• Conventional filtration or direct filtration
• Package system with coated steel, stainless steel, or aluminum tankage
• Sludge recirculation option
• Integrated coagulation, flocculation, sedimentation, and filtration process

Gravity filters are commonly used in applications where liquid-solids separation is required in a variety of different applications for finish water treatment. Gravity filters can incorporate various types of media such as silica sand, anthracite coal, greensand, and granular activated carbon (GAC) to meet each plant’s water treatment needs. In addition, gravity filters allow for easy inspection during operation and typically have a smaller profile.


  • Highly effective for removal of turbidity (usually < 0.1-1 NTU)
  • Cleaning time (backwashing) only takes minutes and filters can be put back into operation instantly
  • Cost-effective, space-saving water treatment system
  • Compact arrangement and simple operation
  • Dual-media filters for improved process performance


Conventional Gravity Filters provide three stages of water treatment consisting of flocculation, clarification, and filtration. Conventional filters are recommended for raw waters with turbidity greater than 5 NTU and/or colour greater than 20 TCU.

Coagulation and Flocculation

First, chemicals are rapidly mixed into the raw water and then gently agitated in a detention tank called the flocculation chamber. The main group of chemicals added are referred to as coagulants. Coagulants when mixed with the raw water form precipitates that trap suspended solids such as dirt, bacteria, viruses, and other microorganisms. The detention time in the flocculator which allows precipitates to form and remove the contaminates must be adjusted accordingly with temperature variations. The colder the raw water, the longer the detention time will need to be due to lower chemical reaction rates. Agitation of the water is required to ensure collisions between the forming precipitates and the remaining contaminant particles, this is usually achieved by the use of a gear motor and vertical propeller blade mixer. Variable speed control is often provided to allow mixing velocity adjustment to provide maximum mixing without damaging the already formed particulate matter (floc).


After flocculation, the water flows into the clarifier, a large chamber where the flow velocity of water is minimized. The low velocity allows the particulate matter to settle out of suspension by gravity. This process is often aided by the addition of tube or plate settlers. Settlers are simple angled plates or tubes spaced closely together, allowing settling particles to quickly make contact with a surface, preventing resuspension. The angle of the plates or tubes (usually 55-60 degrees off horizontal) allows the collected floc particles (called sludge) to flow by gravity to the bottom of the clarifier, which is then periodically removed in a process called desludging. The particulate removal by the clarifier reduces the solids loading on the filter media thereby extending the filter run time between backwashes.

Gravity Filtration

The filter is contained within a filter box, usually made of coated steel, stainless steel or aluminum. Inside the filter box are layers of filter media (sand, anthracite, etc.) and gravel. Below the gravel, a network of pipes makes up the underdrain which collects the filtered water and evenly distributes the backwash water. Backwash troughs help distribute the influent water and are also used in backwashing.

Operation: The influent flows down through the sand and support gravel and is captured by the underdrain. Because of the relatively clear influent water in a rapid sand filter due to coagulation/flocculation and sedimentation, rapid sand filters operate much more quickly and efficiently. In rapid gravity filtration the particulate impurities are removed in or on the media, thus causing the filter to clog after a period. Clogged filters are cleaned by backwashing.

Backwashing: Is the process of reversing the flow of water through the filter media to remove the entrapped solids. Napier-Reid’s backwashing procedure comprises the application of air and water simultaneously to obtain the most efficient wash process. As soon as most particles are washed out and the backward flowing water is clear, the filter is put back to operation. Clearly, relatively large quantities of sludge are generated through backwashing and require some form of treatment before discharge into the environment. The filtered water collects in the clearwell, where it is disinfected and then sent to the customers.

Filter Media: The filter media is the part of the filter which removes particles from the water being treated. The gravel at the bottom of the filter is not part of the filter media, it provides a support between the underdrains and the media and allows an even flow of water during filtering and backwashing.

Filter Media

Filter Media

The sand used in rapid sand filters is coarse sand. In many cases, multiple types of media are layered within the filter.  Typically, the layers (starting at the bottom of the filter and advancing upward) are sand and anthracite coal, or garnet, sand, and anthracite coal.  The picture above shows a cross-section through a dual media filter. The media in a dual or multi-media filter must have varying density as well as varying pore size so that they will sort back into the correct layering arrangement after backwashing.  Anthracite coal is a very light (low density) coal which will settle slowly, ending up as the top layer of the filter.