227 11 The General Model of Protection against Biofouling Until recently, the development of new methods of protection against biofouling has proceeded mostly on an empirical basis. With the development of the basic quanti- tative theory of colonization (Chapter 7) and the concept of anticolonization protec- tion (see Chapter 10), there appears to be a way to analyze the theoretical foundations of protection against biofouling. Let us consider the mathematical model of accumulation (see Chapter 7). On the basis of Equation 7.7, the biomass B of biofouling on a hard surface can be expressed as follows: , (11.1) where α is the coefficient of settlement selectivity, K a is the adhesion coefficient, b is the biomass of dispersal forms in the plankton per unit volume, is the distribution function of the flow rate within the boundary layer, V is the speed of settlement of dispersal forms, and t is time. It should be borne in mind that α denotes the fraction of propagules settling from the plankton onto the hard surface in unit time. The coefficient K a describes the fraction of propagules that adhere to the surface, out of the total number of organisms coming in contact with the surface during time t . The function determines the flow rate within the boundary layer at a point with the coordinates ( x, y ), with U ∞ being the velocity of free (outer) current. Thus, Equation 11.1 describes the fouling of any surface irrespective of its shape and dimensions. The necessary and sufficient conditions of complete protection from biofouling can be found by putting to zero the left part of Equation 11.1, which describes the fouling biomass. Let us assume that B = 0, i.e., fouling is completely absent. From Equation 11.1, this can be the case, in particular, when any of the following condi- tions are met: 1. . 2. . 3. . 4. . 5. . BKbfUxyVt a =+ ∞ α (( ,,) ) fU xy(,,) ∞ fU xy(,,) ∞ α=0 K a = 0 b = 0 fU xy V(,,) , ∞ ==00 VfUxyV=− > ∞ (,,), 0 1419_C11.fm Page 227 Tuesday, November 25, 2003 4:53 PM Copyright © 2004 CRC Press, LLC 228 Marine Biofouling: Colonization Processes and Defenses The case t = 0 is trivial, since it means that the hard surface (object) has not been immersed in water. Other apparent solutions of Equation 11.1 at B = 0 represent combinations of those shown above and may be analyzed by the reader. To suppress the fouling of a hard surface, including that of an engineering object, it is sufficient to meet any of the above conditions. Let us analyze in greater detail the consequences of the five solutions of Equation 11.1 at B = 0. They can be interpreted as general directions of prevention of bio- fouling, protection from it, and extermination of the existing biofouling. Null solu- tions may exist in any of the three situations: prevention of biofouling; suppression of biofouling, or protection proper; and elimination of biofouling that has already formed. Taking these three situations into account, one may consider the theoretical basis of biofouling control as comprising 15 directions. Each of them does not represent a particular method or a technique of protection but, on the contrary, may include a variety of methods. An attempt at formal analysis undertaken here allows one to offer a general model of antifouling protection. It describes what happens to the dispersal forms during the prevention, protection, and elimination of biofouling, leaving room for creative imagination as to how a particular result can be achieved. It has not been been possible in all cases to find published examples of a particular direction of protection. This shows that the proposed model, apart from the ones that we know about, includes essentially new and unexpected approaches to the problem of protection, and therefore it has a certain prognostic value. Cases 1–3: α = 0. The methods are aimed at modifying or suppressing the behavioral (locomotor) reactions of the propagules (microorganisms, larvae of inver- tebrates, and spores of macroalgae) by creating such conditions around the protected surface under which they do not move toward the surface or do not settle on it. Suppression of movement toward the protected surface (case 1) means that some distant-acting measures have been taken to prevent the propagules from approaching the surface. The simplest examples are the distant biocidal action of chlorine in the case of volume-based protection (Yakubenko, 1990) and the immobilizing action of carbon dioxide (Terent’ev et al., 1966). Besides purely chemical protection, physical protection or a combination of the two methods are possible. For example, the barrier may exist in the form of a screen of air bubbles (Rasmussen, 1969a) or by air bubbles combined with underwater spraying of a potent toxin, such as bis (tribu- tyltin) oxide, and a surfactant mixed with kerosene (Gurevich et al., 1989). These substances can be applied onto the submerged part of the hull, killing the dispersal forms and preventing their settlement and attachment. Suppression of settlement with the use of biocides is a fairly common phenomenon in epibiotic relations (see Section 10.1). Case 2 implies active rather than passive protection and characterizes the effect (or effects) aimed at suppressing movement toward the surface. The best example of this type is the use of repellents (see Section 10.3). Case 3 , in my opinion, should be interpreted as movement away from the surface after immediate contact with it or after settlement. This may happen in the natural course of events, when an improper substrate is rejected in the substrate selection phase (see Chapter 4). By analogy with contact-action settlement inductors, one may assume the existence of materials that would suppress settlement after contact 1419_C11.fm Page 228 Tuesday, November 25, 2003 4:53 PM Copyright © 2004 CRC Press, LLC The General Model of Protection against Biofouling 229 chemoreception by the dispersal forms. These hypothetical materials would, in the final reckoning, prevent the