Abstract: A system and method for injecting waste water into a disposal well comprising a plurality of horizontal tanks interconnected to one another in series in a way that fluid communication is established between the horizontal tanks. The tank chambers of the horizontal tanks collectively have a volume to accommodate a capacity of at least 6,000 barrels of waste water in a twenty-four hour period while providing the waste water sufficient retention time to permit water, oil, and solids to separate from one another.