US Patent 7520993 Water treatment process for oilfield produced water

The invention relates to a method and system for treating an aqueous liquid containing dissolved minerals and dissolved hydrocarbons. Method steps and apparatus for treating a waste water feed stream are disclosed which utilize a warm lime softening system in fluid communication with the waste water feed stream, wherein sludge from the warm lime softening system is recycled to improve lime utilization and enhance silica and boron removal without the addition of an external source of magnesium. In addition, a microfiltration system and/or an air stripper system may be used in fluid communication with at least one reverse osmosis system to produce a treatment water that meets state and federal guidelines for surface discharge.