Humic Acid Removal and Tubular Ceramic Micro Filtration Membranes - Essay Example

Running Head: HUMIC ACID REMOVAL AND FOULING USING TUBULAR CERAMIC Humic Acid removal and fouling using tubular ceramic micro filtration membranes combined with coagulation and adsorption [Writer’s Name] [Institution’s Name] Table of Contents Table of Contents 2 Humic Acid Removal and Fouling Using Tubular Ceramic Micro Filtration Membranes Combined With Coagulation and Adsorption 3 Membrane Science and Theory 3 Polymeric Membranes 5 System Concepts 6 Membrane Fouling 7 Ceramic membranes 14 Design Concepts 15 Operations 16 System Costs 17 Residuals Characteristics and Management 19 References 21 Humic Acid Removal and Fouling Using Tubular Ceramic Micro Filtration Membranes Combined With Coagulation and Adsorption Membrane Science and Theory Water transport across clean porous membranes, expressed as flow per unit area, or flux (J), is directly proportional to the transmembrane pressure (ΔP, or TMP) and inversely proportional to the absolute viscosity (µ), as modeled using a slightly modified form of Darcys law (Cheryan, 1998): “in which Q^sub total^ is the filtrate flow, A is the membrane area, and R^sub m^ is the hydraulic resistance of the clean membrane to water permeation. Note that because the viscosity of water varies inversely with temperature, warmer feedwater temperatures will result in either increased flux (for operation at constant pressure) or decreased feed pressure requirements (for operation at constant flux); the converse effect results from colder feedwater.” (Cheryan, 1998) Consequently, temperature is an important consideration in the conceptual design phase, because changes in feedwater temperature over the year may necessitate additional membrane area to maintain the required system capacity, depending on the maximum TMP of the membranes, seasonal water demand, and other site-specific factors. When considering temperature effects on MF/UF membranes, the flux is typically normalized to a reference temperature of 20°C. (Cho et al., 283-298, 2000) The hydraulic resistance of the membrane (R^sub m^) can be related to physical properties of the membrane as follows: Typical units for flux are gallons of water per square foot of membrane area per day or litters of water per square meter of membrane area per hour. The pore density (P^sub pore^) is the number of pores per unit of membrane area, r is the pore radius, τ is the tortuosity factor, and Δz is the pore length. Thus, the resistance to pure water transport across a clean membrane is expected to increase with increasing tortuosity and thickness and with decreasing pore density and pore radius (with a strongly influencing inverse 4th power relationship). An important operational factor that can strongly influence the flux is the foulants that accumulate over a filtration cycle (reversible fouling), between cleaning intervals (reversible fouling), and over the life of a membrane module (irreversible fouling). This fouling can take several forms: particulate/colloidal fouling, organic fouling, and/or bio fouling. Fouling results in the gradual reduction in flux (for constant pressure operation) or increase in TMP (for constant flux operation) because of adsorption or deposition of contaminants either within the pores or on the surface of the membrane. (Hicke, 187–196, 2002) In Eq 1, fouling is incorporated by expanding the resistance term to include additive factors to account for the resistance attributable to various types of fouling (e.g., particulate, organic, biological) as follows: R^sub m^ = R^sub 1^ + R^sub 2^ + R^sub 3^ +.... Particulate fouling increases resistance to the flow of water across the membrane via pore blocking and the subsequent cake layer formation over the course of a filtration cycle. Colloidal interactions play a significant role in particulate fouling, which is the most common cause of the productivity decline in MF/UF membrane processes. (Clever et al., 325-336, 2000) This fouling can typically be removed by periodic backwashing in a manner similar to that used for conventional media filtration. Natural organic matter (NOM), such as Humic substances, has also been recognized as a primary cause of membrane fouling. Organic fouling occurs via complex mechanisms and often requires chemical cleaning to remove. (Cabane et al., 155161, 2002) In addition, the organic matter enhances biological activity in a membrane system, which may increase bio fouling. Generally, bio fouling occurs as a two-step process: (1) attachment of microorganisms to a membrane surface and (2) formation of bio film through cell multiplication. Subsequent extracellular polymer generation (which constitutes the bulk of the bio film) also contributes to bio fouling. It is also worth noting that coagulant aid polymers used in the conventional pretreatment process can cause rapid and potentially irreversible fouling of MF/UF membranes. The risk of fouling is particularly significant with cationic polymers, given that most MF/UF membrane materials have a negatively