Essential Water Treatment FAQs

Essential Water Treatment FAQs

1. What are the water quality standards for tap water?

Answer: Tap water quality indicators must meet the relevant standards in the “Drinking Water Quality Standards” (GB5749-85), which includes 35 indicators. The new standards soon to be implemented will have 88 indicators.

2. How does the color change of pH paper correlate with the pH value when testing solution acidity/alkalinity?

Answer: The relationship between color change and pH value when using pH paper to test solution acidity/alkalinity is as follows:

Acidic Solutions (pH < 7)

The lower the pH value, the stronger the acidity.

The paper color changes from orange to yellow to red, corresponding to a pH range of approximately 1-6. For example, a strong acid solution with pH=1 turns the paper deep red, while a weak acid solution with pH=5 turns it yellow.

Neutral solutions (pH = 7)

The paper color is typically green or yellow, with slight variations depending on the paper type.

Alkaline solutions (pH > 7)

The higher the pH value, the stronger the alkalinity.

The test paper color changes from green to blue to purple, corresponding to a pH range of approximately 8-14. For example, a weak base solution with pH=10 turns the paper blue, while a strong base solution with pH=13 turns it deep purple.

Important Notes:

pH test paper provides only a rough measurement of pH, typically yielding whole-number results.

When using the paper, compare it against a standard color chart. Avoid direct contact with the solution or wetting the paper with water, as this may affect accuracy.

Different types of pH test paper (e.g., universal paper, precision paper) may exhibit slight variations in color range and precision.

3. What are conductivity and resistivity? How are they converted between each other? What is their relationship with TDS?

Answer: One parameter for testing pure water is conductivity or resistivity, reflecting the ion content in the water. Conductivity is measured in (μs/cm) using a conductivity meter (common model: CM-230). Resistivity is measured in (MΩ·cm) using a resistivity meter (common model: RM-230). Total Dissolved Solids (TDS) refers to the aggregate of inorganic salts and organic matter dissolved in water. Its primary components include calcium, magnesium, sodium, potassium ions, carbonate ions, bicarbonate ions, chloride ions, sulfate ions, and nitrate ions.

4. What filter cartridges are used in security filters and precision filters?

Answer: Security filters and precision filters use the following filter cartridges:

(1) PP melt-blown filter cartridges: Used for most industrial or drinking water pretreatment, available in 10“, 20”, 30“, and 40” models.

(2) Spun-bonded filter cartridges (also available in 10“ and 20” models): Primarily used for industrial filtration of coarse particles and silt. Density varies based on tightness.

(3) Activated carbon cartridges: Divided into mesh activated carbon for industrial use and granular activated carbon for reverse osmosis systems. Primarily removes odors and color from water. Activated carbon materials include anthracite, shell-based, and coconut shell-based carbon (commonly called coconut shell carbon). Key differences: a) Varying adsorption capacities; b) Different resistance to pressure washing—coconut shell carbon performs better but is more expensive.

(4) Microfilter Membrane Cartridges: Used for post-treatment of ultrapure or purified water after UV sterilization to remove bacterial debris. Filtration precision ranges from 0.22μm, 0.45μm, 1.0μm, to 2.0μm, with relatively high cost.

5. How does an ion exchange water purifier work?

Answer: Mechanism of action for each treatment unit in ion exchange processes

(1) Sand Filter: Also known as a mechanical filter, it contains refined quartz sand or high-quality anthracite coal. In small-scale equipment, quartz sand typically ranges from 0.5-1.0mm or 1.0-2.0mm. Large-scale equipment uses graded sand, where particle sizes decrease from top to bottom: generally 0.5-1.0mm or 1.0-2.0mm (80%), 2.0-4.0mm, or 4.0-8.0mm, 8.0-16.0mm. This serves as the underlying support layer. Contact filtration is employed to remove suspended solids, colloids, and organic matter from the raw water, reducing the Silt Density Index (SDI) to ≤1.

(2) Activated Carbon Filter: Utilizes high-efficiency activated carbon to adsorb colloids, color, humic substances, and residual chlorine from the water, achieving residual chlorine content ≤0.1 PPM in the effluent.

