Removal of Ammonia Nitrogen from Wastewater Using Modified Activated Sludge Chen Yunnen*, Xiong Changshi, Nie Jinxia School of Resource and Environmental Engineering, Jiangxi University of Science and Technology, Jiangxi Key Laboratory of Mining & Metallurgy Environmental Pollution Control Hongqi Ave. 86, Ganzhou Jiangxi 341000, P.R. China
Received: 9 March 2015 Accepted: 1 December 2015 Abstract The removal of medium-low N-NH4 on activated sludge modified by ferric hydroxide suspension (FHMAS) in batch studies was conducted as a function of pH, concentration of modifier, contact time, and initial concentration. The kinetics study showed that the sorption behavior fit well the pseudo second-order equation. An adsorption isotherms study indicated that FHMAS had a higher adsorption capacity for N-NH4 than other adsorbents. The mechanism of removal of N-NH4 by FHMAS was coexistence of adsorption and cation exchange. Initial N-NH4 concentration being 116 mg/l in metallurgical wastewater was reduced to 11 mg/l after adsorption treatment.
Introduction Eutrophication of surface water is primarily contributed by nitrogen and phosphorus contamination from industry wastewater, farmland fertilization, and municipal sewage, which has been a common environmental issue in many countries . These nutrients cause diverse problems such as toxic algal blooms, loss of oxygen, fish kills, loss of biodiversity, loss of aquatic plant beds and coral reefs, and other problems. Great efforts have been aimed at the removal of N-NH4 from wastewater. Traditional methods include ammoniastripping , chemical precipitation , biological denitrification , dry-ice , and so on. Ammonia stripping makes use of a stripping tower, consumes much
energy, and is only suitable for treating high N-NH4 concentrations. Chemical precipitation needs additional reagents that can introduce new pollutants to water bodies. Biological denitrification is the most common process in the treatment of N-NH4 wastewater. But it is higher efficiency for the removal of low N-NH4 concentration due to the requirement of an appropriate C/N ratio  and need for a long reaction time. It is inefficient, though, for the traditional ways to quickly treat medium-low N-NH4 from wastewater. In recent years considerable attention has been aimed at the study of heavy metals removal from solution by adsorption using wastes, including fruit peels, straw, coconut coir, and so on [7-9]. Generally, the sorption capacities of crude byproducts are low. To improve the adsorption capacity of these by-products, chemical modification has been used. Surplus activated sludge is the by-product of sewage treated by the activated sludge process. Every year a large amount of activated sludge from a sewage plant
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is produced. In this study, activated sludge was used as the biosorbents to remove medium-low N-NH4 from aqueous solution and wastewater. Because of its sorptional nature, ferric compounds often apply to the treatment of wastewater with heavy metals [10-11]. The objectives are to identify the NH4+ ion uptake capacity of the activated sludge modified by ferric hydroxide (FHMAS) to determine the kinetics, and assess the effects of pH, concentration of modifier, contact time, and N-NH4 initial concentration on the removal. Then the adsorption isotherm of NH4+ is obtained, which contains the basic information for exploiting sludge as a water treatment agent.
Material and Methods
All other experiments were conducted at pH 8. As for the factor of modifier concentration, samples were collected after 20 min contact with FHMAS and efficiency of N-NH4 removal was analyzed. Based on the same ferric concentration, a different volume of ferric hydroxide suspension but no FHMAS added was also performed to compare the N-NH4 removal with FHMAS. At the experimental process, the ferric ion concentration in solution also was analyzed. In the contact time experiment, samples were withdrawn at pre-determined intervals and efficiency of N-NH4 removal was analyzed. The percent efficiency of removal of N-NH4 in solution was calculated by the use of the following equation:
Adsorbate Ammonium chloride (99.5%, analytical reagent, Tianjin Kermel Chemical Reagent Development Center, China) was used as the source of N-NH4. Deionized water was used for the preparation of solutions. All other reagents such as ferric chloride, sodium hydroxide (NaOH), and hydrochloric acid (HCl) were analytical grade. Real N-NH4 wastewater was obtained from a Metallurgical Plant of Ganzhou, Jiangxi, China.