charged character. Thus, any pretreatment process-utilizing polymer upstream of polymeric MF/UF membranes should be carefully designed to minimize or eliminate polymer carryover. (Kretzschmar and Sticher, 133-139, 1998) Polymeric Membranes In some cases, the resistance of polymeric membranes has been observed to increase with large TMPs, a phenomenon known as membrane compaction. The compaction process constricts the membrane pores, reducing MF/UF system productivity. Membrane compaction can likewise be incorporated into Eq 1 using an additive resistance factor. (Ning et al., 1-6, 2005) MF/UF processes are expected to become increasingly cost-competitive with media filtration, a trend driven by superior performance, reduced production cost (e.g., because of economies of scale), improved design, significant increase in demand for the technology, and competition. As costs stabilize over the long term, a well-established MF/UF market should be more influenced by traditional factors including supply and demand, materials cost fluctuations, and inflation. (Verbych et al., 237-244, 2005) At this point, for example, a system with innovations, more advanced features, and/or a better performance record should command a higher price rather than serving as an incentive to purchase one comparably priced system over another. In general, for a system exceeding 20 mgd in capacity, the cost of membrane filtration (i.e., equipment only) in 2015 is estimated to range from £0.10 to £0.15/gpd of installed capacity (2004 pounds), with operations and maintenance costs between £0.05 and £0.20/kgal of treated water (2004 pounds). System Concepts The most fundamental component of an MF/UF system is the membrane material. Polyvinylidene fluoride is common in drinking water treatment applications, although numerous other membrane materials are used, each with different properties. Important morphological properties of a membrane are surface porosity, pore size and shape, and surface roughness. MF/UF membrane materials can be manufactured in different geometrical configurations, which are then incorporated into a membrane module (i.e., the smallest distinct filter unit in an MF/ UF system). Commercially available MF/UF membrane configurations include hollow fiber, spiral wound, tubular, and plate-and-frame. However, in municipal drinking water treatment applications, almost all proprietary MF/UF systems use hollow-fiber membranes. (Bhattacharjee, 303-310, 1997) MF/UF systems are generally operated in either constant flux or constant pressure modes. Under constant pressure operation, the membrane flux will decline over the course of a filtration cycle as the membranes foul. Most MF/UF systems, however, are operated at constant flux using variable-speed pumps to meet increasing pressure requirements caused by membrane fouling, which restricts the passage of water. Operating TMPs may span from approximately 3 psi (20 kPa) to as high as approximately 43.5 psi (300 kPa) in some cases, although values that are more typical range from approximately 7 psi (50 kPa) to 30 psi (200 kPa). (Sato et al., 3371–3377, 2002) There are three general operating configurations for MF/UF systems. With encased membranes in cross-flow operation, a portion of the feed is recycled, returning a smaller flow with more concentrated particulate matter to the head of the filtration process. A bleed (i.e., waste) stream may also be used to control the solids concentration in the recirculation loop, as necessary. (Schlautman and Morgan, 4293-4303, 1994) Encased membranes may also be operated in a direct (or dead-end) filtration mode (Suzuki et al., 119-130, 1998) without recirculation or bleed streams. The terms "cross-flow" and "direct filtration" in the context of MF/UF are understood within the industry to apply only to encased membrane filtration systems, which operate under positive pressure. (Matilainen et al., 457– 465, 2002) With submerged (or submersible) membranes, a vacuum is applied to the membrane to create the pressure differential that is the driving force for filtration. Numerous minor operational variants for controlling the flow of water and concentration of solids within the basins may be used in conjunction with submerged membranes, as necessary, according to manufacturer recommendations. (Bowen et al., 417–429, 2002) Membrane Fouling As the membranes become fouled over the course of operation, a combination of both backwashing and chemical cleaning is used to remove foulants to the maximum practical extent, thus ideally restoring operating parameters such as flux (for constant pressure operation) and TMP (for constant flux operation) to baseline levels. For most MF/UF systems, backwashing is typically conducted every 30-60 min for a short duration (e.g., about 1 min) and consists of water and air, or a combination of both. Backwashing may also be initiated based on a TMP set point or a predetermined volume of filtrate produced. (Wolf et al., 289–296,2005) Chemical cleaning (known as clean in place [CIP]) is conducted at a point at which the ability of