LITERATURE REVIEW
Phosphorus (P) pollution and its consequences
Phosphorus is a crucial nutrient that extremely needed for the growth of plant and animals (Han et al, 2015) It is an abundant element in the earth’s crust and also a vital component of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), adenosine triphosphate (ATP), phospholipids, teeth and bones in animal bodies (Nguyen et al., 2012) In addition, phosphorus also plays a key role in industrial processes, it is a major material for several principal industries (e.g fertilizers, metallurgical industry, detergents, paints, and pharmaceuticals) (Nguyen et al.,
Nevertheless, the excessive loading of P in water bodies is a major cause lead to eutrophication, this process is a serious threat to water resources (Ruzhitskaya and Gogina, 2017)
Figure 1.1 Eutrophication caused by P contamination (Chislock et al., 2013)
The concentration of P in the aqueous medium reaches 0.02 mg/ L can cause to eutrophication (Ismail, 2012; Nguyen et al., 2012) This phenomenon is characterized by excessive plant and algal growth The large consumption of oxygen for the dead algae decomposition, resulted in the dissolved oxygen can be reduced dramatically in aquatic medium, and thus threatening the aquatic animal living as discussed by Nguyen et al (2012) Consequently, the reducing water quality, losing biodiversity, damning economic and recreational value and posing significant public health risks (Wilson et al 2006; Tillmanns et al 2008)
Therefore, P should be eliminated from wastewater before discharged into the environment.
Regulations related to P removal
The excessive amount of phosphorus in aqueous medium due to both of natural sources and human activities can cause in negative impacts on ecosystems
Therefore, several guidelines and standards that have been published for protecting and controlling phosphorus pollution in natural water bodies and wastewater effluents (Gibbons, 2009)
Table 1.1 Effluent discharge standards of different countries
Total phosphorus unless otherwise indicated (mg/L)
Colorado Regulation No31 and No85
Guidelines for Effluent Quality and Wastewater Treatment at Federal Establishments
Emission Limit Guidelines for Sewage Treatment Plants that Discharge Pollutants in Fresh and Marine Waters June 2001
2 (10,000 -100,00 PE) 1991 European Union Urban Waste
General Standards for Discharge of Environmental
QCVN 40:2011/BTNMT (Industrial Wastewater Discharge Standards) Source: Nguyen et al., 2013
In the world, to prevent eutrophication of reservoirs, many countries have regulated the level of phosphorus in the surface water is less of 0.05 mg/ L to combat excessive algae growth (Nguyen et al., 2012) According to Ramasahayam,
(2014), to prevent surface water pollution from eutrophication, the maximum allowable concentration of P should be lower than 0.01mg/L For the same purpose, USEPA also recommended that the total level of phosphorus in the inflows to the lake and in the flow should be kept from 0.05 to 0.1 mg/ L, and EPA criterion for the maximum concentration of P discharge into aqueous medium is 0.1 mg/ L (Ramasahayam, 2014)
In order to minimize the pollution burden on surface water as well as to control phosphorus pollution at the source Some countries have developed and applied national regulation of effluent discharge standards as shown in the Table 1.1
It can be seen that the effluent discharge standards vary from one region to others in a country (USA) as well as one country to another This can be explained by the variation in the level of treatment technologies and background phosphorus concentrations in the water bodies in different regions and countries (Nguyen et al.,
2013) To prevent P pollution from the consequences of rapid economic development, China also has developed strictly for phosphorus regulations with the low-acceptable P concentrations (0.5-1 mg/ L) (Wang et al., 2013)
In Vietnam, P regulation is applied in some types of water such as Industrial wastewater Discharge Standards (4-6 mg/ L), the effluent of aquatic Products Processing industry (10-20 mg/ L), Health Care wastewater (6-10 mg/ L), domestic wastewater (6-10 mg/ L) Nevertheless, most of the effluent discharge standards are higher than developed countries and much higher than EPA criterion
Although, each country has the effluent discharge standards is different, however, the most stringent regulations have suggested that the total concentration of P in the effluent only should be kept from 0.5 to 1 mg/ L before being discharged into the water environment (Xu et al., 2011a)
Therefore, in order to meet these stringent limits, the search for technologies treatment of phosphorus is required to protect water bodies from eutrophication (Nguyen et al., 2013).
