II USE OF RED MUD FROM BAYER PROCESS FOR NITROGEN AND PHOSPHORUS REMOVAL

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1 II-93 - USE OF RED MUD FROM BAYER PROCESS FOR NITROGEN AND PHOSPHORUS REMOVAL Alessandra Carucci ( 1) Associate Professor Giovanna Cappai Researcher Aldo Muntoni Researcher Annalisa Onnis PhD student Address ( 1) : DIGITA, Department of Geoengineering and Environmental Technologies, University of Cagliari - Piazza d Armi, 9123 Cagliari, Italy - Tel. +39/7/ Fax +39/7/ carucci@unica.it ABSTRACT Phosphorus and nitrogen are the major nutrients contributing to eutrophication of lakes and other waters. Current P removal technologies, such as chemical precipitation or 'Enhanced Biological Phosphate Removal', are either expensive or perform inconsistently. This paper presents results of an investigation on the removal of phosphorus and nitrogen from a synthetic influent by passage through sand amended with bauxite refining residue (red mud). It s well known in fact that red-mud (aluminum refining residue) is high in iron and aluminum sesquioxides which have a strong phosphorus adsorption capacity. The study was conducted using two columns packed with sand and different proportions of red mud. The adsorption capacities and the removal efficiencies of ammonia nitrogen (NH 4 -N) and phosphorus as orthophosphate (PO 4-3 -P) were obtained for different flow-rates (different contact times) and compositions of the inlet water streams. Batch experiments were also conducted to determine the adsorption capacities of red mud with respect to P, and the experimental data were fitted with the Langmuir and the Freundlich isotherm. The maximum adsorption capacity of the substrate for phosphorus was then evaluated from fits of the Langmuir isotherm to batch adsorption data for contact time 48 h, and that for nitrogen was evaluated through the columns experiment. KEYWORDS: Phosphorus and nitrogen removal, red mud, adsorption capacity, isotherms. INTRODUCTION In the last years several authors have proposed the use of red mud in wastewater treatment either with suspended or fixed bed systems. According to Akiyama [1], in consequence of laboratory experiences conducted with synthetic wastewater, the process of phosphorus removal using red mud is considerably affected by ph. Piga et al. [4] found that the phosphorus removal efficiency using red mud can reach 7-8% but red mud has to be treated with sulfuric acid or with HCl and FeCl 2. Executing batch tests Lopez [3] evaluated the capacity of adsorption of phosphorus as orthophosphate using red mud. The tests showed that the adsorption of phosphorus does not change sensitively for the changes of short contact times (3-6 hours,) while it increases for changes of longer contact times (24-48 hours). Besides, the amount of adsorbed P depends on its concentration in solution, and not on the number of the vacant sites on the adsorbent solid. The same author, by some tests in column, using as fillings a mixture of sand and red mud, verified the possibility of using it in the treatment of civil wastewaters; to do this the secondary effluent of a WWTP has been percolated through the columns, obtaining removal efficiency for ammonium nitrogen of about %, and for the orthophosphate of 1%. ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 1

2 Mediating some continuous tests, using Plexiglas columns filled with different proportions of red mud and sand, Ho et al. [2] verified the possibility of removing phosphorus and nitrogen from primary and secondary effluents of a municipal wastewater treatment plant. He proved that the removal efficiency of both, increased, either increasing the percent of red mud in the mixture, or treating primary waters rather than secondary. The phosphorus was adsorbed with a maximum efficiency of 91% and 99% for secondary and primary water, respectively. The ammonium-nitrogen was removed, partly for adsorption and partly for nitrification and denitrification processes. Experiences showed that average nitrogen removal efficiency was growing from 24 to 74%, by treating secondary and primary effluents, respectively. In the first case the ammonium-nitrogen was partly adsorbed and partly nitrified by the bacteria present in the sewage; considering the low availability of organic carbon in the secondary effluent denitrification was not occurring, and nitrate was present in the final effluent. In the second case, the availability of organic carbon allowed denitrification to occur determining a total decreasing of nitrogen in the final effluent. In this study the removal of phosphorus and ammonium was studied with continuous and batch experiments. Particularly, the influence of contact times on the phosphorus adsorption process and the interference of phosphorus on ammonium adsorption by red mud, were analyzed. To see the effect of the contact times the flow-rate was changed during the experimentation, while to see the effect of phosphorus on ammonium adsorption, two different influent compositions were tested, one containing both sources of nitrogen as ammonium and phosphorus as orthophosphate and another containing only ammonium. MATERIALS AND METHODS Properties of red mud The red mud was taken from the Eurallumina of Portovesme (Cagliari, Italy); its composition is shown in table 1 [5]. Before use, a pre-treatment of the RM was necessary: first it was dehydrated in stove at 4 C, afterwards, to reduce the dimensions of the coarse particles and make the particle size distribution uniform a manual comminution and a subsequent sieving at,85 mm was carried out. Table 1: Chemical composition of the red mud utilized during the experiment Humidity 36,93 Cu ppm 35 % LOI % 12,38 Pb ppm 144 Na 2 O % 12,6 As ppm 62 MgO %,86 Zn ppm 56 Al 2 O 3 % 17,91 Ba ppm 6 SiO 2 % 9,58 Cd ppm 23 P 2 O 5 %,2 Ni ppm 18 K 2 %,13 V ppm 1476 CaO % 7,77 Sb ppm 5 TiO 2 % 8,61 Ce ppm 19 MnO %,1 Zr ppm 115 Fe 2 O 3 % 3,45 Co ppm 5 The sand used, whose dimensions are between,74 and 2 mm, is the non magnetic fraction of a granite ( Ghiandone ); its function is to increase the bed porosity, to obtain a sufficient filtering rate otherwise reduced by the excessive impermeability conferred by high red mud fractions. Continuous adsorption experiments The study was conducted using two Plexiglas columns, with a 91,5 mm internal diameter and a height of 4 mm. The columns were packed to a depth of 3 mm from the bottom of the columns with ceramic beads (diameter 15 mm) and for 36 mm above it with a sand and red mud mixture, 3% (column 1) and % ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 2

