Industrial Water Treatment: Boiler Feedwater, Cooling Water, RO/Process Water, and Wastewater Compliance — Selection + Thai Effluent/ASME/WHO Standards for Plants

A boiler running at full capacity with scale buildup that has cut efficiency by 30% — plant effluent that keeps failing the BOD test required by the Department of Industrial Works — a cooling tower harbouring Legionella bacteria — an RO membrane that failed in under a year because the feed water was never properly prepared. In nearly every case, these failures are not caused by a "bad machine." The root cause is a water treatment system that was selected for the wrong stream, or designed without covering all four water streams from the outset.

Industrial water in a factory must be managed as four fundamentally different streams: boiler feedwater requiring high purity to prevent scale and corrosion — cooling water requiring control of scaling, biofouling, and Legionella — process and RO water (utility water) to match the purity demands of the production process — and wastewater and effluent that must meet regulatory standards before discharge.

This article focuses on treating these four water streams inside the industrial plant. For a detailed look at cooling tower selection and Legionella risk management specifically, see Cooling Tower Selection and Water Treatment — Legionella. For compressed air quality and ISO 8573 dryer/filter systems, see Compressed Air Quality and ISO 8573 Systems.

1. Why Four Streams Must Be Managed Separately — Wrong Treatment for the Wrong Stream = Wasted Investment

The most common mistake in plant water planning is applying a single system to every use, or focusing exclusively on effluent while neglecting the quality of "incoming" water. Each stream has entirely different requirements and treatment processes.

2. Stream 1 — Boiler Feedwater

Why raw water destroys boilers

Raw water contains Ca²⁺ and Mg²⁺ that deposit as CaCO₃ and CaSO₄ on heat-transfer surfaces. Even 1 mm of scale increases fuel consumption measurably; thick scale causes hot spots that rupture tubes. Dissolved oxygen and CO₂ in the feedwater cause corrosion in economiser pipes and the boiler drum — silently costing thousands of baht in early tube replacement.

Key treatment processes

ASME/ABMA boiler water guidelines set increasingly strict limits on TDS, silica (SiO₂), alkalinity, and dissolved oxygen as boiler operating pressure increases. For high-pressure boilers (> ~600 psig), demineralized or RO-quality feedwater is typically required. System designers should reference the specific pressure-range tables in ASME CRTD-Vol.34 or ABMA publications for the actual boiler pressure in use.

Worked example: Blowdown Rate

If TDS in feedwater = 200 mg/L and the maximum allowable TDS in boiler water = 2,000 mg/L → blowdown rate = 200 / (2,000 − 200) × 100 ≈ 11% of steam output. This means approximately 11% of steam-equivalent water must be purged to keep boiler-water TDS from exceeding the limit. The higher the feedwater TDS, the more frequent the blowdown — and the greater the heat and water loss. This is precisely why investing in a softener or RO system pays back through reduced fuel and water costs.

3. Stream 2 — Cooling Water and Cooling Towers

Four risks that must be managed simultaneously

1. Scaling — as water evaporates in the cooling tower, dissolved minerals concentrate. CaCO₃ deposits on heat exchanger surfaces and reduces heat transfer. The tendency to scale is assessed by the Langelier Saturation Index (LSI) or Ryznar Stability Index (RSI): LSI > 0 indicates scaling tendency; LSI < 0 indicates corrosive tendency; a target of −0.5 to +0.5 is typical for most systems. Controlled with antiscalant and COC management.

2. Corrosion — affects copper alloys and mild steel in chiller tubes and piping. Controlled with corrosion inhibitors and pH adjustment to the correct range.

3. Biofouling — algae and slime block nozzles and fill medium, reducing system efficiency. Controlled with oxidising biocides (chlorine, bromine) and non-oxidising biocides rotated to prevent resistance.

4. Legionella — Legionella pneumophila thrives at 25–45°C in warm water with sediment and biofilm, and can cause life-threatening Legionnaires' disease. WHO publishes dedicated risk management guidelines for cooling towers — see also the companion article Cooling Tower Selection and Water Treatment.

