A wastewater flow of 3550 m³/d is to be treated in a facultative pond system. The reaction rate coefficient at the average operating temperature is 0.35 d^-1. The pond system is assumed to behave like a CFSTR. Determine the surface area required for 85 percent removal of soluble BOD for a pond depth of 2 m with (a) a single-cell pond and (b) a four-cell system. The BOD₅ of the wastewater is 500 mg/L.