Minimizing costs of activated‐sludge systems

Abstract

The concentration of mixed liquor suspended solids (MLSS) in aeration tanks affects the required aeration tank volume and the surface area required for secondary settling. As the MLSS increases, the required aeration volume decreases, resulting in decreased construction costs; however, because of solids‐flux limitations, the required clarifier surface area increases with an increase in MLSS, and the cost for clarifiers increases. The total cost of an activated‐sludge system is a minimum at an intermediate MLSS concentration. Previous investigators have shown that the MLSS at which total system cost is minimum depends on sludge settling properties. The optimum MLSS is higher for sludge that settles well than for sludge that settles poorly. This paper demonstrates for the first time that the mass of biological solids that needs to be maintained in the aeration tanks affects the optimum MLSS. Plants with a high mass of biological solids—such as with nitrification or biological nutrient removal—should be designed for higher MLSS than plants with low biological solids. Similarly, plants without primary settling should be designed for higher MLSS than plants with primary settling. The MLSS value at the minimum system cost can be determined using curves for costs of aeration tanks and secondary settling. For plants with multiple units (for example, the Deer Island Treatment Plant in Boston, Mass.), where the cost of each additional aeration tank or secondary clarifier is relatively constant, the impact of additional secondary clarifiers can be assessed in combination with the need for fewer aeration tanks. The ideal MLSS is specific to a particular combination of flows, waste strengths, and cost setting, but the optimum MLSS can be readily determined for particular cases.