Selecting Sediment Filters for Domestic and Rural Water Filtration.
There is a vast range of different sediment filters in the market place, and selecting the right filter for your application can be a real challenge with unforeseen caveats. To decide what sediment filter would be fit for purpose depends on a number of factors particularly the source of water, flow rate required and the application. More often than not, it does involve some trial and error and trialling various types of filters and micron sizes and find which one’s work for you. The purpose of this article is to provide some guidelines to select the right sediment filter that will provide satisfactory results.
Sediment Filters should be installed for a number of reasons, and the application, equipment and entire reticulation circuit should be considered. Sediment can cause:
Excessive wear within pump, especially impeller components.
Damage to seals, gaskets and diaphragms of pumps, valves and other parts.
Damage to other upstream filtration, especially membrane filtration.
Reduced life span of other filters like carbon block.
Excessive wear inside plumbing, and can cause erosion corrosion.
Clog up irrigation sprinkler heads.
Terminology
To start off we need to first cover some terminology, so we can understand some of the features of sediment filters and water quality parameters.
Microns: The smaller the micron number, the smaller the particles that can be filtered out. For point-of-use applications that include reverse osmosis, we normally recommend a 1 or 5 micron sediment filter. The smaller micron filters will filter out smaller particles, however will clog up faster causing pressure drops in the system resulting in decreased flux. One micron is 0.001 mm.
Particle size: Understanding what are the contaminants in your raw water and the relative particle sizes are crucial in selecting the right sediment filter.
Turbidity: Turbidity is a measure of water clarity how much the material suspended in water decreases the passage of light through the water. Suspended materials include soil particles (clay, silt, and sand), algae, plankton, microbes, and other substances. These materials are typically in the size range of 0.004 mm (clay) to 1.0 mm (sand). Turbidity can affect the colour of the water. Turbidity is generally measured by using a turbidity meter and measured in NTUs. (Nephelometric Turbidity Units)
If your turbidity is above 50 NTU you should consider using coagulation and/or flocculation to settle out the finer particles.In colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into a floc. The floc may then float to the top of the liquid (creaming), settle to the bottom of the liquid (sedimentation), or be readily filtered from the liquid. If you have a dam and it takes a long time for the fine material to settle, chances are that there is a significant amount of clay particles in your water. This can cause problems for most filters and hence has to be be removed by techniques such as using a flocculant. It should be noted that flocculants like alum may require pH correction as well, to prevent a decrease in pH. Organics can contribute to increased turbidity by leaching tannins into the water, and these are more difficult to remove. These tannins are complex organic compounds and are best dealt with using ion exchange technology specific to organics(tannin filters).
Total Suspended Solids: Total suspended solids are a total quantity measurement of solid material per volume of water. This means that TSS is a specific measurement of all suspended solids, organic and inorganic, by mass. Turbidity and TSS are the most visible indicators of water quality. These suspended particles can come from soil erosion, runoff, discharges, stirred bottom sediments or algal blooms. Both organic and inorganic particles contribute to the suspended solid content.
Total Suspended Solids (TSS) are solids in water that can be trapped by a filter. TSS can include a wide variety of material, such as silt, decaying plant and animal matter, industrial wastes, and sewage. In commercial aquaculture operations an important component of recirculating water, is to remove suspended solids, typically using a rotary drum filter. This is normally followed up by a biological filter, to ensure that nitrogen and phosphorous is not returned back into the grow-out ponds.
Total Dissolved Solids: “Dissolved solids” refer to any minerals, salts, metals, cations or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.
Total Dissolved Solids cannot be trapped by a filter, unless it is membrane technology like reverse osmosis and nanofiltration which can physically remove ions from water by virtue of particle exclusion.
Nominal Filter Rating: The Water Quality Association (WQA) classifies this filter will filter out at least 85% of the particles of the size it is rated for. In other words, a filter that is rated as a 1 micron nominal can be expected to pick out 85% of the particles that are 1 micron or larger from the water that passes through it.
Absolute Filter Rating: The filter will reject virtually all of the particles of the given size. The usual expectation is a 3-log rejection–or 99.9%. Absolute ratings are usually used for the tightest filters and for purposes where efficiency really matters. For example, if a filter maker promises removal of E. coli, more or less 85% efficiency isn’t good enough. If you’re going to trust your life to the filter, you expect an absolute 3-log or 4-log rating at the very least.
