A vessel is provided for treating flowable materials and methods of use wherein the vessel has a substantially cardioid cross-sectional configuration with a central cusp defining two upper rounded lobes. A front and rear wall enclose the ends of the sidewall with one having an inlet and the other having an outlet, both advantageously situated in different lobes forcing the materials to flow laterally within the vessel as well as longitudinally. A bubbler tube may be provided to longitudinally extend along a lower central portion of the sidewall aiding in the flow of materials within the vessel. Optionally, at least one gas outlet port is in an upper portion of at least one of the upper rounded lobes of the sidewall. Optionally, a flange depends from the sidewall cusp and longitudinally extends the sidewall. A bioremediation process is provided where a plurality of vessels are connected in series where the effluent from one vessel feeds the next vessel in series.