Filtration of water, aqueous solutions and their methods.

Water Filtration Overview

The first recorded attempts at water filtration date back to 2000 BC. The earliest Sanskrit writings described methods for purifying water. These methods ranged from using fibers or fabrics such as a very rustic sand filter, as well as boiling or placing hot metal instruments in the water before drinking.

Water filtration is a basic principle of using a medium (membrane, mesh, screen or other media) to physically filter or trap particles depending on their size. These filters are classified according to the pore size of a mesh or membrane, which is measured in microns; the smaller the number of microns (i.e., the smaller the pore size), the more it will retain sediment from the liquid being filtered.

Filtration is a solid-liquid separation process used in water treatment systems to reduce the concentration of suspended solids (particles) in the water. There are several types of filters and each of them has its application depending on the size and concentration of particles to be retained. In the
Figure 1
the different types of filtration and the typical particle size they retain can be seen.

What is filtration?

Filtering consists of separating solids that are suspended in a fluid. The fluid can be a liquid or a gas. Separation is achieved through a porous medium, such as:

• Perforated metal basket.
• Metallic sieve: it is a mesh. It is also called sieve or sieve.
• Bed formed by granular particles (sand, zeolite, anthracite, garnet...).
• Fabric woven from polymer yarns or natural fibers, such as cotton: used in bag filters or filter presses.
• Paper: as used in laboratories or filter presses.
• Cardboard: as used in filter presses.
• Discs.
• Cartridge of a porous material (yarn, pleated, polymer foam)
• Membranes (microfiltration, ultrafiltration, nanofiltration and reverse osmosis).

The porous medium retains the larger solids at the opening of its pores and allows the passage of smaller particles and liquid.

The chemical engineering career was created at the beginning of the 20th century with the aim of professionalizing the approach (study, design, operation) of industrial processes, and divided them in two:

• Those in which a chemical reaction occurred, he called unitary processes.
• Those in which only physical or physicochemical changes occurred were called unitary operations.

Difference between unitary operations and unitary process.

The term unitary refers to the fact that the principles governing each of the processes or operations in question are the same, regardless of the industry in which they are applied.

For example, the principles governing fermentation are the same, regardless of whether it occurs to make alcohol from sugar cane, grapes or hydrolyzed agave. And the principles governing the distillation of petroleum are the same as those governing the distillation of grape wine, cider or agave fermented wine.

Among the unit operations, an important group is those applied to separate components of a mixture. They are so important that the chemical engineering curriculum devotes more time to them than to unit processes (in which a chemical reaction occurs).

When a manufacturing method has a chemical reaction as an essential step, it is often more challenging and costly to separate the final product, consisting of: the desired compound, unwanted by-products, unreacted reagents and inert compounds that were part of the reagents.

Over time, in the everyday language of engineering and industry, unit operations aimed at separation have been called separation or purification processes.

Perhaps it would be more accurate to call chemical engineering separation process engineering.

Among the unitary operations not intended for separation are those for: transporting fluids, transferring heat (for heating or cooling), stirring, mixing, grinding...

As far as we go: filtration is a unitary operation whose purpose is the separation of solids that are suspended in a fluid. And in colloquial engineering parlance, filtration is a separation process.

Among the separation processes applied by humans, filtration is by far the most widely used. Drying and adsorption on activated carbon are far behind. And far behind the latter is distillation, not to mention others.

What are membrane separation processes?

Since the origin of chemical engineering, the greatest technological innovation in the separation processes are those carried out by means of membranes. These began to be used massively in the last two decades of the 20th century. They are applied to the treatment of water and aqueous solutions . Four types of membranes are produced:

The micron (µ) or micrometer (µm) is one thousandth of a millimeter (mm).

Microfiltration and ultrafiltration membranes are porous. Therefore, they retain particles that are larger than their pore size.

Nanofiltration membranes are porous, but also allow particles (including molecules) to pass through by the dissolution-diffusion mechanism (also known as "translocation"). The latter consists of the particle dissolving in the membrane and dissolving in the water that has passed through the membrane.

Reverse osmosis membranes are non-porous. They reject or allow ions and molecules to pass through, by the dissolution-diffusion mechanism. Therefore, reverse osmosis cannot be considered a filtration process.

