Biofouling and Biocorrosion in Industrial Water Systems

Biofouling refers to the undesirable accumulation of a biotic deposit on a surface. The deposit may contain micro- and macroorganisms. The focus of this paper is microbial fouling biofilms which consist of an organic film composed of microorganisms embedded in a polymer matrix of their own making. The composite of microbial cells and EPS is termed a biofilm. The surface accumulation is often composed of significant quantities of inorganic materials. Complex fouling deposits, like those found in industrial environments, often consist of biofilms in intimate association with inorganic particles (1), crystalline precipitates or scale (2), and/or corrosion products (3). These complex deposits often form more rapidly and are more tightly bound than biofilm alone. These deposits are difficult to characterize at the microscale, i.e. at the cellular level.

Thus while biofilm processes, their kinetics, and their stoichiometry can be described in terms of fundamental, intensive variables, this paper must generally describe observations in terms of performance parameters (e.g. heat transfer resistance or fluid frictional resistance).

Biofouling is understood as the unwanted deposition and growth of living organisms on surfaces. In water treatment, in almost all cases it is caused by microorganisms They can contaminate the water, cover and block surfaces, host pathogens, and attack their support. Biofouling is a biofilm problem. It is defined operationally and refers to that extent of biofilm growth which interferes with the demands of water production or consumption process. Control of biofouling requires effective detection. Water samples give no valuable information to localize biofilms or to assess their extent. Thus, surfaces have to be investigated. Simple field methods are presented as well as laboratory techniques to evaluate the presence of biofilms. Sanitization of biofouling has to include the removal of biofilms rather than the killing of all cells. Sanitization strategies usually have to break the physical stability of the biofilm matrix by chemicals and to remove the biofouling layer by shear forces. Prevention of biofouling depends on a “clean system philosophy”: clean equipment, raw water and chemicals, early detection of biofilm formation and early cleaning measures. In most systems, biofilm development can only be prevented with high expenditures. Thus, the extent of biofilm accumulation has to be kept below the level of interference. The extent of biofilm growth is the result of the balance between growth and detachment and it is dependent on the nutrient situation, the temperature and the shear forces. The control of the nutrient situation in the fluid phase can be much more effective than the control of the cell numbers in order to reduce biofilm thickness. For the extent of biofilm accumulation as well as for cleaning measures, the stability of the biofilm matrix is the crucial factor. It depends on the shear forces, the nutrient situation, stabilizing filaments and particles and destabilizing chemicals and internal processes. “Coexistence with biofilms” requires a continuous awareness and strategies similar to those with which some marine animals prevent microbial colonization on their surface.

Reverse osmosis (RO) membranes used for the treatment of industrial and municipal process waters often become biologically fouled. The development of a microbial biofilm on the feedwater surfaces of RO membranes results in several adverse effects, including: (i) a gradual decline in the membrane water flux, (ii) an increase in the transmembrane operating pressure [i.e. an increase in the membrane delta-p], and (iii) a reduction in membrane mineral rejection. The RO membrane polymer itself may also be directly or indirectly biodegraded by the adherent microorganisms. Bacterial colonization of the permeate [i.e. product-water] surfaces of RO membranes can also occur. Although the extent of biofilm formation on the permeate surface is typically quite low compared to that on the feedwater surface, it can result in microbial contamination of downstream processes, which may be of great concern in ultra-pure water applications.

Over the last decade, the Orange County water District in southern California has conducted basic and applied research on the mechanism of bacterial adhesion and biofilm formation on RO membranes employed in advanced wastewater treatment. Although this research has been performed principally at Water Factory 21, a 0.66 m³/s wastewater reclamation facility incorporating cellulose acetate [CA] type RO membranes, the general conclusions should extrapolate well to most other RO applications. The primary results of the research are summarized below:

Several strategies are currently employed to prevent or control microbial biofilm formation in RO systems. These strategies include: (i) refinement of feedwater pretreatment, e.g. by improving prefiltration or disinfection, (ii) reducing the system operating pressure or recovery, (iii) increasing the frequency of membrane cleaning or improving the cleaning formulation, and (iv) changing the type of RO membrane. Additional research is needed to develop novel RO membrane polymers and module configurations having lower biofouling potentials.