(3) Dual-bed System: Preliminary desalination using strong acid and strong base ion exchange resins extends the mixed-bed regeneration cycle and increases water production per cycle, with effluent conductivity ≤10μS/cm. (4) Mixed-bed System: Advanced desalination using strong acid and strong base ion exchange resins achieves effluent resistivity ≥5MΩ·cm, with a regeneration cycle ≥1 week.

(5) 1μm Microfilter: Filters out worn resin particles.

6. What are the components of the reverse osmosis main unit?

Answer: Process description for reverse osmosis pretreatment: The purpose of the pretreatment and reverse osmosis sections is to provide qualified feedwater for the reverse osmosis unit.

A. Reverse Osmosis System Feedwater Requirements:

1) Silt Density Index (SDI) ≤4 2) Residual chlorine <0.1 ppm 3) Turbidity <1 NTU 4) Fe³⁺ in feed water ≤0.01 ppm 5) Optimal feed water temperature range: 10–30°C 6) Lime Scale Index (LSI) 0

B. Pretreatment employs methods such as coagulation, filtration, and adsorption to ensure the reverse osmosis feedwater meets the above requirements, achieving the following objectives: Prevent scaling on the reverse osmosis membrane surface (including CaCO₃, CaSO₄, SrSO₄, CaF₂, SiO₂, iron and aluminum oxides, etc.); Prevent fouling of reverse osmosis membranes by colloidal substances and suspended solid particles; Prevent fouling and degradation of reverse osmosis membranes by organic substances; Prevent fouling of reverse osmosis membranes by microorganisms; Prevent oxidative damage to reverse osmosis membranes by oxidizing substances;

C. Composition of the Pretreatment System: Raw water contains various impurities such as suspended solids, colloids, organic matter, and inorganic substances. To remove suspended solids, colloids, organic matter, and other contaminants, the raw water pretreatment section incorporates multi-media mechanical filters, activated carbon filters, security filters, and flocculant dosing equipment. Mechanical filters and activated carbon columns effectively remove suspended solids, colloids, organic matter, and other impurities from water. They also adsorb humic substances, color, odor, residual chlorine, etc., from tap water, reducing turbidity and pollution indices. Water treated through these processes becomes clean, odorless, and is termed clear water. Mechanical filters are packed with anthracite and quartz sand, while activated carbon filters contain high-quality activated carbon.

7. What components comprise raw water pretreatment?

Answer: Raw water pretreatment includes: Raw water contains various impurities such as suspended solids, colloids, organic matter, and inorganic substances. To ensure the normal operation of the reverse osmosis pre-desalination section in this system, suspended solids, colloids, organic matter, and other contaminants must first be removed to meet the feedwater requirements for reverse osmosis. Therefore, this system incorporates a raw water pretreatment section. The pretreatment section includes devices such as mechanical filters, activated carbon filters, and security filters.

7.1 Mechanical Filter: Reverse osmosis equipment imposes stringent requirements on feedwater turbidity, particularly demanding a Silt Density Index (SDI) value below 4 and turbidity below 1 NTU. The filter media in the multimedia filter consists of five grades of quartz sand and anthracite coal, designed to remove suspended solids and destabilized colloids from the raw water. Prior to entering the mechanical filter, a high-efficiency flocculant is added to more thoroughly remove organic matter, colloids, and other impurities. The treated water is then sent to the activated carbon filter and security filter for further processing before serving as feedwater for the reverse osmosis system.

7.2 Activated Carbon Filter. Reverse osmosis systems require feedwater residual chlorine levels below 0.1 mg/L. Therefore, activated carbon filters are employed to remove residual chlorine from raw water, preventing membrane fouling. They also further adsorb organic compounds present in the feedwater. The activated carbon filter is packed with refined shell-type activated carbon to adsorb residual chlorine, organic matter, some pigments, and harmful substances from the raw water, thereby reducing chemical oxygen demand (COD).

Activated carbon is widely used for purifying water in domestic applications, food industries, and industrial sectors like chemical manufacturing. Its high adsorption capacity stems from its large specific surface area and numerous micropores averaging 20–30 angstroms in diameter. Additionally, the surface of activated carbon contains numerous functional groups such as hydroxyl and carboxyl groups, enabling chemical adsorption of various organic substances and electrostatic attraction. Consequently, activated carbon can also remove organic substances like humic acid, fulvic acid, and lignosulfonic acid from water. It can also eliminate substances harmful to reverse osmosis membranes, such as residual chlorine, preventing membrane oxidation. Typically, it can remove 63%–86% of colloidal substances, approximately 50% of iron, and 47%–60% of organic matter. The process of activated carbon removing residual chlorine is a simple redox reaction. While this process may cause the activated carbon to break apart, the fragmented carbon does not impair its residual chlorine removal effectiveness, making this process highly efficient.

