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Towards the development of a novel construction solid waste (CSW) based constructed wetland system for tertiary treatment of secondary sewage effluents

This study was conducted to examine the possibility of using construction solid waste (CSW), an inevitable by-product of the construction and demolition process, as the main substrate in a laboratory scale multi-stage constructed wetland system (CWs) to improve phosphorus (P) removal from secondary sewage effluent. A tidal-flow operation strategy was employed to enhance the wetland aeration. This will stimulate aerobic biological processes and benefit the organic pollutants decomposition and nitrification process for ammoniacal-nitrogen (NH+ 4-N) removal. The results showed that the average P concentration in the secondary sewage effluent was reduced from 1.90 mg-P/L to 0.04 mg-P/L. CSW presents excellent P removal performance. The average NH+ 4-N concentration was reduced from 9.94 mg-N/L to 1.0 mg-N/L through nitrification in the system. The concentration of resultant nitrite and nitrate in the effluent of the CSW based CWs ranged from 0.1 to 2.4 mg-N/L and 0.01 to 0.8 mg-N/L, respectively. The outcome of this study has shown that CSW can be successfully used to act as main substrate in CWs. The application of CSW based CWs on improving N and P removals from secondary sewage effluent presents a win-win scenario. Such the reuse of CSW will benefit both the CSW disposal and nutrient control from wastewater. More significantly, such the application can transfer the CSW from a ‘waste’ to ‘useful’ material and can ease the pressure of construction waste solid management. Meanwhile, the final effluent from the CSW-based CWs can be used as non-potable water source in landscape irrigation, agriculture and industrial process.


Introduction
The secondary effluent derived from sewage treatment plant should be further treated when it is reused for industrial and agricultural process. In particular, when it is discharged into a polluted river or lake that requires a higher water quality, tertiary treatment is then a crucial necessity in eliminating the possible danger of causing further pollution of receiving waters. [1−3] However, the most suitable and acceptable tertiary treatment route differs from plant to plant. The selection of the treatment method is highly dependent on process control capabilities and cost. Among the various tertiary treatment processes, constructed wetland (CW) is increasingly applied worldwide as a low-cost, low energy consumption and easy management technology for various kinds of wastewater treatment and/or tertiary treatment. [4,5] Extensive studies have been conducted to improve the removal performance of CW. [6] One of the main aims of research in CW technology today is to discover new medium material that can increase the pollutants removal efficiency since the media in wetland treatment systems plays an important role in integrated physical, chemical and biological processes that remove pollutants in wastewater. [7,8] It is known that phosphorus (P) removal in CWs is poor due to the fact that P removal is limited by the capacity of the media to adsorb, bind or precipitate the incoming P. Therefore, a number of studies have been conducted into screening alternative materials and/or industrial by-products for P removal ability and suitability as main medium in CWs. [7,8] Construction solid waste (CSW), an inevitable byproduct generated in most cases of the construction and demolition process, is disposed of as waste to landfill site. In recent years, the management of CSW has become a significant issue in environmental engineering due to the enormous quantities generated, lack of landfill site and the associated disposal costs. In China, approximately 40 million tons of CSW are generated per year and a two-fold increase has been forecasted by the end of the next decade due to the increased urbanisation. Therefore, seeking alternative pathway for CSW disposal is crucial for construction waste solid management. [11] Most of the research efforts, to date, have been done to investigate the feasibility of reusing such the CSW as pavement or other construction materials. Considering the issues in the final disposal of CSW and P removal in CW, this study attempts to reuse CSW as treatment wetland substrate for P removal from secondary sewage effluent and to demonstrate ammoniacal-nitrogen (N) and organic matter removal performance of the CSW-based CWs.

CSW collection and preparation
The experimental CSW, which contains mainly waste concrete, was collected from a demolition site in Xi'an city, China. Thereafter, the CSW was ground and sieved into particle size of 6-40 mm as the N removal substrate to be used in laboratory-scale CWs. Meanwhile, the waste lightblock CSW that made by coal ash was ground and sieved into particle size of 3-40 mm as the P adsorbent in the CWs.

Secondary effluent collection
The secondary effluent studied was obtained from the disinfection tank of NO.4 Municipal Wastewater Treatment Plant of Xi'an city, China. The WWTP employs an anaerobic/anoxic/aerobic (A 2 /O) process for municipal wastewater treatment. Table 1 summarizes the characteristics of secondary effluent used in this study.

