Water softener (ion exchange unit)

01

Size the capacity from hardness, resin volume and salt dose

Chem-Aqua's technical bulletin explains that softeners remove scale-forming calcium and magnesium before boiler, cooling and reverse osmosis systems. Capacity needs three values: raw-water hardness in ppm, resin volume in cubic feet, and salt dose per cubic foot. Chem-Aqua gives a salt dosage of 15 lbs/ft3 resin (typ.) for an exchange capacity of 30,000 grains/ft3 resin [1]; lower doses reduce capacity but use salt more economically. Hardness in ppm divided by 17.1 gives grains per US gallon, and a 10% safety factor is applied to prevent hardness breakthrough. [1]

The bulletin's example: 118 ppm hardness (6.9 gpg), 6 ft³ of resin at 15 lb/ft³, giving 180,000 grains, 26,087 gallons, or 23,478 gallons after the safety factor. [1]

Editorial calculation in metric units: 6 ft³ is about 170 L of resin; 180,000 grains is about 11.7 kg of hardness as CaCO₃, which at 118 mg/L treats about 99 m³, or about 89 m³ with the 10% safety factor. The salt per regeneration is 6 × 15 = 90 lb, about 41 kg; 15 lb/ft³ is about 240 kg/m³ of resin. [1]

Editorial calculation: 30,000 grains/ft³ is about 1.37 eq/L, against a minimum total capacity of 2.0 eq/L for Purolite C100 resin, so the operating capacity Chem-Aqua gives at 15 lb/ft³ uses about 69% of the resin's total capacity. [1] [2]

Give the supplier the raw-water hardness and its range, iron and other contaminants, daily and peak volume, required soft-water hardness, operating hours, whether service must continue during regeneration, salt type and drain capacity.

02

Check service flow, backwash and resin limits

Purolite C100 is a gel polystyrene strong acid cation resin in sodium form for industrial softening. Its data sheet gives service flow rates of 10 to 40 bed volumes per hour depending on water quality, application and plant design, and calls for up-flow backwash that expands the bed by 50 to 70% for at least 10 to 15 minutes; over-expansion can carry resin out of the top of the vessel. The temperature limit is 120 °C and the particle size range 300–1,200 µm. [2]

Editorial calculation: 170 L of resin at 10–40 bed volumes per hour handles about 1.7–6.8 m³/h. Pressure drop rises with flow, bed depth, particulate matter trapped in the bed and incomplete classification of the bed. [2]

Tank size sets resin volume: Chem-Aqua's table lists, for example, 1 ft³ in a 10 in diameter tank, 4 ft³ in 16 in, 10 ft³ in 24 in and 65 ft³ in a 60 in tank. [1]

03

Industrial plants, valve-and-tank packages and household units

Foren, Chunke, Roagua, Moer and Aoteng list softening systems among industrial water treatment equipment; Canature lists water softeners with control valves and brine tanks; Basidewt lists softener systems with control valves, FRP tanks and resin; Hidrotek and Jubei list household water softeners.

04

Plan for added sodium, regeneration water, iron and salt quality

University of Nebraska–Lincoln Extension's NebGuide on household ion exchange softening gives side effects a buyer should plan for. Sodium is added to the water at about 8 mg/L for each grain per gallon of hardness removed, so water that was 10 gpg of hardness will have about 80 mg/L of sodium after treatment; the guide says people on restricted sodium diets should account for this and consult their physician. [3] Each regeneration uses approximately 50 gallons of water and places additional load on a septic system, and the guide puts the time for the brine to pass through the unit and flush the tank, before soft water is available again, at sixty to 120 minutes. [3] Editorial calculation: 50 US gallons is about 189 L (50 × 3.785).

On feed water, the guide says softeners may remove as much as 5-10 ppm of iron and manganese, that buyers should check the manufacturer's removal rating, and that excess iron (above 5 ppm) or hydrogen sulfide can reduce the effectiveness of the softener, calling for pretreatment such as greensand filters. [3] On salt, it advises using rock salt only if the manufacturer suggests it and only grades averaging less than 1 percent impurities, and notes that potassium chloride adheres more strongly to the resin, reducing exchange efficiency and requiring more salt. [3] For certification, it names the NSF and Water Quality Association programs, which use the same NSF standards. [3]

05

Classify the hardness and choose how regeneration is started

North Dakota State University Extension's WQ1031 (revised May 2022) classifies hardness as calcium carbonate (table) and takes 1 gpg as 17 ppm or 17 mg/L; citing NSF International, it says consumers see a problem once hardness passes 6 to 7 gpg. Some softeners also remove up to 10 ppm of iron and manganese, and water above 10 ppm may need a dedicated iron removal system. [4]

WQ1031 sorts softeners by how regeneration is started: semiautomatic (the operator starts it), automatic on a programmable timer, demand-initiated regeneration (DIR) in response to soft-water demand, which NDSU describes with two softening tanks and a brine tank so one tank softens while the other recharges, and off-site regeneration, where a spent tank is swapped for a recharged one. Recharging too early wastes salt and water; too late, performance falls off. Potassium chloride uses about 10% more salt than sodium chloride. [4]

Editorial calculation: Chem-Aqua's 118 ppm example water sits in NDSU's moderately hard band, and 118 ÷ 17 = 6.9 gpg, matching Chem-Aqua's 6.9 gpg at 17.1. [1] Ask the supplier which of the four regeneration modes it offers and whether soft water continues during regeneration.

06

Resin bed, vessel, control valve and brine tank

The distributor and freeboard row is the one that bites with this resin: without room for 50–70% bed expansion, backwash carries resin out of the vessel. [2]

07

What a softener quotation must state, from flow rate to tank material

Exchange capacity ties back to the sizing above: C100's 2.0 eq/L total against about 1.37 eq/L at 15 lb/ft³ of salt. [2] [1]

08

Send the water, demand and regeneration data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Water: [hardness and range; iron; other contaminants]
Demand: [daily volume; peak flow; hours]
Output: [required soft-water hardness]
System: [single or multi-tank; continuous service]
Regeneration: [salt dose; brine tank; drain]
Site: [pressure; temperature; connections; power]

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