routine backwashing to restore the desired system flux and/or TMP is significantly diminished. A more modest cleaning operation of shorter duration and lower chemical doses may also be conducted on a routine schedule (e.g., daily to weekly) to control fouling on an ongoing basis. (Kooij, 157-166, 1998) These operations are known by the synonymous terms "chemically enhanced backwash" (CEB), "maintenance wash," "maintenance clean," and "enhanced flux maintenance." Important considerations for chemical cleaning include the type and dose of chemical(s) used, frequency, duration, temperature, rinse volume, recovery and reuse of cleaning agents, and residuals neutralization and disposal. (Puebla, 47-55, 2004) A variety of chemicals may be used for cleaning including detergents, acids, bases, oxidizing agents, sequestering agents, and enzymes. (Afonso et al., 157-171, 2004) Chlorine is also commonly used in the chemical cleaning process for both disinfection and oxidation (for compatible membrane materials) at a wide range of doses (e.g., 2-2,000 mg/L). In previous studies, little attention was given to the reversibility of fouling in membranes following precoagulation. Membrane fouling can be divided into two categories: physically reversible fouling and physically irreversible fouling. Physically reversible fouling can be controlled by physical membrane cleaning such as routine hydraulic backwashing whereas physically irreversible fouling evolves even when backwashing is carried out routinely and intensively. For reduction of operation costs of membrane processes, understanding and control of physically irreversible fouling are important (Kimura et al., pp. 3431–3441, 2004). However, in most previous studies on impacts of coagulation on performance of membrane, the two types of fouling were not distinguished except for a few studies (Lahoussine-Turcard et al., pp. 76–81, 1990, Kimura et al., pp. 93–100, 2005 and Choi and Dempsey, pp. 4271–4281, 2004). The increased demand for drinking water has lead many water utilities to use source water containing elevated levels of natural organic matter (Bursill, 1-7, 2001; Skjelkvale et al., 52-115, 2000). This increasing demand, combined with stricter government regulations, necessitates improved drinking water treatment. (Agenson, 91–103, 2003) The presence of natural organic matter (NOM) in the source water is a cause of concern to health professionals and environmental engineers because the reaction of NOM with disinfectants, such as chlorine, results in the formation of disinfection byproducts (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs). Because of their toxicity (Morris et al., 955-963, 1992; Mughal, 287-292, 1992 & Kool et al., 1985), the trihalomethanes and haloacetic acids are regulated by the Environmental Protection Agency. In the United Kingdom, there is an increasing interest in the application of both ozone and membrane filtration for DBP and DBP precursor removal in order to meet the requirements of the Surface Water Treatment Rule (SWTR), the Disinfectant and Disinfectant Byproducts Rule (D/DBPR), and the Long Term 1 Enhanced Surface Water Treatment rule (LT1ESWTR). Several researchers have attempted to combine ozone with polymeric membranes with limited success, in part because the organic membranes, which are commonly used in water and wastewater treatment applications, are prone to destruction by ozone (Shanbhag et al., 4388-4398, 1998; Castro, 50-61, 1995; Shen et al., 597-608, 1990 & Hashino et al., 17-23, 2000). Hashino et al. (Hashino et al., 17-23, 2000) studied the use of ozonation combined with an ozone resistant polyvinylidenefluoride (PVDF) microfiltration membrane. They found that ozone prevented foulants from adhering to the membrane surface, thus decreasing membrane fouling. However, high dissolved ozone concentrations (>1 mg/L) were necessary to obtain high permeate fluxes and prevent membrane fouling. Integrity testing is also a critical MF/UF system feature, enabling the detection and isolation of breaches that would otherwise compromise the effectiveness of the membrane as a barrier for pathogens and particulate matter. (Glucina et al., 205-211, 1998) Almost all commercially available MF/UF systems designed for municipal drinking water treatment are equipped with the ability to conduct an automated pressure decay test using air, which directly challenges the membrane barrier by pressurizing the system modules and monitoring the rate of decay over a short period of time (e.g., 5 min). Decay rates exceeding a given benchmark threshold value (specific to each site and system) indicate an integrity breach, with the magnitude of the response directly proportional to the magnitude of the breach. Because these "direct" integrity tests do not permit the simultaneous production of water, they are typically only conducted once per day on an MF/UF system. (Aiken et al., 1985) Thus, less sensitive "indirect" methods are used as a complementary tool to monitor some aspects of filtrate water quality between periodic applications of a more