TDS: Total Dissolved Solids, generally approximate to mineralization degree.
SDI: Silt Density Index, an indicator for evaluating the effect of system pretreatment. The influent requirement for reverse osmosis (RO) units is SDI < 5.
LSI: Langelier Saturation Index, used to judge the scaling tendency of RO systems.
- LSI = 0: No scaling and no corrosion tendency;
- LSI > 0: Scaling tendency exists;
- LSI < 0: Corrosion tendency exists.
For RO systems, the LSI value shall not exceed 0. The LSI can be reduced by acid dosing or lowering the system water recovery rate.
Ksp: Solubility Product Constant. RO membranes are selectively permeable to solvents and solutes in raw water. Water is concentrated on the concentrate side due to solvent reduction. When the ion concentration product exceeds the solubility product constant, crystals precipitate and cause damage to the RO unit.
The system solubility product constant can be increased by adding scale inhibitors, which improve the solubility of dissolved solids.
2. How to Effectively Control the LSI Index
The LSI index can be controlled effectively by the following methods:
- Reduce the system water recovery rate;
- Dose acid to lower the LSI value;
- Add special chemicals to increase the solubility of dissolved salts in the system, e.g., TRISPE1000 scale inhibitor;
- Remove or reduce scaling-prone ions in raw water in advance, such as softening influent via softening columns.
3. Common Reverse Osmosis Pretreatment Equipment
Mechanical filter, high-efficiency fiber filter, activated carbon filter, cartridge filter, ultrafiltration (UF), microfiltration (MF), sodium ion softener, iron and manganese removal filter, chemical dosing device, raw water tank, aeration tank.
4. Common Desalination Equipment
Electrodialysis (ED) unit, reverse osmosis unit, anion exchanger, cation exchanger, mixed ion exchanger, distillation unit, EDI unit.
5. Selection Principles of Cartridge Filters & Types of Filter Cartridges
Cartridge filter selection matches the total influent flow rate; the diameter is determined according to total water throughput.
- Small-flow 40-inch 5μm filter cartridge: single cartridge water yield ≈ 2m³/h;
- High-flow 40-inch 5μm filter cartridge: single cartridge water yield ≈ 30–60m³/h.
Main cartridge types: Polypropylene cartridge, honeycomb cartridge, melt-blown cartridge, folded cartridge.
6. Methods for Iron Removal from Water
Iron in groundwater is mostly divalent ferrous iron, which must be oxidized to trivalent ferric iron. Aeration enables full contact between water and air to complete natural oxidation; the aerated water then flows into an iron and manganese removal filter for iron removal.
If most iron exists as trivalent iron, aeration is unnecessary, and water can directly enter the iron and manganese removal filter.
7. Why a Decarbonator Must Be Installed after Cation Exchanger
Metal ions in water are exchanged with H⁺ on cation resin, releasing H⁺ into water and making cation exchanger effluent acidic. Most bicarbonate ions (HCO₃⁻) are converted into carbonic acid (H₂CO₃), which further decomposes into CO₂ gas.
Due to the low solubility of CO₂:
- It creates favorable conditions for degassing;
- Without degassing, H₂CO₃ will react with anion exchange resin, increasing the load of anion exchangers and shortening their service cycle.
Normally, decarbonators are installed between cation and anion exchangers, or before RO and other pre-desalination systems. Installation is determined according to raw water quality characteristics.
8. Common Anti-Corrosion Methods
Rubber lining, epoxy coating, plastic lining, enamel coating and other anti-corrosion treatments.
9. Main Components of Reverse Osmosis Units
High-pressure pump, manual/electric outlet valve of high-pressure pump, high/low pressure protection switch, influent flowmeter (optional), product water flowmeter, concentrate flowmeter, product water conductivity meter, membrane assembly (pressure vessel, RO membrane element), concentrate electric valve, concentrate stop valve, influent pressure gauge, inter-stage pressure gauge, concentrate pressure gauge, product water pressure gauge, RO support frame, RO control panel, sampling panel, burst membrane, as well as matching pipelines, clamps and elbows.
10. Essential Instruments and Meters for RO Systems
- Silt Density Index Meter: Measures pretreatment SDI value;
- Concentrate Flowmeter: Matches with product water flowmeter to calculate system recovery rate;
- Product Water Flowmeter: Measures product water output;
- Product Water Conductivity Meter: Monitors product water quality;
- Pressure Gauge: Monitors influent pressure, inter-stage pressure, concentrate pressure and product water pressure;
- Influent Flowmeter: Measures total raw water flow;
- Thermometer: Monitors system operating temperature;
- Influent pH Meter: Tracks pH variation of inlet water;
- Influent Conductivity Meter: Matches with product water conductivity to calculate salt rejection rate;
- ORP Meter: Detects oxidizing substances in influent and assesses system safety risks;
- High/Low Pressure Protection Switch: Prevents system operation under insufficient water supply (low pressure) and overpressure conditions.
RO systems are complex. Instrument configuration depends on process requirements and investment budget. For conventional RO systems, product water flowmeter, concentrate flowmeter, product water conductivity meter, pressure gauges and high/low pressure protection devices are sufficient.
