Phosphate uptake and translocation in a tropical Canna-based constructed wetland
© The Author(s). 2017
Received: 17 January 2017
Accepted: 23 February 2017
Published: 3 April 2017
Considering the problem of eutrophication of the water bodies, phosphate removal from water has emerged as a research of topical interest. The present study aims to investigate the efficacy of Canna lily-based constructed wetland to remove phosphate from wastewater. The translocation of phosphate in plant tissue and its biochemical transformation in sediments is also studied to understand its accumulation and recirculation within the system.
The removal of phosphate stabilized at around 50% in the present study and plant uptake was found to be the major removal mechanism. Average removal was 167 mg/m2 day for total phosphate and 84 mg/m2 day for available phosphate for an initial loading rate of 200 mg/m2 day and 85 mg/m2 day, respectively, at a HRT of 24 h. Most of the phosphate concentrated in above ground tissue of plant and its relative accumulation was maximum in flowers. Fractionation of phosphate in sediments confirmed removal by sediments with an accumulation of apatite phosphate (Ca and Mg bound), but release of non-apatite form (Fe and Al bound).
The study concludes that Canna-based constructed wetland can be an effective tool for phosphate removal from wastewater and sediments particularly under tropical conditions. Regular harvesting of above ground tissue of Canna can result in nutrient export from the system, whereas autochthonous addition may result in recirculation.
Phosphate is a critical parameter to establish the wholesomeness of water in respect of its quality and treatment strategy. Conventional treatment of wastewater removes a significant fraction of suspended impurities and organic carbon, but the removal of nutrients like nitrogen and phosphorus remains limited. Advanced treatment techniques like chemical precipitation, ion exchange, and reverse osmosis are either energy or cost intensive or result in production of secondary sludge (Joshi and Srivastava 2006; Haritash et al. 2015). In such a case, discharge of partially treated wastewater may result in nutrient excess and eutrophication in water bodies (Yadav et al. 2015). Phosphate being an integral component of food and plant waste, fertilizers, detergents etc. is continuously discharged in wastewater, and it reaches to water bodies through various routes leading to a eutrophic state of water body every 10–15 years (Jeppesen et al. 2005). Measures towards restoration of such eutrophic water bodies include chemical application of lime, alum, and ferric salt and introduction of planktons, macrophytes (Gao et al. 2009), and higher organisms to restore a functional biological cycling of phosphorus (Jeppesen et al. 2007). Application of constructed wetlands (CWs) for stabilization of secondarily treated wastewater (Vymazal 2007), and bioretention systems for stormwater, prior to discharge in a water body, has resulted in substantial removal and mobilization of nutrients and restoration of functional nutrient cycling (Sack 2013; Hseih and Davis 2005). Regular harvesting of fish or plants from an under-restoration water body results in export of nutrients, but on-site death and decomposition may lead to recirculation of nutrients in the same environment. Phosphorus being a key limiting nutrient for algal growth in freshwater regulates the biochemical health of a water body (Padma and Nair 2010). Most of the studies report that total phosphorus concentration may not be adequate to determine the ecological risks associated with it. The retention or release of phosphate and its bioavailability are regulated by the chemical state of phosphorus and environmental conditions (Zhang et al. 2012). The release of bound phosphorus to free soluble form, as regulated by pH and redox conditions, is reported in several studies (Kim et al. 2003). Many authors have reported phosphate removal from wastewater by Phragmites, Typha, Eichhornia, etc., but the ultimate treatment/disposal of such plants by composting is difficult. Whereas Phragmites and Typha are hard-stem plants, Eichhornia is an invasive weed and is difficult to harvest. In such a case, Canna lily, a plant with soft tissue, is identified as a potential candidate for phosphate removal without compromising the treatment efficiency, aesthetics, and option of its composting. Some studies on Canna spp. have reported its ability to remove nitrogen and phosphorus efficiently (Polomski et al. 2007), as a result of its higher evapo-transpiration rates against other ornamental plants. The rate of dry weight accumulation was also reported high owing to high rate of nutrient accumulation in plant tissue. Analysis of different plant parts confirmed maximum storage in shoots and roots. Other important factors responsible for nutrient removal are sediments and microorganisms. Gravel-based sediments and plant roots offer suitable substrate for microbial growth. Root exudates in the form of carbohydrates, amino-acids, enzymes, etc. offer oxidizing conditions and support nitrifying conditions during decomposition of organic matter (Gersberg et al. 1986). The chemistry of sediments too compliments the removal of nutrients, especially phosphate. Fragmented limestone waste used as packing material in a phragmites-based CW demonstrated removal efficiency of around 62% for phosphate (Mateus et al. 2012). Calcium and magnesium present in sediments can bind to phosphate to immobilize it temporarily under alkaline conditions, whereas iron and aluminium can dominantly remove phosphate at neutral pH (VanBeusekom and DeJonge 1997). Thus, pH plays an important role in regulating availability of phosphate in an aquatic system. Reducing/acidic conditions may favour resolubilization of phosphate, making a water body prone to eutrophication. Therefore, it is important to monitor the total load, and fractionation of phosphate in a water body to comment upon its trophic status, and to determine the actual efficiency of plants towards removal of phosphate. The present study, therefore, aimed to study the removal of phosphate by plants and sediments, to characterize the different fractions of phosphate in sediments and water, and to quantify the distribution of phosphate in different plant tissues during phytoremediation.
Concentration of total and available phosphate and its removal rate in Canna-based constructed wetland
Total phosphate (TP) (mg/l)
Removal rate (mg/m2 day)
Available phosphate (AP) (mg/l)
Removal rate (mg/m2 day)
Results and discussion
Based on the values of ambient temperature and solar insolation during the study, it was observed that the study area represents tropical to sub-tropical weather conditions which favour optimum activity and growth of vegetation. Average ambient temperature was 30 °C ranging from 24.5 °C (minimum) to 40.2 °C (maximum) during the study. Such conditions promote good rate of uptake of nutrients and hence maximum efficiency towards removal. In order to highlight the role of vegetation and sediments in removal of phosphate from wastewater, this paper discusses the uptake, transformation, and translocation of different chemical forms of phosphate in detail.
Phosphate removal from wastewater
Phosphate in plant tissue
Phosphate in sediments
Based on the observations of the present study, Canna lily-based constructed wetland is found to be an effective treatment option for phosphate from wastewater. The removal of sediment bound phosphate is also facilitated by Canna, but the available phosphate accumulates in sediments which may be released in hypolimnion if pH is acidic or conditions are reducing. Apart from it, though phosphate is translocated to all the tissues, its relative accumulation is observed to be more in flowers which may cause recirculation. Most of the fraction of removed phosphate is accumulated in above ground tissue, and hence, regular harvesting of aerial tissue can result in nutrient export. Considering the nutrient accumulation of harvested tissue, it may be used as raw material for composting if ultimate disposal is aimed at. Within the sediments, AP is accumulated whereas NAIP is released. It confirms easy release of Fe and Al bound phosphate to water though the redox conditions play a crucial role in it. The study concludes that Canna lily removes phosphorus not only from wastewater but also from sediments. Hence, Canna lily-based wetlands are effective in phosphate removal from wastewater especially under tropical conditions.
AKH designed the study, participated in the experimental work, and drafted the manuscript. SD carried out the analysis of phosphate in wastewater and plant. AS did the analysis of sediments and helped prepare the wetland cell for the study. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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