Gravel Bed Hydroponics (GBH) is a constructed wetland system for sewage treatment which has proved effective for tertiary treatment in the UK and secondary treatment in Egypt. Significant improvements in effluent quality have been observed in 100 m long field scale beds planted with Phragmites australis, resulting in large reductions in BOD, suspended solids and ammoniacal N. For such GBH beds, operating optimally with a residence time of about 6 hours, 2 to 3 log cycle reductions in the counts of indicator bacteria, certain bacterial pathogens and viruses are typically obtained. However, the efficiency of mineralisation was strongly influenced by flow-rate and the prevailing temperature. In addition, in the UK, overloading of the treatment system reduced the efficiency of removal of faecal coliforms, probably due to decreased adsorption to biofilms. Faecal coliform counts were also more strongly correlated to BOD than suspended solids. As a secondary treatment process, pathogen removal was consistently better in Egypt than the UK. Although GBH constructed wetlands do not fully satisfy the WHO guidelines for unrestricted irrigation, they can make a significant contribution to the control of pathogens in developing countries.
The potential nitrogen transforming activities of bacteria were measured in the water and biofilms of a Gravel Bed Hydroponic (GBH) sewage treatment system. Within the GBH system, the organic matter and biofilms create complex partitioned micro-environments which promote nitrogen cycling. Biofilms attached to the rhizomes of Phragmites auslralis support higher potential rates of nitrogen transformations per unit surface area than those attached to gravel. Comparison of the transforming activities of the bacteria with the environmental conditions within the bulk water identified the effectors of ammonification, nitrification and denitrification. Rhizome and gravel biofilms respond differently to changes in their environment Nitrogen transformation potential exists throughout the bed, but whereas the potential ammonification and nitrification activities are evenly distributed, those for denitrification are greatest at the top of the bed where the availability of carbon and the local redox conditions promote microbial production of gaseous nitrogen. Thus, the success of GBH beds in treating effluents can, in part, be explained by a stimulation of nitrification/denitrification activities on the surface of the rhizomes. The nature of the effluent will depend on the balance between the rates of these two processes.
A modular wastewater treatment system has been installed for the Colombian Coffee Growers Federation (FEDERACAFE) at the Fundacion Manuel Mejia residential training centre. This system was designed by the University of Portsmouth, UK, and based on previous experience with the Gravel Bed Hydroponic (GHB) constructed wetland system. The steep topography of the site posed constraints on the design and a modular system composed of a rock filter in gabions for primary treatment, GBH beds for secondary treatment and a pond for tertiary treatment was selected. This system was designed to follow the contours of the site and maximise treatment.
The GBH reed bed was planted with Typha angustifolia in early 1997. The system is being monitored by CENICAFE, the research division of FEDERACAFE. Monitoring of the system has shown large BOD removals from more than 350 to less than 20 mg/l. Initial results have also suggested 4 log cycle reductions in indicator bacteria.
It is hoped that the success of this system will result in the replication of the treatment process at many villages in the Colombian coffee growing region where conventional sanitation technologies are impractical.
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