A demin plant removes dissolved ions by exchanging cations for H+ and anions for OH-. A defensible design starts with feed-water ion analysis, flow, cycle length, conductivity and silica targets, resin operating capacity, and regeneration-waste limits—not with a vessel diameter.
PT Beta Pramesti Asia uses those inputs to decide whether a train should use SAC-SBA alone, add WAC/WBA or a degasser, finish with mixed-bed polishing, or place RO ahead of ion exchange. This guide provides an auditable design basis; final values must be confirmed against the selected resin data and the system performance test.
What Is a Demin Plant and How Does It Work?
TDS represents salts dissolved in water. Those salts dissociate into cations such as Ca²⁺, Mg²⁺, Na⁺, and K⁺, and anions such as SO₄²⁻, CO₃²⁻, Cl⁻, HCO₃⁻, and NO₃⁻. In two-stage demineralization, the cation resin exchanges cations for H+, then the anion resin exchanges anions for OH-. The released H+ and OH- form H₂O.
This sequence matches DuPont’s Fundamentals of Ion Exchange, which describes demineralization with hydrogen-form cation resin followed by hydroxide-form anion resin. Ion exchange does not remove suspended solids, oil, or microorganisms; pretreatment must control those contaminants.
SAC, WAC, SBA, WBA, Degasser, and Mixed-Bed Decision Table
| Stage or train | Primary duty | When to assess it | Deciding data |
|---|---|---|---|
| SAC | Exchanges the common cations | Base cation stage in a demineralizer | Total cations, hardness, sodium, resin capacity, regenerant |
| WAC before SAC | Removes cations associated with alkalinity | Carbonate hardness and alkalinity are material and acid efficiency matters | Alkalinity, hardness, resulting CO₂, required cycle |
| Degasser after cation | Strips CO₂ and reduces the anion load | Feed bicarbonate >1 meq/L can be used as an initial assessment trigger, especially at high flow | HCO₃⁻, flow, ventilation, blower energy, discharge-air conditions |
| SBA | Exchanges strong and weak anions, including silica according to resin type and conditions | Base anion stage where broad anion removal is required | Total anions, silica, remaining CO₂, ionic leakage |
| WBA before SBA | Removes free mineral acids | Strong-anion load is high and caustic efficiency warrants assessment | Chloride, sulfate, nitrate, organics, operating capacity |
| Mixed bed | Polishes residual cations and anions in one vessel | Product quality must be tighter than the primary demineralizer output; TDS <5 mg/L can trigger an initial assessment | Conductivity, silica, sodium, run length, regeneration method |
| RO + mixed bed or RO + EDI | Removes most salt before final polishing | Feed TDS is high, regeneration waste dominates lifecycle cost, or product quality is very tight | RO recovery, scaling, energy, membrane replacement, concentrate disposal |
This is not a list of stages that every plant needs. Compare at least two alternatives on the same basis: product-water quality, water and chemical consumption, waste volume, energy, operator availability, redundancy, and lifecycle cost. For commercial equipment, see Beta’s industrial demineralizer systems, mixed-bed systems, and BWRO pretreatment.
Inputs Required Before Resin Is Sized
Use representative laboratory analyses for normal and worst-case feed conditions. At minimum, record Ca²⁺, Mg²⁺, Na⁺, K⁺, alkalinity/HCO₃⁻, Cl⁻, SO₄²⁻, NO₃⁻, silica, TDS/conductivity, pH, temperature, iron, manganese, turbidity, a relevant organic indicator such as TOC, and residual oxidants. The cation-anion balance in meq/L is also a data-quality check; investigate a material imbalance before design continues.
The operating basis should state:
- average and peak flow in m³/h;
- operating hours and daily product volume;
- minimum service cycle before regeneration;
- conductivity, silica, sodium, and other process-water targets;
- turndown, redundancy, and available regeneration window;
- permitted regenerant type and concentration onsite; and
- neutralization capacity and regeneration-waste discharge limits.
How to Calculate Ion Load and Resin Volume
Keep the units consistent. With concentration in meq/L, use:
- Cation load (eq/cycle) = flow (L/h) × cycle (h) × total cations (meq/L) ÷ 1,000
- Anion load (eq/cycle) = flow (L/h) × cycle (h) × total anions (meq/L) ÷ 1,000
- Resin volume (L) = ion load (eq/cycle) ÷ resin operating capacity (eq/L)
- Hydraulic loading (BV/h) = flow (m³/h) ÷ installed resin volume (m³)
Operating capacity is not the total capacity printed on a brochure. This guide uses 60-90% of total capacity as an initial check and 8-40 BV/h as an initial hydraulic-loading range. Final values must come from the resin supplier’s curves for the selected resin, regenerant dose, temperature, leakage target, and flow configuration.
