Subsurface flow constructed wetlands for the treatment of domestic wastewater

This thesis describes in detail the background, methods, results, and conclusions of a three-year PhD research project - “subsurface flow constructed wetlands for the treatment of domestic wastewater”, which was undertaken at Monash University between April 2008 and December 2010. The overall objective of the research was to discover the efficiencies of key pollutant removals (organics, nutrients, solids, and coliforms), when a medium-strength wastewater is treated in different types of subsurface flow wetlands, and how the pollutant removal should be modelled to reflect the biodegradation kinetics of the pollutants. The research was designed and carried out in four phases: In phase 1, a comparative experiment was carried out to understand how wetland media may affect the removal of organics and nitrogen; special focus was on studying the nitrification-denitrification process in hybrid wetland systems with an unconventional organic-containing wetland media (wood-mulch). The results indicated simultaneous nitrification-denitrification in vertical flow wetlands with organic mulch media, primarily due to greater oxygen flux in the wetland matrix and the supply of organic carbon from the media. In contrast, this unconventional media was found to be inefficient when it is applied in horizontal flow wetlands, as anoxic conditions caused excessive leaching of organics from the media. The monitoring of the performances of different wetland units, during this phase, allowed the selection of an effective hybrid wetland system for further experiments, to monitor pollutant removal stability in the next phase. In phase 2, experiments were carried out in three hybrid wetland systems that had identical configurations, plants (Phragmites australis), and media (including gravel, wood-mulch, and zeolite). The three systems were operated under different pollutant and hydraulic loadings, to monitor the removal of nitrogen and organics. The results demonstrated stable nitrogen and organics removal efficiencies within the range of hydraulic and pollutant loading variations. The media of wood-mulch and zeolite were found, or further confirmed, to enhance nitrogen removal in vertical flow wetlands, by enhancing simultaneous nitrification-denitrification and adsorption of ammonia. Horizontal flow wetlands with the traditional gravel media showed abilities to remove nitrate and the excess organics that was leached into the wastewater from wood-mulch media and, within the experiment range, the mass removals of nitrate and COD in the horizontal flow wetlands were found to increase with their loadings. In phase 3, kinetic models were developed to describe the biodegradation of nitrogen and organics removal in wetland systems. Reaction kinetics that were considered in the model development included first-order, Monod and multiple-Monod-kinetics; these kinetics were combined with continuous-stirred tank reactor (CSTR) or plug-flow pattern, to produce equations that link the inlet and outlet concentrations of each key pollutants across a single wetland. Using three statistical parameters, a critical evaluation was made to evaluate the suitability of five potential models for describing pollutant removals in vertical and horizontal flow wetlands. The results recommended the models that were derived from Monod kinetics for nitrogen and organics removal in both vertical and horizontal flow wetland systems. Finally, in phase 4 a simple MATLAB programme was established, based on the model studies in the previous phase, to provide a potential design tool for determining the surface area of a single subsurface (vertical or horizontal) flow wetland, when it is required to achieve a given pollutant removal target. The MATLAB programme can also be used to predict the performances of existing wetland systems. In order to assist readers to track the results of four phases of this research in its logic sequence, the thesis has been arranged to contain seven chapters. Chapters 1-2 describe the scope and objectives of the research and a literature survey on the technical development of subsurface flow wetlands for wastewater treatment. Chapters 3-6 describe in sequence the detailed methods and results of the four phases of the research. Overall conclusions are presented in Chapter 7. Four publications, including three ISI journal papers and one international conference paper, have been produced as a direct result of the research. The papers are attached in Appendix I, and all the original experiment records are attached in Appendix II. Overall, this research improved the current understanding of the removal of ammonia, nitrate, COD and BOD in vertical and horizontal flow wetlands. In addition, it produced kinetic models to describe the removal of these pollutants, potentially allowing model-based design of vertical and horizontal flow constructed wetlands for the direct treatment and reclamation of medium-strength wastewater