Abstract
When seawater is desalted by evaporation method, some polluting materials contained in seawater seem to evaporate and concentrate into the product water. To examine the behaviors of such materials in this process, an experiment was made on ammonium ion (NH4+) by using a 3,000 m3/day multi-stage flash evaporator (Fig.1) and a laboratory-scale distillation flask.
In the plant test, aqueous ammonium chloride solution was added to the make-up line of the evaporator, and the distribution of NH4+ in the plant was traced. When the pH of the circulating brine of the plant was 7.0 or higher, about 65-75% of the added NH4+ was found to concentrate into the product water (the concentration rate: 1.5-1.7)(Table 1, Figs. 6 A, 7 A). In the flask test, after the pH was adjusted at 6.0-9.0 with borate buffer and a small amount of NH4+ was added, the effluent brine of this plant was distilled under the atmospheric pressure. The distillate was taken when its amount reached 3.6% or 14.3% of the total volume, and the distribution of NH4+ in the distillate and in the residue was examined (Figs. 3, 4). The first 3.6% of distillate was found to concentrate NH4+ when the pH of the brine was higher than 6.4, and the first 14.3% of distillate was found to concentrate NH4+ when the pH was over 6.6.
In the plant test where 13.6% of the brine was distilled, the limiting pH to concentrate NH4+ to the product water was around 6.9 (Fig. 8) probably because the distillation took place under various pressures and the gas-liquid equilibria varied from that of the flask test. The pH of the circulating brine in distillation plants is kept at 7.0 or higher in general, and naturally NH4+ is concentrated into the product water. Therefore, in case the distillation is carried out in a plant located near seawaters polluted with NH4+, some proper treatment must be made to remove NH4+ or the operation must be performed keeping the pH of the circulating brine under about 6.
