Reverse osmosis (RO) and nanofiltration (NF) are membrane filtration technologies that are designed to remove solute ions and molecules from a pretreated liquid stream. Well-designed RO/NF units are compact and demand relatively little maintenance, making them an attractive alternative to conventional treatment trains.
Even despite the merits of today’s efficient RO/NF technologies, it pays to understand common pitfalls associated with them. If your facility currently uses RO/NF or is considering purchasing an RO or NF system, the following article will help you to understand the most common problems impacting reverse osmosis and nanofiltration, and some possible solutions.
RO/NF membrane fouling
Fouling occurs when contaminants collect on the surface of a filtration membrane and restrict the flow of water through the membrane’s pores. With the smallest pore sizes of any membrane filtration technology, RO/NF are particularly prone to premature membrane fouling. Without adequate pretreatment and process monitoring steps in place, RO/NF membrane fouling can reduce unit service life, compromise permeate quality, and increase operational costs.
Preventative steps and/or remediation strategies for membrane fouling depend on the types of contaminants present in the process or waste stream. Common types of fouling include:
Particulate and colloidal fouling
Particulate fouling occurs when solid materials build up on a filtration membrane surface, forming a cake layer that blocks water from flowing through the membrane’s pores. In many cases increased pressure differential measurements provide early indication of particulate fouling in RO/NF membranes. Common particulate contaminants include bacteria, viruses, sediment, macromolecules, iron oxides, salts, and colloidal silica. In many cases, particulate fouling of RO/NF units can be prevented by applying appropriate upstream filtration, which can include media filtration, microfiltration (MF), and/or ultrafiltration (UF), depending on the sizes and geometric shapes of particles present.
For streams with colloidal particles, it is sometimes necessary to apply an inorganic coagulant to separate out suspended solids. Commonly-used coagulants include aluminum sulfate, aluminum chloride, sodium aluminate, and ferric chloride, and the use of each will vary depending upon the contaminants present in the feed stream. The use of coagulant chemicals must be monitored closely, however, as coagulants can also lead to RO/NF membrane fouling if allowed to proceed downstream, where they can react with antiscalants or other substances and collect on the membrane.
Biofouling is a process where microorganisms, plants, algae or other biological contaminants grow on RO/NF membrane elements, forming a layer known as biofilm. As biofilm accumulates on the membrane surface, greater pressure is needed to force water through, resulting in higher energy costs, and eventual damage to the RO/NF membrane element. Key symptoms of biofouling include increased differential pressure from feed to concentrate, and decreased membrane flux.
RO/NF membrane elements tend to provide the warm, low-flow environments suitable to biological growth, making them particularly susceptible to biofouling. Some common solutions for control and prevention of biofouling include:
- Biofiltration to remove nutrients from the feed stream;
- Chlorination to chemically destroy biological contaminants; and
- Fouling-resistant membranes that prevent microbials from clinging to the RO/NF element.
Scaling or precipitation fouling
Scaling or precipitation fouling occurs when membrane pores are blocked by crystallized salts, oxides, or hydroxides that have precipitated from solution. Scaling is among the most common forms of fouling in RO/NF elements, especially by divalent calcium (Ca2+) and Magnesium (Mg2+) ions. Like other forms of fouling, scaling and precipitation fouling can compromise the efficiency of an RO/NF unit, and, over time, can irreversibly damage membrane elements.
Control and prevention strategies for scale and precipitation fouling focus on inhibiting crystal growth, resulting in particles that are small enough to be carried away in the reject stream. Control methods include:
- Acid injection to control calcium carbonate scale;
- Water softening, or the addition of lime to feed water as a means of reducing hardness, alkalinity, and silica to prevent scale crystal formation; and
- Scale inhibition, or the injection of a specialized chemical substance into the feed stream to inhibit the growth of salt crystals.
Membrane material compatibility
RO/NF membranes are fabricated from a variety of materials, including cellulose acetate, polyamide, and polysulfone, among others. In order to prevent premature failure, a number of important process factors—including industrial application, pH, substances present, temperature, feed pressure, and biological load—should be considered in the selection of a membrane material. An example of this is the use of polyamide membranes. While widely used in RO/NF units, polyamide membranes are easily damaged by chlorine, making them a poor option in applications where chlorine is needed for disinfection purposes. In order to prevent chemical attack and oxidation, all process characteristics must be carefully considered when selecting a membrane material.
Reject water discharge
While they are extremely effective water purification technologies, RO/NF produce large volumes of wastewater—frequently up to 20-50% of the volume of feed water that they process. Disposal of the concentrated waste streams produced by RO can be challenging, especially if your facility is subject to zero liquid discharge (ZLD) regulations, or if you face high costs for sewer or surface water discharge. A popular solution to mitigate disposal costs is the reduction of RO waste stream volume through evaporation.
Overall impact of proper design on preventing common problems
Many of the common problems impacting RO/NF can be avoided through careful design that takes process conditions into account. While we’ve already discussed how pretreatment is essential for minimizing operational and maintenance issues, care must also be given to other system design elements, including:
- Flux: Flux is the volume of water to pass through a membrane in a given amount of time, often expressed as the number of gallons of water per square foot of membrane per day (Gfd). Flux is used to determine the number of membrane elements needed for an application, and is affected by feed water quality, temperature, and salt concentration.
- Flow rate: Generally measured in gallons per minute (GPM), feed and permeate flow rates are critical measures for efficient RO/NF operation. Good RO/NF system design takes water source into account, for example, when processing surface waters with high colloids, an optimal flow rate may be 10 – 14 GPM/ft2 of membrane.
- Array: An array is the physical arrangement of pressure vessels in an RO/NF system. An array can entail multiple stages, with multiple pressure vessels in each stage. Generally speaking, the higher a recovery rate demanded by a facility, the greater the number of stages in its array.
A trusted engineer can help you to weigh these and other factors to achieve optimal RO/NF performance, maintenance, and energy costs, both immediately and in the long term.