Ammonium Ion Adsorption and Settleability Improvement Achieved in a Synthetic Zeolite-Amended Activated Sludge

1 This is an Accepted Manuscript of an article published by Elsevier on Chinese Journal of Chemical Engineering, Volume 21, Issue 9, on September 2013 available at: https://doi.org/10.1016/S1004-9541(13)60566-2 Copyright Elsevier 2013. En idUS: Licencia Creative Commons CC BY-NC-ND Ammonium Ion Adsorption and Settleability Improvements Achieved in a Synthetic Zeolite-Amended Activated Sludge * Emilia Otal**, Luís F. Vilches, Yolanda Luna, Rodrigo Poblete, Juan M. García-Maya and Constantino Fernández-Pereira Department of Chemical and Environmental Engineering, School of Engineering, University of Seville, Camino de los Descubrimientos s/n, 41092 Seville, Spain Abstract Municipal wastewater treatment plants typically exhibits two classic problems: high ammonium concentration in water after conventional biological treatment and, in some cases, poor activated sludge sediment ability. Potential solutions to these problems were investigated by adding a synthetic zeolite obtained from coal fly ash to different steps of an activated sludge treatment. The experimental results for ammonium removal fit well with the theoretical adsorption isotherms of the Freundlich model and showed a maximum adsorption capacity of 13.72 mg g-1. Utilisation of this kind of zeolite to improve activated sludge sediment ability is studied for the first time in this work. It is found that the addition of the zeolite (1 g L-1) to an activated sludge with settling problems significantly enhanced its sediment and compact ability. This was confirmed by the Sludge Volume Index (SVI), which was reduced from 163 mL g-1 to 70 mL g-1, the V60 value, which was reduced from 894 mL L-1 to 427 mL L-1, and the zeta potential (ζ), which was reduced from -19.81 mV to -14.29 mV. The results indicate that the addition of this synthetic zeolite to activated sludge, as an additional waste management practice, has a positive impact on both ammonium removal and sludge settleability. Keywords Low-cost sorbents, nitrogen, settleability, zeolite adsorption, zeta potential. Received, 2012-09-07, accepted 2013-01-17 * Supported by the Spanish Ministry of Science and Innovation, under the project FOXMORE (CTM2006-05114). ** To whom correspondence should be addressed. Email: [email protected] 2 1 INTRODUCTION Wastewater discharge from municipal wastewater treatment plants (WWTP) usually contains high concentrations of nitrogen and phosphorous that may lead to eutrophication, cause depletion of dissolved oxygen and can be potentially toxic for aquatic life. To prevent these problems, discharge concentration limits that are safe have been adopted in several countries, but achieving these low levels may be a challenge for some WWTP. The reason why secondary effluents from conventional WWTP have high nitrogen and phosphorus concentrations is well known, since the carbon-to-nitrogen and carbon-to-phosphorus ratios are higher in urban wastewater than in the equilibrated metabolism of biomass. To achieve an acceptable level of nutrients (nitrogen and phosphorus) in the secondary effluents of urban wastewater, numerous advanced methods (chemical and biological) have been developed. An additional problem in WWTP is poor operation due to the low settling characteristics of the activated sludge. One reason for this is its charged colloidal structure (commonly in negative charge), which inhibits particle aggregation. In most cases, coagulation and flocculation of these colloidal suspensions allow breaking the colloidal structure and neutralization of the particle zeta potential, thus improving the settleability of the sludge. Moreover, sludge density depends mainly on the ratio of organic to inorganic material, as the inorganic fraction increases sludge density. The addition of inorganic ballasting agents in batch and/or continuous operation as a strategy to solve the problem has been tested [1-10], and it is concluded that the addition of mineral material improves activated sludge settling ability [3-10] and that sludge dewatering can be enhanced by adding the mineral material saturated with some