Phosphorus treatment technologies
There are many technologies for phosphorus removal as shown in Figure 1.2 (Nguyen, 2015) Each method has distinctive characteristics and presents its own merits and demerits
The physical technologies are membrane related processes They include microfiltration, reverse osmosis, and electrodialysis The mechanism of microfiltration related to size exclusion, hence concentration and pressure are not effect to its removal efficiency In contrast, initial concentration, water flux rate and pressure are affect to reverse osmosis due to its primary mechanism is diffusion In the electrodialysis method, ions are moved by an electric field on membrane, they tend to go to through the membrane and concentrate at one compartment, while decontaminated water remains in the other (Karachalios, 2012)
Membrane technology is playing a key role in improving the quality of wastewater treatment, in particular for P elimination
Besides the phosphorus removal in the TSS, dissolved phosphorus also can be removed by membranes According to Reardon (2006), the P concentration of several plants have been reported that they have been achieved less of 0.1 mg/ L in the effluent
In 1970s, magnetic separation method was beginning investigated for phosphorus treatment This is considered as an attractive method because at the same cost with other methods, the phosphorus level in effluent can be reached to 0.1-0.5 mg/ L by magnetic separation
Magnetic separation may be applied as a reliable add-on technology for chemical removal Phosphates in solution are combined with reagent into insoluble compounds And after that, magnetic material is used to isolates phosphate- containing sediment The significant benefits of this process are simple process, and low energy consumption However, it has low elimination efficiency ( 90 % Real wastewater Gnirss et al., 2003
5 94.1 % Domestic wastewater Smith et al., 2014 Magnetic separation
0.5 98 % Synthetic wastewater Zhao et al., 2012
0.2 > 90 % Synthetic wastewater Merino et al., 2011
Ion exchange Tertiary treatment only
500 50 mg/ g Synthetic wastewater Martin et al., 2009
50 51.52 mg/ g Synthetic wastewater Ren et al., 2014
90 % Synthetic wastewater Seo et al., 2013
Primary, secondary or tertiary treatment or activated sludge recycle
5 92 % Synthetic wastewater Ramasahayam et al.,
50 70 % Piggery wastewater My et al., 2017
Crystallization Tertiary treatment or recycle stream
25 91.30 % Synthetic wastewater Xuechu et al., 2009 30-120 96.10 % Synthetic wastewater Menglin et al., 2016
5 91 % - (120 mg/ g) Synthetic wastewater Renman and Renman,
Adsorption Secondary or tertiary treatment
100 > 90 % - (4.75 mg/ g) Synthetic wastewater Nguyen et al., 2013
30 98.20 % Synthetic wastewater Vohla et al., 2010
5 - 25 95% (3.11 mg/ g) Synthetic wastewater Vohla et al., 2010 EBPR
Secondary or tertiary treatment or activated sludge recycle
16.67 99 % Synthetic wastewater Ong et al., 2016
Secondary or tertiary treatment or activated sludge recycle
10 25 % dairy wastewater Hill et al., 2000
5 96 % leachate wastewater Vohla et al., 2005
0.5 - 2 40-75 % landfill leachate Koiv et al., 2009a
Constructed wetlands (CWs) system for wastewater decontamination
Constructed wetlands (CWs) are engineered systems that have been designed and constructed to strengthen the natural processes for wastewater treatment (Vymazal, 2011) These systems comprised of major components such vegetation, substrates, soils, microorganisms and water The various contaminants from wastewater are removed by natural microbial, biological, physical and chemical process in CWs (Vymazal, 2011; Saeed and Sun, 2012)
There are various types of CWs that can be distinguished based on the dominant vegetation type, hydrology and flow direction (Zheng et al., 2014)
Nevertheless, according to the majority of authors, most of CWs are typically classified into the two types: free water surface flow and subsurface flow (Almuktar et al, 2018)
Figure 1.4 The classification of CWs used in wastewater treatments (Wu et al., 2015)
Figure 1.5 The schematic surface flow constructed wetland (Almuktar et al., 2018)
Free water surface flow (FWS) systems are designed similar to natural wetlands, they include an aquatic area with a variety of plants, a sealed basin filled with 20-30 cm of substrates and about 40 cm for the depth of water (Stefanakis et al., 2014) These systems are also considered as expected habitats for many wildlife species
In free surface flow systems, organic compounds in wastewater are effectively removed through primarily the process of sedimentation, filtration and decomposition of microorganisms Nitrogen is effectively treated by denitrification and ammonia volatilization However, phosphorus is unable to effectively removed because the water does not tend to come in contact with soils particles (which adsorb and precipitate with P) as discussed by Taylor et al., 2006
Thus, if phosphorus is the key contaminant of concern, the FWS systems are less suitable for treatment Additionally, the high possibility of human exposure to pathogens and the large area requirement are also disadvantages of these systems
In subsurface flow (SSF) constructed wetlands, water come directly to media and is generally invisible (Vymazal, 2007)
According to the flow direction, SSF might be classified into two types: vertical flow (VF) and horizontal flow (HF) (Almuktar et al., 2018) In HF constructed wetlands, the substrates are flooded by water, while VF constructed wetlands are applied intermittently to gain the high rate of oxygen transfer (Stefanakis et al.,
Figure 1.6 The schematic vertical flow constructed wetland (Almuktar et al., 2018)
Figure 1.7 The schematic horizontal flow constructed wetland (Almuktar et al.,
2018) For wastewater treatment, if phosphorus is the primary contaminant of concern, subsurface flow constructed wetlands can be a greater treatment tool than surface flow constructed wetlands Because they can be controlled by selecting highly P adsorbable substrates as discussed by Pant, 2001 and White 2011
In addition, the role of root-bed media for phosphorus sorption also is very important They facilitate better to remove P from aqueous media for longer time (Pant et al., 2001; Seo et al., 2005) White et al (2011) reported that the P removal of root-bed substrates could be reached around 74 % by using substrates such limestone, oyster shells, crushed brick The viability of the substrate for P removal depends on its maximum adsorption capacity The saturated substrates must be removed and processed, then the new substrates need to be added periodically to maintain P adsorption capacity This is a drawback of SSF constructed wetlands
Hence, the monitoring and evaluating the life of substrates are extremely important and necessary to maintain the P-sorption capacity and to minimize the P export from the saturated (White et al., 2011)
Hybrid systems were developed to overcome the limitations of single stage CWs, because many wastewaters could be difficult to treat in individual systems (Vymazal 2005, 2007).
