Land treatment system design with system parameter optimization support model.
详细信息
文摘
Increasing costs for wastewater treatment provide incentives to evaluate system alternatives, especially among small communities with limited financial resources. Land treatment systems with hybrid poplar tree plantations are becoming increasingly popular as one such alternative in the Pacific Northwest.;Land treatment systems are made up of a complex dual system: the wastewater treatment system and the poplar tree plantation. The complex dual system must account for temporal and spatial variability of natural systems. The goal of the system design is to optimize system parameters, after accounting for system complexity and variability, to maximize economic return without causing adverse effects on the environment and society.;This study presents a system optimization model which accounts for the system complexity and natural variability. The model consists of three submodels: nitrogen loading rate estimation submodel, nitrate leaching submodel and total net revenue estimation submodel. The composite model identifies the maximum total net revenue and determines associated system design parameters.;An optimization example for a hypothetical community with a population of 5,000 is presented. The optimal system with no time horizon or land area constraints is expected to return a net revenue of $773,659 over a 296-year project lifetime. The wastewater treatment system is characterized by the optimal nitrogen loading rate of 49 kgN/ha and the corresponding estimated leachate nitrate concentration of 8.80 mgN/L. The poplar tree plantation is characterized by the optimal rotation period of 8 years and the optimal total field size of 202 ha.;Additional analysis indicates that total net revenue would not be positive if the design system lifetime was shorter than 60 years, or the maximum field size was smaller than 150 ha. Accounting for land depreciation due to wastewater application reduced the optimal nitrogen loading rate by 2 kgN/ha. Costs for an equity loan would reduce system profitability significantly. Sensitivity analysis demonstrated that the model was sensitive to nitrate leaching and poplar tree growth parameters. It was concluded that calibration for site specific nitrate leaching and poplar hybrid specific growth parameters should be evaluated to have more confidence in model performance.