Reclaimed water reuse refers to the technology of reusing water resources by treating domestic sewage, industrial wastewater, etc. to meet specific reuse standards (such as industrial circulating water, green irrigation, municipal miscellaneous uses, etc.). The multi-membrane method is the core technical path for reclaimed water reuse. By combining multiple membrane separation technologies such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), it takes advantage of the selective retention characteristics of different membranes to remove pollutants in water in a step-by-step manner, ultimately achieving water quality compliance for reuse. Its core advantages lie in high treatment efficiency, stable effluent quality and small land occupation, and it is widely used in industrial, municipal, environmental protection and other fields. I. Working Principle The core logic of the multi-membrane reclaimed water treatment equipment is “stepwise membrane separation” : Based on the particle size, molecular weight and chemical properties of pollutants in the water, through the combination of membrane modules with different retention precisions, suspended solids, colloids, microorganisms, dissolved organic matter, salts, etc. are removed in sequence, and ultimately the reclaimed water is purified to the reuse standard. The specific principle is as follows: Pollutant classification and removal. Pollutants in water are classified by particle size from largest to smallest as follows: suspended particles (> 1μm) → colloids (0.001-1μm) → macromolecular organic substances (1000- 100,000 Da) → small molecular organic substances (< 1000Da) → dissolved salts (ion level, < 0.1nm). The multi-membrane method matches the pollutant classification through the “retention accuracy gradient” of the membrane: microfiltration (MF)/ultrafiltration (UF) : It retains suspended particles, colloids, microorganisms (such as bacteria and algae), and macromolecular organic substances (molecular weight > 10,000 Da), serving as a “pretreatment membrane” to reduce the contamination risk of subsequent membranes. Nanofiltration (NF) : It retains small-molecule organic substances (200-1000Da) and divalent salts (such as Ca²⁺, SO₄²⁻), while retaining some monovalent salts. It is suitable for reuse scenarios that are sensitive to hardness and organic substances (such as circulating cooling water). Reverse osmosis (RO) : It retains almost all dissolved salts (desalination rate ≥99%) and small-molecule organic substances (< 200Da), producing low-salinity pure water, which is suitable for high-demand reuse scenarios (such as process supplementary water). The core mechanism of membrane separation: All membrane separation technologies are based on the selective permeability of “semi-permeable membranes”, with the driving force being pressure difference: Microfiltration/ultrafiltration: Driven by a low pressure of 0.1-0.3MPa, it utilizes the pore sieving effect of the membrane to retain large particles, while water molecules and small molecules pass through. Nanofiltration/reverse osmosis: Driven by high pressure of 0.5-1.5MPa, in addition to sieving, it also achieves separation through membrane surface charge repulsion (such as the repulsion of divalent ions by nanofiltration) or dissolution diffusion mechanisms (the preferential permeation of water molecules by reverse osmosis). The single-membrane technology is difficult to handle complex reclaimed water (such as water containing suspended solids, organic matter and salts simultaneously), while the multi-membrane method uses the combination of “pretreatment membrane + fine treatment membrane” : first, UF/MF is used to remove colloids and macromolecules to avoid subsequent NF/RO membrane contamination (colloidal adhesion can cause membrane flux to drop by more than 50%). Then, NF/RO is used to specifically remove organic matter or salts, balancing treatment costs and effluent quality to achieve “purification on demand”. Second, Core Structure: The multi-membrane method reclaimed water treatment equipment is typically composed of a pretreatment system, a membrane separation unit, a post-treatment system, and auxiliary equipment. All parts work in coordination to achieve stable operation. 1. Pretreatment system (Protecting membrane modules and reducing contamination) The core of pretreatment is to remove substances in water that may damage the membrane or cause membrane fouling (such as suspended solids, oils, oxidants, etc.), providing qualified feed water for the membrane separation unit. Grating/filter screen: Made of stainless steel, with a diameter of 1-5mm, it intercepts large volume impurities such as plastic bags and fibers in the sewage, preventing clogging of subsequent pump bodies or membrane modules. Regulating tank: Made of PE or concrete, with a volume of the equipment’s processing capacity for 1-2 hours. It balances the fluctuations in influent water quality/quantity (such as pH and COD fluctuations in industrial wastewater), and some are equipped with agitators or aeration devices (to prevent sedimentation and balance water quality). Coagulation/sedimentation tank (optional) : By adding PAC (polyaluminium chloride) and PAM (polyacrylamide), colloidal particles are coagulated into large flocs, which are then removed by sedimentation in the sedimentation tank (reducing turbidity to ≤5NTU), thereby reducing the filtration load on the membrane. Security filter: 304 stainless steel housing, with 5-10μm PP filter element inside, intercepts the fine particles remaining after pretreatment (such as coagulation flocs fragments), ensuring that the water entering the membrane separation unit is “free from particle damage”. Reducing agent/scale inhibitor dosing system: If the reclaimed water contains residual chlorine (such as municipal sewage), add sodium sulfite to remove it (residual chlorine ≤0.1mg/L, avoid oxidation film material). If high-hardness ions are present, scale inhibitors (such as phosphonates) should be added to prevent scaling on NF/RO membranes.
