2.5 Membrane Filtration: MF, UF, NF & RO Operation, Integrity Testing and Cleaning

2.5 Membrane Filtration: MF, UF, NF & RO Operation, Integrity Testing and Cleaning

The WPI Water Treatment Class 1, 2 and 4 outlines all list membrane filtration under "monitor, evaluate and adjust" treatment processes, and the Class 4 outline adds membrane inspection and maintenance. Membranes are physical barriers, so they behave differently from granular media filters: removal depends on pore size and membrane integrity, and performance is tracked by pressure and flux rather than head loss alone.

The Four Pressure-Driven Membrane Classes

MF and UF work by size exclusion and are used in place of, or after, conventional clarification. NF and RO work by diffusion through a dense membrane, so they remove dissolved substances and need pretreatment to protect the membrane.

Terms You Must Know

• Feed enters the membrane, permeate (filtrate) passes through, and concentrate (reject or retentate) carries the removed material away.
• Flux is permeate flow per unit of membrane area, usually in gallons per square foot per day (gfd):

$$\text{Flux (gfd)} = \frac{\text{Permeate flow (gpd)}}{\text{Membrane area (ft}^2)}$$

• Transmembrane pressure (TMP) is the pressure difference across the membrane. At constant flux, rising TMP means fouling.
• Specific flux is flux divided by TMP, corrected to a standard temperature (often 20°C) because water viscosity changes with temperature. Falling specific flux is the best fouling indicator.
• Recovery is the fraction of feed that becomes permeate:

$$\text{Recovery (\%)} = \frac{\text{Permeate flow}}{\text{Feed flow}} \times 100$$

• Rejection (for RO and NF) is the fraction of a dissolved substance kept out of the permeate:

$$\text{Rejection (\%)} = \left(1 - \frac{C_{\text{permeate}}}{C_{\text{feed}}}\right) \times 100$$

• Log removal value (LRV) is $\log_{10}(C_{\text{feed}}) - \log_{10}(C_{\text{permeate}})$.

Worked example. An RO train receives 500 gpm of feed and produces 375 gpm of permeate. Recovery is $375 \div 500 \times 100 = 75$ percent, and the concentrate flow is 125 gpm. If feed total dissolved solids are 1,200 mg/L and permeate TDS is 30 mg/L, rejection is $(1 - 30 \div 1{,}200) \times 100 = 97.5$ percent. A UF plant producing 2.0 MGD across 50,000 ft² of membrane runs at $2{,}000{,}000 \div 50{,}000 = 40$ gfd.

Fouling, Scaling and Pretreatment

• Particulate fouling and biofouling coat the membrane surface; organic fouling comes from natural organic matter.
• Scaling happens in RO and NF when dissolved salts such as calcium carbonate, calcium sulfate, barium sulfate or silica exceed solubility in the concentrate. Higher recovery concentrates salts further and raises scaling risk.
• RO pretreatment commonly includes antiscalant feed, sometimes acid to lower pH, and cartridge filters (about 5 µm) just ahead of the high-pressure pumps. Many RO membranes are polyamide and are damaged by free chlorine, so chlorine is removed before the membrane when those membranes are used. The silt density index (SDI) test checks whether feed water is clean enough.

MF and UF Operation

1. Filtration cycle: typically 15 to 60 minutes.
2. Backwash: permeate is pushed backward through the fibers, often with air scour, to remove the cake layer.
3. Chemically enhanced backwash (CEB): a short soak with chlorine, acid or caustic on a set schedule.
4. Clean-in-place (CIP): when TMP reaches the manufacturer's limit or specific flux drops, the unit is taken offline and recirculated with cleaning solutions. High-pH caustic, often with chlorine, removes organic and biological foulants; low-pH acid (such as citric acid) removes inorganic scale, iron and manganese. Spent cleaning solutions must be neutralized and disposed of properly.

Integrity Testing under the LT2ESWTR

A broken fiber or failed O-ring lets pathogens bypass the barrier, so 40 CFR 141.719(b) sets integrity requirements for membranes used to earn Cryptosporidium removal credit:

• Challenge testing establishes the removal efficiency the product can achieve.
• Direct integrity testing must be done on each membrane unit at least once each day. The most common method is the pressure decay test: the wetted membrane is pressurized with air below its bubble point, and the rate of pressure loss is compared with a control limit. The test must be able to detect a breach of about 3 µm and verify the log removal credited.
• Continuous indirect integrity monitoring, usually filtrate turbidity, must be recorded at least every 15 minutes. If filtrate turbidity exceeds 0.15 NTU for more than 15 minutes (two consecutive 15-minute readings), a direct integrity test must be performed immediately.
• A unit that fails its control limit must be taken offline, the breach located (for example, by a bubble test on the module) and repaired by pinning or replacing fibers, and the unit retested before returning to service.

Post-Treatment and Concentrate

RO permeate is low in alkalinity and often carries dissolved carbon dioxide, so it is corrosive. Plants strip carbon dioxide in degasifiers, add alkalinity or caustic and a corrosion inhibitor, and often blend with treated bypass water before distribution. Concentrate and CIP wastes must go to an approved outlet, such as a sanitary sewer with the utility's approval or a permitted discharge (Section 3.5).