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Water quality modeling has been widely used as a support tool for water resources management (Elshorbagy and Ormsbee, 2006). In literature, the generic algorithm of Streeter-Phelps equation was utilized in combination with chemical or biological degradations to calculate various water quality indexes in rivers. In many cases, 1st order BOD degradation may not be an appropriate assumption if complicated contaminant discharge existed or DO concentration varied acutely. Therefore, a different BOD degradation assumption was employed to couple with DO dissipation equation to simulate water quality, and the simulated result was compared to the monitored result to show better estimation of water quality in a river system.

In the study, the calculation was based on the steady state assumption. To calculate the BOD and DO in a river, the only reactions of interest are BOD exertion and transfer of oxygen from air to water across air-water interface. The classical Streeter-Phelps algorithm was employed to simulate water quality, which assumed the BOD degradation followed 1st order decay, and the diffusive flux in the river is negligible. In addition, the BOD degradation in modified Streeter-Phelps algorithm was also utilized. In the modification, the BOD 1st order degradation was replaced by the mixed 2nd order decay. The solution for the modified algorithm was shown as D=k1BOD0[DO]k2k1[DO]·(ek1[DO]tek2t)D0·ek2t

Both the classical and modified Streeter-Phelps equations were applied to the calculations of DO and BOD concentrations in Keelung River, Taiwan. In the Keelung River watershed, more than 2 million people resides, and almost 96% of the aquatic pollution source results from the domestic sewage discharge within the watershed. In the simulation, the reaeration constant (k2 ) is calculated using HEC-RAS hydrodynamic model, developed by US Army Corp of Engineers. In the k1 determination, the literature value of 0.16 day−1 was selected (TEPA, 1997) for the classical Streeter-Phelps algorithm calculation. For the modified Streeter-Phelps equation (Equation (4)), the k1 value was determined to be 0.188 L/day-mg by the least square analysis. Also, in this calculation, temperature and salinity correction were considered. For the simulated flow rate, the analysis of flow rate in the upstream Keelung River was performed, and the Q75 flow rate was calculated to be 1.77 cms. The monitored flow rate on 2003/09/05 (1.57 cms) and 2003/12/03 (1.54 cms) were selected for model calibration and verification, respectively.

For the classical Streeter-Phelps algorithm simulation, the calculated DO concentration agreed with the observed data in trend, but the BOD concentration was calculated higher than the observed BOD concentration. This finding implied that the 1st order BOD degradation might not be appropriate to describe the BOD degradation behavior in Keelung River. Generally, the aerobic BOD degradation is a function of BOD concentration itself and DO concentration in the aqueous phase. More DO in the aquatic environment as the electron acceptor should enhance the organic contaminant degradation. Usually for the purpose of simplification, the DO concentration is assumed constant, and such assumption leads to apparent 1st order pattern of BOD decay. The classical Streeter-Phelps algorithm has been widely used in river water quality simulation. In some cases, 1st order BOD degradation assumption may lead to apparent deviation of calculated results from observed ones in pragmatic application. To improve this situation, modified 2nd order BOD degradation pattern may be utilized, which would provide better estimation of water quality with reasonable assumption.
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Document Type: Research Article

Publication date: 01 January 2007

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