Specialist Group on Use of Macrophytes in Water Pollution ... - IWA

Collected samples were analysed for temperature, pH, conductivity, total dissolved solids,
redox potential, dissolved oxygen, suspended solids (SS), and chemical oxygen demand
(COD) at The University of Edinburgh. Ammonia-nitrogen, nitrate-nitrogen, nitrite-nitrogen,
total organic nitrogen, chloride, and molybdate reactive phosphorus (MRP, equivalent to
soluble reactive phosphorus) were tested at the Waterford County Council water laboratory.
Figure 3. Integrated constructed wetland near Waterford (Ireland); a: ICW site 11 treating
farmyard runoff; b: ICW site 7 treating domestic wastewater.
RESULTS
Water treatment performance
In this study, experiments were carried out in five column-scale horizontal free surface flow
ICW systems. Mesocosms 1 and 2 were conducted between February 2009 and October
2010. Whereas mesocosms 3, 4 and 5 were operated between May 2009 and October 2010.
These results indicate that the mesocosms act as sources of COD rather than as sinks (Figures
4 and 5). The sediments released substantially more organic matter in comparison to the
amount of incoming organic matter that could be degraded. This can be explained by the fact
that most organic matter accumulated at the interface between the sediment and clay layers
due to the characteristics of slow organic matter decomposition and low permeability. In
addition, due to relatively high concentrations of ammonia-nitrogen in influent, more than
90% of the above-ground plants in mesocosms were dead after operation of approximately
three months. This change in plant presence considerably affected the oxygen transfer
capacity to the deeper sediment layers and aerobic decomposition processes were thus
reduced. Therefore, the decomposition rate was most likely slower than the primary
productivity rate, which resulted in a net accumulation of organic matter.
In general, the ammonia-nitrogen concentrations at sampling points for all mesocosms were
relatively higher than that of influent, in particular at tap III (Figures 4 and 5). This is
probably because accumulated ammonia-nitrogen is bound loosely to the substrate such as
sediment and subsequently released when water chemistry and other environmental factors
change (e.g. oxygen shortage, low pH and temperature values, and high salinity
concentration). In addition, kinetic ammonification (mineralization) of organic nitrogen
proceeds more rapidly than nitrification, thus creating the potential for an increase in
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IWA <strong>Specialist</strong> <strong>Group</strong> on Use of Macrophytes in Water Pollution Control: Newsletter No. 38 19