cations (Ca or Na) [7]. For ammonium removal, natural zeolites, which are hydrated aluminium-silicate minerals with cation exchange capacity, have been widely used [11-14]. The material can be added at different stages of the wastewater treatment process or used in packed beds. An alternative to natural zeolites is the synthetic zeolites obtained from fly ash, a by-product of coal power plants, which has been used for sustainability purposes in a 3 number of applications, mainly because of their ion exchange and adsorption capacities. The zeolites are synthesized using the hydrothermal [15-22] or the fusion methods [23, 24]. The synthetic zeolite that has been investigated in this work (CV-Z) is mainly composed of NaP1 and low levels of analcime and chabazite zeolites as well as the fly ash remaining after the hydrothermal alkaline activation. The synthetic zeolite has a cation exchange capacity of 2.7 meq g-1, equivalent to 60% zeolite content [25, 26]. CVZ can be considered a low-cost adsorbent [27] when comparing the cost effectiveness of the treatment using the synthetic zeolite to a commercial adsorbent (Zeolite Type A). This material has previously been tested for the removal of contaminants in industrial wastewaters. Those studies show that the use of CV-Z does not significantly improve COD removal efficiency, but it is a promising material for nitrogen and phosphorus removal [25, 27, 28]. The aim of this research was to investigate the ammonium adsorption capacity of CV-Z in urban wastewater, which is a multi-component aqueous solution and some competitive adsorption problems could occur [29]. Another objective of this study is to examine the effect of CV-Z on the activated sludge settling and dewatering performance of the WWTP, which had not been studied, and to assess the feasibility of its application on activated sludge effluents to simultaneously solve or alleviate both problems. 2 EXPERIMENTAL 2.1 Adsorption studies Urban wastewater was collected from the effluent of the primary settling tank of a WWTP in Seville (Spain). The sample was stored at 4 ˚C until use. CV-Z was obtained from the hydrothermal alkaline activation of coal fly ash from the Narcea Power Plant (Spain). To elucidate the ammonium adsorption capacity of CV-Z, batch experiments at room temperature (20 C) were performed in 200 mL volume flasks, using solutions that were prepared by dissolving 10, 25 and 50 mg L-1 of ammonium, as (NH4)2HPO4, in wastewater. For each ammonium concentration, the influence of the adsorbent loading 4 on the ammonium removal was investigated using different CV-Z concentrations, from 1 to 4 mg L-1. Control tests were carried out for all ammonium concentrations using blanks without CV-Z. The flasks were then mechanically shaken for 20 min at 150 r.min1. All tests were performed in duplicate. Ammonium concentrations in the filtered samples were determined using an Ammonium Selective Electrode (Crison Instruments). To determine the NH4+ adsorption capacity of the synthetic zeolite, the Freundlich and Langmuir models were used. The amounts of NH4+ adsorbed onto CV-Z were calculated from the difference between the initial and the remaining ammonium concentrations in the solution. The amount of NH4+ adsorbed per gram of synthetic zeolite, X/m (mg g-1) was given by (Co-Ce)V/m, where X is the mass of ammonium adsorbed onto CV-Z (mg), m is the mass of CV-Z (g), C0 is the initial ammonium concentration (mg L-1), Ce is the equilibrium liquid phase ammonium concentration (mg L-1) and V is the volume of solution (L). The Freundlich model shows that the ratio of ammonium adsorbed to the adsorbent is a function of the equilibrium liquid phase solute concentration. n 1 ee KCm/Xq  (1) where, K (L g-1) and n represent the Freundlich isotherm constants. The Freundlich coefficient K represents an indicator of adsorption capacity; 1/n indicates the adsorption intensity, while its reciprocal n represents the affinity factor for the Freundlich model. The Langmuir isotherm is another useful and simple isotherm for describing both physical and chemical adsorption. It is a two parameter model. )bC1/(bCQ m X qeeoe  (2) where Q0 is the Langmuir monomolecular layer capacity (mg g-1) and b is the Langmuir isotherm constant. 