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The most promising techniques for prevention of biofouling on marine sensors have been investigated in the project. Due to the complexity of the biofouling process which depends on the season, the location (arctic, temperate, tropic) and on the water type (estuary, coast, open ocean) the process of choosing the appropriate antifouling strategy (single method or a combination of different methods) for a special type of sensor is complex as well. In order to aid a potential user a web-based software selection tool has been developed.
The user is guided through a decision tree and finally reaches recommendation for his special application (single antifouling method). In addition, another tool shows which combinations of antifouling methods are valid and which should be avoided.
The tools together with information on BRIMOM results and literature citations are disseminated through a website which will be maintained and updated for several years.
Hydrogels containing the appropriate active compound are effective at preventing both hard and soft marine fouling organisms. Trials in the Clyde estuary have shown that impregnated gels are highly effective at preventing fouling for up to three months.
The development of algal biofilms on optical sensors is initially important and may significantly impair instrument readings. However, subsequent macrofouling may completely obliterate the signals in some instrumentation. Panel trials were specifically designed to test the anti-fouling properties of impregnated hydrogels against microfouling and, more importantly, the peak settlement of a major-fouling organism Semi-balanus balanoides. The hydrogels coatings have been shown to be able to prevent the majority of settlement during the intensive barnacle and mussel settlement seasons.
The time that gel protected instruments are deployed before this settlement season does not appear to effect their macro-antifouling ability, as long as the deployment prior to settlement does not exceed the working life of the hydrogels (c.a. 3 months).
The Ifremer local chlorination technique with metallic mesh electode has been successfully testing on much kind of instruments during BRIMOM project. This technology is compatible with autonomous monitoring in terms of energy need and mechanical design.
Ifremer plan to distribute this know how to instrument manufacturers in the next three years.
First contacts are in progress with instrument manufacturers, additional ones should contact us idf interested.
The Ifremer local chlorination technique with oxide window coating electode has been successfully testing on one optical instrument during BRIMOM project. This technology is very promising. Further collaborations with instrument manufacturers should be engaged in the next three years in order to develop the technique.
BRIMOM initially considered a wide range of instruments but necessarily focussed on several key sensors on which the biofouling assessment studies were conducted:
- Optical transmissometers;
- Optical fluorimeters;
- Optical dissolved oxygen;
- Membrane sensors (dissolved oxygen, pH).
All commercial instruments currently available have been developed for scientific purposes where deployment periods may be several hours or days. None have been designed specifically for long term (3-12 months) deployments.
Chelsea Instruments (CI)recognises that the market for in situ instruments is changing with the emphasis on long term deployments and extended data sets. In response to this CI is assessing which are the most suitable boifouling prevention technologies and how existing instruments may be modified. It may be necessary to design bespoke instruments for long term deployments in preference to modifying existing ones.
Biofouling reduction is only one aspect that must be considered. There is the need for more reliable instruments that are designed for use in long term monitoring programmes.
The modification of existing instruments would include material changes, replacing recessed optical windows for flush mounted versions, coating of existing window geometries to incorporate local chlorination, redesigning wipers and brushes to be more effective and modifications to install replaceable hydrogels. This approach may have a higher cost base than above, but will enable manufacturers an early entry into the market.
A new design enables manufacturers to address all these areas at the early stages of development. Production design and production costs can be assessed in order to keep the instrumentation competitive. The design must be simple to operate and easy to maintain by the user. Downtime must be minimised when returning to the manufacture for service and calibration. Therefore instruments must also be easy for the manufacturer to diagnose faults and repair. Collaboration with scientists and end users will ensure that appropriate technologies are incorporated, with future stretch potential.
There will be significant costs associated with the development and design of new instrumentation. It will be very important to correctly confirm the market requirements for each instrument. This will most likely be a system approach where interface specification data will be as import as the other design criteria.
The project resulted into a marine fouling prevention method feasible for implementation into optical oceanographic instruments for operational monitoring of oceans and coastal seas.
