) indicate that the role of plants goes beyond the sole addition of oxygen, probably by enabling a more diversified and active micro fauna development near the root zone. It is worth pointing out from Table 7 that the increase
in the influent COD concentration had very little effect on the removal efficiency of the system. For instance, the maximum COD removal recorded in experiment I was 97.9% and it was 94.9% in experiment IV. Similarly for NO3−–N the removals were 97.6% and 96.2% in experiments I and IV; for PO4 3−–P it was 96.15% and 90.48% in experiments I and IV,respectively. It is apparent that the difference in removal percentages of the pollutants is within 6% between the systems treating wastewater with an influent COD of ∼450mgl−1 in
experiment I and with ∼1650mgl−1 in experiment IV. The statistical analysis of the data also reveals the difference in removal of nutrients between the experiments is not significant
(Tables 8–10). It is also worth mentioning that frequent change ofwastewater in experiments III and IV also facilitated penetration of air in to the pores present in the gravel bed and
helped in better removals of COD. As COD transformation in such systems is essentially effected by microorganisms whose presence and activities are enhanced by aeration (Mbuligwe, 2004).