2.2 Settling tests Two different sludge samples collected from the same WWTP mentioned above, which differed in sedimentation qualities were used for the experiments. Batch settling 5 tests were performed to investigate the effect of CV-Z on sludge properties, such as sediment ability, monitored as settling sludge volume after 60 min (V60), and SVI. Sludge was used within the first hour after collecting. Settling tests were carried out in a 500 mL settling vessel at room temperature (20 C). The synthetic zeolite was added to the sludge as a dry powder at concentrations of 1, 2, and 3 g L-1. For the blank tests, no zeolite was utilized. The suspensions formed with CV-Z and activated sludge were stirred in a JarTest (Selecta) for 15 min at 100 r.min-1 and then settled for 90 min. Total suspended solids (TSS), V60 and SVI were established according to the Standard Methods for the Examination of Water and Wastewater [30]. Total nitrogen (TN) content was determined using a Shimadzu TOC-V CPH/TOC-V CPN analyzer, with a 5.0% coefficient of variation. Samples were filtered through a 0.45 μm cellulose acetate filter before TN analysis. All the analyses were conducted in duplicate, from which the average was calculated. Solutions of CV-Z in distilled water or activated sludge were centrifuged at 4000 r.min-1 for 10 min and the supernatant was taken and used for the experimental ζ measurements. pH was adjusted with 0.1 mol L-1 H2SO4 or NaOH solutions. The equipment used, Zetaphoremeter IV (ZetaCompact CAD), has a microprocessor unit which determines the electrophoretic mobility of particles, each value being the mean of ten acquisition data. The ζ values were calculated from the electrophoretic mobility values, with the aid of the O’Brien conversion theory. 3. RESULTS AND DISCUSSION 3.1 Ammonium ion adsorption of the synthetic zeolite The influence of adsorbent loading on ammonium removal has been investigated using different concentrations of (NH4)2HPO4 dissolved in wastewater, which ranged from 10 to 50 g L-1. For different initial ammonium concentrations C0, the effect of the adsorbent dose on the variation of ammonium concentration at equilibrium Ce, is shown in Fig. 1. As reported by other authors [31], the ammonium adsorption efficiency increased with the increase in the adsorbent dose for a constant ammonium concentration. The effect of the adsorbent concentration is the strongest: at C0 of 18.5 mg L-1, Ce slightly 6 decreased from 12.2 mg L-1 to 7.5 mg L-1 as the adsorbent dose increased from1 g L-1 to 4 g L-1, while it decreased more evidently, from 48.6 mg L-1 to 27.3 mg L-1, at C0 62.3 mg L-1. Figure 1 Variation of equilibrium ammonium concentration (Ce) in relation to the initial ammonium (C0) and adsorbent concentrations Data of ammonium adsorption to CV-Z were analyzed with the Freundlich and Langmuir models. Freundlich equilibrium isotherms at 20 C for CV-Z range of 1 to 4 g L-1 and the coefficients calculated from the isotherms are shown in Fig. 2. This model fits experimental data adequately, giving R2 values higher than 0.95. As the concentration of zeolite increases, 1/n (intensity of the reaction) increased. Under all the experimental conditions, n values are greater than unity (from 1.1416 to 1.8252), indicating that the ammonium ion is favourably adsorbed to the sorbent surface. It has been reported by other authors that n values between 1 and 10 represent beneficial adsorption [32-35]. As can be seen, the K coefficient decreases with an increase in CV-Z concentration. The maximum ammonium adsorption capacity of the zeolite (qe) is observed to be 13.7 mg g-1 in the 50 mg L-1 of (NH4)2HPO4 solution, for a CV-Z concentration of 1 g L-1 [Fig.2 (a)]. 