sensitive direct test. Although the indirect methods (e.g., turbidity or particle counts) are only generally indicative of membrane integrity, they can be applied on a continuous basis during operation. (Puebla,136-146, 2005) Submerged membrane bioreactor (MBR) is a biological treatment process that uses membrane to replace the gravitational settling of the conventional activated sludge process for the solid−liquid separation of mixed liquor. The submerged membrane unit provides a complete barrier to biomass regardless of its settling characteristic. (Alpatova et al., 155-162, 2004) This attribute enables the MBR to eliminate all the constraints inherently associated with sedimentation. As a result, an MBR can provide greater operation flexibility than the activated sludge process, as the control of mean cell residence time (MCRT) is independent of the hydraulic retention time (HRT). MCRT measures the average time that the biomass is retained inside a bioreactor. One of the important considerations for adopting an operating MCRT is the targeted amount of sludge production. When sludge handling and disposal is a concern, operating a bioreactor at long MCRT would reduce the amount of sludge production. However, some wastewater treatment plants could prefer a short MCRT operation to maximize sludge production for biogas genera tion (Ng, 981-992, 2005). For effective nitrification, a longer MCRT, typically greater than 15 days, is recommended because autotrophic nitrifies are very slow growing microorganisms (Rittmann & McCarty, 2001). A long MCRT operation can also maintain a high concentration of mixed liquor suspended solids (MLSS) in an MBR, resulting in a lower F/M ratio and a smaller reactor footprint. (Mutairi, 115–119, 2004) Membrane fouling is an inevitable phenomenon of all membrane processes. To sustain daily operation due to membrane fouling, membrane maintenance by periodic chemical cleaning or membrane replacement has to be carried out and these will inevitably accrue higher costs. Current methods for controlling membrane fouling in an MBR include: (i) operating with a flux lower than its critical flux (Gander et al., 119-130, 2000), (ii) increasing the cross-flow velocity by controlling the aeration rate (Sofia et al., 67-74, 2004), (iii) operating with an intermediate permeate suction (Yamammoto et al., 43-54, 1989), and (iv) back flushing the membrane periodically with permeate (Bouhabila et al., 123-132, 2001). These approaches are formulated based on the design and operation of a submerged MBR as two separate units: bioreactor and membrane separation. However, with the membrane submerged inside the bioreactor, characteristics of the mixed liquor will have a major impact on the membrane performance. Thus, MCRT, which affects the mixed liquor characteristics, will inevitably influence the rate of membrane fouling. (Hilal and Kochkodan, 97-113, 2003) Currently, there are some mixed reports on the effect of MCRT on membrane fouling of submerged MBR. Several researchers had reported that high MLSS concentration associated with long MCRT would increase the membrane filtration resistance (Yamammoto et al., 43-54, 1989). However, others had demonstrated reduced membrane fouling rate at longer MCRT (Fan et al., 243-250, 2000). Researchers had looked into the effects of extracellular polymeric substances (EPS) and soluble microbial products (SMP) on the membrane performance (Lee et al., 217-227, 2003; Huang et al., 401-406, 2000 & Nagaoka et al., 355-362, 2000). As both EPS and SMP are products associated with microbial activity, their composition and concentration would be influenced by the operating MCRT. Huang et al. (Huang et al., 401-406, 2000) had reported that ac cumulated SMP in the bioreactor can have a negative influence on the membrane permeability. Higher EPS content associated with short MCRT operation was found to cause membrane fouling (Nagaoka et al., 355-362, 2000). To date, limited information is available regarding the impact of the physiological state of mixed liquor at different MCRT on membrane fouling. Most of the related studies focused on operation at long MCRT of more than 20 days (8−9, 11) and the treatment of synthetic wastewater (Ng, 981-992, 2005). In view of the above, the objective of this study is to systematically investigate the effect of MCRT on membrane fouling of a pre-anoxic submerged MBR for domestic wastewater treatment. The membrane flux, mixed liquor characteristics, and SMP and EPS content were monitored to provide a comprehensive understanding of their relationships with the operating MCRT and to elucidate the cause of membrane fouling. (Qdais and Moussa, 105-110, 2004) Application of low pressure driven membranes (i.e., MF/UF) to drinking water treatment has been receiving a lot of attention and already been in practice all over the world. Chemical coagulation can be used as pre-treatment for membrane processes (Wiesner et al., 20–40, 1989; Lahoussine-Turcard et al., pp. 695–702, 1992 and Magara et al., pp. 91–99, 1998). By implementing coagulation with membrane