Each of the three technologies reviewed in this study can functionally treat ballast water aboard a Laker-class commercial vessel to 3log disinfection levels or better, with or without pretreatment filtration. The primary differences in the technologies reviewed are the status of the technical development and the operating conditions necessary to achieve successful disinfection. As such, a side-by-side comparison of the cost, benefits, advantages, and disadvantages of each treatment option is complex.

In this section, we provide a qualitative assessment of the major attributes of each technology to allow for a comparative evaluation. Selecting the most promising secondary-treatment for ballast water will be primarily based on total cost (capital + O&M), biological effectiveness, and risk of failure.

In the previous three sections, we have reviewed seven critical assessment areas for UV, ultrasonics, and heat treatment technologies.

• Technology Description (operational specifications)
• Biological Effectiveness
• Filtration Effects
• Robustness
• Ease of Operation
• Safety
• Environmental Concerns

To compare treatment options, we have grouped defining elements of these assessment areas under two major factors: biological effectiveness and practicality of shipboard application.

As discussed in Section 1, most commercial ships screen ballast water through a 15-mm sieves at the sea chest and/or hull fitting. Thus, the organisms of concern to this evaluation, and that are typical to ballast water, are: bacteria, viruses, phytoplankton and zooplankton (all life stages), and egg and larval stages of higher level biota. The size range typical to these types of organisms is 0.02 : m – 10.0 mm.

All vendor data assembled during this study on the disinfection of microorganisms, as well as much of the technical literature, showed that all three treatment options are capable of disinfection for the size ranges and the organism types of concern. Only the Lloyd’s Register (1995) study discussed evidence of phytoplankton recovery following UV treatment. Neither the data form the surveyed vendors, nor any of the other reviewed scientific literature addressed post-treatment regrowth other than as an endpoint to evaluated treatment efficacy (e.g., post-treatment germination of dinoflagellate cysts).

The likely reason that the "regrowth issue" was not encountered when querying vendors and searching the literature is that these three technologies have not yet been applied to ballast water treatment. A vendor or researcher concerned with drinking water disinfection or wastewater reuse, judges performance of treatment methods based on use or disposal of the treatment stream. Until recently, there has not been a need to evaluate regrowth of microorganisms in conditions found inside ships’ ballast tanks, which can be as variable and numerous as the ships themselves. Clearly, additional research is needed to address this issue.

Biological effectiveness of a secondary treatment relative to pretreatment filtration shows that filtration substantially improves performance in all three technologies. There are several caveats to this conclusion, however. In UV treatment systems, more filtration means less energy and increased microorganism mortality. To maintain mortality targets (i.e., 3log or better), fine filtration is recommended (25 - 100 F m). Removing suspended particulates and microorganisms from the treatment stream dramatically increase the likelihood that an individual small microorganism will be struck and killed by a photon. Without fine-scale filtration, there are blind spots in the treatment stream, and the only way to maintain mortality targets under such conditions is to significantly increase the UV radiation (add energy) or lengthen the exposure period (slow the flow rate). Neither of these options are advisable from the perspective of least-cost, rapid treatment of ballast by a working vessel.

Filtration relative to ultrasonic and heat treatment is a more complicated situation. With ultrasonic treatment, organism mortality can be increased if there are suspended particles in the water that are available for collisions with the microorganisms. Large particles, however, can potentially clog the process cell (refer to Figure 6-1) and restrict flow of the system, which may lead to other performance problems. For these reasons, it seems prudent that approximately 250-F m pretreatment filtration be used in ultrasonic systems. With research and development, future ultrasonic systems might be developed which can handle, and possibly take advantage of, large suspended sediment loads. If such systems become commercially available, doing away with the pretreatment step will be a substantial cost saving to ballast management.

Pretreatment filtration for heat treatment systems might also be avoided by design changes to boilers. The current concern about filtration and heat treatment is sedimentation of suspended particles in the boilers (i.e., the treatment chambers). Sediment buildup in the boilers will reduce the treatment volume and exposure periods. Biological performance will be reduced when the exposure period is reduced. Only by conducting periodic clean-out maintenance or slowing the flow rate can exposure periods and mortality targets be maintained. Thus, filtration for heat treatment is primarily an O&M expense. It is therefore suggested that heat treatment systems have at least 250-F m pretreatment filtration. The pore size of the filtration system might be adjusted (up or down) depending on the amount of sediment buildup experienced under actual operating conditions.

In summary, after review of each of the treatment technologies, it is the opinion of the Battelle researchers that all of the three technologies can meet the biological effectiveness criteria of 3log biological reduction/microorganism mortality. The efficiency of the biological kill rate (as it relates to capital and operational cost), and the engineering limitations of shipboard application (including environmental and safety concerns) are therefore the basis for selecting one treatment option over another.

