Since the discoveries of Louis Pasteur, industries have been using heat treatments for the control of biological contamination. The development of heat treatment technologies for use in the battle against the zebra mussel, Dreissena polymorpha, was developed during the late 1980’s and subsequently led to the testing of heat treatment applications to ballast water.

The Australian Quarantine and Inspection Service (AQIS) study on the effectiveness of heat treatment on dinoflagellate cysts in ballast water concluded that that the energy required to treat the large volumes of ballast aboard commercial ships would make the technology cost prohibitive. Since the AQIS work was done, alternatives to direct heating of ballast water intake or discharge have been evaluated by Rigby and Hallengraeff (1992). One alternative is to use waste heat from the ship’s main plant, which would require connecting heat exchangers to the main engines. While using waste heat to treat ballast water has many advantages (e.g., energy and cost savings), a major disadvantage includes a need to re-plumb ballast tanks on existing ships. The investigation of thermal treatment techniques reported below considered the use of boilers and heat exchangers to treat ballast water on intake only.

Several industrial boiler vendors were contacted during this study to help evaluate the availability of this technology for shipboard application. All contacted vendors report that, as with most industrial applications, a custom-designed boiler system would be required for ballast water treatment. The design of such treatment systems would be technologically simple, and would use off-the-shelf components configured to specific ships. Water would be pumped into the boiler, raised to a desired temperature, and then discharged into the ballast tank. The major factors affecting heat-treatment effectiveness are exposure time, intake water temperature, and target effectiveness temperature (i.e., water temperature at which target organisms succumb).

The source of energy for the boiler(s) is flexible and can depend on what is available on the vessels. Industrial boilers capable of handling high flow rates and temperature relative to ballast water applications can use several sources of fuel, including electricity, No.2 fuel oil (diesel fuel), or No.6 fuel oil (bunker fuel oil). The important caveat is that the energy source must be determined before the system is designed and built. It is expected that on most vessels, only one type of energy source will be readily available to run the treatment boilers, that being the same fuel that is used to run the propulsion plant. Many large carriers use only No.6 fuel oil for powering their main plants (Tagg, 1998). On such ships, a burner designed to use No.6 fuel oil would be installed, along with the appropriate fuel lines and exhaust manifolds.

There are many constraints to consider before installing the large boilers needed to heat large volumes of ballast water. Fuel, space, and operation requirements are particularly acute on existing ships. Rerouting current fuel delivery systems to the treatment boilers may necessitate installing appropriate shielded fuel lines through vessel compartment (e.g., cargo areas) not originally designed for this purpose. Equipment room space is very limited on many vessels. On many vessels it is expected that the size of treatment boilers will require more space than is available below decks (refer to Table 4-1). In such cases, the treatment boilers may need to be located on deck wherever space is available that will not interfere with other shipboard operations. Routing ballast water to topside boilers, or to other parts of the ship, may alter its stability. The associated piping and pumping systems will need to be accessed for installation and maintenance, and exhaust gases will need to be safely vented.

Table 4-1 presents the operating specifications of industrial boilers that are required to achieve final temperatures of ~150°F (65.5°C) for the designated ballast water flow rates.

Performance Criteria

The performance of a specific heat-treatment equipment is based on the equipment’s ability to elevate ballast water temperature to (or above) the thermal-threshold of target organisms. The thermal threshold is the point at which an organism is instantly killed due to either denaturing of cellular proteins or increasing the organism’s metabolism beyond sustainable levels. Thermal threshold is variable among different species, as is species’ ability to endure periods of high temperatures that are below their thermal thresholds. In general, temperatures close to an organism’s thermal threshold can be tolerated for short periods with little nonreversible damage, and temperatures sufficiently cooler than the thermal threshold organisms can be survived for longer periods.

