Title: Optimisation of a hybrid tropical domestic wastewater constructed wetland for nitrogen transformations and removal using a dynamic simulation model.
Author: Jully Seema Senteu
Supervising Institution: EGU - Egerton University, Kenya
Treatment constructed wetlands have been used to improve the water quality of domestic wastewater which has been shown to have increased nutrient concentrations. The efficiency of nutrient removal, especially NH4+-N is important in the design of treatment wetlands as studies show that conventional waste stabilization ponds (WSPs) that are commonly used in Sub-Saharan Africa (SSA) are not efficient in N removal. There are various processes that contribute to the nitrogen (N) transformations and removal in constructed wetlands (CWs) including sequential nitrification and denitrification, mineralisation, decay, accretion and plant uptake. Modelling allows for the in-depth study of these processes and their effect on N removal in treatment systems. The main objective was to model the biogeochemical processes responsible for N transformations in a hybrid CW so as to optimize the system's capacity for N removal. Although the CW study cell was originally designed as a subsurface flow (SSF) system, 80% of the system currently operates as surface flow (SF) with floating macrophytes dominated by Pistia stratiotes and 20% of the cell is SSF with rooted macrophytes predominantly Typha sp and Scirpus lacustris. The cell covers an area of 1051m2. This study was carried out in two phases. The first phase involved characterization of the system with regard to the N removal efficiency. Diurnal variability was also performed to determine the variation in the loading rates as well the physico-chemical parameters in the system over a 24hour period as this variability would also affect the performance of the system. Data analysis was carried out using SPSS 22.0 software. The N removal performance of the system was poor with no significant difference in the N concentrations between the influent and effluent of the system. There was diurnal variation in the temperature in the cell. Oxygen concentrations at the inlet were higher during the day compared to night time probably due to algal activity in the WSPs before the system. The second phase involved the development of an N simulation model as well as scenario analysis so as to determine how the optimum N removal can be achieved in the system. STELLA 10.0.2 software was used to build the N model. Nitrification was found to be the main N transformation process in the system. The main N removal pathways in the system in descending order were denitrification (19.4%), accretion (15.7%) and plant uptake (14.6%). DO concentrations had a greater effect on the N removal in the system compared to harvesting.