The microbial ecology of the root zone method of wastewater treatment.

This study of the root-zone method of wastewater treatment describes the relationships between the type of substrate, i.e. soil or gravel, and the presence/absence of the common reed Phragmites australis, the removal of pollutants, i.e. pathogenic microorganisms, organic matter and the microbial community present within the system. Information concerning BOD (biochemical oxygen demand) and bacteria removal kinetics suggest that the root-zone method is more of a physico-chemical process than a biological process. Organic matter, suspended solids, and bacteria accumulate near the inlet section of the system. The concentration of ciliated protozoa is also higher near the inlet section. Results of this study tend to confirm that reed do not provide enough oxygen for nutrient removal. Removal rates of ammonia were poor in the full-scale systems and very poor in the microcosms. The ciliated protozoan community of various designs of the root-zone method of wastewater treatment is described. Data show that there is a significant difference between gravel beds and soil beds communities, and between planted soil bed and unplanted soil bed communities. The bacterivory of dominant ciliates is estimated by grazing experiments using hydroethidine-labelled Escherishia coli. These experiments confirm that ciliates remove E. coli from the wastewater, and may play an important role in bacteria removal. Comparison of data gathered from full-scale systems and microcosms were limited by the use of synthetic sewage instead of settled sewage, the potential immaturity of microcosm, and the microcosm design. However the full-scale systems the microcosms show the same trends regarding the release of free-living amoebae, and the BOD and bacteria removal kinetics. Gravel systems provide the best removal of suspended solids and free-living amoebae, whilst the planted soil systems provide the best BOD removal. The unplanted soil systems showed a poor removal of bacteria, free-living amoebae, BOD, and suspended solids. A hybrid system, i.e. a soil system followed by a gravel system, might be the optimal design.