,
New Orleans, LA, March 8-12

Fouling Mitigation in Membrane Separations

Copyright ©1998 Zeta Corporation. All rights reserved.
UNPUBLISHED

January 2, 1998

AIChE shall not be responsible for statements or opinions contained
in papers or printed in its publications.

ABSTRACT

One of the major factors limiting the use of membrane separation systems is membrane fouling (Cooper 1980). Membrane fouling places a large economic restriction on membrane systems operation; it can lead to reduced performance, higher energy consumption, and eventually, failure to meet the regulatory standards by the deterioration of the product stream.

Successful results from the application of electrotechnology have been obtained in removal and prevention of redevelopment of biofouling from operating reverse osmosis equipment (RO). Field observations are supported by Robbis device laboratory studies.

INTRODUCTION

A patented ceramic electrode (the Zeta Rod`) uses electrostatic dispersion of mineral and organic colloids to prevent scaling, biofouling, and corrosion in different HVAC equipment. The principles of operation of this capacitor based system are explained in detail in other publications (Pitts, 1995). The purpose of this study was to apply the same technology to RO membranes and see if the anti-scaling and anti-biofouling effects observed in the HVAC systems would have the same effect on RO membranes. This was done both in the field in a bottling plant, and in a controlled environment in a microbiology lab. The results of these studies provide the basis for a new technological approach to prevention of membrane fouling in membrane separation processes.

Membrane fouling can be classified in three main groups:

1. Crystalline fouling, or mineral scaling - results from to the deposition of minerals due to their high concentration in the solution stream.

2. Particle and colloid fouling- relates to the deposition of clay, silt, particulate humic substances, debris, and silica.

3. Microbiological fouling, or "biofouling"- refers to the adhesion and accumulation of microorganisms, forming biofilms (Geesey et al., 1994).

Current techniques to prevent or reduce the rate of membrane fouling involve a set of pretreatment techniques (Zeman & Zydney, 1996, Geesey et al.,1994, Gutman, 1987, Kemmer, 1988, Stumm & Morgan, 1996) and complex processes varying from prefiltration units to water softeners in many different arrays and configurations. Other alternatives include the addition of chemical agents into the feed stream to act as dispersants to aid in the prevention of mineral scaling. Biocides are utilized in some applications to reduce the potential of biofouling.

All these techniques have proven to be inefficient, for it is still considered that periodic membrane cleaning is unavoidable for all membrane filtration equipment. Membrane cleaning requires the unit to be shut down for a chemical and/or a mechanical cleaning. Ironically, these procedures not only cause down time to the unit, and are labor intensive, in some instances they reduce the lifetime and efficiency of the membrane modules themselves due to their incompatibility with the chemical cleansers (Geesey et al., 1994).

TECHNICAL DISCUSSION

In order for biofouling to occur, there has to be an initial adhesion stage of the microorganisms and colloidal particles onto the membrane surface. Primary adhesion is a heterogeneous process, involving three chemical phases:

1. Semi solid phase: microorganisms; including variables such as species composition or microflora, nutrient status, hydrophobicity, surface charge, and extracellular polymeric substances.

2. Liquid phase: the fluid being filtered; with variables such as temperature, pH, dissolved organic and inorganic substances, viscosity, surface tension, and hydrodynamic parameters.

3. Solid phase: the membrane surface; influencing adhesion via chemical composition, surface charge, conditioning film, and biological affinity.

The surface charge is very likely to influence primary microbial adhesion. Most bacteria are slightly negatively charged in aqueous systems. The electrolyte concentration which also influences the electrostatic double layer, has been found to influence initial adhesion (Amjad, 1992). It is at this stage where biofouling should be controlled. By preventing bacteria and mineral deposits from forming or adhering onto the membrane surface, the initial stage of biofouling (which is prerequisite to the remainder of the process) will be eliminated.

Colloidal particles include mineral and biological particles found in all untreated water sources (Stumm & Morgan, 1996). Generally any particle with a radius smaller than 1.0 mm is considered a colloidal particle (Kemmer, 1988). Some organic and biological colloids which cause biofouling of membranes include different types of microorganisms, viruses, biocolloids, fibrils, humus colloids, and aggregates of exudates and macromolecular organic matter (Stumm & Morgan, 1996). It is this wide spectrum of biological materials that has made biofouling control by chemical additions such a difficult task. In part this is because of the tolerance of the different organisms to specific chemical compounds and because of the potential for damage to the membrane by chemical biocides and cleaning agents.

