A Guide to Membrane Operation
The principles of membrane operation involve selective separation by allowing certain substances (water molecules) to pass through while blocking others (e.g., salts, particles) based on size by applying an external pressure.
This article will review the typical operational issues with UF & RO membranes, how they can be monitored through metrics and solutions for issues. For more information about membranes technology and uses follow here.
Data Collection
Modern membrane systems require the collection of a variety of data by a combination of continually monitored live instrumentation and laboratory analysis.
Instrumentation is used to provide live information that can be used to analyse membrane performance during operation such as feed water pressure, temperature and flow, permeate pressure and flow and reject / concentrate pressure and flow. In addition to these parameters, an RO will also have conductivity instrumentation on the feed, permeate and reject streams to measure salt ions. Consumed energy (electrical consumption of pumps) with each membrane system will also be monitored. SCADA systems can record historical data for the plant and set alarms around key metrics to help avoid running into problems.
Lab analysis of samples can also provide insights such as Mixed Liquor Suspended Solids (MLSS) from the feed to a UF system to understand solids concentrations.
Performance Metrics
The membrane performance is effected by scaling and fouling on the membrane face and the data collected from the methods above can be used to provide valuable insight into performance through a number of metrics. Note that the performance metrics can provide valuable insights into fouling and scaling issues of membrane systems, but they do not necessarily pin point the issues to individual membranes within the system. The metrics are as follows;
Membrane Flux measures the rate permeate (Q) passes through a membrane per unit area (A) over time and the smaller the membrane pore size, the less flux and more pressure is needed. Flux is influenced by operating pressure, temperature, feed flow concentrations, membrane fouling and cleaning frequencies. As temperature and pressure also change the flux, its important to normalise against these values for long term review.
Flux (L/m²·h) = Q (L/h) / A (m²)
Specific Energy Consumption (SEC) is a key performance metric that evaluates membrane efficiency by linking energy usage(E) with flux (Q). A higher SEC will produce a higher flux if the membrane is well performing, otherwise an under pefrofming membrane will require higher SEC to produce the same flux.
SEC (kWh/m³) = E (kWh) / Q (m³)
Transmembrane Pressure (TMP) is the driving force in membrane systems, defined as the difference between the average pressure on the feed (Pf), concentrate / reject (Pc) side of the membrane and the pressure on the permeate side (Pp). A higher TMP will produce a higher permeate flow if the membrane is well performing, otherwise an under performing membrane will require higher TMP to produce the same permeate flow. Membranes will have a maximum tolerance for TMP to avoid mechanical failure.
TMP (bar) = ((Pf + Pc) / 2) − Pp
Flux / SEC / TMP Relationship
Gradual TMP Increase + SEC Increase + Decreasing Flux: Indicates scaling is likely the issue.
Sudden TMP Increase + SEC Increase + Sharp Drop in Flux: Suggests fouling, particularly biofouling or particulate fouling.
Recovery Rate (RR) is the percentage of feed water that passes through the membrane as permeate (treated water). Higher recovery rates reduce water wastage but increase concentration polarisation and scaling potential. Lower recovery rates minimise fouling and scaling but produce more brine (wastewater). UF System operating in a membrane Bioreactor (MBR) can recover around 80–95% and an RO system can recover 40% (seawater desalination) to 85% (industrial UF permeate with lower salinity).
Recovery Rate (%) = (Permeate Flow / Feed water Flow) × 100
Concentration Factor (CF) is the ratio of the concentration of solutes in the reject stream to the concentration in the feed water. UF systems typically have a lower CF (1.5 – 2.5) due to the focus on removing suspended solids. RO systems often operate at a CF between 3 – 6, with multi-stage systems achieving a higher CF to optimise water recovery.
CF = 1 / ( 1 - Recovery %)
CF / RR Relationship
Higher Recovery Rate → Higher Concentration Factor. This can increase the risk of fouling and scaling.
Lower Recovery Rate → Lower Concentration Factor. This can reduce the risk of fouling and scaling, but higher reject volume produced. This can be mitigated by using membrane systems in series instead of single pass.