2. The membrane separation unit (core purification module, stepwise pollutant removal) is the “heart” of the multi-membrane method. According to the reuse standards, combinations such as “UF/MF+NF” or “UF+RO” can be selected. The common configurations are “Ultrafiltration (UF) + nanofiltration (NF)” (for industrial circulating water reuse) or “ultrafiltration (UF) + reverse osmosis (RO)” (for high-purity reuse). The core components of the ultrafiltration (UF) system: Hollow fiber membrane module (made of PVDF or PES, resistant to pollution and oxidation), with a membrane pore size of 0.01-0.1μm and a molecular weight cut-off of 10,000-100,000 Da. Structure: The membrane housing is made of fiberglass reinforced plastic or stainless steel. The membrane filaments are arranged in bundles (1-2m in length), and it is equipped with a product water pump, backwash pump and valve group. Function: Remove over 99% of suspended solids, colloids, bacteria, algae and large-molecule organic substances (such as humic acid), with water turbidity ≤0.1NTU and SDI (Pollution Index) ≤3, providing “low-pollution feed water” for subsequent NF/RO membranes. Operating features: It adopts “cross-flow filtration” (part of the concentrated water is recirculated to flush the membrane surface), and backwashing is carried out regularly (30-60 minutes) (using product water to reverse flush the membrane filaments to remove surface contaminants), which extends the membrane life (usually 3-5 years). (2) Core components of nanofiltration (NF)/reverse osmosis (RO) system: Nanofiltration membrane: Roll-type composite membrane (made of polyamide, with a negative charge on the surface), molecular weight cut-off 200-1000Da, divalent salt removal rate ≥90%, monovalent salt removal rate 30-50%; Reverse osmosis membrane: Spiral wound composite membrane (polyamide material), which retains almost all dissolved salts (desalination rate ≥99.5%) and small molecule organic substances (< 200Da). Structure: The membrane housing is made of fiberglass reinforced plastic (with a pressure resistance of ≥2.0MPa), and the membrane elements are arranged in series or parallel (designed according to the water production volume). It is equipped with a high-pressure pump (316L stainless steel, providing a pressure of 0.8-1.5MPa) and a fully automatic valve group (for regulating the recovery rate). Function: Nanofiltration: Removes small-molecule organic substances (such as pesticides and detergents), reduces water hardness (Ca²⁺, Mg²⁺), and the total hardness of the produced water is ≤50mg/L (calculated as CaCO₃), suitable for the reuse of circulating cooling water. Reverse osmosis: Deep desalination (TDS of produced water ≤50mg/L), removal of trace pollutants, suitable for process supplementary water (such as in electronics factories, pharmaceutical factories). 3. Post-treatment system (Ensuring the safety and stability of reuse) Disinfection device: Ultraviolet sterilizer (254nm wavelength, sterilization rate ≥99.9%) or ozone generator (dosage 0.3-0.5mg/L), to kill possible residual microorganisms in the membrane effluent and prevent bacterial growth during reuse (such as biofilm in circulating water pipelines). pH adjustment device: If the pH of NF/RO product water is too low (for example, the pH of RO product water is 5.5-6.5), add NaOH solution to adjust it to 7.0-8.0 to prevent corrosion of the reuse pipeline (especially metal pipelines). Reuse water tank: Made of 304 stainless steel or PE material, with a capacity of 1-2 hours of water production from the equipment. It is equipped with a liquid level sensor (to link the start and stop of front-end equipment) and a circulating pump (to prevent secondary pollution caused by stagnant water). 4. Auxiliary equipment automatic control system: PLC control cabinet (with touch screen), real-time monitoring of parameters of each unit (such as turbidity of UF product water, conductivity of NF/RO product water, pressure, flow rate), automatic control of backwashing, chemical cleaning, dosing and other processes, supporting remote monitoring (optional iot module). Cleaning system: Equipped with acid washing (citric acid) and alkali washing (NaOH) reagent tanks and cleaning pumps, the membrane modules are chemically cleaned regularly (every 3-6 months) to restore membrane flux (flux will decrease by 20-30% after membrane fouling).