(1) …where C0 is N-NH4 concentrations an the start of process (mg/l), Ct is N-NH4 concentrations after time t (mg/l), and t is time of process. At the mentioned optimum conditions, the effects of initial N-NH4 concentration (20-250 mg/l) on the sorption of N-NH4 was studied under the aspects of sorption isotherms. The amount of N-NH4 adsorbed per adsorbent mass was calculated by the use of the following equation:
Adsorbent The activated sludge was obtained from a wastewater treatment plant of Ganzhou City, P. R. China. After being washed many times in distilled water, the sludge was dried at 110°C and sieved through a 245 μm sieve and stored dry until use, labeled as original activated sludge (OAS). Adding NaOH solution to a certain concentration (0.05~0.30 mol/l) of ferric chloride solution, ferric hydroxide suspension was produced. With the ratio of 1 g OAS to 10 ml ferric hydroxide, suspensions were mixed uniformly and placed in an oven at 60°C for 12 h, and the suspension liquid was filtered and washing many times by distilled water until the effluent was cleared up. The filtered solid was dried at 110°C and stored for dry until use and labeled as activated sludge modified by ferric hydroxide (FHMAS).
Batch Experimental Procedures The removal of N-NH4 with FHMAS was performed in batch experiments. Prior to experiments, the pH value of N-NH4 solution was changed from 3 to 10, with 0.1 mol/l NaOH or HCl solution. During the removal process, 0.3 g FHMAS or OAS was suspended in 100 ml wastewater containing N-NH4 (100±5 mg/l). The mixture was agitated on a gyratory shaker at 250 rpm by 20 min. At the end of the experiment the suspension liquid was decanted and filtered through a 0.45 μm cellulose acetate filter and the supernatant was analyzed. Controls were also performed with no adsorbent to compare the efficiency of N-NH4 removal with FHMAS and OAS.
(2) …where qt is the amount of N-NH4 adsorbed on sludge at time t (mg/g), V is the volume of solution (l), and m is the adsorbent mass (g). All experiments were fulfilled at room temperature (293±1 K). Each experiment was run in triplicate and mean values were calculated.
Analysis and Measurements N-NH4 was measured by WT-1 portable apparatus of N-NH4 (Wuhan Water Environmental Protection Company, China), which was based on the standard method . Fe3+ was determined based on the phenanthroline spectrophotometric method. pH was measured with a pH meter (pHS-25, Shanghai Leici Instrument Factory, China).
Results and Discussion Effect of pH on N-NH4 Removal Ammonium is always a pH-dependent balance between soluble ammonium ion NH4+ and dissolved molecular ammonia NH3 in wastewater . In acidic and neutral media, N-NH4 is presented as NH4+. In basic solution, non-volatile NH4+ is converted to NH3. High pH favors ammonia volatilization by moving the equilibrium
Removal of Ammonia Nitrogen from Wastewater... between NH3 and NH4+ to molecular ammonia, as shown in equation (3). In this study the pH from 3 to 10 was changed. (3) In order to determine the desired pH for adsorption of N-NH4 over FHMAS, the uptake of N-NH4 as a function of hydrogen ion concentration was studied, as Fig. 1 showed. From Fig. 1 it can be seen that all three curves have the same trend, namely that inflection points appeared at pH 8. The net sorption removal of N-NH4 by OAS was 9.1, 19.1, 23.9, 30.1, 40.3, 44.3, 42.1, and 40.6% after deducting results of the controls group at pH 3, 4, 5, 6, 7, 8, 9, and 10, respectively. The net sorption effect by FHMAS was 22.3, 32.6, 39.7, 47.9, 60.3, 68, 66.1, and 63.8% at pH 3, 4, 5, 6, 7, 8, 9, and 10, respectively. Under three conditions the removal of N-NH4 increased with pH of solution increasing. Ammonium chloride solution is a strong acid with weak base salt. At low pH, the surface ligands are closely associated with the hydronium ions (H3O+) and restricted the approach of NH4+ cation as a result of the repulsive force. Moreover, the ferric oxide layer on the surface of FHMAS was hydroxylated and positively charged at acidic water environment, which went against N-NH4 adsorption. It was observed that the presence of Fe3+ in solution was detected after contacting ammonium chloride solution with FHMAS, which inferred that ion-exchange between N-NH4 and Fe3+ might exist. All subsequent experiments were conducted with pH 8.
Effect of Concentration of Modifier on N-NH4 Removal The effects of concentration of modifier and ferric hydroxide suspension on the efficiency of N-NH4 removal
Fig. 1. Effect of pH on N-NH4 removal. Initial N-NH4 concentration was 99.45 mg/l, concentration of modifier 0.10 mol/l, contact time 20 min, sorbent dose 3 g/l.
