Biological fouling (biofouling) of seawater piping and heat exchange equipment impacts nearly all oceangoing vessels, resulting in poor performance and excessive maintenance costs. The current approach used by the U.S. Navy for the control of biofouling is electrolytic chlorination of seawater. A level of 200 ppb for 2 hours per day has been established as the minimum chlorine level required to efficiently control biofouling in heat exchangers and condensers, which is well above the Clean Water Act and Uniform National Discharge Standards limit of 100 ppb. In order to comply with these requirements and those that may be imposed in the future, it has become necessary to look to alternative methods for the prevention or control of biofouling.
The objective of this SERDP Exploratory Development (SEED) project was to investigate the use of biodegradable, naturally occurring substances as additives to eliminate or inhibit biological growth in shipboard heat exchange equipment.
Many alternative chemical treatments have been tested, and some have proven to be effective in the control of biofouling. However, in most cases, these materials either are metal-containing (tin, copper, zinc), non-biodegradable, or difficult and costly to use (peroxides, ozone). It is only recently that interest in natural organic products for antimicrobial uses has emerged. Natural plant isolates are generally non-toxic and biodegradable, and substances contained in them are often necessary for required metabolic functions. Three candidate plant extracts were examined for their effectiveness in preventing biofouling of designated metallic materials in seawater systems. Each had been reported to possess antimicrobial properties; however, none had been tested for this specific application. The testing of these additives for use in shipboard heat exchange systems was performed in annular reactors using continuously fed seawater and took into account many variables, including seasonal weather changes (ambient water temperature and quantity/type of sedimentation in influent), geographical location (seawater pH, water quality, local speciation), stagnation (in-port, dead-end water mains), effects on metallic and non-metallic parts, flow rates, temperature flux, and residence time in the system. Based on preliminary testing, optimization of dosing was determined.
The inhibitory effects on the known film-forming bacteria Pseudomonas aeruginosa by selected plant extracts and related compounds were evaluated. Several members of the cinnamic acid group, nicotinic and related acids, benzoic acid derivatives, miscellaneous phenolics, and selected plant extracts were tested. Testing was performed to determine rapid toxicity to Pseudomonas and to determine the lowest observable effective concentration of additive. The family of nicotinic acid derivatives showed significant inhibition of growth of the selected organisms at concentrations suitable for environmental use. One compound in particular, citrazinic acid, showed exceptional growth inhibition and was tested further using Klebsiella pneumoniae and a mixed culture. Based on the results of these tests, this compound has been identified as a potential candidate for use in controlling biofouling in heat transfer equipment systems.
The results of this project support the potential use of plant extracts as biofouling inhibitors. Substantial cost savings may be realized from the reduced manpower required to maintain shipboard heat exchange and plumbing systems. Performance of these systems will also be positively impacted if biofouling is minimized. In addition, significant positive environmental impacts may be realized if these substances are deemed suitable as replacements for chlorination.