Laboratory scale CW set-up and operation
To examine the proposed use of CSW as media in CWs, a four-stage laboratory scale CWs was constructed using Pyrex tubes (100cm in height with an internal diameter of 9.4 cm) to act as 1 st stage and 3 rd stage-while the PVC tanks (100cm long with width of 15 cm and height of 75 cm) were used for 2 nd stage and the 4 th stage of the CW. The 1 st stage, 2 nd stage and 3 rd stage, which were designed for N and organic matter removal, were packed firstly with CSW (with size ranging from 20 to 40 mm) to 5 cm at the bottom to act as supporting layer followed by 60 cm in depth of prepared CSW with particle size ranging from 6 to 12 mm. The 4 th stage, which was designed for P removal, was packed firstly with CSW (with size ranging from 20 to 40 mm) to 10 cm at the bottom to act as supporting layer followed by 55 cm in depth of prepared coal ash-based CSW with particle size ranging from 3 to 20mm. Typha angustifolia was planted in each stage of the wetland system. Figure 1 shows schematic description of the system. The characteristics and configuration of the CSW based multistage vertical subsurface flow (VSF) CWs is summarized in Table 2. A tidal flow strategy was employed in each stage to improve oxygen supply. During the tidal flow operation, each matrix of the CW was alternately filled with effluent from the previous stage and then drained. When the bed is drained, the retreating wastewater acts as a 'passive pump' to draw air from the atmosphere into the matrix. This creates an oxygen-transferring period in each stage of the wetland system. When the bed is filled, the oxygen consumption occurs in each stage of the system.
Such intermittently filling and draining of the bed matrices was generated by peristaltic pumps that were connected to the outlet of previous bed and inlet of next bed. The pumps were controlled by pre-set digital electronic timers. In each stage, the 'tide' took place once every 6 h and gave 4 cycles per day. Each cycle allows 1.76 L of raw water to pass through, giving a hydraulic loading rate (HLR) of

Analysis
The coal ash based CSW obtained from the waste lightblock, which was further oven dried at 103 ± 2 • C, was used for examining its physical and chemical properties. Three parallel samples were digested using the standard method of nitric acid digestion (3030 E, APHA, 1992). Thereafter, the chemical composition of the CSW was determined using Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES) (IRIS Advantage, Thermo Elemental, Franklin, Massachusetts, USA). The surface areas of the CSW samples were conducted using an Accelerated Surface Area and Porosimetry System (ASAP2020, Micromeritics Instrument Corporation, Norcross, Georgia, USA). During the testing period, samples of influent and effluent from each bed of the multi-stage VSF CWs were collected and then filtered using 0.45 µm filter membrane for soluble chemical oxygen demand (SCOD), N and P measurement. The filtered samples were analysed for P concentration using ICP-MS (X series, Thermo Elemental, Franklin, Massachusetts, USA) (for P concentration less than 0.05 mg-P/L) and the stannous chloride method (for P concentration higher than 0.05 mg-P/L) (4500-P D). [12] SCOD and Ammoniacal-nitrogen were analysed using standard methods of 5210 B and 4500-NH 3 C, respectively. [12] Results and discussion

Physical and chemical properties of the coal ash-based CSW
The principal physical and chemical properties of the coal ash-based CSW are shown in Table 3. Specific elemental metal component of aluminium, iron and calcium content (expressed as oxidized metal) in the CSW was 174.7, 96.7 and 127.3 mg/g-CSW, respectively. The other principal chemical components were silicon oxide and orthophosphate (expressed as phosphorus) with mass content of 46.9 and 0.1 mg/g-CSW, respectively. As shown in Table 3, aluminium, iron and calcium are the dominant component that relate to P-metal adsorption and/or precipitation. This can potentially contribute to adsorption and/or chemical precipitation of P onto the CSW. [13] In the adsorption process, the adsorbent must have sufficient surface area that allows the adsorbate to be adsorbed on it. The required surface for phosphate ion is determined to be approximately 23Å 2 . [14] Olsen [15] used radio phosphorus as radioactive tracer to characterize P adsorption on hydroxylapatite and showed that the effective area per phosphate molecule is 21.6Å 2 . Assuming phosphate was adsorbed on CSW surface in monolayers, removal of 1 milligram of phosphate (expressed as phosphorus) from wastewater onto CSW may occupy approximately 4.21 m 2 of surface area. The specific surface area of the coal ash-based CSW, calculated from Brunauer-Emmett-Teller (BET) model, was 48.6 m 2 /g-CSW. This indicates that coal ash-based CSW has sufficient surface area, which allows the adsorbate to be adsorbed on it. Figure 2 presents the P removal performance of the CSW based multi-stage VSF CWs treating the secondary sewage effluent. The phosphate concentration in the secondary ef-  fluent ranged from 0.15 to 3.06 mg-P/L. The phosphate in the effluent of the CWs in the operation period (up to 122 days) was in the range of 0.009 to 0.13 mg-P/L. Accordingly, the P removal efficiency was in the range from 89.9 to 99.7% while the average P removal efficiency was 96.7%. The very low P concentration and very high P removal efficiency achieved at the operation period indicate the excellent P removal ability of the CSW. Figure 3 shows the plot of cumulative P removal against cumulative P loaded in the CWs. It shows a straight line with slope of 0.984. This line indicates that nearly all the loaded P has been adsorbed on to the CSW. Figure 4 shows the ammoniacal-nitrogen removal performance of the CSW-based CWs. The ammoniacal-nitrogen concentration in the secondary effluent ranged from 0.5 to 14.7 mg-N/L with an average of 9.94 mg-N/L. The ammoniacal-nitrogen in the effluent of the CWs at the startup period (0 to 46 days) was decreased from 10.7 to 1.1 mg-N/L while the ammoniacal-nitrogen removal efficiency was increased from 0 to 87.5% in the same period.  This high ammoniacal-nitrogen removal efficiency could be attributed to the enhanced oxygen supply in the multistage VSF CWs. Typically, oxygen can be transferred into the CW system through air diffusion during the filling and/or draining process of the tidal flow operation strategy and the roots of the plant. However, the root of Typha angustifolia was not well developed in this study. Therefore, the amount of oxygen supply through the roots of plant is insignificant. The tidal flow operation strategy may supply sufficient amount of dissolved oxygen and lead to the enhanced nitrification in the treatment system. The theoretical oxygen supply rate of the CWs using tidal flow operation strategy is 137.2 g/m 2 ·d. [8] Figure 5 shows the nitrate removal performance of the CWs. The nitrate concentration in the secondary effluent ranged from 8.1 to 22.9 mg-N/L. While in the effluent of  the CWs, the nitrate at the startup period (0 to 46 days) was decreased from 6.7 to 2.7 mg-N/L. As shown in Figure 5, at the operation period (47 to 122 days), the nitrate in the effluent ranged from 0.1 to 2.4 mg-N/L. This gives 96.0 ± 4.2% of nitrate removal from the secondary sewage effluent through denitrification. Figure 6 shows the nitrite removal performance of the system. The nitrite concentration in the secondary sewage effluent ranged from 0.05 to 1.1 mg-N/L while in the effluent of the CWs it ranged from 0.09 to 2.1 mg-N/L at the start-up period (0 to 46 days). The nitrite in the effluent of the CWs ranged from 0.01 to 0.8 mg-N/L while that of the secondary sewage effluent ranged from 0.2 to 6.4 mg-N/L at the operation period (47 to 122 days).

