Reverse Osmosis filtration is one of those technologies that readers tend to recognize at its extremes, either from industrial-scale applications encountered at work or from tiny-scale consumer systems found at home. In both cases, these filtration systems serve to improve the quality of fluids running through them (typically water). Reverse osmosis filters can be complex, sensitive systems that take lots of attention to keep in tip-top operating shape. Thankfully, this burden can be greatly lessened through the addition of upstream pre-filtration components as we'll explain in detail below.
Let's start our discussion with a high-level overview of membrane filtration concepts:
- Membrane Filtration is a type of fluid filtration equipment that falls under the category of engineered Separation Technologies. Separation systems are all designed to pull out unwanted constituents from a material stream in one way or another.
- There are several different types of membrane filtration designs including Ultra-Filtration (UF), Nano-Filtration (NF), and Reverse Osmosis (RO), which vary by the size and properties of particles that they can filter out.
- RO Filtration is the most restrictive variety of membrane filtration available, designed to separate out virtually all potential contaminants from water streams, resulting in effectively pure water.
- RO membranes work on the principle of applying high pressure to an incoming dirty water stream which forces water molecules through the membrane surface while all contaminating particles are stripped out (e.g. minerals, bacteria, particulates, spores, proteins, gums, salts, fats, sugars, and more).
- RO filtration is so effective at pulling out harmful contaminants from water that it is widely entrusted with filtering water for municipal, medical, food & beverage, pharmaceutical, and similarly sensitive applications.
Because RO filtration is critical to applications like those mentioned above, it is quite common to employ levels of upstream pre-filtration to protect the RO membranes from excess loading, early fouling, and physical damage. Pre-filtration can come in many forms that can target any size and type of soil from large, visible particulates down to microscopic, dissolved gases.
Protecting RO Membrane Systems using Pre-Filtration
RO membrane filtration is an inherently sensitive process, and much care must be taken to keep RO filtration membrane modules operating in top form. Below we'll list out the largest risk factors facing RO membrane filtration systems which pre-filtration can directly address.
- Physical Damage - RO filtration systems operate at high pressures, often in the hundreds or thousands of PSI. As such, large particulates can at times be physically pushed into a membrane's surface, leading to damage that risks the integrity of the entire system. Pre-filtration can remove large particulates before they even make it to the RO system, and usually at a lower cost and complexity than relying on the RO system itself to do all the filtration work.
- Surface Fouling - since RO membranes contain such miniscule pore sizes, they are inherently prone to plugging up or filming over. Solid mineral particulates, organic matter, and piping sediment are just a few of the most common elements that foul membrane modules, all of which can be largely alleviated by upstream pre-filtration.
- Biological Contamination - some membrane materials and their physical structures are more sensitive to biological contamination than others, as they provide favorable conditions for the growth of microorganisms. In addition, some filter materials are not tolerant of cleaning using the high temperatures and strong chemicals necessary to sterilize microbiological activity introduced from feed streams. Pre-filtration that targets biological factors can help address these issues.
- Chemical Concerns - many RO filtration systems are exposed to aggressive chemicals either through their feed streams or by intentional cleaning processes. Membrane filter materials must be compatible with these chemicals, else they'll suffer from premature wear or failure. Pre-filtration can help extract incoming chemicals from feed streams and lower the total load on the RO membranes, requiring less frequent and less aggressive chemical cleaning.
- Thermal Damage - excessively high or low temperature fluids often harm RO membrane materials, leading to reduced performance and risk of complete filtration failure. In addition, temperature directly correlates to pressure and can cause knockdown hydraulic effects. Pre-filtration can help keep membrane loading down and thus reduce high-temperature cleaning requirements, as well as reduce kinetic thermal spikes resulting from abnormally high operating pressures.
Key Considerations when Selecting RO Pre-Filtration Solutions
Buyers starting down the path of evaluating pre-filtration options should keep a few key considerations in mind, including:
- Pre-Filtration Technology - selecting pre-filtration technology begins with understanding the available options, and how they can impact the downstream RO system's performance. Readers should perform ample research on key options including sand filters, charcoal filters, bag & cartridge filters, screen filters, water softeners, multimedia beds, centrifugal separators, gravity settling basins, and alternative membrane filters. Engaging with a knowledgeable filtration expert is highly advisable during this research period.
- Water Chemistry - with a fair understanding of the available pre-filtration technologies in mind, readers should next take a detailed look at their incoming water stream’s chemistry. Each feed stream presents a different mix of risk and operational factors, which will directly drive the selection of an ideal pre-filtration solution. For example, a feed stream with heavy suspended solids may be pre-filtered using an inexpensive bag filter, whereas a feed stream containing organic pathogens may call for a complex sterilizer.
- Contaminant Concentration - when examining feed water chemistry, part of the equation is 'what' is in the water, and a larger part is 'how much'. The concentration of contaminants present in a feed stream significantly impacts an RO system's design and efficacy, with higher loading usually leading to slow flow rates, more filter modules, and higher costs. Pre-filtration can offset these issues by reducing contaminant concentrations in a single pass or through progressive filtration stages.
- Operating Costs - RO filtration systems can be expensive to operate, especially when considering the costs of replacement membrane modules, the energy needed to generate required high fluid pressures, and the utility demands often needed to cool discharge fluid streams. For these reasons, getting the longest possible life out of membrane modules is a major factor in managing an RO system's operating costs. As such, RO system owners should consider pre-filtration options as a way to cut the RO’s long-range lifecycle costs.
Looking Beyond Water Filtration
RO membrane filtration is most widely known to serve water applications, which has been the focus of our article here to keep things simple. Across industries, RO filtration has many more advanced, specialty process applications such as concentrating beverages, recovering antibiotics and propagation enzymes, improving chemical clarity, and more. No matter where reverse osmosis filtration is called for, pre-filtration is almost always an integral part of the solution needed to meet cost, regulatory, quality, safety, and performance objectives.