0 10 20 30 40 50 60 020 40 60 80 C0 (mg L-1) Ce (mg L-1) 1 g L-1 CV-Z 2 g L-1 CV-Z 3 g L-1 CV-Z 4 g L-1 CV-Z 7 Figure 2 Freundlich adsorption isotherm curves and adjustable parameters for different CVZ concentrations at 20 C (error bars: standard deviations) Langmuir equilibrium isotherms at 20 C for CV-Z range of 1 g L-1 to 4 g L-1 solution are presented in Fig. 3. The linear form of the Langmuir isotherm fits experimental data adequately for 1, 2, and 3 g L-1 of CV-Z, giving R2 values above 0.95. However, the R2 value for 4 g L-1 is very poor (0.57). Langmuir constants (Q0 and b) were calculated by least squares analysis. The maximum monomolecular layer capacity, Q0 = 40.8 mg g-1, is obtained with the highest zeolite concentration [Fig.3 (d)]. However, data from this condition fit the Langmuir model very poorly, and the estimated values are always found to be higher than the experimental ones. y = 0.5964x + 0.1381 R2 = 0.9807 K = 1.3744 L g-1 1/n = 0.5964 lgCe lgqe a) 0 0.5 1 1.5 1 1.2 1.4 1.6 1.8 y = 0.5479x + 0.0899 R2 = 0.9506 K = 1.2300 L g-1 1/n = 0.5479 b) 0 0.5 1 1.5 0.8 1 1.2 1.4 1.6 1.8 y = 0.7179x - 0.06 R2 = 0.9994 K = 0.8710 L g-1 1/n = 0.7179 c) y = 0.876x - 0.3055 R2 = 0.9760 K = 0.4949 L g-1 1/n = 0.8760 d) 0 0.5 1 1.5 0.8 1 1.2 1.4 1.6 lgqe lgqe lgqe lgCe 0 0.5 1 1.5 0.8 1 1.2 1.4 1.6 lgCelgCe 8 Figure 3 Langmuir adsorption isotherm curves and adjustable parameters for different CV-Z concentrations at 20 °C (error bars: standard deviations) 3.2 Settling tests The effects of CV-Z concentration on the sludge settling properties were studied at three zeolite doses. During the settling tests, two samples of activated sludge with different settleability were evaluated: an activated sludge with “standard” settleability (AS1) and a sludge coming from the same WWTP but collected after an industrial discharge episode (AS2). Primary data of settleability, nitrogen removal and final pH after the zeolite treatments are shown in Table 1, and settleability curves are shown in Fig. 4. pH values of both types of sludge were increased due to the higher CV-Z dose, as CV-Z contained the remaining NaOH used in the zeolite synthesis. y = 0.0404x+1.5783 R2 = 0.9596 Q0= 24.7525 mg g-1 b= 0.0256 L mg-1 a) y = 0.0663x+1.6806 R2 = 0.9786 Q0= 15.0830 mg g-1 b = 0.0395 L mg-1 b) y = 0.0418x+1.7704 R2 = 0.9933 Q0= 23.9234 mg g-1 b = 0.0236 L mg-1 c) y = 0.0245x+2.4286 R2 = 0.5715 Q0= 40.8163 mg g-1 b = 0.0101 L mg-1 d) Ce(mg L-1) Ce/qe(g L-1) 0 1 2 3 4 020 40 60 Ce(mg L-1) Ce/qe(g L-1) 0 1 2 3 4 5 020 40 60 Ce(mg L-1) Ce/qe(g L-1) 0 1 2 3 4 010 20 30 40 Ce/qe(g L-1) Ce(mg L-1) 0 1 2 3 4 010 20 30 9 Table 1 Settling test parameters for aerobic sludge (AS1 and AS2) using three CV-Z zeolite loadings Test pH NH4+/mg L-1 NH4+ removal/% TN/mg L-1 TN removal/% TSS0/mg L-1 V60/mL L-1 SVI/ml g-1) AS1 7.5 22.9 - - 3,950 110 28 AS1 + 1g L-1 7.7 15.7 31 - - 4,980 105 21 AS1 + 2g L-1 8 10.4 55 - - 5,930 100 17 AS1 + 3g L-1 8.2 7.4 68 - - 6,770 105 16 AS2 8 - - 370 5,470 894 163 AS2 + 1g L-1 8.3 - - 260 30 6,075 427 70 AS2 + 2g L-1 8.6 - - 240 35 7,123 400 56 AS2 + 3g L-1 8.8 - - 210 43 8,140 480 59 (a) Activated Sludge 1 (b) Activated Sludge 2 Figure 4 Height vs. time of activated sludge in settling experiments (data:mean values of three measurements; standard deviations: below 3 mm) Fig.4 (a) shows the standard settling curves obtained for the AS1 sludge. As previously reported [36], the settling curves show the four stages of induction, constant rate, falling rate and compression periods. The AS1 sludge settled properly by itself and settleability raw data (Table 1) indicate that the sludge settles to 1/10 of the original 0 50 100 150 200 250 020 40 60 80 100 time (min) h (mm) a) AS1 AS1 + 1 g L-1 CV-Z AS1 + 2 g L-1 CV-Z AS1 + 3 g L-1 CV-Z 0 50 100 150 200 250 020 40 60 80 100 time (min) h (mm) b) AS2 AS2 + 1 g L-1 CV-Z AS2 + 2 g L-1 CV-Z AS2 + 3 g L-1 CV-Z 0 50 100 150 200 250 020 40 60 80 100 time (min) h (mm) a) AS1 AS1 + 1 g L-1 CV-Z AS1 + 2 g L-1 CV-Z AS1 + 3 g L-1 CV-Z 0 50 100 150 200 250 020 40 60 80 100 time (min) h (mm) a) AS1 AS1 + 1 g L-1 CV-Z AS1 + 2 g L-1 CV-Z AS1 + 3 g L-1 CV-Z AS1 AS1 + 1 g L-1 CV-Z AS1 + 2 g L-1 CV-Z AS1 + 3 g L-1 CV-Z 0 50 100 150 200 250 020 40 60 80 100 time (min) h (mm) b) AS2 AS2 + 1 g L-1 CV-Z AS2 + 2 g L-1 CV-Z AS2 + 3 g L-1 CV-Z 0 50 100 150 200 250 020 40 60 80 100 time (min) h (mm) b) AS2 AS2 + 1 g L-1 CV-Z AS2 + 2 g L-1 CV-Z AS2 + 3 g L-1 CV-Z AS2 AS2 + 1 g L-1 CV-Z AS2 + 2 g L-1 CV-Z AS2 + 3 g L-1 CV-Z 16 zeolites synthesized from coal fly ash for the purification of acid mine waters”, Environ. 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