Figure 1.8 The schematic hybrid constructed wetland (Almuktar et al., 2018) Due to the ability of individual systems could not to provide both aerobic and anaerobic conditions simultaneously, so that its efficiency in nitrogen removal is not high (Vymazal 2011) Thus, hybrid systems have the combination of HF and VF can be obtained higher nitrogen removal efficiency (Zhang et al., 2014)
In recent years, the hybrid systems are used widely for many type of wastewater According to Serrano et al., 2011 this combined system was used effectively for winery wastewater, and wastewater in oil field (Alley et al., 2013), industrial wastewater and grey water (Vymazal 2014; Comino et al., 2013), swine wastewater (Kato et al., 2013; Zhang et al., 2016), pharmaceuticals (Reyes et al.,
In generally, constructed wetland is a potential technology, and the attractive alternatives to traditional wastewater treatment methods The improvements to enhance treatment efficiency for many wastewater types in CW are still being concerned and developed (Zhang et al., 2014)
1.4.3 Application of CWs in wastewater treatment
CWs have been used for quite a long time The first experiment was implemented by Kọthe Seidel at Germany in 1950s with the attempt aimed using CWs to treat wastewater And lots of experiments were conducted and successfully applied in later At the initial stage, most the application of CWs were utilized to treat conventional municipal and domestic wastewater (Wu et al., 2015)
To date , they have been used to various wastewater such as mine drainage (Smith, 1997), landfill leachates (Bulc et al., 1997), seafood processing wastewater (Xeybouangeun, 2011), lake waters (Cui et al., 2011), domestics wastewater (Andreo-Martínez, 2017), compost leachate (Bakhshoodeh et al., 2017), grey water (Ramprasad et al., 2017), dairy wastewater (Adhikari et al., 2015), eutrophic water (Hernández-Crespo et al., 2016), rainfall runoff from a motorway (Shutes at al.,
1999) and agricultural runoff (Wang et al., 2018), and industrial and sewage effluents (Stefanakis, 2018) Additionally, they have been widely developed in various climate conditions (e.g cold, tropical, warm etc) (Vymazal, 2015)
To improve the treatment performance of CWs, there are lots of studies investigated the factors that effect to its treatment efficiencies Many studies examined the effect of hydraulic loading rate (HLR) and hydraulic retention time (HRT) Ramprasad et al (2017) found that elimination of phosphorus efficiency was more at higher HRT In regard of biomass harvest and management, many international studies showed the positive impacts of multiple biomass harvest on the phosphorus removal Specifically, Březinová and Vymazal (2015) observed that the amount of phosphorus removed through multiple biomass harvesting may increase up to 43 % compared to a single harvest Concerning the seasonal variation, Ramprasad et al (2017) released that phosphorus removal rate in the summer was higher than that in other seasons Relating to evapotranspiration, Bakhshoodeh et al
(2017) reported that when evapotranspiration started to rise, phosphorus removal efficiency decreased Since the filter media is the main component of the CWs, it was of great interests of many international researchers Barca et al (2014) found that, using steel slag as filter material can removed over 88 % total phosphorus in influent Andreo-Martínez (2017) obtained extremely high removal efficiency of phosphorus (96.9±1.7 %) by applying blast furnace slags as filter media in the HF- CWs for reclaiming domestic wastewater
Removing P by substrates in CWs
1.5.1 The role and mechanism of P removal by substrates
Substrate plays an important role for phosphorus removal The contaminants can be removed from wastewater through exchange, adsorption, precipitation and complexation process (Wu et al., 2015)
Depend on the physicochemical properties of filter materials that the mechanism will be dominant The phosphorus removal capacities are depended on the contents of calcium, iron, aluminum and their oxides and hydroxides (Yang et al., 2018) such as phosphorus was removed effectively by the use of steel slag (44.3 % of CaO , 16.9 % of Al2O3 and 24.2 % of Fe2O3) (Park et al., 2016), limestone and scallop shells (mainly CaCO3) (Nguyen et al., 2014), BOF-slag (47.08 of CaO, 36.12 % of Fe2O3 8.04 % of SiO2 , 4.94 % of MgO) (Han et al., 2015)
The selection of substrate with high efficiency, abundant availability, durability, appropriate cost, less side effects has been a crucial issue in constructed wetlands
The P adsorption capacity of filter materials depends on its physicochemical properties The chemical composition of materials plays a crucial role for P removal because the phosphorus is mainly removed via adsorption and precipitation process, it means that the Ca, Al, Fe contents is important for the effective P removal However, the chemical composition may also be to side effects when they can affect to the electrical conductivity, pH, and heavy metal release Hence, the filter media should be selected very careful (Vohla et al., 2011)
Besides, the hydraulic conductivity is also considered an important issue for selection of substrate To avoid clogging, the material must have durability, porosity and reasonable size (Vohla et al., 2011)
In addition, CWs are generally characterized as cost-effective technology due to the minimum cost of construction, operation and maintenance To minimize the cost of CWs system, substrate should use locally abundant, available material and no or low cost
In recently, toward to the using of reuse waste as substrate for P removal in CWs is also gaining the special interest Using of reuse waste in CW, not only improving contaminants treatment and reducing the large volume of solid waste in a environmental friendly way but also providing additional economic value to the waste (Yang et al., 2018)
The filter materials used in CWs for P removal can be classified based on their use history, origin or pollutant removal mechanism
For origin of materials, they could be classified into three groups including natural materials, industrial by-products and manufactured products (Cucarelle and Renman, 2009)
Table 1.4 Some filter media used for P removal
Apatite Batch 4.5mg/g - (500mg/L) Molle et al., 2005 Dolomite Batch 7.34mg/g - (60mg/L) Karaca et al., 2004 Gravel Full-scale CW 25.8mg/g - (100mg/L) Tanner et al., 1999 Limestone Batch 0.3mg/g - (25mg/L) Johansson, 1999a
Opoka Batch 0.1mg/g - (25mg/L) Johansson, 1999a
Oyster shell (burnt) Batch, column 0.83mg/g - (320mg/0L) Seo et al., 2005
Sand Full-scale CW 0.165mg/g - (10mg/L) Vohla et al., 2007
Shellsand Batch 4mg/g - (1000mg/L) Roseth, 2000
Coal ash Batch 29.5mg/g - (1500mg/L) Yan et al., 2007 Fly ash (Red mud) Batch 113.9mg/g - (155mg/L) Li et al., 2006
Ochre Batch 2.6mg/g - (1500mg/L) Heal et al., 2003
Sediment of oil shale ash Batch 8.2mg/g - (333mg/L) Vohla et al., 2005 BOF slag column 9.15mg/g - (320mg/L) Kim et al., 2006 Electric arc furnace
(melter) slag Full-scale CW 2.2mg/g - (400mg/L) Shilton et al., 2005
Filtra P Column 3.8mg/g Gustafsson et al.,
2008 Leca (Estonian) Full-scale CW 10mg/g - (333mg/L) ệửvel et al., 2007 Norlite Full scale P removal: 34% Hill et al., 2000
In general, there are various materials that have been studied and used to enhance P removal In addition to natural materials, nowadays the trend of using artificial materials and industrial by-products has been gaining more attention, this waste is also treated by an environmentally friendly way.