filtration, enhanced removal of total organic carbon (TOC) and taste/odour can be expected. In addition, efficient retention of virus is possible by a combination of precoagulation and MF/UF membrane filtration (Matsushita et al., pp. 199–207, 2006 and Fiksdal et al., pp. 364–371, 2006). When the drinking water industry began to utilize membrane filtration, water sources that required minimal treatment are commonly used. As a result of efforts to expand the application of membrane technology, coagulation pre-treatment for membrane filtration is now becoming more common (Howe et al., pp. 133–146, 2006). Chemical coagulation with aluminum/iron salts has been intensively studied in the field of water treatment and the optimum use of this process in the conventional treatment system is well-established (Letterman et al., 1999 and James, 1985). However, this is not the case with the combination of coagulation and membrane processes: the effect of coagulation on membrane performance is not well-understood (Howe et al., pp. 133–146, 2006). Optimum condition of coagulation for a membrane process is not necessarily the same as that for the conventional water treatment particularly in terms of prevention of membrane fouling. (Zidouri, 137-145, 2000) Coagulation conditions will determine the behaviour of foulants and the properties of flocks formed in the process, and subsequently determine the degree of fouling in membrane processes (Cho & Lee, pp. 143–150, 2005). It is widely anticipated that the recent LT2ESWTR and associated Membrane Filtration Guidance Manual (USEPA, 2005), representing the first federal regulatory framework that directly addresses membrane filtration, will result in increasing standardization of the diverse state primacy agency policies for regulating MF/UF. Previous studies on the effect of coagulation on membrane performance have been inconsistent: some studies showed improved membrane performance following coagulation while other studies showed decreased performance. Improvements in membrane performance following coagulation have been noted in several pilot studies (Jack & Clark, pp.83–95, 1998, Guigui et al., pp. 95–100. 2002 and Lee et al., pp. 115–121, 2006). In contrast, there are cases where coagulation pre-treatment has made membrane fouling worse [13], [14] and (Howe et al., pp. 7908–7913, 2006). Schäfer et al. (Schäfer et al., pp. 1509–1517, 2001) reported that coagulation with ferric salt increased fouling of MF/UF membranes during filtration of Suwannee River Humic and fulvic acid solutions. Judd and Hills (Judd and Hillis, pp. 2895–2904, 2001) reported that the low coagulant doses apparently caused incomplete aggregation of colloidal particles such that internal fouling of membranes took place. Carroll et al. (Carroll et al., pp. 2861–2868, 2000) reported that coagulation had no effect on fouling caused by dissolved organic carbon (DOC). One reason for the diverse conclusions on the effect of precoagulation on membrane performance is the variation in experimental conditions (e.g., bench-scale or pilot-scale) in each study. Ceramic membranes Ceramic membranes are ozone resistant, and when these membranes are used in combination with ozone, stable permeate fluxes can be achieved without membrane damage (Kim and Somiya., 7-15, 2001; Kim et al., 131-143, 1999; Karnik et al., 728-734, 2005; Schlichter et al., 307-317, 2004 & Allemane et al., 419-432, 1993). Kim and colleagues used ceramic membranes to investigate the effect of ozone bubbling on flux recovery (Kim and Somiya, 7-15, 2001). The results showed that intermittent ozonation effectively maintained high permeate fluxes and prevented membrane fouling caused by particle accumulation on the membrane surface (Kim and Somiya., 7-15, 2001). Their earlier work demonstrated that stable fluxes could be obtained with ozonation−ceramic membrane filtration. Ultra filtration alone did not achieve the levels of treatment obtained with combined ozonation−membrane filtration. Design Concepts In general, the design approach for an MF/UF system should balance the requirements of the utility and the membrane equipment supplier. The design concepts applicable to most MF and UF processes can be grouped into three categories: site-specific issues, membrane-specific issues, and system-specific issues. Site-specific issues include the capacity, raw water quality and variability, demand profile of the utility, finished water quality goals, redundancy and reliability, site space availability, future expansion requirements, and desirable operational characteristics. (Agashichev, 96-101, 2006) Membrane-specific issues are those that are unique to the individual membrane equipment supplier, such as the flux, type of membrane units (i.e., encased or submerged), backwash approach, cleaning strategy, and process residuals generation. System-specific issues are related to the equipment selection for the facility and include building layout and orientation, materials and construction, control and operational approach, pumping, chemical cleaning system design, backwash