Industrial DI Water System: Design, Sizing & Quality Targets

This engineering guide explains how to plan an industrial di water system—from RO pretreatment and EDI vs mixed bed choices to sizing tables, resistivity targets, polishing steps, instrumentation, and a bill-of-materials template. It is written for factories, laboratories, food & beverage plants, hotels, and surface-treatment shops that need reliable deionized water from wells or municipal supplies.

1) What Is a DI Water System (vs RO & “Ultrapure”)?

A di water system deionizes water by removing dissolved ions to reach a target resistivity or conductivity. RO (reverse osmosis) is usually the primary desalting step; electrodeionization (EDI) or a mixed bed (SAC+SBA resins) then polishes the RO permeate to very low ionic levels. “Ultrapure” is a use-case label (electronics/analytical labs), not a single machine. Final requirements depend on your industry standard and validation plan.

In practice: wells/municipal feed → pretreatment (particulates, organics, hardness control) → RO → EDI or mixed bed → UV (microbial/TOC control) → 0.2 μm final filter → storage and recirculating loop (if required).

2) Quality Targets & Conversions (Resistivity ↔ Conductivity)

Specify quality as resistivity (MΩ·cm) or conductivity (μS/cm). The quick conversion is:

conductivity (μS/cm) = 1 / resistivity (MΩ·cm)

Examples: 18.2 MΩ·cm ≈ 0.055 μS/cm; 1.0 MΩ·cm ≈ 1.0 μS/cm; 0.2 MΩ·cm ≈ 5 μS/cm.

Application (typical)	Common target	Notes
General industrial rinse, boilers makeup (non-high-pressure)	0.1–1.0 MΩ·cm (10–1 μS/cm)	RO + mixed bed often sufficient
Laboratory Type II/III, many F&B utilities	≥ 1–10 MΩ·cm (≤ 1–0.1 μS/cm)	RO + EDI or high-capacity mixed bed
Electronics/analytical Type I	~18 MΩ·cm (~0.055 μS/cm)	EDI + polishing loop; add TOC control

Values above are engineering starting points. Use your sector’s standard (e.g., lab water types, corporate SOPs) for formal acceptance criteria.

3) Di Water System Process Trains that Work in Industry

Storage & loop — stainless steel tank with vent filter, spray ball and recirculation to maintain quality.

Pretreatment — multimedia filtration (MMF) and activated carbon; add softening or antiscalant to protect RO; keep SDI low.

RO desalting — core TDS removal; choose element count/staging for temperature, recovery, and fouling risk.

EDI or Mixed Bed polishing — see Section 4 for selection and sizing.

UV (254 nm) そして 0.2 μm final filter — control microbes and particulates before points of use.

4) Fast Sizing: RO, EDI or Mixed Bed

4.1 RO Sizing (early estimate)

• Design permeate flow Q_p (m³/h) and target recovery R (%) → feed ≈ Q_p/R.
• Initial flux for brackish RO: start around 10–15 LMH (L/m²·h) and adjust per membrane datasheet and temperature.
• Pick vessel/element count after vendor selection; small plants often use 2:1 or 2:2 staging; larger trains may use 3:2, etc.
• Keep recovery conservative if fouling/scaling risk is high; validate with antiscalant software or pilot data.

4.2 EDI vs Mixed Bed — Selection

Criteria	EDI polishing	Mixed bed polishing
Operating mode	Continuous, no bulk chemicals	Batch; requires acid/caustic regeneration
Feed water window	Works best with stable RO permeate (low hardness/CO2)	More tolerant to feed swings
Uptime / staffing	High uptime; skilled commissioning	Simple hardware; periodic regen
OPEX	Electric power; periodic clean-in-place	Acid/caustic, neutralization & safety
Targets	High resistivity with stable operation	Can reach high resistivity; performance depends on resin & regimen

4.3 EDI Sizing (rule-of-thumb)

• Confirm RO permeate is within the vendor window (low hardness, low silica/CO₂, low TOC).
• Select stack size by product flow (m³/h) そして ionic load; apply temperature factor and safety margin for seasonal swings.
• Allow bypass/recirculation for load spikes; instrument with resistivity and flow on each skid outlet.

4.4 Mixed Bed Sizing (rule-of-thumb)

Mixed bed capacity is expressed as equivalents per liter (eq/L). Estimate required resin volume:

Resin volume (L) ≈ [meq/L (ionic load after RO) × flow (m³/h) × run time (h)] ÷ [resin capacity (eq/L)] × safety factor

Set run time to your preferred regeneration interval (e.g., 8–24 h). Use supplier capacity for your resin grade and chemistry.

5) Polishing, Storage & Distribution

• UV at 254 nm for microbial control; add 185 nm UV if TOC reduction is required by SOP.
• Final filter 0.2 μm at point-of-use or before loop return; sterile vents on storage tanks.
• Storage tank in SS304/316L with spray ball, sloped bottom, and recirculation nozzle.
• Loop in SS or sanitary plastic; size for turbulent velocity to maintain quality.

6) Materials & Instrumentation

• Pipework: UPVC/PP for pretreatment; SS316L or sanitary plastics downstream of polishing.
• Key meters: conductivity/resistivity (at RO permeate, polisher outlet, loop return), TOC (if required), SDI, flow, ΔP.
• Safety: chemical dosing bunds, eyewash, neutralization capacity for mixed-bed regeneration waste.

Related products:

• RO membranes & housings, dosing pumps, activated carbon
• Stainless-steel contact/storage tanks
• Water-treatment calculators

7) Bill-of-Materials (BOM) Template

• Pretreatment: MMF, GAC, softener or antiscalant dosing, cartridge filters
• RO skid: high-pressure pump, vessels/elements, interstage instrumentation, CIP ports
• Polisher: EDI stacks or mixed bed resin vessels with regeneration/CIP accessories
• Polishing: 254 nm UV (option 185 nm), 0.2 μm final filter, sanitary valves & gauges
• Storage & loop: SS316L tank with vent filter & spray ball, recirculation pump, loop instruments
• Controls: PLC/HMI, resistivity/TOC transmitters, flow/ΔP, quality interlocks and alarms
• Docs: P&ID, layout, FAT/SAT checklists, IQ/OQ templates

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8) FAQ

Is a di water system the same as reverse osmosis?

No. RO removes most dissolved salts; a di water system adds a polishing step (EDI or mixed bed) to reach very low ionic levels.

EDI vs mixed bed—when should I choose each?

Choose EDI for continuous operation without bulk chemicals and stable RO permeate; choose a mixed bed for wider tolerance to feed swings or when simple batch regeneration fits the site better.

What resistivity should I target?

Use the requirement in your SOP or industry guideline. As a starting point: 1–10 MΩ·cm for many industrial uses; ~18 MΩ·cm for Type I lab water.

Can I retrofit DI after an existing RO?

Yes. Verify RO permeate quality and flow/pressure; then add EDI or a mixed bed skid, plus UV and a 0.2 μm filter before storage/loop.

9) References (External)

• EPA – Drinking Water Basics
• ASTM – Relevant water quality standards (overview)

Stark Water Engineering Team