I. Raw Water Quality Requirements
As a new purely physical desalination process, reverse osmosis is restricted by the structure, material and desalination mechanism of membrane elements, so it has strict requirements on inlet water conditions:
Water temperature: 1 ~ 45℃;
pH value: 2 ~ 11;
Organic content (COD, mg/L): less than 1.5;
Turbidity (NTU): below 1.0;
Silt Density Index (SDI): < 4.0;
Residual chlorine: < 0.1 mg/L (practically controlled at 0);
Iron content (mg/L): when dissolved oxygen > 5 mg/L, Fe < 0.05;
Silica content (mg/L): SiO₂ in concentrated water < 100; LSI: pHb-pHs < 0;
Ions prone to forming sparingly soluble salts such as Sr and Ba: Ipb < 0.8Ksp.
The latter three indicators can be appropriately relaxed by adding scale inhibitors.
If any of the above parameters fails to meet the standard, the reverse osmosis membrane will suffer the following damages:
1. RO membrane contamination by metal oxides;
2. Colloidal fouling;
3. RO membrane scaling;
4. Suspended solids blocking the RO membrane;
5. Contamination by organic matter and microorganisms, resulting in increased effluent COD.
Further adverse impacts on the entire reverse osmosis system include:
Reduced water production capacity; declined effluent quality; increased energy consumption including raw water and power consumption; higher operating costs such as reverse osmosis scale inhibitors, resin regeneration salt and other water treatment chemicals.
Reverse osmosis pretreatment plays an irreplaceable role. Insufficient pretreatment and long-term substandard inlet water will cause irreversible physical and chemical damage to RO membrane elements and greatly shorten their service life.
The purpose of reverse osmosis pretreatment is to solve the following problems and ensure stable operation and long service life of RO equipment:
1. Prevent membrane scaling (including CaCO₃, CaSO₄, SrSO₄, CaF₂, SiO₂, iron and aluminum oxides, etc.);
2. Avoid organic fouling;
3. Prevent microbial contamination and blockage;
4. Reduce blockage caused by colloids and suspended solid particles;
5. Maintain stable water output of the reverse osmosis system.
II. Process Flow
Raw Water Tank
Stores raw water, settles large sediment and other settleable substances, and buffers the impact of unstable water pressure in the pipeline on the water treatment system.
Raw Water Pump
Stabilizes system water supply pressure and flow rate.
Multi-media Filter
Removes sediment, rust, colloids, suspended solids and other particles larger than 20μm in raw water, ensuring stable water quality and extending equipment service life.
Activated Carbon Filter
Activated carbon can adsorb electrolyte ions and perform ion exchange adsorption. It reduces permanganate oxygen consumption (COD) from 15 mg/L(O₂) to 2～7 mg/L(O₂). It also removes color, odor, biochemical organic matter, residual chlorine, pesticide pollutants, trihalomethanes (THM) and other contaminants. The equipment comes with a self-maintenance system with low operating cost.
Water Softening Equipment / Dosing System
To avoid membrane scaling at the concentrated water side, ion softening equipment or appropriate scale inhibitors are adopted before raw water enters the RO membrane module, preventing crystallization of carbonate, SiO₂ and sulfate caused by excessive concentration product.
Security Filter
Removes residual suspended solids, particulate matter and colloids in inlet water to ensure safe and reliable operation of subsequent RO equipment. Equipped with 5μm melt-blown filter cartridges, it intercepts impurities larger than 5μm missed by previous filters, preventing them from entering the RO system and damaging membrane surface and desalination performance.
Reverse Osmosis System
By applying sufficient pressure, solvent (usually water) in the solution is separated through a semi-permeable membrane. Since the direction is opposite to natural osmosis, it is defined as reverse osmosis. RO technology adapts to raw water with various salinity, especially suitable for high-salinity water treatment with excellent technical and economic benefits. It features high desalination rate, high recovery rate, stable operation, small footprint and simple operation. While removing salts, it also eliminates most bacteria, colloids and high-molecular organic matter.
Process Flow:
Raw Water → Raw Water Tank → Raw Water Pump → Quartz Sand Filter → Activated Carbon Filter → Dosing Device → Security Filter → High-pressure Pump → Single-stage Reverse Osmosis Equipment → Pure Water Tank