and chemical cleaning residuals streams disposal, and compressed air systems. (Ducker et al., 1831-1836, 1992) These issues, summarized in the sidebar on page 88, are sorted by category along with several important considerations. (Sofi, 147-156, 2000) Although this sidebar highlights some of the most significant design issues associated with membrane filtration systems, it is not intended to be comprehensive. Because there are numerous membrane treatment equipment suppliers, there may be significant variation in the design features among different products, particularly with respect to the more nuanced aspects. Therefore, prior to the selection of a membrane filtration system, it is important to both consider the design issues (summarized in the sidebar) relative to specific utility needs and to work with the suppliers to understand their respective systems as thoroughly as possible. After selecting a system, utilities and engineers should work closely with the selected equipment suppliers during the design phase. (Zoubi, 42-60, 2007) Operations The two primary goals of membrane systems operations are maintaining productivity and maintaining filtrate quality. Because the ability of MF/UF systems to remove pathogens and particulate matter is independent of raw water quality for an integral membrane barrier, it is easier to achieve high-quality filtered water on a consistent basis with membrane filtration than it is with conventional media filtration. However, lower-quality feedwater does affect the efficiency of system operation. (Wang et al., 153–163, 2001) Consequently, relative to that for a conventional plant, a more significant proportion of operator responsibility is dedicated to maintaining productivity. For example, declining feedwater quality may foul the membranes more quickly and perhaps reduce the effectiveness of routine backwashing, resulting in more frequent chemical cleaning. In such cases, operators may be able to minimize fouling by optimizing pretreatment and/or using a more aggressive backwash process. Under more extreme conditions, operators may need to lower the flux to control fouling, which could reduce production capacity. A marked decline in filtered water quality is generally attributable to breaches in the membrane barrier. Therefore, both direct integrity testing and continuous indirect integrity monitoring are essential for proper membrane system operation. (Tomaszewska and Mozia, 4137–4143, 2002) The primary aspects of MF/UF operations can be categorized as plant control, process monitoring, and data collection and record keeping. In order to ensure effective operations, each of these aspects should be carefully considered and addressed in the water treatment plant operations and maintenance plan. Important factors for each of these three aspects of operations are summarized in the sidebar on page 90, along with key considerations. Membrane technology is a relatively new and rapidly changing field with various commercially available systems and continuous introduction of new and updated membrane products. As a result, operational aspects of specific systems can differ significantly in some cases. Thus, although general operations are summarized in this article and in M53, the membrane system suppliers should be consulted for more detailed, product-specific information. (Karakulski et al., 315-319, 2002) System Costs Although many of the cost considerations for membrane filtration are similar to those for conventional plants, there are some important differences. Foremost among these differences is the proprietary nature of the membrane equipment and the associated procurement process. Because MF/UF equipment represents a substantial portion of the cost of a membrane filtration facility, differences among the various proprietary systems, such as appurtenant equipment, footprint, and installation, can have a significant effect on the overall project cost. For example, submerged membrane systems require tanks, which may be expensive to construct for a new plant. (Harmant and Aimar, 217–230, 1998) However, in retrofit applications in which existing tanks can be used, the need to construct a separate building for an encased membrane system may be more expensive in some cases. Another differentiating factor to consider for membrane filtration is piloting various proprietary systems, which is generally recommended practice in the development of an MFAJF facility but typically not associated with conventional treatment plants. A thorough pilot project may require significant time, some minor construction to accommodate the utility connections for several pilot-scale systems, equipment rental fees, and operator time, all of which have a cost that should be taken into account. (Teixeira and Rosa, 165-175, 2002) Capital cost components of a membrane filtration system that are atypical for conventional treatment plants include membrane equipment, a substantial number of valves, feed pumps (because in-plant pumping is typically required) with variable-frequency drives, strainers, air