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.

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The use of Gravel Bed Hydroponic (GBH) constructed wetlands for wastewater treatment and reuse in semi-arid climates has been evaluated in Egypt with respect to the removal of parasite eggs from domestic wastewaters. Influent and effluent from established 100m GBH reed beds receiving partially treated wastewater, were analysed to establish daily parasite loading rates and removal performance of the system under normal operating conditions (201/min; intermittent 12h on/off flow regime.) The system was then challenged with high numbers of parasite eggs (Ascaris sp., Hymenolepis sp. and Toxocara sp.) equivalent to an influent of 100-500 eggs/1 and a daily loading rate of 1.0-7.2 × 106, representing up to a 110-fold increase over typical mean daily loading rates. Wastewater samples were collected from the inlet, at varying distances along the 100m bed profile, and the outlet. Sampling was also carried out at varying time intervals during the flow regime to determine the spatial and temporal removal of eggs along the bed. In 100m long planted beds, there was significant removal of all selected parasite eggs with the majority being removed within the first 25m. No eggs were detected in the final effluent. These studies demonstrate that GBH wetlands have a capacity for efficient removal of parasite eggs and thus provide a reliable, low cost technology for treating effluent compliant with health related objectives for the treatment of wastewater intended for agricultural reuse.

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A study was performed on the biodiversity of actinomycetes in the Gravel Bed Hydroponic (GBH) system constructed wetland for industrial wastewater treatment. Identification of 102 actinomycete isolates recovered from the biofilm of the treatment system revealed high taxonomic diversity. Forty species belonging to ten genera were identified: Streptomyces,

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