MAGNESIUM CARBONATE AS A COAGULANT, RECOVERY AND REUSE

Abstract

In order to prevent water losses through disposing hydrous sludge during treatment of water, also recovery and reuse of chemicals; treatment by magnesium carbonate as a coagulant was examined in batch and continuous systems. 600 mg/1 MgCO3 as optimum dosage reduced 80% turbidity of a highly turbid water containing 200 NTU. Sulfates decreased, but hardness and alkalinity showed some increases. Recarbonation by CO2 resulted in recovery of 45% of magnesium carbonate which was used again as part of coagulants in continuous system. A mixture of magnesium carbonate and calcium oxide (lime) was examined. 600 mg/1 MgCO3+250 mg/1 CaO as optimum dosage showed 90% reduction in turbidity, 10% reduction in both hardness and alkalinity and almost 60% reduction in sulfates. Recarbonation of sludge resulted in 73.5% recovery for magnesium carbonate and 40% for the lime, which were used again in a continuous system. Six hours operation of a continuous system with a detention time of 1 hour confirmed results obtained from batch analysis. A third trial was experienced with a mixture of magnesium carbonate+sodium carbonate. Although the total hardness showed more decreases, but the turbidity reduction was not satisfactory. As the conclusion, treatment of water by a mixture of MgCO3+ CaO is our choice. There will be no water losses in this method, resulting additional incomes by selling the 15-20% saved water and savings in expenditure and cost of coagulants. Cost of new constructions and CO2 production seems insignificant, comparing to savings and new income.