Valorisation of Phosphorus-Saturated Constructed Wetlands for the Production of Sugarcane

: Constructed wetlands (CW) are a clean and environmentally friendly alternative to conventional wastewater treatment methods, namely in the removal of the nutrients responsible for the eutrophication of receiving water bodies, as is the case of phosphorus compounds. The materials used as CW filling can directly contribute to the removal of phosphorus compounds from wastewater, but with the operating time they tend to become saturated and treatment efficiency decreases. In order to evaluate the viability of producing an energy crop in phosphorus-saturated CW, sugarcane growth was monitored in two pilot-scale CW filled with two different expanded clay aggregates used for 10 years in wastewater treatment. This paper presents the results obtained during the first year of plant development in the plant-cane cycle. Morphologic aspects of sugarcane growth, such as height and average diameter of stems, average leaf area and number of new sprouts, have been monitored. The results obtained are comparable with those cited in the literature for traditional cultivation. Dry biomass productivity of 26.6 ton per hectare per year can be achieved. Estimated sucrose productivity can reach 13.5 ton per hectare per year, and related bioethanol production potential can be between 2.4 and 7.6 cubic meters per hectare per year, depending on the CW filter media used. It is concluded that the cultivation of sugarcane in CW allows to extend the life of these systems by reusing fillers, and simultaneously is an alternative to produce bioethanol raw-material without the use of scarce resources such as arable land, fresh water and plant nutrients.


INTRODUCTION
Sugarcane is the most important feedstock for large-scale production of bioethanol fuel, particularly in tropical and sub-tropical countries where crop productivity is high and agricultural costs are low [1][2][3][4]. Bioethanol obtained from sugarcane may represent a potential reduction of 80% of GHG emissions relative to gasoline, while bioethanol derived from maize may represent only up to 52% reduction in GHG emissions [5].
The production of bioethanol from sugarcane juice or molasses has the advantage of being a welldeveloped and implemented technology, but crop production may require large arable land areas and scarce resources as fresh water and nutrients such as phosphorous compounds [6][7][8].
Constructed wetlands (CW) are an ecological and sustainable alternative to conventional wastewater treatment systems, especially for nutrient removal. CW consist of beds, usually dug into the ground, sealed and filled with filter materials, and planted with *Address correspondence to this author at the Department of Engineering, Instituto Politécnico de Tomar, Estrada da Serra, Quinta do Contador, 2300-313 Tomar, Portugal; Tel: 351 249 328 160; Fax: 351 249 328 167; E-mail: dinamateus@ipt.pt macrophyte plants [9,10]. Wastewater treatment occurs as it passes through the beds by a complex variety of interacting biological, chemical and physical processes. The two main types of CW are free surface or subsurface flow. For sanitary reasons, the subsurface flow is most frequently used because there is no direct contact between the wastewater and the atmosphere [9]. The effectiveness of these systems depends, besides correct dimensioning, on the selection of a filling material with high pollutant-removal capacity and good hydraulic characteristics, as well as the selection of plants species suitable for growing in flooded beds.
Filling materials represent the largest portion of initial capital investment of constructed wetlands. Therefore, its selection is crucial to the viability of these systems and should be based on performance, availability, safety and cost criteria. Although there are many studies on the feasibility of using industrial byproducts or natural filling materials, such as limestone fragments, ceramic waste, slag from iron and steel industries, among others [11][12][13] that are sustainable alternatives because of their cost-effectiveness, the use of various types of expanded clays produced specially for this purpose is very common due to their high efficiency [11,14,15]. However, expanded clays are a relatively expensive material. Therefore, the possibility of regeneration and/or subsequent use as a substrate for growing valuable crops, for example, is an asset and enhances sustainability of the plants using this type of filling material.
Reed (Phragmites australis) is one of the most commonly used macrophytes in CW, due to its high pollutant removal capacity and high resistance to extreme environmental conditions [9]. However, the possibility of growing sugarcane instead of traditional macrophytes has been confirmed in previous studies [16,17] and enhances the sustainability of such wastewater treatment technology through the production of sucrose-rich vegetable biomass, which could be used to produce bioethanol through widely implemented processes. At the same time, the production of this energy crop in CW would avoid the use of agricultural lands and water consumption, as well as the diversion of raw materials from the food supply chain, constraints often referred to in the development of biofuel production [18]. Thus, this study presents data concerning the re-utilisation of CW filled with expanded clay, previously used for phosphorus removal from wastewater, to simultaneously produce bioethanol feedstock (sugarcane) and to perform wastewater treatment.

