here another good reading that report TWs as good wastewater treatment solution:
Wastewater Irrigation and Health Assessing and Mitigating Risk in Low-Income Countries
Published by International Development Research Centre and International Water Management Institute.
Constructed wetlands are usually secondary or tertiary treatment units, in which case they are preceded by a septic tank, Imhoff tank, UASB, anaerobic pond or a conventional wastewater treatment plant. They are used to remove organic matter (BOD), solids and nutrients. Wetlands are generally promoted as a good option to control pathogens.
pathogen removals obtained are available due to the high cost and complexity of the analytical techniques involved. The available information mostly refers only to faecal coliforms. From the small amount of available data, pathogen removal is highly variable and depends on the climate, the type of wetland and the plants used.
Pathogen removal is achieved via filtration, adsorption on to soil or plant roots and predation by micro-organisms (Jiménez, 2007). Wetlands can remove 90–98 per cent of faecal coliforms, 67–84 per cent of MS2 coliphages and 60–100 per cent of protozoa (Jiménez, 2003).
(text and the images are taken in the .pdf version available on the http://www.iwmi.cgiar.org/ web site).