substrate selection and settlement and thus protect the surface from colonization. They can be assigned to neither repellents nor antiadhe- sives. Even though such materials have not been described yet, their existence can be predicted by the general model of antifouling protection and is theoretically quite probable. Cases 4–6: . Measures are taken to prevent or suppress adhesion of dispersal forms to the surface. Adherence can be prevented (case 4), for example, if the surface is insulated from dispersal forms. It has been proposed (Duddrige and Kent, 1985) that the inner walls of sea water conduits can be protected mechanically by means of disposable insulating lining, on which biofouling would be accumulated. Suppression of adherence (case 5) means that either the surface or the integu- ments of the dispersal forms are not sticky. Industrial ship coatings with low adhesion (the so-called fouling-release coatings) are well known. Another possibility would be to create a rapid flow of water around the protected object, with the current velocity exceeding 1 to 2 m/s. This would violate the main condition of biofouling (see Section 7.1): namely, the shear force and lift force combined would exceed the adhesion force. One of the ways to eliminate adhesion (case 6) involves removing the dispersal forms that have adhered to the surface. This can be done by a variety of methods, including mechanical cleaning. A common natural mechanism is peeling of old or dead parts of tegument, together with the foulers attached to them. This is observed in macroalgae and also in animals, in particular during molts (see Section 10.1). Cases 7–9: b = 0. Measures are taken to prevent the dispersal forms from appearing near the surface, to eliminate them, or to suppress their development on the surface. Equally, one may consider the removal of the surface from the area where dispersal forms are present. A number of examples of fouling prevention (case 7) are given below. In order to protect the piping system of Azovstal Iron and Steel Works, E. P. Turpaeva (1987b) proposed using sea water from which macroorganisms have been removed, for example, by filtration. Contact of the protected surface with water containing dis- persal forms of foulers can be avoided by prevention measures, for example, by visits of vessels to freshwater ports, such as the port of St. Petersburg (Zevina, 1990). Suppression of biofouling by creating a propagule-free water layer near the protected surface is theoretically possible. This could be accomplished by drawing off water together with the propagules from the boundary layer and ejecting it beyond the vessel’s contours. Besides antifouling protection, this would increase the speed of the vessel. In fact, such a method of protection is unlikely to be realized in the near future, owing to the high energy demands. Direct elimination of dispersal forms from the protected surface in such a way that they would simply disappear ( b = 0), for example, dissolve, cannot yet be illustrated by appropriate examples. Cases 10–12: . Water flow around the protected object and the movement of dispersal forms are simultaneously prevented or suppressed. The difficulty of finding methods of protection that comply with the above conditions is that the water flow velocity and the movement velocity of the dispersal K a = 0 fU xy V(,,) , ∞ ==00 1419_C11.fm Page 229 Tuesday, November 25, 2003 4:53 PM Copyright © 2004 CRC Press, LLC 230 Marine Biofouling: Colonization Processes and Defenses forms must be simultaneously equal to zero. In principle, this can be achieved by freezing the water around the protected object, so that the vessel would move in an envelope of ice. Incidentally, its outer surface would remain constantly smooth, reducing fuel consumption and possibly compensating for part of the energy spent on creating the ice envelope. Cases 13–15: , V > 0. In my opinion, we are dealing here with prevention or suppression of the movement of propagules relative to the surface, probably within the boundary layer, at a non-zero flow velocity. At present, it is difficult to propose how this method could be realized. Under the above conditions, the movement of propagules must be directed against the water flow. Moreover, the speed of their locomotion must vary at different sites on the surface, while at the same time being exactly equal to the flow velocity within the boundary layer. In other words, the propagules in this case are not carried in any direction within the boundary layer despite the water flow in it. Although this looks like an unsolvable controversy, such paradoxes often give rise to novel creative ideas (Altshuller, 1986). Further analysis of the biofouling processes described by the colonization models (see Chapter 7) allows one to propose a total of 15 independent directions of pre- vention and elimination of biofouling. Each of these directions may be realized in a variety of methods and specific techniques. This indicates vast prospects of further theoretical and practical work. In addition, this demonstrates once more the value of the general approach that I have followed in this book: namely, consideration of biofouling, i.e., the process of colonization of hard surfaces by hydrobionts, as a regular sequence of more elementary events. VfUxy=− ∞ (,,) 1419_C11.fm Page 230 Tuesday, November 25, 2003 4:53 PM Copyright © 2004 CRC Press, LLC . basis. With the development of the basic quanti- tative theory of colonization (Chapter 7) and the concept of anticolonization protec- tion (see Chapter 10), there appears to be a way to analyze. Tuesday, November 25, 2003 4:53 PM Copyright © 2004 CRC Press, LLC 228 Marine Biofouling: Colonization Processes and Defenses The case t = 0 is trivial, since it means that the hard. velocity and the movement velocity of the dispersal K a = 0 fU xy V(,,) , ∞ ==00 1419_C11.fm Page 229 Tuesday, November 25, 2003 4:53 PM Copyright © 2004 CRC Press, LLC 230 Marine Biofouling: Colonization