3.4. Cattail and knotgrass production
Cattail and knotgrass were planted in the tanks with the configuration similar to the surface flow wetland system and tested from June 2005 to September 2005. Cattail was growing in six tanks with size of 0.6 m (width) × 0.6 m (depth) × 6 m (length) (two tanks of 3 m length in series) (Fig. 3). Knotgrass production was investigated in three 0.3 m (width) × 0.3 m (depth) × 3 m (length) tanks. Recycled wastewater from the fish production tanks was pumped into these tanks to supply plants with water and nutrients. The TN concentration in the supply water was about 2 mg/L. Plant biomass was harvested on an irregular schedule depending on the time of year and amount of growth. Cattail was cut to a 10 cm height and knotgrass was cut to a 5 cm height. The biomass was measured and recorded in approximately three different months for calculating the growth rates.
3.5. Data analysis
One way analysis of variance (ANOVA) or t test was conducted to compare means of biomass growth rate at p < 0.05. All statistical analysis was conducted by the software package SigmaStat for Windows version 3.10 (Systat Software, 2004).
4. Results and discussion
4.1. Water hyacinth growth rates
Growth rate of water hyacinth in the 9-tank system is shown in Table 1. A one-way ANOVA was conducted to compare the average water hyacinth growth rate. In this study, there was no significant difference found between the means of water hyacinth growth rate with different treatments of harvest frequency. The overall daily average water hyacinth growth rate was 0.172 kg wet wt./m2, which is equivalent to 629 ton wet wt./ha annually, and approximately 31 ton dry wt./ha annually, based on an average water content of the wet water hyacinth of 95%.