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Engineering aspects, pathways and mechanisms of nitrogen removal in engineered wetland systems

Khajah, Mishari 2017. Engineering aspects, pathways and mechanisms of nitrogen removal in engineered wetland systems. PhD Thesis, Cardiff University.
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Abstract

While constructed wetlands (CWs) have been applied successfully to achieve contaminant removal from several types of wastewater, their efficiency and capacity is still low with respect to nutrients (nitrogen, N and phosphorus, P) removal. Through typical configurations, the removal efficiency for total nitrogen (TN) varied between 40 and 50% and total phosphorus (TP) between 40 and 60% depending on CWs type and inflow loading. This limits their implementation in delivering advanced wastewater treatment. Subsequently, the main aim of this study was to develop innovative CW processes to significantly enhance the nitrogen removal efficiency by investigating a new approach to the configuration of CWs and exploring substitutional operational strategies. If this is accomplished, it could make CWs a main wastewater treatment unit instead of conventional wastewater treatment systems. To accomplish this aim, two systems (multistage and single stage CW) were developed and comprehensively investigated and studied by applying a tidal flow operational strategy with gravel used as the main media for both systems. Tidal flow enhances and promotes the oxygen transfer to the CW system by batch pulse feeding and producing alternant wet/dry conditions of the CW media with wastewater. The multistage configuration is connected by four identical units in series and is divided into two phases depending on the seasonal temperature. The results of the multistage configuration showed average values for removal of 97.3%, 98.1%, 77.3% and 16.6% for chemical oxygen demand (COD), ammonium (NH₄⁺-N), TN and TP respectively for phase 1, and 87.5%, 78.5%, 60.3% and 10.2% for COD, NH₄⁺-N, TN and TP respectively for phase 2. However, TN removal was still not desirable with a mean removal efficiency in both phases 1 and 2, whereas the satisfactory removal for TN is >80%. This is due to the seasonal temperature and because all the stages were under aerobic conditions to a certain extent, which is unfavourable for denitrifying bacteria. Subsequently, to obtain a satisfactory efficiency of TN removal performance, the original multistage configuration system has been modified. To obtain a high and efficient TN removal performance for the multistage system, further investigations and developments have been made regarding system modification and alternative operational options. These have resulted in the development of improved nitrogen ABSTRACT iii removal processes by applying a step-feeding strategy to the system with different step-feeding schemes. As a result, 81.1% of TN removal was achieved. In addition, TP removal was poor because the system was not designed to removal phosphorus. The results of the single stage CW showed average values for removal of 86.7-99.3% and 64.1-74.8% for NH₄⁺-N and TN respectively, depending on the recirculation number (Rn) and inorganic carbon concentrations (IC); a single stage CW was used to follow the new route of nitrogen removal, called the Complete Autotrophic Nitrogen removal Over Nitrite (CANON) process. Microbial community analysis revealed that the dominant phyla for the multistage system were Proteobacteria, Planctomycetes, Firmicutes, and Bacteroidetes, whereas the dominant phyla for the single stage were Proteobacteria, Planctomycetes, Acidobacteria, and Chloroflexi. All these phyla were responsible for nitrogen and organic matter removal in both systems. The difference in microbial community and structure between both systems might attributed to the differences in the operational conditions. Overall, the results of this research study enhance the treatment capacity, which enables CWs to have the potential to work as the fundamental technology in advanced wastewater treatment and to increase our understanding of nitrogen removal in CW systems.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Constructed Wetland; Nitrogen Removal; Wastewater Treatment; Tidal Flow; Step-Feeding Strategy; CANON Process.
Date of First Compliant Deposit: 31 May 2017
Last Modified: 04 Jun 2017 09:51
URI: http://orca.cf.ac.uk/id/eprint/100956

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