Performance of MF and RO processes for water recycling: Advanced treatment after conventional wastewater treatment.

Jawad Al-Rifai a,b, Hadi Khabbaz c, William E. Price d

a Assistant Professor , C ivil Engineering Dept., Engineering Faculty, Philadelphia University, Amman, Jordan ,

c Associate Professor, Australian Geomechanics Society (AGS) , School of Civil and Environmental Engineering , U niversity of Technology, Sydney, City Campus.

d Professor, Australian Institute for Innovative Materials (AIIM) , University of Wollongong, Wollongong.

*Corresponding author. Tel.: 009 62797960453

d Present address: Civil Engineering Dept., Engineering Faculty, Philadelphia University,

Jerash Road, Amman 19392, Jordan.

Abstract

This study attempts to reveal the fate of both organic and inorganic trace contaminants including endocrine disrupting compounds (EDCs), pharmaceutically active compounds (PhACs) and metals in dual membrane processes. For this purpose, water recycling plant operating with microfiltration (MF) and reverse osmosis (RO) membranes was selected. The partition of trace contaminants between RO permeates and brine was investigated. The RO membrane was found to serves as a large reservoir for bulk organic matter as well as trace organic compounds (TOCs) to the adsorption of contaminants on membranes and their likely release in the brine. There was no clear relationship between the content of organic carbon and nitrogen in influencing the distribution of both PHACS and EDCS in the RO streams. This might be due to the complexity of RO feed in treatment plants and the difference between field experiments compared to bench scale experiments where synthetic water was used by other researchers

The RO membrane concentrates the inorganic compounds (anions, cations and heavy metals) in the brine by a factor of between three and five. Furthermore, the concentrations of TOCs reached μg/L levels in the brine with concentration factor ranged between one and five for all detected trace compounds. These results suggest that the tightness of the RO membrane is more significant contributor to compound rejection when compared to compound properties (hydrophobicity, water solubility and molecular weight).

Key words : Mass balance; Pharmaceutically active compounds ; E ndocrine disrupting compounds ; Brine; R everse osmosis .

1. Introduction

Advanced wastewater treatment systems combine a microporous membrane process such as ultrafiltration (UF) and microfiltration (MF), followed by RO membrane. This combination has become the industry standard practice for the reclamation of municipal wastewater for industrial and indirect portable reuse applications.

Many attempts have been made to estimate the performance of membrane separation in order to predict the mass balance through membranes (Williams et al, 1999; Bowen et al, 2002). Most of these models are based on one or more compounds in base water and require sophisticated solution techniques. However, prediction of removal efficiencies for organic constituents is much more challenging than calculations for inorganic compounds (ICs) since the water properties of the compounds and interactions with membrane properties significantly affect the compound’s mass transfer (Williams et al, 1999; Van der Bruggen and Vandecasteele, 2002). Siegrist & Joss

(2012) reviewed principles and capabilities for the most important membrane-based applications for wastewater treatment. Bellona et al. (2004) have conducted a comprehensive survey in order to identify factors affecting the rejection of organic compounds in NF or RO membranes. A complete understanding of the solute and membrane characteristics that influence rejection could lay the foundation for modeling the fate of specific compounds during a high-pressure membrane application (Bellona et al, 2004).

Research investigating the viability of NF/RO membranes has reported the incomplete rejection of organic micropollutants such as endocrine disrupting chemicals (EDCs), pharmaceutically active compounds (PhACs) and others (Kimura et al, 2003; Sch?fer et al, 2003; Kimura et al, 2004; Nghiem et al, 2004). Most of these studies examined the rejection of micropollutants from a bench-scale flat sheet membrane unit or by using a dead end filtration module and high feed water solute concentrations. In addition, most of these experiments utilized deionized water spiked with one or more target solutes and a virgin membrane neglecting solution matrix effects and fouling commonly observed in full scale applications. These factors may lead to overestimation of membrane efficiency due to neglecting such factors. Furthermore, most researchers focus on NF membrane and rejection mechanisms of micropollutants (e.g., PhACs and EDCs) under influence of various factors such as natural organic matter and cations (Comerton et al, 2009); organic fouling (Agenson and Urase, 2007); solute and solution properties (L?pez-Mu?oz et al, 2009); membrane fouling (Yangali-Quintanilla et al.).

The inorganic compounds (ICs) of wastewater are derived from either natural water or added through domestic, commercial and industrial usage. The presence of ICs in water can cause significant concern with respect to drinking water quality, aesthetics and industrial use. The concerns over ICs range from their toxicity to their impact on process operation, product operation and product quality in industrial processes.