MATERIALS AND RESEARCH METHODOLOGY
Materials and equipment
Seven kinds of filter materials have been investigated in this study, including Limestone (LS), Laterite (LA), Coral (CR), Coal slag (CS), Steel slag (SS), White hard clam (WHC) and Okara (OK) These materials were obtained from different locations inVietnam, such as Chuong My, Ha Noi (LS, LA), Buoi market, Cau Giay, Hanoi (CR), Hai Duong thermo power plant (CS), Thai Nguyen steel corporation (SS), Tien Hai, Thai Binh (WHC) and tofu production household (OK)
First, all materials were washed with tap water to remove contaminants on the surface, which may affect on adsorption experiments Next, the washed materials were dried in the oven (Thermo Fisher Scientific REL404A20,) at 800 0 C for 3 h to a constant weight After that, the washed, dried materials were sieved to get the desired particle size of 1.4-2.0 mm
White hard clam Coal slag Steel slag Okara
Figure 2.1 Images of investigated filter materials
WHC: WHC was collected from Thai Binh shellfish Co., Ltd, Nam Thinh commune, Tien Hai district, Thai Binh province (20°23'34.1"N 106°34'28.3"E) It is one of the largest frozen clam processing and exporting factories in the north of Vietnam, with a capacity of over 20 ton/day In recently, the company has exported successfully clams to the European, Japan, Taiwan, South Korea markest It has greatly contributed to the Vietnam's seafood industry After being collected, WHC was washed thoroughly with tap water, and dried in the open air till dry After that it was transported to Chuong My, Hanoi for grinding and sieving to get diferent particle sizes
Figure 2.2 The routine to Thai Binh shellfish Co., Ltd, Tien Hai Thai Binh
Figure 2.3 Procedure to prepare WHC as phosphorous adsorbent
Synthetic and real swine wastewaters:
The real swine wastewater was collected from a pig farm in Chuong My, Hanoi, Vietnam (Coordinates: 20°53'49.6"N 105°42'57.9"E) This farm has a total area of around 1,700 m 2 with 1,100 pig heads It is estimated that approximately 55 m 3 /d was generated and discharged into the surrounding environment after biogas treatment The wastewater was transported to the laboratory right after collection, filtered through 0.45 àm mesh filter paper, and kept in the refrigerator at 4 0 C The pH, turbidity, and DO were measured on site, whereas other parameters, such as COD, BOD, TN, N-NH4, TP, P-PO4 3- were analyzed at the laboratory of Master’s Program in Environmental Engineering (MEE), Vietnam Japan University (VJU)
The real swine wastewater was used for the experiment to assess the WHC-M800 in the actual conditions
Figure 2.4 The pig farm in Chuong My, Hanoi The synthetic swine wastewater was prepared to simulate real swine wastewater For batch and column adsorption tests, synthetic swine wastewater contained only phosphorus In contrast, the synthetic used for constructed wetland comprised both phosphorous and nitrogen The stock phosphorus solution (1000 mg P/ L) was prepared by dissolving KH2PO4 into distilled water Depending on the purposes of different experiments, the working solutions were prepared from stock solution with the appropriate dilution factors For constructed wetland experiment, the synthetic swine wastewater was prepared by dissolving KH2PO4 and NH4Cl in tap water to get the concentration of 12.5 mg P/ L and 125 mg N-NH4/ L
Plant: In this work, Upon paspalum was selected to plant in CWs at the age of 1.5 months, with a height of 25 cm The seedings of the plant was purchased from Buoi market, Caugiay, Hanoi, and grown in a garden in Tien Hai, Thai Binh After planting 1.5 months, the plants were harvested, washed with tap water to remove soil, kept in a bucket of water for 2 days and then a nutrient solution (12.5 mg P/L and 125 mg N/L) for adaptation before being transferred into CWs
All reagents, which were utilized in this study, were purchased from ESQ Co., Ltd (Ba Dinh, Hanoi) These chemicals all have AR grade The solutions with different phosphorus concentrations were obtained by dissolving specific amounts of KH2PO4 into distilled water The distilled water was produced in the MEE laboratory with double distilled water machine (A4000D, Bibby, England)
SEM/EDX NOVA 3000, N32-11 FT/IR 6300typeA
UV/Vis Diode Array Spectrophotome (S2100, Unico) pH meter (S220-Kit, Mettler Toledo)
Figure 2.5 Equipments used in this study
Experiment setting up
Modification of seven filter materials were implemented in order to enhance phosphorus adsorption capacity of investigated materials Depending on the nature of materials, different modification methods were applied In this research, filter materials were classified into three groups, including (1) Ca-rich natural materials,
(2) Fe, Al oxide rich industrial by-product materials, and (3) Cellulose-based agricultural by-product materials As can be seen from Figure 2.6, Ca-rich materials were modified by thermal treatment whereas Fe, Al oxide rich materials and cellulose-based materials were subjected to chemical treatments (with acid or alkaline, metal loading)
Figure 2.6 Modificatin of investigated filter materials Modification of selected filter material (WHC): In order to identify optimum temperature for modification, thermal treatment was carried out with 3 kg WHC at different temperatures (750, 800, and 850 0 C) for the same period of time (3 h)
Then modification tests were conducted with 3 kg WHC at the same temperature
(800 0 C) for various periods of time (1, 2, 3 h) to select the most efficient contact time The optimum modification condition was determined based on (1) phosphorus adsorption capacity of modified WHC, (2) weight loss of WHC after calcination and during adsorption tests The weight loss was calculated using the equation (2.1) as follows:
Loss weight = (m 1 −m 2 ) m 1 100% (2.1) where: m1, the weight of WHC before calcination or adsorption (g) m2, the weight of WHC after calcination or adsorption (g)
