Evapo-transpiration in Ecological Engineering

Abstract

1.1 Constructed wetlands – ecological engineering technologies Odum (1971) described ecological engineering as half science and half engineering: techniques of designing and operating the economy with nature. Ecological engineering or ecotechnology are defined by Guterstam (1996) as the design of human society with its natural environment for the benefit of both. One of the technologies included in ecological engineering is using constructed wetlands for wastewater treatment. Constructed wetlands (vegetation filters or treatment wetlands) are artificial complexes of water, matrix, vegetation and the associated invertebrate and microbial communities designed to simulate the ability of natural wetlands to remove pollutants from water (Brix, 1987; Kangas, 2004). They provide an inexpensive and reliable method for treating a variety of waste waters such as sewage, landfill leachate, mine leachate, agricultural run-off, and are comparatively simple to construct, operate and maintain (Randerson, 2006, Randerson et al., 2007). Based on hydraulic regime CWs may be divided into two main groups: surface flow systems (SF), and subsurface flow systems (SSF) (Kadlec et al., 2000). The latter may employ horizontal flow (HF), vertical flow (VF) or tidal flow (TF) hydraulic regimes and these may be combined in hybrid systems to optimize pollutant removal (Randerson, 2006). Both aerobic and anaerobic processes are involved, but degradation of carbonaceous matter to CO2 and nitrification requires availability of oxygen. That may be achieved most efficiently in compact VF systems; as the surface is flooded, air is forced into the bed, while effluent percolates downwards through the matrix. HF beds typically achieve lower oxygen transfer rates but, with largely anaerobic conditions, they are effective in removing nitrogen to atmosphere via denitrification. SF wetlands most resemble natural wetlands, as the water level is typically above the soil surface (Randerson, 2006). With HF, water flows laterally below the surface, through a gravel bed. Oxygen is consumed by microbial activity, and oxygenation of the bed is limited by surface diffusion and transport via aerenchyma tissue to the root zone, so that anaerobic conditions predominate. Hence nitrification is limited by oxygen, and denitrification is limited by the supply of nitrate and usable carbon compounds. In the VF system, pulses of water flow downwards through layers of increasing particle size. Air is drawn into the bed between each pulse of water. Removal of BOD and