A significant part of the anthropogenic production of ICs ends up in wastewater. Major industrial sources include surface treatment processes with elements such as Cu, Zn, Ni, and Cr, as well as industrial products that at the end of their life are discharged in wastes. Wastewater treatment plants (WWTPs) are expected to control the discharge of ICs to the environment. Karvelas et al. (2003) investigated the occurrence and the fate of eight heavy metals (i.e, Cd, Pb, Mn, Cu, Cr, Zn, Fe, and Ni) during WWT processing in the city of Thessaloniki in Northern Greece. They found that the heavy metals were detectable in the wastewater samples in a range of μg/L with frequency of occurrence of about 90-100%.

This study attempts to reveal the mass balance of both inorganic and organic trace compounds in dual membrane processes through the water recycling plant (WRP). The mass balances of the bulk organic constituents, anions and cations and PHACS AND EDCS s through the RO membrane were evaluated and the partition between RO permeates and brine in the RO was investigated. Mass balance was also assessed in relation to Water properties of the compounds. The quality of different streams of membranes were assessed using the general characteristics (e.g., pH, chemical oxidation demand (COD), nitrogen and phosphorus) to evaluate the treatment processes efficiencies.

2. Materials and Methods

2.1 Facility Overview

WRP investigated in study is located in Australia. The WRP produces 8.8 ML/day of water and is capable of producing of 10.6 ML/day. The product water is delivered to a nearby refinery as cooling tower make up, boiler feed water and other process uses. The WRP returns the rejects (MF back flush and RO brine) back to the head of the WWTPs. The reject flows are 30% or approximately 3.6 ML/day when the plant is producing 8.8 ML/day. The WRP is receiving secondary effluent from municipal WWTP. The municipal wastewater passes through a grit removal unit and a diffused air activated sludge process, while WRP comprises of automatic backwashing 300μm screens, MF and RO membranes as shown in Figure 1.

2.1.1 Microfiltration

The MF membrane itself is protected from gross solids by the 300 μm screens (called Amiad screens). Six MF racks are fitted with 66 filter modules installed in each rack. The MF uses 0.1 micron membranes, with a typical recovery of 97%. It is operated in cross-flow mode with 5-10% recirculation flow to maximize membrane usage and flux. Backwashing is carried out at 20 minute intervals. Compressed air bubbling proceeds every backwashing cycle, shaking off accumulated materials. A clean in place (CIP) process is carried out with NaOH / NaOCl and citric solutions on a monthly basis.

2.1.2 Reverse Osmosis

RO membrane is the next stage taken up after MF. The RO treatment is a 3-stage process per block with an array of 18, 8 and 5 pressure vessels in first, second and third stage, respectively. The RO system consists of six RO blocks. Each RO block has thirty-one pressure vessels. Each pressure vessel houses six RO membrane elements. The elements are formed as Spiral Wound. The membranes are periodically cleaned by flushing water across the membrane and by a chemical CIP process which involves the flushing of HCl and caustic solution through the RO pressure vessels.

The overall recovery from the RO system is 85%. The RO membranes are progressively fouled and therefore a CIP procedure is carried out approximately every 6 months initiated by the operators. In order to prevent bio-fouling, chlorine is added to the MF filtrate in order to maintain it at about 1-3 mg/L. The RO membranes do not have a high tolerance level to free chlorine. Hence, ammonia is added to the MF filtrate to convert free chlorine to chloramine with a level of 2 mg/L. Finally, a post chlorination of the RO permeate is performed to maintain a chlorine residual to meet the product water specification. Furthermore, additional chemical treatment is used to adjust water pH and chlorination. The RO membrane used in the plant is BW30 365 FR (fouling resistant RO) manufactured by DOW/ Filmtec in a flat sheet configuration. The BW30 365 is a polyamide thin film with 99.995% solute rejection.

2.2 Target Analytes

2.2.1 Organic Trace C ompounds

These compounds were categorized in two main groups: EDCs (i.e. bisphenol A and nonylphenol) and PhACs (e.g., salicylic acid, diclofenac and carbamazepine. Selected PhACs represent a wide range of water properties of organic compounds were classified into acidic and neutral compounds. Table 1 shows molecular weight, partitioning coefficient (log Kow), dissociation constant (pKa) and solubility of the target PhACs and EDCs. These characteristics are used in predicting their behavior under clinical conditions and are used in the environmental assessment (Table 1).

2.2.2 I norganic C ompounds

More than thirteen of ICs were included in this study. These compounds were Aluminum (Al), Arsenic (As), Barium (Ba), Boron (Br), Cadmium (Cd), Calcium (Ca), Chromium (Cr), Cobalt (Co), Copper (Cu), Fluoride (F), Iron (Fe), Magnesium (Mg), Manganese (Mn), Molybdenum (Mo), Nickel (Ni), Lead (Pb), Potassium (K), Selenium (Se), Silicon (Si), Sodium (Na), Sulfur (S) and Zinc (Zn).

2.3 Sample Campaigns

Samples were taken from various inlets/outlets of the processes of WRP as shown in (Figure 1). Manual grab-sample collection was accomplished by either pouring directly from a tap into a 2 L amber glass bottle or by the use of a small bucket and pouring into the bottle. The glass bottles placed on ice using an ice box to keep the samples cold and transferred overnight to the laboratory. Seven sets of samples were taken through the whole year.