2.2.2 Characterization of materials a Physical properties
Permeability of filter materials: This experiment aimed at evaluating the ability to conduct water of filter materials, which is known as permeability constant (K) The experiment setting up was demonstrated in Figure 2.3 30 cm of each filter material was packed into a column with the diameter of 10 cm (Principle of Darcy law) K value was estimated based on Darcy law using the following equation (NPTEL, 2003):
L (2.2) where Q, the rate of flow (m 3 /s)
K, permeability of the material (m/s) h, difference in total heads (m)
L, length of the soil mass (m)
A, the cross-sectional area of the soil mass (m 2 )
Figure 2.7 The experiment setting according to Darcy law (NPTEL, 2003)
Porosity of filter materials: The purpose of this test is to determine the ability of holding water of materials, which is known as pore space (porosity) The experiment setting up was demonstrated in Figure 2.8 The porosity was calculated by equation (2.3) as follow:
1 Measure the volume of your sample
3 Saturate the sample with water
4 Record the volume of water used
Figure 2.8 Procedure for determine of porosity (NPTEL, 2003)
Brunauer Emmett Teller (BET) surface area and pore size determination were implemented with Quantachrome NOVA 3000 series (Model N32-11) High Speed Gas Sorption Analyzer
Scanning electronic microscopy (SEM) analysis was carried out with Avomeen’s SEMTech Model 1830 SEM/EDS at the laboratory of Institue of Chemistry, Vietnam Academy of Science and Technology (VAST) to identify the variation in morphology of WHC as the result of thermal modification b Chemical properties
Fourier transform infrared spectroscopy (FTIR) analysis was performed with FT/IR 6300typeA at the laboratory of Faculty of Chemistry, VNU University of Science (HUS), in order to identify the key functional groups reponsbile for adsorption reactions The spectral resolution of the FTIR is 1 cm -1 determined between 4000 and 400 cm -1
Energy dispersive spectroscopy (EDX) measurement was utilized on Avomeen’s SEMTech Model 1830 SEM/EDX at the laboratory of Institue of Chemistry, Vietnam Academy of Science and Technology (VAST) to deterimine change n chemical elemental composition of WHC before and after thermal treatment c Side effects pH: First, adsorption test was conducted at seven filter materials at the following conditions: initial phosphorous concentration of 200 mg P/L; adsorbent dose of 5 g/ 125 mL; shaking speed of 120 rpm, shaking time of 24 h, temperature of 27 0 C Then, the post-adsorption solution was measured to determine pH value with pH meter (Mettler Toledo Seven Compact S220K)
Released heavy metals: The adsorption experiments were performed as the same as those used for evaluating side effect of pH mentioned above The post- adsorption solution was analyzed to determine the contents of heavy metals with AAS Agilent 280FS machine at the laboratory of Center Environmental Monitoring and Modeling (CEMM), VNU University of Science (HUS)
2.2.3 Adsorption experiments a Adsorption batch experiments
A stock phosphorus solution (1000 mg P/ L) was prepared by dissolving
KH2PO4 in distilled water The desired working solutions were made by dilution of stock solution All bacth experiments were conducted in erlenmeyer flasks on the orbital shaker (OS-3000, HACH) at the same speed (120 rpm) and temperature (27
0C) All experiments were triplicated and the mean value was determined
In order to investigate influential factors on adsorption of phosphorous on to WHC and WHC-M800, adsorption tests were implemented by keeping varying the influential factors while keeping all other conditions the same as mentioned above
The pH value of initial phosphorus concentration was varied in the range of 3-11 using H2SO4 and NaOH of various concentrations The temperature was changed from 27 to 40 0 C Adsorbent dose was varied from 0.5 to 5.0 g/ 75 mL
Adsorption isotherm tests Adsorption isotherms features were studied by varying the initial phosphorus concentration (2.5, 5, 10, 25, 50, 100, 200, 400, 500, 800, 1000 mg P/ L) After adsorption, the suspension was filtered through 0.45 àm filter paper and fitrate was used for phosphorus determination using UV-VIS Spectrophotometer (Unico, S2150UV) The adsorption amount was calculated by the difference between initial and residual concentrations The adsorption capacity at equilibrium was estimated as follows: q e = (C 0 −C 𝑒 ) m V (2.4)
Where C0 (mg/L), Ce (mg/L), stand for the initial and equilibrium P concentrations, respectively, V (L) and m (g) represent the volume of the solution and the adsorbent weight While adsorption tests with WHC using adsorbent dose of
3 g/ 75 mL, the dosage of WHC-M800 was 0.5 g/ 75 mL
Analytical methods
The concentration of total phosphorus and ortho phosphorus in wastewater was determined according to the standard operating procedures (365.3-EPA method) The calibration curve was built based on the known phosphorus concentrations and their absorbances By using calibration curve and pre- determined absorbances, the unknown phosphorus concentrations were calculated
Figure 2.11 Calibration curve for phosphorus analysis
Table 2.1 Analysis methods for BOD, COD, TN, NH4
RESULTS AND DISCUSSION
Screening of filter materials for use as substrate in CWs
The substrate plays a crucial role for eliminating of phosphorus Phosphorus can be removed mainly through exchange, adsorption, and precipitation processes (Wu et al., 2015) Hence, selecting of filter materials to apply in CW is an important issue
According to Biswas (2008) and Ning et al (2008), the potential materials for P removal generally must have some characteristics such as high adsorption capacity, abundant availability, low-cost, and potential regeneration Vohla et al (2011) also reported that potential filter materials in CW should have good hydraulic conductivity, reasonable size, less side effects and high durability
