Abstract
High alkalinity of drinking water in tank of ship is thought to be due to calcium hydroxide from cement coating. It is probable that presence of carbonate radical in water accelerates the dissolution of calcium hydroxide from cement coating by converting it into insoluble calcium carbonate, and also this calcium carbonate film covering cement wall forms obstruction of dissolution of calcium hydroxide from cement. Application of dry ice to tank water may make it possible to keep the pH value of water filled after treatment within the permissible range of drinking. To confirm this possibility, a series of experiment was conducted. And the results obtained are summarized as follows: 1. For the water filled in 200l tank coated with 1kg of wash cement, a single supply of 240g of dry ice (=estimated, amount needed to remove calcium hydroxide in 1kg of cement) could keep the water within the permissible range of pH. But this treatment could not keep the water filled after treatment within this range of pH. 2. A single supply of 120g of dry ice to 200l of water in tank coated with 1kg of wash cement could keep the water within the permissible range of pH. This treatment could keep the water filled after treatment within the permissible range of pH. 3. A single supply of 60g or 24g of dry ice to 200l of water in tank coated with 1kg of wash coment was not enough to keep the pH value of water within the permissible range of drinking. The former case needed 3×30g additional supply and the latter 5×24g. These treatment yielded the same results with respect to the prevention of alkalization of water filled after the treatment. 4. It may be said that the repeated supply of dry ice in small doses to the water filled in tank coated with wash cement can keep the water filled after treatment within the permissible range of pH for drinking. Here, it is important to pay full attention to the fact that an excess of dry ice supply to water ended in rather harmful result.
