Use of aquatic and terrestrial plants for removing phosphorus from dairy wastewaters

Abstract

A microcosm study was conducted in Okeechobee, County, Florida, to evaluate the P removal potential of several aquatic macrophytes and “unconventional” terrestrial crops cultured on dairy lagoon wastewaters. Existing conventional spray field crops in this region that are irrigated with dairy wastewaters are estimated to remove 23 mgP/m 2-day. Effects of season and hydraulic retention time (HRT) on P uptake by the floating macrophytes water hyacinth ( Eichhomia crassipes) and duckweed ( Lemna obscura) were evaluated using half-strength primary lagoon effluent as a growth medium. Maximum P uptake rates by water hyacinths in February and July were 59 and 200 mgP/m 2-day, respectively, whereas P uptake by duckweed was identical (20 mgP/m 2-day) during both periods. 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 accounted for by plant uptake was observed in both water hyacinth and duckweed microcosms. Ten emergent macrophytes were screened in secondary lagoon effluent for their P uptake potential, including cattail ( Typha domingensis); pickerelweed ( Pontederia cordata); canna lily ( Canna flaccida); bulltongue ( Sagittaria lancifolia); arrowhead ( Sagittaria latifolia); lizard's tail ( Saururus cernuus); green arum ( Peltandra virginica); giant reed ( Phragmites australis); soft rush ( Juncus effusus); and bulrush ( Scirpus validuus) Most species exhibited increased tissue N and P contents and greater biomass yields when supplemental N and P fertilization was provided. Canna lily and pickerelweed were the two species that provided highest rates of foliage P uptake from the enriched wastewater (uptake rates of 173 and 66 mgP/m 2-day, respectively). Shoot:root biomass ratios for 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 terrestrial crops alemangrass ( Echinochloa polystachia). paragrass ( Brachiaria mutica), floralta limprograss ( Hemarthria altissima) and bermudagrass ( Cynodon dactylon). Effects of primary lagoon effluent application rates (4.5 vs. 8.9 cm/week) and water depth on biomass production and P uptake were evaluated. From February to May, P uptake by bermudagrass, paragrass, alemangrass and floralta limpograss cultured at a 4.3 cm/wk application rate (with drainage of the leachate 24 h after effluent application) was 34, 29, 28 and 16 mgP/m 2-day, respectively. Phosphorus uptake by alemangrass (22 mgP/m 2-day) and paragrass (18 mgP/m 2-day) was reduced by cultivation in 9 cm of standing water. Phosphorus uptake (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 production and increased tissue P concentrations.