Removal of COD and Ammonia Nitrogen by a Sawdust/Bentonite-Augmented SBR Process

Water pollutant removal by biomass adsorbent has been considered innovative and cost-effective, and thus commendable for application in industry. However, certain important aspects have been overlooked by researchers, namely the efficiency in the operation time and pollutant removal. In this research, landfill leachate samples with organic components were treated using a bentonite-enriched sawdust-augmented sequencing batch reactor (SBR) process. By modifying the pH, the sawdust samples were categorized into three groups: the acidic, the alkaline, and the neutral. To bentonite samples, the pH-adjusted sawdust was added at 10%, 20%, and 30% amounts by mass, respectively. At the optimum aeration rate of 7.5 L/min and contact period of 22 h, the treatment achieved 99.28% and 95.41% removal of chemical oxygen demand (COD) and NH3-N with bentonite, respectively. For both pollutants, in the presence of sawdust, the removal was only reduced by about 17% with the contact period reduced to 2 h, which was a considerable achievement.


Introduction
Solid waste disposal by sanitary landfilling is the rational and cost-effective choice for most circumstances.However, despite the advantages, the generation of leachate aided by the precipitation or percolation through the landfill cap is the inevitable consequence of disposal by landfilling, and has been the main problem of this method [1].The presence of a huge amount of organic substances, measurable by the chemical oxygen demand (COD) and the biochemical oxygen demand (BOD), together with ammonia, inorganic salts, and high concentration of heavy metals such as lead and copper could bring health issues to humans [2].Recirculating the leachate back into the landfill, discharging it into the sanitary sewer system, and treating it onsite are some of the options for treatment that could also lead to surface and groundwater contamination.
Leachate and wastewater carry various pollutant concentrations for which the treatment and the improvement procedures in qualifying them for direct discharge into surface water have not been easy.In a high-quality treatment, a combination of physical, chemical, and biological methods is often applied [3,4].The physical method may involve adsorption by clay soil, activated carbon, or zeolites; or the application of membrane techniques or air striping.The chemical method may use coagulation-flocculation techniques, chemical oxidation, or ozonation; while the biological method may also involve the use of activated carbon [5,6].The activated carbon, in particular, could be a carrier of microorganisms, could enhance biomass settling properties, or could provide sorption of heavy metals and toxic organics; thus, scientists have been encouraged to use it as the main purifying substance for landfill leachates.However, the high cost of the material has been the main impediment to its use; thus, alternative solutions such as using mineral additives and mineral clays became attractive [7].
Bentonite is an alluvial clay of weathered and aged volcanic ash and belongs to the smectite group [8].It is formed by the hydrous aluminosilicate minerals, each less than 2 µm in size, which make up the colloidal segments of it.The main reasons that bentonite has been preferred for various applications are its cost-effectiveness and availability throughout the world.Bentonite has been used as a soil additive in agricultural engineering and as a mixture component in construction fluid in civil engineering.It has also been used as a landfill liner in environmental engineering, as an encapsulator of certain waste deposits, and as an adsorbent of different contaminations in the treatment of wastewater [9][10][11][12].Its cation exchange capacity and dissociation capability of the hydrogen ions are the main reasons for bentonite being negatively charged and therefore considered a good adsorbent [13].In addition, bentonite is also capable of adsorbing bacterial constituents of the leachates.
One of the biological systems used for treating pollutants in leachates and wastewater is the sequencing batch reactor (SBR), which merges all treatment units and processes into a single system, while the traditional methods of pollutant removal normally rely on various systems.Typically, a SBR accommodates five processing stages: fill, react, settle, draw, and idle [14].Other than the landfill leachates, the SBR has been used for treating domestic, municipal, industrial, dairy, synthetic, toxic, and slaughterhouse wastewater [15].
Various methods are available in the literature and industry for removing organic pollutants from leachate or wastewater through improving physical, chemical, electrical, thermal, and biological properties [16][17][18][19][20].
Oxidation, reverse osmosis, ion exchange, electrodialysis, electrolysis, and adsorption are just some of the treatment methods and excellent technologies available in the field.However, the cost of water treatment in the aformentioned technologies is at least twice as much as the cost of using the adsorption process [21].Adsorption, due to its naturality, cost-effectiveness, and ease of application, is thus considered the preferred wastewater treatment method for removing soluble and insoluble organic pollutants.
Activated carbon was already known as an adsorbent in the 1940s.However, the costly nature of the material has somewhat limited its use [22].Consequently, the search for low-cost adsorbents has continued [23][24][25][26].The use of waste products in developing low-cost adsorbents has also contributed to the minimization of waste production and recovery of waste materials [27].
Sawdust and other agricultural products have been the low-cost, biomass-based adsorbents used in experiments since the 1980s for removing dyes [28][29][30].The capacity of hardwood sawdust for the adsorption of basic dye increases with increasing temperature; e.g., from 82.2 mg dye/g of wood at 25 • C to 105.7 mg dye/g of wood at 80 • C.
About half of the matter produced by the photosynthetic process in plants becomes the major component of sawdust biomass; also called the lignocelluloses.The lignocelluloses are composed of the three polymers of cellulose, hemicelluloses, and lignin, which are strongly intermeshed and chemically bonded by noncovalent forces and by covalent cross-linkages.The cellulose and hemicelluloses are macromolecules from various sugars, while the lignin is an aromatic polymer synthesized from phenylpropanoid precursors.The composition and percentages of these components differ from one plant type to another.In addition, the composition within a single plant varies with age, stage of growth, and other conditions [31].

