THE
BIoGEORGE SYSTEMBACKGROUND
Microbiologically Influenced Corrosion (MIC) is a significant degradation mechanism for plant materials in many industries, including nuclear and fossil-fueled power plants, chemical processing, refineries, pulp and paper, oil and gas production and distribution, transportation, buildings (HVAC), and the armed services. Essentially, all cooling water and process water applications may be susceptible to MIC. Further, biological fouling, which is always a precursor to MIC, can reduce the efficiency of heat exchangers and fluid distribution systems and can interfere with water-based processes.
MIC and microbiological fouling can produce significant damage to plant components, resulting in increased downtime of equipment and increased operating costs. Mitigation of MIC and microfouling is also costly in terms of increased system maintenance, water treat-ment chemicals and delivery systems, and often, environmental impact. Monitoring for MIC, if done at all, typically relies upon control of the process (e.g., biocide concentrations and addition schedules, corrosion monitoring in sidestreams) or batch type measurements of microbial activity in the fluid or on surfaces. On-line monitoring tools alert the system operator that a biofilm is forming which permits initiation of mitigating actions before significant damage is incurred. Of equal importance, an on-line monitor allows optimization of water treatment concentrations and addition frequencies, and adjustment of maintenance schedules. This avoids over-treatment of the water and reduces operating costs.
The BIoGEORGE system was developed to provide on-line and real-time indications of biofilm activity on typical metallic surfaces. The probe is operated such that biofilm formation is encouraged to form more rapidly on probe surfaces than on plant piping and heat exchanger tubes. As a result, maintaining the probe in a clean condition will assure that the pipe work and heat exchangers are clean.
MIC AND BIOFOULING CONTROLS
The most common approach to MIC mitigation is to chemically treat the water or other fluid. Chemical costs for a large power generation facility are often of the order of $1,000,000 per year. The toxicity of these chemicals is always a concern for plant personnel and for the environment, resulting in close scrutiny and control of effluents and chemical inventories by regulators, environmental agencies, and plant owners.
Probably the second-most common approach to MIC mitigation is to do nothing and simply replace system piping or components when they fail. Several nuclear plants have sustained extensive corrosion damage to their service water systems; much of the damage is the result of MIC. In many cases, extensive and expensive repairs and replacements were required at costs of several million dollars per system. At least three domestic nuclear plants have completely replaced the piping in their service water systems. Costs per plant averaged $30,000,000.
State, federal, and local government agencies require all plants, that discharge to ground waters, obtain permits that restrict the types of chemicals that may be discharged, their concentration, and the number of hours per day that the discharge is allowed. These environmental regulations continue to become more restrictive. Thus, plants are required to control fouling and corrosion, including MIC with lower effluent levels. Users and providers alike are continually searching for control methods that require less chemicals, less toxic chemicals or no chemicals at all. The BIoGEORGE signal is a direct measurement of treatment effectiveness. This permits the reduction or elimination of effluents without compromising treatment effectiveness.
MONITORING
The most common monitoring approaches used as part of the plant's MIC and biofouling controls are:
Do Nothing
The plant operator does not treat the water, cathodically protect key structures,
or utilize preventive maintenance to keep biofilm off surfaces. Basically, repairs
are made when problems are detected.
Water Treatment Service
Company
The plant contracts with a water treatment service company such as Nalco, Betz-Dearborn,
Calgon, or Drew. The water treatment service company provides the chemicals,
delivery system, monitoring, controls, etc., that are considered necessary to
keep microbes under control. Monitoring usually consists of checks for residual
chemicals in the water -- first, to assure compliance with discharge limits,
second, for control of the chemical addition.
Plant Applies Water
Treatment
The treatments are applied by the plant, utilizing a set application schedule
or batch methods (e.g., water samples or deposit samples collected from pipes,
heat exchangers, etc.) to determine if biocide treatment is needed.
THE
BIoGEORGE SYSTEM
The BIoGEORGE system was developed to monitor biofilm activity on metallic surfaces as a means of avoiding MIC. The probe is designed and operated so that viable micro-organisms in the environment are encouraged to settle on probe surfaces well before they settle on heat exchanger tubes or piping.
