Softener, Sand & Carbon: Pretreatment Guide

A sand or multimedia filter is a pressure vessel packed with graded quartz sand, anthracite or both. Water passes down through the bed at 10–15 m/h, and particles down to about 100 µm are captured. From feed up to 100 NTU, the outlet is typically ≤5 NTU. The sand filter series covers 0.5–50 m³/h with motors from 0.37 to 7.5 kW, and each vessel is sized from bed area rather than from a nominal model number.

The filter sits first because everything downstream prefers clear water. Turbidity blinds carbon, coats resin and fouls membranes. A sand filter does not remove dissolved salts, hardness, nitrate or colour; it is a physical barrier only. When the source is above about 50 NTU, runs shorten and backwash water use rises quickly, so coagulation or settling should come before the filter rather than a larger vessel after it.

Hardness is calcium and magnesium, measured as mg/L calcium carbonate. Below 60 ppm, scale forms slowly and a softener is rarely economic. Between 60 and 120 ppm the decision depends on temperature and equipment: a coffee machine, steam boiler or hot-water cylinder may still need protection, while a cold-water line may not. Above 120 ppm, softening is usually justified; above 180 ppm it is normally necessary for heaters, boilers and RO feed.

The common units are easy to confuse. One grain per gallon is 17.1 mg/L as CaCO₃, one German degree is 17.8 mg/L, and one French degree is 10 mg/L. A report in gpg therefore needs conversion before it can be compared with a selection table. Hardness is not a health parameter, but it is a cost parameter: it shortens heat exchanger life and raises detergent use in laundries.

The CK-SF series covers 0.5–15 m³/h in eight catalogue sizes. Resin vessels run from 844 to 3672, resin charges from 30 to 650 kg and brine tanks from 25 to 800 L. Outlet hardness stays below about 1 ppm as CaCO₃ until the resin approaches exhaustion. Automatic valves meter treated volume and start regeneration, which typically takes 60–90 minutes.

Salt is the main consumable. A practical setting is 80–160 g of salt per litre of resin per regeneration, with brine at 8–12%. The theoretical minimum is about 1.2 kg sodium chloride per kilogram of hardness removed; real plants use roughly twice that because regeneration is not perfectly efficient. Twin-tank units alternate so that softened water continues to flow while one vessel regenerates.

Activated carbon removes free chlorine, chloramine, taste, odour and dissolved organics by adsorption. In a pretreatment train its most important job is dechlorination: free chlorine above 0.1 ppm oxidises polyamide RO membranes, and the damage is irreversible. It also protects cation resin, which chlorine degrades over time, and it lowers the organic load that would otherwise foul the membrane.

Design is set by empty bed contact time, not by vessel volume alone. Dechlorination typically needs 10–15 minutes of EBCT; chloramine, taste and odour need 15–20 minutes. Water-treatment carbons commonly have an iodine value of 700–1100 mg/g and a surface area of 500–1500 m²/g, but iodine value describes micropore volume and does not guarantee removal of a specific contaminant.

Carbon is also the stage most often left without monitoring. Chlorine breakthrough can reach a membrane within days of resin exhaustion, so a sample tap and a chlorine or ORP analyser at the carbon outlet are not optional on an RO feed. Replace the carbon on breakthrough, not only on a calendar date.

Specify sample taps before and after every stage, pressure gauges across each bed, and a drain sized for backwash. The sand filter and carbon filter need backwash flow several times the service flow, so the raw-water supply and drain must be checked against that peak, not against the product flow. A train that cannot backwash itself will pass water quality only until the first dirty bed.

Start with the laboratory analysis and the peak hourly flow, not the daily volume. Size the sand filter at 10–15 m/h bed velocity, the carbon filter at 5–15 m/h with 10–15 minutes EBCT, and the softener from peak flow, inlet hardness and the hours available between regenerations. Then check that the backwash peak can be supplied without starving the process. For a 10 m³/h sand filter, backwash may demand 35–45 m³/h for 5–10 minutes.

A train sized only on average flow will pass water quality on paper and fail in summer. Peak demand, backwash demand and regeneration downtime are the three numbers that decide whether the plant keeps running. Write them on the design sheet together with the maximum inlet turbidity, hardness and chlorine, so that a later capacity change can be checked against the original assumptions.

Weekly, record pressure differential across the sand and carbon beds, outlet hardness and carbon outlet chlorine. Monthly, verify backwash flow and duration and look for media in the sight glass. Annually, inspect media depth, clean the brine tank and calibrate the instruments. Replace sand every 3–5 years, carbon on breakthrough rather than on age alone, and resin when capacity stays low after the regeneration settings and salt dose have been verified.