Pleated Sediment Filters: Pleated Sediment Filter Cartridges are most commonly fabricated from pleated reusable polyester fabric. The media has been pleated to provide a very large surface area in order to maximise the cartridge dirt holding capacity and extend the time between cleaning. Pleated cartridges are particularly useful for low pressure applications such as domestic supplies using header tanks or roof water.
Depth Sediment Filters require the water being filtered to pass through a thick wall of filter material and treatment takes place throughout the depth of the sediment filter. Some sediment depth filters, in fact, have what is called a “graded density” structure, meaning that the filter gets tighter as the water passes through the filter wall. Graded sediment density filters remove particles of a variety of sizes. The outside traps larger particles and smaller particles get trapped towards the inner core.
Sediment Depth filters can be of the wound string type or what is called “melt blown.” Wound string is probably the most common. Depth filters are made of a variety of materials, including ceramic, cellulose, polypropylene, acrylic fibre, glass fibre, and polyester.
Ceramic sediment filters are simply extremely tight depth filters made of ceramic. They usually have absolute ratings and are tight enough to filter out some microorganisms.
Cartridge style sediment filters generally have a radial flow pattern where water flows through the outer surface towards the inner core.
A disc filter is a type of water filter used primarily in irrigation, similar to a screen filter, except that the filter cartridge is made of a number of discs stacked on top of each other like a pile of poker chips. The water passes through the small grooves in between and the impurities are trapped behind. These can be manually cleaned or be automated to facilitate a cleaning cycle.
An innovative filtration solution for removing sand , silt and other sediment are the centrifugal spin down filters – popular brands are Rusco Spin Down Filter and Lakos Twist II Clean. The TwistIIClean allows for effective cleaning performance, featuring an easy ¼-twist top handle to create a patented reverse-flushing action to more effectively remove sand/sediment from the screen surface. Overall, that means reduced maintenance, longer run times, less pressure loss, better filtering.
A popular type of sediment filter used especially for bore , dam and river water filtration is the versatile backwash filter . These backwash filters can be automated or manual allowing filtration to as low as 5 microns using Zeolite or activated glass media. It is important to note that where clay is present, coagulant or flocculant dosing would be required to ensure particles are large enough in order to be captured by the media bed. Manual or automated backwash ensures that the media bed is expanded or fluidised resulting in the removal of sediment in the backwash.
Point-of-use (POU) Filtration: This usually encompasses counter top and undersink filtration for example in the kitchen.
Point-of-entry (POE) Filtration: Water is filtered before entering the house and is reticulated to all house hold fixtures that require water.
The Water Source.
The water source plays an important role in selecting the type of sediment filter chosen for a specific application. As a general rule it is recommended to conduct a water analysis, especially if the water supply is from a bore (well), dam, river, creek or rain water tank. There are water test kits available that allows any person to conduct their own testing for selected water quality parameters, including microbiological testing.
Municipal Water: For municipal water, the selection is much easier as this water typically contains low levels of sediment and suspended solids. The sediment filter often serves the purpose of protecting the more expensive carbon block filter which usually has a 5 micron rating. A 1 to 5 micron depth filter is normally chosen for this application, especially if a reverse osmosis is one of the filtration stages.
All private water supplies intended as potable water should include a disinfection step like UV or ozone disinfection.
Rain Water: Rain water quality can be variable and contains a wide variety of contaminants including organic matter like leaves, tannins, and faeces from animals, dust and other pollutants. A 10 to 20 micron pleated filter is normally used prior to the carbon filter. The carbon filter should include a supplementary media like KDF or nano silver to inhibit biofilm formation inside the carbon bed. For point of entry (whole house) applications a jumbo filter housing with suitable cartridge (4.5” x 20”) is highly recommended. The selection of the sediment filter may be a bit trial and error, and if the filter clogs up too quickly, a larger micron sizing may have to be selected. More about rain water filtration.
Bore Water: The selection of sediment filters should only be considered once a water test has been conducted. Bore water may require a number of treatments that will impact the choice of sediment filter, some examples below:
Iron and Manganese: Depending on levels will require a catalytic oxidation treatment to filter out the resulting precipitants in a downstream backwash sand filter. Sediment cartridge filtration not recommended.
Hard Water: Will require an ion exchange water softener or nanofiltration step to remove the hardness salts from the water. Sediment filter will not remove any hardness as the magnesium and calcium salts are in solution. Magnetic, electrical conditioners or any other useless devices are not recommended.
pH of water: If water is acidic or alkaline pH correction will be required. Cartridge filtration not recommended for whole house filter applications.
Heavy metals: Will require a backwash filter with suitable media to remove the contaminant, like Metsorb HMRG.
