Dechlorination equipment development for shipboard pollution prevention

Biological fouling (biofouling) of ship seawater cooling system components has been a persistent problem at several Navy homeport locations. Dockside electrolytic chlorination for biofouling control in ship seawater cooling systems is being utilized at Submarine Base (SUBASE) and Shipyard Pearl Harbor, Hawaii. However, the discharge of chlorine into Pearl Harbor is regulated by the Environmental Protection Agency (EPA) and the State of Hawaii under the National Pollutant Discharge Elimination System (NPDES). Current State of Hawaii discharge standards of 0.1 mg/L chlorine as total residual oxidant (TRO) for two hours a day is, along with its upstream concentration, not wholly effective in controlling biofouling in seawater cooling system components. In addition, more stringent regulation of chlorine point-source discharge into marine waters by the EPA and state environmental protection agencies is anticipated. In response to these requirements, the Navy has developed a portable dechlorination unit for use shipboard. This unit injects a sulfur-based dechlorination chemical that mixes with, and chemically binds the chlorine, making the resulting TRO discharge acceptable under current regulations. A team of scientists from the Naval Research Laboratory (NRL) and CHEM-corr Consultants conducted a concept ship evaluation (SHIPEVAL) of a prototype dechlorination unit aboard the USS Indianapolis (SSN 697) during the period 12-23 September 1994 at SUBASE Pearl Harbor, Hawaii. Results indicate that sodium bisulfite injection downstream of SSN 697 auxiliary sea water (ASW) system heat exchanger components, at an approximate treatment ratio of 2.5:1 was fully effective in dechlorinating 0.1 mg/L TRO to zero prior to system discharge. Lessons learned from the SHIPEVAL will be utilized to develop a fleet-ready, portable dechlorination unit for use at various Navy homeport locations.