Control Of Cyanobacteria Blooms By Iron Based Flocculant In A Drinking Water Supplying Reservoir: Enclosure Experiment

Reservoirs play an important role in both urban and rural drinking water supply, but the water supply is challenged by increasing algal blooms with eutrophication.The control of algal blooms has been considered as a key measurement to maintain water quality for reservoir management.To figure out the controlling effect of an iron based flocculant to the algal bloom in a eutrophic reservoir,we carded out a large enclosure (340 m³) experiment during the period August 25^th of 2008 to October 4^th of 2008. There are three treatments and one control,and each treatment and control have 3 replicates.For the three treatments,the concentration of the iron based flocculant is set to be 15 ppm,30 ppm and 45 ppm,respectively.The enclosures were sampled in a high frequent sampling of every four days.The physical and chemical variables,chlorophyll a concentration,phytoplankton and zooplankton were measured or counted.The algal biomass was largely reduced in all the three treatments.The experiment of controlling cybanobacterial blooms demonstrates the iron-based flocculant can be applied to control algal blooms in the reservoir in a large and real scale.The experiment results showed that,the usage of the iron-based flocculant can obviously reduce pH and chlorophyll a concentration,and improve water transparency. At the early period of the experiment,the concentrations of nitrogen and phosphorus in all the three treatments,especially the phosphate concentration,decreased considerably. It proves that the iron-based flocculant could be used as a solution for emergency treatments,for the fourth day after using flocculant,comparing to before the experiment, 56%of the pH in the enclosure was reduce,93%for chlorophyll a concentration,61% for total nitrogen concentration and 68%for total phosphorus concentration.In the treatment of 30ppm of the flocculant,the transparency of the enclosure rose up 118%of the initial value on the eighth day after treating.In the enclosures,30 taxa phytoplankton were identified in the 11 sampling periods, cyanobacteria and Chlorophyta were the main group by number of species.Anabaena fios-aquae,Microcytis fios-aquae in Cyanophyta,Achnanthses sp.in Bacillariophyta, and Tetraedron minumum,Scenedesmus quadricauda in Chlorophyta,those which frequently occurred and are considered the indicating species of eutrophication,were rarely observed during the experiment.Towards the end of experiment,Monoraphidium sp.in Chlorophyta,Fragilaria sp.and Aulacoseira granulate in Bacillariophyta were new observed species.The sphere shaped algae(Microcytis fios-aquae and Anabaena fios-aquae) were the first dominant species in the phytoplankton community,and their biomass exceed 90%of the total phytoplankton biomass,the usage of flocculant showed a strong elimination to these kinds of alage,at the fourth day after adding flocculant,the elimination of all the three treatments to the sphere shaped algae were nearly 100%,and the abundance of the sphere shaped algae did not recover until at the end of the experiment.The elimination of all the three treatments to sphere algae was above 95% in the whole experimental period.The elimination of the three flocculation treatments to the filamentous algae(Cylindrosper sp.,Limnothrix sp.,Pseudanabaene sp.and Oscillatoria sp.)was 57%,59%and 59%,respectively.It proves that the elimination of the used flocculant to filamentous algae is not as effective as to sphere shaped algae. The cellular shape of algae is a key parameter for the flocculation effect.Both the species number and biomass of Chlorophyta and Bacillariophyta increased in the late period of the experiment,Desmid became the first dominant species in the phytoplankton community.22 taxa of zooplanktons were identified,including Rotifera(8 genera,14 species), Copepods(2 genera,4 species),Cladocera(4 genera,4 species).At the end of the experiment,12 species of zooplanktons were observed,the species number decreased, and all the missing species were from Rotifera.Copepods were the dominant groups, and had an average abundance of 72%of the total zooplankton abundance,and mostly contributed by Copepodites and nauplius.In the high and middle flocculation treatments, the zooplankton abundance was lowered during the experiment,and it gradually recovered at the end of experiment.In the low concentration treatment,only the abundance of Rotifera was reduced.The usage of flocculant caused increase in food reduction and acidication which reduced zooplankton species and abundance.