Space Limitations

Shipboard conditions are more demanding than most industrial conditions. The high cost of shipbuilding and operation drive naval architects and ship owners to maximized revenue-generating space aboard vessels. The consequence is that equipment areas are minimized and available space for treatment systems is small and finite without major retrofitting. This is especially true for the forward and lower compartments of a ship, the most desirable locations for treatment system installation. Most engine and equipment rooms are large enough only to accommodate equipment and access space for repairs and maintenance. For a ballast-water treatment system to be installed below decks, near to the pumping and piping systems serving the ballast tanks, the treatment equipment must be small, efficient, and require limited maintenance. If the treatment plants are distant from current ballasting infrastructure (e.g., on deck for heat treatment boilers), retrofit cost, maintenance, and loss of income-generating cargo space might be prohibitive. While these descriptions of shipboard space utilization are generalities that vary among specific ships, the practicality of using a ballast-treatment system will be often depend on the size and configuration of the treatment system, verses the available pump room and engine room space.

In the ranking of the three technologies reviewed in this study, heat treatment ranks the lowest for size efficiency. A typical heat system requires 66-sq ft in footprint size to treat a 1200 gpm flow. Ultrasound and ultraviolet technologies require substantially less space, ~10 sq. ft and 17 sq. ft respectively, to treat the same flow rate. These footprints account only for the estimated deck space needed to accommodate the treatment technology without consideration for height and weight dimensions.

The heat treatment option also ranks lowest with regard to weight, as it is one to two orders of magnitude greater in weight, and requires much more headroom than the other two technologies. These space requirements probably make heat-treatment installation impossible in retrofits, and will require careful planning for installation on a new vessel. Some UV and ultrasonic systems also require substantial space. In the case of the HPUP technology, scale up to treat 8000-gpm flow would require substantially more space than even heat treatment equipment (278 sq. ft compared to 120 sq. ft). However, the placement of many smaller treatment units may be more feasible than locating a size intensive boiler.

For prototype demonstrations, both UV and ultrasonic treatment systems are the most favorable options for size efficiency. Heat can be demonstrated for a 1200 gpm flow, but it is size intensive even for small flows and it is not practical for scale up. Ultrasound treatment (using the HPUP system as the example) is extremely efficient for a demonstration flow, but not currently size-efficient for large flows. Ultrasound is an emerging technology, and it is possible that advances in the technology will produce small units capable of handling high flows. Pilot-scale demonstrations and further discussions with both UV and ultrasonic system vendors are warranted.

Auxiliary electrical energy on commercial vessels is often as limiting as space requirements. If electrical specifications needed for a treatment system are identified during the design phase for a new ship, a load analysis will determine the needed onboard generating capacity and electrical service. Adding the additional electrical service to a ship in the design and construction phase is far more economical than adding power-hungry treatment equipment to an existing ship. Most commercial cargo vessels have generating capacity only slightly greater than the electric loads of primary equipment (e.g., main engine circuits, bilge pumps, navigational equipment, etc.). Some vessels have substantial auxiliary capacity, but this is commonly dedicated for bow thrusters, refrigeration, or cargo-handling equipment. Unless energy-generating capacity is increased in these vessels, and the necessary circuitry added, the operation of UV or ultrasonic ballast treatment will only be possible when other equipment is idle (R. Tagg, pers. comm., June 1998)

Based on the energy requirements necessary to treat ballast water using either UV or ultrasonic systems (25 kW/hr or 40kW/hr, respectively), auxiliary power capacity may be sufficient if other electrical loads are limited during the treatment plant operation. However, this might not be practicable, as auxiliary power capacity is usually designated for other uses and sufficient reserves might not be available for treatment plant operation when it is needed. As with the issue of space limitation, power demands of a UV or ultrasonic treatment system will need to be reviewed before treatment plant design and installation. Adding generator capacity for vessels retrofitted with these ballast treatment plants is complex and costly, perhaps to the extent where it would be economically unfeasible. Extensive electrical load and wiring analyses will need to be conducted and must comply with U.S. Coast Guard regulations and insurance criteria. This will require substantial engineering time and compliance review time in addition to capital costs.

Although electrical energy can theoretically be used for heat treatment boilers, it is more energy efficient to operate the boilers with fuel oil (refer to Section 4). The logical choice is to operate a heat-treatment ballast system using the same fuel as the main propulsion plant for the vessel. This avoids the need for handling multiple fuels aboard the vessel, and simplifies fuel delivery and maintenance systems for the heat plant. This said, a heat-treatment system is very energy intensive. Fuel consumption for high-flow systems (1,200 gpm) is estimated at over 475 gph to raise the water temperature from 40E F to 150E F. In addition to fuel consumption, installation of oil lines and exhaust systems can be problematic for a heat plant, especially if the heat plant it is not conventionally located to the fuel systems and exhaust stack(s) servicing the propulsion plant.

