An effective means of providing a greater level of disinfection without increasing disinfection byproducts is to improve chlorine contact chamber flow characteristics. The question of the most efficient geometric design of a chlorine contact chamber has been studied in the past. A circular basin is more economical than a rectangular basin, but typical baffle wall arrangements are difficult to fit into a circular shape. The most common baffle designs are serpentine patterns or other combinations of straight chambers, which typically achieve disinfection efficiencies (T10/T) of 70% or less. A new spiral baffle design combines the economics of a circular contact chamber with the extremely high disinfection efficiency and low headloss inherent in a spiral baffle wall arrangement. Although spiral baffle designs have been designed and built in the past, the actual construction of a spiral is often mistakenly thought to be difficult. Most previous designs relied on concentric circular walls inside the outer shell, with a radial wall intersecting the concentric circles. This design is essentially a hybrid of a spiral and a serpentine pattern, and although more efficient than a straight-walled serpentine arrangement, the discontinuity of the circular walls due to the gaps along the radial wall make for difficult analysis, and a substantial boundary member. The spiral design was composed entirely of circular semi-circles or partial circles, of varying radii. This eliminated all sharp corners, allowing the water to always flow in circular pattern, thereby considerably reducing turbulence and head losses. By placing the inlet at the edge of the circular tank and the outlet in the center, concerns of cross connection were eliminated. Includes figures.