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

Concentration (mg/l)
160
140
120
100
80
60
40
20
0
COD Ammonia-N MRP
Variables
Influent I-Tap I
I-Tap II I-Tap III
II-Tap I II-Tap II
II-Tap III
Figure 5. Water quality variables for domestic wastewater mesocosms (I: mesocosm 3; II:
mesocosm 4; Tap I: surface flow water; Tap II: oxidized sediment layer; Tap III: reduced
sediment layer; COD: chemical oxygen demand; Ammonia-N: ammonia-nitrogen; MRP:
molybdate reactive phosphorus).
Plant
Concerning the two mesocosms treating domestic wastewater, mesocosm 3 had higher
reduction removal efficiencies of COD, ammonia-nitrogen, MRP and SS to those of
mesocosm 4. It suggests that the presence of Agrostis stolonifera in planted mesocosms has
limited the diffusion of oxygen (as electron reception) to the lower sediment layers.
Relatively low oxygen concentrations can be identified as the main contributor to the
inadequate removal of COD and ammonia-nitrogen.
Groundwater contamination
An outlet valve was located on the base plate of each experimental mesocosm in to gauge the
possibility of groundwater contamination by infiltration of the polluted water. However, no
leachate was collected throughout the study period. The absence of measurable infiltration
can be explained by the presence of a compact bentonite clay layer, acting as an excellent
barrier to prevent nutrient transfer to potential aquifers. Furthermore, some of
biogeochemical processes play important roles in clogging of the soil matrix. For example,
biomass accumulation and/or insoluble biogas (i.e. methane) formation through soil microbes
(Kellner et al. 2004; Tokida et al. 2005).
Sediment management
In March 2005, sediment accumulations were measured at six ICW sites in Waterford. The
mean accumulation rates were approximately 3 cm per annum for moderately loaded
systems. Previous research (Scholz 2007) indicated that the approximate sludge removal
frequency of cell 1 within ICW site 11 would be nine years considering pond capacity.
However, from the nutrient removal perspective, ICW cells require desludging every 5 to 6
years. For some heavily loaded systems, more frequent desludging appears to be necessary.
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IWA <strong>Specialist</strong> <strong>Group</strong> on Use of Macrophytes in Water Pollution Control: Newsletter No. 38 21