Iii. Main Functions: The multi-membrane reclaimed water treatment equipment achieves the “resource utilization” of reclaimed water through efficient purification treatment. Its core functions are reflected in the following three aspects: Water quality meets standards for reuse, meeting the needs of diverse scenarios. According to the membrane combination method, reclaimed water can be treated to different standards: Industrial circulating water (such as cooling tower make-up water) After “UF+NF” treatment, the turbidity of the produced water is ≤0.1NTU, the hardness is ≤50mg/L, and the COD is ≤30mg/L, avoiding scaling, corrosion or biofilm formation in the circulation system. Municipal miscellaneous uses (such as road sweeping and green irrigation) : After “UF + disinfection” treatment, the produced water has microorganisms ≤100CFU/mL, no odor, and no suspended impurities, meeting the standards of “Water Quality for Urban Sewage Reuse – Urban Miscellaneous Uses” (GB/T 18920-2020). Process supplementary water (such as in the electronics and food industries) : After being treated by “UF+RO”, the resistivity of the produced water is ≥10MΩ · cm and the TOC is ≤50ppb, meeting the requirements of low salt content and high purity. Water conservation and emission reduction, lowering environmental and economic costs. Saving fresh water resources: Industrial enterprises can reduce fresh water consumption by 50-80% through reclaimed water reuse (such as supplementary water for power plant circulating water), alleviating water resource tension (especially in water-scarce areas in the north). Reduce sewage discharge: Reclaimed water can be reused after treatment, reducing the amount of sewage discharged (by 30-60%), lowering sewage discharge fees and environmental protection pressure (such as in heavily polluting industries like chemical engineering and dyeing). Reduce operating costs: The cost of reclaimed water reuse (about 1-3 yuan per ton) is much lower than that of fresh water (3-10 yuan per ton), and long-term operation can significantly reduce the water cost for enterprises. Stable and reliable, it can adapt to complex water quality. Compared with traditional biochemical treatment (such as A/O process), the multi-membrane method has a stronger adaptability to fluctuations in reclaimed water quality: membrane separation is not affected by drastic changes in water temperature and pH (such as the shock load of industrial wastewater). The stepped membrane combination can specifically remove specific pollutants (such as UF for colloids and NF for hardness), and the effluent quality is stable (fluctuation ≤±5%). Fully automated operation reduces manual intervention and is suitable for large-scale and continuous reuse scenarios (such as centralized reuse in industrial parks). The multi-membrane method reclaimed water treatment equipment, through the combined process of “pretreatment + ultrafiltration + nanofiltration/reverse osmosis + post-treatment”, utilizes the selective retention characteristics of membranes to achieve stepwise purification of reclaimed water, ultimately producing water that meets the reuse standards. Its core value lies in efficient water conservation, stable compliance with standards, and adaptability to diverse scenarios. It is a key technical equipment for addressing water resource shortages and promoting the “water circular economy”, and plays an irreplaceable role in the industrial, municipal, and environmental protection fields.
Post time: Aug-07-2025