Common Process Sequences for Reverse Osmosis Pretreatment Systems

Common Process Sequences for Reverse Osmosis Pretreatment Systems

I. Main Text:
Reverse osmosis pretreatment systems typically comprise multiple processes including sand filtration, ultrafiltration (microfiltration), carbon filtration, softening, fine filtration, oxidation, reduction, and temperature adjustment. Beyond optimizing the operational modes and parameters for each individual process, there exists the challenge of arranging these processes in an optimal sequence. The rationality of process sequencing reflects the level of system design and is a crucial measure for maximizing the functionality of each process and enhancing the overall system performance.

Process Overview (Detailed descriptions provided at the end of the document)
① Sand Filtration Process: Utilizes coagulation sand filters to remove suspended solids from the feed water, thereby reducing turbidity and pollution index. ② Ultrafiltration Process: A pressure-driven membrane separation process that retains particles and impurities between 0.001-0.02μm (1-20nm) in diameter through microporous membrane filtration. Effectively removes colloids, silica, proteins, microorganisms, and large organic molecules from water.
③ Carbon Filtration Process: Utilizes activated carbon filters to adsorb organic matter in the influent, reducing organic content in the effluent.
④Softening Process: Employing resin exchange methods to reduce the concentration of insoluble salt ions in the feedwater; or adding scale inhibitors to increase the saturation limit of insoluble salts in the system concentrate.
⑤Disinfection Process: Applying oxidative or non-oxidative disinfectants at the front end of the pretreatment process to eliminate algae and bacteria in the raw water. This directly prevents microbial contamination during each stage of the pretreatment system and indirectly prevents microbial contamination in the membrane system process.
⑥ Reduction Process: For residual chlorine in municipal tap water or oxidizing disinfectants like chlorine dioxide or sodium hypochlorite added to natural water bodies, reduce residual chlorine at the end of the pretreatment system using activated carbon or sodium bisulfite (chlorine reduction agent) to prevent membrane oxidation.
Note: When using non-oxidizing disinfectants, reduction treatment is unnecessary and may even reduce overall microbial contamination risks throughout the system, though these disinfectants are more expensive.
⑦ Iron Removal Process: For groundwater sources containing high levels of ferric iron and manganese, removal is typically achieved using manganese sand, aeration, and filtration processes.
⑧ Temperature Control Process: Reverse osmosis membranes exhibit distinct temperature-dependent operating pressures. At low temperatures, the pressure required to maintain permeate flux increases significantly, leading to substantially higher energy consumption. When the raw water temperature is excessively low and an inexpensive heat source is available, heat exchange can be implemented at the end of the pretreatment system to elevate the inlet temperature of the reverse osmosis system.

(1) Process Positioning of Sand Filtration and Ultrafiltration
① In pretreatment systems centered on coagulation-sand filtration, this process offers the lowest media cost, minimal media loss, and effective retention of suspended solids and turbidity reduction without performance degradation. It is typically deployed as the first stage for treating general raw water. However, due to its limited COD removal capacity, biological processes like aerobic biological filters may be added upstream to reduce raw water COD levels when necessary.
② In ultrafiltration-based pretreatment systems, ultrafiltration primarily retains suspended solids, colloids, and large organic particles—functions similar to coagulation-sand filtration. Its placement in the process flow is comparable to that of coagulation-sand filtration. However, ultrafiltration offers higher filtration precision than coagulation-sand filtration, entails higher process costs, and experiences significant performance degradation upon fouling. For raw water with high turbidity and high COD, efficient pretreatment processes such as disc filtration, fiber filtration, or aerated biological filters are required. Generally, microfiltration processes require pretreatment with a maximum filtration precision of 500 microns, while ultrafiltration processes require pretreatment with a maximum filtration precision of 100 microns.

In summary: Sand filtration and ultrafiltration occupy overlapping positions; either one is sufficient.

(II) Positioning of Carbon Filtration and Softening Processes
① Activated carbon filtration offers dual benefits: adsorbing organic matter and reducing oxidants. For organic adsorption, activated carbon utilizes its vast internal pore surface area to capture small-particle organic compounds while its limited external surface area adsorbs colloidal and large-particle organic matter. Since colloidal and large-particle organic matter adhering to the activated carbon surface can block pathways for small-particle organic matter into the deep pores, the activated carbon process is more suitable for adsorbing small-particle organic matter. Meanwhile, the coagulation-sand filtration or ultrafiltration process protects the activated carbon by retaining suspended solids, colloidal matter, and large-particle organic matter. Activated carbon's removal of small-particle-size organic matter not only protects reverse osmosis membranes from organic contamination but also effectively safeguards softening resins from organic fouling. Consequently, activated carbon processes are typically positioned in pretreatment sequences after coagulation-sand filtration or ultrafiltration and before resin softening.

