New regulations regarding clean water have increased the need for long-term environmental monitoring. As water quality sensor technology improves and becomes more stable, the limiting factor in these projects becomes biological and non-biological fouling of instrumentation. Biofouling of environmental equipment increases site visits and maintenance, which elevate monitoring costs. In addition, biofouling can affect data quality.
The In-Situ TROLL Shield Guard is designed to extend typical deployments by inhibiting fouling on the surface of the TROLL® 9500 Water Quality Instrument. In-Situ Inc. has completed extensive testing of an antifouling solution to establish its effectiveness and usability. Prior to the experiments, a coiled copper guard was installed on a TROLL 9500 and deployed alongside a control TROLL 9500 to quantify performance of dissolved oxygen (DO), conductivity, ORP, pH, and turbidity sensors. Visual inspection and quantitative analysis determined the effectiveness of the antifouling solution. During a two-month deployment, the TROLL Shield Guard extended instrument deployment by up to six weeks longer than the control. See Figure 1.
With the ever increasing need to monitor diminishing water supplies has come the need to understand environmental changes and factors contributing to those changes. By routinely monitoring water quality parameters, possible contamination and changes in water consistency can be detected. During the last two decades, clean water regulations have increased, and regulators, legislators, and citizens want to understand what is discharged into water and by whom. As these requirements increase, it has become important to develop methods for monitoring environmental conditions and for improving existing techniques.
Traditional methods for environmental monitoring include sample collection followed by analysis at a state-regulated laboratory. Although this is an accurate technique, it has some limitations. For example, samples can degrade, which affects analytical accuracy; and spot checking limits understanding of daily changes. These limitations have inspired technological advancements that allow the user to characterize a site continuously rather than by spot sample analysis.
Long-term monitoring provides a daily representation of a site and can immediately indicate a disturbance in the homeostasis of the environment. In addition, the site controller can act quickly if an issue arises. Water quality instruments can measure conductivity, DO, ORP, pH, turbidity, water depth, and nutrient levels of various environments. Each of these parameters can act as an indicator of change in water composition and can trigger further investigation into possible contamination.
Long-term environmental monitoring also has limitations. The leading source of error during long-term deployments is biofouling and its affect on sensor accuracy and performance. Biofouling is the accumulation of waterborne biological organisms on a submerged surface. Biofouling can be microscopic, organic, inorganic, bacterial, algal, plant, or invertebrate. The extent of biofouling varies from site to site with greater impediment in warm areas with saline water such as coastal and equatorial regions.
The degree of biofouling on an instrument is directly proportional to the impact on the performance of the sensors on the water quality instrument. The more biofouling present the greater the impact on sensor performance. Most water quality sensors rely on open exposure to the media in order to produce accurate results. As sensors become covered with biological organisms and debris, exposure to the media decreases, which leads to decreased sensitivity and accuracy. Some of the problems associated with biofouling include:
- Increased site visits to clean and service the equipment
- Inaccurate data due to sensor drift
- Increased time spent post correcting data to
- compensate for fouling
- Data loss due to extensive off-site cleanings
- Permanent damage to the sensors
All of these factors add expense in the form of site visits, downtime, and sensor replacement. Short-term, fouling that covers the sensing material results in slow response and reduced accuracy. Fouled sensors can require full cleaning, service, and user calibration in order to bring performance back within manufacturer specifications. Permanent sensor damage, a long-term implication of fouling, leads to replacement of the sensors or the entire instrument. In either case, biofouling is expensive due to material costs, data loss, and increased staff-hours.
Many manufacturers have tested and implemented a variety of antifouling strategies. Each was designed for a specific budget and application. Methods to address biofouling include the use of mechanical devices, antifouling coatings, biocide-infused coatings or paints, irradiation, and electrochemical devices. Mechanical devices typically consist of a wiper, air, or water blast system, or include a vibration or a shutter mechanism. Mechanical devices are fairly effective but have drawbacks. Most require a power supply in order to function properly, yet most sites do have power readily available. In addition, a wiper can itself become fouled. This can cause additional abrasion of the sensing materials and lead to untimely degradation of the sensor. Mechanical devices can fail, thereby flooding instruments, causing the need for additional maintenance.
Antifouling coatings and paints have proven most effective for marine environments. These coatings do not, however, adhere to all materials and can be ineffective for certain sensors and instruments. In addition, antifouling coatings can be expensive or release hazardous chemicals into the environment. Coatings with infused biocides work well, but are typically too expensive for most manufacturers to use on their equipment or for environmental agencies to purchase.
Irradiation uses ultraviolet light to mitigate biofouling, but is not a viable option due to power constraints and deployment regulations. Lastly, electrochemical antifouling devices use metal oxides, or other chemicals, to deter or neutralize organismal growth. As a relatively inexpensive method, this has become the primary solution for most deployment extensions.