Comparing the Efficiency of Cyperus alternifolius and Phragmites australis in Municipal Wastewater Treatment by Subsurface Constructed Wetland

INTRODUCTION

Increasing world population has led to the emergence of many problems for human being. One of these problems is environmental pollution, especially pollution of water resources by wastewater. This not only causes environmental hazards, but also threatens human health (Vega et al., 2003). Municipal or domestic wastewater is one of the major sources of environmental pollution that imposes a heavy cost on economy of the countries (Tchobanoglous et al., 2003). Today, there is a wide range of wastewater treatment systems; however, most of these systems have major problems, including high manufacturing costs, high energy consumption and complex operation which need sludge treatment and disposal. In most countries, high technology systems with high costs are used for wastewater treatment the implementation of which requires large amount of money and investment. On the other hand, application of natural low-tech wastewater treatment systems such as Constructed Wetland (CW) not only reduces economic costs and energy consumption but also decreases environmental pollution(El-Khateeb et al., 2009; Ye and Li, 2009). Natural wastewater treatment systems use physical, chemical and biological processes which are caused by interactions of water, soil, plants and microorganisms in nature (Maine et al., 2007; Cheng et al., 2009). Constructed Wetlands are considered as cost effective as well as wise-economic method in wastewater treatment due to their simple and inexpensive operation and maintenance. Thus, they can be used in developing countries which are faced with water pollution problem that is caused by wastewater (Mantovi et al., 2003; O’Neill et al., 2011). The Constructed Wetlands can be used to attain these objectives: domestic wastewater treatment and agricultural runoff, industrial wastewater treatment, landfill leachate treatment, flood treatment and urban runoff, post treatment of wastewater, restoration of autotrophic lakes and treatment of water polluted by nutrients such as nitrate and phosphate (Moore et al., 2000; Kivaisi, 2001; Hadad et al., 2006; Vymazal, 2007, 2010; Chung et al., 2008). Constructed Wetlands are used to treat variety of pollutants available in wastewater including organic materials, detergents, nitrogen and phosphorus compounds, heavy metals, suspended solids and trace elements in wastewater such as copper, zinc, aluminum, etc. (Kadlec, 1999; Chung et al., 2008; Kropfelova et al., 2009). Constructed Wetlands can also be used as a final treatment process after biological treatment processes, such as stabilization ponds (Belmont et al., 2004). In general, there are two types of Constructed Wetlands: Free Water Surface wetlands (FWS) and Sub-Surface Flow wetlands (SSF). Wastewater flow can be in upward vertical mode or horizontal mode in subsurface wetlands. Subsurface wetlands’ bed is filled with gravel, sand and proper grading soil. This bed provides an appropriate platform for growth of bacteria. Moreover it leads to high settlement of pollutants (Thurston et al., 2001; Lee et al., 2009). The most important part of wastewater treatment mechanism in wetlands is presence and activities of aquatic plants and microorganisms as well as transportation of oxygen from the air to plant’s roots (Armstrong et al., 2000; Nwuche and Ugoji, 2008; Nwuche and Ugoji, 2010; Di Luca et al., 2011). Studies show that fenny aquatic plants directly and indirectly play an important role in wastewater treatment. These plants transport oxygen to roots, absorb nutrients and directly decompose pollutants (Stottmeister et al., 2003; Cheng et al., 2011). Reed plant or Phragmites australis is one of the most common fenny aquatic plants used in Constructed Wetlands, so that in most of studies wetlands are simply known with this plant. Other fenny aquatic plant used in wetlands is umbrella palm or Cyperus alternifolius. This plant has long, thin and green leaves which grows up to one meter in good condition. It also has strong roots and grows fast. This plant can be used for wastewater treatment in wetland method (Liao et al., 2003).

The necessity of this research is evident for the following reasons: umbrella palm as a fenny aquatic plant might be used for wastewater treatment in wetland method; data in this regard are not enough; necessity of applying treatment using natural methods with the lowest cost possible. So this research was necessary to obtain data regarding wastewater treatment by wetland. On the other hand, this study aimed to identify removal efficiency of chemical and microbial agents by umbrella palm; then compared it with reed’s one in removal of these pollutants in Yazd, Iran.