The fact that many organic and mineral colloidal particles posses a negative charge in aqueous environments causes them to repel one another, thus maintaining the stability of dispersion (Kemmer, 1988, Riddick, 1961). Electrochemical dispersion of colloidal particles has been studied for many years (Pitts, 1995, Riddick, 1961, Pitts, 1992, Elimelech et al., 1994). Several models such as the double layer theory, and the DLVO theory (named after Derjaguin, Landau, Verwey and Overbeek) attempt to explain the stability of colloids.

The double layer theory model (Figure 1) explains the repulsive forces between colloids. It is focused on the effect that the negatively charged colloids have over the positive ions in the bulk solution. The positive ions (also known as counter-ions) form a firmly attached layer around the surface of the colloid known as the Stern Layer. Additional positive ions attracted by the negatively charged colloidal particle face a repulsion force from the counter-ions attached on the Stern layer as well as by other counter-ions approaching the colloid. The density of this layer, known as the diffuse layer, gradually decreases with distance from the colloidal particle, until it reaches equilibrium with the rest of the ions in the solution. It is the diffuse layer surrounding the colloid that creates the most far-reaching repulsive force between colloidal particles; the higher the density of the diffuse layer, the greater the distance over which these repulsion forces are significant.

The primary function of the double layer is to neutralize the negatively charged colloid. This creates an electrokinetic potential between the surface of the colloid and any point in the bulk liquid. This potential (typically in the order of millivolts) is referred to as the surface potential.

The DLVO theory explains the stability of colloids by looking at two opposing forces acting upon colloidal particles. These two forces are the electrostatic repulsion explained by the double layer model and the Van der Waals forces which are weak attractive forces between particles.

This theory explains the tendency of colloids to agglomerate or to remain in a stable state of dispersion by combining the two forces (Figure 2). The net interaction effect, shown in Figure 2 by the curve labeled "Overall interaction energy" is the result of the subtraction of the attraction force from the repulsion force. If the net value is positive a repulsion effect will be observed. In this figure it can be observed how, by increasing the natural ZP of the colloidal particles (lines 1), the overall interaction energy can be given a higher dispersion effect.

The natural ZP typically found in colloids in aqueous suspension ranges from -14 mV to -30 mV. At negative charge values higher than -30mV enough repulsion occurs to favor a stable dispersion. The more negative the charge value, the stronger the dispersion effect (Figure 3). For values ranging between -45 mV and -70 mV stable dispersions are assured.

As the ZP approaches zero, the repulsion effect is lost and agglomeration begins. For values ranging from -10 mV to -15 mV a threshold of agglomeration is observed, and from values between -5 mV to +5 mV strong agglomeration occurs (Riddick, 1961).

Pitts (1995) explains how the capacitive charge of particles can be significantly influenced by elevated voltage fields created by a capacitor system. This results in a sharp increase in the surface charge of wetted surfaces and a reduction of the surface tension of the bulk solution.

FIELD TESTS

Soft Drink Bottling Plant, Tucson , AZ.

An 18" ceramic electrode and a 30 kV DC power supply were installed in the pump suction of the reverse osmosis unit at soft drink bottling plant in Tucson, AZ. The objective of the installation was to study the effect of the high voltage electrostatic field created by the electrode on RO membranes as a means to keep them free of biological deposits (bio fouling).

The RO unit is described as follows:

• Brand: OSMONICS
• Model: OSMO 43CHF-PR216KY/DLX
• Permeate Rate @77°F (25°C): 454 lt./min (120 gpm)
• Concentrate Rate: 135 lt./min (35 gpm)
• Pressure max / min: 2757 kPa (400 psi) (primary) / 1723 kPa (250 psi) final

The unit has a 3-2 series array, with each one of the vessels housing 5 FILMTEC membranes model: BW30-330.

The feed water to the unit is taken from the city of Tucson, AZ well system. It typically contains 300 parts per million (ppm) in total dissolved solids (TDS) (ranging from 250ppm to 500ppm). Pretreatment for the water involves sand filtration, carbon filtration and sodium softening.