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Membrane water treatment is becoming increasingly commonplace for treating municipal water and wastewater throughout the world In the 1970s, membranes were mostly used for industrial applications, with the first documented use of reverse osmosis (RO) membranes being utilized as high purity treatment of a municipal water supply in Texas. In the 1980s, research provided the footing for cost-effective membrane filtration and the advancements in polyamide composite nanofiltration and RO membranes. In the 1990s, membranes for municipal water, wastewater and water reclamation applications became more prevalent. In the U.S., the main areas of membrane use were Florida and California. From there, membrane use has spread to many different locations and applications.
Membrane technology is considered first for many water, wastewater and water reclamation applications. The rise in the use of membranes has prompted the need for more discussion of the operation of membrane facilities and troubleshooting of membranes in real applications. This article will focus on the operation of integrated membrane systems using both membrane filtration (MF), which uses microfiltration/ ultrafiltration membranes, and nanofiltration/reverse osmosis (NF/RO) for the treatment of municipal wastewater effluents. It is important to recognize that membrane plants come in a variety of “flavors,” including MF and RO in wastewater effluent applications, MF for surface water applications, NF/RO for groundwater and seawater applications and even membrane bioreactors (MBR) for the treatment of wastewater.
Routine membrane operation For membrane operation, it is important to understand the role of the membranes in the treatment process. MF systems are typically for particulate and pathogen removal. For RO systems, the goal is removal of total dissolved solids including organic and inorganic constituents. Although the systems are operated differently, there are common operating procedures that can be important to the overall maintenance and performance of the membrane systems. Common procedures are data maintenance, routine preventative maintenance, and repair and corrective maintenance.
Routine monitoring and observation of the installed equipment is critical to assessing system performance using operator senses. For instance, it is important to listen for the sound of normal operation and be able to identify noises that are out of the ordinary. It is equally important to visually inspect the systems, check for leaks, verify levels and compare level information with control system information.
It is necessary to perform routine calibrations of instruments—especially flowmeters, pressure instruments, conductivity particles counters and turbidity instruments. Flowmeters and pressure instruments are important to both MF and RO system performance. Conductivity is important to verifying RO system performance, and particles counters and turbidity are important to verifying the performance of MF/UF membranes.
Membrane systems must maintain thorough and accurate documentation of system information. For example, it is important to have membrane loading/unloading schedules, clean-in-place (CIP) batching information, CIP log sheets and daily operating data, including system flows, pressures, turbidities and conductivities. In addition, it is important to have monitoring data on the feed, filtrate and permeate of the membrane systems on a regular interval. This can be used to assess the seasonality and variability of the water quality, as well as confirm online performance indicators such as turbidity or conductivity.
With routine monitoring, calibration and reliable operating data, it is easy to tell when a membrane system needs more thorough repair and corrective maintenance. For MF systems, it is important to observe log removal values, pressure decay results, particle counts and filtrate turbidity to identify potential broken fibers within the system. It is then necessary to bubble test and pin repair the MF membranes.
For NF/RO systems, it is important to observe pH, conductivity and differential pressures. NF/RO is dependent in many instances on antiscalants and acid addition to ensure the systems are not scaled by sparingly soluble salts. Operators should also verify that chemical feed systems are working properly and operating within acceptable pH ranges. It is also just as important to observe conductivity profile information and identify high or increasing permeate conductivity. This can identify problems with the actual membrane. In addition, staff should observe differential pressures across RO pressure vessels to identify if fouling of the RO membranes is occurring from debris or from biological growth.
Data collection, normalization and review Operating data collection, normalization and review on a regular basis are all essential to facilitate optimized membrane system performance. For MF systems, key elements to analyze and review on a regular basis are trans-membrane pressure, permeability and resistance. In addition to these parameters, it is important to review membrane integrity using pressure decay testing, turbidity and silt density index (SDI). For NF/RO systems, the key elements of analysis are permeability, normalized flux, normalized net driving pressure and differential pressure, as well as membrane integrity indicators such as permeate conductivities and water quality analyses. The values for permeability and resistance should always be reviewed using normalized values for temperature pressure and salinity variations. This will eliminate the influence of variables on operating parameters.
