EFFECT OF LOADING RATE AND PLANTING ON TREATMENT OF DAIRY FARM WASTEWATERS IN CONSTRUCTED WETLANDS .1. REMOVAL OF OXYGEN-DEMAND, SUSPENDED-SOLIDS AND FECAL-COLIFORMS

The effect of influent loading rate on mass removal of BOD, SS and fae cal coliforms (FC) from dairy parlour wastewaters was compared in four pairs of planted (Schoenoplectus validus) and unplanted gravel-bed we tlands (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 fortnight ly over a 20 month period. Hydraulic flows were monitored to enable ca lculation of the mass flows of pollutants. Influent water quality vari ed markedly over the trial period (CBOD5, 20-300 g m(-3); SS, 60-250 g m(-3); FC, 10(3)-10(6) MPN (100 ml(-1)). NBOD was an important compon ent of total BOD, being around 1.5 times higher than the influent CBOD 5, and 2-10 times higher than the effluent CBOD5. Outflow levels of CB OD5, SS and faecal coliforms rapidly mirrored changes in influent load ings. Mean mass removal of CBOD5 increased from 60-75% to 85-90%, tota l BOD (CBOD5 + NBOD) from 50 to 80% and FC from 90-95 to >99% with inc reasing wetland retention time during the first 12 months of monitorin g. Mean annual SS removals of 75-85% were recorded irrespective of loa ding rate. High levels of dissolved humic colour in the wastewaters we re little affected by passage through the wetland at short retention t imes, but were reduced by up to 40% at longer retentions. Mass removal s of CBOD5, SS and FC showed monotonic relationships to mass loading r ates, with little difference between the performance of planted and un planted wetlands, except for CBOD5 at high loadings (> 3 g m(-2) d(-1) ). The planted wetlands showed significantly improved removal rates Fo r CBOD5 at higher loadings, and 1.3 to 2.6 fold higher mass removals o f total BOD.