Electrodialysis System Process Flow Diagram: 2026 Engineering Guide

What an Electrodialysis System Process Flow Diagram Actually Shows

An electrodialysis system process flow diagram is the engineer's working drawing of a unit that moves ions — not water — across a stack of alternating anion- and cation-exchange membranes under an applied DC field. Feed enters the stack, splits into a diluate (product) stream and a concentrate (brine) stream, and exits through separate nozzles while a fourth, closed loop — the electrode rinse — carries gas away from the anode and cathode compartments. Typical industrial ED treats brackish water at ~1 kWh per 1000 USG per 1000 ppm of salt removed plus ~2 kWh/1000 USG for pumping (ScienceDirect, ED/EDR chapter), and requires pretreatment to SDI ≤15, turbidity <2 NTU, TOC <15 mg/L, and pH 2–11 (per ScienceDirect Table 6).

On a PFD, the ED block is drawn as a multi-plate symbol — one rectangle for the membrane stack — fed by a feed/diluate manifold and a concentrate manifold, each with its own circulation pump and conductivity probe. The DC rectifier sits adjacent to the stack as a separate power block, sized at roughly 1–3 V per cell pair (ScienceDirect, ED/EDR chapter). Two electrochemical half-reactions define the electrode loop: at the cathode, 2e⁻ + 2H₂O → H₂ + 2OH⁻; at the anode, 2H₂O → 2H⁺ + ½O₂ + 2e⁻, or in a chloride-bearing feed 2Cl⁻ → Cl₂ + 2e⁻ (ScienceDirect, electrodialysis overview). The four-stream convention — feed, diluate, concentrate, electrode rinse — is what distinguishes an ED PFD from an RO PFD, which typically shows only feed, permeate, and reject. When the ED block sits in an industrial train, it is normally drawn downstream of pretreatment and upstream of an industrial RO system, evaporator, or crystallization skid for brine management.

Unit-by-Unit Walkthrough: From Raw Influent to Diluate and Concentrate Outlets

Drawing the PFD block by block keeps the diagram reviewable. The sequence below follows the flow from the raw inlet to the two stack outlets, with each block keyed to a real equipment selection the engineer can mark on the drawing.

1. Influent feed block. Raw wastewater or process brine enters an equalization tank fitted with a pH probe; the operating window is pH 2–11 (ScienceDirect Table 6). An automatic chemical dosing skid for acid or caustic trim sits on the line ahead of the cartridge filter.
2. Coarse screening. A rotary bar screen protects downstream pumps from rags and large debris — standard practice on any industrial wastewater plant.
3. Multi-media filtration. A multi-media filter drops turbidity below 2 NTU and brings SDI under 15; for food or oily streams an upstream DAF unit strips emulsified FOG to <1 mg/L before the media filter.
4. Cartridge guard. A 5 µm cartridge filter is the last line of defense against particulates that would foul membrane spacers.
5. Feed manifold. Treated water splits into a diluate loop and a concentrate loop. Each loop carries its own circulation pump, flow meter, conductivity probe, and pressure gauge — the four instruments every ED PFD must show.
6. Membrane stack block. Drawn as a single rectangle, the stack houses alternating CEM and AEM sheets (cell pair spacing 0.5–2 mm) with an anode compartment at one end and a cathode compartment at the other. The four nozzles are diluate in, diluate out, concentrate in, concentrate out; the electrode rinse connections tie to the electrolyte loop.
7. Electrode rinse loop. A separate closed loop with Na₂SO₄ or NaCl electrolyte carries H₂, O₂, or Cl₂ gas away from the electrodes; flow must be sized to keep gas blanketing below scaling limits, and the off-gas vents to a safe stack shown on the P&ID.
8. DC rectifier. The power block sits adjacent to the stack, wired through bus bars. Voltage is set to 1–3 V per cell pair; current is capped at roughly 60–80% of the limiting current density (LCD) to avoid water splitting at the membrane interface (ScienceDirect, electrodialysis overview).
9. Diluate and concentrate outlets. Diluate is routed to post-treatment (polishing RO, EDI, or reuse); concentrate is routed to brine management — RO reject, mechanical vapor recompression, or crystallization. For polarity-reversal EDR systems, the PFD adds four automated three-way valves on the diluate and concentrate manifolds — that single addition is what visually distinguishes EDR from conventional ED on the drawing.

ED vs EDR vs EDI: Three Stack Configurations on the Same Flow Diagram

Three ED variants are commonly drawn on industrial PFDs in 2026, and each changes the stack block in a specific way. Conventional ED is the baseline: fixed polarity, suited to clean brackish feed with TDS under ~5,000 mg/L, and the tightest pretreatment requirement. EDR — electrodialysis reversal — flips polarity every 15–60 minutes with simultaneous flow reversal; the periodic back-flushing lifts foulants off the membrane surface, and EDR stacks tolerate SDI up to 12 without appreciable fouling (ScienceDirect citing Allison, 1995). That tolerance is why EDR is the 2026 workhorse for industrial wastewater streams with variable feed. EDI, or electrodeionization, fills the diluate chambers with ion-exchange resin; the resin lowers cell resistance and continuously regenerates under the DC field, polishing diluate to resistivity above 15 MΩ·cm. A 2011 Siemens demonstration combined ED with EDI polishing to take seawater from ~32,000 mg/L TDS to ~500 mg/L at a composite 1.8 kWh/m³, with 30% recovery over a 50 m³/d pilot (ScienceDirect, citing Siemens). Chlor-alkali ED is the fourth configuration: a two-compartment cell with a single CEM between electrodes that drives current utilization close to 100% and produces NaOH above 10 wt.% alongside Cl₂ and H₂ (ScienceDirect, electrodialysis overview).

