Optimal Design of Wastewater Treatment Systems by Enumeration

Wastewater treatment systems were designed to achieve specific requirements at least cost. The trend is to design the most efficient processes. An enumerative mathematical model was used to evaluate two common treatment systems. The model provided designs based on standard criteria and evaluated costs using published equations. It was found that mixed liquors suspended solid (MLSS) levels in activated sludge systems should be 3,000 mg/l. For primary settling tanks preceding activated sludge, the computer always selected the least efficient unit, in terms of BOD and suspended solids removals. Anaerobic digesters were selected over aerobic digesters in all optimal designs, but the cost difference was small. The model is limited by the alternatives considered, the design criteria selected and the constraints imposed, but provides insight into the effects of common design decisions.