Nanofiltration for the Treatment of Coke Plant Ammoniacal Wastewaters

Separation and Purification Technology 76 (2011) 303–307 View metadata, citation and similar papers at core.ac.uk brought to you by CORE Contents lists available at ScienceDirect provided by Elsevier - Publisher Connector Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur Nanofiltration for the treatment of coke plant ammoniacal wastewaters Christa Korzenowski a,b, Miguel Minhalma a,c, Andréa M. Bernardes b, Jane Zoppas Ferreira b, Maria Norberta de Pinho a,∗ a Instituto Superior Técnico, ICEMS, Universidade Técnica de Lisboa (IST/ICEMS/UTL), Lisbon, Portugal b Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de Materiais (PPGEM), Universidade federal do Rio Grande do Sul (UFRGS), Porto Alegre, RS, Brazil c Instituto Superior de Engenharia de Lisboa (ISEL), Lisbon, Portugal article info abstract Article history: This work addresses the treatment by nanofiltration (NF) of solutions containing NaCN and NH4Cl at Received 20 July 2010 various pH values. The NF experiments are carried out in a Lab-Unit equipped with NF-270 mem- Received in revised form 25 October 2010 branes for model solutions that are surrogates of industrial ammoniacal wastewaters generated in the Accepted 26 October 2010 coke-making processes. The applied pressure is 30 bar. The main objective is the separation of the compounds NaCN and NH4Cl and the optimization of this separation as a function of the pH. Mem- Keywords: brane performance is highly dependent on solution composition and characteristics, namely on the pH. Coke plant wastewaters In fact, the rejection coefficients for the binary model solution containing sodium cyanide are always Ammonium Cyanides higher than the rejections coefficients for the ammonium chloride model solution. For ternary solutions Nanofiltration (cyanide/ammonium/water) it was observed that for pH values lower than 9 the rejection coefficients to Fractionation ammonium are well above the ones observed for the cyanides, but for pH values higher than 9.5 there is a drastic decrease in the ammonium rejection coefficients with the increase of the pH. These results take into account the changes that occur in solution, namely, the solute species that are predominant, with the increase of the pH. The fluxes of the model solutions decreased with increased pH. © 2010 Elsevier B.V. Open access under the Elsevier OA license. 1. Introduction eral authors reported difficulties in obtaining complete oxidation of NH3 in this kind of wastewaters. It seemed that nitrifying bacteria In the past years special attention has been given to industrial are inhibited by high cyanide concentrations [4]. The introduction ammoniacal wastewaters generated in the coke-making processes. of such wastewaters to an activated sludge system may result in Wastewaters containing ammonia are generated on coke-making loss of activated sludge viability and may change sludge commu- processes, since ammonium is removed from the exhaust gas by nity structure. Stringent limits for coke oven effluents have been adsorption onto water in order to reduce its concentrations to set in countries where these wastewaters represent a major prob- acceptable levels in the gas outlet of the plant, i.e. to around lem, such as Germany, and the corresponding guidelines include 0.1gm−3. In a typical coke plant, the wastewaters from the gas- the requirements for coke oven effluents before being discharged washing towers and the condensed waters from the coke oven have to a water receiver and prior to mixing with other effluents. Very high ammonia concentration, and besides that they have toxic and low concentrations are required for certain pollutants like cyanides carcinogenic substances, such as phenol, cyanide and thiocyanate, (<0.03 mg L−1) [5]. that are biologically refractory organic compounds and considered In comparison with the large amount of literature regarding priority pollutants [1–3]. The industry uses different methods for conventional treatments for ammoniacal wastewaters contami- ammonium removal and the choice is highly dependent on its con- nated with priority pollutants like cyanides, the literature is very centration and type of contaminants. The more frequently used scarce regarding to non-conventional and more adequate treat- ones are the biological treatments that generally involve nitrifi- ments for ammoniacal wastewaters contaminated with priority cation/denitrification processes. Various bacterial strains are able pollutants [6–10]. to perform cyanide, thiocyanate and NH3 oxidation. However, sev- The pressure-driven membrane separation processes (micro- filtration, ultrafiltration, nanofiltration and reverse osmosis) and namely nanofiltration (NF) have been playing a major role in the ∗ Corresponding author at: Department of Chemical and Biological Engineering, development of advanced wastewater treatments. In fact NF has Instituto Superior Técnico, ICEMS, Universidade Técnica de Lisboa (IST/ICEMS/UTL), the unique feature of selective permeation to target components Av. Rovisco Pais, 1049-001 Lisbon, Portugal. Tel.: +351 218417488; of multi-component solutions, making possible the confinement in fax: +351 218499242. the permeate stream of pollutants like the cyanides and therefore E-mail address: [email protected] (M.N. de Pinho). 