In this study, to select the best potential material for use as substrate in CW, five selection criteria were applied, which include high P adsorption capacity, high hydraulic conductivity, less side effects, low-cost and abundant availability
Based on these five selection criteria, seven filter materials, which were divided into such three groups as natural materials (LA, LS, CR), industrial by-products (SS, CS WHC), agricultural by -products (OK), were investigated (see Figure 3.1 in detail)
3.1.1 Comparing potential materials based on P adsorption capacities
A comparative study was conducted with both raw and modified materials at the same condition using batch experiments Each experiment was triplicated and the average value was determined The results on adsorption capacity of investigated materials were displayed in Figure 3.1
Figure 3.1 Comparison of P adsorption capacity of investigated filter materials (Ci
= 200 mgP/ L, adsorbent dosage: 5 g/ 125 mL, 24 h, 120 rpm, 27 0 C)
It can be seen from the Figure 3.1, most of modified materials had P adsorption capacity higher than that of raw materials It demonstrated that the modification methods were effective to a different extent for various raw materials
In previous-studies, also it was indicated that modification was good solution to enhance P adsorption capacity of materials Méndez (2011) used hydrochloric acid to modify laterite, which led to a 10-time increase in the P adsorption capacity of laterite Pengthamkeerati et al, (2008) also used hydrochloric acid and sodium hydroxide to treat fly ash Yuangsawad and Na (2011) applied high temperature to modify oyster shell
In this study, depending on nature of materials, appropriate modification method was applied For iron, aluminum oxides-rich (LA, CS, SS) and cellulose (OK) materials were used chemical treatment method to modify While Ca-rich materials (CR, LS, WHC) were used thermal treatment method to modify
Modification methods were effective, P adsorption capacity was increased from raw materials with the range of 0.34 -4.08 mg/ g to modified materials with the range of 2.18-6.9 mg/ g The modification efficiency was dramatically increased for Ca-rich materials group (LS, CR, WHC) due to after thermal treatment, the less active
LA LS CR SS CS WHC OK-M
P a ds o rpt io n c a pa cit y ( m g /g )
Raw Modified Natural materials Industrial by-products Agricultural by- products calcium carbonate was converted into more active calcium oxide and calcium hydroxide, which easily combined with phosphate ion, resulting in a significant enhancement of P adsorption capacity
Table 3.1 showed the comparison in P adsorption capacity between WHC, WHC-M800 and other filter materials It can be seen that WHC and WHC-M800 possessed an adsorption capacity, which was 10 times higher than that of materials in previous studies This can be attributed to high CaCO3/CaO content in WHC and WHC-M800 This is supported by Vohla et al., (2011) who found that P adsorption was proportional to high Ca/CaO content of the material
Table 3.1 Phosphorus adsorption capacity of different materials
Broken bricks 0.594 Wang et al., 2013
Sand (I/II) 0.43/0.44 Zhu et al., 1997
Sand (I-IV) 0.13–0.29 Ádám et al., 2007
3.1.2 Comparing filter materials based on their permeability
Since filter materials are used in CWs, they must have good hydraulic conductivity to avoid clogging during operation process (Vohla et al., 2011)
In this study, Darcy law experiment was implemented to find the permeability constant (K), which stand for the water conductivity ability of a filter material This experiment was to evaluate the clogging potential of the studied materials when
Table 3.2 Permeability constant (K) of investigated materials
Materials Permeability constant (K) (cm/ s) Reference
It can be seen that the obtained permeability constants of investigated materials were quite high, in the range of 0.5 -3.1 (cm/ s)
In comparison with conventional materials (gravel, sand) that usually used in traditional CWs, almost investigated materials with the particle size range of 1-2 mm in this study had better good hydraulic conductivity Therefore, these materials with this size can be applied in CW without the risk of clogging
3.1.3 Comparing filter materials based on their side effects
The selection of filter materials should be carefully implemented to avoid undesirable, side effects (Vohla et al., 2011) Since filter materials in this study are rich in metal oxides, it can release metals into solution or affect pH value of the aquatic medium during adsorption process This may pose a threat to the life of plant and aquatic organisms Therefore, this study explored the potential of metal release and pH change as a result of using filter materials a Change in pH of the post-adsorption solution
Using filter materials as a substrate in CW may change the pH of solution into acidic condition (e.g bauxite, fly ash etc.), or alkaline condition (e.g steel slag, limestone, red mud) (Vohla et al., 2011) This may badly affect the growth of plant and aquatic animals Thus, it is necessary to evaluate the change in pH of the post- adsorption solution In order to do that, pH values were measure before and after adsorption The pH of phosphorus solution before adsorption was 4.87 The results on pH of post-adsorption solution were shown in the Figure 3.2
Figure 3.2 pH of post-adsorption solutions (Ci = 200 mg P/ L, adsorbent dosage 5 g/
Intensive investigation of the selected filter materials –white hard clam (WHC)
In order to properly evaluate the applicability of WHC and WHC-M800 as the substrate in CWs, this section intensively investigated WHC in aspects of characteristics, isotherms, and kinetics and column test Before that the optimal modification conditions of WHC were identified