2.4 Analytical Methods

Total organic contents (TOCs) were determined by TOC analysis (Method 5310B) (American Public Health Association (APHA), 2005) using a Shimadzu TOC-VCSH (Total Organic Carbon Analyzer) equipped with ASI-V auto-sampler (Alvarez-Salgado and Miller, 1998). The TOC was determined by measurement of non-purgeable organic carbon (NPOC- the fraction of TOC not removed by gas stripping). The nitrogen content was determined by measuring total nitrogen (TN) using a Shimadzu Total Nitrogen Module (TNM1) coupled with the Shimadzu TOC–VCSH using a chemiluminescence detector (Alvarez-Salgado and Miller, 1998). Ultraviolet absorption (UV) was measured at wavelength of 245 nm according to Method 5910 using Shimadzu UV- Visible Spectrophotometer (model UV 1700 Phara Spec) (APHA, 2005). Turbidity (T) measurements were performed according to Method 2130 (American Public Health Association (APHA), 2005) using a turbidity meter (HACH, model 2100N, HACH, S.A/N.V, USA). Electrical conductivity (EC) measurements were performed according to Method 2510 (Lisitsin et al, 2005) with a conductivity meter as a surrogate parameter. Measurement of pH was performed according to Method 4500-H+ (American Public Health Association (APHA), 2005) with a pH meter.

Trace inorganic compound (ICs) (i.e. cations, anions and heavy metals) were measured by Inductively Coupled Plasma-Atomic Emission Spectroscopy (Method 3120) according to the Standard Methods for the Examination of Water and Wastewater (American Public Health Association (APHA), 2005). Elements and their MDL (mg/L): Al (0.005), As (0.005), Ba (0.005), Br (0.010), Cd (0.001), Ca (0.2), Cr (0.002), Co (0.005), Cu (0.001), F (0.001), Fe (0.002), Mg (0.2), Mn (0.001), Mo (0.01), Ni (0.002), Pb (0.005), K (0.5), Se (0.01), Si (0.05), Na (0.5), S (1) and Zn (0.005).

Upon receipt in the laboratory, samples were filtered using three different filters, GF/D (2.7 μm) and GF/F (0.7 μm) Whatman filters and 0.48 μm Nylon filter membrane (Alltech, Australia). Filtered samples were kept in amber bottles overnight at 4oC. The next day, the samples were allowed to reach room temperature and adjusted to pH 2-3 by addition of 4 M sulfuric acid to enhance trapping of acidic compounds on the solid phase extraction (SPE) sorbent. MilliQ water (1 L) was also spiked with a standard mixture of the investigated compounds to confirm recovery of analytes. Samples were analyzed in batches consisting of 5-6 samples, spiked samples and a blank.

For solid phase extraction (SPE), 60 mg Water Oasis HLB sorbent cartridges (Waters, Australia) were used. The SPE was performed on a 24-fold extraction manifold (Supelco, Visiprep 24). The SPE cartridges were conditioned sequentially with 5 mL methyl tetra-butyl ether (MTBE), 5 mL methanol and 5 mL MilliQ water prior to use. Extraction of the 1 L sample was carried out under vacuum at a flow rate of approximately 15 mL/min. After sample loading the cartridge was washed with 3 mL (5% v/v) methanol in water. In order to eliminate the presence of water from the eluant, a column of anhydrous sodium sulfate was prepared and fitted under the SPE column before the elution procedure started. The SPE columns were eluted with 5 mL (10% v/v) methanol in MTBE. The elution volume was then evaporated to dryness at 39oC under a stream of nitrogen.

In order to determine PhACs and EDCs concentrations, a derivatization step was necessary. The extract residues were dissolved in 300 μL of acetonitrile and then derivatized by adding 100 μL of BSTFA (N,O-bis (trimethylsilyl) trifluoroacetamide) and TMCS (trimethylchlorosilane) (99:1). The analytes were allowed to react for 1 h at 70oC. Finally, 100 μL of fluazifop standards were added to each sample before injection as an instrument internal standard to confirm injection of each sample onto the GC column.

2.4.1 Identification and Quantification of Compounds

A Shimadzu-GC 17A gas chromatograph was used for identification and quantification of compounds, equipped with an auto-injector model AOC-20i, mass detector model QP5000, Phenomenex Zebron ZB-5 column and Split/Splitless injector. The oven temperature program was 100 oC; 30 oC /min.; 150 oC (4 min); 3 oC/min; 19 oC; 1 oC/min; 205 oC (5 min); 30 oC /min; 250 oC (3 min). The injection port was maintained at 270 oC and operated in splitless mode. Helium was used as a carrier gas (flow rate 1 mL/min) and the interface temperature was held at 270 oC. For identification of each analytes, three compound specific ions were recorded in the single ion monitoring mode (SIM).