The system utilizes electrochemical methods for biofilm monitoring functions. By closely tracking biofilm activity on the probe, the operator is alerted to the need to treat his system, to assess the effectiveness of treatments, or to schedule maintenance activities. By controlling biofilm, MIC and biofouling of critical components are eliminated. By closely tracking biofilm activity on the probe, the operator is alerted to the need to treat the system, to assess the effectiveness of treatment, or to schedule maintenance activities.
System Description
The BIoGEORGE system monitors biofilm activity on the probe surface. Biofilm
formation occurs more rapidly on the probe than on plant piping or heat exchanger
tubes. As a result, maintaining the probe in a clean condition assures that
the pipe work and heat exchangers are clean. Indications of biofilm activity
are based upon trends, often over a long term, as opposed to instantaneous readings.
The system consists of a probe, its integrated electronics, interconnecting
cable, display software, a user manual, and product support.
The standard probe is installed into a piping system, heat exchanger water box, cooling tower, or side stream via a 2-inch threaded connection. The probes can also be built to insert into "hot tap" type fittings (slightly larger than one inch access port).
The two-electrode probe, with each electrode comprised of a series of stainless steel or titanium discs, is subjected to intermittent polarization to a preset DC potential. Biofilm activity is detected from an increase in the applied current required to achieve that potential. As a biofilm becomes established, it may also generate a current during times when the applied potential is off. Measurement of this generated current provides a second means of tracking biofilm formation.
Biofilm formation may thus be monitored continuously with the BIoGEORGE system by tracking the current over time, both during polarization (applied current) and while the electrodes are connected through the shunt (generated current).
DESIGN CONSIDERATIONS
Key
considerations reflected in the BIoGEORGE design are:
On-line/Real-Time Monitoring
Most existing biofilm monitoring methods rely upon labor-intensive batch processes
that simply enumerate microorganisms (typically, planktonic organisms). Such
processes are far too slow for most applications, especially those where feedback
(e.g., to water treatment controls) are required. In contrast, the BIoGEORGE
system measures the degree of interaction between the film and the metal surface,
an indicator of the activity of the biofilm, rather than just numbers of organisms.
These features give BIoGEORGE users the capability of controlling treatments
very precisely so that biological control is achieved without over-treating
the environment.
Plant System
The BIoGEORGE probe's simplicity, ruggedness, and sensitivity offer an economical
method for unattended detection of biofilms in a variety of systems and water
chemistries. The capability of the probe to monitor biofilm activity in all
types of systems, from normally stagnant to continuous flow, to intermittent
flow conditions, and its low maintenance require-ments, make it ideal for many
of the treated and untreated systems in a plant.
Operating Conditions
The BIoGEORGE probe works well under all flow regimes and has no moving parts.
Unlike model heat exchangers or other methods that require careful control of
flow and very precise measurements of temperature, the BIoGEORGE probe can operate
under conditions of flow, temperature, etc., that are fully prototypical of
the system that is being monitored.
Biocide Effectiveness
The system has been shown to be particularly useful for biocide optimization
studies. Since the activity of the biofilm is measured, the biocide's penetrating
power and overall effectiveness are readily measured.
The BIoGEORGE system can act as a stand-alone system or can provide an enhancement to specialized monitoring systems used by utilities and other users of cooling water. The real time indications of biofilm activity permit operators to initiate flow or biocide treatments as needed, thereby minimizing the risks of fouling and corrosion while reducing the costs of the mitigation approaches and avoiding over-treatment.
Control Effluents
The BIoGEORGE system permits the operator to effectively control biofouling
and MIC using the minimum quantity of water treatment chemicals; a more environmentally
friendly, "green" approach.
If you would like more information on BIoGEORGE, please contact SI.
™ BIoGEORGE is a trademark of Structural Integrity Associates, Inc. The probes are described by United States Patents 5,246,560 and 5,356,521, owned by EPRI.