Paper Contribution and Organization
The main aim of this study is to investigate the removal efficiency and contact time of two main organic components of leachates, the COD and ammonia nitrogen, by a biomass-clay matrix adsorbent.This paper is organized as follows: Section 2 discusses the materials and methods used throughout this research, results and discussions are presented in Section 3, and the paper is concluded in Section 4.

Na-Bentonite
Bentonite clay provided by AMC Australia was used in this research.It was first washed with deionized water several times in order to remove dust, unwanted materials, and water-soluble impurities.The sample was then oven-dried at 150 to 200 • C for 48 h before being used as an adsorbent.At this point, the grey-colored bentonite was in fine form, passing through mesh no.30.Its physical properties such as specific gravity, particle size distribution, Atterberg limits, organic content, and moisture content were determined according to ASTM D2216 [32].Until the next stage, it was stored in a cool, shaded, and dry environment.The sample was then air-dried and sieved through mesh no.200 prior to chemical analyses, which were in accordance with the laboratory manual of the Geotechnical Research Center, McGill University.The cation exchange capacity (CEC) was determined using the BaCl 2 replacement method [33].
The X-ray diffraction (XRD) analysis was performed based on the method by Moore and Reynold [34].The carbonate content was determined using the titration method [35].The specific surface area (SSA) was determined according to the Brunauer-Emmett-Teller (BET) method [36].The pH was measured for the bentonite-water solution of 1:10 ratio by mass; i.e., one portion of bentonite for every 10 portions of water.The resulting physical and chemical properties of the sample are given in Table 1.

Sawdust
The wood sample was of the "Red Meranti" timber species.The sawdust was washed using deionized water multiple times and was oven-dried at 70 • C for about 48 h in order to remove the moisture content.The dried sample was converted to a fine powder using a ball mill.The resulting material was sieved to the 100-150 µm particle size according to the ASTM D4779 method [37,38].A 0.5 M HCl solution was used to remove the color and water-soluble substances by soaking at room temperature for 6 h.Eventually, the sawdust was filtered and washed multiple times by distilled water and then oven-dried at 70 • C for about 48 h.The adsorbent was stored in an airtight container before use in the application.

Alkaline and Acidic Treatment of Sawdust
Sawdust-water solutions of 1:10 ratio by mass were prepared and tested for pH using the EUTECH 700 pH measurement meter.The water was deionized.The solutions were placed in five different glass containers in the process of adjusting them to pH 2, 4, 6, 8, and 10.To achieve this, the 0.1 M HCl reagent was used for the acidic adjustment, while 0.5 M NaOH reagent was used for the alkaline adjustment.The adjustment was carried out at room temperature while the solutions were shaken continuously.Different amounts of acidic and alkaline reagents were added to the containers based on the targeted pH of each.Until the pH became stabilized, the addition of HCl and NaOH drops continued.Once achieved, the solutions were kept overnight and the pH of each was tested again the next day.This process was continued until the pH became consistent for 2 consecutive days.To eliminate the adsorbed acid or alkali, the sawdust was then washed several times with distilled water and acetone.The alkali-treated and acid-treated sawdust was dried in a hot-air oven at 60 • C for about 72 h and kept in the desiccator for subsequent use.Care was taken to avoid bonding in sawdust that could turn into carbon.