Characteristics of nitrogen removal
As shown in Figures 5 and 6, the CSW-based CWs also presents higher nitrate and nitrite removal efficiency. Regarding to the pathway of ammoniacal-nitrogenn removal, it can be a collective result of plant uptake and nitrification. The resultant nitrate and nitrite are further removed through denitrification. [16] However, the amount of Typha  angustifolia growth in this study is insignificant (approximately 0.2g/m 2 during the operation period). This indicates that plant uptake is not the major pathway for ammoniacal-nitrogen removal. However the microorganisms involved in the nitrification and denitrification process in the CSW-based CWs still need to be studied to reveal the ammoniacal-nitrogen, nitrate and nitrite removal mechanisms in such system. Figure 7 presents the SCOD removal performance of the CWs. The SCOD concentration in the secondary effluent ranged from 19.8 to 53.5 mg/L with an average of 32.83 mg/L. The SCOD in the effluent at the operation period (up to 122 days) ranged from 20.6 to 57.5 mg/L with an average of 35.9 mg/L. This indicates that there was no obvious change in the SCOD in the inflow and outflow of CSW-based constructed wetland. Considering the characteristics and mechanisms of nitrate and nitrite removal through heterotrophic denitrification, the insignificant amount of SCOD removal in the CSW-based CWs indicates that the carbon source, which is necessary for heterotrophic denitrification [17,18] , was not involved in denitrification process. This limited change of SCOD implies that there are some other mechanisms contributed to the N removal in the system. Figure 8 presents the characteristics of pH changes in the effluent of different stages in the CWs. During the operation period, the pH value of the influent ranged from to 6.91 to 7.68 with an average of 7.24. The average effluent pH of different stages is respectively 7.22 (1 st stage), 8.02 (2 nd stage), 7.72 (3 rd stage) and 11.08 (4 th stage). During the adsorption process, the pH value of the effluent is controlled by hydrolysis of the CSW and the release of the hydroxide ion from the CSW into the aqueous phase due to the adsorption of P onto the CSW. The pH increase due to hydroxide ion release can be observed in 2 nd stage and in particular in 4 th stage, where dominating P removal occurs. This provides evidence that P adsorption onto the coal ash based CSW is probably through ligand exchange. [13]

Conclusions
The characteristics of P, N and SCOD removal from the secondary sewage effluent using a novel CSW-based multistage vertical subsurface flow CWs were investigated to present the applicability of CSW reuse as a media in CW. The tidal flow operation strategy was employed to demonstrate N and organic matter removal performance of the CSW-based CW. The results have shown that significant amount of P removal can be achieved using such the system. The CSW presents excellent P removal performance. The ammoniacalnitrogen removal performance was improved due to the oxygen supply capacity and nitrification performance can potentially be enhanced by using the tidal flow operation strategy in the system.
The application of CSW as a media in CWs treating secondary sewage effluent not only improves water quality, but also provide a good showcase for beneficially reuse of the CSW. Such the application can transfer the CSW from 'waste' to 'useful' material and can ease the pressure of construction waste solid management. More significantly, such the application can transfer the CSW from a 'waste' to 'useful' material and can ease the pressure of construction waste solid management. The treated effluent after the CWs could be a water source for non-potable use in landscape irrigation, agriculture and industrial process.