At the same time, the two main problems for the agricultural crops of the Greek islands are the lack of sufficient irrigation water, especially during the summer months and the low organic content of agricultural soils. Problems that could be tackled with the use of domestic and industrially produced wastewater from villages and olive oil factories for irrigation and the sewage sludge as organic fertilizer.
The sewage sludge is a useful source of organic matter, nitrogen and phosphorus which can substitute for inorganic fertilizers in maintaining soil fertility both in the long and short term. Heavy metals, pathogen contamination and stability of the organic fraction are the main factors which needed to be controlled in order to ensure that the addition of sludge to the land has a positive rather than negative effect (Stentiford, 1992). In the past two years a methodology has been developed for the composting of sludge using different local bulking materials, mainly agriculture residues (Manios, et al., 1997). The composting of sewage sludge might be the best way to cope with the sludge disposal problem and also provide a safe and stable organic fertilizer for soil improvement. In addition a proper use of the wastewater could be a recycle this time for irrigation of agricultural crops after a suitable treatment.
Such suitable and cheap method for the treatment of wastewater from villages and olive oil factories, are the constructed wetlands or reedbeds. However, the available fields in the Greek islands for the construction of reedbeds are rocky with small quantities of soil and even smaller quantities of organic matter. For this reason in the synthesis of a wetland substrate, sewage sludge compost could be used in order to provide the substrate with good physical and chemical properties for the growing of the wetland plants and for the better performance of wetland. Hollis et all. (1989) noted that the soil amendments were not necessary in wetlands for plant growth, because hydrophytes grew well in a broad range of soil types. However, when limitations did exist, soil amendments may enhance success. Karathanasis and Thompson (1993) utilized six different mixtures as substrates in an experiment with simulated constructed wetlands. The substrates were 2:1 volume mixtures of ground pine needles with surface soil, peat moss with subsoil, peat moss with surface soil, ground hay with surface soil, sphagnum moss with surface soil and 1:1:1 volume mixture of peat moss mine soil and surface soil. Mitchell and Karathanasis (1995) also used two compost based substrates and two plant species for the treatment of metal-chloride-enriched wastewater by simulated constructed wetlands. One substrate type was a 1:1 by volume mixture of mushroom compost and topsoil; the second a 1:3 mixture of ground leaf litter and topsoil. The two plant species were Typha angustifolia (cattail) and Scirpus validus (bulrush).
The specific objectives of this research was: (1) to study the composting process of sewage sludge using the biomass of one of the most abundant wetland plants in Crete, the Typha domigensis (cattail), as bulking agent ; and (2) to investigate the growth of cattail plants in relation to the sludge compost substrates and the local conditions of the island.