Leachate Sampling and Characterization
Leachate samples were collected from the Pulau Burung sanitary landfill located 10 km away from the Engineering Campus of Universiti Sains, Malaysia, in the city of Nibong Tebal, Malaysia, or approximately 20 km southeast of Penang Island [39].The 64-hectare disposal site, located at 5 • 11 52.5 N 100 • 25 35.8E, was first established in the 1980s.In 1991, it was developed into a Level II semiaerobic sanitary landfill and later in 2001 was upgraded to Level III, using controlled tipping with a leachate reticulation system [40].Only half of the area is currently operational and the daily incoming waste stands at 2200 tons per day.The bed is a natural marine clay which is also the liner.The landfill produces a dark black-green-colored leachate which is classified as stable with high concentrations of COD and ammonia nitrogen and a low BOD/COD ratio [41].
The leachate samples were collected between May 2015 and August 2015.After each collection, the samples were immediately taken to the environmental engineering laboratory of the School of Civil Engineering and were kept in a cold room at 4 • C to avoid any biological and chemical reactions.The properties of the leachate samples are given in Table 2.In order to evaluate the environmental risks of the leachate, the parameters were compared against the Malaysian Environmental Quality

Operation of the Reactors
The treatment was carried out in twenty 1700-mL plexiglass reactors.Each of the cylindrical reactors had a 1000-mL working capacity with a "couple bottom aeration system" or simply two air pressure inputs.The reactors were filled with 1000 mL of leachate sample, while the different mix designs of the Na-bentonite and the pH-adjusted sawdust were as described earlier.The ratio of sawdust plus bentonite against the untreated leachate put together in each reactor was constant at 1:50 by mass.
Therefore, each solid part of the reactor system consisted of 20 g of bentonite plus an odd from the 9 sawdust samples with pH of 2, 6, or 10 and with a percentage of sawdust of either 10%, 20%, or 30% by mass of the 20 g of bentonite.Furthermore, for each group contact time, an additional reactor was run with just the bentonite, while each of the other twenty reactors had the bentonite enhanced with either the acidic or the alkaline sawdust content.
To supply the air, a pump was used with the following specifications: voltage of 240 V, frequency of 50 Hz, input power of 61 W, and pressure of 0.012 MPa.The regulation of the air rate was manually done by an airflow meter.Based on a previous study, an aeration rate of 7.5 L/min was applied with 3 possible contact times of 2, 12, and 22 h [43].The room temperature during tests was set at 30 • C.
Fill, react, settle, draw, and idle were the sequential operation phases of the reactor system.The times for filling and mixing, settling, and drawing and idling times were 20, 90, and 10 min, respectively, thus totaling 2 h for the shortest complete operation.The removal efficiencies of COD and NH 3 -N are defined in Equation (1).
where C i and C f are the initial and final concentrations of the parameters, respectively.

Experimental Design
In this study, the response surface methodology (RSM), and central composite design (CCD) were used for the gathering of mathematical and statistical techniques that are useful for the modeling and analysis of problems, in which responses of interest are influenced by some variables and the statistical design of experiments and data analysis, respectively.The design included k 2 factorial points completed by 2k axial points and a center point, where k is the number of variables.Thereupon, CCD and RSM were applied in order to evaluate the association between the variables, i.e., percentage of sawdust (%), pH of the sawdust, and contact time (h), and their responses.
In other words, the main scope of this design is to optimize the appropriate situation of operating variables to predict the best value of responses.Aeration rate was 7.5 L/min in all the reactors, and 2 dependent parameters (COD and NH 3 -N) were measured as the responses (see Table 3).A reactor was considered without any sawdust in order to compare with the others and to monitor the effect of the sawdust on the COD and ammonia removal.The following equation explains the quadratic equation model for estimating the optimum conditions: where Y is the response; X i and X j are the variables; β 0 is a constant coefficient; β i , β ii , and β ij are the interaction coefficients of the linear, quadratic, and second-order terms, respectively; k is the number of studied factors; and e is the error.The aeration rate was a constant 7.5 L/min for all reactor systems.Another reactor was run free of sawdust as the control.The values of the independent variables and the resulting responses are given in Table 3.