1383-5866© 2010 Elsevier B.V. Open access under the Elsevier OA license. doi:10.1016/j.seppur.2010.10.020 304 C. Korzenowski et al. / Separation and Purification Technology 76 (2011) 303–307 Table 1 Membrane characteristics of NF270. Composition top layer Polyamide MWCO (Da) 270 Roughnessa (A)˚ 28 Contact angle (◦)27 Zeta potential (mV) pH 7 −19 pH 10 −24 a Roughness measured with non-contact mode AFM on a scan area of 1 ␮m × 1 ␮m. where B is a parameter characteristic of a given membrane/solute system and is defined by B = DAm˚/L. In steady-state the flux of solute A trough the membrane is also Fig. 1. Concentration profiles of solute A in a nanofiltration membrane and in the given by: fluid phases adjacent to the membrane. Where L is the membrane thickness, C , Ab J = J C CAm and CAp are the concentrations of solute A in the feed, at the membrane surface A p Ap (4) (fluid phase side) and in the permeate and CAm and CAp are the concentrations of solute A inside the membrane at the feed and permeate side, respectively. where Jp is the permeate flux. Combining Eqs. (3) and (4) and using the definition of intrinsic decontaminating the ammoniacal feed solutions [11,12]. This fea- rejection, the following equation is obtained: ture arises from the fact that NF is ruled not only by sieving J f = p (5) mechanisms but also by electrostatic interactions. Jp + B The main objective of this study is the investigation of nanofiltration capability of fractionating ammonium/cyanide containing 3. Materials and methods solutions, with respect to the confinement of the ammonium and cyanide in two separate streams that can be object of more specific 3.1. Model solutions treatments. The goal is to investigate the influence of the pH solution on the performance of the nanofiltration of coke plant effluents Binary aqueous model solutions containing CN− (MS1) and + namely the rejection coefficients to ammonium and cyanide ions. NH4 (MS2) in a concentration similar to the ones founded on coke For that, NF permeation experiments are carried out for wastewaters were used. Aqueous ternary solutions of ammonium aqueous binary solutions of ammonium and of cyanide (ammo- and cyanide were also prepared (MS3). Model solution 1 contains −1 −1 nium/water and cyanide/water) and for aqueous ternary solutions 0.18 g L of NaCN, model solution 2 contains 18.8 g L of NH4Cl of ammonium and cyanide (ammonium/cyanide/water). The NF and model solution 3 contains 0.18 g L−1 of NaCN and 18.8 g L−1 of experiments are carried in a pH range from 7 to 11. The comparison NH4Cl. The pH was varied between 8 and 11 and sodium hydroxide between binary and ternary solutions results will show if there is solutions were used for pH adjustment. The reagents used were of a change in the solute/membrane interactions when the different analytical purity and the water was deionized. solutes are mixed together as is the case in the real effluent solutions. The solute/membrane interactions may be described by the 3.2. Membrane solution/diffusion model, where a B parameter is used to quantify the affinity of a given solute to the membrane. The nanofiltration membrane selected was the NF270, supplied by Filmtec Corp., Minneapolis, MN (USA). It is a polyamide thin- 2. Theory film composite membrane and was characterized in terms of the hydraulic permeability, Lp, and in terms of rejection coefficients to The diffusive permeation of a component A through a NF mem- reference solutes – NaCl and Na2SO4. These experiments were car- brane (represented in Fig. 1) is described by Fick equation: ried out at a transmembrane pressure of 30 bar, 25 ◦C, a Feed flow − ∂C rate of 9.2 L min 1. Some of the characteristics of this membrane J =−D A (1) A Am ∂x are summarized in Table 1 [15]. where J is the diffusive flux of solute A, D is the solute diffusion A Am 3.3. Nanofiltration permeation experiments coefficient inside the membrane, CA is the solute concentration and x is the distance inside the membrane. The NF experiments were performed in a nanofiltration plant The relationship between solute A concentrations in the fluid (DSS Lab-Unit M20) with 0.072 m2 of membrane surface area. phase adjacent to the membrane and in the membrane side is given Membrane conditioning was carried out through the circula- by the partition coefficient defined as: − tion of deionized water (conductivity <1 ␮Scm 1) pressurized at C 30 bar for 2 h. This avoids pressure effects on membrane structure ˚ = A (2) CA in subsequent experiments. The nanofiltration experiments were carried out in total recircu- The integration of Eq.

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