3.2.1 Identification of the optimal modification conditions of WHC
In order to further enhance the P adsorption capacity of raw WHC, the modification using high temperatures was implemented According to Zang et al.,
2013, under high temperature, CaCO3 in raw WHC was turned into CaO, which was more active in reaction with PO4 3- anions, and thus lowering P concentration in the wastewater
In order to identify the optimal modification condition, three calcination temperature (750, 800, and 850 0 C) and three calcination time (1, 2, 3 h) were investigated Based on P adsorption capacity and the weight loss during calcination and adsorption, the optimal modification condition would be selected
Table 3.5 Effect of calcination temperature
Figure 3.3 Images of raw WHC and WHC modified at different temperatures
It can be observed that the P adsorption capacity of modified WHC increased with an elevation in the calcination temperature Specifically, the q value grew from
12 to 20 mg/ g when the calcination temperature increased from 750 to 850 0 C This can be explained that the conversion of CaCO3 into CaO was more complete at higher calcination temperature (Zang et al., 2013)
Although the high temperature boosted the adsorption capacity of WHC, it also caused the weight loss of the material in both calcination and adsorption processes This is undesirable for application of the material The Table 3.5 indicated that the higher the calcination temperature was, the greater the weight loss of WHC was observed Specifically, the weight loss during calcination was increased from 3.27 to 11% when calcination temperature increased from 750 to
850 0 C Similar trend occurred with the weight loss during adsorption, which increased by 8.6% in the same temperature range as above Taking into consideration of both q value and weight loss, 800 0 C was selected as the best modification temperature
After the best calcination temperature was identified, the WHC was modified at this temperature for different periods of time (1, 2, 3 h) to determine the optimal calcination time The results were shown in Table 3.6 It was found that the weight loss of WHC was quite similar at different calcination temperatures However, the q value was significantly improved from 9.9 to 18.7 mg/ g when the calcination time increased from 2 to 3h Therefore, 3 h was selected as the best calcination time In short, thermal treatment at 800 0 C for 3h was considered as the optimal modification condition
Table 3.6 Effect of the calcination time
In this study, BET analysis was carried out to determine the surface area, pore volume and pore size of WHC and WHC-M800 It was done with Quantachrome NOVA 3000 series (Model N32-11) High Speed Gas Sorption Analyzer at the Institute of Chemistry, Vietnam Academy of Science and Technology (VAST) The result was shown in the Table 3.7
Table 3.7 Brunauer Emmett Teller (BET) analysis
Pore volume (cm 3 / g) Pore size (nm)
The Table 3.7 showed that modification of WHC (800 0 C, 3h) led to a reduction in both surface area and pore size The surface area was decreased from 0.1294 to 0.0948 m 2 / g and the pore size was decline from 2.1041 to 1.7129 (nm)
The pore volume was changed marginally This can be explained by the fact that the calcium carbonate was decomposed due to the high temperature This caused the refining of the particle At 800 0 C, the lattice distortion among the grains of calcium oxide led to the reconstruction of lattice atoms and reduction of the space among molecules (Zhang and Chen, 2013) It was well recognized that the surface area as well as pore size of materials play a decisive role in physisorption In this study, in spite of the reduction in these parameters, WHC-M800 exhibited the improved adsorption capacity This proves that physisorption was not the main mechanism for the retention of P by WHC-M800 In other words, chemisorption might be dominant in this case
Figure 3.4 SEM observation for WHC Figure 3.5 SEM observation for
Table 3.8 Elemental content of WHC
Figure 3.6 EDX spectrum of WHC
Table 3.9 Elemental content of WHC-M800
Figure 3.7 EDX spectrum of WHC-M800
SEM observation was recorded and displayed in Figures 3.4 & 3.5 It was found that the variation in morphology of WHC after modification Specifically, more CaO crystals with smaller particle size could be found on the surface of WHC-M800 compared to raw WHC
EDX spectra, representing the peaks of elements existent in the WHC and WHC-M800 were given in Figure 3.6 and Figure 3.7 It is clear that for both WHC and WHC-M800 mainly composed of oxygen and calcium However, the percentages of both these elements in WHC-M800 were higher than those in raw
WHC 8.54 and 24 % for oxygen and calcium respectively The increase in the percentage of oxygen and calcium elements can be attributed to a decrease in proportion of carbon as the result of conversion of CaCO3 into CaO As observed from the Table 3.8 & 3.9 the atomic percentage of C was decreased from 38.18 to 17.63%
Figure 3.8 FTIR analysis for WHC
Figure 3.9 FTIR analysis for WHC WHC-M800 The functional groups of and WHC and WHC-M800 were determined using Fourier transform infrared (FTIR)-FT/IR 6300 typeA This study was carried out at the laboratory of Faculty of Chemistry, VNU University of Science (HUS) The spectrum was recorded in the range of (4000–400) cm - 1
The spectrum for WHC was shown in Figure 3.8 Accordingly, the peaks at
712, 859, 1082, 1476 cm -1 were dominant, indicating the presence of carbonate groups in the materials The Figure 3.9 represented the spectrum of WHC-M800 It was found that WHC-M800 had the same peaks of CO3 2− band at 712, 874, 1082,
1421 cm −1 However the intensity of these peaks were smaller than that of WHC
