Chris C. Tanner 1
Constructed wetlands have the potential to provide cost-effective low-maintenance treatment systems to improve the discharge quality of agricultural waste stabilisation lagoons. This paper reviews New Zealand studies of dairy farm wastewater treatment using gravel-bed constructed wetlands. Treatment levels achieved over a 20 month period are compared for planted and plant-free wetlands operated at four wastewater loading rates, corresponding to nominal retention times of 2, 3, 5.5 and 7 days. Mean annual suspended solids removals of 75-85% were recorded, irrespective of loading rate or the presence of plants. In the planted wetlands, mean annual mass removal of CBOD increased from 76 to 90%, total BOD (carbonaceous + nitrogenous BOD) from 50 to 80%, faecal coliforms from 95 to over 99%, TN from 48 to 75%, and TP from 37 to 74% as wetland retention time increased. The unplanted wetlands generally showed similar performance to the planted wetlands at long retention times, but poorer BOD, nitrogen and phosphorus removal at short retention times.
