Aquatic Plants, Macrophytes, Halophytes, Agroforestry in Ecological Wastewater Treatment (Chapter 5)

The Role of Macrophyte Roots in Wastewater Treatment
(Refer to Chapter 5)

One of the most important parts of aquatic plants involved in wastewater treatment—in addition to plant leaves that perform photosynthesis, shade out algae, shield the water surface from wind and rain turbulence, and provide a quiescent environment for solids settling—is the roots and rhizomes. Rhizomes are crawling and generally horizontal stems resting at or under the soil surface; they are not roots because they can develop leaves and aerial shoots near the tips and grow roots from their undersurface. Conventional (unplanted) sewage-treatment ponds and containers are generally suspended-growth bioreactors, where communities of microorganisms, bacteria, fungi, and animals are suspended in the water columns while they biodegrade and decompose the waste matter. However, with the presence of aquatic plants, ponds become partly attached-growth treatment bioreactors. The roots provide surface area and substrate for attachment and growth of biofilm and microorganisms—hence, the name, attached-growth treatment—in addition to oxygen for the root zone, or rhizosphere. Without this oxygen diffusion in teh root zone, and with the water surface covered by a dense mat of vegetation, the water columns would be mostly anaerobic and anoxic. Roots are also responsible for mechanical filtration or nutrient assimilation into plants (Soto et al., 1999).

Oxygenation and increased dissolved-oxygen levels at root zones and rhizospheres also facilitate nitrification (the transformation of ammonia into nitrates and nitrites). Researchers have found that biogenic oxygen diffused at the root zone and rhizosphere allows the nitrification of ammonium—which is produced in the decomposition and fermentation of settled solids in the bottom sediments—through mineralization of organic nitrogen (Reilly et al., 2000). It has been hypothesized that the nitrates produced in nitrification is conveyed by diffusion into the surrounding anoxic sediments where it is denitrified (the conversion of nitrates into nitrogen gas) and escapes as nitrogen gas into the atmosphere. Researchers found that nitrogen absorption and oxygen diffusion in macrophyte wastewater-treatment systems are greatest in the non-growing season (Reilly et al., 2000). The root system is, therefore, important in wastewater-treatment processes.

A view of thousands of tiny strands of root hair on which denitrifying bacteria and other microorganisms can attach and remove nitrate in the wastewater. (Photograph by Jo-Shing Yang)