3 (column 2) by weight of red mud, respectively. The construction characteristics of the two columns are summarized in table 2. Table 2: Construction characteristics of the two columns Mass of red mud [kg] Mass of sand [kg] Total mass [kg] Column length [mm] Column diameter [cm] Total volume [cm 3 ] Bulk density [g/cm 3 ] Column 1 Column 2,9 2,1 3, 4 91,5 263,2 1,14,6 2,4 3, 4 91,5 263,2 1,14 A tank of 5 l capacity was provided for each column to feed the influent by a peristaltic pump; the synthetic influent was a standard solution of nitrogen as ammonium-nitrogen (NH 4 + ) and phosphorus as orthophosphate (PO 4-3 ) with tap water, and the concentrations were chosen to simulate the characteristics of a municipal wastewater (6 mg/l of NH 4 -N and 15 mg/l of PO 4-3 -P). Two beakers of 5 l capacity were provided to collect the effluent from the columns. Figure 1 shows the scheme of the equipment used. Figure 1: Equipment used during the three phases. A: feed tank; B: peristaltic pump; C: sand and % red mud mixture; D: sand and 3% red mud mixture; E, F: beakers; G, H: ceramic beads. The study was divided into three phases. In Phase 1 the removal of nitrogen and phosphorus in mixtures of 3 and % red mud was compared. In Phase 2, the flooding cycles were conducted using the same concentrations of nitrogen and phosphorus in the influent, but the flow-rate was doubled. In order to avoid possible interference of phosphorus, due to the fact that red mud is high in iron and aluminum sesquioxides which have a strong affinity with phosphorus ions, and to increase the retention time in nitrogen adsorption, in Phase 3 the influent was changed, using a solution of a known concentration of ammonium-nitrogen, without any source of orthophosphate, and the flow-rate was reduced again. ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 3

