ARCHITECTURE CIVIL ENGINEERING E NVIRONMENT The Silesian University of Technology No. 2/2019 doi : 10.21307/ACEE-2019-030 REMOVAL OF HARDNESS IN WASTEWATER EFFLUENT USING MEMBRANE FILTRATION Mariusz DUDZIAK a, Edyta KUDLEK b a Prof.; Faculty of Energy and Environmental Engineering, Institute of Water and Wastewater Engineering, Silesian University of Technology, Konarskiego 18, 44-100 Gliwice, Poland E-mail address: [email protected] b PhD; Faculty of Energy and Environmental Engineering, Institute of Water and Wastewater Engineering, Silesian University of Technology, Konarskiego 18, 44-100 Gliwice, Poland E-mail address: [email protected] Received: 15.04.2019; Revised: 9.05.2019; Accepted: 9.05.2019 Abstract The effluents from urban wastewater treatment plant are characterized by high concentrations of both calcium and mag - nesium salts which contribute to the hardness of this particular water flux. It applies primarily to places where the distri - bution systems draw water from underground sources. Using hard water, for instance, in households causes the domestic wastewater to be hard as well. The hardness of wastewater is not a normative indicator. However, it is an important scien - tific aspect in the field of water reclamation. As part of this work, research of the reduction of the overall hardness of efflu - ent from the selected urban wastewater treatment plant in the Upper Silesia (Poland) was commenced. After the prelimi - nary tests it was determined that, according to the common water hardness classification, the hardness of effluent from the researched treatment plant equals the hardness of hard water (350–550 mg CaCO 3/L). In order to reduce the hardness of wastewater effluent a membrane filtration, including nanofiltration and reverse osmosis, was proposed. The processes were performed comparatively with the use of composite pipe membranes of PCI Membrane System Inc. (USA). The membrane used for nanofiltration was AFC-30 and the one for reverse osmosis was AFC-80. In both cases the transmembrane pres - sure was 2.0 MPa, while temperature and feed linear velocity amounted to 20°C and 3.4 m/s, respectively. It was determined that after both the reverse osmosis and nanofiltration the treated wastewaters were very soft. Therefore, the use of these processes, for instance, for productive purposes, may be considered. It should also be borne in mind that the nanofiltration process was more favorable in terms of membrane effectiveness. Keywords: Effluent from wastewater treatment plant, Hardness of wastewater, Membrane filtration. 1. INTRODUCTION The effluents from urban wastewater treatment plant may be characterized by high concentrations of both Hardness is a measure of the amount of calcium and calcium and magnesium salts which contribute to the magnesium salt that is present in water. In general, hardness of this particular water flux [2]. It applies pri - surface water is characterized by lower hardness than marily to places where the distribution systems draw groundwater. Water with a hardness of up to water from underground sources. Using hard water, 100 mgCaCO 3/L is regarded as very soft, between for instance, in households causes the domestic waste - 100 and 200 mgCaCO 3/L as soft, between 200 and water to be hard as well. The hardness of wastewater 350 mgCaCO 3/L as medium-hard, between 350 and is not a normative indicator. However, it is an impor - 550 mgCaCO 3/L as hard and above 550 mgCaCO 3/L tant research aspect in the field of water recovery. as very hard [1]. In general, there are five types of water softening 2/2019 ARCHITECTURE CIVIL ENGINEERING ENVIRONMENT 141 M. Dudziak, E. Kudlek methods: distillation and groups of thermal, physical, cules that are to be removed from the water. The chemical and physico-chemical processes. membranes are characterized by increasingly smaller Distillation demineralizes the water flux completely pores and lower value of the volumetric flux of the because it removes all salts from the water [3]. In permeate, depending on whether it is the process of thermal method the breakdown of calcium and mag - microfiltration, ultrafiltration, nanofiltration or nesium salts is triggered by temperature exceeding reverse osmosis. Theoretically, the most concise 37°C [4]. The physical methods encompass various membranes, used in the process of reverse osmosis, high pressure membrane filtration processes, includ - pass through only water, the nanofiltration mem - ing the processes of reverse osmosis and nanofiltra - branes allow for the separation of ions of different tion [5, 6]. The chemical methods consist of chemical valence and the separation of organic substances, precipitation of insoluble precipitate or binding the while the ultrafiltration ones retain small suspen - calcium and magnesium ions into complex com - sions, colloids, bacteria and viruses. Microfiltration pounds with the help of various reagents, such as cal - membranes, the ones with the largest pores, allow to cium hydroxide (lime), sodium bicarbonate (soda), retain microsuspensions. Due to the physical struc - sodium hydroxide (caustic soda), phosphates, barium ture the hydraulic resistance of membranes is rising salts and others. [7, 8]. Physico-chemical methods are which makes it necessary to apply increasingly higher based on various ion exchangers that are capable of pressures. exchanging their own ions with ions found in the sur - The pressure membrane filtration is used in the fol - rounding solution [9, 10]. However, taking into lowing areas of water treatment [11–16]: account the fact that only membrane filtration is able • production of drinking water from seawater and to lower both the overall hardness and the amount of brackish water by reverse osmosis (RO) – desalina - other non-organic and organic substances, only this tion, process was taken into consideration for the treat - ment of effluents from the urban wastewater treat - • production of drinking water from groundwater ment plant. and surface water with the use of microfiltration and/or ultrafiltration (MF/UF), Membrane filtration allows for the separation of pol - lutants on molecular or ion level. This process is usu - • production of drinking water from groundwater ally used for the desalination of seawater and brack - and surface water with the use of nanofiltration ish water, in the preparation of ultra-pure water and, (NF) with and without the preliminary treatment more rarely, for water softening and the removal of by coagulation (C) and microfiltration (MF), radionuclides, heavy metals, nitrate ions and organic • preliminary preparation of seawater and brackish substances, including low molecular weight micropol - water with the help of MF/UF and/or NF in the lutants [11–16]. The data on the use of this process production of drinking water by RO, for the treatment of wastewater is also scarce. In • treatment of the process waters for closing water membrane filtration the driving force is the differ - circuits in industry by MF/UF and/or NF/RO, ence in the chemical potentials on both sides of the • production of ultra-pure water for industrial pur - membrane which may be obtained by different values poses by MF/UF and RO, of pressure, concentration, temperature or electric potential. During membrane filtration the feed flux • production of industrial water or drinking water by (feed) is divided into two fluxes: permeate passing wastewater treatment with the help of membrane through the membrane (filtrate) and the remaining bioreactors (MBR) and RO. solution (retentate or concentrate). The processes of The aim of this paper was evaluating the degree of membrane filtration may be run in dead-end or cross- reduction of the overall hardness of effluents from flow systems. In the dead-end system the feed passes the selected urban wastewater treatment plant in the perpendicularly to the membrane and in the cross- Upper Silesia (Poland) after membrane filtration, flow system in parallel to the membrane. The reten - including reverse osmosis and nanofiltration. tate may be recirculated or fed back into the system. The processes used in the membrane filtration of water fluxes are primarily those in which the driving force depends on the pressure difference on both sides of the membrane. The selection of an appropri - ate process depends on the size of pollutant mole - 142 ARCHITECTURE CIVIL ENGINEERING ENVIRONMENT 2/2019 REMOVAL OF HARDNESS IN WASTEWATER EFFLUENT USING MEMBRANE FILTRATION e T 2. MATERIALS AND METHODS N E Analytical methods M N The evaluation of the quality of the tested effluent O R from the urban wastewater treatment plant (WWTP) I V before and after membrane filtration was performed by N analyzing selected physico-chemical indicators E (Table 1). The conductivity and pH value of samples was measured with multifunctional measurement device CX-461 (ELMETRON). The absorbance in ultraviolet ( = 254 nm) was measured with UV VIS 1000 (Cecil Instruments) with 1 cm optical path length. The turbidity of samples was measured with Turbidimeter TN-100 (Eutech Instruments). The color measuremenλt was performed with spectrophotometer UV-VIS Spectroquant ® Pharo 300 ( = 340 nm) (Merck). The overall hardness was determined by means of versenate hardness test, in accordance with Polish Norm, PN – ISO 6059:1999 [17]. λ Table 1. The physico-chemical characteristics of effluent from waste - water treatment plant Indicator Unit Value Figure 1. pH - 7.41 The installation TMI 14 (1 – water coat, 2 – manometer, Color mgPt/L 90.00 3 – thermometer, 4 – flowmeter, 5 – regulating valves, 6 – high-pressure pump, 7 – control box with inverter, Turbidity NTU 0.86 8 – tank, 9 – drain from the tank, 10 – membrane module, Conductivity µS/cm 1031.20 11 – permeate outlet) Absorbance in UV 254 1/cm 0.234 Total hardness mgCaCO 3/L 358.00 The installation was made completely of steel and equipped with an intermediate tank with a volume of 20 L, high pressure pump with a capacity between 0.5 and The physico-chemical characteristics of effluent 3.0 m 3/h (type CRN 3) produced by Grundfos The tested effluent came from the selected urban (Denmark) and a control and measurement apparatus.