421 are shown in Fig. 2. The two curves in Fig. 2 show that the efficiency of N-NH4 removal onto FHMAS was higher than that of ferric hydroxide suspension. The effect of removal always increases with the Fe3+ concentration increasing for FHMAS, indicated that perhaps increasing adsorption sites can increase the efficiency of N-NH4 removal by adsorption. After the experimental process, the analyzed Fe3+ concentration in solution showed that the content of Fe3+ increased with N-NH4 efficiency of removal increasing by FHMAS, but Fe3+ concentration was almost zero by ferric hydroxide suspension, which inferred the existence of cation exchange in the experimental process with FHMAS. Analysis of the data showed that the number of removed N-NH4 was more than that of Fe3+ in solution, which meant that cation exchange and adsorption are taking place at the same time. Fig. 2 also showed that the efficiency of removal of N-NH4 by FHMAS was first increased and then decreased with the concentration of increasing modifier. Maybe a small amount of ferric hydroxide can enhance the cation exchange capacity with NH4+. The excess ferric hydroxide may be part of the sludge pore blockage and lead to the decline of FHMAS adsorption capacity for N-NH4. All subsequent experiments were conducted with 0.15 mol/l ferric hydroxide suspension.
Kinetics Study The efficiency of removal rate of N-NH4 by FHMAS is shown in Fig. 3. The sorption process was rapid in less than 20 min and reached a plateau of some 87% in 30 min. Various sorption kinetic models have been used to describe the removal of metals. The Lagergren first-order kinetic process has been used for reversible reaction with an equilibrium between liquid and solid phases. The rate equation for the reaction may be represented by the following expression:
Fig. 2. Effect of concentration of modifier on N-NH4 removal. Initial N-NH4 concentration was 101.83 mg/l, pH 8, contact time 20 min, adsorbent dose 3 g/l.
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Fig. 3. Effect of contact time on N-NH4 removal. Initial N-NH4 concentration was 103.38 mg/l, pH 8, concentration of modifier 0.15 mol/l, adsorbent dose 3 g/l.
Fig. 5. Pseudo second-order sorptional kinetics of N-NH4 by FHMAS.
(4) …where t is contact time (min), qt is quantities of N-NH4 adsorbed at time t (mg/g), qe is quantities of N-NH4 adsorbed at equilibrium (mg/g), and k1 is sorption rate constant. From Eq. (4), a plot of lg (qe -qt) versus t should give a straight line to confirm the applicability of the first-order kinetic model (Fig. 4), in which the correlation coefficient (R2) was 0.76. However, a pseudo second-order kinetic model has been considered to be the most appropriate over the past few years . As a result, if a pseudo second-order equation is adequate, the rate equation for the reaction may be represented by the following expression:
Fig. 4. Lagergren first-order Sorptional kinetics of N-NH4 by FHMAS.
(5) …where k2 is sorption rate constant (mg/(g·min)). From Eq. (5), a plot of t/qt versus t should give a straight line to confirm the applicability of the second-order kinetic model (Fig. 5). Table 1 contains the sorptional kinetics parameters of N-NH4 by FHMAS. From the correlation coefficient (R2) of two lines it can been seen that the pseudo second-order kinetic (R2 = 0.99) was more fitted than that of Lagergren first-order kinetic (R2 = 0.76) for the sorption process. Adsorption Isotherms Langmuir adsorption isotherm model was tried to fit to the experimental adsorption data by linear estimation based on least-square method and the equation shown below:
Fig. 6. Langmuir adsorption model of N-NH4 by FHMAS.
Removal of Ammonia Nitrogen from Wastewater...
