Introduction

Those in the industrial zero-discharge and wastewater resource recovery industry know that reverse osmosis concentrate is the toughest part of the entire system. After concentration by the reverse osmosis membrane, calcium and magnesium hardness, silica, total salinity, and alkalinity are all multiplied, resulting in complex water composition and a strong tendency to scale. Insufficient pretreatment can easily lead to scaling and fouling of the subsequent membrane system, decreased evaporator efficiency, and abnormal operation of the crystallization system. The dual-alkali method, as the mainstream technology for deep hardness removal of reverse osmosis concentrate, can achieve synergistic control of hardness and silica. Combined with enhanced separation by silicon carbide ceramic membranes, it can further improve the stability of the effluent and the reliability of the system.

I. Basic Knowledge of Hardness and Characteristics of Reverse Osmosis Concentrate

Total hardness refers to the total concentration of calcium and magnesium ions in the water, which is a core indicator for assessing the risk of scaling and the operational stability of the membrane system.

Based on its composition, hardness can be divided into two main categories: One is carbonate hardness, also known as temporary hardness, which mainly exists in the form of calcium bicarbonate and magnesium bicarbonate. It can be removed by heating and boiling to precipitate and remove the hardness.

The other is non-carbonate hardness, also known as permanent hardness, which exists in the form of sulfates and chlorides. It cannot be removed by heating and must be effectively removed through chemical precipitation, ion exchange, or membrane methods.

In the context of reverse osmosis concentrate, the characteristics are obvious: Reverse osmosis systems have a trapping and enriching effect on salts, hardness, and silicates. The ion concentration on the concentrate side is typically concentrated 3 to 8 times, and the TDS is generally between 10,000 and 60,000 mg/L, reaching over 100,000 mg/L under high operating conditions. Total hardness is consistently maintained at 500 to 3,000 mg/L, with silica and alkalinity also being high, and the Langrier saturation index and sulfate scaling index significantly exceeding the standards. The biggest problem with this type of water quality is that it easily forms insoluble scale such as calcium carbonate, magnesium hydroxide, calcium sulfate, and silicates, which adhere to the membrane surface, pipe inner walls, and evaporator heat exchange tube walls. This causes membrane flux decline, a surge in transmembrane pressure difference, a decrease in heat exchange efficiency, and deterioration of crystallized salt quality, which is a key pain point restricting the long-term operation of zero-discharge systems.

II. Introduction to Dual-Alkali Precipitation Technology

In layman's terms, the dual-alkali method is a chemical precipitation process in which calcium hydroxide and sodium carbonate are added in steps. It is currently the most applicable and economical pretreatment technology for reverse osmosis concentrate with high hardness, high silica, and high alkalinity.

1. Process Reaction Principle

(1) Adding calcium hydroxide (Ca(OH)2) mainly serves to adjust alkalinity, remove magnesium and silica, and reduce the bicarbonate alkalinity of the water. Magnesium removal and alkalinity reduction reaction equations:

Silicon removal chemical reaction equation (under high pH conditions):

By adding lime to raise the water pH to above 11.0, magnesium ions are converted into magnesium hydroxide precipitate. Simultaneously, dissolved silica in the water is converted into silicate ions under strongly alkaline conditions, which then combine with calcium and magnesium ions in the water to form insoluble calcium silicate and magnesium silicate precipitates, achieving efficient silicon removal. At the same time, bicarbonate ions are neutralized, reducing the alkalinity baseline and lowering the risk of subsequent hardness and silica scaling.

(2) Adding sodium carbonate (Na₂CO₃), sodium carbonate is mainly used to remove calcium ions from the water and remove difficult-to-treat non-carbonate hardness. Calcium hardness removal reaction equation:

By utilizing the combination of carbonate ions and calcium ions to form calcium carbonate crystal precipitates, residual calcium hardness is further reduced, and the total hardness is stably controlled within an acceptable range. 2. Technological Advantages and Traditional Shortcomings

The dual-alkali method has significant advantages: readily available reagents, low operating costs, and the ability to perform large-scale continuous treatment. It can simultaneously complete hardening, alkali reduction, and desiliconization, and has strong resistance to high-salt and high-alkali water quality impacts, making it highly suitable for extreme conditions such as reverse osmosis concentrate.

However, the traditional dual-alkali method also has obvious shortcomings: the calcium carbonate, magnesium hydroxide, and silicate flocs generated in the reaction have fine particle sizes, resulting in slow natural settling rates and incomplete solid-liquid separation in conventional sedimentation tanks; the effluent turbidity and suspended solids are relatively high, and fine crystal particles can easily penetrate the filter unit, still posing a scaling risk to the downstream membrane and evaporator; moreover, hardening and desiliconization exhibit competitive inhibition, making it difficult for ordinary sedimentation processes to simultaneously optimize both indicators.

III. Introduction to the Dual-Alkali Method + Silicon Carbide Ceramic Membrane Coupling Process

To address the shortcomings of the traditional dual-alkali method, the dual-alkali method + silicon carbide ceramic ultrafiltration membrane coupling process has emerged, providing dual protection for concentrate pretreatment. Overall Process Path:

The entire process has a clear logic: The front end relies on a dual-alkali method to complete a chemical reaction, converting calcium and magnesium ions and silicides into solid precipitates; the back end relies on a silicon carbide ceramic membrane for high-precision retention, replacing traditional high-density sedimentation tanks and multi-media filtration processes.

The silicon carbide ceramic membrane has nanoscale pores, which can completely retain calcium carbonate, magnesium hydroxide, silicate micro-flocs, colloids, and suspended particulate matter, fully meeting the stringent influent requirements of high-pressure membranes and evaporation crystallization equipment.

The advantages of this process are as follows:

1. Stable and Reliable Operation: The silicon carbide ceramic membrane is resistant to harsh operating conditions, anti-fouling, and not easily clogged. It requires low cleaning frequency, and there is no significant performance degradation over long-term operation, enabling continuous and stable operation and reducing downtime impact.

2. Outstanding Economic Efficiency: The dual-alkali method has low reagent costs, the silicon carbide ceramic membrane has a long lifespan and does not require frequent replacement, backwashing water consumption is low, and long-term maintenance costs are controllable.

3. Wide Adaptability: Adaptable to various industries and concentrations of reverse osmosis concentrate, meeting both concentrate reuse and zero-discharge pretreatment requirements.

4. Simple and Easy-to-Implement Process: Simplified process, small footprint, can be integrated into a single unit, highly automated, easy to maintain, and can operate stably without a specialized team.

5. Green and Environmentally Friendly: Low sludge production and comprehensive utilization, no secondary pollution, enabling concentrate quality improvement and reuse, helping enterprises save water and reduce pollution, and achieve resource recycling.

The alkaline method + silicon carbide ceramic ultrafiltration membrane coupling process retains the economic efficiency and shock resistance advantages of the dual-alkali method, while leveraging the silicon carbide ceramic membrane to address the shortcomings of traditional processes. It not only shortens the process flow, saves space, and reduces manual maintenance, but more importantly, it completely intercepts scale-forming substances at the source, stably controlling the hardness and silicon content of the effluent, significantly reducing the risk of scaling and fouling in the downstream membrane system and evaporator, extending equipment operating cycles, and reducing reagent consumption and downtime maintenance costs. It has become a mature and preferred process for zero-discharge pretreatment of reverse osmosis concentrate.