Ideally, a heat-treatment system will elevate the temperature of incoming water above the thermal thresholds for all the organisms of concern. This would include microorganism stages that can tolerate extreme conditions (e.g., dinoflagellate cysts). This study uses 150°F (65.5°C) (considered by many to be above the thermal threshold for all aquatic organisms of concern) as the temperature for operation of a 1,200 and 8,000 gpm ballast water systems. This does not mean that heat treatment systems are ineffectual at lower operating temperatures. It is possible for a heat treatment system to operate efficiently and effectively (all target organisms killed) at temperatures in the 110-150°F (43.3-65.5°C) range, if the exposure periods are proportionately increased. In practical terms, lengthening the exposure period (the residence time a unit of water held at temperature) means slowing the flow rate and/or increasing the size of the boiler tank.

Also in their evaluation of exposure period verses effectiveness, Bosch and Hallegraeff found that while 45°C (113E F) water could reduce germination to 0% with a 30-second exposure time, exposure to 40°C (104°F) water required 90 seconds to achieve 0% germination. Thus, if a ballast water treatment system were designed for only G. catenatum cysts (a relatively hardy microorganism of concern) killing, 45°C (113E F) water with a 30 second exposure period in the boiler is sufficient. However, the same result can be achieved at 40°C (104°F) if the exposure period is 90 seconds. Slowing the flow rate or proportionately increasing the size of the boiler to achieve the 90-second exposure period could save considerable energy (i.e., fuel) costs, if such changes are economical for the ships operators.

In summary, heat-treatment boiler technology is well established and is an extremely effective application. The principal limits for rapid heat treatment of incoming (or discharging) ballast water are engineering/design and energy-consumption related. A balance of temperature and flow rates will be necessary for flow-through boiler treatment to be feasible on the scale of a commercial vessel.

Assuming that the ballast water will be heated to 150°F (65.5°C), filtration is unnecessary for treatment systems operating within design limits. However, filtration will reduce the amount of sediment and biological detritus that will collect in the bottom of the boiler. The accumulation of sediment in the boiler will gradually reduce the exposure period of the oncoming ballast water. Thus, pretreatment filtration will theoretically reduce boiler maintenance requirements (to remove sediment accumulations) and lengthen exposure periods of the incoming water. The smaller the pore size of the pretreatment filter, the less sediment will deposit in the boiler.

Most industrial heating-system applications are adequate for shipboard treatment systems, depending on location and exposure to weather if placed outside. If a heat treatment system is installed onto the deck of a vessel, it will require shelter from the elements of seawater and salt air in order to maintain system integrity and operational status. If located in the engine room it is assumed that boiler system will not require substantial modifications to adequately treat ballast water.

The heat treatment systems researched in this study, once installed, are stand-alone units that do not require extraordinary supervision during operation. The units are equipped with external pressure and temperature gauges that can be wired to a main control panel for monitoring. The repair and maintenance of the system could be conducted by one of the ships’ engineers, with minimal need for extended training. The boilers would require some general maintenance such as burner upkeep and tank cleaning. Tank cleaning would be required for the removal of sediments and "scaling" (i.e., of mineral deposits that form from the heating of seawater).

The use of a heat treatment system creates specific safety concerns. The use of large quantities of heat to treat ballast water will create a hot-water hazard. Shipboard operations normally produce this type of hazard but it is generally associated with the ship’s main boiler system and is restricted to the boiler room. The potential for locating an industrial boiler outside of the boiler room, either in a different area of the ships’ compartments or on deck will produce a hot-water hazard located in a different area. This may require special engineering considerations during design and installation of the system. If located on the deck of the vessel, the treatment system would need to be protected for environmental impacts such as wave action. It would also need to be mounted away from potential contact with industrial machinery during the loading and unloading (de-ballasting and ballasting) procedures.

If a heat recovery system is used to preheat water coming into the treatment system, there should be no significant safety concerns associated with filling ships’ ballast tanks with heated water. However, if the waste heat is not captured from the boiler discharge, and very hot water is pumped directly into empty ballast tanks, the resulting expansion and contraction of steel structures may compromise the structural integrity of the ship. Such weakening of ships’ structures would need to be evaluated by naval architects to determine if substantial safety risk is presented in this design scenario.

