Microfiltration for Second-Stage Softening Provides Many Benefits to an Automotive Assembly Plant in Mexico

General Motors de Mexico (GMM) owns and operates an assembly plant in Silao, Mexico. The plant is located in an arid region, and approximately 350-km northwest of Mexico City. The source of the water supply is six wells located on the GMM site. The well water is characterized as moderately hard, with both calcium and magnesium present. It also contains a high concentration of dissolved silica.Due to an impending expansion of the assembly plant in 1999 and plans to build a new engine plant in 2001, GMM initiated a Water Management Project (WMP) in 1997. The project included water conservation, and the expansion and upgrade of the existing water and wastewater treatment facilities. This paper concentrates on the water treatment plant expansion and upgrade.Prior to the WMP, Silao plant utilized single-stage softening with lime. The key components of the water treatment plant were two solids contact clarifiers (SCC's). The SCC's operated at a pH of 10 with lime addition. The clarifier effluent was neutralized by using sulfuric acid, and then filtered, disinfected and stored in a (potable water) storage tank. The sludge generated in the reactor/clarifiers was pumped into a sludge holding tank, and then dewatered by a plate-andframe filter press. The treatment system was effective in the removal of calcium from the groundwater. However, silica and magnesium were only partially removed by the treatment system.The upgrade focused on improving the removal of total hardness (i.e., magnesium) and silica, while employing those processes that conserved water and minimized wastewater generation. To determine optimal conditions, laboratory and pilot tests were performed. The testing determined that a two-stage softening process was needed to achieve GMM's water quality requirements. The two-stage process was realized by installing recarbonation and microfiltration equipment downstream of one of the SCC's and operating the first-stage softening process at a pH of 11.5. The other softening train remained a conventional single-stage softening operating at a pH of 10. The two-stage system has produced a water supply that is low in calcium, magnesium and silica. This has enabled GMM to minimize blowdown from process equipment and conserve water.