The magnesium carbonate process of water treatment has replaced alum in a portion of two water plants in full scale studies conducted over the past two and one-half years. This new water treatment technology was compared to the presently used alum process in parallel treatment using identical units in Montgomery, Alabama and Melbourne, Florida. All water is recycled within the process along with the three basic water treatment chemicals - lime, magnesium bicarbonate, and carbon dioxide. Other advantages found were increased floc settling rates, simplicity of operation and control, reduced costs when sludge treatment and disposal costs are considered, and more complete disinfection. In Melbourne's case, considerable energy would be conserved by on-site lime recovery.