2.1. Experimental layout
Fig. 1 is a schematic diagram of the experimental layout. Due
to the different growing conditions required, two sequential
steps are involved in the production of high-starch duckweed:
biomass production and starch accumulation. Duckweed plants from each step were sampled for starch analysis. Since
protein is the other important component of duckweed and
a valued ingredient of the residual duckweed meal that can be
used to produce valuable by-products such as animal feed
supplements or organic fertiliser (Ahmad, 1990; Islam et al.,
2004), its change was also examined. The postharvest highstarch
duckweed was subjected to enzymatic hydrolysis for
fermentable sugar production, and then the hydrolysate was
fermented anaerobically by yeast to produce ethanol. The
total reducing sugars in the hydrolysate and the ethanol in the
fermentation liquor were measured, with the reaction efficiencies
determined to evaluate the performance of the
overall conversion.
2.2. Biomass production
A pilot-scale duckweed culture pond of 300 m2 in surface area
and 0.6 m in depth was operated for biomass production at
Barham Farm, Zebulon, NC, USA (Fig. 2a). Pig effluent from the
pig rearing buildings on the farm flows into an ambient
temperature anaerobic digester for primary treatment. The
treated effluent was stored in a lagoon from which the high
nutrient liquid was pumped regularly into the duckweed
culture pond to maintain the ammonium (NH4eN) concentration
of pond water at about 20 mg l1. The newly produced biomass from duckweed growth was harvested from the pond
three times a week, keeping the pond covered by approximately
two layers of duckweed fronds. The biomass yields
were recorded and the compositions of duckweed plants were
analysed for four consecutive weeks from August 30 to
September 26, 2010 to evaluate the performance of the duckweed
culture system. The nutrient removal ability of the
duckweed system was evaluated by carrying out a mass
balance of NH4eN and o-PO4eP to the culture pond.