Study On Aquatic Plant Filter Bed System For Eutrophicated Water Quality Improvement

Aquatic plant filter bed (APFB) is a new type of technique combining plant purification and physical filtration for constructed wetlands without any substrate that applied mainly to purification of hyper-eutrophic surface water.A pilot scale APFB established at the lakefront of Taihu Lake was tested to treat polluted river water.The following results were obtained:Ipomoea aquatica and Oenanthe javanica are identified as the most suitable edible aquatic vegetable species transplanted in APFB during summer/autumn and winter/spring separately.The characteristics of APFB for nitrogen and phosphorus were fully manifested. Under the optimal operation conditions with hydraulic loading rate(HLR) of 4.0 m3/(m2·d) and water surface level of 10cm, the removal loading rates of total nitrogen(TN),total phosphorus(TP) are 1.539 g/(m2·d) and 0.232 g/(m2·d) respectively for all-year average, the removal efficiency are accordingly 17.3% and 24.1%. The removal efficiencies for TN and TP are better than dissolved nitrogen and dissolved phosphorus. The removal loading rate of nitrogen and phosphorus are also influenced by the frequency of harvesting, and the proper frequency of harvesting for Ipomoea aquatica is once a month. The APFB can output economic profit affirmatively when operation under the HLR between 3.0 to 4.0 m3/(m2·d).By the dynamics evaluation of nitrification and denitrification potential for sediment accumulating in the bottom of APFB, it is found that the maximum nitrification and denitrification potential appears in the middle part and front part of APFB separately. This distribution causes unfavorable effect on microorganism denitrification process. It is found that the distribution of nitrification potential accords with the ammonium-oxidizing bacteria density using fluorescence in situ hybridization (FISH) to detecting ammonium-oxidizing bacteria and nitrite-oxidizing bacteria density in sediment The sum of nitrogen removal loading rate by denitrification and plant uptake is equal to the amount released by sediment, which is the reason of low removal efficiency for dissolved nitrogen. Analysis of the mass balances of nitrogen and phosphorus in the planted channel shows that uptake by plant, denitrification and block precipitation contribute nearly equal to each other for removal of TN. However, the block precipitation is the foremost removal mechanism of TP.APFB was used to pretreat eutrophicated source water for the first time. The optimal HLR is 2.5~3.0 m3/(m2·d) and the optimal water surface level is 10~20cm when APFB used as source water pretreatment process. The CODMn and algae density removal efficiencies can reach 20% and 90% respectively in summer, while 11% and 85% (calculated as chlorophyll.a) in winter. The NH4+-N and turbidity removal rate can consistently reach 30% and 90% separately all year around. It can be concluded that APFB is a optional pretreatment process for suburb small waterworks.The characteristic of APFB for microcystin (MC) removal was investigated. The average removal efficiencies of total microcystin (TMC), MC-RR and MC-LR separately reach 59.4%, 55.2% and 66.0% under the proper HLR of 4.0 m3/(m2·d). The attenuation of MC-RR concentration in APFB is rapid than that of MC-LR. The removal efficiency of TMC is positive linear correlation with the removal efficiencies of total algae cells, chlorophyll.a (Chl.a), permanganate index (CODMn) and TP. Static experiments shows that the present of plant is benefit to MC-RR removal while sunlight avails of MC-LR removal.Food hygiene security of the aquatic vegetables cultivated in APFB was investigated. The content of some heavy metals including Cd,Cr,Cu,Hg,As and pesticide including BHC and DDT in vegetables esculent part were all kept to national food hygienic standard. But the content of Pb is sometimes over the limit value of national food hygienic standard. By field experiments and indoor static tests, the mode of plant arrangement and periodical harvestry to ensure food hygiene security for aquatic vegetables were put forward, further more validated in pilot scale experiment.