Effect of Reed and Hydraulic Retention Time on the Lead Removal in Horizontal Subsurface Flow Constructed Wetland

 
Introduction
Wetlands are classified into four kinds of flows as subsurface, surface, vertical, and hybrid flows. Usually, wetlands are planted with common reed (Phragmites Australis), a rhizomatose plant of the Graminae which produces a good yield in green biomass and whose roots can reach a considerable depth and plays a significant role in the wetland self-purification. The treatment efficiency of these systems mainly depends on the wetland design, hydraulic loading rate (HLR), and type of contaminant, microbial interactions and the climatic factors. For best treatment efficiency these systems require a low hydraulic loading rate and a long hydraulic retention time. The hydraulic retention time, including the length of time the water is in contact with the plant roots, affects the extent to which the plant plays a significant role in the removal or breakdown of pollutants. Whereas plants significantly affect the removal of pollutants in horizontal subsurface systems with long hydraulic retention times used to clean municipal wastewater, their role is minor in pollutant removal in periodically loaded vertical filters, which usually have short hydraulic retention times. With respect to the necessity of the research and above descriptions, the main purpose of this study was to evaluate the effect of reed and the increasing hydraulic retention time from 1 day to 10 days on lead removal efficiency in horizontal subsurface-flow constructed wetland to reduce the negative impact generated by lead in the environment.
 
Matherials and Methods
    Experiments of research were conducted in the Agriculture Faculty of Islamic Azad University, Dezful, Iran (48°25´E, 32° 16´ N) under ambient conditions. Climate of this region is warm with Mediterranean rainfall regime. Artificial reed-bed system was containing three metal boxes made of galvanized iron in mesocosm scale (1×0.3×0.35m). Dimensions of bed were 1 m long, 0.3m width and 0.35 m height that filled by river sand with a diameter of 0.01-5 mm and a depth of 30 cm. A large number of young and healthy Phragmites Australis seedlings were gathered from local irrigation canals and were immediately cultivated a number of 20 plants in each wetland in June 7, 2013. While planting seedlings, at least a space 10 cm from each other and a depth 20 cm between the roots was considered which increased the buds and shoots during the reeds growth and prevented them from dying out. The surface loading rate and hydraulic retention time were selected as four different retention time variables including 1, 3, 5, and 10 days. The wastewater containing lead with a concentration of 10 mg pb/l was selected because was above standard of Iran Environmental Protection Agency, World Health Organization (WHO) and FAO for irrigation and this dose was also chosen in similar studies in the range 1-20 mgpb/l. All influent and effluent samplings and plant samples were measured by Perklim Elmer A Analyst 700 atomic absorption.
 
 
Results and Discussion
Models and Lead removal efficiencies
 According to measurements, removal efficiency (RE) and relative concentration of inflow to outflow (c/c₀) in synthetic wastewater were calculated. In addition, removal time ratio (RTR) was calculated at different retention times. Accordingly, the relationship between the lead removal efficiency and HRT was examined that the relevant equation was derived as the exponential equation by using CurveExpert1.4 software, so that among the fitted equations had the highest correlation coefficient (r=0.975) that its results were presented in Table 1. Meanwhile, Measurements showed the more hydraulic retention time increases, the more removal efficiency also increases but the increasing rate of RE was decreasing in the range of 88.2% to 84.1 during different retention times.
 
Table1. Regression coefficients of exponential equation of lead removal efficiency





Standard error


     n


r


c


b


a




0.004


12


0.975


0.718


10.39


0.085





 
Effect of Hydraulic Retention Time
    Changes trend of removal efficiency in relation to HRT showed that the more retention time increased, the more removal efficiency also increased. It seemed that with the increasing HRT, the hydraulic loading rate and lead surface loading rate decreased thus, enough opportunity was provided for physical, biological, and chemical processes for lead removal in the reed-bed systems. In order to investigate the significance effect of HRT on the removal efficiency, observed means were compared using SPSS18 software and Duncan’s test which the relevant results are shown in Table 2. According to the results a significant difference was observed between removal efficiencies at retention time 1, 3, 5 days (p˂0.05) but no significant difference was observed between efficiencies at retention time 5, 10 days (p˂0.05). Therefore, 5-day retention time is suggested in optimum conditions (Table 2).
 