Concentration Polarisation (CP) occurs when solutes accumulate near the membrane surface, creating a higher concentration at the membrane interface compared to the bulk feed water. This boundary layer can reduce membrane performance and increase the risk of fouling and scaling. CP can be mitigated by utilising a cross flow membrane system or using spacers to create turbulence in the feed. Operating at a low velocity (less scouring), high feed pressures with clean /new membranes that are very permeable can lead to a higher CP. This is an important metric, but its not easy to produce as samples from the membrane surface are not always possible. Instead, the more obtainable factors mentioned previously can more easily achieved and can diagnose and predict issues such as fouling and scaling.
Fouling
Fouling occurs when unwanted substances accumulate on membrane surfaces, reducing performance by decreasing flux, increasing TMP, and shortening membrane life. Fouling is caused by several factors and they affect both UF and RO membranes.
Biofouling is the accumulation of microorganisms (bacteria, algae, fungi) on the membrane surface, forming a biofilm that reduces flux and effects both UF and RO membranes. This can be caused by the presence of microbial nutrients (organic matter, phosphates, nitrates) in feed water and warm temperatures that accelerate microbial growth. Biofouling can be resolved by using Cleaning In Place (CIP) with Sodium Hydroxide (Alkaline) or a biocide. The feed water could be disinfected using UV or chlorination as pre treatment for RO modules, but dechlorination will be needed to avoid membrane damage.
Colloidal Fouling occurs when tiny, insoluble particles (colloids) such as clay, silt, and organic matter accumulate on the membrane surface, clogging pores and is mainly a risk to UF membranes but could also effect RO membranes.This is caused by high levels of suspended solids in the feed water, inadequate pre treatment or coagulation / flocculation failures in the upstream biological process. Colloidal fouling can be resolved by back flushing to dislodge materials (UF) and carrying out a CIP with Sodium Hydroxide (Alkaline). Often in industrial waste water treatment its common to have UF as pre treatment for RO, but there is a risk that a membrane can be damaged and allow particles to feed into the RO. Its important to sample the feed and permeate flows form systems to look for such issues. Also a cartridge filter installed on the feed of the RO can provide an additional barrier.
Organic fouling results from the accumulation of organic materials, such as fats, oils, grease (FOG), proteins, and natural organic matter (NOM), on the membrane surface and is mainly a risk to UF membranes. This is caused by high concentrations of organic matter in the feed water and inadequate pre treatment to remove FOG. Silicone in wastewater is hydrophobic and adheres to membrane surfaces, forming a resistant layer and would require specialist CIP to remove. Its important to first identify if such hydrophobic constituents are present in waste water through sampling prior to system design. Organic fouling caused by FOG Colloidal fouling can be resolved by back flushing to dislodge materials (UF) and carrying out a CIP with Sodium Hydroxide (Alkaline).
Scaling
Scaling refers to the precipitation of dissolved salts or minerals on the surface of membranes. This buildup creates a hard, crystalline layer that obstructs water flow, increases operating pressure, and reduces membrane performance. Scaling is a critical issue in RO and to a much lesser extent for UF operating in an MBR. Scaling can lead to reduced flux, higher TMP and membrane damage. As water passes through the membrane, ions become more concentrated in the concentrate stream. Each ion has a solubility limit and when this concentration is exceeded, they precipitate on the membrane surface. Temperature and pH effect the solubility limit and its important to know which salts are present as the solubility limit changes. Detailed salt analysis should be undertaken of the feedstock prior to system design to ensure that the correct recycle rates are selected and feedstock changes will often require new analysis.
Scaling can be managed by dosing with an anti scalent continuously to prevent crystal formation and by monitoring pH and temperature. Higher recovery rates increase the concentration of ions in the concentrate stream, so recovery should be set with this in mind.
Summary
Fouling and scaling are the biggest challenges with the operation of membrane technology. Processing plants therefore need careful monitoring to keep track of key parameters. Many plants today provide a lot of detailed information from instrumentation and also from lab analysis. Operators need to keep track of changes and they can be flagged with alarms through the SCADA and a remote alarm system if needed. There can be many potential causes of reduced performance and a reduction in flux can be caused by fouling or scale. The speed of changes can give an indication of the issue, but often operators need to monitor changes in the quality of the feed stock, check calibrate instruments and take an holistic view. In service back flushing, basic cleans and anti scalent dosing and pH correction systems can be automated to avoid issues arising. However, if they do occur an approach of trying successive more aggressive cleans to rule out issues may be needed.