Generally, backwash sand filters are recommended for bore water applications. Cartridge style sediment filters can be considered for secondary point-of-use treatments.
Surface Waters: The water quality parameters of surface waters can be variable and heavily contaminated from run-off. A wide variety of contaminants like organic matter, pesticides and high silt content may require a host of filtration stages. High turbidity water will require clarification, flocculation and backwash sand filtration to allow for further treatment steps. A cartridge style filter like a pleated washable filter can be considered in the final stages, for instance prior to disinfection. For UV – disinfection 5 micron filtration is required in order to optimise the penetration of UV-light.
Cartridge style sediment depth filters can be considered for secondary point-of-use treatments.
Flow Rate Required
In general, flow rates for sediment filters are higher than for equally sized carbon or media filters. The tighter the filter, the slower the recommended flow rate and the higher the pressure drop. As a general guideline the table below suggests flow rates (litres per minute) vs microns for a pleated 9.75″ X 2.5″ filter cartridge:
| Microns | Flow Rate (lpm) |
| 1 Micron Absolute | 11 |
| 2 Micron Nominal | 15 |
| 5 Micron Nominal | 26 |
| 20 Micron Nominal | 30 |
| 50 Micron Nominal | 38 |
Tabled below, are suggested maximum flow rates in lpm for the four most common cartridge sizes. These are indicative flow rates and may vary depending on the manufacturer. These are washable and reusable (5 micron and larger) pleated cartridges.
| Micron Rating | 2.5 X 9.75 | 2.5 X 20 | 4.5 X 10 | 4.5 X 20 |
| 1 Absolute | 11 | 23 | 30 | 45 |
| 0.35 Nominal | 15 | 30 | 34 | 49 |
| 1 Nominal | 15 | 30 | 38 | 57 |
| 5 Nominal | 26 | 53 | 57 | 95 |
| 20 Nominal | 30 | 61 | 57 | 95 |
| 50 Nominal | 38 | 76 | 57 | 95 |
Pressure Drop
Most manufacturers of cartridge filters can provide graphs that show the relationship of flow rate vs. pressure drop, and that needs to be taken into consideration. The pressure drop does progressively increase, as the cartridge takes up more dirt. The pressure drop across a cartridge filter is used to determine the condition and effectiveness of the cartridge. A high pressure drop across a cartridge filter, over the maximum recommended pressure drop, indicates that the cartridge needs attention. A 70 kPa pressure drop is normally an indicator that the cartridge needs to be replaced. No pressure drop at all indicates that the filter is either breached or that the seals are not working, and again needs attention. In most cases the attention needed is cartridge replacement. With some of the pleated filter cartridges, a cleaning can restore an acceptable pressure drop.
Pressure gauges are recommended upstream and downstream of the sediment filter, as that is a useful diagnostic tool to evaluate the condition of the cartridge.
In commercial applications where backwash filters or other automated self cleaning filters are used, the backwash can be initiated by differential pressure. This means as the filter progressively builds up with sediment, the overall supply pressure drops to a point where an automated backwash and rinse cycle is initiated.
Conclusion -Sediment Filter Selection:
Cartridge sediment filters are effective in the removal of moderate amounts of particles from liquids in the size range of 50 to 0.035 microns. When filtering large amounts of solids at high flowrates, other treatment methods such as self-cleaning multimedia filters may be required as pre-filters, followed by cartridge sediment filtration, if needed.
The selection of sediment filters depends on a number of factors including water quality, flow rate required and application. In many cases, it does involve some trial and error, and frequent filter replacement may require an increase in micron rating to reduce the pressure drop across the filtration stages. For domestic point-of-use reverse osmosis systems a one to 5 micron depth sediment filter is recommended. For whole house point-of-entry applications, pleated sediment filters can be used, depending on the turbidity and quality of the water.
For high flow applications, as a general rule it is recommended that backwash media filters should be used. Sand separators could also be an option. The decision may also be an economical one where the higher cost of backwash filtration equipment has to be weighed up against the cheaper cartridge –style filtration. Backwash sediment filtration, especially when automated offer the advantages of minimising operator intervention. There are some very efficient media available that can filter to 1 micron including activated glass media .
Search Terms:
Cartridge Filters, Sediment Filters, Depth Filters, Multi Media Filters, Pleated Filters, Sediment Removal, Iron Removal Filters, pH correction, Self-Cleaning Filters, Backwash Filers, Sand Filters, Pressure Gauges, Heavy Metals removal, Bore water treatment, Water Softener, Rain Water Treatment ,River Water Treatment, activated glass media