At a Glance
Diafiltration uses the same membrane system as UF or MF. Additional cost is primarily wash water (buffer) consumption. Use untangle.bio for project-specific estimates.
How Diafiltration Works
Diafiltration is a membrane filtration technique where wash water (or buffer) is continuously added to the retentate while permeate is removed. This washes permeable solutes (salts, sugars, small impurities) out of the retentate without changing the retentate volume. The product, retained by the membrane, remains in the retentate at constant concentration while impurities are exponentially diluted.
The Exponential Wash-Out Equation
For a fully permeable solute (rejection = 0), the concentration after N diavolumes follows:
CN = C0 × e−N
Diavolume Removal Table
| Diavolumes (N) | Remaining Fraction | Cumulative Removal |
|---|---|---|
| 1 | 36.8% | 63.2% |
| 2 | 13.5% | 86.5% |
| 3 | 5.0% | 95.0% |
| 5 | 0.67% | 99.3% |
| 7 | 0.091% | 99.9% |
| 10 | 0.0045% | 99.995% |
Design Guide — Modes & Parameters
The choice between constant volume and discontinuous diafiltration depends on system constraints and buffer consumption targets.
Diafiltration Modes
| Mode | Description | Advantages |
|---|---|---|
| Constant Volume (CVD) | Wash water added at the same rate as permeate removal — retentate volume stays constant | Simplest to operate and model; optimal buffer usage for fully permeable solutes |
| Discontinuous (dilute & concentrate) | Alternate cycles of dilution (add buffer) and concentration (remove permeate) | Uses less buffer than CVD when solute rejection is partial (10–50%); easier with manual systems |
| Variable Volume | Wash water rate differs from permeate rate — volume changes during DF | Combines concentration and buffer exchange in a single step |
Key Design Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Number of diavolumes | 5–10 | 5 DV for 99.3% removal; 7–10 DV for trace-level clearance |
| Membrane MWCO | 10–100 kDa | Same as the UF step — diafiltration uses the same membrane |
| Wash buffer composition | Application-specific | Pure water for desalting; target buffer for buffer exchange |
| Retentate concentration | 5–50 g/L protein | Concentrate first, then diafiltrate to minimize buffer volume |
| TMP during DF | 1–3 bar | Maintain same TMP as concentration step for consistent flux |
| Temperature | 2–25 °C | Cold for biologics stability; room temp acceptable for robust molecules |
Best Applications for Diafiltration
| Application | Diavolumes | Wash Buffer | Use Case |
|---|---|---|---|
| Buffer exchange | 5–7 | Target formulation buffer | Transfer protein from purification buffer to formulation buffer |
| Desalting | 5–7 | Pure water (WFI) | Remove NaCl, ammonium sulfate, or other salts after chromatography |
| Sugar removal | 5–7 | Water or buffer | Remove glucose, lactose from protein solutions |
| Detergent removal | 7–10 | Water | Remove Triton X-100 or other detergents after viral inactivation |
| Solvent exchange | 5–7 | Target solvent | Exchange organic solvents (e.g., after RPC elution) |
| Impurity wash-out | 5–10 | Water | General small-molecule impurity clearance |
Cost Considerations
Capital Cost (CAPEX)
Diafiltration uses the same TFF membrane system as ultrafiltration — no additional membrane hardware is required. The primary CAPEX addition is a wash buffer inlet line with flow control (pump or pressurized vessel) and a buffer preparation/storage tank. In untangle.bio, diafiltration is modeled as the wash water inlet on UF/MF operations.
Key Cost Drivers
| Factor | Impact |
|---|---|
| Buffer consumption | Primary cost — 5–10× the retentate volume per DF cycle |
| Buffer preparation | WFI or buffer salts, preparation tank, and quality testing |
| Process time | DF adds 30–120 minutes depending on diavolumes and flux |
| Membrane area | Same as UF step — sized for flux at target protein concentration |
Operating Cost (OPEX)
Buffer (water or formulation buffer) is the dominant operating cost. At 5 diavolumes with a 50 L retentate, 250 L of buffer is consumed per batch. For pharma applications using WFI at several dollars per liter, this is significant. Energy costs are minimal (same pumps as the UF step).
Frequently Asked Questions
What is the difference between diafiltration and ultrafiltration?
Ultrafiltration concentrates the retentate by removing permeate (volume decreases). Diafiltration adds wash buffer at the same rate permeate is removed, maintaining constant volume while washing out permeable impurities. In practice, a UF/DF process first concentrates (UF mode), then exchanges buffer (DF mode) using the same membrane system.
How many diavolumes do I need?
For fully permeable solutes: 5 diavolumes removes 99.3%, 7 removes 99.9%, and 10 removes 99.995%. For partially rejected solutes (10–50% rejection), more diavolumes or discontinuous mode may be needed. untangle.bio caps at 10 diavolumes as the practical maximum.
Should I concentrate before or after diafiltration?
Always concentrate first, then diafiltrate. Buffer consumption equals N diavolumes multiplied by retentate volume. Concentrating 2-fold before DF cuts buffer usage in half. The exception is when high protein concentration causes viscosity issues or gel polarization on the membrane.
Can diafiltration remove partially rejected solutes?
Yes, but less efficiently. For a solute with rejection R, the effective removal per diavolume is (1 − R) × 63.2%. A solute with 50% rejection needs roughly twice as many diavolumes as a fully permeable solute. Consider discontinuous (dilute-concentrate) mode for better efficiency with partially rejected solutes.
Related Separation Techniques
Design a Diafiltration Step Into Your Process
Connect wash water to UF or MF operations in your flowsheet and simulate buffer exchange with real mass balance.
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