Membranes are manufactured from synthetic organic polymers, such as: polyamides (PA), polyamides/polyacrylonitrile (PA/PAN), polyethersulfone (PES), polyvinylidene fluoride (PVDF), polysulfone (PSF), polyethylene (PE)...

The membranes retain smaller particles than those retained by the rest of the filter media.

Figure 1. Comparison of water filtration processes and particle size ranges they retain (Weiner, 2012).

What is a solid?

It is a state of aggregation of matter in which the molecules that form it are together, maintain great cohesion and resist changes in shape and volume. Liquids only resist changes in volume. Gases do not resist changes in shape or volume.

What is the smallest particle size of a solid suspended in water?

It is not possible to precisely define the smallest particle size from which it can be considered a solid. The American Water Works Association (AWWA), the Water Environmental Federation (WEF) and the American Public Health Association (APHA) publish a standardized method for determining the total suspended solids content of water in the Standard Methods for the Examination or Water and Waste Water. The method uses a glass fiber filter with a nominal pore size of 1.5 µ. From this perspective, it does not consider suspended solids that are not retained in the filter.

Therefore, the gravimetric method of the Standard Methods for the Examination or Water and Waste Water considers total dissolved solids, not only those actually dissolved, but also those that are not retained in the glass fiber filter.

For reference, the smallest particle detectable by the human eye has a diameter of between 50 and 100 µ (i.e., between 0.0 and 0.10 mm).

The average diameter of human hair is between 70 µ and 120 µ.

What is the range of particle size retained by a water filter?

Through the filtration process, particles ranging from 0.001 microns (in a nanofiltration membrane) to about 2.5 cm (in a basket strainer) can be retained.

Smaller particles can be retained by reverse osmosis. And larger particles are usually retained by metal grids.

What are the most important parameters that define the design of a liquid filter?

What is surface filtration and depth filtration?

Filter media retain particles in two ways:

1. Surface filtration occurs when the particles are larger than the pores of the filter media. These are deposited on the outer surface of the medium and form a "cake".

Figure 2. Surface filter. (Donaldson)

2. Depth filtration occurs when the particles are smaller than the pores of the filter media. Retention occurs in the internal structure of the medium.
depth filtration
(
Figure 3
).

Figure 3. Depth and non-fixed pore filter. (Donaldson)

The surface filters can be cleaned by scraping off the retained cake or by flushing with water.

Depth filters cannot be cleaned efficiently unless the filter media is decompacted.

In depth filtration, granular media are made of multiple layers of media(multimedia). When the fluid passes through the filter, particles larger than the spaces within the filter media are retained, accumulating mainly in the different layers of granular media in the filter.

These are filters formed by fibers or materials compressed to form a matrix that retains the particles in the superficial and internal layer, these can have micronages graduated from higher to lower, to avoid a premature clogging of the cartridge.

For very high flow rates cartridge filters are not the best option, the recommendation is to use deep bed filters or disc filters.

Surface filters are inherently uniform structures that, like a sieve, retain all particles at the same pore size, giving you precise control over their surface, most commonly using bag filters or pleated filters.

What are the methods of water filtration?

Water filtration consists of separating suspended solids through the use of a filter media (filter or screen) that retains the larger particles and allows the fluid to pass through. This process is one of the most widely used methods of separating substances and, in practice, encompasses different filtration techniques or systems, each designed to achieve different ranges of accuracy depending on the specific industry or application.

Among the most common methods are filtration with cartridge filters, silica sand filtration, the use of multi-bed (or multimedia) filters, disc filters, ultrafiltration, nanofiltration and reverse osmosis. The choice of method depends on factors such as the quality of the input water, the type of contaminants present and the level of purity required in the process.

Water filtration with cartridge filters

They are one of the most widely used filtration products because of their usefulness in removing sediments and particles in the water. Solids in the water are often caused by stormwater runoff and bad pipes that can leave sediment in the water. Typically, water that comes out of bad pipes tends to discolor and makes it unusable in homes and businesses.