Activated carbon filters utilize the adsorption properties of activated carbon to remove impurities from liquids, achieving purification. Its adsorption capabilities manifest in the following aspects: 1. It can adsorb organic matter, colloidal particles, and microorganisms in water. 2. It adsorbs non-metallic substances such as chlorine, ammonia, bromine, and iodine. 3. It adsorbs metal ions including silver, arsenic, lead, hexavalent chromium, mercury, antimony, and tin. 4. It effectively removes color and odor, eliminates pyrogens in pharmaceutical water, and extends the service life of ion exchange resins.

7.3 Security Filter: Security filters are microfiltration devices serving as safeguards in pretreatment systems. They act as a final checkpoint when mechanical filters malfunction, ensuring qualified water supply under all conditions. To prevent particulates from water or pipelines entering pumps and reverse osmosis systems, security filters are specifically installed as the last filtration stage. Replace the filter cartridge when the inlet/outlet pressure difference exceeds 0.1 MPa (as filtered media directly enters the microporous membrane pores, acid/alkali cleaning rarely restores flux). The filter design allows for quick cartridge replacement. Quick calculation for cartridge flow rate: 20-inch 5μ = 1 ton/hour; scale up accordingly.

8. What is the process flow of a reverse osmosis pure water machine? What are the components and functions of a reverse osmosis desalination system?

1. Process Flow of Reverse Osmosis Water Purifiers. Water contains various inorganic salts that cannot be removed by conventional filtration. Traditional ion exchange methods face challenges such as high acid/alkali consumption, short regeneration cycles, labor-intensive operations, and severe environmental pollution. Reverse osmosis technology, an emerging high-tech innovation over the past two decades, utilizes the principle of reverse osmosis. It employs highly selective reverse osmosis membranes to achieve up to 99% removal of inorganic salts from water. Consequently, it offers advantages such as simple operation, low energy consumption, and zero pollution, making it widely adopted in pure water production.

1.1 High-Pressure Pump: The high-pressure pump is installed to provide sufficient inlet pressure for the reverse osmosis process, enabling water molecules to overcome osmotic pressure and pass through the reverse osmosis membrane into the freshwater layer. The high-pressure pump is a stainless steel multistage centrifugal pump. Premium options include Danish Grundfos pumps, while commonly used models include domestically produced Hangzhou Nanfang pumps and Danish Grundfos pumps. All wetted parts are made of 316 stainless steel (actually 304). The electrical control system is designed with various user-friendly functions, including low-pressure automatic shutdown, high-pressure automatic shutdown, water shortage protection, and full-water shutdown.

1.2 Reverse Osmosis Membrane Modules and Pressure Vessels: Reverse osmosis systems effectively remove the vast majority of inorganic salts, particulates, bacteria, viruses, and other dissolved substances from water. The reverse osmosis membrane modules utilize CPA3 high-rejection low-pressure membranes manufactured by Hydranautics (USA). Constructed from aromatic polyamide composite membrane material, each membrane achieves a rejection rate of 99.7%, representing the world's highest rejection rate for reverse osmosis membrane modules. The system achieves a rejection rate exceeding 97% with a recovery rate ranging from 85% to 60%.

Membrane specifications: 2-inch membranes: e.g., 2021, 2015 4-inch membranes: e.g., 4040 8-inch membranes: e.g., 8040 4-inch membrane diameter: 106mm; 8-inch membrane diameter: 201mm. During production, concentrate discharge typically accounts for 20–30% of total feedwater. Its characteristics include low turbidity, low color, and salt content five times higher than tap water. In electroplating plants, it is generally used as pretreatment water, with quality superior to tap water. Alternatively, it can be stored for landscaping, toilet flushing, or car washing. Under normal operating conditions, the average service life of this membrane module is 3 years. The module is designed to operate within a pressure range of 1.4–1.7 MPa. The pressure vessel—the housing providing the operating pressure environment for the reverse osmosis membrane module—is an 8"-5W stainless steel vessel rated for 120 PSI (0.9 MPa). Each pressure vessel can accommodate 5 reverse osmosis membrane modules.