4 Phase 1 During this first phase, the two columns were fed with the synthetic influent, whose concentrations of ammonium-nitrogen and phosphorus-orthophosphate were kept constant, for 52 days with a constant flooding rate of 32 ml/h. At the end of every flooding cycle the ammonium-nitrogen and phosphorus-orthophosphate concentrations in the effluent were measured using the Standard Methods [6]. Besides, for every flooding cycle the outlet volumes through the columns were determined. When ammonium removal stopped in both columns, and the ammonium already absorbed started to be left again, the second phase was initiated. Phase 2 This phase was characterized by the increase of the flooding rate, that changed from 32 ml/h to 62 ml/h. The aim of the changement was to see the effect of the flooding rate on the orthophosphate adsorption capacity of red mud, as well as to reach more rapidly the phosphate saturation of the columns. Higher flow rate determined a shorter retention time causing a decrease of phosphorus removal in both columns. The concentration of phosphorus-orthophosphate, as well as the outlet volumes, ph and flooding rate were measured daily, while the concentration of ammonium nitrogen was measured only occasionally. Phase 3 During this phase a new column was arranged, which replaced the column 1; the same procedure and the same proportion of sand and red mud as in the first two phases were used. The inlet to this column was changed, using as influent a standard solution of nitrogen as ammonium-nitrogen (6 mgnh 4 + -N/l) with tap water with a flooding rate of about 32 ml/h. The inlet to column 2 was kept constant. Batch adsorption experiments Mechanisms responsible for the phenomena of adsorption of phosphorus by red mud determine very long experimentation period. For this reason adsorption of phosphorus as orthophosphate by the RM was examined using also some batch tests, realized in parallel to the experimentation in column to determine the maximum capacity of adsorption, which was already found in the first phase for ammonium. Aggregated RM (1 g) was suspended in 25 ml of KH 2 PO 4 solution containing, 4, 8, 15 or mg/l of phosphorus (P) and stirred for 48 hours. At the end of the contact time the suspension was centrifuged and the P in the supernatant was determined. The amount of phosphorus adsorbed by RM was calculated by the difference between the added P and P remaining in solution at the end of the incubation. RESULTS AND DISCUSSION Continuous adsorption experiment: Phase 1 During the first phase an average of 34% nitrogen removal was obtained with 3% red mud, 27% removal with % red mud. The decrease in nitrogen removal with decreasing red mud content was caused by a decrease in the adsorption capacity. Nitrogen reached the saturation capacity after the 48 th day in the column with 3% red mud, and after the 39 th day in the column with % red mud, and the adsorption capacity was found to be 868 mg NH 4 + -N/kg RM, and 793 mg NH 4 + -N/kg RM, in the column 1 and 2, respectively. The difference of adsorption capacity of the red mud in the two columns could be due to the higher amount of red mud of column 1, with respect to column 2, which determined a lower permeability and a consequent longer contact time between solute (NH 4 + ) and adsorbent material. Figure 2 shows the ammonium-nitrogen concentration profiles for phase 1 in both columns. ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 4

5 NH 4 + -N (mg/l) Influent N column 1 column2 Figure 2: Ammonium concentration profiles during the 1 st phase An average of 1% of phosphorus removal was obtained in each column during the whole phase. The high removal efficiencies for the orthophosphate, which are obtained even after reaching the saturation capacity for the ammonium, can be explained with the higher affinity that red mud has with this ion than with the ammonium ion. Figure 3 shows the ammonium and phosphorus concentration profiles in the column with 3% of red mud. PO 4-3 -P (mg/l) NH 4 + -N (mg/l) Influent P Effluent P Influent N Effluent N Figure 3: Ammonium and Phosphorus concentration profiles in the column 1 during the 1 st phase Phase 2: During this phase the flow-rate was doubled; despite such increase, in the column with the higher amount of red mud the out-let concentration from the column was almost zero for further 16 days of feeding. Altogether, during the first phase and the first 12 days of the second, 1164,7 mg PO 4-3 -P/kg RM were removed. In column 2 the orthophosphate concentration in the effluent started to increase since the fourth day. Referring to the trend of the experimental data of input and output concentrations of phosphorus shown in Figure 4, we can observe, except for a transient time in which the orthophosphates passed from to about 7 mgp/l (in column 1 between the 16 th and the 35 th day, in the column 2 between the 4 th and 15 th day from the beginning of phase 2) an almost constant concentration of 7-9 mgp/l in the effluent for a long time period, with a constant P removal efficiency between 4 and 5%. The decrease of the adsorption capacity highlighted from the initial increase followed by a constant amount of phosphorus measured in the effluent, can be due to either the partial saturation of the column or to the increase of the flooding rate, that determines an increase of the interstitial rate and reduces the contact time and so the amount of ions that can reach the vacant sites on the surface of the adsorbent. ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 5

6 PO 4-3 -P (mg/l) Influent column 1 column 2 Figure 4: Orthophosphate concentration profiles during the 2 nd phase Phase 3: During this phase the influence on ammonium adsorption process of phosphorus, present in the feed to the columns in the previous phases, was studied. At this aim the influent was changed, using a solution of a known concentration of ammonium-nitrogen, without any source of orthophosphate. Figure 5 shows the ammonium nitrogen concentration profiles in the effluent of the column 1 (3% of red mud), during the first phase (influent containing both ammonium and orthophosphate sources) and the third phase (only ammonium source). The figure clearly shows that the amount of ammonium adsorbed by red mud is higher in the last phase; in fact the concentration of ammonium in the effluent, after more than 5 days, is kept constant to a value of 9-1 mgn/l, while in the first phase the saturation of the column was already reached after 48 days. The ammomium removal efficiency passes from an average of 34% in the first phase to 83% in the third phase; the maximum adsorption capacity of red mud with respect to ammonium was not calculated for this last phase because the saturation was not yet reached, but it is higher than 16 mg NH N/kg RM, that is already a value double respect to the value found in the first phase of the experimentation. NH 4 + -N (mg/l) Influent N Effluent N 1st phase Effluent N 3rd phase Figure 5: Ammonium concentration profiles during the 1 st and the 3 rd phase During this phase the trend of phosphorus concentration in the effluent of column 2 was still monitored. Figure 6 shows the phosphorus concentration profiles during the whole experimental study. It has to be noticed the increase of adsorbed phosphorus during the third phase, where the flow rate was reduced again to the initial value of 32 ml/h. This fact confirms that higher contact times allow higher adsorption capacities, even though the effluent concentration has not returned to the value of phase 1. ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 6

