TIBTECH- DECEMBER 1990 [Vol. 8] 337

Biofilms, biofouling and biocorrosion

Biofilms form on surfaces of solid materials. A biofilm is essentially a community of microorganisms that is embedded in a matrix of polymer produced by some or all of these microorganisms. Such biofilms lead to problems related to fouling and corrosion of certain industrial ma- terials. This recent Workshop* aimed to provide a comprehensive review of state-of-the-art techniques for de- tection, treatment and prevention of biofouling and biocorrosion in drinkable and purified water sys- tems, heat exchangers and other industrial water circuits.

Kevin Marshall (University of New South Wales, Sydney, Australia) introduced the concepts of biofoul- irig and biocorrosion by discussing the role of bacterial adhesion and growth in biofilm formation on sur- faces exposed to flowing water. Possible strategies for avoiding bio- film development by preventing bacterial adhesion were considered.

In terms of biofouling problems, Hans-Curt Flemming discussed bio- film formation in highly purified water systems. The apparent an- omaly of active bacterial growth in waters with extremely low levels of organic carbon can be explained by the accumulation of sufficient organic compounds at surfaces under flowing conditions to allow adherent bacteria to grow and to form biofilms. He emphasized the fact that biofilms are not static en- tities, and their stability may be increased or decreased by a wide range of operating conditions. Early detection and cleaning of systems was deemed essential, although bio-

*The First International Workshop on Industrial Biofouling and Biocorrosion, organized by Hans-Curt Flemming (Uni- versity of Stuttgart, FRG) and Gill Geesey (California State University, Long Beach, CA, USA), was held at the University of Stuttgart, 13-14 September 1990.

film re-growth following most clean- ing regimes is often faster than the initial biofilm development.

Problems specific to reverse os- mosis (RO) membranes were consid- ered by Harry Ridgway (Orange County Water District, CA, USA). Despite the use of cross-flow systems to provide a high degree of hydro- dynamic shear, RO membranes foul to varying extents resulting in a severely reduced flux across the membranes. In coastal Orange County, RO filtration is employed to reclaim wastewaters and the finished product, at drinking-water standard, is re-injected into aquifers to prevent seawater intrusion. Myco- bacteria dominate as the primary fouling organisms, probably because they are hydrophobic, very resistant to biocides and nutritionally ver- satile: Sometimes, biodegradation of the semi-permeable RO membrane material is observed. A degree of control of membrane fouling was obtained by pre-treatment of the water (filtering plus biocide), im- proved cleaning of the membranes, and changing the type of membrane employed.

The current status of biocides in controlling biofouling in water systems was assessed by Mark Le Chevallier (American Water Works Service Co., IL, USA). He noted that biofilms resulted in a loss of residual chlorine, increased bacterial counts and hydraulic roughness, reduced dissolved oxygen, and occasional alterations in taste, odour and colour of waters. Chlorination tends to induce biofilm sloughing leading to higher initial bacterial counts in the water phase. Proposed factors affecting the appearance of bac- terial populations with increased resistance to biocides following treatment include selection of in- herently more resistant organisms, attachment to surfaces, capsule for- mation, cell aggregation and initial

~) 1990, Elsevier Science Publishers Ltd (UK) 0167 - 9430/90/$2.00

depth of biofilm. Although hypo- chlorous acid is more effective for bacteria in the aqueous phase, chlor- amines are superior against biofilms. A strategy for control of biofilms requires attention to good engineer- ing practice, an understanding of the water chemistry and its effect on biofilm development, and modelling of biocide performance within dif- fusion-limited biofilms. Some of these points were elaborated on by Marc Mittleman (University of Knoxville, TN, USA) in considering plant design in preventing biofoul- ing in high purity (18 MOhm cm -1) waters. A wide variety of bacteria have been isolated from biofilms in high-purity and distilled waters. New techniques described for de- tecting the early phases of biofilm formation include attenuated total reflectance-Fourier transform infra- red and surface enhanced Raman spectroscopy, photon scanning tunnelling microscopy, and bio- luminescence markers in bacteria combined with the use of a light pipe and photomultiplier system.

Bill Characklis (Montana State University, MO, USA) proposed a rational approach to solving biofoul- ing problems based on such funda- mental parameters as rate and extent of transport, and net absorption of soluble and particulate materials to the wetted substratum, microbial reactions at the substratum or within the biofilm, and net detachment of cells or sloughing ofbiofilm from the wetted substratum. A rational ap- proach is deemed necessary to deter- mine the effects of changing critical variables in any given system, to compare biofouling processes in dif- ferent systems, and to develop stan- dard practices related to design, operation and control.

The topic of biocorrosion was introduced by Gill Geesey (Cali- fornia State University, CA, USA), who emphasized the role of different populations and consortia of biofilm bacteria in creating conditions con- ducive to biocorrosion. Possible mechanisms of microbially induced corrosion include the formation of differential aeration cells, cathodic or anodic depolarization, hydrogen embrittlement, metal reduction, metal oxidation and precipitation, acid production, and the sequestering of metal ions by polyanionic poly- mers excreted by biofilm bacteria.

