Mathematical Modelling of a Domestic Wastewater Treatment System Combining a Septic Tank, an Up flow Anaerobic Filter, and a Constructed Wetland

Abstract

Wastewater treatment is essential for environmental protection, public health, and to ensure water supply for future generations. Conventional treatment technologies, such as activated sludge, are energy-intensive and require constant maintenance as they were designed for large cities. These technologies are not suitable for wastewater treatment in small communities and rural areas. Decentralized treatment approaches combining Anaerobic Reactors (AR) and Constructed Wetlands (CW) have proven more appropriate in these cases. These passive treatment methods require low investment and maintenance. Additionally, they allow for onsite water reuse and energy generation. The combined configuration of an AR sequentially coupled with a CW have proven to be an efficient system by which the limitations of the individual units (AR and CW) are overcome. Chapter one provides the motivation related to global needs of wastewater treatment, problem statement and context, which is the deficit of treatment and current technologies deficiencies. The research question and solution overview provided by the mathematical modeling applied to decentralized treatment technologies. Finally, the main contributions of this work to the state of the art are detailed. In Chapter 2, several systems combining three types of AR (Up-flow Anaerobic Sludge Blanket, Anaerobic Baffled Reactor, and Up-flow Anaerobic Filter) with CW are reviewed as well as their capabilities and drawbacks. An emphasis was made to analyze their performances, characteristics, and the processes involved in pollutant removal (solids, organic matter, nutrients, and sulfate). In Chapter 3, the experimental work of this thesis is presented. Mathematical modeling of the pollutant removal processes occurring in wastewater treatment plants can provide detailed and valuable information. More profound knowledge provided by these models is useful for predicting the future behavior of the treatment systems and can be applied to optimize the operation and facilitate monitoring. Optimal operation procedures and feasible monitoring are essential to ensure the successful implementation of these technologies in rural areas and small communities where investment budgets are limited. The system's efficiency and robustness can also be increased by applying mathematical modeling in design and control. The experimental system evaluated consisted of a Septic Tank (ST), an Up-flow Anaerobic Filter, and a Horizontal Flow Constructed Wetland (HFCW). Sampling was done biweekly for three months. BOD5, COD, TSS, TKN, ON, NH4+, NO2- and NO3- levels were measured. The experimental data was used to develop three different mathematical models: First order kinetics models, Linear Multiple Regression models, and Mass balance models. The first-order kinetic models developed were efficient to predict pollutant removal with significant precision. Multiple linear regression models were found to help reduce the cost and time of monitoring procedures. These models also reflected physical, chemical, and biological processes involved in pollutant removal in a logical manner. Finally, mass balance models indicated that the system is highly tolerant to influent wastewater variations.