is more likely to sink and thus less likely to pose aesthetic problems. If necessary and possible, weights should be added to promote sinking. Compacted bales of garbage should be discharged over deep water (50 m or more) to prevent rapid loss of their structural integrity due to wave action and currents.

4.6.2 Floating cargo-associated waste that is not plastic or otherwise regulated under other MARPOL annexes may be discharged beyond 25 nautical miles from the nearest land. Cargo-associated waste that will sink and is not plastic or otherwise regulated may be discharged beyond 12 nautical miles from the nearest land. Most cargo-associated waste may be generated during the loading and unloading process, usually at dock side. It is recommended that every effort be made to deliver these wastes to the nearest port reception facility system prior to the ship's departure.

4.6.3 Maintenance wastes are generated more or less steadily during the course of routine ship operations. In some cases, maintenance wastes may be contaminated with substances, such as oil or toxic chemicals, controlled under other annexes or other pollution control laws. In such cases, the more stringent disposal requirements take precedence.

4.6.4 To ensure timely transfer of large quantities of ship-generated garbage to port reception facilities, it is essential for ships or their agents to make arrangements well in advance for garbage reception. At the same time, disposal needs should be identified in order to make arrangements for garbage requiring special handling or other necessary arrangements. Special disposal needs might include off-loading food wastes and associated garbage which may carry certain disease or pest organisms, or unusually large, heavy, or odorous derelict fishing gear.

5.1 The range of options for garbage handling aboard ships depends largely upon costs, personnel limitations, generation rate, capacity, vessel configuration and traffic patterns. The types of equipment available to address various facets of shipboard garbage handling include incinerators, compactors, comminuters and their associated hardware.

5.2 Grinding or comminution

When not in a special area, the discharge of comminuted food wastes and all other comminuted garbage (except plastics and floatable dunnage, lining and packing materials) may be permitted under regulation 3(1)(c) of Annex V beyond 3 nautical miles from the nearest land. Such comminuted or ground garbage must be capable of passing through a screen with openings no greater than 25 mm unless such comminuters or grinders comply with international or governmentally accepted standards which effectively accomplish this. It is recommended that garbage not be discharged into a ship's sewage treatment system unless it is approved for treating such garbage. Furthermore, garbage should not be stored in bottoms or tanks containing oily wastes. Such actions can result in faulty operation of sewage treatment or oily-water separator equipment and can cause sanitary problems for crew members and passengers. Options for grinding or comminution include the following:

5.2.1 A wide variety of food waste grinders are available in the market and are commonly fitted in most modern ships' galleys. These food waste grinders produce a slurry of food particles and water that washes easily through the required 25 mm screen. Output ranges from 10 to 250 liters per minute. It is recommended that the discharge from shipboard comminuters be directed into a holding tank when the vessel is operating within an area where discharge is prohibited.

5.2.2 Size reduction of certain other garbage items can be achieved by shredding or crushing and machines for carrying out this process are available for use on board ships.

5.2.3 Information on the development and use of comminuters for garbage aboard ships should be forwarded to the Organization.

5.3 Table 3 shows compaction options for various types of garbage.

5.3.1 Most garbage can be compacted; the exceptions include unground plastics, fibre and paper board, bulky cargo containers and thick metal items. Pressurized containers should not be compacted since they present an explosion hazard.

5.3.2 Compaction can reduce the volume of garbage into bags, boxes, or briquettes. When these compacted slugs are equally formed and structurally strong, they can be piled up in building block form; this permits the most efficient use of space in the storage compartments. The compaction ratio for normal mixed shipboard garbage may range as high as 12:1.

5.3.3 Some of the available compactors have options such as sanitizing, deodorizing, adjustable compaction ratios, bagging in plastic or paper, boxing in cardboard (with or without plastic or wax paper lining), baling, etc. Paper or cardboard tends to become soaked and weakened by moisture in the garbage during long periods of on-board storage. There have also been problems due to the generation of gas and pressure which can explode tight plastic bags.

5.3.4 If grinding machines are used prior to compaction, the compaction ratio can be increased and the storage space decreased.

5.3.5 A compactor should be installed in a compartment with adequate room for operating and maintaining the unit and storing trash to be processed. The compartment should be located adjacent to the areas of food processing and commissary store-rooms. If not already required by regulations it is recommended that the space have freshwater washdown service, coamings, deck drains, adequate ventilation and hand or automatic fixed fire-fighting equipment.

5.3.6 Information on the development and use of shipboard compactors should be forwarded to the Organization.

5.4 In comparison with the technology of land-based incineration, the state of the art in marine incinerators is not highly advanced, primarily because the technology has not yet been subject to constraints on air emissions nor to the types of materials that could be incinerated. Marine incinerators in current use are predominantly designed for intermittent operation and hand stoking and typically do not include any provisions for air pollution control. Control of air pollution is normally required in many ports in the world. Prior to using an incinerator while in port, permission may be required from the port authority concerned. In general, the use of shipboard garbage incinerators in ports in or near urban areas should be discouraged as their use will add to possible air pollution in these areas. Special considerations for incinerators are listed below:

5.4.1 Table 4 presents options for incineration of garbage, including considerations for special handling by vessel personnel, combustibility, reduction of volume, residual materials, exhaust, and on-board storage space. Most garbage is amenable to incineration with the exception of metal and glass.

5.4.2 In contrast to land-based incinerators, shipboard incinerators must be as compact as practicable, and with operating personnel at a premium, automatic operation is desirable. Most shipboard incinerators are designed for intermittent operation: the waste is charged to the incinerator, firing is started, and combustion typically lasts for three to six hours.

5.4.3 Commercial marine incinerators currently available vary greatly in size, have natural or induced draught, and are hand fired. It should be noted that incinerator ratings are usually quoted on the basis of heat input rate rather than on a weight charged basis because of the variability of the heat content in the wastes. Some modern incinerators are designed for continuous firing, and can handle simultaneous disposal of nearly all shipboard waste.

5.4.4 Some of the advantages of the most advanced incinerators may include that they operate under negative pressure, they are highly reliable since they have few moving parts, they require minimal operator skill, they are low in weight, and they have low exhaust and external skin temperatures.

5.4.5 Some of the disadvantages of incinerators may include the possible hazardous nature of the ash or vapour, dirty operation, excessive labour required for charging, stoking and ash removal, and they may not meet air pollution regulations imposed in certain harbors. Some of these disadvantages can be remedied by automatic equipment for charging, stoking and ash discharge into the sea outside areas where such discharge is prohibited. The additional equipment to perform these automatic functions requires more installation space.

5.4.6 The incineration of predominantly plastic wastes, as might be considered under some circumstances in complying with Annex V, requires more air and much higher temperatures for complete destruction. If plastics are to be burnt in a safe manner, the incinerator should be suitable for the purpose, otherwise the following problems can result: