PP

A PP filter cartridge is made of polypropylene microfibers through a melt-blown process. It is typically used as an economical purification method in the pretreatment stage, effectively trapping large amounts of rust, sediment, and suspended solids, while protecting downstream purification equipment from contamination and clogging. Its filtration accuracy ranges from 1 to 10 μm.

Activated carbon

Activated carbon is a key adsorption material capable of removing from water: dissolved organic matter; substances that cause unusual color and odor; residual chlorine and disinfection by-products; and some heavy metal ions. This helps prevent contamination of downstream filtration membranes and ion exchange resins. Activated carbon filter cartridges require regular replacement to prevent contamination resulting from saturation.

KDF

KDF is a high-purity copper/zinc alloy particulate that performs water treatment through micro-electrochemical oxidation-reduction reactions. It can remove up to 99% of chlorine and dissolved metal ions and compounds—such as lead, mercury, nickel, and chromium—from water. It is highly effective at suppressing the growth of bacteria, fungi, scale, and algae.

Reverse osmosis (RO) membrane

Reverse osmosis membranes are typically used to remove contaminants larger than 1 nm in diameter. Typical reverse osmosis processes can eliminate 90–99％ of ionic contaminants, the majority of organic substances, and nearly all particulate pollutants. Theoretically, this method can remove 100% of molecules with a molecular weight greater than 300 Daltons, as well as particles including colloids and microorganisms; however, dissolved gases cannot be removed by reverse osmosis membranes.

Deionization (DI) cartridge/polishing cartridge

The cartridge is filled with a mixed-bed ion exchange resin (a uniform mixture of cation and anion exchange resins). It effectively removes ions from water through ion exchange with H+ and OH- ions. It is usually installed downstream of the RO membrane to produce higher purity water and extend the resin's service life. The output water resistivity is 16–18.2 MΩ·cm. While the ion exchange method effectively removes ions, it does not effectively remove most organic substances or microorganisms.

EDI

Continuous Electrodeionization (CEDI) is a technology that combines ion-exchange resins and ion-selective membranes with direct current to remove ionized impurities from water. It is commonly used as a replacement for deionization cartridges, offering stable water quality, long service life, and strong resistance to contamination. The resistivity of the produced water generally reaches 5–17 MΩ·cm, with a total organic carbon (TOC) content below 30 ppb.

UV lamp

UV lamps serve as sterilization devices that decompose and photo-oxidize organic contaminants, causing their polarization or ionization. They are typically installed upstream of ion exchange cartridges to facilitate the adsorption and removal of these processed contaminants. Their primary functions include sterilization and total organic carbon (TOC) reduction.

Ultrafiltration membrane

The pore size of ultrafiltration (UF) membranes typically ranges from 1 to 50 nm. As tough, thin, and selectively permeable membranes, they can block most molecules above a specific size, including colloids, microorganisms, and pyrogens, ensuring the stable high quality of ultrapure water. The internationally accepted molecular weight cut-off (MWCO) for UF membranes used in ultrapure water systems is 5,000 Daltons.

Point-of-use filter

A point-of-use filter physically blocks particles and microorganisms in water, with a typical pore size of 0.2 μm. It is generally installed at the point of use in a purification system to remove residual trace resin fragments, carbon particles, colloidal particles, and microorganisms.

Internationally recognized pure water standards primarily include: International Organization for Standardization (ISO), College of American Pathologists (CAP) Reagent Water Standards, American Society for Testing and Materials (ASTM), Clinical and Laboratory Standards Institute (CLSI, formerly NCCLS), and United States Pharmacopeia (USP), etc.
China has also established corresponding ultrapure water standards, including:

GB/T 6682-2008 Specifications and Test Methods for Water Used in Analytical Laboratories;

The updated standard GB/T 33087-2016 Specifications and Test Methods for High-Purity Water for Instrumental Analysis;

GB/T 11446.1-2013 Electronic Grade Water.

Reagent-grade water standards by College of American Pathologists (CAP)/Water standards by Clinical and Laboratory Standards Institute (CLSI, formerly NCCLS)

I. Purified water

Purified water H2O 18.02 This product is potable water prepared from drinking water by distillation, ion exchange, reverse osmosis, or other suitable methods, free of any additives.

II. Test items as per the Pharmacopoeia of the P.R.C (2010)

1. Characteristics

This product is a colorless, tasteless, and clear liquid.

2. Acidity and alkalinity

Take 10 mL of this product, add 2 drops of Methyl Red Indicator Solution—no red color shall develop. Take another 10 mL of this product, add 5 drops of Bromothymol Blue Indicator Solution—no blue color shall develop.

3. Nitrates

The limit for nitrate is 0.000006%. Take 5 mL of this product, transfer it to a test tube, cool in an ice bath, add 0.4 mL of a 10% potassium chloride solution and 0.1 mL of a 0.1% diphenylamine sulfate solution, and shake well. Slowly add 5 mL of sulfuric acid dropwise, shake thoroughly, and then place the test tube in a 50 °C water bath for 15 minutes. The resulting blue color must not be deeper than that of a reference solution. This reference is prepared by mixing 0.3 mL of the standard nitrate solution (see preparation below) with 4.7 mL of nitrate-free water and treating the resulting mixture by the same method. Preparation of the standard nitrate solution: Dissolve 0.163 g of potassium nitrate in water and dilute to 100 mL. Accurately measure 1 mL of the solution, dilute with water to 100 mL. Then accurately measure 10 mL of the resulting solution, dilute with water to 100 mL, and shake well. This final standard solution contains 1 µg of NO₃⁻ per mL.

4. Nitrite

The limit for nitrite is 0.000002%. Take 10 mL of this product, transfer it to a Nessler cylinder, add 1 mL of a sulfanilamide solution in dilute hydrochloric acid (1→100) and 1 mL of a N-(1-naphthyl)ethylenediamine dihydrochloride solution (0.1→100). The resulting pink color must not be deeper than that of a reference solution. This reference is prepared by mixing 0.2 mL of the standard nitrite solution (see preparation below) with 9.8 mL of nitrite-free water and treating the resulting mixture by the same method. Preparation of the standard nitrite solution: Dissolve 0.750 g (calculated on the dried substance) of sodium nitrite in water and dilute to 100 mL. Accurately measure 1 mL of solution, dilute with water to 100 mL. Then accurately measure 1 mL of resulting solution, dilute with water to 50 mL, and shake well. This final standard solution contains 1 µg of NO₂⁻ per mL.

5. Ammonia

The limit for ammonia is 0.00003%. Take 50 mL of this product, add 2 mL of alkaline potassium iodomercurate test solution (Nessler's Reagent) and allow it to stand for 15 minutes. If a color develops, it must not be deeper than that of a reference solution. This reference is prepared by mixing 1.5 mL of an ammonium chloride solution (prepared by dissolving 31.5 mg of ammonium chloride in ammonia-free water and diluting to 1000 mL) with 48 mL of ammonia-free water, then adding 2 mL of the alkaline potassium iodomercurate test solution.

6. Conductivity

Conductivity ≤ 2 μS/cm (resistivity ≥ 0.5 MΩ .cm)

7. Total organic carbon (TOC)

Not more than 0.50 mg/L (Appendix VIII R).

8. Oxidizable substances

Take 100 mL of this product, add 10 mL of dilute sulfuric acid, and bring to a boil. Add 0.10 mL of potassium permanganate volumetric solution (0.02 mol/L), boil for another 10 minutes; the pink color must not disappear completely.

9. Non-volatile residues

Place 100 mL of this product in an evaporation dish previously dried to constant weight at 105 °C, and evaporate to dryness on a water bath. Then, dry the residue to constant weight at 105 °C; the residue must not exceed 1 mg.

10. Heavy metals

The limit for lead is 0.00001%. Take 100 mL of this product, add 19 mL of water, and evaporate to 20 mL. After cooling, add 2 mL of acetate buffer (pH 3.5) and sufficient water to produce 25 mL. Then add 2 mL of thioacetamide test solution, mix well, and allow to stand for 2 minutes. The color produced must not be deeper than that of a reference solution prepared by treating 1.0 mL of standard lead solution with 19 mL of water in the same manner.

11. Microbial limit

Take this product, process using the membrane filtration method, and examine as directed (Appendix XII J). The total aerobic microbial count (bacteria, molds, and yeasts) must not exceed 100 per 1 mL.

1. Resistivity of water
The electrical conductivity of water is related to its resistance: a higher resistance indicates poorer conductivity, while a lower resistance indicates better conductivity. According to Ohm's law, at a constant water temperature, the resistance (R) of water is directly proportional to the distance (L) between electrodes and inversely proportional to their cross-sectional area (F), as expressed by the formula: R=ρ·L/F, where ρ represents resistivity (or specific resistance). Resistance is measured in ohms (Ω), or microohms (μΩ). 1 Ω equals 10⁶μΩ. The SI unit for resistivity is the ohm-meter (Ω·m). If the electrode cross-sectional area F is 1 cm² and the distance L between two electrodes is 1 cm, the resistance value equals the resistivity. The resistivity of water depends on its salt content, ion concentration, ion charge number, and ion mobility. Therefore, pure water has high resistivity, and ultrapure water exhibits even greater resistivity. The purer the water, the higher its resistivity.

2.Conductivity
The reciprocal of resistivity is conductivity, denoted as L. The ability of a liquid to conduct electricity is usually measured by conductivity—the reciprocal of resistance. Conductivity (L) is calculated as L=l/R=S/l, and its unit is the mho, also known as the siemens (S). Since the siemens is a relatively large unit, millisiemens (mS) and microsiemens (μS) are commonly used, where 1 S=10³ mS=10⁶ μS.
Effect of temperature on conductivity
The resistance of a solution decreases with increasing temperature. That is, at a constant concentration, the conductivity of a solution increases with temperature at a rate of approximately 2% per °C. Furthermore, for electrolytes of the same type, the temperature coefficient varies with concentration. At low concentrations, the relationship between conductivity and temperature can be expressed as: L₁=L₀[1+α(t-t₀)+β(t-t₀)²]. Since the value of the second term, β(t-t₀)², is small, it can generally be neglected. Thus, at lower temperature ranges, the relationship can be approximated as: L₁=L₀ [1+α(t-t₀)]. Therefore, temperature compensation must be applied in practical measurements.
The temperature coefficient of conductivity for most ions is approximately 1.4% to 3% per °C. For H⁺ and OH⁻ ions, the temperature coefficients are about 1.5% and 1.8% per °C, respectively. Given that the required accuracy for conductivity measurements is typically 1% or better, the effect of temperature cannot be overlooked.