Cycles-of-Concentration (COC) — the core metric for cooling water management

COC is the ratio of mineral concentration in the recirculating cooling water to that in the makeup water, measured by Cl⁻, conductivity, or SiO₂:

COC = [concentration in recirculating water] / [concentration in makeup water]

Worked example — COC and makeup water volume:

A 500 RT (refrigeration-ton) cooling tower system with an approximate evaporation rate of 0.75% of circulation flow:

• Evaporation rate ≈ 500 × 3.5 L/min·RT × 0.0075 ≈ 13 L/min
• At target COC = 4: Blowdown = Evaporation / (COC − 1) ≈ 13 / (4−1) ≈ 4.3 L/min
• Makeup water = Evaporation + Blowdown ≈ 13 + 4.3 ≈ 17.3 L/min
• If COC dropped to 3: Blowdown rises to 6.5 L/min, Makeup = 19.5 L/min → more water consumed

A COC of 3–5 is suitable for most systems. Too high a COC without an inhibitor programme accelerates scaling and Legionella risk. The right COC target must account for the local makeup water quality — which varies significantly across Thailand between surface water, groundwater, and municipal supply.

4. Decision Map: Choosing the Treatment Process for Each Stream

flowchart TD
    A["Identify the water stream
requiring treatment"] --> B{"Which stream?"}
    B -->|"Boiler feedwater"| C{"Boiler operating
pressure?"}
    B -->|"Cooling water"| D["Multimedia filter →
Antiscalant + Inhibitor + Biocide
COC + Blowdown control"]
    B -->|"Process / RO water"| E["Multimedia filter →
Activated carbon →
Softener → RO → UV/Cl₂"]
    B -->|"Wastewater / effluent"| F{"Nature of
wastewater?"}
    C -->|"Low–medium pressure
(< ~600 psig)"| G["Softener (ion exchange) +
Deaeration + O₂ scavenger +
Blowdown control"]
    C -->|"High pressure
(> ~600 psig)"| H["RO / Demineralization +
Deaeration +
Condensate polishing"]
    F -->|"Abnormal pH,
suspended solids"| I["pH neutralization →
Coagulation/Flocculation →
Clarifier / Sedimentation"]
    F -->|"Contains oil"| J["Oil interceptor / DAF
(Dissolved Air Flotation)
as first step"]
    F -->|"High BOD/COD"| K["Biological treatment:
Activated Sludge / MBR / MBBR
→ Sludge dewatering"]
    D --> L["Legionella risk assessment
per WHO guidelines"]
    E --> M["Online TDS / conductivity
monitoring"]
    I --> N["Verify DIW/PCD limits
before discharge"]
    J --> N
    K --> N

5. Stream 3 — Process and RO / Utility Water

When does process water require RO or DI quality?

Typical RO system sequence

1. Multimedia filtration — removes suspended solids and turbidity (sand, gravel, anthracite media)
2. Activated carbon (AC) — adsorbs chlorine, organics, colour, and odour. Critical step: chlorine must be removed before RO membranes because chlorine destroys polyamide membrane
3. Water softener — reduces Ca/Mg to prevent membrane scaling and fouling
4. RO membrane — pressure drives permeate through a semi-permeable membrane; ion, bacteria, and endotoxin rejection > 99.5% for quality membranes
5. Post-treatment — UV disinfection or chlorination for biological safety; pH adjustment as required

Frequently overlooked: RO produces reject water (concentrate) equal to roughly 20–40% of feed. Failing to plan for this stream is a common mistake. Reject can often be directed to non-critical uses (floor washing, landscaping) or blended into the wastewater treatment system. If discharged directly, high-TDS reject may itself require treatment to meet DIW/PCD limits.

6. Stream 4 — Wastewater and Effluent: Thai DIW/PCD Standards and Treatment Processes

Thai industrial effluent standards

The Department of Industrial Works (DIW) and the Pollution Control Department (PCD) jointly regulate industrial effluent in Thailand. Key parameters that must be controlled include:

Important notice: The precise numeric limits for each parameter differ by factory category as registered with the Department of Industrial Works (per Notification No. 37/2543 and subsequent amendments). Operators must verify current applicable limits directly with DIW at www.diw.go.th and PCD at www.pcd.go.th — these are updated periodically.

