Summary
The five-pond integrated constructed wetland (ICW), which received farmyard dirty water (FDW) and dairy washings twice daily from the 140 cow dairy herd and three times daily from the 30 strong dairy-cream herd at the College of Agriculture, food and Rural Enterprise (CAFRE), was intensively monitored from it’s inception in November 2005 until February 2008. The principal aim of the study was to observe, sample, analyse and record the efficiency of the ICW for retention/removal of phosphorus (P), nitrogen (N) and reduction of 5 day biochemical oxygen demand (BOD5) from the FDW. The study was conducted for the whole system and for each of the individual five ponds.
Results of water samples taken from the ponds between November 2005 and February 2008 indicated that from the initial FDW influent piped into the system, water discharged from pond 5 after a hydraulic retention time (HRT) of 60-100 days (flow affected), had a significant (P=0.001) mean annual reductions in total phosphate (TP) of 94.9 to 97.1%, ammonia nitrogen (NH4 -N) 92.9 to 99% and BOD5 of 99.0%. The largest reduction for BOD5 and TP occurred in pond 1, whereas NH4 -N levels declined more significantly in pond 2. Soluble reactive phosphorus (SRP) and total soluble phosphorus (TSP) were also significantly reduced during passage through the ponds by 94.5% and 95.9% respectively. Other FDW nutrient contents, total oxidisable nitrogen (TON) and nitrite nitrogen (NO2 -N) were less consistently recorded but reductions were of a similar order. Across the ponds, pH remained relatively constant (mean pH 7.26 ± 0.07), with the largest variation (max pH 9.48, min pH 4.29) recorded in the inlet FDW. Conductivity was continuously reduced across the ponds, with the inlet mean of 760 ± 34 (max 1942) μScm-1 reduced to 291 ± 12 μScm-1 at the outlet. Faecal coliform (presumptive) and total coliform (presumptive) counts were also significantly lowered (up to log 5 reduction). Although relative percentage reductions in TP concentrations at the ICW outlet remained consistent, actual outflow concentrations increased each year. There was no correlation (r = <.1) between TP and BOD5 concentrations measured at the inflow and the outflow from the ICW. The ratio of CW size to watershed area was sufficient to meet the target requirements for FDW nutrient removal and BOD5 reduction.
During summer 2006, a period of low rainfall, dry hot weather and high levels of potential evapotranspiration (PET) effectively caused pond water to fall below the levels of the outfall pipes, resulting in zero flow conditions in all ponds except ponds 1 and 2, which continued to receive FDW. The bioremedial efficacy of these two ponds and the system as a whole was not seen to be damaged or diminished during this period, reflected in the recorded data which showed that although flows between ponds had ceased, BOD5 levels remained below discharge limits and nutrient concentrations (especially P) reduced significantly (P=0.001). During 2007, the very wet summer resulted in only intermittent pond water level reductions whereas 2008 saw a prolonged period (from April to September) with zero flow between ponds and no discharges from the CW.
Alterations to the wetland FDW catchment area drainage occurred during spring 2008, changing the flow of rainfall and dilution of the FDW. This in turn, increased concentrations of contaminants (see 2008 - 2009 Update: page 59). To avoid inclusion of data resulting from changed parameters, the main body and findings of the report are confined to the period ending March 2008.
