Cost Optimization of Wastewater and Septage Treatment Process

Analysis of the current knowledge has revealed the lack of a method for increasing the cost-effectiveness of wastewater and septage treatment in plants overloaded by contamination. This was the premise for undertaking research on the process of septage pre-treatment in a subsurface vertical flow constructed wetland (SS-VF) prior to its input into the biological section of a municipal treatment plant. In previous research the authors have indicated that this allows for a significant reduction in the value of pollution indicators. The objective of this paper is to assess the cost-effectiveness of this process by means of an optimization model. The decision variable was the coefficient of septage stream distribution into the quantity directed to the SS-VF bed in relation to its total quantity. The optimization criterion was the minimization of the expected annual cost of wastewater and septage treatment. Verification of the model has shown that it is reasonable to subject all septage to the pre-treatment in a SS-VF bed for small wastewater treatment plant (WWTPs) located in rural areas. The bigger the septage pollution load is, the greater the reduction in the treatment costs. The proposed solution is less cost-effective in urban areas, where the construction of a SS-VF bed requires land purchase and additional costs of its adjustment. Optimization results largely depend on the cost function, so it is important to build it on reliable local data.