Experimental Method
All experiments were carried out according to the Standard Methods for the Examination of Water and Wastewater [44].A spectrophotometer (DR/2800 HACH) was used for measuring color (Pt.Co), NH 3 -N (mg/L), magnesium (mg/L), total iron (mg/L Fe), nickel (mg/L Ni), and arsenic (mg/L As).COD concentration was determined using the closed reflux and colorimetric method no.5220D.YSI 556 MPS (YSI incorporated, Yellow Springs, OH, USA) was used for recording the values of pH, temperature ( • C), and TDS (%).

Adsorption by Clay Mineral
For an SBR system with 20 g of bentonite, no sawdust, 1000 mL of leachate, and an aeration rate of 7.5 L/min, the COD removal was 12% after 2 h and 99% after 22 h.For a system with 20 g of bentonite and 20% sawdust of pH 6, with all other conditions remaining the same, the COD removal was 83% after 2 h and 84.2% after 12 h.Eventually, the maximum COD removal was 99.4% after 22 h, with 10% sawdust of pH 2.More detailed results are given in Table 3 and Figure 1.
A decrease in the nonbiodegradable organic pollutants in wastewater translates to a decrease in COD [45].The complex interaction between the clay minerals and the organic compounds results in a decrease in COD in a process which involves the roles of the silicate layers, the inorganic cations, the water, and the organic molecules.First, between the organic matter and the clay minerals, there is the ion exchange, which is either the cation exchange or the anion exchange.Cation exchange is the term describing the adsorption of the organic cations by the clay minerals with the inorganic cations leaving.The interaction of positive charge sites, at oxide surfaces or edge sites of clays with carboxylate (COO − ), sulfonate (SO 3 − ), and phenolate (aromatic O − ) groups, is by replacement of univalent exchangeable anions (e.g., Cl − or NO 3 − ) bound to a protonated surface hydroxyl (OH 2 + ) [46,47].
Because of the weakness of the electrostatic bonds, this organic adsorption mechanism is not observed often.On the other hand, the presence of the metal oxides at clay fractions can make this mechanism complete [48].Adsorption by dispersion forces is very important, since it is not only acts separately, but also as a supplier in the other mechanisms.There is a parallel relationship between the adsorption degree of this mechanism and the molecular weight of the adsorbate.Van der Waals interaction occurs between the adsorbed molecules rather than the surface of the adsorbed molecules [49,50].
The mechanism of the protonation of organic molecules at clay surfaces makes many compounds cationic.The acidity of the clay surface is the main factor causing the organic compound to be in cationic form.The main source of the protons and the protonation reactions are exchangeable H + ions occupying cation exchange sites, water associated with metal cations at the exchange sites, and proton transfer from another organic cation [51].The hemisalt formation mechanism occurs while neutral organic bases are adsorbed on H-clays in excess over the number of protons available, and in this case, two molecules are competing for the proton [52,53].Coordination by cation bridging and water bridging are the other means of organic matter adsorption by clay minerals.Organic molecules may be coordinated to the inorganic saturating cations by a direct bonding, which is called cation bridging, or through water, which depends on the water content of the clay mineral and is known as water bridging.When the water content is reduced in the system, there will be competition between the water and the polar molecules for ligand position around the metal cation, initially through a water bridge and then through a direct interaction [46][47][48].
Clean Technol.2018, 1, x FOR PEER REVIEW 7 of 15 compounds cationic.The acidity of the clay surface is the main factor causing the organic compound to be in cationic form.The main source of the protons and the protonation reactions are exchangeable H + ions occupying cation exchange sites, water associated with metal cations at the exchange sites, and proton transfer from another organic cation [51].The hemisalt formation mechanism occurs while neutral organic bases are adsorbed on H-clays in excess over the number of protons available, and in this case, two molecules are competing for the proton [52,53].Coordination by cation bridging and water bridging are the other means of organic matter adsorption by clay minerals.Organic molecules may be coordinated to the inorganic saturating cations by a direct bonding, which is called cation bridging, or through water, which depends on the water content of the clay mineral and is known as water bridging.When the water content is reduced in the system, there will be competition between the water and the polar molecules for ligand position around the metal cation, initially through a water bridge and then through a direct interaction [46][47][48].Surface complexation of organic anions is formed between the organic ions or molecules and functional groups and divided into two main groups.If no water intervened between the surface functional group and the ion it binds, the complex is inner-sphere, while in the presence of the water, it will be the outer-sphere one.The outer-sphere surface complexes involve electrostatic bonding mechanisms and are less stable than the inner-sphere ones, which involve ionic and covalent bonding.Surface complexation of organic anions is formed between the organic ions or molecules and functional groups and divided into two main groups.If no water intervened between the surface functional group and the ion it binds, the complex is inner-sphere, while in the presence of the water, it will be the outer-sphere one.The outer-sphere surface complexes involve electrostatic bonding mechanisms and are less stable than the inner-sphere ones, which involve ionic and covalent bonding.
One of the most important adsorption mechanisms in clay soils is hydrogen bonding, which occurs at edge and surface sites.Mortland [46] suggested three main groups for this mechanism, as follows: 1.
Water bridging involves the linking of a polar organic molecule to an exchangeable metal cation through a water molecule in the primary hydration shell.One or both protons of the water molecule can participate in the bonding.

2.
Organic-organic hydrogen bonding occurs when the exchangeable cation on the clay is an organic one.The organic-clay hydrogen bonding mechanism acts when the hydroxyls of the exposed siloxane (Si-O) and aluminol (Al-OH) planes of clay minerals interact with a polar organic molecule [49].

3.
Adsorption by donation of π electrons [54] and hydrophobic bonding [55] are the other mechanisms of organic adsorption with clay minerals.

Sawdust Adsorption Mechanism
In this study, various mix designs of bentonite and pH-adjusted sawdust are considered with different operation procedures in order to monitor the effect of sawdust on the melioration of adsorption efficiency.As given in Table 3 and Figure 1, the sawdust has a positive effect in reducing the contact time with the same high removal efficiency.In a 2-h period, the addition of 10% sawdust with pH 2 increased the COD removal by bentonite alone from 12.13% to 83%.On the other hand, in a 12-h period, the addition of 20% sawdust with pH 6 increased the removal by 1%, while in a 22-h period, the addition of 10% sawdust with pH 2 decreased the removal by 0.2%.
Chemical and physical adsorption mechanisms are the two main adsorption mechanisms of sawdust in contact with unwanted materials.Chemically, adsorption is related to the structural components of sawdust relating to the lignin, cellulose, hemicellulose, and active groups on the surface, while physical adsorption behavior is activated by the treatment of sawdust with chemical materials.The cell wall of the sawdust mainly consists of cellulose, lignin, and hydroxyl groups, which are active ion exchange compounds [47].Ion exchange, hydrogen bonding, and the electron-donating nature of O-, S-, and P-containing groups in sawdust materials are the main indirect organic matter adsorption mechanisms of sawdust, with cation and anion exchanging and different bridging mechanisms, respectively.As explained earlier, the optimum efficiency in the early and final contact time occurred with sawdust of pH 2. An acidic treatment of sawdust from pH 6 to 2 led to the generation of cylindrical tubes which could capture the organic matter, as shown in Figure 2. The size of the organic matter and the diameter of the tubes can affect the physical adsorption mechanism.
donating nature of O-, S-, and P-containing groups in sawdust materials are the main indirect organic matter adsorption mechanisms of sawdust, with cation and anion exchanging and different bridging mechanisms, respectively.As explained earlier, the optimum efficiency in the early and final contact time occurred with sawdust of pH 2. An acidic treatment of sawdust from pH 6 to 2 led to the generation of cylindrical tubes which could capture the organic matter, as shown in Figure 2. The size of the organic matter and the diameter of the tubes can affect the physical adsorption mechanism.

Clay Mineral Adsorption Mechanisms
The NH 3 -N removal from the leachate sample is in the range of 35% to 98.3% (Table 3 and Figure 3).The maximum removal of ammonia nitrogen occurred with 10% sawdust of pH 2 in a 22-h contact time with constant operating conditions.In contrast, 20% sawdust of pH 2 in a 12-h contact time leads to a negative effect on the efficiency of the system.

