Filtration For Water
Filtration for water can reduce harmful microorganisms, chemicals and particulates. It also removes odors, bad taste and improves the appearance of the water.
Filtration is achieved by using a filter medium (thin or thick barrier) and a fluid with suspended solids to separate the solids from the liquid. Screen filters are the first step in filtration, followed by membranes.
Screen Filters
Screen filters are woven from ultra-fine metal wires that let water and liquid pass but traps even the smallest sediment. They are durable, resistant to a variety of temperatures and moisture and won’t unravel or rust. They are used to separate sand and fine particles from irrigation water for agricultural and industrial applications.
Unlike disc filters, they operate under gravity force and do not require water pressure to operate. When a screen filter is full, a backflush valve is activated to reverse the water flow for a short period, releasing collected debris into the backflush line. A timer can also be used to initiate the backflushing cycle at pre-determined intervals, reducing the risk of losing too much water during flushing.
Depending on the size of the soil particle load, either a coarse or fine mesh screen can be used. Disc and screen filters work well with both high and low water flow rates, though disc filters are better equipped for handling larger sediment. Disc filters have a more complicated cleaning process, requiring dismantling and manual cleaning of the discs (Figure 6). Blowdown screen filters simplify this process, and are cleaned by opening a valve that diverts water flow through the filter, releasing trapped particles. They only need to be rinsed occasionally with clean water to maintain their effectiveness.
Membrane Filters
Membrane filters allow fluids and gases to pass through while preventing the passage of particles or microorganisms. They are made of porous plastic films that retain particulates larger than a certain pore size. Particles that are too large to be trapped by the membrane surface are pushed to the membrane’s reject side and discarded from the purified water or gas stream.
The membrane’s pore size structure determines what kinds of particles it can trap, and it must be rated according to its ability to keep bacteria and filtration for water other unwanted organisms away from drinking water. Choosing the right pore size is important, because smaller pores mean that more contaminants will be allowed through, including dangerous pathogens like E.coli and Salmonella.
In membrane filtration, water is forced through the membrane under pressure. This pressure is supplied by a pump, and it helps to overcome the osmotic pressure caused by the buildup of unwanted constituents on the membrane surface. The pump also helps the system achieve a desired flux rate.
This is determined through feed water quality evaluations and pilot testing, and the information is programmed into the membrane systems operating logic. As a result, the system operates the membrane racks to maintain the desired flux rate. To prevent the membrane from becoming clogged by the material accumulating on the dirty side, the system must be backwashed regularly.
Reverse Osmosis
Reverse osmosis uses pressure to overcome osmotic forces that favor even distribution of water molecules. This process forces water molecules through a semipermeable membrane that removes dissolved solids from the water and concentrates it in a storage tank. This process can also remove organic contaminants like bacteria and certain viruses that may be in the water.
Most reverse osmosis systems come with a prefilter to remove sediment and chlorine from the water before it is forced through the RO membrane. After the water exits the membrane it is often run through a filtration for water factory postfilter to remove any residual tastes or odors in the water. The most common RO system consists of 3 stages but some can be made up of as many as 5 stages depending on the needs of the household.
Once the water is filtered it is typically stored in a storage tank where it is held until it’s ready for use. Some reverse osmosis systems are built with multiple tanks that can be teed together to increase the amount of water the system can deliver at once. The storage tank can also be fitted with an air pump that will increase the pressure of the water in the tank if needed.
It is important to keep in mind that a reverse osmosis system requires regular maintenance and replacement of the filters. To prevent biofouling on the membrane it is recommended that homeowners who use a reverse osmosis system flush the entire system every two months to clear away any microorganisms that may have grown on the membrane.
Ultrafiltration
In a typical ultrafiltration (UF) system, hydrostatic pressure is used to force water through a semi-permeable membrane with pore sizes ranging from 103 – 106 daltons. The process removes emulsified oils, metal hydroxides, colloids, pyrogens, proteins and other high molecular weight molecules. It is particularly well-suited for clarification of solutions containing large amounts of suspended solids and bacteria. UF is also a good choice for potable water and tertiary wastewater applications.
Membrane pore size is often measured using molecular-weight cutoff. However, for clarity purposes it is more common to measure the permeate rate of the membrane, or the flux, which is proportional to the applied pressure. UF can be performed with tubular membranes that utilize cross-flow separation or hollow fiber membranes. Some systems operate from the outside-in, while others use a vacuum to draw the water into the membrane.
Unlike conventional filters that act as depositories for the suspended and dissolved material, UF membranes concentrate the materials on their surface, which makes them much easier to clean. This also reduces the need for prefiltration and helps to prevent deep membrane clogging. In addition, the low pressure drop of a UF system means that it can be easily combined with other purification technologies for complete water treatment. The Neo-Pure TL3 ultrafiltration system is a great example of this. It can be installed in-line with a refrigerator, ice maker or through-door dispenser, but it is most popular for point-of-use drinking water. It is easy to install with 1/4” quick-connect fittings and can be run directly off of your cold-water line.