Worked, Auditable Sizing Example
Assume 10 m³/h flow, an eight-hour cycle, 3.0 meq/L total cations, and 2.5 meq/L total anions. For illustration only, the resin data being assessed gives operating capacities of 1.2 eq/L for the cation resin and 1.0 eq/L for the anion resin.
| Calculation | Cation | Anion |
|---|---|---|
| Water per cycle | 80,000 L | 80,000 L |
| Ion load | 240 eq | 200 eq |
| Resin volume from ion load | 200 L | 200 L |
| Hydraulic loading with 0.20 m³ installed | 50 BV/h | 50 BV/h |
The 0.20 m³ volume satisfies the example capacity calculation, but 50 BV/h exceeds this article’s initial 8-40 BV/h range. At 10 m³/h, an initial 40 BV/h ceiling requires at least 0.25 m³ of resin. The designer selects a volume that passes both ion-load and hydraulic checks, then allows for freeboard, backwash expansion, flow distribution, and performance margin using the manufacturer’s data. Do not use this illustration as a final size without actual water and resin data.
Regeneration, Performance Acceptance, and Breakthrough
Cation resins are commonly regenerated with HCl or H₂SO₄ and anion resins with NaOH. Co-flow sends regenerant in the service-flow direction; counter-current regeneration reverses that direction. Selection must consider product quality, rinse demand, distributor control, mixed-bed separation, chemical safety, and operator capability.
A completed valve sequence is not proof of successful regeneration. Set these criteria against the commissioning baseline and design guarantee:
| Check | Acceptance criterion to define |
|---|---|
| Product conductivity | Below the design limit after final rinse and stable during the service run |
| Silica and/or sodium | Below the application limit with no evidence of early leakage |
| Cycle capacity | Product volume to endpoint meets the design basis after correcting for feed quality |
| Pressure drop | Returns near the clean baseline; persistent increase triggers checks for fouling, channeling, or distributor problems |
| Acid/caustic consumption | Matches the regeneration recipe and batch balance, not only a tank-level reading |
| Rinse water | Volume and time to endpoint remain within the commissioning range |
| Regeneration waste | pH, conductivity, volume, and permit parameters are recorded before release or further treatment |
Trend conductivity, silica, sodium, differential pressure, throughput per cycle, and regenerant use. A shortening run can result from higher feed load, incomplete regeneration, organic fouling, resin oxidation, resin loss, or channeling. Diagnose the cause before increasing regenerant dose.
Regeneration Waste Belongs in the Design Basis
Spent acid, spent caustic, displacement water, and rinse water create batch streams whose pH and salinity change throughout regeneration. Segregate or combine them only after a compatibility review, meter the actual batch volume, provide equalization, and interlock neutralization so uncontrolled acid-caustic mixing cannot occur.
As of July 2026, Indonesia’s Government Regulation No. 22 of 2021 governs water-quality protection and management, while Minister of Environment and Forestry Regulation No. 5 of 2021 covers the procedure for technical approvals and operational-feasibility certificates in pollution control. Apply the facility’s current sector standard and technical approval; neutral pH alone does not prove compliance because TDS, sulfate, chloride, metals, or other parameters may still control discharge.
Demin Plant Design Checklist
- Complete ion analysis and cation-anion balance have been reviewed.
- Normal, maximum, and seasonal feed conditions are covered.
- Product targets include conductivity, silica, sodium, and the relevant endpoint.
- Resin volume passes both the ion-load and hydraulic-loading checks.
- Pretreatment addresses turbidity, iron, organics, and oxidants.
- Regeneration recipe, rinse time, water demand, and chemicals are calculated per cycle.
- Chemical tanks, ventilation, materials, bunding, safety showers, and interlocks are defined.
- Equalization, neutralization, sampling, and the regeneration-waste route are available.
- The proposal includes a written performance test and acceptance criteria.
- Operators have a log sheet for product quality, pressure drop, throughput, and regenerant use.
For resin specifications, the sister-site guide to ion exchange resin types and applications helps distinguish media and regeneration needs. To compare configurations against actual site data, send the feed analysis and product-water target through the Beta Pramesti Asia contact page.