For an EDR retrofit, our own internal commissioning notes mirror the practice described in the forward osmosis commissioning protocol — sequence the polarity-reversal valves last, after hydraulic stabilization, to avoid pressure transients on the stack.

Feedwater Quality Limits and Pretreatment Equipment Translation

The ScienceDirect influent table is the binding spec the engineer must translate into equipment on the PFD. Each row of the table becomes one or two named blocks upstream of the stack, sized to the parameter and interlocked to the ED shutdown circuit.

The pH adjustment block is most often a packaged automatic chemical dosing skid feeding sulfuric acid or NaOH into the equalization tank. For sites already running a packaged integrated purification unit, the dosing panel and multimedia filter are pre-piped, which shortens PFD drafting by one or two blocks.

Energy, Current Density, and Stack Sizing on the PFD

Where the energy numbers belong on the drawing matters as much as the values themselves. The ScienceDirect ED/EDR chapter gives two figures that should be annotated directly on the PFD: separation energy of approximately 1 kWh per 1000 USG per 1000 ppm of salt removed at 18–22°C, written next to the rectifier block, and pumping energy of approximately 2 kWh per 1000 USG of product water, written next to each circulation pump. Current density is the lever that ties capex to opex: the limiting current density (LCD) is proportional to the target diluate concentration, and the stack area is inversely proportional to operating current density (ScienceDirect, electrodialysis overview). In practice, designers set operating current at 60–80% of LCD to leave headroom for feed variability — push above LCD and water splitting at the membrane interface destroys current efficiency; sit well below LCD and membrane area (and capex) balloon.

The economic ceiling is sharp. Seawater desalination from 36,000 mg/L to 800 mg/L by ED requires ~26 kWh/m³ (~93.6 MJ/m³) per Korngold (1982), as cited in ScienceDirect — that figure is roughly an order of magnitude above seawater RO and is why ED is reserved for brackish water and brine concentration, not open-ocean desalination. The same physical limit is what makes ED attractive for inland ZLD front-ends and chlor-alkali feed, where the concentrate can be valorized rather than discarded.

2026 Engineering Checklist for Drafting the ED Process Flow Diagram

1. Confirm influent meets SDI 15, turbidity <2 NTU, TOC <15 mg/L, Fe 0.3 mg/L, pH 2–11, free Cl₂ 0.5 mg/L continuous.
2. Select ED or EDR based on feed variability; default to EDR when SDI₅ sits between 5 and 12 or feed composition swings.
3. Size the rectifier at 1–3 V per cell pair and the current at 60–80% of the limiting current density.
4. Specify the four-stream manifold — feed, diluate, concentrate, electrode rinse — with isolation valves on each.
5. Route concentrate to brine management (RO reject, MVR evaporator, or crystallization); route electrode off-gas to a safe vent on the P&ID.
6. Document the energy budget on the PFD: separation (1 kWh/1000 USG per 1000 ppm) + pumping (~2 kWh/1000 USG) + transformer losses.

Frequently Asked Questions

What streams are shown on an electrodialysis PFD?

Four internal streams: feed/diluate in, diluate out (product), concentrate in, concentrate out, plus a closed electrode rinse loop carrying Na₂SO₄ or NaCl electrolyte. The electrode loop vents H₂, O₂, or Cl₂ to a safe stack (ScienceDirect, electrodialysis overview). The anode and cathode half-reactions are 2H₂O → 2H⁺ + ½O₂ + 2e⁻ and 2e⁻ + 2H₂O → H₂ + 2OH⁻ respectively.

What influent limits apply to an ED stack?

SDI 15 (5-min), turbidity <2 NTU, TOC <15 mg/L, oil/grease <1 mg/L, Fe 0.3 mg/L, Mn 0.1 mg/L, free Cl₂ 0.5 mg/L continuous (15–20 mg/L spikes tolerable), pH 2–11 (ScienceDirect Table 6). EDR extends tolerable SDI to 12 but does not remove the need for multimedia and cartridge filtration.

How much energy does an industrial ED system use?

About 1 kWh per 1000 USG per 1000 ppm of salt removed for separation, plus about 2 kWh per 1000 USG of product for pumping, at 18–22°C (ScienceDirect, ED/EDR chapter). Seawater at 36,000 mg/L feed to 800 mg/L product runs ~26 kWh/m³ and is generally uneconomic; brackish water and brine concentration are the economic sweet spot.

How is EDR different from ED on a PFD?

The PFD adds four automated three-way valves on the diluate and concentrate manifolds so the flow paths swap when the DC polarity is reversed, typically every 15–60 minutes (ScienceDirect, electrodialysis overview). That single change is what visually distinguishes an EDR PFD from a conventional ED PFD, and it is why EDR tolerates SDI up to 12 without appreciable fouling.

Further Reading

• DAF system process flow diagram walkthrough
• ZLD sizing guide for machining coolant blowdown