b) Suspended Solids
From Table 2, the average SS removal
efficiency was highest in the lotus unit (67-70%)
and slightly less in hydrilla (53-56%) and control
(22-33%) units, respectively. Since most of the
effluent SS concentrations in aquatic ponds are
normally from algae, this situation suggested that
the leaves of floating-leaf plants in lotus unit above
the water surface can prevent wind action and
suppress sunlight. These conditions are unfavorable for growth of suspended algae in the water
column and also enhance sedimentation (Jiang and
Xinyaun, 1998). In hydrilla and control units, there
was sufficient light to stimulate the growth of
algae, which resulted in higher concentrations of
effluent SS. Adsorption and filtration of SS by
submerged plant in hydrilla unit could increase SS
removal efficiency to be higher than in the control
unit without plant. The longer hydraulic retention
time of 10.5 days could effect a higher SS removal
efficiency than 5.4 days because SS concentrations
are easily settled in conditions of slow water flow
in the aquatic system.
The Effluent Standards of Thailand specified
SS in effluent at 30 mg/l. Both experimental runs
of lotus unit could meet this regulation while