Clay Mineral Adsorption Mechanisms
The NH3-N removal from the leachate sample is in the range of 35% to 98.3% (Table 3 and Figure 3).The maximum removal of ammonia nitrogen occurred with 10% sawdust of pH 2 in a 22-h contact time with constant operating conditions.In contrast, 20% sawdust of pH 2 in a 12-h contact time leads to a negative effect on the efficiency of the system.Ion exchange and adsorption are two main mechanisms of the ammonia removal from wastewaters by clay minerals.One of the advantages of using clays as the adsorbent for ammonia removal is that the layered structure of the clay allows expansion (swelling) when in contact with water, which exposes an additional mineral surface capable of ammonium ion (NH4 + ) adsorption [56].Specific surface area (SSA), cation exchange capacity (CEC), and the functional groups of the different fractions of soils are the surface characteristics of the soil solids.Some exchangeable cations, such as K + , Na + , Mg 2+ , and Ca 2+ , are available in the structure of bentonite molecules, and these cations can easily exchange with other cations such as NH 4+ .The stage of the cation in the lyotropic series, concentration of the cations, and the diameter of the cation molecule are the effective factors in the ion exchange mechanism.Similar to the other mechanism, the reaction between the cation and the negatively charged surface of the soil fractions leads to the adsorption of the cation by the clay.Consequently, bentonite, which includes montmorillonite clay minerals with high cation exchange capacity (CEC) and specific surface area (SSA), is one of the most important clay adsorbents.Ion exchange and adsorption are two main mechanisms of the ammonia removal from wastewaters by clay minerals.One of the advantages of using clays as the adsorbent for ammonia removal is that the layered structure of the clay allows expansion (swelling) when in contact with water, which exposes an additional mineral surface capable of ammonium ion (NH 4 + ) adsorption [56].
Specific surface area (SSA), cation exchange capacity (CEC), and the functional groups of the different fractions of soils are the surface characteristics of the soil solids.Some exchangeable cations, such as K + , Na + , Mg 2+ , and Ca 2+ , are available in the structure of bentonite molecules, and these cations can easily exchange with other cations such as NH 4+ .The stage of the cation in the lyotropic series, concentration of the cations, and the diameter of the cation molecule are the effective factors in the ion exchange mechanism.Similar to the other mechanism, the reaction between the cation and the negatively charged surface of the soil fractions leads to the adsorption of the cation by the clay.Consequently, bentonite, which includes montmorillonite clay minerals with high cation exchange capacity (CEC) and specific surface area (SSA), is one of the most important clay adsorbents.

Sawdust Adsorption Mechanism
In this study, the maximum adsorption of ammonia nitrogen, at 98.3%, was achieved by the bentonite with 10% sawdust of pH 2 and after 22 h of contact, as given in Table 3 and Figure 3. Thus, the conditions for maximum ammonia nitrogen removal were the same as for maximum COD removal.The main point, however, was achieving an acceptable removal rate with a reduced contact time.Thus, the bentonite with 20% sawdust of pH 6 could remove 78.3% ammonia nitrogen after 2 h, while the bentonite without sawdust could remove 37.5% after the same 2 h, 68.8% after 12 h, and 95.3% after Clean Technol.2018, 1 135 22 h.With only 17% more removal after 22 h, the bentonite with 20% sawdust of pH 6 was obviously a design with considerable benefit in terms of savings in the contact time.At lower pH, the positively charged ammonium has competition with the H + , and by the ion exchange mechanism, is adsorbed into the surface of the sawdust.With an increase in pH, the ammonium may be adsorbed by the hydrogen bonding mechanism, along with ion exchange.
The basic chemical phenylpropane units of lignin (primarily syringyl, guaiacyl, and p-hydroxy phenol) are bonded together by a set of linkages to form a very complex matrix.This matrix comprises a variety of functional groups, such as hydroxyl, methoxyl, and carbonyl groups, which impart a high polarity to the lignin macromolecule.So, lignin is the constituent of lignocellulose materials which participates in the adsorption process.Figure 4 illustrates the FTIR results of the sawdust of pH 2 and pH 4. The graph shows an increase in the peaks at 1599, 1511, 1467, 1429, 1157, and 1054, which belong to lignin.A strong aromatic ring stretch, phenylpropanoid polymer deformation (methyl and methylene), glycosidic linkage, and vibrational stretching are indicative of a number of bonds which are present in the lignin [57,58].

