USE OF AQUATIC AND TERRESTRIAL PLANTS FOR REMOVING PHOSPHORUS FROM DAIRY WASTEWATERS

A microcosm study was conducted in Okeechobee, County, Florida, to eva luate the P removal potential of several aquatic macrophytes and ''unc onventional'' terrestrial crops cultured on dairy lagoon wastewaters. Existing conventional spray field crops in this region that are irriga ted with dairy wastewaters are estimated to remove 23 mgP/m(2)-day. Ef fects of season and hydraulic retention time (HRT) on P uptake by the floating macrophytes water hyacinth (Eichhornia crassipes) and duckwee d (Lemna obscura) were evaluated using half-strength primary lagoon ef fluent as a growth medium. Maximum P uptake rates by water hyacinths i n February and July were 59 and 200 mgP/m(2)-day, respectively, wherea s P uptake by duckweed was identical (20 mgP/m(2)-day) during both per iods. During July, wastewater total P concentrations were reduced from 7.3 mgP/l to 0.2 mgP/l by water hyacinth and to 2.4 mgP/l by duckweed at a 7-day HRT. Wastewater P and N removal in excess of that accounte d for by plant uptake was observed in both water hyacinth and duckweed microcosms. Ten emergent macrophytes were screened in secondary lagoo n effluent for their P uptake potential, including cattail (Typha domi ngensis); pickerelweed (Pontederia cordata); canna lily (Canna flaccid a); bulltongue (Sagittaria lancifolia); arrowhead (Sagittaria latifoli a); lizard's tail (Saurunus. cernuus); green arum (Peltandra virginica ); giant reed (Phragmites australis); soft rush (Juncus effusus); and bulrush (Scirpus validus). Most species exhibited increased tissue N a nd P contents and greater biomass yields when supplemental N and P fer tilization was provided. Canna lily and pickerelweed were the two spec ies that provided highest rates of foliage P uptake from the enriched wastewater (uptake rates of 173 and 66 mgP/m(2)-day, respectively). Sh oot:root biomass ratios canna lily and pickerelweed were 1.2 and 0.7, respectively, so ''whole-plant'' P uptake rates were likely twice the values shown above. Several studies were conducted with the terrestria l crops alemangrass (Echinochloa polystachia), paragrass (Brachiaria m utica), floralta limprograss (Hemarthria altissima) and bermudagrass ( Cynodon dactylon). Effects of primary lagoon effluent application rate s (4.5 vs. 8.9 cm/week) and water depth on biomass production and P up take were evaluated. From February to May, P uptake by bermudagrass, p aragrass, alemangrass and floralta limpograss cultured at a 4.5 cm/wk application rate (with drainage of the leachate 24 h after effluent ap plication) was 34, 29, 28 and 16 mgP/m(2)-day, respectively. Phosphoru s uptake by alemangrass (22 mgP/m(2)-day) and paragrass (18 mgP/m(2)-d ay) was reduced by cultivation in 9 cm of standing water. Phosphorus u ptake (based on foliage harvests) by floralta limpograss (41 mgP/m(2)- day) and alemangrass (49 mgP/m(2)-day) was enhanced by cultivation at a 8.9 cm/week application rate, due both to increased biomass producti on and increased tissue P concentrations.