Choosing the Right Dirty Water Filter for You

Rotorflush Filters Limited –experts in filtering suspended solids from water

Help choosing your pump filter, filterpump, suctionFaraday Giving his card to Father Thames and thinking 'we need a bigger Dirty Water Filter'

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 filterpumps to recycle farmyard runoff.  Others use our filters for keeping fountains running, for irrigation, or anwhere 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 filterpumps.  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 560229 or email sales@rotorflush.com to discuss your needs.

Common Filter Types

Basket Strainer - traditional Dirty Water Filter
Simple Basket Strainer
Cutaway Y-strainer - can be a v high maintenance dirty water filter if there are a lot of suspended solids
Cut-away of ‘Y’-strainer
cartridge filter, good for not very dirty water filter
Cartridge Filter
Hydrocyclone filter dirty water filter used a lot for separating solids in industrial processes
Hydrocyclone filter
Media / sand filter - a dirty water filter that does a good job but uses a lot of water when you need to back flush the media.
Media Filter
Rotorflush sel-cleaning dirty water filter - low maintenance efficient solids separation
Rotorflush Self-cleaning Filter
Run down screens - great dirty water filter for removing big bits, better with a Rotorflush filterpump to wash it down
Run-down screen
filterpresss
Filter Press
Separator
slurrypress
Slurry Press

 

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 filter will depend on the water quality and what it is in the water that causes the most trouble.

Hard solids

A hydro cyclone can often be 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.

Blocked Filter Mesh
Blocked Filter Mesh

Soft Solids

There are many different types of solids: straw and animal manure, lint in laundries, paper pulp, fats etc.  Furthermore, they can all affect the efficiency of a dirty water filter in different ways.  Soft solids can be filtered by all the filters pictured but tend to be more difficult to remove from filters and filter media than harder material.   Because softer contaminants tend to clump together they can choke filter mesh and media. Consequently, most filters will block up more quickly when filtering softer solids.

Many filters have mechanisms for backwashing the filter mesh or rinsing the filter media.  Rotorflush Filters are excellent for dealing with softer suspended solids in water. This is because their self-cleaning mechanism is continuous and therefore increases the efficiency of filtration without interrupting the flow.

Too much fat can cause problems as it binds with other fibrous contaminants and is not easily dispersed.  As a result, screen filters will struggle to filter water if it is viscous or contains much oil and fat.

Tadpoles not blocking a pump or filter
Tadpoles not blocking a pump or filter

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 anti-fouling.

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 Filter Pumps, Hydro cyclones (for heavy solids)

  • Run down screens/parabolic screens are simple systems but often need some manual cleaning.
  • Rotorflush filters and filter pumps 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 runoff 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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