Buoy systems for marine environmental monitoring suffer from biofouling. Biofouling reduces the quality of the measurements, reduces the time between maintenance visits by boat and reduces the life time of instruments. Reduction or prevention of biofouling improves measurements and reduce operational and capital costs with 25 to 50%. Two biofouling prevention methods (hydrogel coating and local chlorination)have been tested under realistic conditions with optical instruments (transmissometers).
The local chlorination method proved to be effective and operational feasible. Implementation of the method and redesign of the instruments is being studied. Technical and financial issues will assessed before a final decission on redesign of the instrumentwill be taken. In case of a possitive outcome marketing of the redesigned instrument is expected within one year after the end of the project.
Pulsed illumination of the surface with UV-C from a light source external to the instrument is a very effective way to protect free-standing deployed oceanographic instruments from biofouling.
Based on bioassay based test programs TNO conducted a range of full-scale instrument trials with UC-C protected turbidity sensors and SCUFA fluorescence sensors. The trials with the turbidity sensor were conducted in flow-through systems with natural biofouling pressure and in the Harbor of Den Helder (NL). The SCUFA tests were conducted in Den Helder Harbor and in a marina on Helgoland (DE). Six runs with UV-C protected turbidity and fluorescence sensors were conducted with varying UV-C illumination regimes (ranging from 45 min on/15 min off to 1 min on/29 min off). In all cases the result was comparable: reference surfaces and non-protected instruments became heavily fouled within a few weeks, resulting in rapid instrument failure. The protected instruments were effectively protected by the UV-C illumination and continued to perform well.
A UV-C illumination regime of 1 min/59 min off gives 100% protection of the optical windows and a nearly 100% protection of the surrounding housing as shown in the trials. 100% effectiveness is not needed, as some degree of biofouling is acceptable. The illumination regime can therefore easily be reduced further, especially when combined with a periodic UV boost to kill any fouling that may have settled. The currently used Pen-rays provide good protection, but the radiation is not very effective as the radiant energy is spread in all directions. Fitting a reflector around the bulb will direct the radiation towards the target surface. This will reduce the power consumption. It will also significantly reduce the potential risks to humans and non-target organisms.
UV-C has been used successfully by TNO for protecting turbidity sensors and SCUFA fluorescence sensors, but the results can be fully extrapolated to any other sensor surface that needs protection. The lifetime of the system is in fact only limited by the durability of the UV bulbs and the available power source.
Antifouling techniques for automated monitoring stations based on flow through systems were established and tested in the river Elbe and the North Sea. Depending on the location of the station and the season optimal antifouling techniques with minimal energy and reagent consumption were found. Standard antifouling strategies for in-situ sensors cannot be used for such flow through systems, because the whole system has to be protected.
One method is the variation the flow rate:
With flow rates of less than 0.1m/s in the summer in the German Bight a reduction of the flow rate by 50% shows 70% less biofouling. For very high flow rates the reverse effect will be found. Different cleaning procedures are needed depending on the area: Starting from automated tap water risning (for artic waters) over acidification to pH 3 (for temperate waters) up to a combination of acidification and chlorination (for tropical waters).
The latter was carried out by establishing an in-situ electrolytic production of chlorine in the flow through system. This reagent-free antifouling method is easy to install and the maintenance is safe. By this procedure more than 95% of biofouling can be prevented over several months providing a sufficient data quality.
Laboratory trials were conducted with commercially available, Pen-Ray® low-pressure UV-C light sources of different intensities. These experiments mimicked the effects of having UV-C light sources located behind the optical window in an instrument that needs protecting from biofouling. Results showed that low levels of pulsed UV-C light shining through an optical window (Spectrosil®) and illuminating the surface that needs protecting can significantly reduce bacterial fouling on the optical surface.
Although at the developmental stage, results indicated that when commercial UV-C light emitting diodes (with low power requirements and high UV-C intensity) become commercially available that this may be an effective antifouling technique which could be located internally within optical instruments. Further research and development is required before this technique is at the level of commercial exploitation.
Following further R&D results will be disseminated in the form of peer-reviewed publications.
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