Incidents such as clogging and degradation of water heaters, refrigerators, shower fixtures, valves are very likely to occur. For these cases, water treatment operators use cartridge filters to ensure the use of clean water.

These cartridges use a mechanical filtration process that physically prevents the flow of solids in the water, acting as a kind of water. If these filters are of high quality, they contain porosity of a suitable size for the water to flow through without allowing sediments to pass through.

The cartridges can pick up large amounts of dirt if used on large surfaces. Cartridge filters are capable of removing suspended particles by using a gradient density depth construction.

Water filtration with silica sand

Silica sand serves as a support for drinking water, sewage and wastewater treatment, deep wells, aquariums and decorative. This type of sand is widely used in industrial filters to process wastewater and filter clean water.

A special feature of silica sand is that it has a similar grain size to each other and is a material that does not degrade and resists water impurities such as solvents, acids, organic compounds and pollutants.

This material is used for filtration of water with low to medium contaminant loads that require retention of particles up to 20 microns in size.

Filtration quality may vary depending on parameters such as filter shape, filter bed height, characteristics and granulometry of the filter media. Finally, it should be clarified that their main function in filters is to trap contaminants that float in the liquid and allow water to pass through.

An advantage of the sand filter is that it requires a higher backwashing flow than other deep bed filters, and this means that the filter cleaning is done in a shorter time, thus saving water and time.

Filtration with multi-branch or multimedia filters

This type of filter has the mission to remove suspended solids in the water with a size of up to 15 micrometers or microns. This means that dust, pollen, small debris, insects, etc. that is larger than the aforementioned size will be retained in filters to be disposed of down the drain during backwashing.

This type of filters are also known as deep bed filters where the upper bed of filtering material has a larger size of fragments, then the water passes to a layer of smaller size to finally pass through a thin layer of fine fragments in the lower one supported by gravel.

Usually the shape of these filters is cylindrical and vertical, they are made of steel, carbon or stainless steel and contain polyethylene on the inside and fiberglass on the outside.

Broadly speaking, these filters work as follows: the water enters through the upper part of the filter and flows downwards through the beds, the water is collected in the lower part of the bed thanks to diffusers. Multi-bed filters must be backwashed after a certain period of service in order to expel the retained solids and cause movement in the different filter beds to avoid cementation.

Usually the bed or multi-bed is formed by:

• Anthracite
• Silica sand
• Garnet
• Supporting silica gravel.

Filtration with disc filters

The main function of disc filters is the retention of solids in water. At the beginning they were thought to filter irrigation water and as it was used it was found the variety of functions that it has in the filtration of industrial and domestic water.

The discs are ring-shaped and when placed one on top of the other they leave a central gap and each disc has grooves that are more open on the outside perimeter and narrower on the inside. Their filtration range is from 5 to 400 microns, although they are typically used for large volumes with a filtration of between 100 and 130 microns. Their efficiency is not only due to their slots, but also to their geometry, angle, intersections, length and quantity.

As for its operation, the water flows from the outside to the center channel, the solids remain in the channels, the larger ones in the outer part and the smaller ones reach the inner part. This operation minimizes the frequency with which it will be necessary to clean discs, therefore, it has an operation similar to a depth filter.

The biggest advantage of disc filters in saving space, as they can form a stack of discs that can grow by modules vertically or horizontally, but the best is the water savings, which approximately 10 to 30% savings vs. deep bed, but the option of disc filters assisted or mixed with pressurized air can be 80 to 90% savings, compared to water using deep bed filters.

Filtration with ultrafiltration membranes

Ultrafiltration is a membrane separation process used in water treatment that makes possible the mechanical separation of suspended or dissolved solids between 0.01 to 0.1 μm (micrometers).

Ultrafiltration membranes are in contact with two liquid phases at different hydrostatic pressure and there will be components of the liquid phase that will go from the high pressure side to the low pressure side.

Small particles that are dissolved in the liquid can pass through the porous membrane, while large dissolved molecules and suspended solids will be retained.

Ultrafiltration is generally used as pretreatment in surface water, seawater, groundwater, graywater treatment, biologically treated effluents and as pretreatment of water for re-treatment with membrane demineralization systems.

