What is GAC in Water Treatment

GAC (granular activated carbon) is a filter which is proven to remove organic chemicals from water. This type of filter can also be used to remove chemicals that release objectionable odours and tastes to water including chlorine and hydrogen sulphide. GAC is also a hybrid mixture of graphite platelets that are interconnected by non-graphitic carbon bonds, making it an environmentally responsible product that can be reactivated by thermal oxidation and be used multiple times for the same application.

GAC is developed by using raw organic materials that are high in carbon such as coconut shells, coal, dried corn stalks, wood, lignite, etc. In BWT products, it is primarily coconut shells. In the absence of oxygen, heat is used to increase and activate the surface area of the carbon, which is why we generally call them  “carbon” filters. Activated carbon has the ability to absorb certain dissolved chemicals in water that is passing through a filter containing GAC. All treatment systems require correct installation and periodic maintenance; it is therefore very important that the concentration of contaminants, the type of concentration, and average water usage is known – to determine the correct size and components of the systems for proper installation. Eventually, the ability of the GAC to bind and remove chemicals is used up and the GAC will need to be replaced. The replacement of the GAC is dependent on the contaminant levels and water use. Some filters may last for several years as the contaminant level and water usage is low, while some filters will require more frequent replacements.

History of GAC for water purification

GAC began around the beginning of the 20th century. During 1906, the activation process was applied to charcoal and the first application/usage of activated carbon in the form of GAC was during 1910 for the dechlorination of chlorine infested water.

The first installation of GAC filters took place in Germany during 1929, targeting odour and taste issues. We are proud to state that H2O | BWT was first to market in supplying GAC filters in South Africa.

GAC Structure

Each carbon source produces a GAC with a unique pore structure, including micro-pores, meso-pores, and macro-pores. Micro-pores are very small and are typically smaller than a molecule (5-1000 Angstroms). A higher percentage of micro-pores is found within coconut shells which makes it a good choice for disinfection by-products and small organics. Wood carbons have a high percentage of macro-pores, making it a good choice for the removal of larger organics and decolourisation. GAC’s ability to remove organic matter is derived from it’s large surface area. One single gram of GAC may have a surface area exceeding 1000 m2. We kid you not!

Recommendations for Using the GAC Filter

A typical GAC filter is susceptible to the same kinds of mechanical disturbances as a typical multimedia filter. In order to avoid sudden changes in water velocity during production, during backwash, or when starting up the filters, it is important to avoid sudden changes in water velocity. Backwash water in the filter should be allowed to flow for a minimum of 30 seconds, depending on the filter design. It is recommended to bring the filter back to full flow within 10 minutes after the unit is brought back online.  Observe periodic backwash events and account for seasonal variations in water density when backwashing.

The filter should be sampled once a year to ensure proper maintenance. In this procedure, a core sample of the filter is collected in order to get an accurate result.  An iodine number test can be used to determine whether residual activity exists in the GAC. Based on historical data, the GAC should be reactivated or exchanged as soon as the iodine level is between 450 and 550.

How to determine which GAC Water Treatment to Use

Buying a water filter is seldom an off-the-shelf purchase. Determining which water treatment to use is dependent on:

  1. Water usage,
  2. The amount and type of contaminants in the water,
  3. Other chemicals present in the water,
  4. Exposure pathways that need to be eliminated.

This is quite a science and it is why we are here to assist you. We can help you buy the right filter for whatever your water challenge requires. 

Is GAC hazardous?

Water treatment with GAC is not considered hazardous and does not create hazardous outcomes if used correctly. However, if used incorrectly, it may be hazardous as wet activated carbon depletes oxygen in an enclosed environment. So be careful when using it in your luxury-for-two submarine. Other than that you should be fine!

(Please take note that for all water filter installations, you should only use a licensed water treatment contractor or engineer).