338 TIBTECH - DECEMBER 1990 [Vol. 8]

Interestingly, some biological pol- ymers bind copper in the Cu ÷ form, some in the Cu 2÷ form, and some fail to bind copper ions. Most of the evidence for microbial in- volvement in metal corrosion is cir- cumstantial, but Geesey believes that recent understanding of microbial biofilm processes should make it easier to relate specific biochemical activities of the organisms to the electrochemical reactions leading to corrosion. Bob Tatnell (DuPont de Nemours & Co., DE, USA) provided a series of case histories of bio- corrosion in various metals in dif- ferent industrial situations and under different environmental conditions. He noted that, although precise figures are not available, biological factors probably play a role in about half of the metal-corrosion events reported worldwide.

Allan Hamilton (University of Aberdeen, UK) noted that the sul- phate-reducing bacteria (SRB) are widely implicated in biocorrosion in natural and industrial environments.

These anaerobic bacteria function efficiently at the biofilm-substratum interface where oxygen availability is limited both by slow diffusion through the biofilm matrix and by bacterial utilization closer to the biofilm-water interface. In the ab- sence of oxygen, protons may serve as electron acceptors at the cathode leading to the production of mol- ecular hydrogen, which the SRB subsequently oxidize during sul- phate reduction, giving rise to sul- phide and the potential for metal sulphide corrosion products. The reactions are very complex, as SRB- induced corrosion is stimulated by oxygen and involves consortium relationships between the SRB and other microorganisms; points em- phasized by both Allan Hamilton and Bill Costerton (University of Calgary, Alberta, Canada). Costerton described a variety of newly devel- oped techniques for the detection, monitoring and control of biocor- rosion. Some of the techniques have found ready acceptance (e.g. the

Robbins device for monitoring bio- film and biocorrosion development), whereas others remain to be more thoroughly tested (e.g. a hydrogen- ase test for the presence of SRB).

The fact that this workshop was held in Germany indicates an in- creasing awareness in Europe, in recent years, of the importance of biofouling and biocorrosion. Work- shops similar in scope have been held during the last five years by Water Micro Associates in the USA. Problems resulting from biofilm formation in flowing water systems are of worldwide occurrence, and the costs resulting from biofouling and biocorrosion are enormous. It will be interesting to note develop- ments over the next few years as more engineers and microbiologists realize the importance to industry of these microbial processes.

KEVIN MARSHALL

School of Microbiology, The University of New South Wales, Kensington, Sydney, Australia.

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Separations for biotechnology Robust and efficient separation pro- cesses are essential for the manu- facture of biochemical products. Over the past decade, the demand for improved processes to purify proteins has increased dramatically following the successful molecular cloning and expression of recom- binant proteins. In response to this demand, research activities in down- stream processing have increased, focusing on purification of high mol- ecular-weight macromolecules (pri- marily proteins), developing a better understanding of individual unit operations; integration of process strategies for purification, and devel-

* The Second International Conference on Separations for Biotechnology was held at the University of Reading, UK, 10-13 September 1990. The proceedings of this conference have been published in Separations for Biotechnology, edited by D. L. Pyle, Elsevier Applied Science Press, London, 1990.

oping new and innovative methods for purification. The papers and posters presented at the recent con- ference on Separations for Biotech- nology* clearly reflect these current trends. The plenary speaker was Prof. A. S. Michaels (Chestnut Hill, MA, USA) who placed the state and direction of research in downstream processing in perspective. Achieving reproducible high standards of prod- uct purity and quality must remain the goal for those in the business of biochemical process manufacturing. Today's goal is to make high purity therapeutic compounds. In the future, a new challenge will emerge: to extend our capability to make a wider range of materials for 'special- ity' as well as 'commodity ' markets. The technical challenge is to main- tain high product standards while adapting new techniques to larger- scale operation; competitiveness will depend increasingly on efficient im- plementation of process technology.

(~) 1990, Elsevier Science Publishers Ltd (UK) 0167 - 9430/90/$2.00

The conference organization fol- lowed closely the sequence of unit operations in biochemical processes.

Cell harvesting Since many biochemical products

of commercial interest are intra- cellular, the initial papers dealt with methods of cell harvesting and cell disruption. The two common ap- proaches to cell harvesting are centrifugation and filtration. P. N. Ward (ICI Biological Products, Billingham, UK) described work clone in collaboration with M. Hoare (University College London, UK) on elucidating the dependence of de- watering performance in a scroll decanter centrifuge on rheological properties of the suspension. An understanding of the chemistry, as well as the physics of the problem is essential for rational optimization of performance. Viscoelasticity and net- work property characterization were shown to provide a basis for the design of a de-watering operation. The viscoelasticity of borax-floccu- lated cells was characterized as both shear moduli and loss angle as a