EFFECT OF LOADING RATE AND PLANTING ON TREATMENT OF DAIRY FARM WASTEWATERS IN CONSTRUCTED WETLANDS .2. REMOVAL OF NITROGEN AND PHOSPHORUS

The effect of influent loading rate on mass removal of nitrogen and phosphorus from dairy parlour wastewaters was compared in four pairs of planted (Schoenoplectus validus) and unplanted gravel-bed wetlands (each 19 m(2)). The wetlands were operated at nominal retention times of 7, 5.5, 3 and 2 days, with in and outflows sampled fortnightly over a 20 month period. Hydraulic flows were monitored to enable calculation of the mass flows of nutrients, and plant biomass and tissue nutrient levels sampled to evaluate plant nutrient uptake. Influent water quality varied markedly during the trial period (TN, 10-110; NH4-N, 5-70; and TP 8-18 g m(-3)). As theoretical wastewater retention times increased from 2 to 7 days, mean reduction of TN increased from 12 to 41% and 48 to 75% in the unplanted wetlands and planted wetlands, respectively, and TP removal increased from 12 to 36% and 37 to 74%, respectively. In the planted wetlands, mean annual removal rates of TN (0.15-1.4 g m(-2) d(-1)) and TP (0.13-0.32 g m(-2) d(-1)), increased gradually with mass loading rates. The unplanted wetlands showed a marked decline in TN and TP removal at high loadings. Net storage by plants in the first year of monitoring accounted for between 3 and 20% of the greater N removal and between 3 and 60% of the greater P removal in the planted wetlands.