Pure Water System Buyer’s Guide 2025: How to Choose the Right RO/DI Equipment

This engineer-to-engineer guide explains what a pure water system is, how RO/DI/EDI trains fit into modern plants, and how to size, specify, install and maintain systems with confidence. You’ll find decision maps, component questions, a quick sizing worksheet, a cost model, and an RFQ checklist.

Reviewed by Stark Water Process Engineering Team • Last updated: 20 Oct 2025

1) What a pure water system is & where it fits

A pure water system removes salts, hardness, organics and microbes to meet a defined product-water spec. Common trains:

• RO only (polishing elsewhere): general process water, beverage make-up, lab pre-treatment.
• RO → DI (mixed bed): high quality with regeneration chemicals; robust when CO₂/silica are controlled.
• RO → EDI: continuous high resistivity without acid/caustic; needs good RO and interstage conditioning.
• RO → polishing (carbon/UF/UV/0.2 μm): low TOC and microbiological risk for bottling & point-of-use.

Anmeldung	Typisches Ziel	Indicative train
Boiler make-up	5–20 μS/cm	Pretreatment → RO → (optional) softener/MB
Food & beverage	1–10 μS/cm; low TOC	Pretreatment → RO → carbon/UV → final filter
Lab/process	0.055–1 μS/cm (18–1 MΩ·cm)	Pretreatment → RO → EDI or MB → polishing
Elektronik/UPW	18 MΩ·cm; low silica/TOC	Pretreatment → RO → EDI → mixed-bed → UPW loop

2) Pure Water System Quick decision map

1. Target resistivity/conductivity: ≤1 μS/cm → add DI/EDI; ≥1 μS/cm → RO with polishing may be enough.
2. Chemical policy: no acid/caustic on site → choose EDI; otherwise DI mixed bed is viable.
3. CO₂ & silica: if high, add interstage pH/degassing or choose DI mixed bed polishing.
4. Micro/TOC risk: add carbon/UV/0.2 μm and hygienic loop design.

3) Source water drives design

Test TDS, hardness, alkalinity/CO₂, iron/manganese, turbidity/SDI, chlorine/oxidants, and bacteria. Results decide pretreatment and RO recovery limits.

• Hardness → scale risk → softener/antiscalant + recovery limits.
• Iron/Mn → oxidize & filter before RO; pick non-fouling media.
• High SDI → multimedia or UF pretreatment.
• Free chlorine → activated carbon and/or SBS dosing to protect polyamide RO.

4) Key components & vendor questions

RO-Membranen

Ask for brand class, array (4040 vs 8040), design flux, recovery and validated replacement cycles. See 4040 vs 8040.

Hochdruckpumpe

Check duty point, efficiency, NPSH, seals and noise; match to temperature-corrected RO curve.

Vorbehandlung

MMF for solids, activated carbon for chlorine/organics, softener or antiscalant for hardness, UF when SDI is unstable.

Instrumentation & controls

Pressures & ΔP, flows, temperature, feed/permeate conductivity, ORP, pH (interstage), tank levels, event logging.

Skid hygiene

CIP ports, drainable low points, labeled piping and safe chemical access.

5) Sizing essentials

Schritt	Faustformel	Anmerkungen
Average flow	From process demand (m³/h)	Prefer measured data
Erholung	60–85% typical	Respect scale/fouling limits
Peak factor	1.3–1.6×	Size tank + loop to cover peaks
Loop velocity	0.9–1.5 m/s	Micro control via turbulence
Final filter	0.2–0.45 μm integrity-testable	Beverage & sensitive tools

6) Materials & workmanship

• SS316 for sanitary/high-temp loops; SS304 for frames/common headers.
• uPVC/PPR acceptable on cold non-sanitary runs within rating.
• Compatible elastomers (EPDM/FKM/PTFE), clean welds, drainability.

See stainless-steel tanks & housings.

7) Installation, training & warranty

• Commissioning with instrument calibration and baseline logs.
• Operator training—start/stop, CIP/SIP, chemical safety, daily checks.
• 12-month warranty from start-up; optional extensions.
• Spares: cartridges, O-rings, SBS/antiscalant, pump seal kit, membrane end-caps.

8) Cost of ownership

Model electricity (pump kWh), chemicals (SBS/antiscalant/cleaners), membranes & cartridges, labor and sanitation cadence. Quick checks: RO OPEX-Rechner.

9) Compliance & documentation

• P&IDs, wiring diagrams, instrument ranges & alarm setpoints.
• FAT/SAT with acceptance data and baseline logs.
• PM schedule and validated CIP/SIP procedures.
• Align claims to WQA resources und ASTM D1193 when applicable.

10) RFQ template (copy & paste)

Company & site	-
Average / peak demand	— m³/h / — m³/h
Target quality	— μS/cm (or — MΩ·cm); silica ≤ — mg/L; TOC ≤ — ppb
Speisewasser	TDS —; hardness — as CaCO₃; SDI —; Fe/Mn —/—; free Cl₂ —; pH —
Preferred train	RO / RO+DI / RO+EDI / RO+polish
Materials	SS316 / SS304 / uPVC / PPR; tank & loop spec
Utilities	Power —, compressed air —, drain —
Kontrolliert	PLC/HMI brand, remote I/O, data logging
Delivery & services	Delivery date, installation/commissioning, training, warranty

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11) FAQs

RO vs DI vs EDI—what’s best for my plant?

RO for bulk desalting; DI for highest resistivity with chemical regeneration; EDI for continuous high resistivity without acid/caustic when interstage CO₂/silica are controlled.

How long do RO membranes last?

Typically 3–5 years with stable pretreatment and periodic CIP; see our membrane care guide.

Stainless vs plastic piping?

SS316 for sanitary/high-temp loops; SS304 for frames/headers; uPVC/PPR for cold non-sanitary runs as allowed.

Internal links: RO membrane housing diagram - Stainless-steel tanks & housings - LSI-Rechner - Chlorine dosage calculator

Technical guidance only—verify with your feed tests and OEM instructions.