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BALLAST WATER TREATMENTdisinfection processes can be further classified into physical orchemical treatment processes. The latter may involve residualcontrol to meet discharge limits for active chemical substancesor physical enhancement technologies to improve treatmentperformance.As shown in Figure 2, the global market for BWT systemsis dominated by two treatment technologies -- UV and electrochlorination.These are manufactured by 26 equipmentsuppliers and compose 85% of the approximately 1,500 IMOtype-approved BWT systems installed to date. The basic processtrains are generally described above.UV Disinfection: This is a two-stage treatment processwith a pressure-rated filter unit (typically with micron-rateddisks or membrane elements) to remove sediment and largerorganisms, followed by a UV disinfection unit to inactivatesmaller plankton, bacteria and viruses. No chemicals are requiredto operate the system. During ballasting, water is typicallyprocessed through both the filter and UV stages as wateris pumped into the ballast tanks. Solids captured by the filtersare discharged at the ballasting location. During de-ballasting,the filter is typically bypassed and water is treated by the UVunit only before discharging overboard.Electrochlorination: This is a two-stage treatment processwith a micron-rated pressure filter or strainer followed by anelectrochlorination unit. The latter is essentially an on-demandchemical system, which uses electrolytic cells to generate asodium hypochlorite (bleach) solution from seawater and electricity.Some treatment systems pump the entire ballast waterflow through the electrolytic cells, whereas others use a smallslipstream to generate a concentrated hypochlorite solution,which is then recombined with the main flow. During ballasting,water is processed through both the filter and electrochlorinationunit as water is pumped into the ballast tanks. A chlorineresidual (or residual oxidant) is maintained in the tanks fora minimum contact time (typically a few days) to improve disinfectionand eliminate regrowth of organisms during transit.(Image courtesy of Severn-Trent deNora)Shipboard installation ofelectrochlorination systemfor generating sodium hypochlorite.56 | Maritime Professional | 4Q 2013