
Membranes are the essential element of any water recycling system as they serve as the filter for removing solids and chemicals. When choosing a membrane, the first consideration is what types of suspended solids will be filtered. Total dissolved solids (TDS) and total suspended solids (TSS) are essential considerations across industries including food processing, laundry and many others. Every industry will have specific needs for suspended solid removal. However, to optimize separation technologies, the membrane selected needs to provide the proper filtration for the solids needing removal. The Kemco team has outlined how to select the proper membrane for optimal filtration and ROI.
Membrane Types
Membrane separation relies on the principle of size exclusion; essentially, the membrane serves as a barrier for particles that are too large to pass through its pores. There are four primary types of membrane filtration, each characterized by a specific range of pore sizes.
Membrane types include microfiltration, ultrafiltration, nanofiltration and reverse osmosis (RO). Reverse osmosis is the most selective filtration type as it removes almost all minerals and monovalent ions. However, before a stream can become clean enough for the selectivity of RO, it ideally is pre-treated by ultrafiltration with a ceramic membrane (CMF).

Tubular Membranes and CMF
Tubular membranes feature an outer housing containing a bundle of tube-shaped membrane elements. They are particularly well-suited for streams with high total suspended solids (TSS) and total dissolved solids (TDS) as a pre-treatment for RO treatment.
Spiral-Wound Membranes and RO
These filters are made up of layers of membrane material tightly wound around a perforated core. While they offer the highest surface area relative to their size, their dense packing makes them most suitable for the removal of small particles.
Ultimately, when selecting the proper membrane, you need to know what size particles you are filtering for. Particles are measured in microns or one-millionth of a meter or one-thousandth of a millimeter. The smaller the micron, the more selective your membrane must be.

Microfiltration
0.1 – 5.0 microns – Microfiltration membranes are effective for the removal of TSS, flocculated particles and bacteria. For example, it is ideal for removing sand, clay and algae. Microfiltration is used to remove suspended solids and larger colloids while allowing dissolved solids and macromolecules to pass through. Microfiltration membranes are typically rated by nominal pore size, which ranges from approximately 0.1–10 µm. MF processes operate at low pressures, usually at 10 psi or less.

Ultrafiltration
0.01 – 0.1 microns – Ultrafiltration will remove both TSS and TDS including plastics, silica, silt, colloids, as well as certain viruses. UF membranes are designed to retain larger dissolved substances (e.g., proteins, starches) and suspended solids (e.g., colloids, viruses) while allowing smaller dissolved organic compounds and salts to pass through. UF membranes are defined by their molecular weight cutoff (MWCO), which specifies the smallest molecular weight at which at least 90% of the solute is retained.
Nanofiltration
0.002-0.005 microns – Nanofiltration will remove larger organic molecules and multivalent ions like calcium and magnesium, as well as most viruses and pesticides. Nanofiltration membranes filter out multivalent ions and small molecules within the nanometer range, such as sulfate ions and sugars. Nanofiltration operates at higher pressures compared to the more porous ultrafiltration and microfiltration membranes.
Reverse Osmosis
0.001 microns – Reverse osmosis will theoretically remove nearly all minerals and monovalent ions. Reverse osmosis uses the most restrictive membranes for liquid separation, effectively rejecting dissolved salts, inorganic solutes and organic molecules with molecular weights over approximately 100 Dalton (Da) while allowing water to pass through. Reverse membranes typically reject 95–99.8% of dissolved salts like sodium chloride. Operating pressures for RO processes generally range between 100–1,000 psi.

Challenges for Filtration
Filtration needs for streams vary across industries. Using a combination of different membrane filtration types enables highly selective separation of streams. This approach enhances operational efficiency, minimizes clogging and fouling and facilitates the recovery of byproducts. For facilities looking to minimize their haul-off costs, a combined filtration system is ideal for removing TSS and TDS.
Streams with elevated levels of TSS can obstruct flow or lead to increased pressure, resulting in reduced performance and a higher risk of premature membrane failure. Filtering viscous liquids such as oils, sugar syrups, chocolate, paints, solvents, waxes, adhesives, coatings, silicone, glycol and similar products presents challenges due to their natural resistance to flow, which makes it difficult to pass them through a filtration membrane.
Additionally, PFAS (per- and polyfluoroalkyl substances) are becoming increasingly concerning for industries because precursor chemicals, such as FTOH, can transform into PFAS. Consequently, these chemicals persist in water systems and are expected to be addressed in upcoming regulations. In April 2024, PFAS compounds PFOA and PFOS were classified as CERCLA hazardous substances, and they, along with seven other PFAS compounds, may soon be designated as RCRA hazardous constituents. Reverse osmosis systems offer an optimal solution for enhancing treatment processes and ensuring regulatory compliance as they are customized to a facility’s needs.
How the KLEENWATER™ System Optimizes CMF and RO Membrane Filtration
Kemco’s patented KLEENWATER™ System provides both Ceramic Microfiltration (CMF) and Reverse Osmosis (RO) membrane filtration types. CMF is the first step in filtration as it targets more challenging streams. The KLEENWATER™ System uses CMF as a pre-treatment before RO then further filters TDS and TSS. The CMF tubular membrane channels remove solids like oils, syrups and solvents whereas the RO spiral-wound membrane filters out smaller suspended solids and chemicals.
Through the combined power of thermal stability lessening the demand on boilers and reducing natural gas consumption, the KLEENWATER™ system yields reuse rates of 75-85% for facilities’ water resources and thermal energy.
Considering membrane technology for your facility? Consult with our team and learn how Kemco filtration systems can be customized to your facility’s specific membrane needs.
