Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands—I. Removal of oxygen demand, suspended solids and faecal coliforms

The effect of influent loading rate on mass removal of BOD, SS and faecal coliforms (FC) from dairy parlour wastewaters was compared in four pairs of planted (Schoenoplectus validus) and unplanted gravel-bed wetlands (each 19 m2). 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 pollutants. Influent water quality varied markedly over the trial period (CBOD5 20–300 g m−3; SS, 60–250 g m−3; FC, 103–106 MPN (100 ml−1). NBOD was an important component of total BOD, being around 1.5 times higher than the influent CBOD5, and 2–10 times higher than the effluent CBOD5. Outflow levels of CBOD5, SS and faecal coliforms rapidly mirrored changes in influent loadings. Mean mass removal of CBOD5 increased from 60–75% to 85–90%, total BOD (CBOD5 + NBOD) from 50 to 80% and FC from 90–95 to >99% with increasing wetland retention time during the first 12 months of monitoring. Mean annual SS removals of 75–85% were recorded irrespective of loading rate. High levels of dissolved humic colour in the wastewaters were little affected by passage through the wetland at short retention times, but were reduced by up to 40% at longer retentions. Mass removals of CBOD5, SS and FC showed monotonic relationships to mass loading rates, with little difference between the performance of planted and unplanted wetlands, except for CBOD5 at high loadings (> 3 g m−2 d−1). The planted wetlands showed significantly improved removal rates for CBOD5 at higher loadings, and 1.3 to 2.6 fold higher mass removals of total BOD.