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Copper-Alloy Biofouling Control On Marine Water-Monitoring Systems
Anti-Fouling Methods Using Copper Alloys Reduce Biological Fouling, Extend Time Between Maintenance Visits for Water-Monitoring Systems

AUTHORS:

Mike Cook
Application Engineer
YSI Inc.
Yellow Springs, Ohio


Although biological fouling has always been an issue for water-quality data collection, longer deployment times are now more common because of advances in electronics and equipment design. Considering this, manufacturers have been actively pursuing anti-fouling methods to reduce the impact of biofouling on the quality of collected data.

Researchers with YSI Inc. conducted experiments from 2006 to 2008 to test how copper, a metal with natural anti-microbial properties, could be incorporated into YSIís existing anti-fouling control methods. The researchers tested several anti-fouling systems at six different sites—two freshwater and four marine—throughout the United States. Through testing, YSI researchers narrowed down the specific combination of components that most successfully deterred biofouling buildup. Early tests using commercially available copper tape led to further experiments using a variety of custom brass and copper-alloy components fitted specifically to YSI equipment. The exact ratio of copper, brass and other alloys was adjusted to find the best combination of chemical properties. Researchers also worked to optimize physical components in the sensors to help reduce and remove biofouling.

Data collected indicated that the hardware provided viable data for extended deployments of more than 40 days. In contrast, biofouling degraded the data quality collected by the control instruments more rapidly, in some cases in just nine days.

Results from the experiments informed YSI researchers of the optimal components for the companyís anti-fouling kit, which was released commercially in June 2008 and updated with new anti-fouling items in May 2010. The anti-fouling kits can be retrofitted to existing equipment or applied to new equipment.


Biofouling Impacts
Collecting viable water-quality data always presents challenges. Water conditions, weather, site location and site access can each present obstacles to collecting data and performing maintenance. Improved sensor stability and battery life, combined with lower power consumption, have allowed for longer sensor deployment times at lower operating costs. These technology changes, while beneficial, have raised questions about sensor maintenance intervals; the longer an instrument stays in the field, the greater the risk for biofouling.

Biofouling, the accumulation of microorganisms, plants, algae and small animals on underwater equipment, impacts data quality by interfering with readings, causing false positives and increasing noise and sensor failure. For example, optical turbidity sensors emit infrared light and then measure the scatter of light caused by particles in the water; as biological growth on the optics also causes light scatter, an impacted sensor will measure artificially high turbidity values.

With this knowledge, water managers must consider the biofouling rate at specific sites and during particular seasons when planning site maintenance visits. A typical site visit requires multiple employees, vehicle usage, time and resources for travel, equipment preparation, equipment cleaning, readings verification and instrument redeployment. Therefore, more frequent site visits require more resources and, thus, more money to collect quality data. The Alliance for Coastal Technologies estimates biofouling-related maintenance costs consume 50 percent of the operational budgets of long-term water-quality monitoring programs.

Optical dissolved oxygen data from the Florida test site. The control sonde showed evidence of fouling after 11 days of deployment, while the anti-fouling sondes were not affected by fouling for 40 days.

Biofouling Control Methods
Methods to combat biofouling on submerged sensors have evolved from the use of toxic chemicals and pumps to more mechanical systems that use wipers or shutters, and more recently, to combination systems that employ both mechanical and other technologies, such as ultrasonic and chlorine-generation systems. The latter systems are often more effective and typically do not have a negative impact on the environment.

Generally, marine biofouling presents more of a challenge than freshwater biofouling because of the presence of hard fouling organisms such as barnacles and mussels, which can completely cover the optics on the probe, corrupting data and damaging sensing equipment. Soft fouling organisms typically characterize freshwater systems and are often more easily controlled by mechanical wipers. Marine applications may also have faster settlement and more aggressive growth.

All YSI optical sensors utilize mechanical wipers and brushes to keep sensors clean and free of debris; brushes remove debris from the wipers and nonoptical sensors. Both wiping mechanisms mount to a central wiping shaft, and when the shaft rotates, the wiper/brush moves in a circular motion, removing the debris. Wiping precedes all sampling intervals. Wipers and brushes provide adequate biofouling protection in moderately productive environments; in highly productive environments and during prolonged deployments, however, systems require additional protection.

One method to control biofouling has been to use copper. Copper-containing paints have been used as a biofouling countermeasure, but there are significant limitations; paint interferes with sensing technologies, pollutes the environment and must be reapplied on a regular basis. Copper-based alloys, however, can be used as a good alternative. Many alloys have low dissolution rates and little impact on the environment, and they can last for long periods of time. Materials with these anti-fouling properties are increasingly being coupled with mechanical systems that physically wipe or guard the sensing element between sampling intervals to provide optimal protection.


Anti-Fouling Experiments
Researchers field tested a variety of prototype anti-fouling systems, which were used in conjunction with the sensor wipers and brushes employed on YSIís 6-Series multiparameter water-quality sondes. Over 18 months, the research team tested and refined the anti-fouling components so they could select specific materials that provided optimal durability and anti-fouling properties.

After preliminary testing to determine the proper copper alloys and conducting some early experiments, the 6-Series was modified in a number of ways to improve its anti-fouling capability and was then tested against a control device to determine if the modifications were effective.

Conductivity/Temperature Probe. The 6-Series measures conductivity and temperature through a single probe. The temperature thermistor, which is located on the tip of the conductivity/temperature probe, is especially important because all data collected from the sonde is temperature-compensated. YSIís conductivity sensor houses four nickel electrodes inside the sensor body with channels that allow water to flow freely through two boreholes along the sensorís axis. The internal location of the electrodes presented an anti-fouling challenge, so the solution required a combination of treatments. Copper tape treated the outside of the sensor housing, and eventually a small copper mesh screen was used around the conductivity/temperature probe to prevent the settlement of biofouling organisms. In addition, the conductivity electrodes were treated with C-Spray, a nanopolymer coating used to inhibit the attachment of fouling organisms.

