Reduction of Sediment in Ballast Water Tanks through Design and Operational Measures

There must be equal emphasis on preventing sediment accumulation as there is on treatment of ballast water in ships. Ships should be designed and constructed with target to minimize the uptake and undesirable entrapment of sediments, facilitate their removal and provision of safe access allowing removal without compromising safety or operational efficiency. IMO has developed guidelines to assist ship designers, shipbuilders, owners and operators to design ships to minimize the retention and accumulation of sediment (IMO, 2012). Ballast water treatment systems (BWTS) do nothing to reduce the volume of sediment. 65 per cent of the sediment is smaller than 20μm. A 50μm filter will reduce sediments by just 10 per cent. Sediment increases by about 2 to 3 cm each year. Some species notably toxic dinoflagellate cysts, can survive in the sediment for months (Hallegraeff and Bolch. 1992), forcing some port authorities to restrict stripping operations at port. Sediment allows the survival of benthic and planktonic organisms on the bottom of tanks. Sediment may render a BWTS ineffective. In the presence of sediment it cannot be guaranteed that compliance with the D2 standard will be met. The sediment itself prevents the elimination of the microorganisms that are located below the deposits. B The turbidity and presence of sediments in ballast water can have a significant effect on the effectiveness of UV treatment. Besides hosting the organisms, ballast tank sediments presumably act as a buffer in increasing the salinity tolerance of the organisms in the ballast tanks (Drake, 2005). The accumulation of sediment, sand, water and other impurities in the bottom of the tanks develop a very corrosive environment (Bilgin et al, 2016). It is thus necessary to apply simple and inexpensive design and operational solutions to improve the management of sediment on ships. Equivalently sediment accumulation is reduced when the ballast water residuals are reduced. Strength and structural continuity are the primary considerations in ship design and construction. Unfortunately, drainage is a secondary consideration. The design must ensure the structural and operational integrity of ships but at the same time facilitate the flow of ballast and removal of sediments. Until the IMO Ballast Water Treatment Convention came into center stage, cleaning of the ballast tanks has not received the attention it deserves.