Table 1. Sorptional kinetics parameters of N-NH4 by FHMAS. Sorptional kinetics
Sorption rate constant
Lagergren first-order kinetic
Pseudo second-order kinetic
Table 2. Langmuir and Dubini-Radushkevich isotherm parameters estimated with the equilibrium adsorption data for N-NH4 by FHMAS at 20ºC. Langmuir
(7) (6) …where Ce is equilibrium concentration (mg/l), qmax is maximum adsorption capacity (mg/g), and b is relative energy of sorption (l/mg). The value of qmax and b were calculated from the intercept and slope of the lines in diagram of (1/qe) versus (1/Ce). As shown in Fig. 6, the regression correlation coefficients of plot of (1/qe) versus (1/Ce) gave a straight line for N-NH4 adsorption by FHMAS. The Langmuir model fit well the experimental data based on the obtained determination coefficients (R2 = 0.997) and isotherm parameters shown in Table 2. The maximum adsorption capacity (qmax) of FHMAS, obtained in this research for N-NH4, can be compared with other adsorbents’ N-NH4 uptake as shown in Table 3. It can be observed that the FHMAS had adsorption capacities for N-NH4 of 7.43 times higher than that of the bentonite . In addition, the Dubini-Radushkevich isotherm model (Eq. 7) was tried to fit to the experimental adsorption data by lineal estimation based on the least-square method:
Table 3. Comparison of qmax of N-NH4 on different adsorbents. Adsorbent
Rare earth absorbent
…where qm is Dubinin-Radushkevich monolayer capacity (mg/g), β is constant related to adsorption energy (mol2/ J2), ε is Polanyi potential (J/mol) related to equilibrium concentration Ce. (8) …where R is gas constant (8.314 J/mol) and T is absolute temperature. The constant β is related to the mean free energy of adsorption per mol of the adsorbate (E, kJ/mol) when it is transferred to the surface of the solid from the solution, and E can be estimated as follows:
(9) Table 2 shows the Dubini-Radushkevich isotherm parameters for N-NH4 by FHMAS. The qm value shows less adsorption capacity in comparison to Langmuir isotherm. Moreover, the free energy of adsorption values (4-40 kJ/mol) suggests that physical the adsorption process occurs at the adsorbent surface . Treatment of Metallurgical Wastewater High concentrations of N-NH4 are commonly present in industrial wastewater such as metallurgical, tannery, textile, landfill leachate, and fertilizer wastewater . The real wastewater used in this study was obtained from a metallurgical plant in Ganzhou City, P. R. China. The metallurgical wastewater quality before and after treatment by FHMAS is listed in Table 4.
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Table 4. Water quality before and after treatment of metallurgical wastewater by FHMAS. Items
Emission standard 
After ammonia stripping the concentration of N-NH4 was found to be about 116 mg/l. Conventional treatment methods could not effectively remove the medium-low concentration of N-NH4. The FHMAS allows an alternative technique and showed encouraging performance on treating N-NH4 from the wastewater. The results after treatment by FHMAS were list in Table 4. When ferric hydroxide 0.15 mol/l, dosage 3.5 g/l, and pH 8 contact 30 min at room temperature, initial N-NH4 concentration and COD being 116 mg/l and 105 mg/l; ammonia in actual metallurgical wastewater was reduced to 11 mg/l and COD to 58 mg/l for N-NH4 and COD after treatment by adsorption achieves pollutant concentration limits in solution for a new enterprise in the “Emission Standards of Pollutants from Rare Earths Industry” (GB264512011) . From the adsorption capacity for N-NH4 30 mg/g, which was smaller than 32.7 mg/g at adsorption equilibrium for simulated N-NH4 solution, it can be seen that some cations and organic constituents in metallurgical wastewater can affect the efficiency of N-NH4 removal. And the concentration of Fe ion in wastewater increased because of cation exchange. Therefore, FHMAS was preliminarily feasible for treating N-NH4 from real wastewater.
Conclusions The presented research was carried out to explore a new adsorbent – modified activated sludge – for treating medium-low concentrations of N-NH4 from wastewater. The experimental results showed that sludge modified by 0.15 mol/l ferric hydroxide can greatly improve the removal for N-NH4. Influencing factors such as pH, concentration of modifier, contact time, and N-NH4 initial concentration showed the serious influence of N-NH4 removal. The kinetics study indicated that the sorption process was better described by the pseudo second-order equation than that of Lagergren’s first-order equation. An adsorption isotherm study indicated that FHMAS had higher adsorption capacities for N-NH4 than that of the clinoptilolite. The mechanisms of removal of N-NH4 by FHMAS were coexistence of adsorption and cation exchange. Metallurgical wastewater containing mediumlow concentrations of N-NH4 was treated by FHMAS. Initial N-NH4 concentration of 116 mg/l was reduced to 11 mg/l in 30 min by 3.5 g/l FHMAS at pH 8.
Acknowledgements The authors gratefully acknowledge financial support from the project of the National Natural Science Fund of China (No. 51568023), the National Science and Technology Pillar Project of the People’s Republic of China (No. 2012BAC11B07), and the Jiangxi Provincial Department of Education of China (No. GJJ14419). Additionally, the authors would like to express their sincere appreciation to the anonymous reviewers for their helpful comments and suggestions.
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