3. Conductivity of pure water
Even in pure water, H⁺ and OH⁻ ions are present. While pure water is commonly described as a poor conductor of electricity, it is more accurately characterized as a very weak electrolyte. It establishes the following dissociation equilibrium:
H₂O←→H⁺+OH or 2H₂O←→H3+O+OH^-
. The equilibrium constant is:
KW = [H+].[OH-]/H₂O=10^-14
, where KW is the ionic product of water. Thus,
[H⁺]2 =[OH^-]2=10^-14
∴[H⁺]2=[OH^-]2=10^-7
lH₂O,0=λOH-,0=349.82+198.6=548.42S/cm.mol2.
Given that the density at 25°C for H₂O is d=0.9970781 g/cm³, the actual molar concentration of water molecules is 0.99707 mol/L, whereas a value of 1 was assumed in the original simplification. In fact, only a fraction of 0.99707 of water molecules dissociate, producing [H⁺] and [OH^-] ions each at a concentration of 0.99707×10⁻⁷ mol/L. The total conductivity of the [H⁺] and [OH^-] ions, denoted as KH₂O, is calculated as follows:
KH₂O=CM/1000λH₂O
=(0.99707.10^-7/1000).548.42
=0.05468μ S.cm-1≈0.054 μS.cm-1
∴ρH₂O=1/KH₂O=1/0.05468×10-9
=18.29(MΩ.cm)≈18.3(MΩ.cm). From the ion product of water being 10^-14, the theoretical limiting conductivity of high-purity water is derived as 0.0547 μΩS.cm⁻¹, with a resistivity of 18.3 MΩ.cm at 25°C.
The temperature coefficient of water's electrical conductivity varies across different conductivity ranges. For commonly used distilled water with a conductivity of 1 μS/cm, the coefficient is approximately +2.5% per °C.

4. TDS
Total Dissolved Solids (TDS), measured in milligrams per liter (mg/L), indicates the mass (in milligrams) of dissolved solids present in one liter of water.
The concept of TDS is widely used in the water treatment industries of countries and regions such as the United States. It is typically measured with a TDS meter, which operates by determining water's electrical conductivity to estimate the TDS value. Physically, a higher concentration of dissolved solids in water results in a higher TDS value, which enhances its electrical conductivity and thus yields a higher conductivity reading.
In common terms, the TDS value represents the concentration of dissolved impurities in water. A higher TDS value indicates a greater amount of impurities, and conversely, a lower value indicates a lesser amount.
To use a TDS meter: Remove the probe cover and press the "ON/OFF" button. Once the LCD screen turns on, immerse the probe into the water sample. After the reading stabilizes, press the "HOLD" button to lock the value. Remove the meter from the water and record the reading. After use, gently wipe the probe dry with a soft cloth or paper.
Factors affecting TDS meter measurements:
Water temperature:​The meter should not be used to measure high-temperature water, such as boiling water.
Water flow: It is not suitable for measuring water with significant turbulence or flow.
Water contamination:​The meter should not be used in water with a high concentration of contaminant.

5. Water hardness
In water, certain metal cations combine with some anions. During the heating process, evaporation and concentration cause them to easily form scale, which adheres to heated surfaces and impairs heat transfer. The total concentration of these metal ions in water is referred to as water hardness. In natural water, the most common metal ions are calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). They combine with anions in water, such as carbonate (CO₃^2-), bicarbonate (HCO^3-), sulfate (SO₄^2-), chloride (CL-), and nitrate (NO^3-) ions, forming calcium and magnesium carbonates, bicarbonates, sulfates, chlorides, and nitrates, which constitute water hardness. Metal ions such as iron (Fe), manganese (Mn), and aluminum (Al) in water can also contribute to hardness. However, as their concentrations in natural water are typically very low, they are generally considered negligible. Therefore, the total concentration of Ca²⁺ and Mg²⁺ is regarded as water hardness. Water hardness significantly impacts boiler operation. Therefore, feed water must be softened or demineralized according to the specific water quality requirements specified for different boiler parameters.

6. Relationship between TDS and conductivity
For most water sources, the conductivity/TDS ratio ranges between 1.2 and 1.7. Using a factor of 1.4 for seawater and 1.3 for brackish water typically yields a close approximation.

7.How temperature affects permeate flow

Higher temperatures result in higher permeate flow, and vice versa. When operating at elevated temperatures, the operating pressure should be reduced to maintain a constant permeate flow rate, and vice versa.