The two main environmental concerns of heat treatment systems are thermal pollution and air pollution. Thermal pollution is primarily a concern if the ballast water is treated on discharge rather than intake. If intake water is treated, waste heat energy from the treatment process is transferred to the ballast tanks. Some undesirable effects associated with ballast tank expansion and contraction might result (refer to Safety discussion); however, thermal pollution to the environmental is assumed minimal. The heat energy in the tanks will gradually dissipate during the ship’s voyage. This is not the case, however, if the treatment is applied to the ballast tank discharge. In this case, large volumes of very warm water will be discharged outside the ship, which could conceivably cause localized effects to the environment. If the discharging ship is underway at moderate speed, the affects will be minimized. However the if the discharge is conducted in confined waters (e.g., while at berth), the affects could be significant (e.g., heat stress to native flora and fauna). The design and construction of an efficient system can reduce the concern of thermal pollution, as well as save energy, by using the heated water leaving the treatment boiler to preheat the water coming into the boiler.

Onix Corporation presented information to Battelle indicating that substantial energy recovery can be achieved by preheating of boiler-intake water. With an assumption that the intake water is 35°F (2°C), and water exiting the boiler at 170°F (77°C), Onix reports that the intake water can be elevated to approximately 140°F (60°C). Aside from substantial benefit of energy conservation [needing only to elevate treatment water 30°F (D 16°C), instead of 105°F (D 75°C)], the reduction of boiler water temperature from 170°F to 45°F will limit potential negative effects associated with expansion/contraction of ballast tank structures that could occur if the 170°F water were pumped directly to the ballast tanks. Correspondingly, if heat treatment is applied to ballast water as it is discharged from the ship, scavenging heat from water exiting the boiler will substantially lessen thermal pollution to the environment.

As for boiler exhaust impacts, we assumed that the heat treatment boilers would be kept in good operational conditions. However, considering that it is very likely for the treatment process to be applied in port (when ballast is being taken up or discharged), regional air impact could be significant. Air pollution from many ships in port undergoing heat treatment could be a significant incremental pollution impact, as many port regions have preexisting air-quality problems.

Regarding venting of boiler exhaust gases, we assume that in most cases treatment boiler exhaust will be either routed to the main stack onboard the ship or a separate stack will be constructed (particularly if topside boiler installation is necessary). Adding an auxiliary exhaust stack to a ship might present engineering problems (Tagg, 1998), safety problems, and obstruct deck activities, but it is not, in itself, expected to cause additional environmental impacts.

Nonenergy O&M costs for heat treatment could not be clearly determined for this technology relative to ballast water treatment. As previously discussed, stand-alone boilers, such as would be necessary ballast treatment, do not require extraordinary supervision during operation. Repair and maintenance of the system can presumably be conducted by one of the ships’ engineers, with minimal need for extended training. The boilers would require some general maintenance such as burner upkeep and tank cleaning, which is similar in scope and complexity as other shipboard systems. Total O&M cost, including part replacement, will be probably be driven by the size of the boiler, which, as previously discussed, is a function of water flow rate and operating temperature.

Similarly, energy costs are difficult to quantify. Recovering heat energy from boiler discharge to preheat boiler intake water will show dramatic energy/fuel savings. The expected problem, however, will be the size and complexity of the heat exchangers necessary to retrieve the lost heat. As previously discussed, heat treatment boilers are relatively large devices and equipment room space may preclude their installation below decks. Similarly, space for heat exchangers may be at a premium and significant O&M costs may result if these devices cannot be readily inspected and serviced

Heat exchangers are marketed for a variety of industrial purposes and come in a range of sizes and designs depending on their application. The purpose of these devices is to transfer heat between two fluids, such that one fluid is heated and the other fluid cooled. Heat exchangers used in a ballast-water heat treatment system can increase total treatment plant performance and energy efficiency. Heat sources for heat exchangers can be independent treatment-system boilers, coolant water (‘jacket water") from the ships’ propulsion engines, or auxiliary steam from the ships’ boilers. The feasibility of using any of these three heat source options is dependent on ship specifications, desired treatment rate, and target temperature and exposure period to achieve necessary biological effectiveness. To simplify the presentation of data and calculations, the following discussion focuses primarily on the use of an independent boiler to heat ballast water to 150°F (65.5°C) at 1,200 GPM. Technical specifications are presented in Table 4-2.

