Microbial biocoenosis on constructed wetlands

Subsurface flow constructed wetlands are used worldwide as a simple, low cost means of wastewater treatment. Vertical flow systems with intermittent loading, which use a sandy substrate for the main layer, represent the latest generation of subsurface constructed wetlands and are the state of the art for treatment of domestic wastewater. Although there is a lot of knowledge in design and operation of these systems, existing design criteria are mostly based on "rules of thumb" providing a specific area per people equivalent. Up to now subsurface flow constructed wetlands are often seen as a "black box". Therefore basic scientific research is necessary to understand the degradation and transformation processes in more detail. The major part of the removal processes regarding the main constituents of wastewater (organic matter, nitrogen) is carried out by micro- organisms. Therefore, within this project, investigations at a pilot-scale are carried out to measure the quantity and activity of the microbial biocoenosis in subsurface flow constructed wetlands. It is necessary to adapt microbiological measurement methods common in soil microbiology to the special characteristics of the sandy substrate used for the main layer of subsurface flow constructed wetlands. Combining various techniques from different disciplines (microbiology wastewater chemistry, soil physics) will enhance the knowledge gained of the different processes. This interdisciplinary approach is a necessity to successfully measure kinetic parameters and microbial removal efficiency in constructed wetlands for wastewater treatment. The measured data are necessary to calibrate a simulation tool for constructed wetlands. The use of a fully calibrated simulation tool for checking existing design criteria will help to improve the design criteria in terms of area demand, and will promote a more detailed insight into the black box "constructed wetland" that can be utilised for the optimisation of removal efficiency.

Subsurface flow constructed wetlands are used worldwide as a simple, low cost means of wastewater treatment. Vertical flow systems with intermittent loading, which use a sandy substrate for the main layer, represent the latest generation of subsurface constructed wetlands and are the state of the art for treatment of domestic wastewater. Although there is a lot of knowledge in design and operation of these systems, existing design criteria are mostly based on "rules of thumb" providing a specific area per people equivalent. Up to now subsurface flow constructed wetlands are often seen as a "black box". Therefore basic scientific research is necessary to understand the degradation and transformation processes in more detail. The major part of the removal processes regarding the main constituents of wastewater (organic matter, nitrogen) is carried out by micro- organisms. Therefore the overall goal was to get more insight into and understanding of the processes. The goal was tackled in three ways: by characterisation of the quantity and activity of the microbial biocoenosis, by investigation of bacterial removal efficiencies and processes and by numerical simulation of the processes in the main layer. The microbial community in the main layer of vertical flow beds has been investigated regarding their biomass and community composition. Biomass analysis revealed that about 60 % of the microbial biomass could be found in the first cm and about 96 % of the microbial biomass was located in the first 10 cm of the main layer. It was also demonstrated that the microbial biomass is quite high compared to soils. Community composition analysis revealed that the system was dominated by the same bacterial community than soils (mainly by Actinobacteria and ß- Protebacteria). The measurements indicated that the community of ammonia oxidizing bacteria found in vertical flow beds is similar to that in horizontal flow beds, but differs from common wastewater treatment plants regarding the presence of Nitrosospira Spp. The measurements further showed a high removal rate for pathogens. Most of the elimination is taking place in the first 20 cm of the main layer. A numerical simulation tool was successfully applied for simulating the constructed wetland systems investigated in this study as well as for other applications such as tertiary treatment of wastewater and treatment of combined sewer overflow. Much information was gained within the project on the elimination processes in the main layer of subsurface flow constructed wetlands. The project therefore contributed to the optimization of constructed wetland technology. However, a lot of new questions has been arising showing that there is still a great need for further investigations.