Water-Quality Probes. Data from optical water-quality probes, which include those measuring dissolved oxygen, turbidity, blue-green algae and chlorophyl, are susceptible to biofouling because the exposed optical surface is sensitive to particles that are in the light path. In addition, multiple potential growth surfaces, such as the sonde guard and wiper assemblies, surround the optical sensors and provide an opportunity for fouling to impact these surfaces and affect readings. To reduce biofouling, engineers used copper alloys for the optical sensor housings and wiper assemblies.


St. Petersburg Harbor Experiment
One of the six experiments was conducted in collaboration with the University of South Florida at a deployment site in St. Petersburg Harbor, Florida.

The St. Petersburg experiment tested anti-fouling treatments with three sonde configurations: a control sonde, a sonde with just copper-alloy components, and a sonde with copper-alloy components and a copper mesh screen.

The control sonde did not utilize any anti-fouling hardware other than the 6-Seriesí standard wipers. The copper-alloy sondes utilized copper-alloy sensor guards plus copper-alloy sensor housings or copper tape on the housings. The copper-alloy and mesh sondes also included a copper wire mesh on the inside of the sonde guard.

Deployments were to a depth of 0.5 meters, with the sondes configured to collect data every 15 minutes. A sonde-mounting apparatus and test platform used in the study allowed researchers to deploy multiple sondes in close proximity at the same depth, which would expose the sondes to the same conditions.

The platform allowed easy access to the sondes for biofouling checks and instrument servicing, and visual inspection, photographic documentation and data downloads were completed on a weekly basis.

After a 40-day deployment, YSI researchers compiled the data from all three sonde configurations to verify whether the copper-alloy treatments helped lengthen the time during which readings were accurate.

Temperature data from all three configurations did not exhibit biofouling effects and accurately represented site conditions.

All sondes exhibited similar specific conductivity trends for the first 22 days, after which the control sonde began to deviate from the two copper-alloy sondes, which continued to provide accurate data throughout the 40-day deployment.

Optical dissolved oxygen data from the test site in Florida indicated that both copper-alloy sondes performed well over the 40-day deployment, while the control sonde diverged from the norm 11 days after deployment.

Like dissolved oxygen, turbidity data was similarly compromised on the control sonde, which began to deviate from the copper-alloy sondes after only nine days, according to the data. In contrast, the copper-alloy sondes collected accurate data throughout the 40-day deployment.

Visual inspections corroborated well with the data. While the sondes treated with the copper alloy showed evidence of biofouling on the mounting apparatus, the sensor surfaces, wiper bodies and brushes were free of biological growth. In contrast, the control sonde displayed fouling on sensor surfaces as well as wiper and brush bodies, which ultimately affected the quality of the data.

These promising results were also confirmed by an experiment in Marion, Massachusetts. Here, YSI researchers also found data that indicated the effectiveness of a conductivity/temperature probe with anti-fouling protection. The probe was initially deployed in mid-summer with only YSIís basic anti-fouling guards, and biofouling impacted the unprotected conductivity sensor after 18 days. After cleaning the sensor, researchers installed a copper mesh sleeve on the conductivity/temperature probe and treated the conductivity electrodes with C-Spray. In late summer the sondes were redeployed. The results indicated that treating the conductivity sensor led to 93 days of maintenance-free, accurate data.


Developing an Anti-Fouling Kit
With the information from these experiments, YSI has designed and successfully employed a combination of anti-fouling measures for commercial use that significantly lower the cost of maintenance through protecting multiparameter water-quality systems in long-term deployments.

Through field trials, YSI found that treating all surfaces near the sensing elements was critical. Otherwise, biofouling organisms could attach to any unprotected surface, such as the foam wiper pad or sensor guard, and grow many inches from that point, where they could interfere with the sensing elements.

YSI developed a complete copper-alloy anti-fouling system that worked with the existing sensor wipers and brushes. This anti-fouling system includes copper-alloy optical wipers, which clean the entire surface of the sensor rather than only the optical portion. This prevents biofouling organisms from colonizing the edge of the probe and migrating toward the sensing element. The system also includes sonde guards made from a durable copper alloy, optical sensors with copper-alloy housings that are rated for submersion to 200 meters, copper-alloy ROX™ optical dissolved oxygen membrane caps and copper-alloy locking nuts and port plugs. Finally, the system uses a copper mesh screen around the conductivity/temperature probe as well as C-Spray solution, which creates a slippery surface on the internal conductivity electrodes.


Conclusions
As in-situ sensor technology advances, biofouling is the principal limiting factor of long-term deployments and prevents further reduction in costs for environmental monitoring programs. Biofouling can isolate water-quality sensors from the measuring environment and interfere with light transmission, both of which can compromise data. Two years of testing anti-fouling materials at freshwater and marine sites indicated that anti-fouling hardware effectively provided viable data for deployments longer than 40 days. Without anti-fouling hardware, sensors were affected by fouling in as few as nine days. By using anti-fouling components, the St. Petersburg site decreased its maintenance visits by 66 percent and saved $10,000. Overall, installing anti-fouling components on water-quality instruments effectively extends deployment times and enables water managers to collect high-quality data.


Acknowledgments
The author would like to thank Michael Lizotte of YSI Integrated Systems and Services and Sherryl Gilbert of the University of South Florida for their work on the anti-fouling test platform.



Mike Cook is an applications engineer with YSI Inc. and SonTek, a YSI subsidiary. He has been involved in environmental monitoring projects for more than 15 years, which has included work as an environmental and agricultural consultant in the United States and Spain.




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