Heat exchangers passively transfer heat between two liquids by circulating them through a stack of metal plates. Heat moves across the plates (across the thermal gradient), lowers the temperature of the hot liquid, and raises the temperature of the cold liquid. The effectiveness of the heat transfer depends on the thermal gradient, which equates to the temperature and amount of heated liquid (i.e., water) or steam that is circulated through the hot side of the exchanger, as well as the temperature and amount of cold liquid circulated through the other side (Figure 4-2). For example, data provided by Alfa Laval, Inc. indicate that 1,700GPM of water heated to 170E F is needed to raise the temperature of seawater from 40E F to 150E F at a rate of 1,200GPM.

On a ship, there are three possible heat sources for temporarily elevating the temperature of the ballast water in a "closed" system: (1) steam from the main boiler system, (2) hot water from the engine cooling jackets, and (3) a standalone boiler specifically for ballast water treatment plant.

The principles that govern boiler efficacy are the same for heat exchangers. Biological performance is dependent on temperature and exposure period. The speed at which an exchanger can raise the temperature to the desired endpoint for biological inactivity is in relation to the fuel consumed and the size of the exchanger system. A longer exposure (treatment) period allows for lower final temperatures and less fuel consumption. Short exposure periods require higher temperatures and more fuel to achieve equivalent biological effectiveness.

Industrial heat exchanger systems are constructed from hardened steel, stainless steel, and titanium. These systems should prove adequate for onboard treatment of ballast water. If below-deck space limitations require that components of the system be located on deck, a structure will be needed to provide protection from harsh elements of the maritime environment.

Heat exchanger systems require minimum attention during operation. They are configured such that the heating source fluid and the object fluid are concurrently pumped through the system to receive treatment. The repair and maintenance of the units should not require special training for any of the crew, and would presumably be conducted by one of the ships’ engineers. The system would require intermittent cleaning, to remove "scaling," and fouling that may occur from the processing of seawater.

The safety issues discussed under Section 4.1 are applicable to heat exchangers as well as boilers.

As discussed in Section 4.1 relative to boilers, the environmental issues associated with heat plants are air pollution and thermal pollution. There will be minimal increases in air pollution with the use of heat exchangers that draw their thermal energy from the main plant in the form of steam or jacket water. This is because the ship is already producing this energy. If the ship does not have the available capacity to provide this energy, a specific boiler is needed to produce the energy, which will result in an air pollution contribution.

The discharge of heated water directly into ballast tanks could create safety concerns based on potential expansion and contraction of the steel structure of the ship (See Section 4.1 Safety). However, the use of a recovery heater removes much of the heat from the treated water and reduces this risk to a negligible level. The residual heat that enters the ballast tanks is expected to dissipate gradually from the ship, and there will be no significant thermal impact to the aquatic environment when the ship deballasts.

Nonenergy O&M costs for heat exchangers could not be definitively determined for this technology relative to ballast water treatment. As previously discussed, heat exchangers, such as would be necessary for ballast treatment, do not require extraordinary supervision during operation. Repair and maintenance of the system can presumably be conducted by one of the ships’ engineers, with minimal need for extended training. The exchangers would require some general-maintenance plate inspection and cleaning, which is similar in scope and complexity as other shipboard systems. Total O&M cost, including part replacement, will be probably be driven by the size of the system, which is a function of water flow rate and operating temperature. Labor costs for O&M will be a function of available shipboard manpower.