The various properties and states of water significantly impact the performance of separation membranes. Therefore, a deeper understanding of water quality characteristics is crucial for the better application of membrane water treatment technology. After determining the selected influent water source, a comprehensive and accurate water quality analysis must be conducted. This is the key basis for establishing the pretreatment scheme and can provide a reference for system design.
1. Water Temperature
Water viscosity decreases as temperature increases, meaning it flows more smoothly. Changes in water temperature are quickly reflected in the permeate flow rate of membrane elements. To ensure a stable permeate flow rate, the system operating temperature is crucial. Conversely, excessively low temperatures reduce the solubility of silicates and sulfates, increasing the risk of scaling and operating costs. Increased water temperature leads to a decrease in desalination rate, primarily because the diffusion rate of salt through the membrane accelerates with increasing temperature.
2. Electrical conductivity
Electrical conductivity is an indicator of the ability of ions in water to conduct electricity, measured in microsiemens per centimeter (µS/cm). The conductivity value increases with increasing ion concentration. However, conductivity does not accurately reflect the types of ions present; in fact, ions such as silicon ions may not affect conductivity changes.
3. Ionic Components
Water often contains various ions such as sodium ions, potassium ions, calcium ions, magnesium ions, chloride ions, and sulfate ions. To characterize the content of certain ions, concepts such as alkalinity (characterized by hydroxide and carbonate ions) and hardness (characterized by calcium and magnesium ions) are derived.
4. Organic components
Organic matter in water is typically characterized by indicators such as BOD, COD, and TOC. Its main components include bacteria, pesticides, hormones, and surfactants. Different organic substances have varying effects on membranes; certain specific organic substances may accelerate organic fouling in the system, leading to shorter fouling time and increased cleaning frequency.
5. Raw Water Classification
The types of raw water entering the RO/NF pretreatment system can be classified according to total salinity (TDS) and total organic matter (TOC):
- The TDS of the low-salinity, high-purity permeate from the first-stage RO system is up to 50 mg/L;
- Low-salinity tap water with a TDS of less than 500 mg/L;
- Groundwater with low natural organic matter (NOM) content and TDS less than 5,000 mg/L with moderate salinity;
- Brackish water with moderate salinity and a TDS of less than 5,000 mg/L;
- The wastewater is of medium salinity with high TOC and BOD content and TDS less than 5,000 mg/L.
- High-salinity brackish water with a TDS between 5,000 and 15,000 mg/L;
- Seawater with a TDS of around 35,000 mg/L.
Different types of water sources require different pretreatment processes and different types of membrane elements. For small-scale projects that do not have the conditions for water quality analysis or testing, the pretreatment of projects that have been put into operation with the same type of water source can be used as a reference for design. However, for large-scale projects, a comprehensive water quality analysis must be carried out.
The quality of the raw water determines the pretreatment process flow, and comprehensive water quality analysis data is crucial for designing a reasonable pretreatment system and ensuring the long-term stable operation of the reverse osmosis system. Table 4.1 is a list of water quality analysis items recommended by Shandong Jozzon Membrane Technology Co., Ltd. for reverse osmosis systems.
Table 4.1 Raw Water Quality Analysis Table for Reverse Osmosis System
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Engineering Company Name: Engineering company address: Sampling time: Sampler: Analysis time: Analyst: Raw water source characteristics: |
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electrical conductivity |
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Water temperature (°C) |
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Pollution Index (SDI 15 ) |
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Turbidity (NTU) |
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pH value |
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Ionic components |
numerical values |
Ionic components |
numerical values |
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Calcium ions ( Ca²⁺ ) |
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Chloride ions ( Cl⁻ ) |
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Magnesium ions ( Mg²⁺ ) |
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Sulfate (SO 4 2- ) |
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Sodium ions ( Na⁺ ) |
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Carbonate ( CO3 2－ ) |
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Barium ions ( Ba²⁺ ) |
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Bicarbonate ( HCO3− ) |
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Strontium ion ( Sr²⁺ ) |
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Phosphate ( PO₄³⁻ ) |
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Potassium ions (K + ) |
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Fluoride ions ( F⁻ ) |
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Ferrous ion ( Fe²⁺ ) |
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Nitrate (NO 3 – ) |
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Iron ions ( Fe³⁺ ) |
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Silicon dioxide ( SiO₂ ) |
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Aluminum ions ( Al³⁺ ) |
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Other ions (such as boron ions) |
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Total cations |
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Total anions |
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Total dissolved solids (TDS) (ppm) |
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Biological oxygen demand (BOD) |
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Total suspended solids (SS) |
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Total organic carbon (TOC) (ppm) |
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Hardness (CaCO3 ppm ) |
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Chemical oxygen demand COD (ppm) |
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Total alkalinity (mmol/L) |
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Bacterial count (cFU/mL) |
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Phenolphthalein alkalinity (mmol/L) |
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Residual chlorine concentration (ppm) |
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Note:
Please specify the units for analytical items, such as mg/L, ppm, meq/L, and expressed as CaCO3 , etc. , Ba²⁺ and Sr²⁺ must be detected at mlevels of 1 g/L (ppb) and 1 mg/L (ppm), respectively .