MATERIALS AND METHODS

This is an applied-interventional study in which the efficiency of two plants including Cyperus alternifolius and Phragmites australis was studied in sub-surface constructed wetland method to remove following parameters: BOD₅, COD, TSS, NO₃-N, NH₃-N, PO₄-N, Total Coliform (TC) and Fecal Coliform(FC). It should be noted that control parameters including pH and temperature were measured too. At first, three reactors were built as pilot by method of Constructed Wetland with sub-surface flow and four retention days. Pilots’ dimensions were as the following: 2 m length, 1.5 m width and 60 cm depth. In the two reactors, 50 seedlings of Cyperus alternifolius and Phragmites australis which were grown up to 15 days were planted and one pilot was considered as a control with no plant grown in. Soil gradation of all three reactors’ bed was sand in which three different diameters of this soil was used so that sands with coarse size (10-25 mm) were located near reactor’s outlet valve at the bottom, sands with medium size (8-14 mm) and small size (1-4 mm) were located in the middle and on top of the reactor respectively. Some clay was also added to the reactors in order to create an appropriate environment for growth of plants’ roots. The wastewater which was used in this research was effluent of septic tank (as pretreatment). Input flow of each one of these wetlands was set to 20 L min^-1. Sampling was done 20 days after beginning of wetland operation with four retention days in summer season. Ninety samples were collected in two months period, i.e. in each step of sampling, 6 samples from input and output were collected using special containers. Samples were kept in ice in order to avoid their adverse reactions; then they were immediately transferred to the laboratory and tested according to standardized methods. After various tests, obtained results were analyzed by SPSS software and Paired Sample Test statistical analysis.

RESULTS

Average concentration of influent and effluent parameters including BOD₅, COD, TSS, NO₃-N, NH₃-N, PO₄-N, TC and FC in Constructed Wetland is shown in Table 1. The data indicated the average concentrations of BOD₅, COD and TSS in the influent of wetlands to be 197, 413 and 111 mg L^-1, respectively. There was the lowest amount of these parameters in the effluent of Cyperus alternifolius wetland, that is, regarding BOD₅, COD and TSS, they were 53, 104 and 17 mg L^-1, respectively (Table 1). Also the highest removal efficiency for BOD₅, COD and TSS turned out to be 73, 74 and 84% , respectively (Fig. 1).

Table 1:	Average concentration of influent and effluent parameters in constructed Wetlands


Fig. 1:	Removal Efficiency (%) of BOD₅, COD and TSS in constructed Wetlands

Average concentration of NO₃-N, NH₃-N and PO₄-N in the influent of wetlands was shown to be 9.1, 59.5 and 7.4 mg L^-1, respectively. There was the lowest amount of these parameters in the effluent of Cyperus alternifolius wetland so that for NO₃-N, NH₃-N and PO₄-N they were 5.7, 37.6 and 4.9 mg L^-1, respectively (Table 1). Thus the highest removal efficiency in terms of NO₃-N, NH₃-N and PO₄-N reached 40, 36 and 33%, respectively (Fig. 2). Average concentration of TC and FC in the influent of wetlands stood at 2.8x10⁶ and 2.6x10⁵ MPN 100 mL^-1 , respectively. There was the lowest amount of these parameters in the effluent of Phragmites australis wetland, that is, TC and FC turned out to be 1.8x10⁵ and 1.3x10⁵ MPN 100 mL^-1, respectively (Table 1). Therefore the highest removal efficiency in terms of TC and FC reached 93 and 50% (Fig. 3), respectively. The data represented that average of temperature and pH in the influent and effluent of all three CW was almost the same and was between 30-30.2 c and 7.4-7.7, respectively (Table 1).

Statistical analysis: demonstrated that reduction rate of the parameters including BOD₅, COD, TSS, NO₃-N, NH₃-N,PO₄-N and TC in all three CW was significant (p = 0.05). The reduction rate of FC, however, was not significant (p = 0.05).


