**ABSTRACT**

Design of horizontal subsurface flow constructed wetlands using the state of the art *k-C* *first order model requires the use of correct model constants to ensure that the design systems meets the required treatment standards. The aim of the study was to estimate the model constants for the design of a field scale horizontal subsurface flow constructed wetland to treat slaughterhouse wastewater using palm kernel shell as substrate. A 40 days column incubation was conducted and the model was fitted to experimental data. Rate constant (*k _{20}*) values of 0.604/day, 0.623/day, 0.278/day, 0.323/day and 0.306/day were obtained for BOD, TSS, NH

_{4}-N, NO

_{3}-N and PO

_{4}

^{3-}. Estimated model constants were compared to the universal values in the literature which revealed very significant variations in the obtained wetland surface area.

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