Table 2.Effect of retention time on the average removal efficiency





HRT(day)


RE (%)


Duncan Test^*




1


84.1


A




3


87.2


B




5


88.1


C




10


88.2


C





* Different letters indicate significant difference between HRT at the level of P˂0.05.
   Similar studies have been done on the lead removal efficiency in constructed wetlands by different researchers and similar results have been obtained, so that the lead removal efficiency was mainly concluded in the range 76 to 95%. Moreover, the formation of insoluble sulfides, filtration of solids and colloids, sedimentation, combining with iron and manganese oxide have been reported as the main process in lead removal from waste water in CWs. In a research conducted in subsurface-flow constructed wetland in a pilot scale by cultivating Typha and Phragmites, similar result was obtained. In this study, the lead concentration of influent was in the range 1-20 mg/l and the removal efficiency was measured in the range of 75-96%. In another research conducted on lead and cadmium, was observed that with the increasing retention time from 2 to 6 days, removal of the heavy metals increased, too and the maximum removal efficiency was 75%.
 
Bioaccumulation Factor
The linear relationship between HRT and bioaccumulation factor (BAF) in the dry tissue of reed was examined. Accordingly, a linear relationship with high correlation coefficient (r²=0.988) was obtained based on the equation of  in which HRT was hydraulic retention time in day and BAF was lead bioaccumulation factor in mg/kg of dry weight. The results showed that there was a significant relationship between hydraulic retention time and BAF so that with the increasing HRT, absorption of plant root increased up to 116 mg/kg d.w. of root tissue at 10-days HRT. Thus, reed is highly capable of absorbing and accumulating of Pb and plays an important role in the removal of lead in subsurface wetland system. Aquatic plants are among the main biological processes in wetlands because they not only directly absorb oxygen but also make it enter around the root zone which leads to nutrition absorption, oxidation and direct spoilage of contaminations and more activities of microorganisms. Consequently, different plant species play important roles in the heavy metals removal. With regard to the mass balance of lead in wetland, the percentage of bed and plant contribution in the lead removal was calculated, separately. As a result, the contribution of reed in lead removal during four retention times in the range of 8-22% of influent lead  from 1 to 10 days, respectively so that the contribution of reed increased in relation to HRT but in contrast, the contribution of bed decreased in the range of 77 to 91% of influent lead. It seemed that the contribution of bed at lower retention time was more effective due to higher HLR and when the hydraulic retention time increased, the storage capacity of bed decreased. Considering that the used bed was made of fine sand and the sand lacked cation exchange capacity (CEC), so the removal mechanisms were mostly biological and physical mechanisms resulted from plant and microorganisms uptake and hydraulic characteristics, therefore with the increasing reed bioaccumulation factor (BAF) in relation to HRT, a decrease in the contribution of bed seems to be reasonable. Biological removal is an important pathway for heavy metal removal in the CWs, it includes plant and microbial uptake. The rate of metal removal by plants varies widely, depending on plant growth rate, plant species and concentration of the heavy metals in the wastewater. Maximum concentration of metals in plants has been observed in under-ground organs.
 
Conclusions
   The relative Concentration of effluent to influent (C/C₀) decreased significantly due to the increasing HRT and the decreasing HLR from 19.8 to 3.5 cm/day and as a linear relationship with high correlation coefficient (R²=0.991). Therefore, the more retention time increased and the surface loading decreased, the more time was provided for chemical and biological processes affecting the lead removal in subsurface wetland system. Moreover, statistical analyses showed that there was a significant difference between removal efficiencies at retention times 1, 3, 5 days (p˂0.05) but there was no significance difference between efficiencies at retention time 5, 10 at 5% level. Therefore, 5-day retention time was suggested in optimum conditions with the removal efficiency 88.1%. Meanwhile, the results of the measurement of reed tissue showed that there was a direct relationship HRT and BAF, so that the more retention time increased, the more bioaccumulation factor increased up to 116 mg/kg d.w. of below organs during experiment period. Moreover, the contribution of reed in the lead removal was in the range of 8-22% of influent lead from 1 to 10 days, respectively so that the contribution of reed increased in relation to retention time. Consequently, reed played an important role in the removal of lead in horizontal subsurface wetland.