Fig. 2:	Removal efficiency of NO₃-N, NH₃-N, PO₄-P in constructed Wetlands


Fig. 3:	Removal efficiency of TC and FC in constructed Wetlands

DISCUSSION

Results of this study indicated that wetland with Cyperus alternifolius plant had higher efficiency compared to Phragmites australis in removal of chemical parameters including NH₃-N ,NO₃-N ,TSS, COD ,BOD₅, PO₄-P. Moreover, results of this study identified that Phragmites australis wetland had higher efficiency compared to wetland of Cyperus alternifolius in the removal of microbial parameters including total coliform and fecal coliform. Knight et al. (2000) performed a study on animal wastewater treatment using Constructed Wetlands. Results of this study showed that removal rate of parameters including BOD₅, SS, NH₄-N, TN and TP were 65, 53, 48, 42 and 42%, respectively (Knight et al., 2000). Liao et al. (2003) did another research on capability of Cyperus alternifolius and vetiver to treat pig field wastewater in China. Results of this study demonstrated that these two plant species removed COD, BOD₅, TP and NH₃-N parameters up to 64, 68, 18 and 20%, respectively in four retention days (Liao et al., 2003). Results of another research which was carried out by Song et al. (2006) indicated that using Constructed Wetland for wastewater removal could remove COD_,BOD₅ and TSS up to 62.2%, 70.4%9 and 71.8% respectively (Song et al., 2006). Juang et al. (2008) in a research on pollutants treated by Constructed Wetland identified the average of BOD₅, SS and NH₃to be 71.8, 36.6 and 41.7%, respectively. In this constructed wetland, a variety of plants such as Cyperus alternifolius were used. Retention time was specified 3-4 days (Juang et al., 2008). El-Khateeb et al. (2009) conducted a study in which Constructed Wetland with sub-surface flow was used for final wastewater treatment after one anaerobic process; the removal rate of BOD_5,COD and TSS with Constructed Wetland turned out to be 53, 66.6 and 68.2% , respectively (El-Khateeb et al., 2009). Comparison of the above-mentioned results with results of this study demonstrated that removal efficiency of chemical parameters was higher in this study. This increase may be the result of using Cyperus alternifolius plant. Thus, application of Cyperus alternifolius plant instead of Phragmites australis plant in wetland may be an appropriate alternative. Kadlec et al. (2010) performed a study on Constructed Wetlands with Phragmites australis plant in Columbia. Results of this study showed that the wetlands under study removed 98% of fecal coliforms and 95% of E. coli. Moreover, these wetlands had an effective role in removal of chemical agents (Kadlec et al., 2010). Results obtained from this study are parallel to above-mentioned study. This indicates that Phragmites australis wetland has higher efficiency in removal of microbial parameters compared to Cyperus alternifolius. Yang et al. (2006) conducted a research on a Constructed Wetland in northern China. Results specified that wetland can remove efficiently heavy metals from industrial wastewater in long-term application. Furthermore, it has removed more than 98% of suspended solids (Yang et al., 2006). Vymazal (2009) performed a study showing that application of Constructed Wetlands was effective in industrial wastewater treatment as well as waste leachate treatment (Vymazal, 2009). In view of similar studies and the results obtained from this study in representing high efficiency of Cyperus alternifolius wetland in removal of chemical parameters, it is clear that Constructed Wetland with Cyperus alternifolius plant can be used not only for Municipal wastewater treatment, but also for industrial wastewater treatment and waste leachate treatment. The results, moreover, indicated that Cyperus alternifolius has appropriate efficiency for removal of nutrients including nitrogen and phosphorus. Thus, this plant can be used for treatment of water and wastewater polluted by nutrient such as nitrate and phosphate (Stearman et al., 2012).

CONCLUSION

In general, this research detected that Cyperus alternifolius plant has acceptable efficiency for treatment of Municipal wastewater. Moreover, using a combination of Phragmites australis and Cyperus alternifolius wetland may lead to high efficiency of removal of chemical and microbial parameters. Of course, it is essential to perform additional studies on efficiency of these two plants in removal of nutrients, heavy metals, parasites’ semen and intestinal viruses. Furthermore, capability of this plant in various climate conditions as well as industrial wastewater treatment with high organic loading should be investigated too.