Sawdust Adsorption Mechanism
In this study, the maximum adsorption of ammonia nitrogen, at 98.3%, was achieved by the bentonite with 10% sawdust of pH 2 and after 22 h of contact, as given in Table 3 and Figure 3. Thus, the conditions for maximum ammonia nitrogen removal were the same as for maximum COD removal.The main point, however, was achieving an acceptable removal rate with a reduced contact time.Thus, the bentonite with 20% sawdust of pH 6 could remove 78.3% ammonia nitrogen after 2 h, while the bentonite without sawdust could remove 37.5% after the same 2 h, 68.8% after 12 h, and 95.3% after 22 h.With only 17% more removal after 22 h, the bentonite with 20% sawdust of pH 6 was obviously a design with considerable benefit in terms of savings in the contact time.At lower pH, the positively charged ammonium has competition with the H + , and by the ion exchange mechanism, is adsorbed into the surface of the sawdust.With an increase in pH, the ammonium may be adsorbed by the hydrogen bonding mechanism, along with ion exchange.
The basic chemical phenylpropane units of lignin (primarily syringyl, guaiacyl, and p-hydroxy phenol) are bonded together by a set of linkages to form a very complex matrix.This matrix comprises a variety of functional groups, such as hydroxyl, methoxyl, and carbonyl groups, which impart a high polarity to the lignin macromolecule.So, lignin is the constituent of lignocellulose materials which participates in the adsorption process.Figure 4 illustrates the FTIR results of the sawdust of pH 2 and pH 4. The graph shows an increase in the peaks at 1599, 1511, 1467, 1429, 1157, and 1054, which belong to lignin.A strong aromatic ring stretch, phenylpropanoid polymer deformation (methyl and methylene), glycosidic linkage, and vibrational stretching are indicative of a number of bonds which are present in the lignin [57,58].

Statistical Analysis
Response surface methodology (RSM) is used to analyze the correlation between the variables, i.e., contact time, pH, and sawdust content, and COD and NH3-N removal.Considerable model terms were preferred to achieve the best fit in a particular model.CCD permitted the development of mathematical equations where predicted results (Y) are evaluated as a function of contact time (A), pH (B), and sawdust (C).The results are computed as the sum of a constant, three first-order effects (terms in A, B, and C), three interaction effects (AB, AC, and BC), and three second-order effects (A 2 , B 2 , and C 2 ), as shown in Equation (1), Tables 3 and 4. In order to determine the accuracy of fit, the results were analyzed by ANOVA statistical technique.Equations from the first ANOVA analysis are adapted by neglecting the terms found to be statistically irrelevant.The reduced quadratic models in terms of actual factors are illustrated in Table 4.

Statistical Analysis
Response surface methodology (RSM) is used to analyze the correlation between the variables, i.e., contact time, pH, and sawdust content, and COD and NH 3 -N removal.Considerable model terms were preferred to achieve the best fit in a particular model.CCD permitted the development of mathematical equations where predicted results (Y) are evaluated as a function of contact time (A), pH (B), and sawdust (C).The results are computed as the sum of a constant, three first-order effects (terms in A, B, and C), three interaction effects (AB, AC, and BC), and three second-order effects (A 2 , B 2 , and C 2 ), as shown in Equation (1), Tables 3 and 4. In order to determine the accuracy of fit, the results were analyzed by ANOVA statistical technique.Equations from the first ANOVA analysis are adapted by neglecting the terms found to be statistically irrelevant.The reduced quadratic models in terms of actual factors are illustrated in Table 4.According to the probability value (less than 0.05), all models were significant at the 5% confidence level.R 2 values close to 1 were favorable, and a high R 2 coefficient ensured acceptable modification of the quadratic model to fit the experimental data.The correlation coefficient (R 2 ) gave the proportion of total variation in the response predicted by the model, indicating the ratio of sum of squares due to regression (SSR) to total sum of squares (SST).Adequate precision compared the range of the predicted values at the design points to the mean prediction error [59].Figure 5 shows the predicted versus actual-value plots of the response parameters for the COD and ammonia removal.These plots signified a sufficient agreement between the real data and the values achieved from the models.According to the probability value (less than 0.05), all models were significant at the 5% confidence level.R 2 values close to 1 were favorable, and a high R 2 coefficient ensured acceptable modification of the quadratic model to fit the experimental data.The correlation coefficient (R 2 ) gave the proportion of total variation in the response predicted by the model, indicating the ratio of sum of squares due to regression (SSR) to total sum of squares (SST).Adequate precision compared the range of the predicted values at the design points to the mean prediction error [59].Figure 5 shows the predicted versus actual-value plots of the response parameters for the COD and ammonia removal.These plots signified a sufficient agreement between the real data and the values achieved from the models.