Using heat exchangers to augment boiler options will significantly reduce the energy required to elevate the water temperature. Additional substantial fuel savings is possible if the heat-treatment boiler can be replaced or reduced in size if waste shipboard steam or engine cooling water is used to preheat the incoming ballast. For example, to operate an exchanger system such as the one detailed in Section 4.2 (1,200 gpm), energy consumption would be half that of a corresponding flow-through boiler system (475 gph vs. 250 gph). Additional fuel can be saved if steam or engine water can be used for preheating. However, the more components and complexity in a treatment plant, the higher will be installation and O&M costs, indicating the need for a case-by-case evaluation to judge potential economy of fuel saving features. It is probable that retrofitting existing ships with many types of heat plants will be prohibitive from compliance and/or cost perspectives.

It is a conservative assumption that significant modification and retrofit expenses will be incurred if any of the three reviewed treatment technologies are installed aboard an existing ship. These modifications could be substantial, especially for a heat treatment system. Potential modifications for each treatment option are listed in Table 5-1.

Most of the modifications associated with UV and ultrasonic equipment are less than modifications necessary for a heat treatment system. For heat-plant installation, substantial costs include the fuel system, exhaust system, heat exchangers (for transfer and recovery of heat), and plumbing (especially for an off-line location).

A boiler system may require substantial rerouting of the ballast piping based on boiler locations onboard. Safety issues may arise if the boiler is placed on deck and could pose a potential hazard to the crew. In addition, fuel systems may need to be modified to supply the burner with fuel and could include the addition of a specific supply tank. The use of a fuel-fired boiler will require changes to accommodate a new exhaust source. It is possible that the exhaust could be piped into the existing stack if existing piping configurations allow it. More than likely it will be necessary to design and install a new exhaust system specifically for the burner.

Other impacts associated with retrofits include loss of income-producing cargo space and/or use of remaining space. Similarly, a related retrofit expense if the period that the vessel is out of service for installation and testing of the new treatment system. It is likely that the installation period for an off-line boiler and heat exchangers for a heat-treatment plant will incur considerably more shipyard expenses than equivalent installations of UV or ultrasonic plants in engine/equipment compartments. Correspondingly, testing and adjustment of large ultrasonic plants, which are a newer and less developed technology, will likely delay getting a ship back in service.

The cost of treatment plant purchase is quite variable among the technologies and among vendors within technologies. The price ranges for capital purchase of plant equipment only are presented in Table 5-2. As discussed earlier, installation, O&M, supplies, and labor will substantially add to the total purchase and operating costs of these treatment plants. Additional expenses will be incurred if the installed plant interferes with normal shipboard operations, schedules, or cargo capacity/type.

On a straight capital cost basis, the technologies ordered least-to-most cost are: UV, heat, and ultrasonics.

There are three identified environmental concerns related to installation of ballast-treatment plants aboard vessels.

The second environmental concern is the increased energy consumption/air pollution generated by a heat plant. This concern is discussed in Section 4.

The third environmental concern is thermal pollution from heat treatment plants. This concern only applies if treatment is applied to ballast water when it is discharged, and a substantial amount of waste heat is not recovered. Thermal pollution is discussed in Section 4.

Our analysis of the three major technologies revealed no substantial safety or failure risk beyond that already managed abroad commercial ships. Operating principles and equipment used for the technologies are well understood by ships’ crews. The complexity and lack of development of the ultrasonic technology may lead to failure of the biological performance of the equipment, but it should not lead to increased safety hazards to the ship or ships crew.

The single incremental safety risk shared across the three technologies relates to the substantial increase in pumps and pipework, and associated control and operation, required for all treatment plants. With added complexity, comes increased risk of catastrophic failure should a major pipe, valve, or equipment component burst and flood critical shipboard equipment or compartments.

In summary, Table 5-3 compares and contrasts the three technologies across the major elements of biological effectiveness and practicality of shipboard application.

In conclusion, UV is currently the technology best suited for secondary treatment of ballast water. UV treatment technologies are well developed, there are many vendors producing equipment for a variety of applications, and the technology has clear potential for shipboard demonstration (including retrofitted existing vessels with 1,200 to 8,000-gpm flow rates). Innovatech, Inc. currently markets shipboard UV plants for drinking water disinfection and, although their products are designed to treat smaller flows (300 gpm), they claim experience with design and installation of treatment units for shipboard design compliance (R. LaFrenze, Innovatech, pers. comm., July 1998). Another vendor of UV technology, Aquionics reports that they are certified by Lloyds Register for shipboard application of their treatment systems and have many maritime clients (T. Schweitzer, Aquionics, pers. comm., May 1998).