We investigated the effects of the presence of four duckweeds, Spirodela polyrhiza, Lemna minor, Lemna aequinoctialis, and Wolffia arrhiza, on phenol removal from environmental waters. Phenol was more rapidly removed from water with duckweeds than without duckweeds. In contrast, phenol concentrations did not decrease in autoclaved Hoagland solutions with sterilized duckweeds. These results indicated that all the duckweeds could mediate the acceleration of phenol removal from water and that the acceleration was not caused by phenol uptake by the duckweeds. S. polyrhiza and W. arrhiza were associated with the greatest acceleration of phenol removal. A variety of phenol-degrading bacteria, including Pseudomonas spp., Delftia spp., Azospirillum spp., Acinetobacter spp., and Zoogloea spp., were isolated from S. polyrhiza and W. arrhiza. Extracts from the duckweed plants promoted the cell growth and/or rate of phenol degradation by the bacterial isolates. These results suggest that duckweeds can provide a habitat for phenol-degrading bacteria, stimulate their growth, and enhance their phenol-degradation activity. The results of this study have helped to identify duckweed species best suited for treatment of phenol-polluted waters.

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To clarify the mechanism of submerged macrophytes disappearance and supply the necessary theory for the vegetation restoration in eutrophic Lake Taihu, effects of the moderate nutrients concentration, excessive NH₄⁺－N and PO₄^3－－P in the water column on the antioxidant defense system in submerged macrophyte, Vallisneria asiatica, were studied with the 20－day aquarium experiments. The results showed that the moderate concentration of nutrients can promote the metabolism of V. asiatica. Either excessive NH₄⁺－N or PO₄^3－－P could cause the oxidative stress to cells of V. asiatica, expressed as decreased contents of chlorophyll a (Chl.a) and protein, and the enhancement of catalase (CAT) activities in leaves of V. asiatica. In addition, 0.6 mg・l^－1 of PO₄^3－－P caused more oxidative damages to V. asiatica than 3.5 mg・l^－1 NH₄⁺－N. The results indicated that the antioxidative protection mechanisms could be activated but still could not prevent the damage of the metabolism system in V. asiatica exposed to either excessively high concentrations of NH₄⁺－N or PO₄^3－－P. Therefore, it is necessary to establish a dual control strategy of N and P for the restoration of V. asiatica in eutrophic Lake Taihu.

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Immobilized activated sludge dominated by insensitive nitrifiers was acclimated with a supply of high ammonium-nitrogen. The effect of temperature on nitrification was examined using accumulated pellets. The nitrification was nitrite type and the activated energy was 44.8 kJ/mol. The accumulated pellets were used to treat NH₄－N at a concentration of 40 mg/l as a test of their utility in domestic wastewater treatment. Synthetic wastewater with a NH₄－N concentration of 40 mg/l was treated with a removal ratio of 95 ％ under a loading rate of 0.36 kg－N/m³/d at 10 ℃. Nitrification is stable under temperature oscillations during the four seasons. The nitrification changes from nitrite type to nitrate type after summer. Influent NH₄－N with concentration of 40 mg/l can be reduced to or below 1.0 mg/l at levels of 0.32 kg－N/m³/d at 10 ℃.

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In this study, quick and effective sampling method of dissolved gas in the water was developed, and the potential of generation and emission of N₂O and CH₄ in a shallow eutrophic pond was evaluated. On-site quick and effective dissolved gas sampling method was developed through following steps: 1） take water sample into just a half volume of the gas-sampling vessel, 2） shake the vessel and take the gas-phase inside the vessel into a gas-saving bag, 3） simultaneously measure the water temperature for gas solubility calculation and take the around air for back ground gas concentration adjustment. The results indicated that in shallow eutrophic ponds, generation and emission of N₂O could be reduced as the result of phytoplankton’s propagation on competition of inorganic nitrogen uptake with nitrifying microbes, especially under nitrogen-limited condition. In such ponds, generation and emission of CH₄ could also be reduced as the result of keeping whole water layer of the pond oxic by both DO supply by phytoplankton and mixing the pond water by wind. It was suggested that the existence of phytoplankton had a large effect on generation and emission of N₂O and CH₄, and that they also made a contribution to control of N₂O and CH₄ to no small extent.

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