Challenges of Mitigating Membrane Fouling Brochure

26th ANNUAL WATEREUSE SYMPOSIUM CHALLENGES OF MITIGATING MEMBRANE FOULING THROUGH CHEMICAL PRE-TREATMENT Michael Chan, P.E., Duraflow, Tewksbury, MA Abstract The most important factor in water recycling and reuse with Reverse Osmosis (RO) is the pretreatment that protects the RO membrane against fouling, scaling or degradation. The recent trend to the widespread application of low-pressure cross-flow membrane Microfiltration (MF) prior to Reverse Osmosis is not without challenges. Like RO, MF is subject to fouling by various organic, mineral, and chemicals that are unique to the specific membrane property. Pre-treatment chemistry was demonstrated as a critical part of the membrane based water recycle process in maintaining the operation efficiency by removing the unwanted or incompatible contaminants or converting them into MF membrane-compatible species. Advancement made in membrane fouling mitigation technique results in substantial reduction in both CAPEX and OPEX. This paper provide a review of field performance data for pretreatment chemistries applied in ZLD water recycle systems to achieve membrane fouling reduction and flux rate enhancement for the MF pretreatment process. A case study is presented for the pretreatment chemistry developed for cooling water blowdown recycle in a power plant. Introduction Use of reverse osmosis (RO) membrane filtration for wastewater recycling and reuse is no longer a new idea. Thousands of RO systems were installed for various industrial and municipal applications on a global basis. This concept has been expanded and become a key part of today’s zero liquid discharge (ZLD) technology. Over the past two decades, ZLD technology has grown in the industrial manufacturing sector. Despite the negative impact of the recent recession on many sectors of the economy, industry analysts predict a cumulative annual growth rate (CAGR) for water recovery/ reuse systems in excess of 200% over the next decade. The growth is fueled by a number of environmental, economic and social factors. Of which, the major ones include the continued concern over quantity and quality of water supply, progressively strict wastewater disposal regulations, public pressure of industrial impact on the environment, and ever increasing cost of the valuable resource – water. ZLD is an integration of multiple technologies to achieve elimination of discharge of wastewater. The equipment needed to achieve ZLD varies depending on the characteristics of the wastewater as well as the wastewater volume. Management of the entire water cycle for industrial applications is driving efficiency and innovation towards ZLD. A typical approach to ZLD is to process the main wastewater stream through pre-treatment followed by membrane filtration, specifically RO for dissolved solid separation. Concentrated RO brine solution (reject) is sent to an evaporator/crystallizer system for further water recovery. In many cases, falling film evaporation is used to further concentrate the RO brine prior to crystallization. Falling film evaporation is a typical method to concentrate the water up to the crystallization point. The resultant brine then enters a forced-circulation crystallizer where the solution concentrates beyond the solubility of the contaminants. As a result, crystals are formed in this process. The crystal laden brine is dewatered with the liquid returned to the crystallizer. Permeate and condensate generated from the treatment process can be, in most cases, returned to the production operations (source) or used as cooling tower/boiler makeup and water source for plant maintenance, eliminating the discharge of liquids. The dissolved solids in the wastewater are essentially all converted into insoluble solids for proper off-site disposal. The overall ZLD concept is presented in Figure 1. Figure 1 – Typical ZLD Process Concept Each industrial effluent stream presents its own unique challenge when designing an entire water treatment system to efficiently and effectively minimize waste or eliminate the discharge of wastewater. The ZLD concept has been proven in the field as a technically viable approach. However, the economic value of the ZLD technology has yet to reach a generally acceptable level in a number of industries. To optimize the CAPEX and OPEX of the ZLD design, extensive R&D effort has been conducted to improve the recovery efficacy and performance reliability of the RO operation. In this case, a smaller RO reject volume will substantially reduce the size and operating cost of the more capital- and energy-intensive evaporator/crystallizer systems. Moreover, mitigating the membrane fouling will lower the life-cycle cost in favor of the investment value of the ZLD system. Membrane Fouling Management Membrane Fouling RO membranes are well suited for removal of dissolved solids, but adversely affected or fouled by suspended solids, colloidal material, bacteria or scale. Common examples of such foulants are calcium