Integrity and permeability
There are two aspects of membrane performance that must be considered and monitored thoroughly: integrity and permeability. There are various tools used in membrane system troubleshooting and performance optimization that will assist with monitoring these parameters. Physical or chemical damage to the membrane can result in a breach of membrane integrity, which results in poorer product water quality.
For MF systems, when membrane integrity issues are observed or suspected, there are both direct and indirect integrity test methods for evaluating the system. The direct integrity tests include pressure hold tests, sonic integrity tests and bubble point tests. Indirect integrity tests include turbidity, particle counts and SDI testing. RO system troubleshooting tools for membrane integrity include conductivity profiles, vessel probing and flow tests, CIP effectiveness evaluation and normalized performance data evaluation.
Probably the most important tool that evaluates membrane issues in a MF system is the pressure hold test. It is a timed sequence of applying a low-pressure air to one side of a membrane and monitoring the pressure decay over time. It allows for testing of all modules within a membrane unit and sensitivity on the order of single fiber break. It is considered the standard for MF systems, is automated and has widespread acceptance in the industry. The test allows for testing of both the membrane and piping for leaks.
Another test that is important for evaluating MF system integrity is the SDI test. SDI data is useful in integrated membrane plants and can be used to link the performance of the two membrane systems. The test will tell when the MF system is having integrity problems even before turbidity indicates it. It will also reveal changes in RO feed water quality that could result in higher RO fouling rates.
In addition to permeate water quality analyses, permeate conductivity is an online indication of the NF/RO membrane system performance and can identify when there are problems with the membranes’ salt rejection. These problems are typically a consequence of membrane age, chemical damage, physical damage, O-ring disconnects or O-ring deterioration. Conductivity profiles and vessel probing are useful tools in assessing the integrity of RO systems.
Membrane permeability refers to a membrane’s ability to pass water relative to the applied pressure. The parameter is an indication of the membrane’s fouling rate and can be used to assess the system’s ability to continuously make design capacity. For membrane systems, the permeability can be evaluated from a variety of normalized equations, including specific flux, membrane resistance, and normalized flux or flow. Permeability is a good indication of the membrane performance and can identify when problems are occurring, such as tightening of the membrane, fouling, scaling or loss of flow.
For MF systems using dead-end filtration, transmembrane pressure (TMP) is typically used to directly monitor the fouling rate of the membrane. TMP is the difference between feed and filtrate pressure, and it can be used with the temperature-corrected flux to calculate the permeability of an MF system. As membrane fouling occurs, the TMP increases, while the permeability decreases.
For NF/RO systems, a variety of issues can contribute to membrane fouling and the loss of permeability. Biological, colloidal, organic or inorganic fouling of RO membrane is typical and can result in an increase in feed pressure or differential pressure across the stages of an NF/RO system. As feed or differential pressures increase in an RO system, membrane permeability can decrease.
In addition to the data evaluation for identifying issues with the membrane systems, there are evaluations of the CIP data that can identify effectiveness of the cleaning, including lower feed pressure, lower differential pressure and temporary effect on product water quality. The ideal cleaning will return the membrane to the startup permeability value. However, it is more practical to expect an effective CIP to return the system to a consistent post-clean permeability without a negative impact to the membrane integrity.
While membrane integrity is critical to ensure product water goals are achieved, acceptable membrane permeability is critical to ensure product capacity goals are achieved. Membrane permeability is inversely proportional to the operating pressures and energy costs associated with operating a membrane system. As membrane permeability decreases, the energy costs associated with maintaining design capacity increases.
As membrane systems are used more and more in water, wastewater and water reclamation, it is vital to identify and establish good operating procedures and practices for the treatment systems. There are routine operations and monitoring procedures that must be performed in order to collect information that facilitates routine maintenance, calibration and repair. Thorough and appropriate data collection and management allow for identifying and analyzing problems as well as optimizing system performance.
Lastly, there are many additional system-specific tools that can allow for troubleshooting and ultimately correcting issues with the membrane systems. When taken together, these procedural building blocks can be implemented to ensure the long-term health and viability of membrane systems.