Degradable organic matter in wastewater, whether soluble or suspended, is ultimately removed through microbial degradation. The microbes responsible for this degradation are generally associated with slimes or films that develop on the surfaces of soil particles, vegetation and litter. In general reedbeds are designed and operated to maintain aerobic conditions so that degradation is performed predominately by aerobic microorganisms. This is due to the fact that aerobic decomposition tends to be more rapid and complete than anaerobic, and potential odours associated with anaerobic degradation are avoided (Aggelakis and Tchobanoglous, 1995; Brix, 1993; Reddy and D'Angelo, 1997; Tchobanoglous and Burton, 1991; Wittgren and Maehlum, 1997).
Green and Upton (1994) presented COD removal data from five different reedbeds in the UK, for two consecutive years. The beds were used for tertiary treatment of domestic wastewater. All beds were aiming in producing a final effluent of less than 60 mg/l. The best performance was achieved by the reedbed in Himley, in 1991, with a 60.3 % removal and a final effluent of 31.7 mg/l. Three, similarly constructed reedbeds in Germany were used for the secondary treatment of domestic wastewater. Their performance for removing COD was compared based on the hydraulic loading since different hydraulic loading produces different retention times, effecting in this way the bed's performance. The German beds performed considerably better than the British, with average COD removal above 70 %, and a maximum one of 95 % when the hydraulic loading was only 4-12 mm/day (Netter, 1993).
Under the term 'total solids' one can include all those materials, organic or inorganic, that are present in the wastewater and remain as residue upon evaporation at 105 oC. The solids can be separated in a number of types: suspended solids (SS), volatile suspended solids (VSS), fixed suspended solids (FSS), total volatile solids (TVS), filterable solids (FS), volatile filterable solids (VFS), fixed filterable solids (FFS) and total fixed solids (TFS) (Tchobanoglous and Burton, 1991).
Solids removal is achieved mainly through filtration followed by either aerobic or anaerobic microbial degradation on the surface or within the media respectively (Batchelor and Loots, 1997; Wittgren and Maehlum, 1997). According to Davies and Cottingham (1994) the majority of the solids (almost 75 % of the original concentration) existing in an wastewater are retained in the first third of a reedbed. Similarly Zachritz and Fuller (1993) suggested that 60 % of the TSS were removed in the first one third of an subsurface reedbed.
Experiments were conducted in the UK for evaluating the performance of two reedbeds, used for secondary and tertiary treatment of domestic wastewater. The beds were gravel based (gravel diameter 10 to 20 mm) planted with Phragmities and their performance evaluation included solids removal. The secondary system achieved removal performance of the suspended solids up to 78 %, for an effluent value of 20 mg/l and an influent value of 90.6 mg/l. The tertiary treatment system achieved removal performances of up to 61 % for an effluent value of 10 mg/l and influent value 25.8 mg/l (Williams et al., 1995).
Karathanasis and Thompson (1991) and Mitchel and Karathanasis (1995) used organic materials and compost of organic materials in the substrates of pilot scale reedbeds. Their aim was to improve the substrate in order to be able to treat wastewater with high concentration of heavy metals. The performance achieved from the majority of the materials was above 90 % depending on the metal. There is a number of similar publications on the use of compost in order to improve constructed wetlands substrate thus enabling a system to remove a variety of pollutants (Goldstein, 1993, Mungur et al., 1997).
Reedbeds, though, are commonly used for treating domestic wastewater produced mainly by small communities, especially in the UK (Green and Upton , 1994). In order to consider the use of composted materials in a bed's substrate, data concerning the removal of pollutants like COD and TSS should be obtained. This paper is presenting some of the results of a research which was aiming in determining the ability of mature sewage sludge compost in improving the performance of pilot scale reedbeds treating domestic wastewater.