Materials
Two pilot scale CW were built in an inner courtyard of the campus of the Polytechnic Institute of Tomar (Central Portugal, 39°35'57.7'' N, 8°23'26.1"W) under the conditions of a Mediterranean climate, classification Csa according to the Köppen-Geiger climate classification [19]. They consist of rectangular PVC tanks above the soil surface (1.2 m long x 1.0 m wide x 0.53 m deep) with a slope of 1% and a drainage system composed by two longitudinal perforated pipes at the bottom. Each tank contains a first 0.10 m layer of gravel that covers the drainage system, and a second 0.37m layer of expanded clay Filtralite (Maxit Portugal), with Filtralite® MR 3/8 (CW-MR; round, 500 to 600 kg/m 3 , 3 to 8 mm) and another with Filtralite® NR 3/8 (CW-NR; round, 300 kg/m 3 , 3 to 8 mm). Filtralite® MR has higher density than Filtralite® NR, shows better capabilities for wastewater treatment but is more expensive [15].
The CW have been in operation for about 10 years as phosphorus filters without plants, in a continuous vertical subsurface flow mode. In May, six plants of the Saccharum officinarum species (sugarcane) with 3month germination have been transferred to each tank. Plants were selected at a similar stage of development and planted about 0.15 m deep, following a 2×3 distribution with equal interspacing, in both beds. The experiments were conducted without any pre-treatment of the filler materials and with no addition of fertilizers or pesticides.

Sugarcane Growth Monitoring
The sugarcane plants were monitored regularly until late November. Stalk diameter was measured at the first internode from each stalk base. Stalk height was measured from the bottom of the canes near the surface of the filler material to the base of leaf +1 (first well-developed leaf counted from the plant´s top).
The number of fully-expanded green leaves for each plant was counted and the length and width of the leaf +3 measured (third well-developed leaf counted from the plant´s top). Average plant´s leaf area was estimated using the model proposed by Silva et al. [20].

Biomass Productivity
In November the sugarcanes with about 10 months' growth were cut and biomass yield and sucrose content of the stalks determined. Above-ground biomass was harvested, separated into stalk and leafs and the fresh weight of stalks was measured. Representative samples of plant stems were finelychopped using a cutter-grinder, and dried at 60 ºC to constant weight to evaluate the dry matter content.
Wet and dry basis biomass annual productivity was estimated on a per hectare basis, considering the fresh biomass measured and dry matter content calculated in each CW divided by CW superficial area multiplied by a scale factor of 1.5. The scale factor was used to take into account the extra area required for the movement of personnel and machinery in a real scale CW.

Sugarcane Sucrose Content and Bioethanol Production Potential
Sucrose production was estimated according to the correlation proposed by Muchow et al. [21], which correlates sucrose accumulation in the sugarcane stalk with crop biomass, on a dry weight base.
Bioethanol potential production was estimated from the sucrose to ethanol conversion of 0.570 cubic meters of ethanol by metric ton of processed sugar, which value corresponds to the lower end of the usual conversion range for modern ethanol production facilities [22].
Sugarcane and bioethanol productivities were converted to annual values per hectare as for biomass productivity calculations.

Constructed Wetland Performance Evaluation
Total Phosphorus (TP) removal in CW was monitored between January and November. During this period the CW were fed continuously with secondary wastewater effluent, at the hydraulic load average of 41±2 Lm Every two weeks, samples were collected at input and output CW streams and analysed to determine the concentration of TP. Analysis followed standard methods for the examination of water and wastewater [23]. 5 mL water samples were autoclaved in an acid potassium persulfate solution at 121 ºC for 30 min. After digestion and cooling, liberated orthophosphates were quantified spectrophotometrically at 880 nm, by ascorbic acid method. Figure 1 presents the sugarcane average height in the two pilot-scale CW during the growth period, compared to sugarcane growth in traditional soil irrigated cultures [20]. The sugarcane height growth was weaker than the one observed for soil cultures, following the same trend until the seventh month of growth. Growth evolution was significantly better for sugarcanes planted in the CW-NR (P<0.001).   [20]. After a lower growth at first months, CW sugarcane growth was better than the one observed for soil cultures. Diameter evolution was significantly better for sugarcanes planted in the CW-NR (P=0.012).  Table 1 shows the average dimensions of the sugarcanes at the time of cutting (10 months). All biometric indicators are better for plant growth in CW-NR, but not significantly different for average height and average diameter (P=0.244 and P=0.850), and with low significance for the average foliar area (P=0.046).

