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Reverse osmosis (RO) and nanofiltration (NF) have long been used for desalination, softening and contaminant removal. However, as groundwater, freshwater and “clean” water sources deplete globally, RO and NF are being applied to surface water, wastewater and groundwater. These sources contain more particulate matter, organic substances and other solids that may lack compatibility with RO and NF membranes. Since RO and NF technologies are built to remove salt and dissolved ions, proper pretreatment is critical to the performance, life expectancy and operating costs of these systems.
The salt rejection characteristics of RO and NF membranes are well known to the industry. However, feedwater particulate matter compatibility, fouling/scaling rates, impact on membrane life and membrane degradation due to poor source waters are all problems still being analyzed. Some of these problems are found after facilities are built and put into service—a problem because multimillion dollar investments are at stake—and improper application gives membranes a bad name. Those in the membrane industry strongly believe that it is not membranes that fail but improper application or inadequate pretreatment that causes them to fail.
Source Water Assessment
The first and most important step in RO system planning and design is assessing source water quantity and quality. It is crucial to get adequate information for source water quality—not just a snapshot but also a historical view of it. For some parameters, such as TOC, TSS, temperature, and pH, historical data helps establish the minimum, average and maximum expected values needed to properly plan and design the system. For some parameters, measurement should be done on site since the property will change with time and method of sample handling.
Primarily, pretreatment is used to make the feedwater to the RO compatible with the membrane. Pretreatment increases the efficiency and life expectancy of the membrane by minimizing fouling, scaling and degradation.
Fouling refers to entrapment of particulates, such as silt, clay, suspended solids, biological slime, algae, silica, iron flocs and other matter, on the surface or even worse, in the membrane pores. Typically, fouling initially occurs in the lead elements of the first stage and then works through the rest of the membrane’s elements. Depending on operating conditions and water chemistry, some metals, such as soluble iron and manganese, oxidize once they are within the membrane system and can precipitate throughout the RO system. Microbes can grow and spread in the same way. Microbiological and organic fouling are perhaps the most common types of foulants and are more difficult to control in surface water and wastewater applications.
Scaling refers to precipitation and deposition of sparingly soluble salts, such as calcium sulfate, barium sulfate, calcium carbonates, silica, calcium fluoride and any other supersaturated salt on the immediate surface of the membrane. Typically, scaling starts on the tail elements of the last stage (on the reject side), since the water there is being treated with the highest concentrations of ions. Once a crystal of scale forms within the membrane element, it acts as a nucleation site for additional scales to form, and the rate of scale formation increases exponentially.
Inadequate pretreatment often means a need for frequent cleaning to restore product flux and salt rejection. This results in excessive chemical cleaning costs and increased system downtime. In severe cases, it results in permanent performance loss, membrane degradation and shorter membrane life.
The type and extent of a pretreatment system will depend on the type of source (i.e. well water, beach wells, open sea, surface water or partially treated wastewater). The major differences are the feedwater composition and water quality variability from seasonal factors, climate conditions and/or surface water activities.
Table 2 shows a general guideline for acceptable feedwater to a RO or NF system. However, no set standard exists in the industry for these criteria because system design and operational parameters play a role in the potential fouling and scaling of an RO system. For example, systems with higher recovery tend to foul quicker and may have a higher potential for scaling, since that membrane actually gets a higher concentration of ions and impurities.
Most element manufacturers have similar guidelines, but they may not be as stringent. In general, lower values mean more reliable performance, less frequent cleaning and longer membrane life.
Some facilities will have no RO pretreatment while others will have a complex, comprehensive system for poor raw waters. The pretreatment systems can be chemical, mechanical or a combination of the two.
Pretreatment generally sufficient when membrane cleaning is limited to 3 to 4 times per year or less, membrane elements last more than 5 years, and the productivity and salt rejection are maintained within expected ranges.
More frequent cleaning can sometimes “wash away” the impact of poor pretreatment but it is not a substitute for good pretreatment practices. More frequent cleaning often results in higher downtime, labor and chemical costs and causes membranes to age prematurely.
For most municipal RO systems, cartridge filters (1 to 5 microns) should be the minimum pretreatment, even for the cleanest groundwater sources. That is because foulants/scalants sometimes come from other sources even if they are not in the groundwater. Examples include cement lining, corrosion of steel and ductile iron raw-water piping, well casing failure, colloidal sulfur from oxidation of hydrogen sulfide, well-drilling fluids that may exist months after start up, and pretreatment failure or upset. In these occasional but not unusual cases, the cartridge filter will act as an insurance policy on the membrane. Therefore, cartridge filters should not be viewed as a pretreatment but as a last defense for RO elements.
The more comprehensive and complex the pretreatment becomes, the more it should be viewed as a separate system and not a side process.
There are many solutions for an acceptable RO or NF pretreatment system. The solution depends on raw water composition, seasonal and historical water quality changes and the operational parameters of the RO or NF system. The guidelines in this article are only suggestions and are subject to debate.
However, the importance of system approach and adequate pretreatment needs cannot be overemphasized. Additionally, it has been proven that relying on frequent cleaning to wash away the pretreatment inadequacy is not an optimum solution or an industry-acceptable practice.