precipitates, metal oxides, colloidal silica coating, and organics. Industrial wastewater typically contains most, if not all, of these fouling substances. The implications of membrane fouling are a decrease in flux Pretreatment Industrial Production Operations RO Membrane Evaporator / Crystallizer Solid By-product Disposal Raw Wastewater Recycle Water ZLD and salt rejection, an increase in feed pressure and energy consumption and often irreversible membrane damage. Once fouled, limited by its inherent membrane properties, only mild cleaning chemicals including citric acid and detergent can be used. Stronger or more effective cleaning chemicals, such as sulfuric acid, hydrochloric acid, bleach, and peroxide will cause detrimental damages to the RO membrane. In wastewater recycling applications, RO can hardly function on its own without any protection from the fouling materials. Appropriate pretreatment must be provided to achieve stable performance of RO membranes. Microfiltration (MF) Pre-treatment MF has demonstrated the ability to significantly reduce membrane fouling and provide stable, predictable RO performance. The success of MF for this application can be attributed to the following key reasons: ? MF membrane is designed to remove suspended solids and colloidal particles (RO foulants). Some of the MF products, such as the one with tubular configuration, can handle very high TSS of >5000 mg/L in the influent. ? MF membrane can be made of a variety of polymeric materials, including PVDF, which exhibit strong resistance to concentrated chemicals. As a result, the membrane can be cleaned with mineral acids, oxidizers (bleach, peroxide), caustic and selected organic solvents for removal of different persistent fouling elements, both inorganic and organic, which are difficult to be removed with weak cleaning chemicals. ? MF filtration produces a quality product water stream with NTU (<1.0) and SDI (<3.0) values in full compliance with the feed water criteria specified by all RO manufacturers. ? The chemical or physical characteristic of the foulants in the wastewater can be converted into particles suitable for removal by MF filtration. ? In addition for fouling control, the MF can be designed to remove regulated contaminants in the raw water to make the RO reject more manageable for recovery or discharge downstream. Various fouling reduction or flux enhancement techniques are incorporated into the design of MF products. Most commercial MF products employ one or more of the techniques outlined below. It is important to understand these features in the selection of the proper MF products. Fouling Reduction Technique Description Backwash Periodic reversal of filtrate into the flow channels. Cleaning chemicals are dosed into the filtrate backwash to enhance the cleaning effectiveness. Turbulence Promotion Use of insertion (baffles, wire rings, etc.) or moving sponge balls to promote turbulence to break down the boundary layer adjacent to the membrane surfaces. Polarization Minimization Operate at high feed velocity and low trans-membrane pressure (TMP) to minimize polarization and compaction, respectively. Surface Modification Modify membrane charge and/or hydrophilic/hydrophobic property of the membrane to accommodate specific foulants. Pore Size Modification Vary membrane pore size to fit specific foulant size. Chemical Pre-treatment Modify the chemical and physical characteristic of foulants to mitigate the fouling effect on the selected membrane. While most of the above techniques can address one or two specific types of fouling contaminants, the chemical pre-treatment approach can provide a wide range of flexibility to complement the others. For example, particle size of foulants can be manipulated through addition of proper chemicals to satisfy the pore size and surface charge of the selected membrane. MF Chemical Pre-treatment Chemical pre-treatment mitigates the effects of the fouling components on membrane because it induced a change in the characteristics of the foulants. In the design phase, the following criteria must be met to achieve minimum fouling. ? All precipitable fouling components must be converted to the solid form with particle size greater than the membrane pores. To meet this objective, interfering chemicals such as chelating agents or organic compounds are typically destroyed or destabilized. ? Non-precipitable dissolved fouling components must be removed via adsorption or chemically altered to membrane compatible forms. ? The treatment chemicals used must be chemically compatible with the membrane and with each other. ? If pass through the MF, the chemicals used must be compatible with the RO membrane. Following are examples of basic MF pre-treatment chemistries applied for different industrial wastewater: Fouling Elements Industries / Source MF Pre-treatment Chemistry Hardness (Ca & Mg) Power Generation Cooling Tower Blow-down ? Chemical softening - Lime & soda ash Heavy Metals Automotive Metal Finishing Metal Refinery Steel/Iron ? Hydroxide precipitation – Caustic/lime ? Sulfide precipitation – Sodium sulfide ? Organo-sulfur precipitation – Sodium Dithiocarbamate Fluoride Semiconductor ? De-fluoridation – Calcium/aluminum salts Sulfate Mining Concrete ? Sulfate precipitation - lime and aluminum salt. Silica Groundwater ? Chemical adsorption – Magnesium salt Oil & Grease Petrochemical Industrial Laundry ? Adsorption – Lime / activated carbon ? Acid Cracking – Sulfuric acid Organic All Industries ? Adsorption – Lime / activated carbon ? Coagulation – Aluminum/iron salts ? Oxidation destruction – Bleach / Peroxide Case Study – Power Plant ZLD Project Data Plant Type/Location: Gas-Fired Power Plant in California Project Objective: 100% recycle of cooling tower blowdown (CTB) Years of Operation: 8 Years Average CTB Flow: 300 GPM Treatment Concept: The process starts with chemical softening and microfiltration followed by RO. The RO permeate is returned to the cooling tower, and the reject stream is fed to a two-stage thermal system that evaporates the RO reject into crystalline solids. The solids are disposed of as landfills, the distillate is used as makeup water for the heat recovery steam generators (HRSG), and the balance is returned to the cooling tower with an evaporation rate of over 3,000 GPM. . Major RO Foulants: Hardness (Calcium and Magnesium), Silica, Organic (Anti-scalant & Dispersants) and total suspended solids (TSS) Fouling Mitigation Process Chemical Softening: Reaction I – Ferric salt (Organic coagulation) Na2CO3 to pH 8.5 (pH adjustment) NaOCl (Bio-growth control) Magnesium salt (Silica adsorption) Reaction II - Na2CO3 & Lime to pH 10.5 (Hardness precipitation) Microfiltration: The MF membranes are manufactured in a tubular configuration designed to handle high solid concentration. The membranes, made of PVDF, are cast on the surface of porous polymeric tubes to produce a nominal pore size of 0.1 micron. Figure 2 illustrates one of the MF membrane modules installed in the system. Bleach and hydrochloric acid are typically used for membrane cleaning. Figure 2 – MF Membrane Module Figure 3 – MF System Configuration The chemically pre-treated wastewater is processed through the MF membrane modules designed for separation of the precipitates from water. The wastewater is pumped at a velocity of 12 – 15 ft/sec through the membrane modules connected in series as shown in Figure 3. The turbulent flow, parallel to the membrane surface, produces a high-shear scrubbing action which minimizes deposition of solids on the membrane surface. During operation, filtrate permeates through the membrane, while the suspended solids retained in the re-circulation loop are periodically purged for further de-watering. An automatic back-pulse mechanism is an integral part of the operation design to provide physical surface cleaning by periodically reversing the filtrate flow direction. The MF has been operated with an average flux of 300 GFD. The entire treatment process is depicted in Figure 4. Process Performance Data The removal efficiency for the fouling components is presented as follow: RO Membrane Foulants Influent (CTB) (mg/L as ion) MF Filtrate (mg/L as ion) RO Permeate (mg/L as ion) Ca 260 <25.0 <1.0 Mg 130 <10.0 <0.5 SiO2 120 <10.0 <1.0 COD 400 <150 <5.0 TSS 250 <1.0 ND SDI >Max. SDI Test Value <3.0 ---- Conclusion Industrial wastewater is known to consist of complex membrane fouling constituents. Most of them are obvious and defined, but many are hidden and unknown to the design engineers. In many cases, it only takes one incompatible fouling element to paralyze the entire system operation. Chemical pre-treatment that transforms wastewater into membrane-compatible form provides the needed flexibility to cope with the complicated and fast-changing waste streams. To a large extent, membrane fouling management is based on scientific data and methodology. However, the know-how developed from empirical field experience is essential to the overall result of a successful membrane based water recycle system. References 1. Cheryan, Munir, Ultrafiltration and Microfiltration Handbook, 2nd edition, CRC Press, Boca Raton, Florida, 1998, pp. 237 – 254. 2. Duraflow, Internal Technical Files, 2005 – 2011. 3. Hydranautics, RO Membrane Foulants and Their Removal from PVD RO Membrane Elements, Technical Service Bulletin, 2009. Figure 4 - Cooling Tower Blowdown ZLD Process Flow Diagram CW Blowdown Mg Salt Lime Softening Chemicals Lime Softening Chemicals Heat Exchange Concentration Tank Power Generation Operation EQ Tank Cooling Towers Reaction 2 Tank Reaction 1 Tank Sludge Slurry Chem Additives Anti-Scalant HCl Makeup Water Treat Chemicals 2-Stage Chemical Softening DF Tubular Microfiltration Two-Stage RO Water Treatment Plant Sludge Thickening Tank RO Pre-Conditioning RO Permeate RO Brine Water Recycle Tank Brine Feed Tank Two-Stage Evaporator / Crystallizer Wells / Aqueduct (Water Source) Filter Press Recycle Water to Cooling Tower Dry Salt Crystals (Landfill) Sludge Cake (Disposal) Distillate