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Softening resins are commonly used as filter media in water softening equipment, but in practice, the same batch of resin can perform differently for different customers. The issue of softened water not meeting standards can be analyzed in two parts:
One. Initial commissioning of water softening equipment with hardness exceeding standards is mainly due to the following reasons:
A. The O-ring at the connection between the central tube and the fully automatic softening control valve does not form a seal. In this case, you should check:
(1) Whether the length of the central tube is sufficient and whether the outer diameter meets the requirements;
(2) Whether the O-ring was forgotten;
(3) Whether the O-ring is damaged;
(4) Whether the central tube is damaged or cracked.
B. The ratio of raw water hardness to the height of the ion exchange resin layer is too high. The feed water hardness for a primary sodium ion exchanger should be less than 8 mmol/L.
C. The flow rate of softened water is too fast. It's strictly forbidden to operate at the equipment's maximum allowed flow for long periods. Usually, the flow rate for co-current regenerated fixed beds is 20–30 m/h, and the upper limit is just an instantaneous maximum, so long-term operation at this rate is not allowed.
D. There's a lot of gas in the resin tank, which might come from air in the feed water or improper closure of the air check valve during slow washing.
E. No large-grain industrial soft water salt was used.
F. The automatic soft water control valve is leaking internally: typically, when the control valve leaks internally, water flows out simultaneously from both the soft water outlet and the waste water outlet.
Two. The reasons why the softened water produced by the running equipment exceeds hardness standards are mainly as follows:
A. The resin softening regeneration cycle is set too long, or the softened water flow meter is faulty and causes inaccurate measurement, preventing the cation exchange resin from regenerating in time when it should. Before replacing the resin, there wasn’t a corresponding standard match. Also, if the customer previously used electricity-rated or imported resin but replaced it with domestic water-standard resin, the water quality will differ from before.
B. The backwash time is too short, so some of the waste brine that should have been washed away ends up in the softened water tank.
C. Unstable raw water pressure leads to insufficient water in the salt tank, insufficient salt uptake, and inadequate backwash. Any of these can cause the softened water to exceed hardness standards after resin regeneration, affecting the water quality in the softened water tank.
D. When there's very little salt in the salt tank and it’s not added in time, the regeneration of the ion exchange resin won't work well.
E. Operational mistakes, like closing the raw water valve or opening the bypass ball valve during the ion exchange resin regeneration process, or leaks.
F. The ion exchange resin can get poisoned and lose its exchange capacity. High levels of Fe³⁺, Al³⁺, and manganese in the raw water can poison the resin (at this point, the resin turns dark red), reducing the resin's exchange capacity and decreasing the water production per cycle.
Water production calculation formula:
Cycle water production (m³) = {Resin working exchange capacity (mol/m³) × Resin volume (m³)} ÷ Raw water hardness (mmol/L)
1. Resin Working Exchange Capacity (Eg)
Meaning: How many hardness ions a liter of resin can actually exchange; it's a key performance indicator.
Value: Usually taken as 1000 mmol/L (1 mol/L) for calculations, which represents a theoretical ideal value. The actual resin volume exchange is about 60% of this (depending on water quality, process, etc.).
2. Resin Volume (V)
Meaning: The actual amount of resin packed inside the tank.
Calculation: Cylinder volume formula π × radius² × height. Make sure the units are consistent, either liters (L) or cubic meters (m³).
3. Raw Water Hardness (H)
Meaning: The total concentration of calcium and magnesium ions in water.
Unit conversion: If the water quality report lists the unit as mg/L (as CaCO₃), you need to convert it to mmol/L. Conversion formula: Hardness (mmol/L) = Hardness (mg/L as CaCO₃) ÷ 50
For example: 370 mg/L ÷ 50 = 7.4 mmol/L.
Calculation example:
Suppose the raw water hardness is 300 mg/L (as CaCO₃), the resin tank is filled with 500L of resin, and the working exchange capacity is taken as 1000 mmol/L.
Unit Conversion: Raw water hardness = 300 ÷ 50 = 6 mmol/L.
Calculating Water Production: Total exchange capacity = 1000 mmol/L × 500 L = 500,000 mmol.
Cycle water production = 500,000 mmol ÷ 6 mmol/L = 83,333 L (about 83 tons).
Attention! Attention! Attention!
Theoretical vs Actual: The formula calculates the theoretical value. Actual water production can vary due to inlet water temperature, pH, regeneration effectiveness, outlet standards, etc. It's recommended to use the calculation as a reference and adjust based on operational monitoring data.
Units Must Be Consistent: This is the easiest mistake to make. Before calculating, make sure hardness is in mmol/L, and resin volume is in L or m³. Otherwise, the results can be way off.