Filtration with nanofiltration membranes

This filtration method called nanofiltration is usually used in waters with low solids that are totally dissolved. This system could be consideredbetween reverse osmosis and ultrafiltration due to the levels of separation it allows and the pressures required for its application, the solids retained are between 0.001 to 0.01 μm (micrometers).

This process is generally used for water softening or softening, to remove heavy metals in wastewater, to decontaminate wastewater, to pre-treat water prior to reverse osmosis, to remove nitrates or to remove colors.

It works by means of pressure in membranes that allow the concentration of organic solutes, therefore, low molecular weight solutes are retained, but salts can pass totally or partially through the membrane.

There are two types of membranes for nanofiltration: spiral wound membranes and tubular membranes, the former are more sensitive to pollution, although their quality is inferior to tubular membranes.

Water filtration with reverse osmosis membranes

In this filtration method, water is passed through a concentrated solution through membranes that are semi-permeable. The pressure applied to the water is higher than the osmotic pressure and thus the salts are retained.

Semi-permeable membranes only allow certain types of ions to pass through, as water passes through the membranes and dissolved ions are retained. It should be clarified that the pressure in the water is equivalent to the salt contained in the water.

Reverse osmosis has 3 streams, the feed, the concentrate:

• The feed stream is the flow of the incoming water.
• There are two types of water streams in the permeate which is purified water and concentrate.
• Concentrate refers to the water that could not pass through the membrane and also contains the rejected salts.

Reverse osmosis filtration is the most widely used purification method in our times, as it is very effective in eliminating almost all ions from the water, and due to the massification of its use, it has become cheaper and has been replicated by many manufacturers.

References

• Wakeman, Richard J., Solid/liquid separation: principles of industrial filtration, Elsevier, 2005. principles of industrial filtration. Elsevier.
• AWWA, Water Quality &Treatment a Handbook of Community Water Supplies, McGraw-Hill, 1999. Water Quality &Treatment a Handbook of Community Water Supplies. McGraw-Hill.
• Test for total suspended solids. Accessed on the Internet on January 20, 2023 (https://www2.gov.bc.ca/assets/gov/environment/research-monitoring-and-reporting/monitoring/emre/methods/solids_total_suspended_tss_-_pbm.pdf)
• Spanish Association of Desalination and Reuse. Consultado en internet el 20 enero 2023 (https://aedyr.com/diferencias-microfiltracion-ultrafiltracion-nanofiltracion-osmosis-inversa/#:~:text=La%20principal%20diferencia%20entre%20ambos,en%20el%20proceso%20de%20filtraci%C3%B3n).

Preparation and revisions

Prepared: GGC 23 Jan 2023

Reviewed:

Are you interested in learning about a specific filtration separation method?

Keep reading the following articles:

Microfiltration:

Microfiltration is a filtration process using a microporous media that retains the suspended solids of a fluid. The pore size of the membrane ranges from 0.1 to 1 micron or microns.

Microfiltration filtration in the market is typically done by cartridge filters, disc filtration and deep bed filters.

What is microfiltration?

Nanofiltration:

Nanofiltration is a filtration process using a nanoporous membrane that is used in waters with low total dissolved solids. The purpose is to remove polyvalent ions, in addition to disinfecting it by retaining organic matter.

What is nanofiltration?

Ultrafiltration:

It is a separation process based on a porous membrane with openings between 0.01 and 0.1 microns.

Ultrafiltration membranes are more closed compared to microfiltration, but more open than nanofiltration and reverse osmosis. The membranes work with low pressure, which results in lower operating costs. In addition, they are very effective as a pretreatment for reverse osmosis, and also have a backwashing system, which gives them a longer life span.

What is Ultrafiltration?

Reverse osmosis:

Reverse osmosis, although it could be called a very fine purification method, is ultimately by filtration at the Angstroms level.

Reverse osmosis (RO) is a process in which the flow rate through a semipermeable membrane is reduced and a buoyancy force in excess of the osmotic pressure is exerted in the opposite direction to the osmosis process (Figure 1). In this way, the substances found in the water are separated on one side of the membrane (concentrate) and on the other side a diluted solution low in dissolved solids is obtained (permeate).

What is reverse osmosis?