Drinking Water Softening Treatment Method - Membrane Softening Method

On the one hand, raw water is increasingly polluted; on the other hand, people have higher and higher requirements for drinking water quality. With the improvement of membrane separation technology, membrane softening technology is receiving increasing attention. Although the construction and operating costs are higher than lime softening, membrane softening can produce high-quality drinking water. Even if future water quality standards are further raised, membrane methods can still meet the requirements, making the advantages of membrane softening quite obvious. Membrane softening can treat raw water of different qualities and produce drinking water of various qualities according to people's requirements. Lime softening treatment cannot meet future needs; when building water plants, membrane technology should be chosen from a long-term perspective. However, membranes are currently expensive. As membrane prices decrease, membrane softening will be widely used. Some high-hardness water also contains high color and high natural organic matter content. For these raw waters with poor quality, membrane softening should be considered. Membrane softening can effectively remove color, total organic carbon, and trihalomethane precursors (THMFP) simultaneously. Membrane softening is suitable for applications with poor feed water quality but high softening water quality requirements. Membrane softening offers advantages such as no regeneration required, no sludge production, complete removal of organic matter, simple operation, and small footprint, which are unmatched by other softening processes. Commonly used membrane technologies include electrodialysis, nanofiltration, and reverse osmosis.

Reverse osmosis, during softening, can essentially remove all calcium and magnesium ions from the water. However, completely softened water is harmful to human health when consumed. In reverse osmosis softening, a portion of the water bypasses the reverse osmosis unit, while the other portion undergoes membrane treatment. The two portions are then mixed; the ratio is called the mixing ratio. By adjusting the mixing ratio, the hardness of the effluent reaches the desired value. Two rural areas in Florida, USA, used reverse osmosis membranes and a mixing method for softening well water. The main difference between these two reverse osmosis membrane water separation plants lies in the well water quality and effluent water quality targets. Area A uses a shallow aquifer with lower hardness and total dissolved solids, and has lower effluent water quality requirements. Therefore, it allows for a larger amount of water to bypass, resulting in a higher mixing ratio and reduced water production costs. Two water plants have similar annual operating costs, but Plant A has a higher water production capacity. The third option is to use a low-pressure softening membrane. This membrane is sufficient to soften the water, remove disinfection byproducts, reduce total dissolved solids, and remove all color to meet drinking water quality standards. Due to the lower pressure, the effluent hardness is higher, resulting in a lower mixing ratio. The total treatment cost is similar to the first two options. Utilizing a lower mixing ratio, more raw water passes through the membrane, allowing for better control of trihalomethane precursors.

Electrodialysis plays a crucial role in membrane separation and is the primary method for drinking water production in some areas. When using electrodialysis to reduce water hardness, pretreatment processes are essential. First, alkaline agents should be added to remove some non-carbonate hardness from the water, ensuring the pretreated water hardness meets drinking water requirements and reducing membrane scaling. Pretreatment also involves reducing turbidity and color to meet the feed water quality requirements for electrodialysis, minimizing membrane fouling, and extending membrane lifespan. In electrodialysis, it's crucial not to remove all calcium and magnesium ions from the water. Completely softened water is not suitable for drinking; a hardness of 170 degrees is generally considered optimal for human health. To prevent scaling on the electrodialysis membrane, frequent electrode reversal can be used, and the discharged water can be neutralized with an alkaline solution. Electrodialysis is a deep treatment technology that reduces hardness while also lowering the total dissolved solids (TDS) content in the water.

Nanofiltration membranes, based on organic polymer nanotechnology, are ideally suited for softening drinking water. Nanofiltration membranes have a loose surface layer structure. Due to the presence of both amino and carboxyl groups within the membrane, they exhibit high removal efficiency for low concentrations of salts, achieving high water flux at relatively low pressures (0.5–1 MPa). Nanofiltration membranes achieve a TDS removal rate of 50–70%, with particularly high removal rates for Ca²⁺, Mg²⁺, and SO₄²⁺. They are suitable for softening water with high SO₂⁺ content, while retaining harmless sodium and potassium salts essential for the human body. Nanofiltration requires nearly turbid water as its feed water, generally with an SDI ≤ 3, making it suitable for softening hard groundwater. However, because nanofiltration removes most of the hardness, the effluent can cause some corrosion to the pipe network. Therefore, subsequent treatment after membrane treatment is crucial, including chlorination to remove gases such as chlorine dioxide and hydrogen sulfide, and to control corrosion.

The membrane softening process should be tailored to different feed water qualities, product water quality, and quantity requirements, selecting appropriate softening membrane modules and establishing the operating process flow and parameters. Membrane softening requires strict pretreatment, typically involving multiple pretreatment processes such as pre-sedimentation, coarse filtration, and fine filtration. High hardness can cause excessive CaCO3 foaming and scaling during nanofiltration membrane operation. Due to its ease of construction, fully automated operation, and convenient management, membrane separation water plants provide better water quality compared to lime-softening plants. The membrane can remove Giardia lamblia, Cryptozoa, radionuclides, sulfur, nitrogen, metals, and most organic carcinogens. With the presence of chlorination disinfection byproducts and waterborne bacteria such as Cryptosporidium in drinking water, membrane softening has become a preferred method. Membrane water purification plants are primarily for small-volume applications, but the maximum daily production capacity of membrane softening can currently reach tens of thousands of tons.

Various softening methods have their own characteristics and are influenced by various factors. Lime-enhanced coagulation pretreatment is simple, but the effluent quality is not as good as membrane softening and management is more complex. Ion exchange softening and membrane softening have strict pretreatment requirements and are more expensive. If membrane softening does not require a bypass to achieve the same effluent quality as lime softening, its operation and maintenance costs are lower than those of lime softening.