| Summary: | 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. === Graduation date: 2004
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