Optimization of Wastewater Treatment Design under Uncertainty and Variability

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Title: Optimization of Wastewater Treatment Design under Uncertainty and Variability
Author: Doby, Troy A. Jr.
Advisors: Daniel H. Loughlin, Committee Member
S. Ranjithan, Committee Member
E. Downey Brill, Committee Member
John W. Baugh, Jr., Committee Chair
Sarah K. Liehr, Committee Member
Francis L. de los Reyes, Committee Member
Abstract: Typical goals of domestic wastewater treatment are a low cost process that is reliable in terms of meeting effluent quality standards. Designers using traditional steady-state design with modeling of domestic wastewater treatment plants use scalar values as inputs. The inputs are typically of two types — (1) design loadings based upon historical data and (2) stoichiometric and kinetic parameters based upon literature values. There is typically not a way of knowing a priori the reliability of the design or whether the design is least cost using the traditional design approach. Designers using deterministic optimization with modeling of domestic wastewater treatment plants also use scalar values of the two types of inputs as with traditional design. While the designer may now know that the design is least cost given the inputs, there is no way of knowing a priori whether the design is the most reliable for the cost. It is possible to take an existing design — whether obtained by traditional design methods, deterministic optimization, or by any other design method — and determine the reliability of the design. To do so, however, requires characterization of both uncertainty and variability of the data. Uncertainty arises because of a lack of knowledge about an input value and its statistical distribution. Variability arises because of the heterogeneity of the processes determining the input value. In this particular case, the historical input loadings are known and thus the variability is characterized. The characterization of the load variability is then used for future predictions of behavior. The stoichiometric and kinetic parameter values for the particular case are not typically known and thus are uncertain. An approach to the uncertainty of these values is proposed. Different loading criteria (based on percentiles of historical flow and waste concentration data) were used in deterministic optimization. It was determined that the higher the flow percentile, the more expensive the design. However, a more reliable design could be found at a lower cost (and at a lower flow percentile). A different design procedure using stochastic programming is illustrated taking both cost and reliability into account during the design procedure. As a result, a reliability-cost tradeoff curve is generated. This curve is characterized by (1) a very steep portion where slight increases in cost lead to large improvements in reliability; and (2) a very flat portion where large increases in cost lead to small improvements in reliability. This design procedure also allows determination of the value of an experimental program characterizing the uncertainty and variability of the stoichiometric and kinetic parameters and their statistical distribution.
Date: 2005-04-19
Degree: PhD
Discipline: Civil Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/4887

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