| Eutrophication is one of the global environmental problems,and eutrophication of lakes and reservoirs in China are quite prominent.There is an increasing demand for economic and effective ecological engineering to restore aquatic ecosystems.Jiulong River(Jiangdong reservoir)was selected as the study site,Jiulong River is the drinking water source of Xiamen and Zhangzhou.In this study,an experimental scale Algal Turf Scrubber(ATS)system was designed to test the efficiency of ATS(algal productivity,nitrogen and phosphorus removal rate)under various flow conditions in terms of hydraulic loading and flow intermittent frequency.The optimal flow condition was determined to obtain the best performance of ATS.Based on the experimental results,a large-scale ATS unit was further designed to treat the water of the Jiangdong reservoir in the future and its cost under three engineering schemes was estimated.The main results are as followings:First,hydraulic loadings influenced the efficiency of ATS.The largest algal producivities and nutrient(nitrogen,phosphorus)removal rates are generally observed under the hydrauliac loadings of 120 LPM m-1.Seven groups of experiments with different hydraulic loadings(from 60 to 180 LPM m-1)were set.There is a hyperbolic relationship between nutrient ramoval rates and the hydraulic loadings.A optimal value of hydralic loading(120 LPM m-1)exixts.In another word,the efficiency of ATS increased with flow rate when hydraulic loadings less than 120 LPM m'1,but turned to be declined when flow rate continute to increase.We concluded that slow inflow(low hydraulic loading)did not stimulate algae growth;However,too fast inflow with a large water shear force might make algae difficult to attach to the ATS and increase shedding of mature algae.Second,the intermittent frequency of inflow also influenced the efficiency of ATS.In total,the efficiency of ATS with intermittent inflow under optimal hydraulic loading is lower than that with continuous inflow.Three intermittent inflow(4 h-1,12 h-1,60 h-1)were set under the optimal hydraulic loading(120 LPM m-1).The experimental results showed that the efficiency of ATS were the highest with intermittent frequency of 4 h-1.The ratio of nitrogen removal rate of intermittent flow(4 h-1,12 h-1,60 h-1)to nitrogen removal rate of corresponding continous flow were 0.99,0.83,0.80,respectively.The ratio of phosphorus removal rate of ntermittent flow(4 h-1,12 h-1?60 h-1)to corresponding phosphorus removal rate of continous flow were 0.91,0.67,0.60,respectively.The ratios were all less than 1,suggesting that intermittent flow can save electric power but reduce the efficiency of ATS.Third,schemes of hydraulic loading 120 LPM m-1 and intermitent flow frequency 4 h-1 have the potential for large-scale engineering application.Given a one-hectare of ATS system to treat the water of Jiangdong reservoir,it was estimated that the nutrient(nitrogen,phosphorus)removal cost of scheme A(continuous flow,60 LPM m-1),scheme B(continuous flow,120 LPM m-1),scheme C(intermittent flow 4 h'1,120 LPM m'1)were?170,226,137 kg-1(for nitrogen),and ?474,580,384 kg-1(for phosphorus),respectively.The cost of scheme C appeared to be lowest.Furthermore,in order to meet the National Criterion of Environmental Quality for Surface Water(GB3838-2002)(TN and TP less than 1 mg N L'1 and 0.05 mg P L-1 for reservoir water),the treated cost for each tonne water would be ?0.26,0.35,0.21 Scheme C has highest cost-effectiveness and present good potential for engineering application. |
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