Sugarcane Growth Indicators
Although the average final stalk height was lower than the one reported by Silva et al. [20], Figure 1, experimental values are within the range reported in the literature for sugarcane with equal growth time, between 1.18 and 2.22 m [24,25].
The average stalk diameters obtained in both CW are superior to those reported by Silva et al. [20] and also by Caione et al. [25], located between 0.0218 and 0.0220 m.
Average foliar areas observed in both CW are above the range reported in the literature, between 0.166 and 0.600 m 2 /plant [26,27].

3.2.
Biomass Production and Bioethanol Production Potential Table 2 shows the indicators of sugarcane productivity in both CW. Although sugar content is higher for the canes in CW-MR, productivity in terms of fresh biomass and sucrose is about three times lower than for CW-NR. This difference is due to the fact that in the CW filled with Filtralite® NR the canes showed a better development (cf. Table 1) and produced more new sprouts.
Biomass yield are within the range of results published for conventional culture which, depending on the cultivation conditions, can vary between 30 and 226 tons per hectare per year, wet basis [28,29]. Bioethanol productivity of traditional sugarcane plants ranges from 7 to 8 cubic meters per hectare per year [30,31]. So, estimated productivity of CW-NR is well within the range of reference, whereas productivity of CW-MR is far below the commercial range.
The feasibility of using CW vegetation to produce bioethanol by biomass fermentation after cellulose hydrolysis was assessed by He et al. 2010 [32]. For the common CW vegetation Phragmites australis these researchers obtained a yield of 0.01 kg of ethanol per kg of dry plant biomass. Considering the typical Phragmites australis productivity in CW of up to 1.8 kg/m 2 , the estimated potential of bioethanol production may be calculated as 0.23 cubic meters per hectare per year [33,34]. Although this route also represents a sustainable production of bioethanol, the productivity is expressively lower than sugarcane bioethanol and requires a not yet well-developed lignocellulose conversion technology.

Phosphorus Removal Efficiency of the Constructed Wetland
Special attention was dedicated to phosphorous removal capabilities of sugarcane planted CW due to the importance of phosphorous compounds on water bodies eutrophication problems. Average TP removal efficiencies were 44±12% for CW-NR and 50±9% for CW-MR. The values obtained for phosphorus removal are within the range of values found in the literature for the macrophyte based CW, ranging between 38% and 99% [9].
Although the expanded clays were already saturated in phosphorus, it was found that the growth of sugarcane allowed these materials to be reused as CW filling materials for wastewater treatment, thus prolonging their lifetime.

CONCLUSIONS
It can be concluded that sugarcane can be produced in CW saturated filling media, extending the lifetime of expanded clays applied in wastewater treatment.
Growth biometrics of sugarcane plants in CW conditions is comparable to growth of plants in traditional lands. Growth indicators and productivity of sugarcane was different for the two tested filling media. Growth indicators were better and productivity was higher for sugarcane growth in Filtralite® NR than in Filtralite® MR, which is more expensive.
Estimated bioethanol productivity is 7.6 and 2.4 m 3 per CW hectare per year, respectively for Filtralite® NR and Filtralite® MR fillings. The higher value is in the range of typical productivities for traditional ethanol production from sugarcane. Besides the lower value obtained for the Filtralite® MR filling, sugarcane production on this filling media represents an improvement in the sustainability of CW technology.  In addition to demonstrating the feasibility of producing a first-generation bioethanol feedstock, the results showed that the growth of sugarcane in exhausted filter media ensures CW wastewater treatment capabilities and contributes to reducing the use of arable land and the consumption of scarce resources such as fresh water and phosphorus, among other nutrients used as fertilizers in traditional sugarcane production.