②Oxidizing agents play a dual role in reverse osmosis systems: they cause oxidative degradation of reverse osmosis membranes and softening resins while also inhibiting microbial contamination in pretreatment processes and pipelines. After the activated carbon process reduces the oxidizing agents, subsequent processes in the system flow will no longer be protected by them. When the system's raw water temperature is high or microbial content is elevated, the downstream exchange resins will face microbial threats. Therefore: - For high microbial loads, high raw water temperatures, and low oxidant concentrations, place the activated carbon filter after ion exchange. - For low microbial loads, low raw water temperatures, and high oxidant concentrations, place the activated carbon filter before ion exchange.
In summary: In most cases, carbon filtration should precede softening processes.

III) Precision Filtration and Its Process Position
Traditional pretreatment systems—including sand filtration, carbon filtration, and softening—utilize granular media. During system operation, media particle shedding is an ongoing concern, with potential risks of accidental media discharge. Additionally, improper dosing of coagulants may pose risks to membrane systems. To prevent contamination from leached filter media and flocculants, fine filtration is typically the final stage in traditional pretreatment systems. Operating at minimal load here, it is often termed a security filter.
Summary: Fine filtration commonly serves as the final stage in traditional pretreatment processes.

(4) Positioning of Scale Inhibitor Dosing
In small-to-medium pre-treatment systems, softening processes are commonly used to address insoluble salts, while large systems often employ scale inhibitor processes. Scale inhibitors are typically dosed after sand filtration and before fine filtration. This placement prevents effective chemical solution from being retained by the sand filter, utilizes fine filtration to remove impurities from the chemical solution, and leverages fine filtration for secondary chemical mixing.
In summary: Scale inhibitor dosing is commonly positioned after sand filtration (or after carbon filtration if present) and before microfiltration.

(5) Process Position for Disinfectant Dosing Raw water in large and medium-sized systems typically lacks residual chlorine or other disinfectants. To prevent microbial contamination in pre-treatment and membrane systems, pre-treatment systems must incorporate disinfection processes, with a primary focus on the sand filtration stage.
① When carbon filtration is present in the system, oxidative biocides should be dosed before sand filtration. This prevents microbial proliferation in the sand filter while forming a typical bioactive carbon process with the activated carbon.
② When no carbon filtration exists and oxidizing disinfectants like chlorine dioxide or sodium hypochlorite are used, sodium sulfite must be added as a reducing agent to prevent oxidative damage to reverse osmosis membranes. To protect the entire pretreatment system, oxidizing disinfectants should be dosed before sand filtration, and reducing agents before polishing filtration.
③ When ultrafiltration is integrated into this disinfection loop, it effectively prevents microbial fouling of ultrafiltration membranes. ④ Non-oxidizing disinfectants require no reducing agents. Moreover, non-oxidizing disinfectants entering the reverse osmosis membrane system can effectively inhibit microbial contamination of the membrane system.
In summary: Disinfectant dosing is typically positioned before sand filtration.

(VI) Summary of Pretreatment Sequence
Relatively speaking, reverse osmosis systems using municipal tap water as feedwater have a relatively fixed pretreatment sequence and operate at lower pressures. Conversely, systems using natural water sources (rivers, lakes, etc.) require special attention to periodic (seasonal) water quality fluctuations, particularly the potential for algal blooms at the system inlet, and vigilance against microbial contamination. When industrial wastewater or municipal reclaimed water serves as the source, the pretreatment system becomes more complex. Beyond the common pretreatment processes mentioned above, it must be supplemented with physical-chemical treatment, biological treatment, and other wastewater treatment processes to meet or substantially meet the operational requirements of the reverse osmosis system.
In summary, pretreatment systems operate in complex environments with diverse processes. The relative positioning of each process is closely tied to specific raw water conditions and membrane system requirements, allowing for a degree of flexibility in designing the sequence of processes.