Typical wastewater treatment process train

flowchart LR
    A["Wastewater from
production process"] --> B["Equalization tank
(flow balance + pH
pre-equalisation)"]
    B --> C{"Contains
oil/grease?"}
    C -->|"Yes"| D["DAF / Oil separator
(remove oil first)"]
    C -->|"No"| E["pH neutralization
(adjust to 6–8)"]
    D --> E
    E --> F["Coagulation +
Flocculation
(settle SS + colour)"]
    F --> G["Clarifier /
Sedimentation
(remove settled sludge)"]
    G --> H{"BOD/COD
elevated?"}
    H -->|"Yes (food / chemical
processes)"| I["Biological treatment
Activated Sludge / MBR / MBBR
organic degradation"]
    H -->|"Low or already
compliant"| J["Sand filter /
polishing"]
    I --> J
    J --> K["Lab analysis
before discharge"]
    K --> L["Discharge —
passes DIW/PCD
limits"]
    G --> M["Sludge thickener +
dewatering
(filter press / centrifuge)"]
    M --> N["Sludge cake
→ licensed disposal"]

Biological treatment process options

7. Integrating All Four Streams in a Single Plant

flowchart TD
    R["Raw water / municipal supply"] --> S1["Shared pre-treatment:
Multimedia + AC filter"]
    S1 --> P1["Softener →
Deaerator →
Boiler feedwater"]
    S1 --> P2["Softener → RO →
Process / utility water
→ production lines"]
    S1 --> P3["Cooling tower makeup
+ Antiscalant / Biocide
+ COC control"]
    P1 --> W["Boiler blowdown
→ wastewater system"]
    P2 --> W
    P3 --> W
    W --> WT["Wastewater treatment:
pH neutral → DAF → Clarifier
→ Biological → Polishing"]
    WT --> D["Discharge (meets DIW/PCD)
or partial reuse"]

8. Checklist to Ask Your Water Treatment Contractor Before Signing

Provide the left column before requesting a quote, and obtain the right column confirmed before signing the contract.

9. What Buyers Most Often Overlook

Raw water analysis before design: a water treatment system designed without a raw water analysis is almost always either undersized or overbuilt. Water quality across Thailand varies significantly — between surface water, groundwater, and municipal supply, and between regions. Minimum analysis: TDS, hardness, pH, alkalinity, iron, manganese, silica, and coliform count.

RO reject water planning: an RO system produces 20–40% reject. "Forgetting" this stream is a surprisingly common mistake. If the reject TDS is very high, it may need additional treatment before it can be discharged legally through the effluent system.

Sludge disposal costs: a wastewater treatment plant continuously generates sludge that must be disposed of legally — usually requiring a licensed hazardous or industrial-waste disposal contractor. Sludge disposal is a significant operating cost that is often omitted from financial evaluations.

Continuous monitoring systems: manual grab-sample lab analysis provides a snapshot in time. Investment in online analysers (pH, conductivity, ORP) and basic SCADA gives early warning of upsets and provides the compliance audit trail that DIW inspectors increasingly expect.

Cross-stream interactions: boiler blowdown, RO reject, and filter backwash all flow to the wastewater system — the wastewater treatment plant must be designed to handle the peak combined flow, not just production-process wastewater alone.

The energy-water nexus: effective boiler and cooling water treatment directly reduces the plant's energy consumption. Less scale on heat exchangers means lower electricity and fuel costs — and this linkage makes water treatment investment straightforward to justify through an energy-saving ROI calculation. This connects directly to ISO 50001 energy management frameworks.

WHO standards for food and pharmaceutical process water: for factories where water contacts the product directly, reference the WHO Guidelines for Drinking-water Quality alongside US EPA guidance and Thai TIS/มอก. standards to ensure coverage of both chemical and microbiological parameters.

Consult the Engineering Team

The correct water treatment system begins with a raw water analysis and a clear definition of each stream's requirements — not equipment selection from a catalogue. Send us the flow rates for each stream, a raw water analysis report (if available), your factory's DIW registration category, and a description of current problems, and the engineering team will help size the system, specify the right process for each stream, and review your contractor's proposal before you sign.

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