In extreme cases where water is completely lacking or inadequate, and no other conventional supplies are available, it may be necessary to transport water by tanker from another source far removed from the point of use. When such water transfers require shipment across the sea, motorized water tanker vessels or barges are commonly used. Islands which suffer regular droughts should consider providing permanent barge off-loading facilities, including storage, as a component of their water distribution systems.
Barging of water involves the physical transportation of water from one location to another by sea, using a barge or similar tank vessel. Barges should contain storage tanks of adequate size to maximize the value of the volume of water transported relative to the cost of transportation. The storage tanks must be suitably constructed and cleaned to prevent contamination of the water; generally, they should be single-purpose vessels and not used for the transportation of other liquids. Barges may be self-propelled, but are generally towed by another vessel such as a tugboat. Once the barge arrives at a suitable port, it is secured and the water transferred by pumps to storage tanks or vehicles on land. The water is then either pumped directly into the water distribution system from the storage tanks, or distributed to consumers using tanker trucks. Protecting the purity of transported drinking water is essential, and the quality of the water should be monitored.
Extent of Use
Marine vessels were used in Antigua during the drought of 1982-1983. More than 20 million gallons of water were barged during that emergency. Currently the Morton Salt Company in Inagua, Bahamas, and the Bahamas Water and Sewerage Corporation in New Providence use vessels to transport water. The Water and Sewerage Corporation has chartered a 5 000 deadweight ton (dwt) water tanker and a 14 000 dwt motorized barge/water tanker on time charter, to operate continuously between Andros and New Providence. In New Providence, 54% of all water consumed comes from the island of Andros.
Operation and Maintenance
The main operational problem experienced in the use of marine vessels is weather delays. Based on the experience in the Bahamas, barges are unable to operate on an average of approximately 25 days per year. The second most frequent problem experienced is mechanical breakdown of the vessels, which can halt water transportation for a period of 1 to 7 days per incident. Approximately 15 days per year are lost due to mechanical problems.
The Water and Sewerage Corporation on Andros employs one person, periodically assisted by a second, to manage the charter operation. The need of the Corporation for spare parts is minimal (repairs are undertaken by the charter operator) and the skill level required to fill and empty the barge is very basic. Of greatest concern to the Corporation is assuring the purity of the transported water. The Corporation maintains its own laboratory to test the water, and treatment facilities are available to provide any necessary treatment before the water is introduced into the supply system.
Level of Involvement
The level of government participation in the conveyance of water using marine vessels is usually very high. The scale of this type of operation is so large that only organizations involved in public water supply or large resort operators could consider it as an option.
Transporting water by marine vessels is generally more costly than other alternatives. However, this form of waterborne transport does have merit during emergencies.
The cost of barging water from the island of Dominica to the island of Antigua is $20/1 000 gal landed in Antigua; to transport the 1 000 gal by truck from the port of St. John costs between $25 and $50.
The key to low-cost water transportation by barge or tanker is transporting large quantities using large tankers continously over the long term. Economies of scale significantly reduce the unit cost of water transported in this manner. However, for this type of transportation to be effective, there must be very efficient loading and unloading facilities. If these do not already exist, they can be very expensive to construct. The shipment cost of water transported in the Bahamas between Andros Island and New Providence is about $3.41/1000 gal, including fuel costs. Factoring in the cost of the shore facilities (the Water and Sewerage Corporation owns both the production facility on Andros and the receiving facility in New Providence), the total cost of the water is approximately $5.84/1 000 gallons shipped.
Effectiveness of the Technology
The transport of water from Andros to New Providence started in 1976 after the failure of the reverse osmosis and distillation plants on New Providence, which had produced up to 2 mgd each. The production and cargo landing sites and vessels (tugs and barges) were placed in operation within a year, and began transporting 1.8 mgd. This was planned as a temporary solution to the problem, but since it remains the least costly option for providing New Providence with good quality water, the practice continues. Andros now produces 5 mgd of freshwater for New Providence. (While groundwater extraction on New Providence has a lower unit cost than water shipped from Andros, the volume of groundwater available has remained constant for the past 20 years mainly because additional land for well-field expansion cannot be acquired; thus, increased water demands in the future will have to continue to be met by the shipment of water from external sources.)
This technology also was effective in augmenting the water supply in Antigua during the severe drought of 1982-83. However, it was determined that it could not supply the needs of the island on a continuing basis because of the prohibitive transportation costs. For this reason, a desalination plant was constructed in 1987 to provide an assured water supply.
This method of transporting water is suitable for most coastal areas where there are suitable berthing facilities for barges and the infrastructure is in place to store or distribute the water after it is unloaded.
· The technology does not require highly skilled personnel to operate it.
· It may be cost-effective, depending on the costs of the available alternatives.
· There is a lag period before the technology can be implemented; start-up times to charter a ship are generally about 3 to 6 months.
· Operations are affected by the weather; shipping may be halted when winds are greater than 27 knots and the seas higher than 11 ft.
· The cost of transportation is high and in some cases may be prohibitive.
· Transportation times are relatively slow.
· The quality of the water at the point of use may be difficult to assure, owing to possible contamination by seawater and/or other contaminants during transportation.
· Water must be distributed from the barge to the consumers.
The use of this technology is well accepted in the Caribbean islands where the water bome transportation of water is feasible.
Further Development of the Technology
In order to make water conveyance by marine vessels more efficient, infrastructure must be put in place to allow for the immediate distribution of barged water to consumers once the barge arrives in a port. This requires that pumps, treatment or disinfection facilities, and transmission lines be in place at the port. Considering that in many cases this infrastructure might only be used every 5 to 7 years, during drought periods, it becomes difficult to justify such an investment. Thus, inexpensive portable off-loading facilities that can be used in times of emergency would be a desirable future development.
Cadrington Coleby, Water and Sewerage Corporation, Ministry of Works and Lands Building, Post Office Box N3905, Nassau, Bahamas. Tel. (809)428-3451. Fax (809)429-5292/424-9228.
General Manager, Water and Sewerage Corporation, Post Office Box N3905, Nassau, Bahamas. Tel. (809)323-3944. Fax (809)322-5080.
John Bradshaw, Water Manager (Ag.), Antigua Public Utilities Authority, Post Office Box 416, Thames Street, St. John, Antigua. Tel/Fax (809)462-2761.
Vincent Sweeney, Sanitary Engineer, c/o Caribbean Environmental Health Institute (CEIH), Post Office Box 1111, Castries, Saint Lucia. Tel. (809)452-2501. Fax (809)453-2721. E-mail: email@example.com.
Rogers, Milton. 1989. "The Public Water Supply in Antigua," APUA Review, 1(3).
Woodroffe, Mike. 1992. "The General Manager Says...," APUA Review, 4(1).