Almost all disinfection theory and practice has dealt with inactivation of organisms suspended in a water column. Increasingly, microbiologists recognize that most organisms exist at solid-liquid interfaces. This paper considers disinfection of attached bacteria in potable water systems. Studies detail the mechanisms of disinfection resistance for attached organisms and how these organisms can exist in the presence of a disinfectant residual. Data are presented on the efficiency of various disinfectants for inactivation of biofilm bacteria. The influence of corrosion rates and water chemistries are also examined. A mechanistic model describing the interaction between various biocides and biofilm disinfectant demand shows that faster reacting biocides may be largely consumed before penetrating the biofilm and inactivating attached bacteria. Finally, a combined approach of disinfection and nutrient limitation is presented for optimum biofilm control.

Purified water used in product formulation, cleaning, and cooling operations has evolved from a process fluid to an essential raw material. Levels of biological and abiological contaminants which were undetectable five years ago are now regarded as unacceptable for many products and processes. While recent advances in analytical chemistry and fine particle physics have resulted in dramatic reductions in abiological contaminants, biological contamination of purified waters used in these critical industries remains a significant challenge to future product development. In the semiconductor industry, 1 pm and smaller line-width devices are subject to fatal defects as a result of bacteria present in “ultrapure water.” Active ingredient degradation and pyrogen contamination of heat labile biological and medical devices have been linked to purified water contamination. The extent of bacterial growth and biofilm formation in 18 MOhmcm waters is a function of materials selection, systems design, and preventative maintenance protocol Limiting essential growth factors — C, N, P, S, trace elements, light — in purified water systems is an important key to the control of biological contamination. Development of on-line, real time biofouling detection systems is currently underway. These evolving systems, which include supercritical fluid extraction of signature biomarkers and electrochemical impedance spectroscopy should provide insight into conditions pro-moting the development of fouling biofilms. Future applications of novel detection and treatment systems will include advanced life support systems such as those found on the space station.

Microorganisms growing on surfaces perform a variety of metabolic reactions, the products of which may promote the deterioration of the underlying substratum. These reactions refer to biocorrosion when the substratum consists of a metal or metal alloy. The effect of corrosive microbial products on an underlying metal surface is exacerbated when their concentrations are permitted to increase to high levels as may occur when the microorganisms grow on the surface in a biofilm. The biofilm contains exopolymers which impede the diffusion of solutes and gases between the surface and the bulk aqueous phase. The biofilm also permits the development of highly structured microbial communi-ties on the surface. The various species are able to collectively carry out metabolic activities that are potentially more corrosive to the underlying surface than could be achieved by a single species acting alone. These features of sessile microbial growth represent important prerequisites of biocorrosion.

Biocorrosion is a well-established, highly destructive phenomenon. Published cases link bacteria and fungi to accelerated corrosion of steel and cast iron, copper alloys, stainless steels, aluminium and nickel alloys. In addition, microorganisms can cause the destruction of plastics, stone, concrete and, of course, wood. While the exact figures are not available, biological factors probably play a role in about half of all the corrosion and materials degradation that occurs in the world.

This is a brief discussion of a variety of biocorrosion cases which occurred in industrial water systems. The focus will be on cast iron — where biocorrosion is com-monplace and usually not properly diagnosed — and on stainless steels, where corrosion failures are, for the most part, unexpected and very expensive.

Sulphate-reducing bacteria (SRB) are the microorganisms most widely implicated in cases of biocorrosion arising in a wide range of natural and industrial environments. Models for their mechanism of action have concentrated on cathodic stimulation of the electrochemical process by hydrogen oxidation and/or the production of iron sulphide corrosion products. Preventatitive measures are largely confined to cathodic protection by sacrificial anode or impressed current, or the use of biocides in contained systems. Although SRB are strictly anaerobic organisms, they can be responsible for extensive biocorrosion under aerobic environmental conditions.

Over recent years two features characteristic of biocorrosion have become domi-nant in our effort to first understand and then control the processes involved.

Bacteria grow preferentially in biofilms. This mode of growth allows these organisms to set up highly structured, physiologically cooperative communities because they remain in stable juxtaposition to the colonized surface and to each other. Planctonic bacteria cannot establish these highly organized consortia. As a consequence, the focused bacterial biodeterioration of insoluble substrates and microbially influenced corrosion of metals are both dependant on biofilm formation. Furthermore, the biodeterioration of complex soluble organic substrates requires more than one species of bacteria; biofilm formation is therefore also a sine qua non in these processes. We must understand biofilms if we are to understand and control biodeterioration.