7 PO 4-3 -P (mg/l) Phase 1 Phase 2 Phase Influent P column 1 column 2 Figure 6: Phosphorus concentration profiles during the 3 phases Batch adsorption experiments Non linear Langmuir (1) and Freundlich (2) equations were fitted to the experimental adsorption data: Q K L Ce q = (1) 1 + K L Ce where q is the mass of P adsorbed per unit mass of the aggregated RM, Q is the maximum capacity of the substrate for adsorption of phosphorus, K L is a constant related to the energy of the adsorption-desorption process and C e is the concentration of P in solution at the end of the contact time. 1 = n (2) q K F C e where K F and n are constants. The experimental data and the isotherms fitted to them are shown in Figure P adsorbed (mg P/g RM) Experimental data Freundlich isotherm Langmuir isotherm ( / ) Figure 7: Best fits of the Freundlich and Langmuir isotherms to the experimental data of the batch test for a contact time of 48 h The values of the constants of the two isotherms, used to fit the experimental data, together with the corresponding determination coefficients are listed in Table 3. The isotherm that better fits the experimental data is the Langmuir. The maximum adsorption capacity of red mud with respect to phosphorus was found to be,24 mg P/g RM, using the best fits of the Langmuir isotherm. ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 7

8 Table 3: Fitting constants, and determination coefficients (R 2 ) Freundlich isotherm K F 7,425 Q,24 1/n,18 Langmuir isotherm K L,7 R 2,959 R 2,836 CONCLUSIONS In continuous experiments, using 2 columns packed with sand and different proportions of red mud, the effect on nitrogen and phosphorus adsorption, of the contact time, the kind of influent as well as the content of red mud in the columns, was analyzed. During the first phase an average of 34% nitrogen removal was obtained with 3% red mud, 27% removal with % red mud. The maximum adsorption capacity for ammonium was found to be 868 mg NH 4 + -N/kg RM, and 793 mg NH 4 + -N/kg RM, in the column 1 and 2, respectively. An average of 1% of phosphorus removal was obtained in each column during the whole phase. During the second phase the increase of the flow rate, that determines shorter contact times, caused the decrease of the removal efficiency of phosphorus that passed from 1% to a value between 4-5%. In the third phase a influent containing only an ammonium source was percolated through a new column with 3% of red mud. The nitrogen removal efficiency was higher than in the first phase and assumed a constant value of 83%. Batch experiments examining the adsorption of phosphorus as orthophosphate by red mud indicated that it has strong affinities with this ion. The maximum adsorption capacity, evaluated from fits of the Langmuir isotherm to the experimental data for a contact time of 48 hours was,24 mg P/g RM. These results confirm that red mud could be efficiently reused as filter material to remove P from wastewaters, due to its high adsorption capacity with respect to this element. Contact time has resulted an important parameter to be considered in the filter design. As to ammonium, the lower removal efficiency demonstrated by red mud, treating a mixture of P and N has been influenced by the higher affinity to P; in the case of a water only contaminated by NH 4 -N, the removal potential of red mud could be considered for a specific treatment application. BIBLIOGRAPHIC REFERENCES 1. AKIYAMA, M., TAKAHASHI, J., KOBAYASHI, KENJI. Removal of phosphorus from wastewater. Jpn. Kokai Tokyo Koho, 3 pp. Japanese Patent. 2. HO, G. E., MATHEW K. AND GIBBS R. A. Nitrogen and phosphorus removal from sewage effluent in amended sand columns. Water Resour. Research, v. 26, n. 3, p , LOPEZ E., SOTO B., ARIAS M., NUNEZ A., RUBINOS D. AND BARRAL M.T. Absorbent properties of red mud and its use for wastewater treatment. Water Research. v. 32, n. 4, p , PIGA L., POCHETTI F., STOPPA L. Recovering metals from red mud generated during alumina production. Hazard Ind. Wastes, 22 nd, p , AA.VV. Riutilizzo dei fanghi rossi Euroallumina. Piano di disinquinamento per il risanamento del territorio del Sulcis-Iglesiente, APHA. Standard methods for the examination of water and wastewater. 17 th ed. Washington DC, ABES - Associação Brasileira de Engenharia Sanitária e Ambiental 8