Optimal levels of remineralisation for desalinated waters

Abstract

The purpose of remineralization is generally to produce water with a Langelier saturation index (LSI) very close to zero that remains stable in contact with the atmosphere. It is also important to accomplish this with the lowest possible consumption of CO2 to minimize operating costs. This study consideres the way to adequately remineralize desalinated water for a range of temperatures of between 14°C and 25°C. At most desalination plants, CO2 needs to be added following reverse osmosis and prior to remineralization. The concentration of CO2 in osmotized water required to reach the proposed levels of remineralization depends on the permeate alkalinity prior to remineralization. In this article, the dosage is shown that is required for each water according to the pH and alkalinity of the permeate prior to remineralization with calcite beds. In the case of using Ca(OH)2, the dosage of CO2 should be doubled. Underdosing of CO2 produces water with low alkalinity. This implies a low buffering capacity and, therefore, a tendency to easily acidify through uptake of CO2 from the atmosphere. This fact, coupled with the low calcium content, makes this type of water unsta-ble, with a tendency to be corrosive. Overdosing of CO2 implies higher alkalinity and hardness in remineralized water, as well as a CO2 content that tends to decrease in contact with the atmosphere, causing the pH of the water to increase and thus giving rise to a slightly positive LSI. It is noted that the hardness of the remineralized water does not usually reach 100 mg CaCO3 while the LSI is between 0.5 and 1.0, even at temperatures of 40°C. In many water distributions networks LSI values of between +0.1 to +0.5 are required to protect the facilities. This is achieved in the case of CaCO3 post-treatment by slightly increasing the pH through the addition of NaOH (2–4 mg NaOH/L). In Ca(OH)2 facilities this is done with a slight overdose of Ca(OH)2 . On some occasions, technical limitations in controlling the exact dose of Ca(OH)2 may cause an increase in turbidity above 1 NTU.