Available groundwater is depleting globally. In the U.S., large-scale development and pumping have led to concerns over groundwater availability for environmental, agricultural, industrial and domestic needs. Groundwater is a valuable resource and is essential to sustain streamflow and maintain ecosystems. As a result, reverse osmosis and nanofiltration technologies are being increasingly applied in wastewater and surface water treatment.

However, these sources have an increased number of suspended particles, organic matter and solids that are often incompatible with membrane filtration. Reverse osmosis and nanofiltration have been commonly used in water softening, contaminant reduction and removal, as well as desalination and were not typically designed to handle water feeds with a higher degree of contaminants. Effective pre-treatment is crucial in maintaining the life expectancy and performance of membrane filtration systems, and in reducing costs.

Figure 1. Wastewater treatment facilityFigure 1. Wastewater treatment facility

Microfiltration, ultrafiltration, nanofiltration and reverse osmosis

Membrane filtration is a fast, efficient and economical water treatment process that uses a fine mesh membrane to separate particles and microorganisms from the water. The water that passes through the membrane is free of particles and microorganisms, while the water that remains behind is clean and safe for use. Membrane filtration is a well-established method for drinking water treatment and can be used in both municipal and industrial wastewater treatment.

Figure 2. Reverse osmosis desalination plant.Figure 2. Reverse osmosis desalination plant.

Membrane filtration varies based on pore size; with larger pores some types of elements are not removed. The type of membrane chosen depends on the industry and feed water. Microfiltration is typically used to remove solids from water and has pore sizes of approximately 0.1 microns. This membrane filter removes bacteria and suspended solids and is often used as a pre-treatment in other water treatment processes as well as in processing dairy products while still allowing proteins through.

Ultrafiltration systems have pore sizes of around 0.01 microns and can prevent viruses, in addition to bacteria and solids, from passing through. This membrane is commonly used to clarify fruit juices and remove pathogens from milk and in wastewater treatment.

Nanofiltration membranes have a smaller pore size at around 0.001 microns and are often used as a pre-treatment for reverse osmosis and softening water. It is similar to reverse osmosis but does not require high pressures. Reverse osmosis removes all impurities resulting in pure water, can be applied in desalination and is commonly used to polish drinking water.

Figure 3. Comparison of various membrane filtration methods. Source: Jody DascaluFigure 3. Comparison of various membrane filtration methods. Source: Jody Dascalu

Types of wastewater pre-treatment

Pre-treatment is the process that takes place before final membrane filtration. This should be the first step in the membrane filtration of highly contaminated feed water and can be used to reduce the number of solids and microorganisms present in water.

Effective pre-treatment can reduce frequent cleaning and system downtime. Sufficient pre-treatment occurs when membrane cleaning is reduced to at least four times a year. The extent of the pre-treatment processes depends on the quality of the raw unfiltered water. It can be vital in ensuring the longevity of a reverse osmosis plant.

Reverse osmosis membranes can foul and scale without regular maintenance. Fouling refers to the entrapment of various particulates on the surface of, or within, the membrane. Suspended solids, iron flocs, silica, biological slime and clay are some examples of particulates associated with fouling.

Organic and microbiological fouling are some of the most common foulants and are among the most difficult to control when filtering wastewater and surface water. Microbes can spread throughout the whole filtration system when fouling goes unchecked.

Scaling occurs when salts such as calcium sulfate, calcium carbonates, barium sulfate and other supersaturated salts precipitate on the surface of the membrane. Scaling often starts on the tail-end of the membrane that contains the highest concentration of ions. Once a scale begins to form, it acts as a nucleation site and causes the amount of scaling to increase exponentially.

Membrane filtration by itself may be used as a form of pre-treatment as economic viability has increased.

Options for pre-treatment

Pre-treatment systems are mechanical or chemical and can be a combination of the two. Some common chemical pretreatment options include:

Acids — Reduce the scaling potential of compounds such as carbonates by lowering the pH.

Antifoulants — Help to keep iron in suspension.

Antiscalants and scale inhibitors — Reduce scaling by modifying the kinetics of nucleation.

Bisulfites — Used in dechlorination; free chlorine reacts with sodium bisulfite to form sodium bisulfate, ammonium chloride and hydrochloric acid.

Coagulates — Typically added to improve solids removal in flocculation/coagulation processes. The typical chemicals used as coagulates include iron and aluminum salts; due to their positive charge they neutralize negative charges of dissolved particles, causing them to bind together. Larger particles settle to the bottom of the tank.

Some mechanical pretreatment options include:

Pre-screens — Sand and large object removal.

Cartridge filters — Protect membrane elements.

Clarifier — Settling tanks that reduce suspended solids; settled particulates can be continuously removed.

Media filtration — Granular material such as anthracite and sand is used to remove suspended solids.

Activated carbon — Dechlorination and organic removal through absorption into the pores of the activated carbon.

Ozone — Reduces biological activity and removes organic compounds by oxidizing contaminants such as viruses, bacteria and some metals.

Ultraviolet (UV) light — Deactivates microorganisms with electromagnetic radiation. The DNA of bacteria, fungi, parasites and viruses are destroyed by UV light so they can longer reproduce.

Membrane integrity testing

Regular maintenance can keep a membrane lasting longer but throughout its use, membrane integrity needs to be monitored. Membrane integrity testing is used to ensure that the membrane is not damaged or compromised. Common testing procedures include turbidity monitoring, particle counting, bubble point testing, sonic wave sensing and biological monitoring.

As groundwater levels continue to decline, cost-effective and environmentally friendly ways to manage water supplies are becoming essential. Relying on frequent cleaning is not the optimum solution for membrane filtration systems. Raw water composition as well as seasonal quality changes need to be considered when scoping an effective pretreatment solution.

About the author

Jody Dasculo is a freelance writer in the technology and engineering niche. She studied in Canada and earned a Bachelor of Engineering. Jody has over five years of progressive supply chain work experience and is a business analyst. As an avid reader, she loves to research upcoming technologies and is an expert on a variety of topics.

To contact the author of this article, email engineering360editors@globalspec.com