1.3 Automatic Flushing System: During reverse osmosis operation, concentration processes and concentration polarization cause the solid content concentration on the membrane surface to significantly exceed the bulk concentration of the feedwater. Therefore, an automatic low-pressure flushing system is equipped to periodically flush the reverse osmosis membranes at low pressure before startup, after shutdown, or after a continuously adjustable operating period. This removes minor contaminants adhering to the membrane surface. Upon completion, the system automatically resumes its pre-flush state. The reverse osmosis unit incorporates an automatic low-pressure flushing protection device at the inlet. This device automatically initiates membrane flushing when the system starts up, ensuring minimal membrane fouling.
Below are essential water treatment topics covering water quality standards, process principles, equipment maintenance, and troubleshooting:

I. Water Quality Standards and Testing
Tap Water Quality Standards
Tap water must comply with the “Drinking Water Quality Standard” (GB5749). Current standards include multiple indicators such as microorganisms, chemical substances, and sensory characteristics. Key parameters like residual chlorine, turbidity, and pH must be monitored to ensure water safety.

Industrial Pure Water Requirements
Different industries demand varying pure water qualities. For example, electronics require resistivity ≥18MΩ·cm, while electroplating necessitates conductivity <10μS/cm. Select appropriate treatment processes based on specific application scenarios.

Water Quality Testing Methods
Common testing indicators include conductivity, resistivity, TDS (Total Dissolved Solids), and SDI (Silt Density Index). Proficiency in operating and maintaining testing instruments such as conductivity meters and pH test strips is essential.

II. Water Treatment Processes and Equipment
Pre-Treatment Processes

Mechanical Filtration: Removes suspended solids and colloids using media like quartz sand or anthracite.
Activated Carbon Adsorption: Removes residual chlorine, organic compounds, and odors. Requires periodic carbon replacement.
Softening Treatment: Reduces water hardness to prevent scaling. Typically employs ion exchange resins.
Advanced Treatment Processes

Reverse Osmosis (RO): Eliminates inorganic salts, microorganisms, and organic matter. Requires controlled feedwater quality (SDI ≤ 4, residual chlorine < 0.1 ppm).
Ion Exchange: Advanced desalination using cation, anion, or mixed-bed configurations. Requires periodic regeneration.
Disinfection: Common methods include chlorination, UV irradiation, and ozone treatment. Disinfection byproducts must be controlled.
Equipment Maintenance

Reverse Osmosis Membranes: Regular cleaning and replacement to prevent scaling and fouling.
Water Pumps: Inspect impellers and bearings to prevent wear and cavitation.
Filters: Regular backwashing and filter cartridge replacement to prevent clogging.
III. Common Faults and Remedies
Sludge Issues

Sludge Bulking: Excessive filamentous bacteria growth reduces sludge settling performance. Adjust nutrient ratios, pH, and dissolved oxygen levels.
Sludge Aging: Lightened sludge color and increased fine sludge particles. Increase sludge discharge frequency and supplement nutrients.
Water Quality Abnormalities

Turbidity Increase: Poor pretreatment efficiency. Inspect filters and adjust flocculant dosage.
Excess Residual Chlorine: Overdosing of disinfectant. Adjust dosage.
Odor: May result from microbial growth or organic contamination. Enhance disinfection and deodorization.
Equipment Failures

Aeration System Failures: Clogged diffusers or motor malfunctions. Clean diffusers and inspect motors.
Pump Malfunctions: Impeller wear or cavitation. Replace impellers and adjust installation position.
IV. Safety and Environmental Protection
Disinfection Byproduct Control
Chlorine disinfection may generate byproducts like trihalomethanes. Optimize disinfection processes, such as using ozone or UV-assisted disinfection.

Sludge Treatment
Properly manage sludge to prevent secondary pollution. Methods include thickening, dewatering, and incineration.

The above issues require flexible handling based on actual application scenarios. It is recommended to conduct regular training for operators and establish comprehensive water quality monitoring and equipment maintenance systems.