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Choosing the Right Self-cleaning Filter

C J Hiscock

Published

Help choosing your pump filter, filter pump, suction strainer, dirty water filter or other filtration equipment

There are almost as many types of dirty water filter as there are circumstances when you may need to use one. This page presents some common filter types and their typical uses in various conditions.

The uses for Rotorflush self-cleaning filters and submersible filterpumps™ are many and various. Farms, factories, and water treatment works use our filters. Water companies use our filters for sample preparation for water analysis, and to keep inlet screens clear. Farmers use our filters and filter pumps to recycle farmyard run-off. Others use our filters for keeping fountains running, for irrigation, or anywhere that dirty water needs to be pumped without blocking pumps and other equipment.

It can be tricky sorting out the best dirty water filter and filtration system to meet your requirements. Fortunately, we have a wealth of knowledge and expertise in the installation and use of pump intake filters and filter pumps. Furthermore, we are more than happy to give technical advice about pressure and flow rates, as well as practical information about filters for particular applications.

Call us on +44 (0)1297 740126 or email sales@rotorflush.com to discuss your needs.

The Basics

Set Up Diagram for Rotorflush filter

If you just need a self-cleaning strainer or filter for a surface pump you already have, then you need to know:

  • The flow rate you are pumping
  • How finely you need to filter (top tip, always go for the largest size mesh you can get away with – reduces maintenance)
  • What you are trying to remove

If you want a submersible pump with a built-in self-cleaning intake then you need to know all of the above plus:

  • The static head (the height above water level you wish to pump)
  • The pipe run from the pump to where you need the water
  • The diameter of the pipe
  • The pressure you need at the point of delivery
Rubi Filterpump™ diagram

How dirty is the water, and what’s in it?

Filtration can be problematic. It is important to know what you wish to filter for, and equally what your dirty water filter can handle. This is because most dirty water contains a mix of contaminants, unless it is dirty as a result of a specific industrial process. Therefore, the type of filter will depend on the water quality and what it is in the water that causes the most trouble.

Hard solids

A hydrocyclone can often be used to spin hard solids out of the water. Hard solids are the easiest to filter if they are denser than water. Screen filters will filter out hard solids that are a similar density to water. Another reason hard solids are easier to filter is because they are less likely to coagulate and block filter mesh compared with softer impurities in water. If hard solids are very fine – for example, a fine silt, water can be cleaned using a cartridge filter after a screen.

Living organisms.

Living organisms – algae, biofilms, crustaceans, weed, tadpoles etc – can all affect the performance of filters. Also, many filters present an ideal habitat for many aquatic and marine life forms. The problem with this type of contamination is that it will stick to filter screens and media and is difficult to clean off. Living organisms also slowly grow and cover the filter, eventually causing it to block. Any dirty water filter can be susceptible.

Most filters deal with this well, with regular maintenance to ensure biofilms and algae that slowly grow on the filter screens are kept at bay. In addition, filters used in seawater can be treated with antifouling.

The continuous backwash in Rotorflush filters slows the build-up of biofilm on the filter’s surfaces. Another positive benefit of these filters is that their low-velocity intake and constant backwash prevent many smaller forms of aquatic life from becoming stuck to the filter mesh or sucked into pumps.

How Dirty is the Water?

One of the most common ways to assess the level of contamination in dirty water is to measure the Total Suspended Solids (TSS) in the water. This is the measure of suspended solids by dry weight per litre of water. Wikipedia has a concise description HERE. The turbidity or haziness of the water serves as a rough guide to the TSS if it cannot be measured.

Of course, water quality is not always consistent and may change as flow rates or other factors vary. What follows are general considerations to be taken into account when deciding on methods of filtration and which type of dirty water filter to use.

Water With Very Low Concentration of Solids

For example mains water, well and spring water, swimming pools etc.

Suitable Filters: Y-strainers, Cartridge filters, Sand and other Media Filters

  • Y-Strainers are usually coarse strainers with apertures greater than 1mm. Y-filters are used in-line as part of the pipework design.
  • Cartridge filters usually filter more finely, typically between 5 and 500 microns.
    They need to be dis-assembled and manually cleaned when they become blocked. They are good and economical where the fluid contains very few particles, and are usually used as an insurance to protect upstream equipment.
  • Sand filters are often used for filtering water for swimming pools

Nearly all self-cleaning filters on the market are in line filters that are positioned in the pipeline after the pump. Consequently, they periodically need to go into a cleaning cycle when the filter becomes blocked. Water and solids are discharged from the filter for disposal. Examples of these are media and sand filters, and self-cleaning screen filters.

Water with Low – medium Concentration of Solids

For example, river water, lake water, seawater.

Suitable Filters: Y-Strainers, Cartridge Filters, Rotorflush Self-cleaning Filters, Sand and other Media Filters, Self-Cleaning Screen Filters, Hydrocyclones (for heavier solids)

  • Basket strainers installed on pump intakes also block quickly, starving the pump of water and significantly shortening the life of the pump. Traditional, inexpensive but inefficient dirty water filter.
  • Y-strainers and cartridge filters are often specified in these situations, but are not usually very satisfactory. A relatively low concentration, particularly of softer solids, can very quickly accumulate. These types of filters need manually cleaning very often if a continuous flow is required. Some type of self-cleaning filter must be used if frequent cleaning is not an option.
  • Hydrocyclones are a very good and economical way of separating heavy solids (for example, water containing sand). They do require pressure (usually 2 bar and above) so do use energy.
  • Media and sand filters – these are very good at removing fine solids, down to 5 microns. Media filters work best for small particles. The media trapos the particles within it. However, large solids will quickly settle on the surface of the media and block it. As a result, media filters have to frequently backwash, losing a lot of water.
  • Rotorflush filters and filterpumps™ provide very effective pre-filtration for media filters and prolong the intervals between backwashes.

If the levels of solids are too high, the filters are continually in a cleaning cycle and not filtering effectively. Rotorflush Filters are a good solution for solid separation as they clean and filter at the same time leaving the solids in the water source. Because the filters are filtering on intake, they also protect the pump from damage.

Water with a Medium to Heavy Concentration of Solids

Medium Levels of Solids, e.g. wastewater from livestock units, vegetable washing and food processing, laundries, mines etc.

Suitable Filters: Run Down/Parabolic Screens, Rotorflush Filters and Filterpumps™, Hydro cyclones (for heavy solids)

  • Run down screens/parabolic screens are simple systems but often need some manual cleaning.
  • Rotorflush filters and filterpumps™ work well in these situations and are particularly good at filtering water with fibrous contamination. Good examples are their successful use for providing backwash for inlet screens at water treatment works, and their efficacy in dealing with dirty water run-off on farms. The self-cleaning mechanism built in makes the Rotorflush a very efficient dirty water filter.
  • Hydrocyclones are very good where solids are hard and heavy – so would be good for separating sand from water.

The filtration options start to become more limited and expensive the more suspended solids there are in the water.

Water with a Heavy Concentration of Solids

High Levels of Solids. This is where the viscosity is higher than water owing to the high solids content) e.g. agricultural slurry, sewage sludge.

Suitable Filters: Slurry separators, Centrifuges, Filter presses.

  • A slurry separator can dewater livestock manure.
  • Centrifuges separate heavily contaminated liquids.
  • Filter presses dewater sewage sludge and industrial sludges.

Rotorflush Filters are not usually suitable for use where the fluid viscosity is higher than water.

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