Nutrient Transformations in a Natural Wetland Receiving Sewage Effluent and the Implications for Waste Treatment

Abstract

The processes influencing nutrient (phosphorus and nitrogen) renovation in a natural wetland which had received oxidation pond effluent for twelve years were studied, and compared with current literature perceptions. Mass transport studies showed that 30–70% of the influent P was removed from the water column, which was much greater than published values suggest could be predicted for this highly loaded (∼ 34 g P m−2 y−1) system. Sediment traps studies showed that deposition of particulate P immediately downstream of confluences with arms of the wetland not impacted by sewage effluent (natural wetlands) was the dominant cause of P removal. Separation of the deposited-P into chemically definable fractions along with studies on the water chemistry, suggested that P deposition was associated with iron-organic complexes contributed dominantly from the natural wetlands. Considerable spatial and temporal heterogeneity was also demonstrated for nitrogen transformations. During summer most of the influent-N was in nitrate form which was all transformed during passage through the wetland. Isotope (15N) dilution studies indicated that ∼ 60–70% was denitrified, 25–35% converted to ammonium (dissimilatory reduction), and 5–10% assimilated. For most of the year, however, influent N was mainly in reduced forms. Despite this, significant quantities of nitrate were exported from the wetland especially at higher flows in spring-early summer. Assays on the sediment showed that there was a marked increase in nitrification activity at the confluence with natural wetlands. It is suggested that marked changes in sediment redox potential at these confluence sites provide ideal conditions for nitrification of sorbed ammonium which is subsequently flushed from the system in ‘flood events’. The distribution and type of nutrient processing observed in this wetland are attributable to its configuration. The implication of these results to the sustainability of nutrient renovation in wetland treatment systems is discussed.