Determine the appropriate number of units and dimensions for settling basins to treat 75,000 m3/d at an overflow rate of 0.8 m/h. Assume that the settling-basins will be constructed of reinforced concrete and that the cost of forming and pouring circular walls is 1.25 times the cost of forming and pouring straight walls. What will be the relative costs of using circular tanks compared to rectangular tanks utilizing common walls where possible?