compression equipment (as necessary for pneumatic valves, integrity testing, backwashing, and similar processes), and chemical cleaning equipment (i.e., tanks, metering pumps, and similar equipment), including facilities for spent cleaning solution neutralization. (Bacchina et al., 77-91, 2002) As previously noted, tanks may also be required for submerged membrane systems if appropriately sized existing tanks or basins (e.g., in the case of a retrofit) are not available. Special considerations for the operation of a membrane filtration facility include energy, chemical cleaning, and membrane replacement. (Bottino et al., 75-79, 2001) Generalized cost curves for membrane equipment, membrane filtration facilities (including construction and installation as well as the membrane equipment), and facility operation as a function of system capacity are tested. (Starov and Churaev, 145-167, 1993) Because both the capital and operating costs of a membrane filtration facility vary with the flux at which the system is operated (i.e., fewer membrane modules are required at higher fluxes), all three tests indicate curves for high, medium, and low fluxes. Additional detailed information regarding the management of membrane residuals is provided in an AWWA report (AWWA Residuals Management Research Committee Subcommittee on Membrane Residuals Management, 2003).Note that what is considered a "high," "medium," or "low" flux varies among different proprietary systems. The costs cited in each membrane are estimated to be accurate within ± 25%. Additional details regarding what factors are incorporated, as well as applicable assumptions, are provided in M53. Costs should always be escalated for the current year relative to that of the original publication of the manual. (Abdessemed and Nezzal, 367-373, 2002) Residuals Characteristics and Management The primary residuals associated with MF/UF systems are the waste streams generated from backwashing and chemical cleaning operations (either as-needed and more aggressive intermittent CIP or more modest and routine CEB). Encased MF/UF systems operating in a cross-flow mode also generate concentrated waste streams, which are similar to backwash residuals. (Braghetta et al., 628-641, 1997) With the exception of those systems operating in cross-flow mode (or similar variant with a bleed stream), MF/UF systems typically produce residuals on an intermittent basis because of regular backwashing. However, for larger systems, the combination of multiple unit’s backwashing at staggered intervals may generate a waste stream on a more continuous basis. (Abdi et al., 1750- 182, 1996) Backwash residual characteristics depend on the quality of the membrane system feedwater as well as on the recovery of the membrane system. At typical recoveries of 90-97%, the solids in the feed are concentrated by a factor of 7-20 in the backwash residuals. Backwash flows generally represent about 95-99% of the total residuals volume from MF/UF systems; the remainder is generated from the cleaning processes used to control fouling. Waste cleaning solutions reflect the chemicals used in the cleaning process combined with the extracted foulants. (Aoustin, 63-78, 2001) The volume of CEB waste is typically very low; for example, a daily CEB generally represents < 0.2-0.4% of the plant flow. Residuals from a CIP conducted monthly (i.e., a typical industry benchmark interval) will normally be < 0.05% of plant flow. These volumes may be reduced if the cleaning solutions are recycled or reused. The characteristics of typical backwash and chemical cleaning wastes. Note that pH adjustment and/or dechlorination may be necessary prior to disposal depending on the chemicals used and the regulations applicable to the method of disposal employed. (Aiken and Malcolm, 2177–2184, 1987) Disposal of MF/UF residuals is generally governed by the same regulations that control disposal of conventional treatment residuals. Additional state and/or local regulations that protect sensitive ecosystems, such as cold-water fisheries or pristine watersheds, may also be applicable. (Anne, 67-77, 2001) The selection of residuals disposal methods depends on several site-specific factors: climate, availability of land for facilities, size of the MF/UF installation, feasibility, and federal, state, and local requirements. (Pokrovsky and Schott, 141– 179, 2002) According to a US Bureau of Reclamation-sponsored survey of 65 MF/UF installations (Mickley, 2003), the most common methods for backwash residuals disposal were surface water discharge (38%); sewer discharge (25%); land application, such as percolation ponds or irrigation (22%); and recycling (14%). Treatment was provided at a small number of locations to meet specific discharge requirements. Only one small (0.12-mgd [0.45-ML/d]) installation in Oklahoma used an evaporation pond. (Chahboun et al.,235-244, 1992) The most common methods of CIP residuals disposal were sewer discharge (37%), land disposal (25%), and surface water discharge (11%). 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