Optimization of the System
The final experimental results involving COD and NH3-N removals with various mix designs of adsorbents and operating conditions were analyzed using RSM and CCD in order to determine a specific operation system that could lead to the optimum removal of both organic compounds.
The resulting responses from optimizing the COD and NH3-N removals are given in Figure 6.Accordingly, the optimized conditions were the sawdust content of 29.3% with pH of 3.3 and the contact time of 21.8 h, which would result in 99.7% COD removal and 98.4% NH3-N removal.

Optimization of the System
The final experimental results involving COD and NH 3 -N removals with various mix designs of adsorbents and operating conditions were analyzed using RSM and CCD in order to determine a specific operation system that could lead to the optimum removal of both organic compounds.
The resulting responses from optimizing the COD and NH 3 -N removals are given in Figure 6.Accordingly, the optimized conditions were the sawdust content of 29.3% with pH of 3. According to the probability value (less than 0.05), all models were significant at the 5% confidence level.R 2 values close to 1 were favorable, and a high R 2 coefficient ensured acceptable modification of the quadratic model to fit the experimental data.The correlation coefficient (R 2 ) gave the proportion of total variation in the response predicted by the model, indicating the ratio of sum of squares due to regression (SSR) to total sum of squares (SST).Adequate precision compared the range of the predicted values at the design points to the mean prediction error [59].Figure 5 shows the predicted versus actual-value plots of the response parameters for the COD and ammonia removal.These plots signified a sufficient agreement between the real data and the values achieved from the models.

Optimization of the System
The final experimental results involving COD and NH3-N removals with various mix designs of adsorbents and operating conditions were analyzed using RSM and CCD in order to determine a specific operation system that could lead to the optimum removal of both organic compounds.
The resulting responses from optimizing the COD and NH3-N removals are given in Figure 6.Accordingly, the optimized conditions were the sawdust content of 29.3% with pH of 3.3 and the contact time of 21.8 h, which would result in 99.7% COD removal and 98.4% NH3-N removal.
Regulations 2009: Control of Pollution from Waste Transfer Station and Landfill, which were developed under the Malaysia Environmental Quality Act 1974 [42].

Figure 1 .
Figure 1.Plot of COD removal (A) with sawdust content of 10%, (B) with sawdust content of 20%, and (C) with sawdust content of 30%.

Figure 1 .
Figure 1.Plot of COD removal (A) with sawdust content of 10%, (B) with sawdust content of 20%, and (C) with sawdust content of 30%.

Figure 3 .
Figure 3. Plot of ammonia removal (A) with sawdust content of 10%, (B) with sawdust content of 20%, and (C) with sawdust content of 30%.

Figure 3 .
Figure 3. Plot of ammonia removal (A) with sawdust content of 10%, (B) with sawdust content of 20%, and (C) with sawdust content of 30%.

Figure 4 .
Figure 4. Fourier transform infrared spectroscopy (FTIR) of sawdust of pH 2 and sawdust of pH 6.

Figure 4 .
Figure 4. Fourier transform infrared spectroscopy (FTIR) of sawdust of pH 2 and sawdust of pH 6.

Figure 5 .
Figure 5. Design-expert plot; predicted vs. actual values plot for (A) COD removal and (B) NH3-N removal.

Figure 6 .
Figure 6.Desirability response of the removal system.

Figure 5 .
Figure 5. Design-expert plot; predicted vs. actual values plot for (A) COD removal and (B) NH 3 -N removal.
3 and the contact time of 21.8 h, which would result in 99.7% COD removal and 98.4% NH 3 -N removal.Clean Technol.2018, 1, x FOR PEER REVIEW 12 of 15

Figure 5 .
Figure 5. Design-expert plot; predicted vs. actual values plot for (A) COD removal and (B) NH3-N removal.

Figure 6 .
Figure 6.Desirability response of the removal system.Figure 6. Desirability response of the removal system.

Figure 6 .
Figure 6.Desirability response of the removal system.Figure 6. Desirability response of the removal system.

Table 1 .
Characteristics of bentonite sample.

Table 2 .
Characteristics of leachate sample.

Table 3 .
Experimental variables and results for sequencing batch reactor (SBR).

Table 4 .
ANOVA results for response parameters., first variable, Contact time: h; Adeq.P, adequate precision; Adj.R 2 , adjusted R 2 ; B, second variable, sawdust pH; C, third variable, Sawdust content: %; cv, coefficient of variance; Press, predicted residual error sum of square; Prob, probability of error; R 2 , correlation coefficient; SD, standard deviation. A