This may be due to the loss of CaCO3 in the form of CO2, as the result of thermal decomposition The same findings were reported by Kim et al (2018) and Khiri et al (2016)
The P treatment performance in the integrated CWs – adsorption system
The main purpose of this experiment was to compare P removal efficiency by
CW with the substrate was WHC (adsorptive material) and yellow sand (conventional material) In addition, the contribution of CW unit (plant, substrate) and adsorption unit was also evaluated
In this study, the WHC was used at the range size of (0.6-2 mm) as main layer
(23 cm) and the range size of (1.4 - 4 mm) as small bottom layer (4.5 cm) Due to the producing material process, the range size of (0.6-2 mm) was dominant whereas the amount of volume at range size (1.4 - 4 mm) was very small Moreover, the range size of (0.6-2 mm) could be ensured to minimize clogging
The obtained results showed in Figure 3.29 and Table 3.12 & 3.13
Figure 3.29 The change of phosphorus in the effluent over the time
Table 3.12 The phosphorus concentrations before and after treatment with horizontal flow lab-scale constructed wetlands
Wastewater sampling point Tank 1 Tank 2 Tank 3 Tank 4
10-May 15-May 20-May 25-May 30-May 4-Jun 9-Jun
Tank2 Tank1 Tank3 Tank4 Adsorption unit 1 Adsorption unit 2
Table 3.13 The phosphorus removal efficiency and pH after treatment with horizontal flow lab-scale constructed wetlands
Tank 1 Tank 2 Tank 3 Tank 4 Adsorption unit 1
The CW system was operated during 29 days The basic difference between the phosphorus removal efficiency of WHC and yellow sand was found
In the small-scale, using WHC as substrate had the average phosphorus removal efficiency 21 % higher than using yellow sand as substrate In the initial stage, the difference in eliminating phosphorus efficiency of both WHC and sand was insignificant, but the longer time, the difference is obvious Because the P adsorption capacity of WHC (11.6 mg/ g) was much higher than P adsorption capacity (0.13 mg/ g) of sand (Ádám et al., 2007) resulted in the saturation of sand was faster
After one month operation, the P concentration in effluent of CW based on WHC (0.24 mgP/ L) was far below Vietnamese effluent discharge standards (6mgP/
L), and still represented a stable trend whereas the P concentration in effluent of
CW based on yellow sand was increase rapidly, it reached over Vietnamese effluent discharge standards
WHC was proven as a potential material for setting of horizontal flow CW to remove phosphorus because it had high P removal efficiency and less opportunity of clogging with the range size quite small (0.6-2 mm)
The use of adsorption column with WHC-M800 as a polishing unit could keep
P removal efficiency up to 98.7 % after tank 1 and 95 % after tank 3
In addition, Ubon paspalum plant also contributed to the phosphorus removal processes in CW system by consuming nutrient for their growth (Cooke, 1992)
However its contribution was low (1.9-3.5 %) because this was the adaptation stage of plants in CW system
In comparison at the tank 1 and tank 3, the contribution of plant at tank 3 (3.5
%) was slightly higher than tank 1 (1.9 %), however the pH at the tank 3 (7.7) was lower than that of tank 1 (8.1) According to Cerozi and Fitzsimmons (2016), the plants has good uptake at the range pH 5.5 - 7.5 Therefore with the pH at the tank 1 was higher than those of tank 3 might be caused to this difference of plant uptake
In short, in the initial stage, the trial application of WHC in CW showed that P treatment performance was very good, which was reached 98% and still demonstrated a stable trend While, P removal efficiency in CW system with yellow sand only reached 77% and showed a trend that was fastly close to saturation state.
CONCLUSION AND RECOMMENDATION
CONCLUSION
This study has selected potential filter material (WHC), intensively investigated the selected material, and evaluated the trial application of WHC for P removal in CWs The key findings can be summarized as follows
(1) WHC was superior to other investigated materials for use as substrate in CW
(2) WHC and WHC-M800 possessed suitable physicochemical properties for use as substrate in CW The maximum P adsorption capacity of WHC-M800 (39.04 mg/g) was significantly higher than that of WHC (11.3 mg/g) Batch experiments indicated that WHC and WHC-M800 could work effectively in a wide range of pH (3-11) Isotherm data was best fitted by the Langmuir model with high R 2 values (0.996 and 0.994 for WHC and WHC-M800) The kinetic and column test showed the high P removal efficiency for both WHC and WHC-M800 WHC-M800 obtained high removal efficiency with real swine wastewater (80%)
(3) The WHC-based CW exhibited the high P treatment efficiency (98%), and represented a stable trend The WHC-based CWs demonstrated higher P removal efficiency than the yellow sand-based CW (around 21%).
RECOMMENDATION
In this study, the hybrid CW-adsorption treatment system has been operated for a short period of time with only one hydraulic and P loading regime Thus, the optimal operation conditions and P mass balance should be the focus of the future work
This research was conducted mainly with synthetic wastewater In the future research, the treatment performance of the CW-adsorption system should be evaluated with real swine wastewater
This study focused only on P removal efficiency of WHC and WHC-M800
The additional study should perform to investigate the simultaneous removal of several pollutants (organic, TSS, P and N)
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APPENDICES a) CW system in Higashikagura,
Hokkaido b) CW system in Biei, Hokkaido c) CW system in Mombetsu, Hokkaido d) Pilot scale CW system in Koka, Shiga
Appendix 1: Visiting some CW systems during internship in Japan a) Dumping site of WHC shell b) Sampling swine wastewater c) Collecting WHC shell d) Packing sample Appendix 2: Preparing WHC as the substrate in CWs