Characteristics And Influence Factors Of Pollutant Removal And Greenhouse Gas Emission In Duckweed-based Wastewater Treatment Ponds

In recent years,water pollution and global warming caused by the increase of greenhouse gas emissions have become two global environmental problems recognized by the international community.Duckweed ponds are widely used in water pollution control because of its advantages in low cost,easy harvest and the ability to convert wastewater nutrients(such as nitrogen and phosphorus)into duckweed biomass which is rich in protein and starch.However,as a part of aquatic ecosystem,duckweed ponds can also emit greenhouse gases into atmosphere.It is very important for duckweed-based wastewater treatment ponds to investigate the characteristics and influence factors of pollutant removal and greenhouse gas emission in the large-scale application.This study is based on several pilot-scale duckweed-based wastewater treatment ponds(surface areas of 12 12 or 8 m2)located near the Diancli Lake in Kunming.It aims to identify the effects of natural growth(no duckweed harvest),influent load,coverage density and water depth on pollutant removal and greenhouse gas emissions in the duckweed-based wastewater treatment ponds by setting up four experimental groups and conducting long-term continuous operation(more than one year).The results showed that:(1)In duckweed ponds with the natural growth of duckweed(no duckweed harvest),coverage density of duckweed was higher,which led to massive death of duckweed and low growth rate(the annual average of 2.66 g/m2/d).In addition,higher coverage density reduced the DO,pH of water and ORP of sediment by blocking air exchange between water and atmosphere,and reduced the removal of COD,TN,TP,NH4+-N and turbidity(the annual average removal rate were 28.76%,17.25%,23.14%,8.01%and-98.05%,respectively).Additionally,natural growing duckweed reduced CO2 and N2O emissions but increased CH4 emission,resulting in a high greenhouse gas global warming potential(GWP)of duckweed ponds(4035.56 g CO2/m2/yr),which was much higher than that of oxidation ponds covered without duckweed(2749.96 g CO2/m2/yr).The results showed that the long-term natural growth of duckweed was not conducive to the application of duckweed ponds,and it was necessary to investigate and effectively control the main parameters,such as coverage density,nutrient load and water depth,in practical application of duckweed ponds.(2)Through the regular artificial salvage and control of the duckweed cover density at 825 g wet weight/m2(about 300%),the annual growth rate of duckweed increased to 6.66 g/m2/d,the removal rate of COD,TN,TP,NH4+-N and turbidity of sewage increased to an average annual 45.48%,34.74%,59.15%,33.39%and 65.73%.Also,effectively reduced greenhouse gas emissions and reduced GWP(2815.22 g CO2/m2/yr).(3)The study on the effect of the water intake nutrient load found that the high nutrient load significantly inhibited the growth rate of the duckweed in the duckweed-based wastewater treatment ponds(7.47g/m2/d and 6.77g/m2/d at an average annual level of low and high nutrient load,respectively),significantly reduced the water DO and ORP,as well as the removal rate of pollutants.However,the removal rate of pollutants was higher,and the cultivation of high nutrient content(annual carbon about 39%,crude protein about 30%,phosphorus about 1.2%)of the duckweed.Although the high nutrient load duckweed-based wastewater treatment ponds significantly reduce the N2O emissions(low and high loads are 194.01μg/m2/h and 150.24μg/m2/h,respectively),it does not significantly affect the emissions of CO2 and CH4.However,the GWP(low and high loads are 3984.39 g CO2/m2/yr and 4120.52 g CO2/m2/yr)increased numerically,indicating that high loads were not conducive to comprehensive greenhouse gas emission reduction.(4)The study on the effect of the coverage density found that the high coverage density due to space constraints,accelerated the death and settlement of Duckweed,was not conducive to the growth of duckweed(high and low coverage annual growth rate of 7.47 g/m2/d and 6.66 g/m2/d),and reduced dissolved oxygen and redox potential of water.However,the removal rate of other pollutants other than NH4’-N was increased,and the average annual removal rates of COD were 37.05%and 45.48%,TN 34.5%and 59.15%,TP 57.88%and 34.74%,and the turbidity 59.78%and 65.73%,respectively,under high and low coverage.Also,high coverage densities significantly increased CO2 emissions(low and high coverage densities were 31.67 mg/m2/h and 52.83 mg/m2/h.respectively),significantly reduced CH4 and N2O emissions(CHa were 15.71 mg/m2/h and 10.04 mg/m2/h,N2O 102.40 pg/m2/h and 59.08 μg/m2/h,respectively),which comprehensively reduced the GWP(high and low load is 3984.39 g CO2/m2/yr and 2815.22 g CO2/m2/yr,respectively).(5)The study on the effect of the water depth found that the inlet amount per unit area(the surface load of the water inlet),the increase of depth was not conducive to the growth of duckweed in the duckweed-based wastewater treatment ponds(the growth rate of deep and shallow systems were 6.59 g/m2/d and 7.47 g/m2/d)and the absorption and fixation of nutrients.Accelerate the death and settlement of duckweed due to nutritional restrictions.As a result of the increase of water depth caused by the total amount of water,hydraulic stayed long,water quality was higher,the removal rate was higher(the average annual removal rate of COD were 45.73%and 37.05%,TN 44.66%and 34.5%,NH4+-N 51.02%and 35.95%,turbidity 90.79%and 59.78%,respectively)but the removal rate was indeed low.In addition,the depth of water did not significantly affect the discharge of CO2 and N2O in the duckweed pond,but the increase in water depth did significantly reduce CH4 emissions(from 15.71 mg/m2/h to 4.43 mg/m2/h),and the combined reduction of the GWP of the duckweed-based wastewater treatment ponds was significantly effective,from 3984.39 g CO2/m2/yr to 1380.58 g CO2/m2/yr.(6)Comparatively speaking,CH4 was the most important greenhouse gas emitted by the duckweed pond system,and its contribution to the greenhouse effect potential of most duckweed ponds was more than 80%,which should be the key control object of greenhouse gas emission reduction in duckweed pond.In general,different factors showed different effect on pollutant removal and greenhouse gas emission in duckweed ponds,and the factors should be controlled according to the specific purpose in practical application.For example,if primary concern is the growth of duckweed,relatively low coverage density,inflient load and wate depth are suitable;if primary concern is effluent quality and pollutant removal efficiency,relatively high coverage density,low inflient load and high wate depth are suitable;if primary concern is pollutant removal rate,relatively high coverage density,inflient load and low wate depth are suitable;relatively low influent nutrient load,high coverage density and water depth can significantly reduce the GWP of duckweed pond.This study identified characteristics and influence factors of pollutant removal and greenhouse gas emission in duckweed-based wastewater treatment ponds at pilot-scale.The results provide guiding measures and scientific basis for improving the removal of pollutants and reducing the emission of greenhouse gas in duckweed-based ponds,and have theoretical and practical reference value for further large-scale application of duckweed-based ponds.